united states patent patent no.: smart et al. date of ... · invention there is provided an...
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(12) United States Patent Smart et al.
(54) LITHIUM-ION ELECTROLYTES CONTAINING FLAME RETARDANT ADDITIVES FOR INCREASED SAFETY CHARACTERISTICS
(75) Inventors: Marshall C. Smart, Studio City, CA (US); Kiah A. Smith, Milwaukee, WI (US); Ratnakumar V. Bugga, Arcadia, CA (US); Surya G. Prakash, Hacienda Heights, CA (US); Frederick Charles Krause, San Francisco, CA (US)
(73) Assignee: California Institute of Technology, Pasadena, CA (US)
(*) Notice: Subject to any disclaimer, the term of this patent is extended or adjusted under 35 U.S.C. 154(b) by 918 days.
(21) Appl. No.: 12/543,495
(22) Filed: Aug.18, 2009
(65) Prior Publication Data
US 2010/0047695 Al Feb. 25, 2010
Related U.S. Application Data
(60) Provisional application No. 61/189,415, filed on Aug. 19, 2008, provisional application No. 61/201,842, filed on Dec. 16, 2008.
(51) Int. Cl. H01M6116 (2006.01)
(52) U.S. Cl. USPC ........... 429/332; 429/330; 429/199; 429/200;
429/307; 429/331; 252/62.2 (58) Field of Classification Search
USPC ................. 429/332, 330, 199, 200, 307, 331; 252/62.2
See application file for complete search history.
(lo) Patent No.: US 8,795,903 B2 (45) Date of Patent: Aug. 5, 2014
(56) References Cited
U.S. PATENT DOCUMENTS
6,399,255 B2 6/2002 Herreyre et al. 6,492,064 B1 12/2002 Smart et al.
2005/0277027 Al * 12/2005 Kim et al . ..................... 429/330 2006/0073391 Al * 4/2006 Kim .............................. 429/329 20 09/00 173 86 Al * 1/2009 Xu et al . ....................... 429/331
OTHER PUBLICATIONS
Doughty, D.H., et al., "Effects of Additives on Thermal Stability of Li Ion Cells", Journal of Power Sources, 2005, 146, pp. 116-120. Ma, Y, et al., "A Phosphorous Additive for Lithium-Ion Batteries", Electrochemical and Solid State Letters, 2008, 11 (8), pp. A129-A131. Nam, T., et al., "Diphenyloctyl Phosphate and Tris(2,2,2-trifluoroethyl) Phosphite as Flame-Retardant Additives for Li-Ion Cell Electrolytes at Elevated Temperatures", Journal of Power Sources, 2008, 180, pp. 561-567.
(Continued)
Primary Examiner Laura Weiner
(57) ABSTRACT
The invention discloses various embodiments of Li-ion elec-trolytes containing flame retardant additives that have deliv-ered good performance over a wide temperature range, good cycle life characteristics, and improved safety characteristics, namely, reduced flammability. In one embodiment of the invention there is provided an electrolyte for use in a lithium-ion electrochemical cell, the electrolyte comprising a mixture of an ethylene carbonate (EC), an ethyl methyl carbonate (EMC), a fluorinated co-solvent, a flame retardant additive, and a lithium salt. In another embodiment of the invention there is provided an electrolyte for use in a lithium-ion elec-trochemical cell, the electrolyte comprising a mixture of an ethylene carbonate (EC), an ethyl methyl carbonate (EMC), a flame retardant additive, a solid electrolyte interface (SEI) film forming agent, and a lithium salt.
9 Claims, 40 Drawing Sheets
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https://ntrs.nasa.gov/search.jsp?R=20150003337 2018-06-09T09:56:38+00:00Z
US 8,795,903 B2 Page 2
(56) References Cited
OTHER PUBLICATIONS
Smart, M.C., et al., "Improved Performance of Lithium-Ion Cells
With the Use of Fluorinated Carbonate-Based Electrolytes", Journal
of Power Sources, 2003, 119-121, pp. 359-367.
Smith, K.A., et al., "Lithium-Ion Electrolytes Containing Flame-Retardant Additives for Increased Safety Characteristics", ECS Transactions, 2009, 16 (35), pp. 33-41, The Electrochemical Society.
Smith, K.A., et al., "Lithium-Ion Electrolytes Containing Flame-Retardant Additives for Increased Safety Characteristics", 214th Meeting ofthe Electrochemical Society, Honolulu, HI, Oct. 15, 2008. Wang, X., et al., "Nonflammable Trimethyl Phosphate Solvent-Con-taining Electrolytes for Lithium-Ion Batteries", Journal of the Elec-trochemical Society, 2001, 148 (10), pp. A1058-A1065.
Wang, X., et al., "Nonflammable Trimethyl Phosphate Solvent-Con-taining Electrolytes for Lithium-Ion Batteries", Journal of the Elec-trochemical Society, 2001, 148 (10), pp. A1066-A1071. Xu, K., et al., "An Attempt to Formulate Nonflammable Lithium Ion Electrolytes with Alkyl Phosphates and Phosphazenes", Journal of the Electrochemical Society, 2002, 149 (5), pp. A622-A626. Xu, K., et al., "Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate", Journal ofthe Electrochemical Society, 2002, 149 (8), pp. A1079-A1082. Xu, K., et al., "Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries", Journal of the Elec-trochemical Society, 2003, 150 (2), pp. A161-A169. Xu, K., "Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries", Chem. Rev., 2004, 104, pp. 4303-4417. Zhang, S.S., et al., "Tris(2,2,2-Trifluoroethyl) Phosphite as a Co-Solvent for Nonflammable Electrolytes in Li-Ion Batteries", Journal of Power Sources, 2003, 113, pp. 166-172.
* cited by examiner
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TEMP (°C)
RATE CURRENT (A) CAPACITY (Ah)
WATT-HOURS (VV h)
ENERGY (Wh/Kg)
l% OF ROOM I TEMP
CAPACITY (Ah)
WATT-HOURS (Wh)
ENERGY (WhIKg)
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20 °al Initial
C/5 1.800 6.9080 25.073 103.95 100 6.8947 25.03 104.06 100
20 0 c 7.000 6.2737 22.020 91.30 90.82 6.3852 22.667 94.25 92.61 cr2 3.500 6.5372 23.322 96.69 94.63 6.5249 1 23.295 96.86 94.64 C15 1.400 6.9152 24.877 103.14 100.10 6.9055 24.856 103.35 100.16
C/10 0.700 7.1899 25.884 107.31 104.08 7.1895 25.892 107.66 104.28 C/20 0.350 7.3629 26.527 109.98 1 106.58 7.3678 26.551 110.40 106.86 C;50 0.140 7.4035 26.660 110.53 107.17 7.4345 26.770 111.31 107.83
10 0 c 7.000 5.8089 20.206 83.77 84.09 5.9083 20.860 86.73 85.69 c12 3.500 6.0681 21.605 89.57 87.84 6.0524 21.572 89.70 87.78 Cr5 1.400 6.4461 23.241 96.35 93.31 6.4372 1 23.223 96.56 93.37
C/10 0.700 6.7325 24.343 100.92 97.46 6.7299 24.342 101.21 97.61 C/20 0.350 7.0210 25.370 105.18 101.64 7.0269 25.396 105.60 101.92 C/50 0.140 7.2964 26.292 109.01 1 105.62 7.3254 26.397 109.76 106.25
00 c 7.000 5.4378 18.526 76.81 78.72 5.5533 19.271 80.13 80.54 C12 3.500 5.6957 20.009 82.96 82.45 5.6758 19.953 82.96 82.32 cry 1.400 6.0224 21.626 89.66 87.18 6.0129 21.603 89.82 87.21
Clio 0.700 6.2889 22.750 94.32 91.04 6.2847 22.742 94.56 91.15 C;20 0.350 6.5637 23.808 98.70 95.02 6.5725 23.844 99.15 95.33 C150 0.140 6.9334 25.076 103.96 100.37 6.9652 25.192 104.75 101.02
-10* C 7.000 5.0073 16.430 1 68.12 72.49 5.1258 17.227 71.63 74.34 c12 3.500 5.2855 18.081 74.96 76.51 5.2613 17.998 74.83 76.31 Cr5 1.400 5.5653 19.750 81.88 80.56 5.5501 19.701 81.92 80.50
Clio 0.700 5.8634 21.075 87.37 84.88 5.8591 21.065 87.59 84.98 C;20 0.350 6.1313 22.205 92.06 88.76 6.1315 22.211 92.35 88.93 C150 0.140 6.4670 23.493 97.40 93.62 6.4913 23.584 98.06 94.15
-20 0 c 7.000 4.7786 14.257 59.11 1 69.18 4.6554 14.844 61.72 67.52 D2 3.500 4.8704 15.881 65.84 70.50 4.8393 15.753 65.60 70.19 C15 1.400 5.1634 17.657 73.20 74.75 5.1371 17.551 72.98 74.51
C/10 0.700 5.4255 19.056 79.00 78.54 5.4142 19.011 79.05 78.53 C/20 0.350 5.6926 20.362 84.42 82.41 5.6887 20.346 84.60 82.51 C/50 0.140 6.0245 21.813 90.44 87.21 6.0432 21.881 90.98 87.65
-30° C 7.000 4.5880 1 11.984 49.68 66.42 4.0878 11.885 49.42 59.29 c12 3.500 4.3046 13.065 54.17 1 62.31 4.2251 12.736 52.96 61.28 C15 1.400 4.6824 15.065 62.46 67.78 4.6663 14.964 62.22 67.68
10 0.700 4.9371 16.516 68.47 71.47 4.9252 16.461 68.44 71.44 0.350 5.2225 18.038 74.78 75.60 5.2133 17.993 74.82 75.61 0.140 5.5779 19.813 82.14 80.75 5.5942 19.873 82.63 81.14
-40 0 C 7.000 0.5644 1.396 5.79 8.17 3.8324 9.297 38.66 55.59 C12 3.500 3.6000 9.754 40.44 52.11 3.5997 9.757 40.57 52.21 Cr5 1.400 4.1115 12.038 49.91 59.52 4.0825 11.905 49.50 59.21
Clio 0.700 4.3587 13.415 55.62 63.10 4.3394 13.319 55.38 62.94 C;20 0.350 4.6906 15.109 62.64 67.90 4.6809 15.054 62.59 67.89 C/50 0.140 5.0578 17.150 71.10 73.22 5.0783 17.203 71.53 73.66
100 0.070 5.2632 18.422 76.37 1 76.19 5.3032 1 18.547 77.12 76.92 -50° C12 3.500 1.5415 3.486 14.45 22.32 1.5253 3.502 14.56 22.12
Cr5 1.400 2.8741 7.328 30.38 41.61 2.8355 7.199 29.93 41.13 Clio 0.700 3.4388 9.195 38.12 49.78 3.4039 9.063 37.68 49.37 C;20 0.350 3.9363 11.204 46.45 56.98 3.8984 11.056 45.97 56.54 C/50 0.140 4.4235 13.677 56.70 64.03 4.4448 13.719 57.04 64.47
100 0.070 4.6809 15.292 63.40 67.76 4.7114 15.361 63.87 68.33 .600 C12 3.500 0.0004 0.001 0.00 1 0.01 0.0004 1 0.001 0.00 0.01
cry 1.400 0.6521 1.378 5.71 1 9.44 0.6537 1 1.376 1 5.72 9.48 C/10 0.700 1.1357 2.563 10.63 16.44 1.0732 2.420 10.06 15.57 c/20 0.350 2.1648 5.090 21.10 31.34 1 2.1100 1 4.948 1 20.57 30.60 c/50 0.140 3.4634 9.048 37.51 1 50.14 1 3.4477 1 8.977 1 37.33 50.01
U.S. Patent Aug. 5, 2014 Sheet 38 of 40 US 8 ,795,903 B2
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U.S. Patent Aug. 5, 2014 Sheet 40 of 40 US 8,795,903 B2
Fig. 39
US 8,795,903 B2 2
LITHIUM-ION ELECTROLYTES CONTAINING FLAME RETARDANT
ADDITIVES FOR INCREASED SAFETY CHARACTERISTICS
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/189,415, filed Aug. 19, 2008, which is incorporated herein by reference in its entirety, and this application also claims priority to U.S. Provisional Patent Application Ser. No. 61/201,842, filed Dec. 16, 2008, which is also incorporated herein by reference in its entirety.
