DOI: 10.1002/cphc.201402175

Recent Progress in Research on High-Voltage Electrolytesfor Lithium-Ion BatteriesShi Tan,[a, b] Ya J. Ji,[a] Zhong R. Zhang,[a] and Yong Yang*[a]

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1. Introduction

With the rapid development of the lithium-ion-batteriesmarket, people have started to expect better electrochemicalperformance from batteries, judged by metrics such as energydensity and power density. This especially holds true for batter-ies used in 3G/4G portable devices, electric vehicles, andenergy-storage systems in smart electric grids. However, thedevelopment of higher energies and power densities in lithi-um-ion batteries is meeting some technical bottlenecks at thisstage. For example, it is quite difficult to find stable electrodeand electrolyte materials that can work at high potentials(~5 V vs. Li+/Li). In pursuit of new electrode systems, novelcathode materials such as LiCuxMn2-xO4 (4.9 V vs. Li+/Li),[1, 2]

LiNi0.5Mn1.5O4 (4.7 V vs. Li+/Li),[3–6] LiNixCo1-xPO4 (4.8–5.1 V vs.Li+/Li),[7] and Li2CoPO4F (5.1 V vs. Li+/Li)[8–10] have attractedmuch attention due to their high operating voltage. Althoughthese electrode materials can cycle quite well at such high vol-tages, keeping their structures stable, the working voltage ofthese new materials is beyond the electrochemical window ofconventional organic carbonate-based electrolytes (<4.3 V vs.Li+/Li).[11–13] Therefore, the stability of electrolytes at high vol-tages has become a constraining factor for the developmentof high-energy-density cathode materials, and research onhigh-voltage electrolytes at the 5 V level is urgently necessary.In this review, we first introduce some background knowledgeabout conventional carbonate-based electrolyte systems; then,recent progress in the research of high-voltage electrolytes issummarized and reviewed, with consideration of new recipesof conventional carbonate solvents and additives and high-voltage electrolytes with new types of solvents. The contentsof solid electrolytes such as inorganic or polymer electrolytesthat can work at high voltages are, however, not included. Thefuture direction and prospects for the research and develop-ment of high voltage electrolyte systems are also briefly dis-

cussed. Previous reviews on common electrolytes for Li or Li-ion batteries can also be found in the literature, while novelhigh voltage electrolyte systems have been rarelydiscussed.[14–17]

1.1. The Conventional Carbonate-Based Electrolyte System

It is well known that the cathode material, anode material, andelectrolyte are the three basic components of battery systems.The role of the electrolyte is to act as a charge-transfermedium, allowing ions to move between the cathode andanode (positive and negative electrodes, respectively). Further-more, the electrolyte also plays an important role in contribu-ting to and sustaining the battery’s charge capacity, cyclingperformance, safety performance, and so on. In general, theelectrolyte of a lithium-ion battery is composed of organic sol-vents, lithium salts, and functional additives. As one of themost important components, an ideal electrolyte solvent forlithium-ion batteries should meet the following basic require-ments:[15, 18] 1) It should have a wide electrochemical window(0~5 V). 2) It should be compatible with the battery electrodematerial (no side reactions between electrode and electrolyte).3) It should be chemically inert with regards to all cell compo-nents during cell operation. 4) It should have a high ionic con-ductivity, and typically a low viscosity and a high dielectricconstant. 5) It should exist as a liquid in a wide temperaturerange. In particular, its melting point should be as low as possi-ble, while the boiling point should be as high as possible. Forany single solvent, it is difficult to meet these requirements atthe same time, and properties such as a high dielectric con-stant and low viscosity are usually not integrated into a singlecompound (in this case, with the exception of some organicnitrogen-containing compounds).[19] Therefore, a compromiseof these properties is arrived at via the mixing of solvents. Forexample, commercial lithium-ion-battery electrolytes are usual-ly composed of salts and different ratios of solvents such asLiPF6 + [ethylene carbonate (EC): linear alkyl carbonate (ethyl-methyl carbonate (EMC), dimethyl carbonate (DMC), diethylcarbonate (DEC), etc.)] .[20] Figure 1 shows several structural for-mulae of relevant alkyl carbonates. Peculiarly, such mixtures ofelectrolytes have a slightly wider electrochemical window thana single solvent, it is probably due to the synergistic effect ofcyclic carbonate and linear carbonate.[15] For example, the oxi-dation potentials of single carbonate solvents are ordered by

