Available online at www.sciencedirect.com

Review Article

Recent advances in the research of functional electrolyte additives for lithium-ion batteries

Weimin Zhao

1 , # , Yajuan Ji 2 , # , Zhongru Zhang

2 , Min Lin

1 , Zeli Wu

2 , Xi Zheng

2 , Qi Li 2 and Yong Yang

1 , 2 , ∗

Functional electrolytes with additives are one of the key materials which affect the electrochemical performance of the Li-ion batteries such as energy density, power density and

cycling performance. This paper gives a short overview of recent works on some new functional electrolyte additives involve P/N/F/S-containing and new type of lithium salts. The insights of working mechanism of these additives are summarized. The newly progress of in situ spectroscopic techniques and theoretical tools to characterize the composition, structure and growth of solid electrolyte interfaces (SEIs) are also briefly reviewed.

Addresses 1 College of Energy, Xiamen University, Xiamen 361005, China 2 State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

∗Corresponding author : Yang, Yong ( yyang@xmu.edu.cn )

#They contributed equally to this work.

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Current Opinion in Electrochemistry 2017, 6 :84–91

This review comes from a themed issue on Batteries and Superca- pacitors

Edited by Seung Mo Oh

For a complete overview see the Issue and the Editorial

Available online 16 October 2017

https:// doi.org/ 10.1016/ j.coelec.2017.10.012

2451-9103/© 2017 Elsevier B.V. All rights reserved.

ntroduction

lectrolyte is one of the most important components in

etermining the electrochemical performance of Li-ion

atteries. Compared with routine electrolytes containing

olvents and lithium salts only, advanced electrolytes or unctional electrolytes with different additives have be- ome increasingly versatile and strongly affected critical arts governing battery performance parameters such

s deliverable capacity, power, cycle-life, storage and

afety performance, etc. This is mainly due to the roleshich played by the corresponding additives [1

••,2] :1) stabilizing solid/electrolyte interface (SEI) film to

urrent Opinion in Electrochemistry 2017, 6 :84–91

educe electrolytes reaction (oxidation or reduction) and

rotect transition metal ion dissolution from cathode ma-erial, (2) improving physical properties of the electrolyte

uch as ionic conductivity, viscosity and wettability, (3)educing the flammability of organic electrolytes, (4) roviding overcharge protection, (5) enhancing thermal tability of LiPF 6 against the organic solvents and son. However, it is well known that it is impossible toatisfy all the required battery properties by addition ofingle additive in the electrolyte, and the mainstream ofdditives development is to combine multiple additives ogether in the recipe of practical electrolytes or develop

ultiple-functional additives.

n LIB, SEI film is an important component because theyetermine the reversibility of Li-intercalation chemistries hile reflecting the kinetics of overall cell reactions [3

••] .herefore, improving the battery performance via stabi-

izing the SEI is a main focus of the additive research.hese additives include the unsaturated organic com- ounds (double or triple bonds, cyclic structures, phenyl,tc.), organic phosphorus/nitrides/fluoride/sulfide, new

ype lithium salts, etc. (as seen in Figure 1 ).

n this paper, we will review the research progress oftate-of-the-art additives and discuss their functions in

mproving LIB battery performance. Both the progress f theoretical calculation and characterization techniques f electrolyte additives are also briefly summarized.

unctional electrolyte additives

hosphorus-containing additives

hosphorus-containing organic compounds are well nown and practically used as flame-retardant (FR) aterials that suppress the flammability of liquid elec-

rolytes in lithium-ion battery based on the flame retar-ation mechanism of physical char-forming process and

hemical radical-scavenging [2] . Trimethyl phosphate

nd triethyl phosphate with high content of phosphorus re among the earliest investigated FRs. Unfortunately,oth of them are unstable against the low reductive po-ential on the graphite anode surface. By replacing alkylroups with the aryl (phenyl) groups [4] , fluorinated alkyl5] and cyclophosphazene [6] , the strategy can effectivelymprove the reductive stability of phosphates. In addi- ion, Cao et al. [7] found that phosphamide, containing

–N bonds, not only increased the FR effectivenessut also enhanced thermal stability of SEI on anode.

