Solid State Ionics 267 (2014) 32–37

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Solid State Ionics

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Compatibility of polymer electrolyte based onN-methyl-N-propylpiperidinium bis(trifluoromethanesulphonyl)imideionic liquid with LiMn2O4 cathode in Li-ion batteries

Agnieszka Swiderska-Mocek ⁎, Dominika NaparstekFaculty of Chemical Technology, Poznan University of Technology, PL-60 965 Poznan, Poland

⁎ Corresponding author. Fax: +48 61 662 571.E-mail address: agnieszka.swiderska-mocek@put.pozn

http://dx.doi.org/10.1016/j.ssi.2014.09.0070167-2738/© 2014 Published by Elsevier B.V.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 April 2014Received in revised form 24 June 2014Accepted 4 September 2014Available online xxxx

Keywords:Polymer electrolyteIonic liquidLiMn2O4 cathodeLi anodeLi4Ti5O12 anodeLi-ion battery

LiMn2O4 (LMO) was examined as a cathode material for the lithium-ion battery, working together with a poly-mer electrolyte (PE) based on the PVdF polymer network, MePrPipNTf2 + LiNTf2 ionic liquid (MePrPip:N-methyl-N-propylpiperidinium cation, NTf2 is bis(trifluoromethanesulphonyl)imide anion) and vinylene car-bonate (VC) as the SEI forming additive. The polymer electrolyte was prepared by the casting technique. TheLiMn2O4 cathode in LMO/PE/Li and LMO/PE/Li4Ti5O12 (LTO) cells was characterized by scanning electronmicros-copy (SEM), cyclic sweep voltammetry (CV), electrochemical impedance spectroscopy (EIS), cyclic sweep volt-ammetry (CV) and galvanostatic charging/discharging. Room temperature ionic conductivity of the LiNTf2 +MePrPipNTf2 + PVdF and LiNTf2 + MePrPipNTf2 + PVdF + VC polymer electrolytes was 0.46 mS cm−1 and4.4 mS m−1, respectively. The LiMn2O4 cathode with quaternary PE exhibited a good specific capacity of117mAh g−1 and 105mAh g−1 at low andmedium rates. The full LMO/LTO cell using PE as a separator displayedgood cycling performances (ca. 103mAhg−1 after 160 cycles). This result indicates that the non-flammable, qua-ternary LiNTf2 + MePrPipNTf2 + PVdF+ VC polymer electrolyte can be safely used in combination with a man-ganese cathode in Li-ion batteries.

© 2014 Published by Elsevier B.V.

1. Introduction

Safety is one of themost important requirements for commercializa-tion of lithium ion batteries, i. e. the use of portable electronic devicesand hybrid electric vehicles (HEVs). To this end, it would lead to thereplacement of conventional, volatile and flammable organic alkyl car-bonate electrolytes with ionic liquid (IL)-based solutions [1–3]. ILs arenon-volatile, non-flammable and have good conductivity. Generally,conductivity of the order of 10–12 mS cm−1 is typical of ionic liquidsbased on the imidazolium cation. In the case of ionic liquids frequentlyused in lithium-ion cells (tetraalkylammonium, pyrrolidinium,piperidiniumcations), their conductivity ismuch lower, in the range be-tween 0.1 and 5 mS cm−1. These materials also exhibit an outstandingchemical and thermal stability from−90 to 350 °C and electrochemicalstability up to 5 V [4–7].

The use of a polymer electrolyte eliminates the need for the contain-ment of the liquid electrolyte, which simplifies the cell design, aswell asimproves safety and durability [8–10].

In turn, the inclusion of an ionic liquid into the polymer electrolyteprovides additional benefits (apart from safety). Namely, polymer elec-trolytes, resulting from a polymer matrix together with an ionic liquid

an.pl (A. Swiderska-Mocek).

solution, represent an attractive solution, since they combine mechani-cal and chemical stability of the polymer component with the intrinsicgood conductivity, non-flammable nature and high thermal stability ofthe ionic liquid component.