GOVERNMENT RIGHTS
The invention described herein was made in the perfor-mance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.
BACKGROUND
a. Field of the Invention The invention relates to electrolytes and organic solvents
for electrochemical cells. In particular, the invention relates to lithium-ion electrolytes and organic solvents for lithium-ion cells.
b. Background Art Lithium-ion ("Li-ion") cells typically include a carbon
(e.g., coke or graphite) anode intercalated with lithium ions to form LixC; an electrolyte consisting of a lithium salt dis-solved in one or more organic solvents; and a cathode made of an electrochemically active material, typically an insertion compound, such as LiCo02 . During cell discharge, lithium ions pass from the carbon anode, through the electrolyte to the cathode, where the ions are taken up with the simultaneous release of electrical energy. During cell recharge, lithium ions are transferred back to the anode, where they reintercalate into the carbon matrix.
Future NASA missions aimed at exploring Mars, the Moon, and the outer planets require rechargeable batteries that can operate effectively over a wide temperature range (-60° C. (Celsius) to +60° C. (Celsius)) to satisfy the require-ments of various applications, including: Landers (lander spacecraft), Rovers (surface rover spacecraft), and Pen-etraters (surface penetrator spacecraft). Some future applica-tions typically will require high specific energy batteries that can operate at very low temperatures, while still providing adequate performance and stability at higher temperatures. In addition, many of these applications envisioned by the ESRT (Exploration Systems Research and Technology) program will require improved safety, due to their use by humans. Lithium-ion rechargeable batteries have the demonstrated characteristics of high energy density, high voltage, and excellent cycle life. Currently, the state-of-the-art lithium-ion system has been demonstrated to operate over a wide range of temperatures (-40° C. to +40° C.), however, abuse conditions such as being exposed to high temperature, overcharge, and external shorting, can often lead to cell rupture and fire. The nature of the electrolyte can greatly affect the propensity of the cell/battery to catch fire, given the flammability of the organic solvents used within. Therefore, extensive effort has been devoted recently to developing non-flammable electro-lytes to reduce the flammability of the cell/battery.
Desired properties for Li-ion electrolytes can include high conductivity over a wide temperature range (e.g., 1 mS (milli- Siemens) cm- ' from —60° C. to +60° C.); good electrochemi- cal stability over a wide voltage range (e.g., 0 to 4.5V (volts))
5 with minimal oxidative degradation of solvents/salts; good chemical stability; good compatibility with a chosen elec- trode couple, including good SEI (solid electrolyte interface) characteristics on the electrode and facile lithium intercala- tion/de-intercalation kinetics; good thermal stability; good
io low temperature performance throughout the life of the cell, including good resilience to high temperature exposure and minimal impedance build-up with cycling and/or storage; and low toxicity. Since the flammability of the electrolyte solution in Li-ion batteries is a major concern, significant research has
15 been devoted to developing electrolyte formulations with increased safety. Known electrolytes used in state-of-the-art Li-ion cells have typically comprised binary mixtures of organic solvents, for example, high proportions of ethylene carbonate, propylene carbonate or dimethyl carbonate, within
20 which is dispersed a lithium salt, such as lithium hexafluoro- phosphate (LiPF 6). Examples may include 1.0 M (molar) LiPF 6 in a 50:50 mixture of ethylene carbonate/dimethyl car- bonate, or ethylene carbonate/diethyl carbonate. More recently, electrolytes have also been developed which com-
25 bine more than two solvents and/or have incorporated the use of electrolyte additives to address specific performance goals.
Fluorinated esters have been incorporated into multi-com- ponent electrolyte formulations and their performance was demonstrated over a wide temperature range (-60° C. to +60°
30 C.) (see U.S. application Ser. No. 12/419,473 filed Apr. 7, 2009, for "Lithium Ion Electrolytes and Lithium Ion Cells with Good Low Temperature Performance", Smart et al.). The fluorinated ester co-solvents were employed due to their favorable properties and improved safety characteristics,
35 mainly associated with their low flammability associated with their halogenated nature. Since the flammability of the electrolyte solution in Li-ion batteries is a major concern, significant research has been devoted to developing electro- lyte formulations with increased safety. To achieve this, a
4o number of approaches have been adopted, including the use of low-flammability solvents and the use of electrolyte addi- tives. Regarding the first approach, the use of halogenated solvents (Smart, et al., "Improved Performance of Lithium Ion Cells with the use of Fluorinated Carbonate-Based Elec-
45 trolytes", Journal of Power Sources, 119-121, 359-367 (2003)) and ionic liquids (Xu, Kang, "Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries", Chemical Reviews, 104(10), 4303-4417 (2004)) have been pursued. With respect to the use of electrolyte additives, the
50 main focus has been upon the use of phosphorus containing additives, including trimethyl phosphate (Wang, et al., "Non- flammable Trimethyl Phosphate Solvent-Containing Electro- lytes for Lithium-Ion Batteries", Journal oftheElectrochemi- cal Society, 148(10), A1058-A1065 (2001); Wang, et al.,
55 "Nonflammable Trimethyl Phosphate Solvent-Containing Electrolytes for Lithium-Ion Batteries", Journal of the Elec- trochemical Society, 148(10), A1066-A1071 (2001)), triethyl phosphate (Xu, et al., `An Attempt to Formulate Nonflam- mable Lithium Ion Electrolytes with Alkyl Phosphates and
60 Phosphazenes", Journal of the Electrochemical Society, 149 (5), A622-A626 (2002)), triphenyl phosphate (Doughty, et al., "Effects of Additives on Thermal Stability of Li-ion Cells", Journal of Power Sources, Vol. 146, Issues 1-2, pp. 116-120 (2005)), tris(2,2,2-trifluoroethyl) phosphate (Xu, et
65 al., "Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate", Journal of the Electrochemical Society, 149(8), Al 079-Al 082 (2002); Xu, et al., "Evaluation
US 8,795,903 B2 3
of Fluorinated Alkyl Phosphates as Flame Retardants in Elec-trolytes for Li-Ion Batteries", Journal of the Electrochemical Society, 150(2), Al61 -Al 69 (2003)), and bis(2,2,2-trifluoro-ethyl) methyl phosphonate (TFMPo) (Xu, et al., "Nonflam-mable Electrolytes for Li-Ion Batteries Based on a Fluori- 5
nated Phosphate", Journal of the Electrochemical Society, 149(8), Al 079-A1082 (2002); Xu, et al., "Evaluation of Flu-orinated Alkyl Phosphates as Flame Retardants in Electro-lytes for Li-Ion Batteries", Journal of the Electrochemical Society, 150(2), Al61 -Al 69 (2003)). 10
In addition, known improvements have been made to the safety characteristics of Li-ion electrolytes by the addition of flame retardant additives, such as triphenyl phosphate (re-ferred to as TPhPh or TPP or TPPa), tributyl phosphate (re-ferred to as TBP or TBuPh), triethyl phosphate (referred to as 15
TEP or TEtPh), and bis(2,2,2-trifluoroethyl) methyl phos-phonate (referred to as BTFEMP or TFMPo) (see NPO-46262, May 8, 2008). A number of electrolytes based upon these approaches have delivered good performance over a wide temperature range, good cycle life characteristics, and 20
improved safety characteristics, namely reduced flammabil-ity. Since the flammability of the electrolyte solution in Li-ion batteries is a major concern, significant research has been devoted to developing electrolyte formulations with increased safety. To achieve this, a number of approaches 25
have been adopted, including the use of low-flammability solvents and the use of electrolyte additives. As discussed above, regarding the first approach, the use of halogenated solvents (Smart, et al., "Improved Performance and Safety of Lithium Ion Cells with the Use of Fluorinated Carbonate - 30
Based Electrolytes", Journal of Power Sources, 119-12,359- 367 (2003)) and ionic liquids (Xu, Kang, "Nonaqueous Liq-uid Electrolytes for Lithium-Based Rechargeable Batteries", Chemical Review, 104(10), 4303-4417 (2004)) have been pursued. With respect to the use of electrolyte additives, the 35
main focus has been upon the use of phosphorus containing additives, including trimethyl phosphate, triethyl phosphate, triphenyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, and bis(2,2,2-trifluoroethyl) methyl phosphonate (TFMPo) (see all above). In addition, known phosphorus based flame retar- 40
dant additives have been investigated. Some of these electro-lyte additives have been demonstrated to perform well in multi-component Li-ion battery electrolytes. For example, tris(2,2,2-trifluoroethyl) phosphate (see FIG. 5e) has been used in electrolyte solutions consisting of LOM LiPF 6 in 45
EC+EMC (1:1 wt %) in varying concentrations (Xu, et al., "Nonflammable Electrolytes for Li-Ion Batteries Based on a Fluorinated Phosphate", Journal of the Electrochemical Soci-ety, 149(8), A1079-A1082 (2002); Xu, et al., "Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Elec- 50
trolytes for Li-Ion Batteries", Journal of the Electrochemical Society, 150(2), A161-A169 (2003)). Tris(2,2,2-trifluoroet hyl) phosphite (see FIG. 5f) has been used in concentrations ofup to 15% in solutions of LOM LiPF 6 PC+EC+EMC (3:3:4 wt %) (Zhang, et al., "Tris(2,2,2-trifluoroethyl) Phosphite as 55
a Co-Solvent for Nonflammable Electrolytes in Li-Ion Bat-teries", Journal of Power Sources, 113 (1), 166-172 (2003)), while other have investigated in 1.15M LiPF6 in EC+EMC (3:7 vol %) (Nam, et al., "Diphenyloctyl Phosphate and tris (2,2,2-trifluoroethyl) Phosphite as Flame-Retardant Addi- 60
tives for Li-ion Cell Electrolytes at Elevated Temperature", Journal of Power Sources, 180 (1), 561-567 (2008)). Triph-enylpho sphite (see FIG. 5g) has been investigated in solutions consisting of LOM LiPF 6 in EC+DEC+DMC (1:1:1 wt %) using concentrations of 10 wt % FRA (Ma, et al., A Phos- 65
phorous Additive for Lithium-Ion Batteries", Electrochemi-cal and Solid State Letters, 11(8), A129-A131 (2008)).
4 Accordingly, there is a need for lithium-ion electrolytes
containing flame retardant additives having increased and improved safety characteristics.
SUMMARY
This need for lithium-ion electrolytes containing flame retardant additives having increased and improved safety characteristics is satisfied. The invention discloses various embodiments of Li-ion electrolytes containing flame retar-dant additives that have delivered good performance over a wide temperature range, good cycle life characteristics, and improved safety characteristics, namely, reduced flammabil-ity.