[a] S. Tan,+ Y. J. Ji,+ Dr. Z. R. Zhang, Prof. Dr. Y. YangState Key Lab for Physical Chemistry of Solid SurfacesCollege of Chemistry and Chemical EngineeringXiamen University, Xiamen, 361005 (China)E-mail : yyang@xmu.edu.cn

[b] S. Tan+

Ningde Amperex Technology LimitedNingde 352100 (China)

[+] These authors contributed equally to this work

Developing a stable and safe electrolyte that works at voltagesas high as 5 V is a formidable challenge in present Li-ion-bat-tery research because such high voltages are beyond the elec-trochemical stability of the conventional carbonate-based sol-vents available. In the past few years, extensive efforts havebeen carried out by the research community toward the explo-ration of high-voltage electrolytes. In this review, recent prog-ress in the study of several promising high-voltage electrolytesystems, as well as their recipes, electrochemical performance,electrode compatibility, and characterization methods, are

summarized and reviewed. These new electrolyte systems in-clude high-voltage film-forming additives and new solvents,such as sulfones, ionic liquids, nitriles, and fluorinated carbo-nates. It appears to be very difficult to find a good high-volt-age (~5 V) electrolyte with a single-component solvent at thepresent stage. Using mixed fluorinated–carbonate solvents andadditives are two realistic solutions for practical applications inthe near term, while sulfones, nitriles, ionic liquids and solid-state electrolyte/polymer electrolytes are promising candidatesfor the next generation of high-voltage electrolyte systems.

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DMC (5.0 V) > EC (4.8 V) � propylene carbonate (PC) (4.8 V) >EMC (4.6 V) on a Pt working electrode, but the mixed electro-lytes have oxidation potentials of 5.1 V for EC-DMC, 4.9 V forEC-EMC, and 5.0 V for EC-DEC.[21]

Aside from the solvent type, the oxidative stability of anelectrolyte is also greatly affected by the type of lithium saltsin the solvent as well as the working electrodes it is pairedwith.[22–24] Table 1 lists the oxidation potential of somecommon lithium salts in different solvents and working elec-trodes. As shown in Table 1, using the same solvent composi-tion [(EC: diethoxyethane (DEE) (1:1)] and working electrode(Lil-xMn2O4), several common lithium salts can be ranked ac-cording to their oxidation potential in the following way:ClO4

�>N(CF3SO2)2�>CF3SO3

�>AsF6�>PF6

�>BF4� . However,

when DMC replaces the unstable ether DEE, the order changesto: ClO4

�~PF6�~BF4

�>AsF6�>N(CF3SO2)2

�>CF3SO3� . It is be-

lieved that the reactivity of DEE toward the Lewis acids PF6�

and BF4� contributes to the early decomposition of the electro-

lytes.[24] However, when the working electrode was switchedwith activated carbon(AC), the order changes to:AsF6

�>BF4�>

PF6�>N(CF3SO2)2

�>CF3SO3� .[25]

The critical constraining factor in lithium-ion batteries re-garding the working voltage, specific capacity, and energydensity is the cathode (positive electrode). For those well-func-tioning high-voltage cathode materials, in addition to thefactor of their own structural stability, their electrochemicalperformances are influenced by the anodic stability of the elec-trolyte system. At the same time, the anodic stability of car-bonate-based electrolyte systems, especially lithium salts, isclosely correlated with the cathode materials investigated.Therefore, when using a LiPF6-based carbonate electrolyte ina high-voltage battery system, the targeted cathodes shouldbe considered carefully. For example, LiNi0.5Mn1.5O4-based cath-ode materials with an upper cut-off voltage of 5.2 V showsa good cycling performance in 1 m LiPF6 + (EC:DMC) (1:1) elec-trolytes, because both Ni2+ /3 + and Ni3+ /4 + redoxcouples arepinned at the top of O2� :2p band.[21] However, for some Co-containing compounds such as LiCoO2 or LiNi1/3Co1/3Mn1/3O2,LiPF6-based carbonate electrolyte cannot be used. This ismainly attributed to the overlap between the Co3+/Co4 + : t2g

band and the top of O2� : 2p band, which brings a significant

Figure 1. Structural formulae of relevant alkyl carbonates.