Polymer electrolytes with ionic liquids, generally based on PEO[11–19], homo- and co-polymers of poly(vinylidene fluoride) (PVdF)[20–25], poly(ethylene glycol) dimethyl ether [26] and other polymers[27–31], have been studied over the last few years. Typically, polymerelectrolytes with ionic liquids are formed by dissolving polymers in anionic liquid composed of a lithium salt and IL. The next step may be touse polymerized ionic liquids (PILs), which in the wake of polymeriza-tion of its monomer provide matrices for polymer electrolyte systems[32,33].

Ionic liquid based on N-methyl-N-propyl-piperidinium (MePrPip)as the cation and bis(trifluoromethanesulfonyl)imide (NTf2) as theanion has a relatively high ionic conductivity of 2.3 mS cm−1 at 25 °Cand sub-ambient melting temperature. Moreover, it shows an electro-chemical window of about 5.0 V, stability reduction potential (about−0.3 V vs. Li/Li+) and a high thermal decomposition temperature ofmore than 400 °C [4,34].

Spinel-type lithium manganese oxide LiMn2O4 (LMO) is one of thefrequently studied cathode materials in terms of its use in Li-ion batte-ries. It has a theoretical capacity of ∼148 mAh g−1; however, it is oftenobserved to have a practical capacity of 110–120mAh g−1. The cathode

Table 1Compositions and electrochemical properties of the polymer electrolytes (PEs) with ionicliquid investigated in this work.

Electrolyte Composition Ionicconductivity

Activationenergy

LiNTf2wt.%

MePrPipNTf2wt.%

PVdFwt.%

VCwt.%

mS cm−1

at 25 °CkJ mol−1

PE 1 2.5 67.4 30.1 0 0.5 26.1PE 2 2.4 65.5 29.1 3.0 1.0 23.7PE 3 2.3 63.4 28.8 5.5 4.4 12.4PE 4 2.3 62.8 27.8 7.0 3.0 14.6

33A. Swiderska-Mocek, D. Naparstek / Solid State Ionics 267 (2014) 32–37

material has been studied for many years, since it exhibits a potential of4.0/4.2 V vs. Li+/Li when cycled over the composition range of LixMn2O4

(0 b x b 1) and presents a higher thermal stability [35–38]. Thermalstability is a promising aspect for the improvement of battery cell safety.In addition, LiMn2O4 is nontoxic and low-cost, it exhibits good structuralstability and is environmentally friendly. LiMn2O4 has been shownto have good performance properties in lithium batteries basedon ionic liquids. This spinel was examined in an ionic liquid basedon trimethylhexylammonium [39], N-methyl-N-propylpiperidinium[40] 1-cyanomethyl-3-methylimidazolium [41] cations and thebis(trifluoromethanesulphonyl)imide anion.

The main problemwith the application of lithium as the anode is itshigh reactivitywhichduring a prolonged operation of the cell (cyclic de-position and dissolution of Li) leads to the formation of dendrites on themetal surface, causing serious problemswith safety and efficiency of thesystem. Carbonaceous materials are used widely, particularly for theirlow power applications [2]. On the other hand, dendritic lithium growthwas observed on the graphite anode surface at potentials approaching0 V vs. Li at the end of Li insertion [1]. A solution to this problem couldbe provided by the use of electrochemical redox coupled with higherequilibrium potentials, which make Li formation thermodynamicallyless favorable. Such a material is spinel lithium titanate, Li4Ti5O12

(LTO). Due to a reduction potential of LTO of around 1.55 V vs. Li/Li+,safety concerns such as e.g. dendrite formation, lithium plating, andelectrolyte decomposition can be avoided. In addition, LTO is character-ized by a high structural stability with an almost negligible volumetricchange (the so-called “zero strain” insertion material) in the Li+ inser-tion/extraction process and a flat operating voltage [42,43]. Finally,Li4Ti5O12 can be charged up to Li7Ti5O12, which corresponds to a theo-retical capacity of 175 mAh g−1 [44,45]. The replacement of lithiumwith LTO in a lithium-ion battery will result in a reduction in the oper-ating cell voltage, which reduces overall energy density at the celllevel. This can lead to their improved cycle-life, safety, while additional-ly the LMO/LTO chemistry may be attractive for high power battery ap-plications [46,47].