In one embodiment of the invention there is provided an electrolyte for use in a lithium-ion electrochemical cell. The electrolyte comprises a mixture of an ethylene carbonate (EC); an ethyl methyl carbonate (EMC); a fluorinated co-solvent; a flame retardant additive; and, a lithium salt. Pref-erably, the electrochemical cell operates in a temperature range of from about —50 degrees Celsius to about 60 degrees Celsius. The fluorinated co-solvent preferably comprises 2,2, 2-trifluoroethyl butyrate (TFEB), di-2,2,2-trifluoroethyl car-bonate (DTFEC), 2,2,2-trifluoroethyl methyl carbonate (TFEMC), mono-fluoroethylene carbonate (FEC), ethyl trif-luoroacetate (ETFA), 2,2,2-trifluoroethyl acetate (TFEA), 2,2,2-trifluoroethyl propionate (TFEP), ethyl-2,2,2-trifluoro-ethyl carbonate (ETFEC), propyl-2,2,2-trifluoroethyl car-bonate (PTFEC), methyl-2,2,2,2',2',2'-hexafluoro-i-propyl carbonate (MHFPC), ethyl-2,2,2,2',2',2'-hexafluoro-i-propyl carbonate (EHFPC), fluoropropylene carbonate (FPC), trif-luoropropylene carbonate (TFPC), methyl nonafluorobutyl ether, 2,2,3,3,3-pentafluoropropyl methyl ether, 1,1,3,3,3-pentafluoro-2-trifluoromethyl ether, 2,2,3,3-tetrafluoropro-pyl difluoromethyl ether, 1, 1, 2,3,3,3 -hexafluoropropyl ethyl ether, or perfluoropolyether. The flame retardant additive preferably comprises triphenyl phosphate (TPhPh/TPP/ TPPa), tributyl phosphate (TBP/TBuPh), triethyl phosphate (TEP/TEtPh), bis(2,2,2-trifluoroethyl) methyl phosphonate (BTFEMP/TFMPo), triphenyl phosphite, tris(2,2,2-trifluoro-ethyl) phosphate, tris(2,2,2-trifluoroethyl) phosphite, diethyl ethylphosphonate, and diethyl phenylphosphonate.
In another embodiment of the invention there is provided an electrolyte for use in a lithium-ion electrochemical cell. The electrolyte comprises a mixture of an ethylene carbonate (EC); an ethyl methyl carbonate (EMC); a flame retardant additive; a solid electrolyte interface (SEI) film forming agent; and, a lithium salt. Preferably, the electrochemical cell operates in a temperature range of from about —50 degrees Celsius to about 60 degrees Celsius. The flame retardant additive preferably comprises triphenyl phosphate (TPhPh/ TPP/TPPa), tributyl phosphate (TBuPh), triethyl phosphate (TEtPh), bis(2,2,2-trifluoroethyl) methyl phosphonate (TFMPo), triphenyl phosphite, tris(2,2,2-trifluoroethyl) phosphate, tris(2,2,2-trifluoroethyl) phosphite, diethyl eth-ylphosphonate, and diethyl phenylphosphonate. The solid electrolyte interface (SEI) film forming agent preferably comprises vinylene carbonate (VC), vinyl ethylene carbonate (VEC), dibutyl pyrocarbonate (DBPC), dimethyl pyrocar-bonate (DMPC), mono-fluoroethylene carbonate (FEC), lithium bis(oxalate)borate (LiBOB), lithium oxaltodifluo-roborate (LiODFB), ethylene sulfite, propylene sulfite, buty-lene sulfite, lithium tetrafluorooxalatophosphate (LiPF 4 (C204)), vinylene acetate, acrylic acid nitcile, ethyl isocyanate, 2-cyano furan, divinyl adipate, maleic anhydride, 2-vinyl pyridine, and vinyl-containing silane based com-pounds.
US 8,795,903 B2 5
In another embodiment of the invention there is provided an electrolyte for use in a lithium-ion electrochemical cell. The electrolyte comprises a mixture of an ethylene carbonate (EC); an ethyl methyl carbonate (EMC); a phosphorus con-taining flame retardant additive; and, a lithium salt. Prefer-ably, the electrochemical cell operates in a temperature range of from about —50 degrees Celsius to about 60 degrees Cel-sius. The phosphorus containing flame retardant additive preferably comprises triphenyl phosphate (TPhPh/TPP/ TPPa), tributyl phosphate (TBuPh), triethyl phosphate (TEtPh), bis(2,2,2-trifluoroethyl) methyl phosphonate (TFMPo), triphenyl phosphite, tris(2,2,2-trifluoroethyl) phosphate, tris(2,2,2-trifluoroethyl) phosphite, diethyl eth-ylphosphonate, and diethyl phenylphosphonate.
In another embodiment of the invention there is provided a lithium-ion electrochemical cell comprising an anode; a cath-ode; and, an electrolyte interspersed between the anode and the cathode. The electrolyte comprises a mixture of an ethyl-ene carbonate (EC); an ethyl methyl carbonate (EMC); a fluorinated co-solvent preferably comprising 2,2,2-trifluoro-ethyl butyrate (TFEB), di-2,2,2-trifluoroethyl carbonate (DT-FEC), 2,2,2-trifluoroethyl methyl carbonate (TFEMC), mono-fluoroethylene carbonate (FEC), ethyl trifluoroacetate (ETFA), 2,2,2-trifluoroethyl acetate (TFEA), 2,2,2-trifluoro-ethyl propionate (TFEP), ethyl -2,2,2 -tri fluoroethyl carbonate (ETFEC), propyl-2,2,2-trifluoroethyl carbonate (PTFEC), methyl-2,2,2,2',2',2'-hexafluoro-i-propyl carbonate (MH-FPC), ethyl-2,2,2,2',2',2'-hexafluoro-i-propyl carbonate (EH-FPC), fluoropropylene carbonate (FPC), trifluoropropylene carbonate (TFPC), methyl nonafluorobutyl ether, 2,2,3,3,3-pentafluoropropyl methyl ether, 1,1,3,3,3-pentafluoro-2-trif-luoromethyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, or perfluo-ropolyether; a flame retardant additive preferably comprising triphenyl phosphate (TPhPh/TPP/TPPa), tributyl phosphate (TBuPh), triethyl phosphate (TEtPh), bis(2,2,2-trifluoroet-hyl) methyl phosphonate (TFMPo), triphenyl phosphite, tris (2,2,2-trifluoroethyl) phosphate, tris(2,2,2-trifluoroethyl) phosphite, diethyl ethylphosphonate, and diethyl phe-nylphosphonate; and, a lithium salt. Preferably, the electro-chemical cell operates in a temperature range of from about —50 degrees Celsius to about 60 degrees Celsius.
In another embodiment of the invention there is provided a lithium-ion electrochemical cell comprising an anode; a cath-ode; and, an electrolyte interspersed between the anode and the cathode. The electrolyte comprises a mixture of an ethyl-ene carbonate (EC); an ethyl methyl carbonate (EMC); a phosphorus containing flame retardant additive preferably comprising triphenyl phosphate (TPhPh/TPP/TPPa), tributyl phosphate (TBuPh), triethyl phosphate (TEtPh), bis(2,2,2-trifluoroethyl) methyl phosphonate (TFMPo), triphenyl phosphite, tris(2,2,2-trifluoroethyl) phosphate, tris(2,2,2-tri-fluoroethyl) phosphite, diethyl ethylphosphonate, and diethyl phenylphosphonate; and, a lithium salt. The electrochemical cell operates in a temperature range of from about —50 degrees Celsius to about 60 degrees Celsius.
The features, functions, and advantages that have been discussed can be achieved independently in various embodi-ments of the disclosure or may be combined in yet other embodiments further details of which can be seen with refer-ence to the following description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be better understood with reference to the following detailed description taken in conjunction with
6 the accompanying drawings which illustrate preferred and exemplary embodiments, but which are not necessarily drawn to scale, wherein:
FIG. 1 is an illustration of a table summary of charge- s discharge characteristics of experimental lithium-ion cells
containing various electrolytes subjected to formation cycling according to the invention;
FIG. 2 is an illustration of a table summary of discharge capacity at low temperature for MCMB-LiNiCo0 2 contain-
10 ing electrolytes with flame retardant additives according to the invention;
FIG. 3 is an illustration of a graph showing discharge capacity at —20° C. (expressed as percent of room tempera-
15 ture capacity) of MCMB-LiNiCo0 2 cells containing electro-lytes with flame retardant additives according to the inven-tion;
FIG. 4 is an illustration of a graph showing cycle life performance of MCMB-LiNiCo0 2 cell containing electro-
20 lytes with flame retardant additives according to the inven-tion;
FIG. 5a is an illustration of the chemical structure of flame retardant triphenyl phosphate;
FIG. 5b is an illustration of the chemical structure of flame 25 retardant tributyl phosphate;
FIG. 5c is an illustration of the chemical structure of flame retardant triethyl phosphate;
FIG. 5d is an illustration of the chemical structure of flame retardant bis(2,2,2-trifluoroethyl) methyl phosphonate;
30 FIG. 5e is an illustration of the chemical structure of flame retardant tris(2,2,2-trifluoroethyl) phosphate;
FIG. 5f is an illustration of the chemical structure of flame retardant tris(2,2,2-trifluoroethyl) phosphite;
FIG. 5g is an illustration of the chemical structure of flame 35 retardant triphenylphosphite;
FIG. 5h is an illustration of the chemical structure of flame retardant diethyl phenyl phosphonate;
FIG. 5i is an illustration of the chemical structure of flame retardant diethyl ethyl phosphonate;
40 FIG. 6 is an illustration of a table summary of charge- discharge characteristics of experimental MCMB-LiNiCo0 2 cells containing electrolytes with flame retardant additives subjected to formation cycling according to the invention;
FIG. 7 is an illustration of a graph showing fifth discharge 45 of formation cycling forcells containing flame retardant addi-
tives according to the invention; FIG. 8 is an illustration of a table summary of discharge
capacity at low temperature of a number of MCMB-LiNi-0002 cells containing electrolytes with flame retardant addi-
50 tives according to the invention; FIG. 9 is an illustration of a graph showing discharge
capacity at —20° C. (expressed as percent of room tempera-ture capacity) of MCMB-LiNiCo0 2 cells containing electro-lytes with flame retardant additives according to the inven-
55 tion; FIG. 10 is an illustration of a graph showing discharge
capacity at —40° C. (expressed as percent of room tempera-ture capacity of MCMB-LiNiCo0 2 cells containing electro-lytes with flame retardant additives according to the inven-
60 tion; FIG. 11 is an illustration of a graph showing cycle life
performance of MCMB-LiNiCo0 2 cells containing electro-lytes with flame retardant additives according to the inven-tion;
65 FIG. 12 is an illustration of a summary table of formation characteristics of electrolytes with flame retardant additives and fluoroesters according to the invention;
US 8,795,903 B2 7
FIG. 13 is an illustration of a graph showing cycle life of electrolytes with flame retardant additives and fluoroesters according to the invention;
FIG. 14 is an illustration of a summary table of results obtained from Electrochemical Impedance Spectroscopy 5
(EIS) measurements for cathodes in contact with electrolytes with flame retardant additives and fluoroesters according to the invention;