Shi Tan earned his bachelor’s degree

at Dalian University of Technology and

then a Master of Science in Physical

Chemistry from Xiamen University in

2013. Now he is employed as design

engineer at Advanced Product Devel-

opment, Ningde Amperex Technology

Limited (ATL). His research interests

focus on materials for energy storage

and conversion applications and elec-

trochemical processes in these sys-

tems. His current research is fully de-

voted to the commercial application of Si alloys for improving the

energy and power density of lithium-ion batteries.

Yong Yang obtained his Ph.D. in Physi-

cal Chemistry at Xiamen University in

1992. Except for a one-year (1997–

1998) academic visit at Oxford Univer-

sity, he has been at the State Key lab

for Physical Chemistry of Solid Surfaces

at Xiamen University since 1992,

where he now works as a distinguished

professor in Chemistry. His main re-

search interests are R&D of new elec-

trode/electrolyte materials, in situ

spectroscopic techniques, interfacial

and reaction mechanism studies in electrochemical energy-storage

and conversion system, especially for Li-ion batteries. He has ob-

tained several national/international research awards since 1996 in-

cluding the 2014 Technology Award by the IBA (International Bat-

tery Association).

Zhongru Zhang graduated from the

Department of Chemistry, Xiamen Uni-

versity, China, in 1999. He then ob-

tained his Ph.D. degree in Physical

Chemistry from Xiamen University in

2004. Currently, he is a senior engineer

at the College of Chemistry and Chem-

ical Engineering, Xiamen University,

China. His research interests focus on

the research and development of cath-

ode materials and electrolytes for lithi-

um-ion batteries.

Yajuan Ji earned a bachelor’s degree at

Northwest A&F University in 2012.

Now, she is a Ph.D. student at the Col-

lege of Chemistry, Xiamen University.

Her research interests focus on high-

voltage electrolytes for lithium-ion bat-

teries. At present, she is mainly engag-

ed in studying the mechanism of elec-

trolytes based on dinitrile solvents and

the synthesis of new high-voltage elec-

trolytes and additives.

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amount of holes into the O2� : 2p band at high voltages andresults in the loss of oxygen from the electrode lattice and theoxidation of the electrolyte.[26] For example, it was reportedthat carbonate-based electrolytes are not stable at 4.5 V andabove (vs. Li+/Li) on LiCoO2 and LiNixMnyCozO2 (where x + y +

z = 1) electrodes.[11–13] Oh et al. also reported on the electro-chemical performance of olivine LiCoPO4/C composite withhigh electronic conductivity in the potential range of 3.0–5.0 V,showing an unsatisfactory cycling performance of LiCoPO4/Cwith a capacity retention of only 70 % after 50 cycles.[27] Wuet al. demonstrated the cycling performance of Li2CoPO4F/Lihalf cells in a 1 m LiPF6 + (EC:DMC)(1:1) electrolyte system, withthe voltage range of 2.0–5.4 V and a current density of143 mA g�1; however, the capacity retention was only 36 %after 50 cycles. Ex situ XRD and electrochemical impedancespectroscopy (EIS) clearly show that the poor cycling per-formance may not be due to structural degradation but toa deteriorated electrode/electrolyte interface, which is causedby the sustained decomposition of electrolyte at high volt-ages.[10] Therefore, the oxidative resistance of electrolytes athigh voltage has become one of the key factors restricting theenergy density of lithium-ion batteries. In the following sec-tions, we will review the recent progress of high-voltage elec-trolytes, which are extending the electrochemical window toa wider potential range with the following strategies : first, theconstruction of a stable cathode/electrolyte interface with ad-ditives or surface coating; and second, the investigation ofstable solvents such as sulfone, nitrile and ionic liquids whichcan tolerate 5 V anodic potential. Due to space limitations,some promising high-voltage electrolytes such as solid inor-ganic/polymer electrolytes are not included in the presentreview.