The general aim of this paper was to study the LiMn2O4 cathode in anon-flammable polymer electrolyte based on a poly(vinylidene fluoride)polymer (PVdF), lithium bis(trifluoromethanesulphonyl) imide (LiNTf2)salt, N-methyl-N-propylpiperidinium bis(trifluoromethanesulphonyl)imide ionic liquid (MePrPipNTf2) and vinylene carbonate (VC) as theSEI forming additive. The MePrPipNTf2–LiNTf2–PVdF or MePrPipNTf2–LiNTf2–PVdF–VC polymer electrolytes were prepared by the castingtechnique. The electrochemical properties of LiMn2O4 were tested incombination with Li metal and Li4Ti5O12 anodes.

2. Experimental

2.1. Materials

Lithium manganese oxide (LiMn2O4, Aldrich), lithium titaniumoxide (Li4Ti5O12, BET surface area 32.6 m2 g−1, b200 nm particle size,Aldrich), poly(vinylidene fluoride) (PVdF, MW = 180 000 Fluka),carbon black (CB, Alfa Aesar), vinylene carbonate (VC, Aldrich),N-methyl-2-pyrrolidinone (NMP, Fluka), N,N-dimethylacetamide(DMA, Aldrich), dimethyl carbonate (DMC, Aldrich) and lithiumbis(trifluoromethanesulphonyl)imide (LiNTf2, Fluka) were used asreceived.

N-methyl-N-propylpiperidinium bis(trifluoromethanesulphonyl)imide (MePrPipNTf2) was prepared according to the literature [48].The water content in the ionic liquid was found to be less than 50 ppmaccording to the standard Karl Fisher titrationmethod (Aldrich). The liq-uid electrolyte was produced by the addition of LiNTf2 salt to liquidMePrPipNTf2 to a concentration of 0.4 M. This operation was carriedout in a dry-box under argon atmosphere.

The polymer electrolyte and composite electrodeswere prepared bythe casting technique. Preparation of polymer electrolytes (PEs) by

casting method was previously described [49]. First, the polymer(PVdF) was swollen in DMA at 50 °C. Thereafter, 0.4 M LiNTf2 inMePrPipNTf2 was added to the PVdF N,N-dimethylacetamide solution.The resultant viscous solution was cast onto a glass plate. In order toevaporate the volatile solvent (DMA), the plate was placed in a desicca-tor. The slow drying process (in the stream of dry argon, first at roomtemperature, then at 60 °C and finally at reduced pressure) was termi-nated after complete evaporation of the solvent and after the free-standing membrane was produced. The composition of the final foilwas ca. 30 wt.% of the polymer (PVdF) and ca. 70 wt.% of the electrolyte(solution of 0.4 M LiTNTf2 in MePrPipNTf2). The resulting gel-type poly-mer electrolytes were optically homogeneous and transparent mem-branes. Polymer electrolytes were soaked of vinylene carbonate (VC,3%–7%) before cell assembly. After the addition of the polymer electro-lyte vinylene carbonate was left for 2 h, then VC visible excess was re-moved by filter paper. The content of VC in the electrode wasestimated by weight. Polymer electrolyte as a disc of 1 cm in diameterwas weighed before and after the addition, and the final removal of ex-cess vinylene carbonate. The compositions of the final polymer electro-lytes are listed in Table 1. Electrodes were prepared from a slurry ofLi4Ti5O12 or LiMn2O4, carbon black (CB), PVdF and 0.4 M LiTNTf2 inMePrPipNTf2 in N-methyl-2-pyrrolidone (NMP, Fluka). The ratio ofcomponents (LTO):(CB):(PVdF)(Li salt in IL) and (LMO):(CB):(PVdF)(Lisalt in IL) was 50:5:10:35 (by weight). Cu (Hohsen, Japan, area0.785 cm2) andAu (area 0.785 cm2) foilswere used as current collectorsfor the anode and cathode, respectively. Electrodes typically contained2.8 mg–3.5mg of LTO and LMO after vacuum evaporation of the solvent(NMP) at 120 °C. A round-shaped lithium electrode (Aldrich, 0.75 mmthick) was used as the counter and reference electrode.