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Functional electrolyte additives for lithium-ion batteries Zhao et al. 85

Figure 1

Chemical structure of some electrolyte additives for li-ion batteries.

The corresponding decomposition process of the additiveis that the P

–N is broken and connected to the –OR groupduring the discharging process by usage of 31 P MAS NMR[8] . On the other hand, the phosphorus-containing addi-tives were particularly attractive as cathode/electrolyteinterfaces (CEI) film-forming additives in LIB whenadded within an appropriate concentration. For example,tri(hexafluoro-iso-propyl) phosphate (HFiP) [9] , triethylphosphite (TEP) [10] , tris(pentafluorophenyl) phosphine(TPFPP) [11] , tris(trimethylsilyl) phosphate (TMSP)[12] , phosphazene compounds [13] and so on can signif-icantly improve the cycle stability of Li-ion cells. Thoseresults can be ascribed to the formation of a robust andprotective film derived from the preferential oxidationof the phosphorus-containing additive on the surfaceof the electrode to suppress the electrolyte subsequentdecomposition and related transition metal dissolution,thereby enhancing the structural stability of materialsand cycling performance [14] .

Nitrogen-containing additives

Nitrogen-containing additives mainly include nitrilesand nitrogen-containing heterocyclic compounds [15] .Dinitriles have superior electrochemical stability windowof 7 V (Li + /Li) [16] . Song et al. [17] demonstrated that

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the introduction of nitriles to electrolytes exhibited greatimprovements in both cycling performance and safety atelevated temperature, which is attributed to the modi-fied electrode/electrolyte interface by strong interactionbetween nitrile functional group (–CN) and transitionmetal ions of cathode material to suppress thermallyaccelerated interfacial side reactions. Our recent work[18,19

••] also confirms that the suberonitrile (SUN) as anadditive can enhance the electrochemical performance ofcathode materials at higher potentials. XANES spectrumanalyses indicate that the charge-transfer from SUN toLiCoO 2 may result in lower valence of Co

4 + and lowerCo-O covalence, which shall stabilize the correspondingelectrode/electrolyte interface (as shown in Figure 2 )[20] . The chemical images by PEEM technique revealthat there was at least two types of divalent Co on elec-trode surface, CoF 2 and Co

2 + -SUN (through chemicalinteraction between LiCoO 2 and SUN via sXAS andPEEM), protecting the high charged-state Co ions fromcatalyzing the further decomposition of electrolytes atthe high working voltage. Nitrogen-containing hete-rocyclic compounds can be also used as additives forovercharge prevention, which will polymerize at voltagesgreater than the maximum operating voltage and producean insulating layer of polymer, thereby increasing the

Current Opinion in Electrochemistry 2017, 6 :84–91

86 Batteries and Supercapacitors

Figure 2

PEEM image of LiCoO 2 cycled in (a) Base and (b) Base + LiBOB + SUN

electrolyte, (c) O K-edge and (d) C K-edge XANES of uncovered large LiCoO 2 in pristine electrode and cycled electrode with/without SUN/LiBOB [20] .