FIG. 15 is an illustration of a summary table of results obtained from Electrochemical Impedance Spectroscopy io (EIS) measurements for anodes in contact with electrolytes with flame retardant additives and fluoroesters according to the invention;
FIG. 16 is an illustration of a graph showing the Tafel polarization measurements performed on MCMB anodes in 15
contact with electrolytes with flame retardant additives and fluoroesters and data was obtained from MCMB-LiNio $COo z0z cells containing lithium reference electrodes according to the invention;
FIG. 17 is an illustration of a graph showing the Tafel 20
polarization measurements performed on LiNiCo0 2 cath-odes in contact with electrolytes with flame retardant addi-tives and fluoroesters and data was obtained from MCMB-LiNiCo02 cells equipped with lithium reference electrodes according to the invention; 25
FIG. 18 is an illustration of a table summary of DC micro-polarization measurements for the cathode of cells containing flame retardant additives and fluoroester electrolytes accord-ing to the invention;
FIG. 19 is an illustration of a table summary of DC micro- 30
polarization measurements for the anode of cells containing flame retardant additives and fluoroester electrolytes accord-ing to the invention;
FIG. 20 is an illustration of the chemical structure of SEI (solid electrolyte interface) enhancing agent vinylene carbon- 35
ate; FIG. 21 is an illustration of a table summary of formation
characteristics of electrolytes with flame retardant additives and SEI enhancing agent according to the invention;
FIG. 22 is an illustration of a table summary of evaluation 40
of low temperature performance of cells containing flame retardant additives and SEI additive according to the inven-tion;
FIG. 23 is an illustration of a table summary of results obtained from Electrochemical Impedance Spectroscopy 45
(EIS) measurements for the cathodes of cells containing flame retardant additives/VC according to the invention;
FIG. 24 is an illustration of a table summary of results obtained from Electrochemical Impedance Spectroscopy (EIS) measurements for the anodes of cells containing flame 50
retardant additives/VC according to the invention; FIG. 25 is an illustration of a table summary of linear
polarization measurements for the anodes of cells containing electrolytes with flame retardant additives/VC according to the invention; 55
FIG. 26 is an illustration of a table summary of linear polarization measurements for the cathodes of cells contain-ing electrolytes with flame retardant additives/VC according to the invention;
FIG. 27 is an illustration of a table summary of formation 60
characteristics of experimental MCMB-LiNiCo0 2 cells con-taining electrolytes with flame retardant additives subjected to formation cycling according to the invention;
FIG. 28 is an illustration of a table summary of discharge capacity at low temperature of a number of MCMB-LiNi- 65
C002 cells containing electrolytes with flame retardant addi-tives according to the invention;
8 FIG. 29 is an illustration of a graph showing discharge
performance at 0° C. (expressed as percent of room tempera-ture capacity) of a —400 mAh MCMB-LiNiCo0 2 cell con-taining 1.0 M LiPF 6 in EC+EMC+TPP (20:70:10 v/v %) according to the invention;
FIG. 30 is an illustration of a graph showing discharge performance at 0° C. (expressed as percent of room tempera-ture capacity) of a MCMB-LiNiCo0 2 cell containing 1.0 M LiPF 6 in EC+EMC+DTFEC+TPP (20:50:20:10 v/v %) according to the invention;
FIG. 31 is an illustration of a graph showing the Tafel polarization measurements of LiNiCo0 2 electrodes from MCMB-LiNiCo02 cells containing electrolytes with 10% TPP and with and without the addition of DTFEC according to the invention;
FIG. 32 is an illustration of a graph showing the Tafel polarization measurements of MCMB electrodes from MCMB-LiNiCo02 cells containing electrolytes with 10% TPP and with and without the addition of DTFEC according to the invention;
FIG. 33 is an illustration of a table summary of discharge capacity at low temperature of a number of MCMB-LiNi-0002 cells containing electrolytes with 10% TPP and TFEMC and/or FEC according to the invention;
FIG. 34 is an illustration of a graph showing discharge performance at 0° C. using a —C/8 discharge rate to 2.0 V (volts) according to the invention;
FIG. 35 is an illustration of a table summary of the forma-tion characteristics of a number of Li Li (Lio.i7Nio.25N4uo.5a)O2 cells containing various electrolytes with TPP and TPPi flame retardant additives according to the invention;
FIG. 36 is an illustration of a table summary of discharge performance of a 7 Ah Li-ion cell containing LOM LiPF 6 in EC+EMC+TPP+VC (19.7/73.9/4.9/1.5 v/v %) according to the invention;
FIG. 37 is an illustration of a graph showing discharge rate performance at 20° C. of a 7 Ah Li-ion cell containing LOM LiPF 6 in EC+EMC+TPP+VC (19.7/73.9/4.9/1.5 v/v %) according to the invention;
FIG. 38 is an illustration of a graph showing 100% depth of discharge (DOD) cycling of 7 Ah Li-ion cell containing flame retardant additive containing electrolyte [LOM LiPF 6 in EC+EMC+TPP+VC (19.7/73.9/4.9/1.5 v/v %)] and the base-line electrolyte [LOM LiPF 6 in EC+DMC+DEC (1:1:1 v/v %)] according to the invention; and,
FIG. 39 is an illustration of a partially exploded view of a lithium-ion electrochemical cell constructed according to one embodiment of the invention.
DETAILED DESCRIPTION
Disclosed embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all disclosed embodiments are shown. Indeed, several different embodiments may be provided and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the disclosure to those skilled in the art.
The invention discloses various embodiments of Li-ion electrolytes containing flame retardant additives that have delivered good performance over a wide temperature range, good cycle life characteristics, and improved safety charac-teristics, namely, reduced flammability. In addition, the invention discloses lithium-ion electrochemical cells com-
US 8,795,903 B2 9
10 prising anodes; cathodes; and embodiments of the electrolyte aerospace applications (e.g., astronaut equipment, satellites, of the invention interspersed between the anodes and the planetary rovers, and other suitable applications), and mili- cathodes. tary applications (e.g., communications devices, aircraft bat-
FIG. 39 is an illustration of a partially exploded view of a teries, back-up power sources, and other suitable applica- lithium-ion electrochemical cell 10 or battery constructed 5 tions). according to one embodiment of the invention. The electro- lytes and organic solvents described herein may be used in the
EXAMPLES AND ELECTROCHEMICAL
construction of the improved Li-ion electrochemical cell, MEASUREMENTS characterized by good performance over a wide temperature range, good cycle life characteristics, and improved safety 10 Li-ion Electrolytes Containing FRA and characteristics. The electrochemical cell preferably operates
Incorporating Fluorinated Ester Co-Solvents
in a temperature range of from about —50 degrees C. (Celsius) to about 60 degrees C. (Celsius). The lithium-ion electro- Description. Experiments were conducted on Li-ion elec- chemical cell 10 comprises an anode 12. The anode 12 may trolytes containing flame retardant additives (FRA) and comprise mesocarbon microbeads (MCMB) carbon, lithium 15 incorporating fluorinated ester co-solvents. As part of a con- titanate (Li4Ti5O 12), carbon graphite, coke based carbon, tinuing effort to develop advanced electrolytes to improve the lithium metal, or another suitable material. Carbon is the safety and performance of Li-ion cells, especially over a wide preferred anode material for lithium-ion rechargeable cells operating temperature range, a number of Li-ion electrolytes due to its low potential versus lithium (of the lithiated com- that contain flame retardant additive in conjunction with flu- pound), excellent reversibility for lithium intercalation/de- 20 orinated ester co-solvents were developed to provide a safe, intercalation reactions, good electronic conductivity, and low wide operating temperature range system. The safety charac- cost. Three broad types of carbonaceous anodic materials are teristics of these electrolytes were further improved by the known: (a) non-graphitic carbon, e.g., petroleum coke, pitch
addition of flame retardant additives, such as triphenyl phos-
coke, (b) graphitic carbon, e.g., natural graphite, synthetic phate (TPP or TPPA or TPhPh), tributyl phosphate (TBP or graphite, and (c) modified carbon, e.g., mesocarbon micro- 25 TBuPh), triethyl phosphate (TEP or TEtPh), and bis(2,2,2- bead carbon material. The lithium-ion electrochemical cell
trifluoroethyl) methyl phosphonate (BTFEMP or TFMPo). A
further comprises a cathode 14 such as an insertion-type number of electrolytes based upon these approaches have cathode. The cathode 14 may comprise lithium cobalt oxide
delivered good performance over a wide temperature range,
(LiCO02), lithium nickel cobalt oxide (LiNi o $Coo 202), good cycle life characteristics, and improved safety charac- lithium manganese oxide (LiMn2O4) 1 lithium nickel cobalt 30 teristics, namely reduced flammability. aluminum oxide (LiNiCoA1O 2), lithium metal phosphate
The following electrolyte formulations were investigated
(LiMPO4) where the metal may comprise iron, cobalt, man- and demonstrated in experimental MCMB carbon- ganese, or another suitable metal, lithium nickel cobalt man- LiNio $Coo 202 cells: (1) 1.0 M LiPF 6 in EC+EMC+TFEB+ ganese oxide (LiNiCoMnO 2), high voltage electrodes such as
TPP (20:55:20:5 v/v %); (2) 1.0 M LiPF 6 in EC+EMC+
Li(Lio 17Nio 25Mn(1 58)02 and LiMn2O4 spinel, or another 35 TFEB+TBP (20:55:20:5 v/v %); (3) 1.0 M LiPF 6 in suitable material. Suitable cathode materials include transi- EC+EMC+TFEB+TEP (20:55:20:5 v/v %); (4) 1.0 M LiPF 6 tion metal oxides, such as insertion-type metal oxides. In
in EC+EMC+TFEB+BTFEMP (20:55:20:5 v/v %); (5)1.0 M
lithium-ion cells, the cathode functions as a source of lithium
LiPF 6 in EC+EMC+TPP (20:75:5 v/v %); (6)1.0 M LiPF 6 in for the intercalation/de-intercalation reactions at the anode
EC+EMC+TPP (20:75:5 v/v %)+1.5% VC; (7) 1.0 M LiPF 6
and the cathode. Also, it is preferable that the cathode material 40 in EC+EMC (20:80 v/v %)+1.5%VC; and, (8)1.0 M LiPF 6 in in lithium-ion cells have a high voltage versus lithium (>3.0
EC+EMC (20:80 v/v %) (Baseline).