1.2. Stabilizing the Cathode/Electrolyte Interface withAdditives and Surface Coating

In general, in Li-ion batteries, the electrolyte solvents and saltsundergo decomposition on the surface of the anode, mainlyduring the initial charge process, and form a passive layer con-sisting of inorganic and organic decomposition products fromthe electrolyte.[28] This film is a good electronic insulator thatprevents further electrolyte degradation while still allowing Liions to pass through it during cycling.[29] This essential passiva-tion layer has appropriately been named “solid electrolyte in-terface” (SEI), which was first proposed by Peled et al.[29]

In a comprehensive review published several years ago, Xu dis-cussed in detail some effects of the passivation film on the

performance of the anode material.[15] Usually, if an appropriatesubstance is added to the electrolyte (in an amount below 5 %,either by weight or by volume) and this substance is domi-nantly reduced on the graphite electrode, an SEI film will beformed that prevents solvent reduction and co-intercalation.The trace substance added is named an SEI-forming additive.The use of SEI-forming additives is one of the most effectiveand economical methods for controlling and improving theperformance of lithium-ion batteries. Such additives have thefollowing characteristics: 1) a significant improvement in theperformance of lithium-ion batteries should be achieved byusing a relatively small amount of additive. They should alsoexhibit : 2) an excellent compatibility with the solvent and elec-trode materials; 3) cost-effectiveness, and 4) environmentalfriendliness.

Compared with the research on the anode/electrolyte inter-face, there have been relatively few studies dedicated to theunderstanding of the cathode/electrolyte interface, let alone itbeing accepted as a key part of the investigation of lithium-ionbatteries. It is likely that the solvated Li+ cannot be insertedinto the positive electrode and would not cause exfoliation ofthe electrode as that of graphite electrodes, due to the layeredoxides’ structure being held together by coulombic interac-tions between oppositely charged slabs composed of metalcations and oxide anions.[30, 31] Goodenough and co-workerswere perhaps the earliest authors to suggest that a surfacefilm exists on the cathode/electrolyte interface.[32] There is anincreasing amount of evidence indicating that this protectivelayer, which possesses similar physicochemical properties asSEI on the surface of graphite, does indeed exist on the surfaceof cathode material and also has a significant influence on theelectrochemical performance of the battery materials.[33, 34] It isgenerally accepted that the coulombic efficiency, rate capabili-ty, and cycling performance of the battery are highly depen-dent on the SEI of the anode, while the surface film can alsoimprove the stability of the cathode/electrolyte interface athigh potential. This reduces the side-reactions between cath-ode material and electrolyte, such as electrolyte oxidative de-composition, and therefore results in a high coulombic effi-ciency.[35–37] Furthermore, mixing a film-forming additive withconventional carbonate-based solvent is an economical and ef-fective method to achieve this aim. According to recent re-ports,[38–40] cathode film-forming electrolyte additives are oxi-dized predominantly on the cathode electrode surface, similarto how anode additives function, and then form a stable SEIfilm on the cathode surface that prevents direct contact be-tween electrolyte and electrode. In this way, the oxidative de-

Table 1. Anodic stability of some conventional lithium salts.

Solvent Working Oxidation potential (vs. Li+/Li) [V]electrode ClO4

� PF6� BF4

� AsF6� N(CF3SO2)2

� CF3SO3�

EC/DEE[24] Li1 + xMn2O4 4.55 3.8 3.4 3.9 4.4 4.1EC/DMC[24] Li1 + xMn2O4 >5.1 >5.1 >5.1 4.7 4.35 3.2EC/DMC[25] AC - 4.55 4.78 4.96 4.33 4.29

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composition of the electrolytes and side-reactions betweenelectrode and electrolyte are suppressed.

As an example of the preceding discussion, Cresce et al.[41]