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nternal resistance of the battery sufficiently for protec- ion [21] . In addition, many heterocyclic compounds, in

hich the N core with lone-pair electrons acts as theeak base site to deactive the reactivity of PF 5 to inhibit

he further formation of HF, have been reported as theiPF 6 stabilizer [22] .

luorine-containing additives

ntroduction of fluorine atoms into the molecules results n the drop of energy levels for the highest occupied

olecular orbital (HOMO) and the lowest unoccupied

olecular orbital (LUMO). The former effect can im- rove the various electrochemical and physical properties uch as polarity, oxidation durability, liquidous temper- ture range and nonflammability because fluoride has strong electronegativity and weak polarity. However,he lower LUMO will lead to poor resistance against eduction and thereafter fluorinated additives can be

lso used as effective film-forming additives which

odify the structure and components of SEI layer and

uppress the further decomposition of electrolytes and

hen improve the cycling performance of the battery

23] . Typically, fluoroethylene carbonate (FEC) has been

uccessfully applied to the commercial electrolytes to

mprove cell performance, based on a reduction mech- nism where FEC reduces to form vinylene carbonate

VC) and LiF, followed by subsequent VC reduction

nd then produced a better conductive and stable SEI lm [24] . Abraham et al. [25] synthesized a series oferfluoroalkyl-substituted ethylene carbonates and

urrent Opinion in Electrochemistry 2017, 6 :84–91

mall amounts of 4-(perfluorooctyl)-1,3-dioxolan-2-one

PFO-EC) was found to be an effective additive inessening cell performance degradation during extended

ycling, ascribing to the formation of double-layer pas-ivation film via decomposition of PEO-EC. Chain

uorocarbonates, methyl-(2,2,2-trifluoroethyl) carbonate

FEMC) [26] , as additive can form a stable solid elec-rolyte interface layer which includes plenty of metaluorides and –CF-containing species formed by additive

ecomposition, and effective passivation of cathode sur- ace to suppress the dissolution of transition metal ionsnd lattice oxygen out of the structure of the destructionhen improved cycling performance.

ulfur-containing additives

rganic additives containing sulfur have attracted big

nterest due to the stronger electronegativity of sulfur.herefore, they are more likely to attract electron and be-

ng reduced on the anode compared with carbonate-based

lectrolytes.

t is reported that 1,3-propanesultone (PS) as an elevated-emperature additive which can reduce the ethylene

assing to suppress cell swelling during the cycling [27] .rop-1-ene-1,3-sultone (PES) [28] and vinyl ethylene

ulfite (VES) [29] , combined the sulfone with dou-le bond in one molecule, can form effective film touppress the PC co-intercalation into graphite anode.,3,2-Dioxathiolane-2,2-dioxide (DTD) as an additive

n electrolytes could form a PEO like polymer withhe inner layer comprised of Li 2 S-like compounds onraphite surface resulting in higher rate capacity [30

••] ,hile methylene methanedisulfonate (MMDS) could

ead to the changes of impedance [31] . Even a numerousesearch about additives containing different main ele-

ents such as P, N, F, S have been studied extensively inhe last 10 years, it is far from fully understanding of theoles of functional groups in the additives and working

echanism of single additives or combination of several dditives. Especially, the total effects of several additives n practical electrolytes are just the simple addition ofach additive. There is thereby an urgent need fill theap but it is still a significant challenge to investigate thexplicit mechanism and connection between properties nd functional group of these additives.

ew lithium salts

iPF 6 is the dominant salt used in the commercial LIBanufactured today. Nevertheless, the chemical and

hermal instability of LiPF 6 are two most conspicuous ssues [32] . The development of new lithium salt haseen a core topic to enhance the interfacial stabilityetween positive/negative electrode and electrolyte.

orates and boron-based cluster ithium bis(oxalate)borate (LiBOB), the first nonflu- rinated salt attracted significant interest, which was

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Functional electrolyte additives for lithium-ion batteries Zhao et al. 87

reported in a German patent by Metallgesellschaft AGin 1999 [33] . Subsequently, researchers found that theexcellent ability of film formation derived from LiBOBcan protect graphite from exfoliation even in neat propy-lene carbonate [34] . This salt is easily soluble in nitrileand ester solvents, but has a relatively low solubility incarbonate solvents [35] . Therefore, this salt is generallyused as additive to LiPF 6 electrolytes and can offer anexcellent capacity while cycled at elevated temperatureand high working voltage up to 5 V (vs. Li + /Li), which isattributed to the borate film formation on the electrodesderived from the decomposition of LiBOB [36] . Uponthe investigation of solving the compatibility problem be-tween LiBOB and LiCoO 2 by using nitriles such as SUN,Ji et al. [18] also found that the borate decomposed has astrong electron-withdrawing ligand character to capturedF