V) to compensate for voltage losses due to the use of alternate
In general, many of the formulations displayed good per- lithium anode materials (having reduced lithium activity)
formance over a wide temperature range, good cycle life
such as lithiated carbon. Lithiated cobalt oxide is a preferred
characteristics, and were expected to have improved safety compound because of its ease of preparation and reversibility. 45 characteristics, namely low flammability. Of the electrolytes Lithiated nickel oxide, lithiated manganese oxide, and other studied, 1.0 M LiPF 6 EC+EMC+TFEB+TPP (20:55:20:5 v/v suitable lithiated metal oxides are good alternatives. The
6/%) (where TPP=triphenyl phosphate) was identified as being
anode 12 may be separated from the cathode 14 by one or a promising non-flammable electrolyte, due to reasonable more electrolyte-permeable separators 16, with the anode/
low temperature performance and superior life characteris-
separator(s)/cathode preferably cylindrically rolled up in 50 tics. In addition, the electrolyte consisting of 1.0 M LiPF 6 "jelly roll' fashion and inserted into a can or case 18, which is
EC+EMC+TPP (20:75:5 v/v 6/%)+1.5%VCwas demonstrated
sealed or closed by a cap 20. The Li-ion electrochemical cell
to have even further improved life characteristics, due to the 10 further comprises an electrolyte, such as one of the elec- incorporation of an SEI (solid electrolyte interface) promoter trolytes discussed in detail below, interspersed between the
(i.e., VC=vinylene carbonate), which appears to inhibit the
anode and the cathode. Both the anode 12 and the cathode 14 55 decomposition of the TPP. are bathed in the electrolyte which is able to pass through the
A number of experimental lithium-ion cells, consisting of
separator(s), allowing ion movement from one electrode to
MCMB carbon anodes and LiNi o BCoo 202 cathodes, have the other. The electrolyte that may be used in the Li-ion
been fabricated to study the described technology. These cells
electrochemical cell may comprise one of the below dis- served to verify and demonstrate the reversibility, low tem- cussed mixtures. Other features, such as one or more gaskets, 60 perature performance, and electrochemical aspects of each anode and cathode tabs, safety vents, center pin, and other electrode as determined from a number of electrochemical features known in the art may be included as deemed appro- characterization techniques. The electrolytes selected for priate, in accordance with known battery design and fabrica- evaluation are listed above (see electrolytes 1-8). FIG.1 is an tion. The electrolytes of the invention may be used in the
illustration of a table summary of charge-discharge charac-
above-described electrochemical cell or battery, and may also 65 teristics of experimental lithium-ion cells containing various be used in such batteries as automotive Li-ion cell batteries, electrolytes subjected to formation cycling according to the computer laptop Li-ion cell batteries, and can be used for
invention. As shown in FIG.1, all cells displayed good revers-
US 8,795,903 B2 11 12
ibility at room temperature and minimal reactivity during the formation cycling. The high coulombic efficiency and com-parable irreversible capacity losses were indirectly related to the overall stability of the solutions and the electrode filming characteristics. As further shown in FIG. 1, reasonable revers- 5
ibility was observed with the cells containing all of the elec-trolyte variations, when compared after the formation cycling. It should be noted that some variation in capacity was due to different electrode weights, and not electrolyte type, so most comparisons were expressed in terms of percentage of io initial capacity under ambient temperatures. Of the additives investigated, the triphenyl phosphate (TPP) displayed the lowest irreversible capacity losses and highest coulombic efficiency, suggesting that it is not electrochemically decom-posing and participating in the electrode filming process del- 15
eteriously. Low Temperature Performance. FIG. 2 is an illustration of
a table summary of discharge capacity at low temperature for MCMB-LiNiCoO 2 containing electrolytes with flame retar-dant additives according to the invention. When the cells 20
described were evaluated at low temperatures, as shown in FIG. 2, reasonable low temperature performance was gener-ally observed. This finding was significant since the use of flame retardant additives was anticipated in decreasing the power capability and low temperature performance, due to 25
decreased ionic conductivity of the media and increased elec-trode film resistance with the possible reactivity of the FRA.
As expected, the low temperature performance was some-what compromised upon the addition of the flame retardant additives, especially at temperatures below —20° C. (Celsius). 30
Of the FRA-containing electrolytes the formulation contain-ing tributyl phosphate delivered the best performance at low temperature, with over 68% of the room temperature capacity being delivered at —20° C. using a —C/16 (approximate full capacity over 16 hour discharge rate), as illustrated in FIG. 3. 35
FIG. 3 is an illustration of a graph showing discharge capacity at —20° C. (expressed as percent of room temperature capac-ity) of MCMB-LiNiCoO2 cells containing electrolytes with flame retardant additives according to the invention. The x-axis shows percent of room temperature capacity (%) and 40
the y-axis shows voltage (V). Cycle Life Performance. As mentioned previously, the
general expectation was that the cycle life performance would be compromised with the addition of FRAs due to the pos-sible reactivity with the electrode interfaces leading toimped- 45
ance build-up and capacity loss. To mitigate this, incorpora-tion of a "film forming" additive to the electrolyte to prevent excessive reactivity of the FRA, especially at the anode, was investigated. FIG. 4 is an illustration of a graph showing cycle life performance of MCMB-LiNiCoO 2 cell containing elec- 50
trolytes with flame retardant additives according to the inven-tion. The x-axis shows cycle number and the y-axis shows percent initial capacity. As illustrated in FIG. 4, when vinylene carbonate (VC), a well-known SEI "film forming" additive, was added to an electrolyte containing an FRA 55
additive (e.g, triphenyl phosphate), much improved cycle life performance was obtained. It is anticipated that this approach can be extended to other FRA-containing electrolytes which have different solvent blends, salt types, and/or which employed other SEI film forming agents. It should also be 60
noted that the cycle life performance is anticipated to be much better in aerospace quality lithium-ion cells, due to the fact that the data reflects experimental cells which are a flooded design, and not hermetically sealed.
FIG. 12 is an illustration of a summary table of formation 65
characteristics of electrolytes with flame retardant additives and fluoroester according to the invention. Addition of flame
retardant additives did not significantly increase irreversible capacity loss or reduce coulombic efficiency of the cells dur-ing the formation cycles. FIG. 13 is an illustration of a graph showing cycle life of electrolytes with flame retardant addi-tives and fluoroester according to the invention. The x-axis shows cycle number and the y-axis shows percent initial capacity. As shown in FIG. 13, phenyl branches improved cyclability of FRA electrolytes, and short aliphatic branches of the FRA reduced the electrochemical and life cycle stabil-ity of the electrolyte. FIG. 14 is an illustration of a summary table of results obtained from Electrochemical Impedance Spectroscopy (EIS) measurements for cathodes in contact with electrolytes with flame retardant additives and fluo-roesters according to the invention. As shown in FIG. 14, addition of FRA did not significantly increase the impedance growth at the cathode. FIG. 15 is an illustration of a summary table of the results obtained from Electrochemical Impedance Spectroscopy (EIS) measurements for MCMB anodes in con-tact with electrolytes with flame retardant additives and fluo-roesters according to the invention. As shown in FIG. 15, significant impedance growth at the anode occurred with the addition of the FRA, and the film resistance growth was significantly higher than the baseline. FIG. 16 is an illustra-tion of a graph showing the Tafel polarization measurements performed on MCMB anodes in contact with electrolytes with flame retardant additives and fluoroesters and data was obtained from MCMB-LiNio $Coo 202 cells containing lithium reference electrodes at a temperature of 23° C. according to the invention. The x-axis shows current (Amps) and the y-axis shows anode potential (mV versus Li'/Li). FIG. 17 is an illustration of a graph showing the Tafel polar-ization measurements performed on LiNiCO0 2 cathodes in contact with electrolytes with flame retardant additives and fluoroesters and data was obtained from MCMB-LiNiCoO 2 cells equipped with lithium reference electrodes at a tempera-ture of 23° C. according to the invention. The x-axis shows current (Amps) and the y-axis shows cathode potential (V versus Li'/Li). FIG. 18 is an illustration of a table summary of DC (direct current) micro-polarization measurements for the cathode of cells containing flame retardant additives and fluo-roester electrolytes according to the invention. FIG. 19 is an illustration of a table summary of the results obtained from DC micro-polarization measurements for the anode of MCMB-LiNio $Coo 202 cells containing flame retardant additives and fluoroester electrolytes according to the inven-tion. FIGS. 18-19 further highlight that the deleterious action of the flame retardant additives was resultant from their action at the anode.
FIG. 20 is an illustration of the chemical formation and structure of SEI (solid electrolyte interface) enhancing agent vinylene carbonate. It is generally held that vinylene carbon-ate undergoes a radical polymerization process at the elec-trode surface, i.e., especially under reductive conditions at the anode, to produce poly(vinylene carbonate) which imparts beneficial properties to the SEI (i.e., facile transport of lithium ions while offering protection against excessive sol-vent reduction). SEI enhancing agents can improve the SEI layer by reducing gas generation against extended cycling, reduced capacity loss and SEI stabilization. The mechanism is electrochemically-induced polymerization, and it is termi-nated by a radical anion reaction with a solvent molecule. The SEI enhancing agent can reduce the decomposition of flame retardant additives. FIG. 21 is an illustration of a table sum-mary of formation characteristics of electrolytes with flame retardant additives and SEI enhancing agent according to the invention. As shown by FIG. 21, the addition of the SEI enhancing additive, vinylene carbonate, increased irrevers-
US 8,795,903 B2 13
14 ible capacity loss. However, continued degradation can be preferred low temperature characteristics. Larger branches of minimal after the formation cycling due to the protective
FRA offered increased electrochemical stability leading to
nature of vinylene carbonate. FIG. 22 is an illustration of a improved cycle life. Deleterious effects of FRA were prima- table summary of evaluation of low temperature performance rily seen at the anodes, such as the build-up of surface films at of cells containing flame retardant additives and SEI additive 5 the interface which limits the kinetics of lithium intercalation/ according to the invention. As shown by FIG. 22, the addition de-intercalation as determined by Electrochemical Imped- of vinylene carbonate contributed to improved low tempera- ance Spectroscopy (EIS) and the DC (direct current) polar- ture and increased rate performance. FIG. 23 is an illustration ization techniques. These findings can lead to the mitigation of a table summary of results obtained from Electrochemical strategy of incorporating additional electrolyte additives to Impedance Spectroscopy (EIS) measurements performed on 10 minimize the continued reaction of the FRA at the interface. LiNiCO02 cathodes of MCMB-LiNiCoO 2 cells containing electrolytes possessing flame retardant additives/VC accord- Li-ion Electrolytes Containing Phosphorus-Based ing to the invention. As shown by FIG. 23, the impedance
FRA
growth at the cathode was not significantly influenced by the addition of vinylene carbonate or FRA. FIG. 24 is an illus- 15 Description. Additional experiments were conducted with tration of a table summary of results obtained from Electro- Li-ion electrolytes containing more phosphorus-based flame chemical Impedance Spectroscopy (EIS) measurements per- retardant additives. As part of a continuing effort to develop formed on MCMB anodes of MCMB-LiNiCoO 2 cells advanced electrolytes to improve the safety and performance containing electrolytes possessing flame retardant additives/
of lithium-ion cells, especially over a wide operating tem-
VC according to the invention. As shown by FIG. 24, the 20 perature range, a number of Li-ion electrolytes that contain influence of both FRA and vinylene carbonate was seen at the
flame retardant additives optimized for operation over a wide
anode and was exacerbated at low temperatures (i.e., larger temperature range were developed. Phosphorus-based flame film resistance, R. and larger charge transfer resistance, Rat). retardant additives were investigated, including (a) tris(2,2, FIG. 25 is an illustration of a table summary of linear polar- 2-trifluoroethyl) phosphate (see FIG. 5e showing the chemi- ization measurements that were performed on MCMB anodes 25 cal structure), (b) tris(2,2,2-trifluoroethyl) phosphite (see from MCMB-LiNiCoO2 cells containing electrolytes pos- FIG. 5f showing the chemical structure), (c) triphenylphos- sessing flame retardant additives/VC according to the inven- phite (see FIG. 5g showing the chemical structure), (d) tion. FIG. 26 is an illustration of a table summary of linear
diethyl ethylphosphonate (see FIG. 5i showing the chemical
polarization measurements performed on LiNiCoO 2 cath- structure), and (e) diethyl phenylphosphonate (see FIG. 5h odes from MCMB-LiNiCoO 2 cells containing electrolytes 30 showing the chemical structure). These phosphorus-based possessing flame retardant additives/VC according to the
flame retardant additives, as shown in FIGS. 5e-5i, were
invention. As shown by FIGS. 25 -26, the polarization resis- added to an electrolyte composition intended for wide oper- tance of the MCMB anodes increased much more than the ating temperature range, namely LOM LiPF 6 in EC+EMC cathodes for cells containing FRA. (20:80 v/v %).