2.2. Procedure and measurements

The batterieswere prepared by sandwiching the polymer electrolytefilms between an anode (Li metal or Li4Ti5O12) and a composite cathode(LiMn2O4). Cells were assembled in a dry argon atmosphere in a glovebox. Electrodes were separated by a polymer electrolyte soaked withVC and placed in an adapted 0.5″ Swagelok® connecting tube.

Conductivity of polymer electrolytes, sandwiched between two goldblocking electrodes, was measured in the Swagelok® connecting tubeplaced in an air thermostat. During conductivity measurements of theelectrolyte impedance values were recorded between 1 Hz and 10 kHzat the amplitude of 10 mV. Electrochemical impedance spectroscopy(EIS) was performed with the use of the G750 Potentiostat/GalvanostatMeasurements System (Gamry, USA).

The electrochemical characteristic of the LMO/PE/Li and LMO/PE/LTO cells was investigated using cyclic voltammetry (CV), galvanostaticcharge/discharge tests. Cyclic voltammetry of the LMO electrode wasmeasured in the potential range of 3.2–4.3 V vs. Li/Li+ with the scanrate of 0.01 mV s−1 and at 25 °C. Cycling measurements were takenwith the use of the ATLAS 0461 MBI multichannel electrochemical sys-tem (Atlas-Solich, Poland) at different current densities: C/10–1 C and at25 °C for the LMO electrode. Constant current charging/discharging cy-cles for the LMO/PE/Li cell were conducted between 3.2 and 4.3 V versus

1323334353637383

-8.2-7.9-7.6-7.3-7

-6.7-6.4-6.1-5.8-5.5-5.2-4.9-4.6-4.3

2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5

T / OC

ln (σ

/ S c

m-1

)

1/T 1000/ K-1

PE 3

PE 1

PE 4

PE 2

Fig. 2. Ionic conductivity as a function of temperature for polymer electrolytes.

34 A. Swiderska-Mocek, D. Naparstek / Solid State Ionics 267 (2014) 32–37

the lithium reference. However, the galvanostatic charge/dischargetests for the full LiMn2O4/PE/Li4Ti5O12 cell were carried out in the volt-age range of 1.8–2.7 V vs. Li/Li+ at current densities: C/10–C/2.

Scanning electron microscopy (SEM) as well as energy dispersivespectroscopy (EDX) of electrodes was performed with a Tescan Vega5153 apparatus. After electrochemical measurements, the cells weredisassembled, and the electrodes were washed with DMC and dried invacuum at room temperature.