− from LiF, then effectively decreased the amount ofthe LiF in the CEI film and greatly suppress the increasesof interface resistance. Additionally, the strong interactionbetween SUN and borate groups might help stabilizingthe decomposition of BOB

−. Other new boron-based saltof inexpensive and chemically, electrochemically andthermally stable salts based upon boron chelate complexanions with aromatic or aliphatic diols or carboxylic acidslike lithium difluoro(oxalate)borate (LiODFB) [37

••] ,lithium bis(2-fluoromalonato)borate (LiBFMB) [38] ,lithium 4-pyridyl trimethyl borate (LPTB) and lithium2-fluorophenol trimethyl borate (LFPTB) [39] , lithiumcatechol dimethyl borate (LiCDMB) [40] have been alsodeveloped.

Phosphate-type salts

With the aim of mitigating its chemical/thermal in-stability of PF 6

−, while maintaining its merits, themodified version of LiPF 6 such as lithium tetrafluo-rooxolatophosphate (LiF 4 OP) was firstly synthesizedby Lucht and co-workers [41] . The properties of thissalt combine the benefits of both PF 5

− and BOB

−,could display high ion conductivity and thermal stability[42] . More importantly, LiF 4 OP as a salt-type additivecould improve the cycling performance of cells as wellas the thermal stability of lithiated negative electrodes,which was ascribed to the oxalate group reacting andform a polymerize passivation film on both electrodessurface [43] .

Lithium difluoro(bisoxalato) phosphate (LiDFBP),like LiF 4 OP, but has two oxalate groups in one phos-phate salt molecule. Choi et al. [44] introduced it asa novel electrolyte additive for lithium-rich cath-odes and confirmed that LiDFBP can enhance therate capability and cycling performance at both roomtemperature and 60 °C, due to the uniform and elec-trochemically stable SEI on the cathode. They alsofound that a large relative fraction of Li x PO y F z on thecathode while precycled in the LiDFBP-containingelectrolyte.

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Another promising salt is PO 2 F 2 −, which has been re-

ported by Choi and co-workers [45] . They firstly foundthat LiPO 2 F 2 combined with VC used as electrolyteadditives is able to achieve satisfied rate capability andcycling stability with highly pressed graphite electrodeswith high mass loading. That is due to the formation of amore stable and highly Li + conductive SEI layer derivedfrom LiPO 2 F 2 and VC which allows faster kinetics forLi + insertion. Recently, we also found that LiPO 2 F 2 usedas additive can form CEI film on the cathode surface,which can effectively improve the cycle performance ofLiNi 0.5 Co 0.25 Mn 0.25 O 2 electrode at room temperatureand elevated temperature.

Theoretical calculation of functional additives

with the first-principles

First-principle simulations have been conducted toidentify possible reaction pathways and intermediatesby reaction energy barrier and evaluation of potentialadditives. It helps us to understand the role of additivesfrom atomic scale and benefits to search and develop newfunctional additives. However, high-quality calculationand refined method is important in this strategy. VCadditive was firstly investigated under cluster boundaryconditions by implicit solvation model [46] . Recently,Jung et al. [47] show that Li + ion significantly impactsthe reductive decomposition of PES, the most kinet-ically favored process is S

–O bond breaking, but Li +

ion prevents further decomposition by forming a Li + -participated seven-membered ring. For ES in PC-basedsolvent, Sun and Wang [48] establish a more complexcluster model, [(ES)Li + (PC) 2 ](PC) n ( n = 0, 6, 9), inorder to compensate overestimation of solvent effectsarising from the PCM for the naked (ES)Li + (PC) 2 ,and the theoretical reduction potential (1.90–1.93 V) forthe additive ES agrees well with the experimental one(1.8–2.0 V).