Summary. The safety characteristics of Li-ion electrolytes 35 The following electrolyte formulations were investigated has been further improved by the addition of flame retardant and demonstrated in experimental MCMB carbon- additives (FRA), such as triphenyl phosphate (TPP or TPPa or
LiNio $Coo 202 cells: (1) 1.0 M LiPF 6 in EC+EMC+TFPa
TPhPh), tributyl phosphate (TBP or TBuPh), triethyl phos- (20:75:5 v/v %); (2)1.0 M LiPF 6 in EC+EMC+TFPi (20:75:5 phate (TEP or TEtPh), and bis(2,2,2-trifluoroethyl) methyl
v/v %); (3) 1.0 M LiPF 6in EC+EMC+TPPi (20:75:5 v/v %);
phosphonate (BTEMP or TFMPo). A number of electrolytes 40 (4)1.0 M LiPF 6 in EC+EMC+DEP (20:75:5 v/v %); (5)1.0 M based upon these approaches have delivered good perfor- LiPF 6 in EC+EMC+DPP (20:75:5 v/v %); and, (6) 1.0 M mance over a wide temperature range, good cycle life char- LiPF 6 in EC+EMC (20:80 v/v %) (Baseline), (where acteristics, and improved safety characteristics, namely
TFPatris(2,2,2-trifluoroethyl) phosphate, TFPitris(2,2,2-
reduced flammability. Of the additives investigated, the triph- trifluoroethyl) phosphite, TPPi=triphenyl phosphite, enyl phosphate displayed the lowest irreversible capacity 45 DEP-diethyl ethylphosphonate, and DPP-diethyl phe-losses and high coulombic efficiency, suggesting that it dis- nylphosphonate). plays the least amount of electrochemical decomposition and
In general, many of the formulations investigated in this
does not participate in the electrode filming process to a great study displayed good performance over a wide temperature extent. Of the FRA-containing electrolytes, the formulation range, good cycle life characteristics, and were expected to containing tributyl phosphate delivered the best performance 5o have improved safety characteristics, namely low flammabil- at low temperature, with over 68% (percent) of the room
ity. Of the electrolytes studied, 1.0 M LiPF 6 in EC+EMC+
temperature capacity being delivered at –20° C. using a
DEP (20:75:5 v/v %) and 1.0 M LiPF 6 in EC+EMC+DPP –C/16 discharge rate. With respect to cycle life performance, (20:75:5 v/v %) displayed the best operation at low tempera- improved life characteristics were observed with the incor- tures, whereas the electrolyte containing triphenylphosphite poration of film forming additives, which serve to prevent 55 displayed the best cycle life performance compared to the excessive reactivity of the FRA at the electrode interfaces, baseline solution. It is anticipated that further improvements especially at the carbon anode. Further optimization of these can be made to the life characteristics with the incorporation electrolyte formulations was anticipated to yield improved
of a SEI promoters (such as VC, vinylene carbonate), which
performance. It was also anticipated that much improved
will likely inhibit the decomposition of the flame retardant performance would be demonstrated once these electrolyte 6o additives, as demonstrated in the previous study. solutions were incorporated into hermetically sealed large
Description. A number of experimental lithium-ion cells,
capacity, prototype cells, especially if effort is devoted to consisting of MCMB carbon anodes and LiNi o $Coo 202 ensuring that all electrolyte components are highly pure. The cathodes, were fabricated to study the described technology. fluorinated esters imparted desirable physical characteristics
These cells served to verify and demonstrate the reversibility,
on the base electrolyte solvent system. The structure of the 65 low temperature performance, and electrochemical aspects of FRA significantly influenced the performance characteristics each electrode as determined from a number of electrochemi- of the cells. Short aliphatic chained phosphates imparted
cal characterization techniques. The electrolytes selected for
US 8,795,903 B2 15
evaluation are listed above (electrolytes 1-6). FIG. 6 is an illustration of a table summary of charge-discharge charac-teristics of experimental MCMB-LiNiCoO 2 cells containing electrolytes with flame retardant additives subjected to for-mation cycling according to the invention. As shown in FIG. 5
6, all cells displayed good reversibility at room temperature and minimal reactivity during the formation cycling. The high coulombic efficiency and comparable irreversible capacity losses were indirectly related to the overall stability of the solutions and the electrode filming characteristics. 10
FIG. 7 is an illustration of a graph showing fifth discharge of formation cycling for cells containing flame retardant addi-tives according to the invention. The x-axis shows discharge capacity in Ah (Ampere hours) and the y-axis shows voltage (V). As shown in FIGS. 6-7, reasonable reversibility was 15
observed with the cells containing all of the electrolyte varia-tions, when compared after the formation cycling. It should be noted that some variation in capacity was due to different electrode weights, and not electrolyte type, so most compari-sons were expressed in terms of percentage of initial capacity 20
under ambient temperatures. Of the additives investigated, the electrolytes containing the diethyl ethylphosphonate and diethyl phenylphosphonate displayed the lowest irreversible capacity losses and highest coulombic efficiency, suggesting that it was not electrochemically decomposing and partici- 25
pating in the electrode filming process deleteriously. Low Temperature Performance. FIG. 8 is an illustration of
• table summary of discharge capacity at low temperature of • number of MCMB-LiNiCoO 2 cells containing electrolytes with flame retardant additives according to the invention. 30
When the cells described were evaluated at low temperatures, as shown in FIG. 8, reasonable low temperature performance was generally observed. This finding is significant since the use of flame retardant additives was anticipated in decreasing the power capability and low temperature performance, due 35
to decreased ionic conductivity of the media and increased electrode film resistance with the possible reactivity of the FRA.
Of the additives investigated, the diethyl ethylphosphonate (DEP)-containing electrolyte resulted in cells that displayed 40
the best low temperature performance of the group. Conduc-tivity studies performed on the individual electrolyte solu-tions lead to the finding that some of the solutions containing FRA actually displayed higher ionic conductivity at lower temperatures compared with the baseline solution which did 45
not contain any flame retardant additive, such as with triethyl phosphate. This finding suggests that the FRA can serve to lower the viscosity of the medium, in addition to imparting flame retardant properties. Thus, the high conductivity of the electrolyte, coupled with potentially minimal decomposition 50
of the additive on the electrode surfaces, can lead to good low temperature performance.
This is illustrated in FIG. 9, in which the performance of cells containing electrolytes with various FRAs are shown when discharged at —20° C. using a CA 6 discharge rate. FIG. 55
9 is an illustration of a graph showing discharge capacity at —20° C. (expressed as percent of room temperature capacity) of MCMB-LiNiCoO 2 cells containing electrolytes with flame retardant additives according to the invention. The x-axis shows percent of room temperature capacity (%) and the 60
y-axis shows voltage (V). Of the FRA-containing electro-lytes, the formulation containing diethyl ethylphosphonate delivered the best performance at low temperature, with over 85% of the room temperature capacity being delivered at —20° C. 65
Similar trends and excellent performance was obtained at —40° C., as shown in FIG. 10, in which the cells were dis-
16 charged using a C/16 discharge rate, following charging at room temperature. FIG. 10 is an illustration of a graph show-ing discharge capacity at —40° C. (expressed as percent of room temperature capacity) of MCMB-LiNiCoO 2 cells con-taining electrolytes with flame retardant additives according to the invention. The x-axis shows percent of room tempera-ture capacity (%) and the y-axis shows voltage (V). As shown, the cell containing the electrolyte with the DEP additive again displayed the best performance at low temperature.
Cycle Life Performance. As mentioned previously, the general expectation is that the cycle life performance will be compromised with the addition of FRAs due to the possible reactivity with the electrode interfaces leading to impedance build-up and capacity loss. However, generally good cycle life performance was obtained in experimental cells, as illus-trated in FIG. 11, with the cell containing the electrolyte with triphenyl phosphite additive delivering the best performance. FIG. 11 is an illustration of a graph showing cycle life per-formance of MCMB-LiNiCoO 2 cells containing electrolytes with flame retardant additives according to the invention. The x-axis shows cycle number and the y-axis shows discharge capacity in (mAh) (milli-Ampere-hours). To further improve the cycle life characteristics, the addition of SEI "film form-ing" additives is anticipated to impart enhanced stability to the system. It should also be noted that the cycle life perfor-mance is anticipated to be much better in aerospace quality lithium-ion cells, due to the fact that the data reflects experi-mental cells which are a flooded design, and not hermetically sealed.
Summary. The safety characteristics of Li-ion electrolytes have been further improved by the addition of flame retardant additives, such as tris(2,2,2-trifluoroethyl) phosphate, (b) tris (2,2,2-trifluoroethyl) phosphite, (c) triphenylphosphite, (d) diethyl ethylphosphonate, and (e) diethyl phenylphospho-nate. A number of electrolytes based upon these additives have delivered good performance over a wide temperature range, good cycle life characteristics, and improved safety characteristics, namely reduced flammability. Of the addi-tives investigated, the diethyl ethylphosphonate displayed the lowest irreversible capacity losses and high coulombic effi-ciency, suggesting that it displayed the least amount of elec-trochemical decomposition and did not participate in the elec-trode filming process to a great extent. Of the FRA-containing electrolytes studied, the formulation containing diethyl eth-ylphosphonate also delivered the best performance at low temperature, with over 85% of the room temperature capacity being delivered at the —20° C. using a C/16 discharge rate. With respect to cycle life performance, the formulation con-taining the triphenylphosphite electrolyte additive displayed the best performance. Further optimization of these electro-lyte formulations is anticipated to yield improved perfor-mance. It is also anticipated that much improved performance will be demonstrated once these electrolyte solutions are incorporated into hermetically sealed large capacity, proto-type cells, especially if effort is devoted to ensuring that all electrolyte components are highly pure.