and Tan et al.[42] reported that the electrochemical window oftraditional carbonate-based electrolytes could be significantlyimproved by adding 1 % tri(hexafluoro-iso-propyl)phosphate(HFiP) to either LiNi0.5Mn1.5O4 or Li-rich layered cathode materi-als such as Li1.2Mn0.56Ni0.16Co0.08O2. This is attributed to thestable SEI film formed by the decomposition of HFiP additiveat high voltages. Electrodes cycled in electrolyte with HFiP ad-ditive have relatively more stable SEI impedance and chargetransfer resistance than those cycled in additive-free electro-lytes, which enhances the electrochemical performance of highvoltage cathode materials, either in cyclic stability or rate capa-bility. Figure 2 shows the cycling performance of Li/LiNi0.5Mn1.5O4 half cells with a new electrolyte additive, tris(pen-tafluorophenyl) phosphine (TPFPP). The capacity retention ofLiNi0.5Mn1.5O4 after 55 cycles is 85.0 % for 0.5 % TPFPP addedelectrolytes, which is higher than the 70.3 % of the referenceelectrolyte. However, it is worth noting that the TPFPP additivecontent should be strictly controlled, because a high percent-age of additive would result in the formation of a thick SEIlayer and deteriorated material cycling performance.[43] Abeet al. measured the thickness of the cathode surface film withand without the additive biphenyl (BP) by Auger electron spec-troscopy (AES). As shown in Figure 3, the film thickness raisedalong with the amount of BP content, with a 0.1 wt % additionof BP leading to the formation of a ~20 nm surface film.[39] Thisclass of additives includes 3,4-ethylenedioxythiophene (EDT),o-terphenyl (OTP), furan,[39] tris(trimethylsilyl) borate (TMSB),[44]

methylenemethanedisulfonate(MMDS),[45] trimethylphosphite(TMP),[35] 3-hexylthiophene[46] and so on. In addition to organicadditives, recent reports demonstrate that some inorganicsmall molecules, such as LiBF2(C2O4) and tetramethoxytitanium(TMTi), can also be used as elec-trolyte additives for the in situgeneration of a passive film toenhance the performance of thecathode material.[47] As with theorganic additives, these inorgan-ic additives are designed to bepreferentially oxidized to forma stable SEI layer on the cathodesurface. Then, the stable cathodeSEI serves as a protective layerpreventing the further oxidationof the electrolyte and allowingthe cathode to achieve a betterelectrochemical performance athigh voltage.

In addition to using electrolyteadditives, previous studies haveshown that the surface modifica-tion of cathode materials is alsoa powerful method for the stabi-lization of the electrode/electro-lyte interface, which could great-

ly improve a battery’s electrochemical performance.[27, 48–54] Forexample, Zhang et al. showed that TiO2-coated LiNi0.8Co0.2O2

electrodes exhibited a better cycling performance than thepristine material. The effects could be attributed to the pres-ence of TiO2 on the surface of LiNi0.8Co0.2O2 particles, which isable to suppress electrolyte decomposition, and results in theformation of different surface oxide films through an electro-oxidation mechanism involving solvents and the electrodes.[55]

Wu et al. reported that Li2CoPO4F cathode material witha Li3PO4 coating can also isolate the active material with highcatalytic activity from the electrolyte, and then stabilize theelectrode/electrolyte interface. Lithium bis(oxalate)borate(LiBOB) shows a synergistic effect with the Li3PO4 coating layerthat further stabilizes the interface. In this way, long-term cy-cling stability with 83.8 % capacity retention after 150 cycleswas achieved for Li2CoPO4F cathode material.[56] Besides opti-

Figure 2. Cycling performance of Li/LiNi0.5Mn1.5O4 half cells using a blankelectrolyte (STD) and 0.5 % and 1 % TPFPP-containing electrolytes at C/5 and1 C cycling rates in the potential range of 4.9–3.5 V.[43]

Figure 3. AES analysis of the LiCoO2 cathode SEI layer thickness: Results are given for LiCoO2 electrodes used incylindrical cells after 200 cycles at 45 8C in 1 m LiPF6 + (EC:MEC)( 3:7) with 0 %, 0.1 wt %, and 2 wt % BP additive.[39]

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mizing the electrode/electrolyte interface and suppressing sidereactions between electrode and electrolyte, surface modifica-tion can form a “buffer” layer on the surface of unstable cath-ode materials in order to prevent oxygen atoms being extract-ed out with high activity from the structural lattice to the elec-trode’s surface, enabling an improved electrochemical per-formance to be achieved.[57] For example, the discharge capaci-ty of AlF3-coated Li[Li0.12Mn0.54Ni0.13Co0.13]O2 increased from 249to 267 mA h g�1, while the irreversible capacity loss (ICL) de-creased from 76 to 47 mA h g�1. At the same time, the cyclingperformance of AlF3-coated material greatly improved underboth room temperature and elevated (50 8C) temperatures.[51]