Additives decomposition has been reinvestigatedby ab initio molecular dynamics (AIMD) and elec-trode/electrolyte interface model, which provides moreaccurate insight due to considering explicit solvation andelectrode effects. Tateyama et al. [49

••] proposed that VCpreferentially reacts with the EC anion radical to sup-press the two-electron (2e) reduction of EC and enhancethe initial SEI formation, contrary to the conventionalscenario in which VC is sacrificed and its oligomerizationform SEI [46] . For FEC additive on silicon surface,AIMD simulations suggest that initial reduction decom-position products are F

−, CO, C 2 H 3 O 2 − or C 3 H 3 O 3

−,F

−, and no HF or CH 2 CHF is released ( Figure 3 ) [50] .Another calculation suggests that the most likely productof the FEC reduction decomposition is LiF [51] , whichis consistent with this work. However, further reactionscannot be investigated directly by AIMD simulationsdue to its short time scale. In the future study, the

Current Opinion in Electrochemistry 2017, 6 :84–91

88 Batteries and Supercapacitors

Figure 3

(a) FEC molecule with labels. C, O, H, and F atoms are depicted as gray, red, white, and purple spheres. (b) and (c) Expanded views of decomposed molecules on the top and bottom sides of panel, omitting the Li 13 Si 4 slab. (d) and (e) Configurations at times t = 0 ps and t = 4 ps into an AIMD simulation of a Li 13 Si 4 slab immersed in liquid FEC/EC. Si and Li atoms are depicted as yellow and blue spheres. In (d), 3 FEC and 1 EC molecules at the surface, about to react, are depicted as ball-and-stick models. In (e), these 3 FEC and 1 EC that have reacted. (f) Liquid FEC in contact with Li 4 SiO 4 coated Li 13 Si 4 anode slab, also leading to two-e − reduction reactions [50] .

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eep understanding of functional additives depends on

ore realistic models, electric double layer should be

ncluded.

dvanced spectroscopic characterization

nd imaging techniques

large variety of traditional surface-analytic tools, like

EM, TEM, AFM, FTIR, IRAS, Raman, XANES, XPS,RD, DSC, ARC, etc., have been used for analyzing

he SEI film ranging from spectroscopy to microscopy

o diffraction and thermoanalysis ( Table 1 ), an excellenteview was published by Novák et al. [52

••] . Here, weurther modify and summarize several relevant analytical echniques for electrolyte additives in Table 1 . Recently,umerous advanced techniques aiming at probing and

maging LIB have emerged like X-PEEM, STXM,

EMS and ssNMR. e

urrent Opinion in Electrochemistry 2017, 6 :84–91

-ray photoemission electron microscopy (X-PEEM) 53] and scanning transmission X-ray microscopy (STXM) 54] are two powerful tools and capable of imaging sur-ace structures with tens of nanometer spatial resolution

n elucidating the SEI nature on the electrodes. Theechniques help us better understanding of the spatiallyaried chemical composition and electronic structure

f SEI on various electrode components and their in-erplay. Differential electrochemical mass spectrometry

DEMS) [55] is an electrochemical technique by cou- ling mass spectrometry. It is mainly used to detectolatile and gaseous products (CO, CO 2 , SO 2 , ethylene,tc.), which were caused by decomposition of electrolyte.ani Hashemi and La Mantia [55] introduced a cellodel where users can get accurate DEMS result, for

oth organic and aqueous systems, and then deduce theeaction mechanism of electrolyte. Our group also use

EMS to monitor the release of gaseous products wheni[Li 0.2 Mn 0.54 Ni 0.13 Co 0.13 ]O 2 was delithiated at 4.6 V