3. Results and discussion

Ternary LiNTf2 + MePrPipNTf2 + PVdF polymer electrolytesprepared by the casting technique had a form of free-standing, flexibleand transparent foils of 0.16 mm–0.20 mm in thickness. Despite ahigh content of the ionic liquid, and a relatively low content ofthe polymer the resulting membranes have good mechanicalproperties. Fig. 1 shows a picture of the produced PE. QuaternaryLiNTf2+ MePrPipNTf2+ PVdF+VC polymer electrolyte was producedafter soaking the vinylene carbonate membrane previously obtained bythe casting technique. A small addition of VC (3%–7%) does not changethe goodmechanical properties of the foil. Additionally, it was observedthat the presence of vinylene carbonate in IL-based polymer mem-branes enhances their conductivity and highly stabilizes the interfacewith a lithiummetal electrode by forming a protective solid electrolyteinterface (SEI) film. It is widely known for the role of VC as the SEIforming additive [7,50,51]. The good electrochemical and interfacialproperties of this type of hybrid membranes (with PVdF-co-HFP) havebeen demonstrated in our laboratory [49].

Fig. 2 shows the temperature dependence of ionic conductivity ofpolymer electrolytes in the form of Arrhenius plots. As it can be seen,ionic conductivity increases with the increasing temperature and allthe plots appear to be linear in the temperature range measured. Themembrane without VC (PE 1) displays the lowest conductivity in thewhole range of temperatures investigated in this study, attaining valuesof 0.5 and 1.5mS cm−1 at 25 and 69 °C, respectively. The activation en-ergy of the conducting process was 26.1 kJ mol−1. However, the lowconductivity of the ternary LiNTf2 (2.5 wt.%), MePrPipNTf2(67.4 wt.%), PVdF (30.1 wt.%) polymer electrolyte is comparable tothe systems of PEO or PVdF-co-HFP [16,24,25]. The addition of VC tothe polymer electrolytes causes conductivity to increase. This is mainlydue to a decrease in viscosity. The conductivity value of PE 3 (with3 wt.% VC) and PE 4 (with 7 wt.% VC) at 25 °C is 1.0 and 3.0 mS cm−1,

Fig. 1. Photographs of polymer electrolyte.

respectively. Ionic conductivity reaches the maximum value,4.4 mS cm−1, when the PE contains 5.5 wt.% of VC. Then activation en-ergy decreases to 12.4 kJ mol−1, suggesting that the ions in the PE con-taining 5.5% VCmigrate more easily than in the other systems (Table 1).

The electrochemical behavior of the LMO/PE/Li system was investi-gated using CV. The obtained voltammograms of the LiMn2O4 electrodein PE 1 and PE 3 are shown in Fig. 3. In the case of the polymer electro-lyte with the addition of VC (PE 3) two pairs of oxidation current peaksand reduction current peaks for the cathode can be observed. This illus-trates the typical characteristics of two-stage reversible phase transfor-mation of the spinel LiMn2O4 [52]. The oxidation peaks (Li+

deinsertion) are observed at potentials of ca. 4.05 V and 4.18 V, whilethe reduction peaks (Li+ insertion) at potentials of ca. 3.95 V and4.07 V, respectively. The peaks in subsequent cycles are sharp, narrowand almost identical, indicating excellent reversibility of Li ion insertionand extraction for the LiMn2O4 spinel and the detected two-plateaucharge/discharge profile [38]. In turn, for the LMO/PE 1/Li cell we cansee only one set of well-defined current peaks at 3.35 and 3.60 V(3.52 V in the next cycles). The position of the anodic and cathodicpeaks is not consistent with the redox potential of the electrochemical

- 0.35

- 0.25

- 0.15

- 0.05

0.05

0.15

0.25

0.35

3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6

I / m

A

E / V vs. Li/Li+

1st cycle PE 1 2nd cycle PE 1

1st cycle PE 3 2nd cycle PE 3

Fig. 3.Cyclic voltammogramof LiMn2O4withPE1 and PE3vs. Li/Li+. Scan rate: 0.01mVs−1,at 25 °C.

Fig. 4. SEM images of the LiMn2O4 cathode: pristine electrode (a and b), after 200 charge/discharge cycles (c, d). Electrolyte: PE 4. Magnification: 5000× (a, c) and 20 000× (b, d).