Li-ion Electrolytes Containing Increased Concentrations of FRA
Description. A number of additional electrolyte formula-tions containing flame retardant additives were investigated and demonstrated in experimental MCMB carbon-LiNio $Coo 202 cells. These electrolytes included: (1) formu-lations which incorporated greater concentrations of the flame retardant additive; (2) the use di-2,2,2-trifluoroethyl carbonate (DTFEC) as a co-solvent; (3) the use of 2,2,2-
US 8,795,903 B2 17
18 trifluoroethyl methyl carbonate (TFEMC) as a co-solvent (4)
LiPF 6 in FEC+EMC+TPP (20:70:10 v/v %) displaying the
the use of mono-fluoroethylene carbonate (FEC) as a co- highest values of 85.9% and 85.6%, respectively. Also of note solvent and/or a replacement for ethylene carbonate in the
is the fact that the FEC and TFEMC-containing carbonate
electrolyte mixture; and, (5) the use of vinylene carbonate
blends displayed the highest coloumbic efficiency by the fifth (VC) as an "SEI promoting" electrolyte additive, to build on 5 cycle, all exhibiting over 98% efficiency (compared to –(ap- the favorable results previously obtained. The use of higher proximately) 97% shown by the baseline solutions), indica- concentrations of the flame retardant additive is known to tive of good stability. Furthermore, some of the solutions reduce the flammability of the electrolyte solution, thus, a
displayed much lower cumulative irreversible capacities
range was investigated (e.g., 5% to 20% by volume). The compared to the baseline formulation, especially with the desired concentration of the flame retardant additive is the io FEC containing electrolyte formulations which all displayed greatest amount tolerable without adversely affecting the per- less than 104 mAh in contrast to the baseline solutions which formance in terms of reversibility, ability to operate over a exhibited greater than 126 mAh and 136 mAh, respectively. wide temperature range, and the discharge rate capability. Effect of Increased TPP Concentration and Incorporation Fluorinated carbonates, such as mono -fluoroethylene carbon- of DTFEC on the Discharge Performance at Different Tem- ate (FEC), may be used as both a fluorinated ester-based 15 peratures. When the cells containing electrolyte possessing co-solvent, as well as a flame retardant additive. 10% TPP with and without the incorporation of DTFEC
The electrolytes developed which embodied these
described were evaluated at different rates and low tempera- approaches included: (1) Approaches based on increased
tures, as shown in FIG. 28, reasonable low temperature per-
FRA concentrations: (a) 1.0 M LiPF 6 in EC+EMC+TPP (20:
formancewas generally observed. FIG. 28 is an illustration of 70:10 v/v %); (b) 1.0 M LiPF 6 in EC+EMC+TPPi (20:70:10 20 a table summary of discharge capacity at low temperature of v/v %); and (c)1.0 M LiPF 6 in EC+EMC+TPPi (20:80:20 v/v a number of MCMB-LiNiCO0 2 cells containing electrolytes %); (2) Approaches based on the incorporation of di-2,2,2- with flame retardant additives according to the invention. This trifluoroethyl carbonate: (a) 1.0 M LiPF 6 in EC+EMC+DT- finding was significant since the use of flame retardant addi- FEC+TPP (20:50:20:10 v/v %); and (b) 1.0 M LiPF 6 in tives was anticipated in decreasing the power capability and EC+EMC+DTFEC+TPP (20:30:40:10 v/v %); (3) 25 low temperature performance. As illustrated in FIG. 28, when Approaches based on the use of 2,2,2-trifluoroethyl methyl
10% TPP was added to one of the baseline electrolytes,
carbonate (TFEMC) and (4) mono-fluoroethylene carbonate namely LOM LiPF 6 in EC+EMC (20:80 v/v %), minimal (FEC): (a) 1.0 M LiPF 6 in EC+EMC+TFEMC+TPP (20:50:
impact upon the low temperature discharge rate capability
20:10 v/v %); (b) 1.0 M LiPF 6 in FEC+EMC+TFEMC+TPP
was observed, being essentially comparable at 0° C. over a (20:50:20:10 v/v %); and (c)1.0 M LiPF 6 in FEC+EMC+TPP 3o range of discharge rates investigated. The performance of the (20:70:10 v/v %); (5) Approaches based on the use of
cell containing 1.0 M LiPF 6 in EC+EMC+TPP (20:70:10 v/v
vinylene carbonate (VC): (a)1.0 M LiPF 6 in EC+EMC+TPPi
%) at 0° C. is displayed in FIG. 29. FIG. 29 is an illustration (20:75:5 v/v %)+1.5% VC; and (b)1.0 M LiPF 6 FEC+EMC+ of a graph showing discharge performance at 0° C. (expressed TFEMC+TPP (20:50:20:10 v/v %)+1.5% VC; (where as percent of room temperature capacity) of a –400 mAh TPP=TPPa=triphenyl phosphate, TPPi=triphenyl phosphite, 35 MCMB-LiNiCO0 2 cell containing 1.0 M LiPF 6 in DTFEC-Ii-2,2,2-trifluoroethyl carbonate, TFEMC=2,2,2- EC+EMC+TPP (20:70:10 v/v %) according to the invention. trifluoroethyl methyl carbonate, FEC=mono-fluoroethylene
The x-axis shows percentage of room temperature capacity
carbonate, VC–vinylene carbonate, EC–ethylene carbonate, (%) and the y-axis shows cell voltage (V). When the perfor- EMC–ethyl methyl carbonate). mance of a cell containing an electrolyte possessing both 10%
A number of experimental lithium-ion cells, consisting of 4o TPP and 20% DTFEC was evaluated under identical condi- MCMB carbon anodes and LiNi o $Coo 202 cathodes, were tions, as shown in FIG. 30, somewhat decreased performance fabricated to study the described technology. These cells was observed, being more dramatic at higher rates and lower served to verify and demonstrate the reversibility, low tem- temperatures. FIG. 30 is an illustration of a graph showing perature performance, and electrochemical aspects of each
discharge performance at 0° C. (expressed as percent of room
electrode as determined from a number of electrochemical 45 temperature capacity) of a MCMB-LiNiCoO 2 cell containing characterization techniques. The electrolytes selected for
1.0 M LiPF 6 in EC+EMC+DTFEC+TPP (20:50:20:10 v/v %)
evaluation are listed above (electrolytes 1-5). FIG. 27 is an according to the invention. The x-axis shows percentage of illustration of a table summary of formation characteristics of
room temperature capacity (%) and the y-axis shows cell
experimental MCMB-LiNiCoO 2 cells containing electro- voltage (V). Thus, although this system was anticipated to lytes with flame retardant additives subjected to formation 5o have increased safety in contrast to an electrolyte with only cycling according to the invention. As shown in FIG. 27, in
TPP present, it was observed to have diminished performance
whichthe formation characteristics are shown for a number of
capability at higher rates. When increased DTFEC was added cells containing electrolytes with triphenyl phosphate (TPP)
to the system (e.g., 40% DTFEC by volume), this trend of
as a flame retardant additive, all cells displayed good revers- decreased rate capability at lower temperature was further ibility at room temperature and minimal reactivity during the 55 magnified. formation cycling. The high coulombic efficiency and com- In order to further elucidate the nature of the performance parable irreversible capacity losses were indirectly related to characteristics of the cells containing the different electro- the overall stability of the solutions and the electrode filming
lytes, Tafel polarization measurements were performed on
characteristics. both the anodes and cathodes at different temperature to For comparison, two baseline electrolytes were investi- 6o determine the lithium intercalation/de-intercalation kinetics.
gated which do not contain any flame retardant additive or
The measurements were conducted on the cells while they fluorinated solvents, namely LOM LiPF 6 in EC+DMC+DEC
were in a full SOC (state of charge) (OCV (open circuit
(1:1:1 v/v %) and LOM LiPF 6 in EC+EMC (20:80 v/v %). As voltage)–>4.07V) on each electrode while utilizing a lithium shown in FIG. 27, notable features are that the FRA-contain- reference electrode. In all of these Tafel plots, there were ing electrolytes possess comparable couloumbic efficiency 65 distinct charge-transfer controlled regimes, where the over- on the first cycle (e.g., above 83%), with 1.0 M LiPF 6 in potential increased linearly with log (I). The effect of mass EC+EMC+DTFEC+TPP (20:50:20:10 v/v %) and 1.0 M
transfer seems to be relatively insignificant, such that kinetic
US 8,795,903 B2 19
20 parameters, i.e., exchange current and transfer coefficients, can be derived. FIG. 31 is an illustration of a graph showing the Tafel polarization measurements of LiNiCO0 2 electrodes from MCMB-LiNiCoO 2 cells containing electrolytes with 10% TPP and with and without the addition of DTFEC according to the invention. The x-axis shows current (Amps) and the y-axis shows cathode potential (V versus Li'/Li). As illustrated in FIG. 31, in which the Tafel polarization mea-surements have been performed on the LiNiCO0 2 cathodes at room temperature, the cell containing the 1.0 M LiPF 6 in EC+EMC+TPP (20:70:10 v/v %) electrolyte displayed enhanced lithium kinetics (i.e., higher limiting current den-sities) compared to the baseline system that does not contain the flame retardant additive. When the cell containing both the 10% TPP and 20% DTFEC was evaluated comparable kinetics to the baseline was observed. In contrast, when 40% DTFEC was added to the electrolyte the resultant limiting current densities of the cathode were noticeably lower.
FIG. 32 is an illustration of a graph showing the Tafel polarization measurements of MCMB electrodes from MCMB-LiNiCO0 2 cells containing electrolytes with 10% TPP and with and without the addition of DTFEC according to the invention. The x-axis shows current (Amps) and the y-axis shows anode potential (V versus Lim/Li). When the Tafel polarization measurements were performed on the MCMB anodes at room temperatures, as shown in FIG. 32, both the incorporation of 10% TPP and DTFEC were observed to decrease the lithium de-intercalation kinetics compared to the baseline composition. These results sug-gested that these compounds react at the anode interface to produce solid electrolyte interface (SEI) layers that do not allow facile movement of lithium-ions at high rates. To miti-gate this behavior "SEI promoting" additives can be added to the electrolyte to further improve the nature of the interface.
Effect of Increased TPP Concentration and Incorporation of TFEMC and FEC on the Discharge Performance at Differ-ent Temperatures. In addition to investigating trifluoroethyl butyrate and di-2,2,2-trifluoroethyl carbonate as co-solvents in multi-component mixtures, other fluorinated carbonates were incorporated into electrolyte formulations with the intent of further lower the flammability of the system, such as 2,2,2-trifluoroethyl methyl carbonate and mono-fluoroethyl-ene carbonate. As mentioned previously, when electrolytes containing these co-solvents and 10% TPP were added to MCMB-LiNiCO0 2 cells, good reversibility, high coulombic efficiency and low irreversible capacities were observed. When the cells were evaluated at different rates and different temperatures, good performance was observed being compa-rable to the baseline formulations, as shown in FIG. 33. FIG. 33 is an illustration of a table summary of discharge capacity at low temperature of a number of MCMB-LiNiCoO 2 cells containing electrolytes with 10% TPP and TFEMC and/or FEC according to the invention.
FIG. 34 is an illustration of a graph showing discharge performance at 0° C. using a —C/8 discharge rate to 2.0 V (volts) according to the invention. The x-axis shows percent of room temperature capacity (%) and the y-axis shows Volt-age (V). As illustrated in FIG. 34, good performance was delivered from the cells containing 10% triphenyl phosphate and trifluoroethyl methyl carbonate and/or fluoroethylene carbonate when evaluated at a —C/8 discharge rate (50 mA) at 0° C. In all cases, the cells were observed to deliver more capacity than the baseline system, with the cell containing the 1.0 M LiPF 6 in FEC+EMC+TFEMC+TPP (20:50:20:10 v/v %) electrolyte exhibiting the best performance.
Evaluation of FRA-Containing Electrolyte Formulations in Conjunction with High Voltage, High Capacity Cathode
Materials. In addition to evaluating the electrolytes in MCMB-LiNiCO0 2 cells, a number of the formulations were investigated in Li Li(Lio.i2Nlo.25N4uo.5a)O2 cells to deter-mine their compatibility with high voltage, high capacity
5 cathode materials. These systems are typically charged to a much higher potential (e.g., 4.80V) than the commonly used lithium nickel cobalt oxide materials.
FIG. 35 is an illustration of a table summary of the forma-tion characteristics of a number of Li Li
10 (Lio.i2Nio.25N4uo.5a)O2 cells containing various electrolytes with TPP and TPPi flame retardant additives according to the invention. As illustrated in FIG. 35, Li Li
(Lio.i2Nio.25N4uo.5a)O2 cells containing a "baseline" electro- 15 lyte that does not contain any flame retardant additives deliv-
ered an average of 234 nhkWg reversible capacity when cycled to 4.80V vs Li+/Li using C/20 charge and discharge rates. When triphenyl phosphate (TPP) was incorporated into the electrolyte formulation very little loss in reversible capac-
20 ity is observed, with the cells displaying an average of 228 and 221 mAh/g being displayed with 5% TPP and 10% TPP, respectively. Slightly lower reversible capacity was observed with the incorporation of 10% TPP and 20% DTFEC (e.g., 216 mAh/g). In contrast, the use of triphenyl phosphite (TPPi)
25 was accompanied by a sharp decline in the performance, resulting in much lower reversible capacity and an increase in the irreversible capacity. Similar trends in the rate capability at room temperature were observed for the cells.