56] . As a nondestructive and quantitative method, solid-tate nuclear magnetic resonance (ssNMR) could charac- erize SEI compositions by detecting multiple nuclei ( 1 H,3 C, 19 F, etc.). Michan et al. [57] showed EC/DMC decom-osition products on Si/C composite electrodes indepen- ently by using selectively

13 C-labeled electrolytes and

D correlation experiments, which PEO-type oligomers nd CH 3 OCO 2 Li were the dominated products,espectively.

onclusions and outlook

esearch and development of functional additives for IB batteries has made significant progress in last severalears. It overcomes and amends some shortcomings and

isadvantage of the routine electrolytes such as solventsnd salts for the active demand of fast-growing of LIB bat-eries with high performance. We have also accumulated

ome knowledge about the working mechanism of these

mportant additives, though mostly on the single addi- ive’s effect. In general, more attention should be paido seek or design some novel organic compounds withultifunctional groups in a molecule or some composite

ithium salts, and the resulted SEI layers on the electrodeshould be more flexible and stable. And possibly in someays, we can prepare some SEI layers on the electrodesith some functional roles by using a simple combinationf electrolyte additives. However, the detailed character- zation of interfacial chemistry, especially the SEI or CEIlm has been especially challenging, because of the elu-ive and complex nature of the interface and absence offfective and noninvasive in situ techniques until recently.hus, more sensitive, less invasive, and reliable surface

nalytical tools are urgently needed. More systematic nd multiple-tools analysis of the SEI films and electro-hemical reaction mechanism of the additives should be

ncouraged.

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Functional electrolyte additives for lithium-ion batteries Zhao et al. 89

Table 1

Characterization techniques for electrolytes and additives and SEI films.

Techniques For electrolytes and additives For SEI films

Atomic spectroscopy AES, AAS, AFS, XRF XAS, XANES Molecular spectroscopy FTIR, Raman, UV–vis FTIR, Raman, UV–vis Photoelectron spectroscopy XPS, UPS, EDS XPS, UPS, EDS Mass spectrometry GC-MS, LC-MS, TG-MS, DEMS TOFMS Nuclear magnetic resonance Liquid state NMR Solid state NMR

Chromatography GC, LC, HPLC, IC LC, HPLC, IC

Thermal analysis TG/TGA, DTA, DSC TG/TGA, DTA, DSC

Electrochemical analysis CV, Galvanostatic method, LSV, potentiostatic method, chronoamperometry

EIS

Microscopy techniques / TEM, SEM, AFM, X-PEEM, STXM

Note: the full name of some techniques: AES (Atomic emission spectroscopy) AAS (Atomic absorption spectroscopy) AFS (Atomic fluorescence spectrometry) XRF (X-ray fluorescence) XAS (X-ray absorption spectroscopy) XANES (X-ray absorption near edge structure) FTIR (Fourier transform infrared spectroscopy) UV–vis (Ultraviolet–visible spectroscopy) XPS (X-ray photoelectron spectroscopy) UPS (Ultraviolet photoelectron spectroscopy) EDS (Energy dispersive spectrometer) TOFMS (Time of flight mass spectroscopy) GC (Gas chromatography) LC (Liquid chromatography) HPLC (High performance liquid chromatography) IC (Ion chromatography) TG/TGA (Thermogravimetric analysis) DTA (Differential thermal analysis) DSC (Differential scanning calorimetry) CV (Cyclic voltammetry) LSV (Linear sweep voltammetry) EIS (Electrochemical impedance spectroscopy) TEM (Transmission electron microscope) SEM (Scanning electron microscopes) AFM (Atomic force microscope)

Acknowledgments

This work is financially supported by National Key Research and Develop- ment Program of China (grant no. 2016YFB0901500) and National Natural Science Foundation of China (grant nos. 21233004 , 21428303 , 21621091 ).

References and recommended reading

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• Paper of special interest. •• Paper of outstanding interest.

1. ••

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