35A. Swiderska-Mocek, D. Naparstek / Solid State Ionics 267 (2014) 32–37

process (4.0/4.2 V vs. Li+/Li) determined from charging/dischargingtests. Therefore, only the quaternary polymer electrolyte will work ina later stage of the manganese cathode tested. An additional advantageis its high conductivity, rarely seen in the case of polymer electrolytes.The presence of vinylene carbonate in the polymer electrolyte stabilizesthe interface with the lithium and manganese electrodes, because dur-ing SEI formation this additive is converted into a component of theinterface.

The next part will show compatibility of quaternary LiNTf2 +MePrPipNTf2 + PVdF+ VC polymer electrolyte for systems comprisinga LiMn2O4 cathode and Li metal or Li4Ti5O12 anodes.

Fig. 4 shows scanning electron microscopy (SEM) images of a pris-tine LMO electrode and those after electrochemical cycling working

Fig. 5. EDX spectrum of pristine and cycled LiMn2O4 electrode.

with polymer electrolyte. The pristine cathode (Fig. 4a and b) consistsof many crystals with clearly marked edges. However, after electro-chemical cycling interparticle boundaries disappear and the particlesare transformed into a much more uniform structure (the effect offlooding or bonding). These changes may indicate the formation of aSEI layer [53]. Also, differences in the composition of the surface in thepristine and cycled cathode indicate the formation of a passivationlayer on its surface. The EDX analysis (Fig. 5) suggests that fluorineand sulfur are present on the electrode surface (from electrochemicaldecomposition of the NTf2 anion). However, larger peaks of carbonand oxygen are likely to result from the polymerization of vinyl carbon-ate to the cathode surface of the electrochemical charge and discharge.

Fig. 6 shows the charge–discharge curves of the LMO/Li halfcell using the LiNTf2 (2.3 wt.%), MePrPipNTf2 (63.4 wt.%), PVdF

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

0 20 40 60 80 100 120 140

E /

V

Capacity / mAh g-1

C/10C/5C/21 C

Fig. 6.Galvanostatic charge/discharge profile of LiMn2O4/PE 3/Li cell at different C rates, at25 °C.

0

20

40

60

80

100

120

140

160

0

20

40

60

80

100

120

140

160

1 5 9 13 17 21 25 29 33 37 41

Cap

acity

/ m

Ah

g-1

Cycle number

discharge capacity

charge capacity

efficency

Cul

ombi

c ef

ficen

cy/ %

Fig. 7.Cycle performance of the LiMn2O4/PE 3/Li cell at C/10 ratewith coulombic efficiencyat 25 °C.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 20 40 60 80 100 120 140

E / V

Capacity / mAh g-1

C/10C/5C/2

Fig. 9. Galvanostatic charge/discharge profile of LiMn2O4/PE 3/Li4Ti5O12 cell at differentC rates, at 25 °C.

36 A. Swiderska-Mocek, D. Naparstek / Solid State Ionics 267 (2014) 32–37

(28.8 wt.%), and VC (5.5 wt.%) polymer electrolyte tested under differ-ent current rates (C/10–1 C). It can be seen that the charge/dischargecurves of the LiMn2O4 electrode clearly exhibit two potential plateausonly at a low and medium charge/discharge rate of C/10 and C/5(1 C = 120 mA g−1). The curves registered for high current rates (C/2and 1 C) deviate from the characteristic shape of the LMO cathode.The capacity of the discharging and charging processes at the firstcycle was 132 and 122 mAh g−1, respectively, resulting in a coulombicefficiency of 92% (Fig. 7). In subsequent cycles, the coulombic efficiencyincreased to 99.7 % after 40 cycles (reversibility capacity was ca.117 mAh g−1). Then the reversible capacity is not low and accountsfor 97% of the theoretical value, which is a good result and providespromising prospects for further research. The cycle-life performance ofthe LMO/PE 4/Li cell was evaluated at different rates as shown inFig. 8. These results confirm good cycle life capability of the cell and arelatively high specific capacity, 117 mAh g−1 and 105 mAh g−1,under low and medium C rates (C/10–C/5), respectively. It wasobserved that the significant capacity decreased to 76 mAh g−1 at theC/2 rate. A further increase in current density (1 C) causes an even larger