Demonstration of FRA-Containing Electrolytes in High 30 Capacity Prototype Li-Ion Cells. To further assess the perfor-
mance characteristics of candidate electrolyte formulations, a number of high capacity, hermetically sealed Li-ion cells were fabricated with one of the promising electrolyte solu-tions, namely LOM LiPF 6 in EC+EMC+TPP+VC (19.7/73.9/
35 4.9/1.5 v/v %). Thus, a number of 7 Ah MCMB-LiNiCO0 2 Li-Ion cells were obtained from Yardney Technical Products, Inc. of Pawcatuck, Conn., that incorporated this electrolyte. After the formation process at the vendor, the cells were subjected to conditioning cycling at various temperatures
40 (20° C., 0° C., and —20° C.) to determine the reversible capacity, the specific energy, and the impedance of the cells. After completing the initial characterization testing, a num-ber of performance tests were performed on the cells, includ-ing evaluating: (a) the discharge rate performance at various
45 temperatures, (b) the charge rate performance at various tem-peratures, and (c) determining the cycle life characteristics. FIG. 36 is an illustration of a table summary of discharge performance of a 7 Ah Li-ion cell containing LOM LiPF 6 in EC+EMC+TPP+VC (19.7/73.9/4.9/1.5 v/v %) according to
50 the invention. As shown in FIG. 36, the discharge rate char-acterization testing was performed over a wide temperature range (20° to —60° C.) and a wide range of discharge rates (e.g., C1100 to 1.0 Q. The cells were charged at room tem-perature and discharged to 2.OV (the initial measurement at
55 C15 and 20° C. was performed only down to 2.75V). As illustrated, the cells displayed good rate capability down to very low temperatures, being able to support moderate rates to temperatures as low as —50° C.
FIG. 37 is an illustration of a graph showing discharge rate 60 performance at 20° C. of a 7 Ah Li-ion cell containing LOM
LiPF 6 in EC+EMC+TPP+VC (19.7/73.9/4.9/1.5 v/v %) accordingto the invention. Thex-axis shows discharge capac- ity (Ah) and the y-axis shows cell voltage (V). In FIG. 37, the discharge rate performance of the cell is displayed at 20° C.,
65 illustrated that superior performance was obtained in the pro- totype cells compared to the data generated with the experi- mental three electrode cells. This trend has also been
US 8,795,903 B2 21
22 observed with a number of electrolyte formulations that have pentafluoropropyl methyl ether, 1,1,3,3,3-pentafluoro-2-trif- been scaled up to industrially manufactured Li-ion cells. luoromethyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl
As mentioned above, charge rate characterization testing ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, and perfluo- was also performed on the 7 Ah Li-ion cells, which consisted
ropolyether.
of evaluating the cell at different temperatures (20° C., 10° C., 5 3. The electrolyte of claim 1 wherein the flame retardant 0° C., and —10° C.) and different charge rates (ranging up to additive is selected from the group consisting of bis(2,2,2- a C rate charge). Charging consisted of employing constant
trifluoroethyl) methyl phosphonate (BTFEMP/TFMPo),
current to a constant potential of 4.10V, followed by taper triphenyl phosphite, tris(2,2,2-trifluoroethyl) phosphate, tris current cut-off of Cl 100. It was observed that the cell could be
(2,2,2-trifluoroethyl) phosphite, diethyl ethylphosphonate,
effectively charged over the range of conditions investigated, io and diethyl phenylphosphonate. without the evidence of lithium plating occurring which can
4. The electrolyte of claim 1 wherein the lithium salt is
be deleterious to cell health. selected from the group consisting of lithium hexafluoro- In addition to these tests, a 100% depth-of-discharge phosphate (LiPF 6), lithium tetrafluoroborate (LiBF 4), lithium
(DOD) test was implemented using C15 charge and discharge
bis(oxalate) borate (LiBOB), lithium hexafluoroarensate (Li- rates (over a voltage range of 2.75V to 4.1 OV). FIG. 38 is an 15 AsF6), lithium perchlorate (LiC1O 4), lithium trifluo- illustration of a graph showing 100% depth of discharge romethanesulfonate (LiCF 3 SO3), lithium bistrifluo- (DOD) cycling of 7 Ah Li-ion cell containing flame retardant romethanesulfonate sulfonyl imide (LiN(S0 2CF 3)2), and additive containing electrolyte [LOM LiPF 6 in EC+EMC+ mixtures thereof. TPP+VC (19.7/73.9/4.9/1.5 v/v %)] and the baseline electro- 5. The electrolyte of claim 1 wherein the electrolyte com- lyte [LOM LiPF 6 in EC+DMC+DEC (1:1:1 v/v %)] accord- 20 prises from about 10% to about 60% by volume fluorinated ing to the invention. The x-axis shows cycle number and the co-solvent. y-axis shows discharge capacity (% of initial capacity). As
6. The electrolyte of claim 1 wherein the electrolyte com-
illustrated in FIG. 38, excellent cycle life performance has prises from about 5% to about 15% by volume flameretardant been obtained thus far, with approximately 400 cycles being additive. completed while still retaining 95% of the initial capacity. 25 7. The electrolyte of claim 1 wherein the electrolyte com- The cycle life observed was actually superior to that obtained
prises 1.0 M LiPF 6 in 20% by volume ethylene carbonate
with a similar cell containing the baseline electrolyte formu- (EC) plus 55% by volume ethyl methyl carbonate (EMC) plus lation, namely LOM LiPF 6 in EC+DMC+DEC (1:1:1 v/v %). 20% by volume 2,2,2-tri-fluoroethyl butyrate (TFEB) plus
It is expected that other FRAs in the same or similar classes
5% by volume triphenyl phosphite. of compounds as discussed above, can work in a similar 30 8. A lithium-ion electrochemical cell comprising: fashion in the invention, i.e., phosphates, phosphites, and
an anode;
phosphonates. a cathode; Many modifications and other embodiments of the disclo- an electrolyte interspersed between the anode and the cath-
sure will come to mind to one skilled in the art to which this ode, wherein the electrolyte comprises a mixture of: disclosure pertains having the benefit of the teachings pre- 35 an ethylene carbonate (EC); sented in the foregoing descriptions and the associated draw- an ethyl methyl carbonate (EMC); ings. The embodiments described herein are meant to be • fluorinated co-solvent selected from the group consist- illustrative and are not intended to be limiting. Although
ing 2,2,2-trifluoroethyl butyrate (TFEB), di-2,2,2-tri-
specific terms are employed herein, they are used in a generic
fluoroethyl carbonate (DTFEC), 2,2,2-trifluoroethyl and descriptive sense only and not for purposes of limitation. 40 methyl carbonate (TFEMC), mono-fluoroethylene
What is claimed is: carbonate (FEC), ethyl trifluoroacetate (ETFA), 2,2, 1. An electrolyte for use in a lithium-ion electrochemical
2-trifluoroethyl acetate (TFEA), 2,2,2-trifluoroethyl
cell, the electrolyte comprising a mixture of: propionate (TFEP), ethyl -2,2,2 -tri fluoroethyl carbon- an ethylene carbonate (EC); ate (ETFEC), propyl-2,2,2-trifluoroethyl carbonate an ethyl methyl carbonate (EMC); 45 (PTFEC), methyl-2,2,2,2',2',2'-hexafluoro-i-propyl • fluorinated co-solvent selected from a group consisting of
carbonate (MHFPC), ethyl-2,2,2,2',2',2'-hexafluoro-
fluorinated carbonates, fluorinated ester co-solvents, i-propyl carbonate (EHFPC), fluoropropylene car- and fluorinated ethers;
bonate (FPC), trifluoropropylene carbonate (TFPC), • flame retardant additive selected from a group consisting methyl nonafluorobutyl ether, 2,2,3,3,3-pentafluoro-
of fluorinated phosphate flame retardant additives, phos- 50 propyl methyl ether, 1,1,3,3,3-pentafluoro-2-trifluo- phite flame retardant additives, and phosphonate flame romethyl ether, 2,2,3,3-tetrafluoropropyl difluorom- retardant additives; and, ethyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether,
• lithium salt and perfluoropolyether; wherein the electrolyte comprises from about 5% to about • flame retardant additive selected from the group con-
30% by volume flame retardant additive. 55 sisting of triphenyl phosphate (TPhPh/TPP/TPPa), 2. The electrolyte of claim 1 wherein the fluorinated co- tributyl phosphate (TBuPh), triethyl phosphate
solvent is selected from the group consisting of 2,2,2-trifluo- (TEtPh), bis(2,2,2-trifluoroethyl) methyl phospho- roethyl butyrate (TFEB), di-2,2,2-trifluoroethyl carbonate nate (TFMPo), triphenyl phosphite, tris(2,2,2-trifluo- (DTFEC), 2,2,2-trifluoroethyl methyl carbonate (TFEMC), roethyl) phosphate, tris(2,2,2-trifluoroethyl) phos- mono-fluoroethylene carbonate (FEC), ethyl trifluoroacetate 60 phite, diethyl ethylphosphonate, and diethyl (ETFA), 2,2,2-trifluoroethyl acetate (TFEA), 2,2,2-trifluoro- phenylphosphonate; and, ethyl propionate (TFEP), ethyl -2,2,2 -tri fluoroethyl carbonate • lithium salt, (ETFEC), propyl-2,2,2-trifluoroethyl carbonate (PTFEC), wherein the electrochemical cell operates in a temperature methyl-2,2,2,2',2',2'-hexafluoro-i-propyl carbonate (MH- range of from about —50 degrees Celsius to about 60 FPC), ethyl-2,2,2,2',2',2'-hexafluoro-i-propyl carbonate (EH- 65 degrees Celsius and further wherein the electrolyte com- FPC), fluoropropylene carbonate (FPC), trifluoropropylene prises from about 5% to about 30% by volume flame carbonate (TFPC), methyl nonafluorobutyl ether, 2,2,3,3,3- retardant additive.
US 8,795,903 B2 23
9. A lithium-ion electrochemical cell comprising: an anode; a cathode; an electrolyte interspersed between the anode and the cath-
ode, wherein the electrolyte comprises a mixture of: an ethylene carbonate (EC); an ethyl methyl carbonate (EMC); a fluorinated co-solvent selected from the group consist-
ing 2,2,2-trifluoroethyl butyrate (TFEB), di-2,2,2-tri-fluoroethyl carbonate (DTFEC), 2,2,2-trifluoroethyl methyl carbonate (TFEMC), mono-fluoroethylene carbonate (FEC), ethyl trifluoroacetate (ETFA), 2,2, 2-trifluoroethyl acetate (TFEA), 2,2,2-trifluoroethyl propionate (TFEP), ethyl -2,2,2 -trifluoroethyl carbon-ate (ETFEC), propyl-2,2,2-trifluoroethyl carbonate (PTFEC), methyl-2,2,2,2',2',2'-hexafluoro-i-propyl carbonate (MHFPC), ethyl-2,2,2,2 1,2',2'-hexafluoro-i-propyl carbonate (EHFPC), fluoropropylene car-bonate (FPC), trifluoropropylene carbonate (TFPC), methyl nonafluorobutyl ether, 2,2,3,3,3-pentafluoro-propyl methyl ether, 1,1,3,3,3-pentafluoro-2-trifluo-
24 romethyl ether, 2,2,3,3-tetrafluoropropyl difluorom-ethyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, and perfluoropolyether;
a flame retardant additive selected from the group con- s sisting of triphenyl phosphate (TPhPh/TPP/TPPa),
tributyl phosphate (TBuPh), triethyl phosphate (TEtPh), bis(2,2,2-trifluoroethyl) methyl phospho-nate (TFMPo), triphenyl phosphite, tris(2,2,2-trifluo-roethyl) phosphate, tris(2,2,2-trifluoroethyl) phos-
10 phite, diethyl ethylphosphonate, and diethyl phenylphosphonate; and,
a lithium salt, wherein the electrochemical cell operates in a temperature
range of from about —50 degrees Celsius to about 60 15 degrees Celsius and further wherein the electrolyte com-
prises 1.0 M LiPF6 lithium salt in 20% by volume eth-ylene carbonate (EC) plus 55% by volume ethyl methyl carbonate (EMC) plus 20% by volume 2,2,2-tri-fluoro-ethyl butyrate (TFEB) plus 5% by volume triphenyl
20 phosphate (TPhPh/TPP/TPPa).