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160 180 200 220

Dis

char

ge c

apac

ity /

mA

h g-

1

Cycle number

C/10

C/5

C/2

1 C

C/10

Fig. 8. Discharging capacity of LiMn2O4/PE 4/Li cell at different C rates, at 25 °C.

decrease in capacity to 47 mAh g−1. However, the return to the lowercurrent density (C/10) results in an increase in specific capacity(100 mAh g−1).

Voltage–capacity plots of the LiMn2O4/Li4Ti5O12 cell using a polymerelectrolyte with 5.5% VC at different current densities (C/10, C/5, C/2)are shown in Fig. 9. The charge/discharge voltage two-stage plateau oc-curs. The cell provides a 2.5 V operating voltage. The LMO/PE 3/LTO cellshows a very good cycling performance at low and medium rates(Figs. 10 and 11). The first charge capacity and the discharge capacityare 127 and 120mAh g−1 (based on themass of LiMn2O4), respectively.The initial coulombic efficiency is only 94%. After 40 cycles, the dis-charge capacity is 114mAh g−1 (approaching 95% of the theoretical ca-pacity) and the coulombic efficiency exceeds 99.5%. When the currentdensity is doubled (from C/10 to C/5), the reversible capacity decreasesto 92mAh g−1. At the C/2 rate the cell yielded 97% coulombic efficiencyand achieved a reversible capacity of only 55 mAh g−1. In contrast,returning to the lower current density (C/10) again results in an in-crease of specific capacity to 103 mAh g−1.

0

20

40

60

80

100

120

140

160

0

20

40

60

80

100

120

140

160

1 5 9 13 17 21 25 29 33 37 41

Cap

acity

/ m

Ah

g-1

Cycle number

discharge capacitycharge capacityefficency

Cul

ombi

c ef

ficen

cy /

Fig. 10. Cycle performance of the LMO/PE 3/LTO cell at C/10 ratewith coulombic efficiencyat 25 °C.

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160 180

Dis

char

ge c

apac

ity /

mA

h g-

1

Cycle number

C/10

C/5

C/2

C/10

Fig. 11. Discharging capacity of LMO/PE 4/LTO cell at different C rates, at 25 °C.

37A. Swiderska-Mocek, D. Naparstek / Solid State Ionics 267 (2014) 32–37

4. Conclusions

A non-flammable, polymer electrolyte, incorporating MePrPipNTf2ionic liquid has been prepared successfully by the casting technique.The ternary LiNTf2 + MePrPipNTf2 + PVdF polymer electrolyte showsgoodmechanical properties. The addition of VC to themembrane result-ed in a large increase of ionic conductivity (4.4 mS cm−1 at room tem-perature) without lowering its safety and good mechanical properties.

The LiMn2O4 electrode tested in this study shows good capacity andcyclability, which points to a good electrode/electrolyte interface con-tact. It is perhaps due to its very high compatibility of quaternary PEwith the electrode material. Preliminary battery tests have shown thatLiMn2O4/PE/Li cells are capable of delivering relatively high specific ca-pacity, at 117 mAh g−1 and 105 mAh g−1, under low and medium Crates (C/10–C/5), respectively. The full LiMn2O4/Li4Ti5O12 cell using PEas a separator displayed good cycling performances (ca. 103 mAh g−1

after 160 cycles). This result indicates that the non-flammable, quater-nary LiNTf2 + MePrPipNTf2 + PVdF + VC polymer electrolyte can besafely used in combination with the manganese cathode in Li-ionbatteries.

Acknowledgments

Support of grant 31-277/2014 DS PB is gratefully acknowledged.

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