Regardless of the methods used, such as organic additives,inorganic additives or surface modification, the stabilization ofthe electrode/electrolyte interface has been found to controlthe decomposition of the electrolyte and reduce the side reac-tions between electrode and electrolyte; thus, it should be thekey factor to improving the electrochemical performance oflithium-ion batteries. This SEI layer on the cathode must meetsome requirements. For example, the thickness and composi-tion of the SEI layer (which determines its ionic/electronic con-ductivity) should be carefully controlled, as it has a greatimpact on the intercalation/de-intercalation rate of Li+ into/from the electrodes. The density and homogeneity of SEI filmdetermines whether or not it will reduce aggressive side reac-tions and stabilize the electrode/electrolyte interface. There-fore, further investigation into interfacial reactions is worth-while and can be accomplished by using different in situ andex situ analysis techniques. For example, X-ray photoelectronspectroscopy (XPS),[58–60] time-of-flight mass spectrometry (TOF-MS),[61–63] Raman,[64, 65] AES,[39, 66] Fourier-transform infrared(FTIR),[66, 67] and so on, are some common and powerful surfaceanalysis techniques and can provide valuable surface informa-tion. In addition, scanning microscopy techniques such asscanning electron microscopy (SEM),[62, 68] transmission electronmicroscopy (TEM),[42, 69] and atomic force microscopy (AFM)[70]

are used for imaging the morphology of the SEI layer, whichprovides some direct data for the analysis of surface films. Elec-trochemical techniques such as cyclic voltammetry (CV)[71] andelectrochemical impedance spectroscopy (EIS)[72] are alsouseful tools for the study of the SEI layer at different potentials,as they can provide indispensable in situ information aboutthe formation, growth, and transformation process of the SEIlayers. Most of these techniques have also already been sum-marized in detail in the previous review paper.[15]

1.3. New Types of Solvents for High-Voltage Electrolytes

Despite the dominance of traditional carbonate-based electro-lytes and the development of methods for the stabilization ofthe electrode/electrolyte interface, great efforts are still re-quired to develop and study novel high-voltage electrolytes.Although researching into new high-voltage electrolytes is ur-gently necessary for the use of 5 V-level lithium-ion batteries,higher standards and more strict requirements are involvedwhen compared with conventional electrolytes. Besides thebasic requirements for solvents, such as having a wide liquid-

phase temperature, high ionic conductivity, high thermal sta-bility, and chemical inertness and so on, novel-type high-volt-age solvents have to show extraordinary anodic stability athigh voltage and form a stable SEI layer on the anode’s sur-face. Recently, sulfone based solvents,[11, 73, 74] ionic liquid sol-vents,[75] nitrile based solvents[76] and derivatives of carbonatebased solvents[77] have opened the door for next generation5 V-level batteries due to their outstanding high oxidation po-tentials and excellent physicochemical characteristics.

1.4. Sulfone-Based Solvent Systems

As a byproduct of the petroleum industry, sulfone-based elec-trolytes possess some advantages: not only are they of lowcost, but they also have a wide electrochemical window withmore than 5 V (vs. Li+/Li) ;[11, 73, 80–82] they have thus becomepromising high-voltage electrolytes. Usually, sulfone-based sol-vents are cyclic or acyclic from a structural point of view, butcan also be classified into symmetric and unsymmetric with re-spect to their substituent groups. Recently, sulfolane and/or or-ganic sulfites (such as dimethyl sulfite, diethyl sulfite, ethyl sul-fite, propylene sulfite, and vinyl ethylene sulfite) were alsoused as solvents or additives in electrolytes for lithium-ion bat-teries.[40, 83–86] Table 2 summarizes the data on selected sulfone-based electrolytes from previous work.[11, 78–80, 87, 88] Unfortunate-ly, most sulfone-based solvents have a high melting tempera-ture, especially those with molecular symmetry. For example,TMS has a rather high melting point, at 27 8C, but the meltingpoint of DMS is drastically higher, at 110 8C. Consequently,most of the sulfone-based solvents fail to be evaluated asa single solvent. However, it was found that the breaking ofmolecular symmetry could lead to a much lowered meltingpoint. For example, with the introduction of a methyl groupinto DMS, the corresponding generated unsymmetric sulfone,EMS, has a substantially lower melting point of 36.5 8C.[11] Addi-tionally, Sun et al. synthesized oligoether-containing sulfones,such as ethyl methoxyethyl sulfone (EMES), which shows a lowmelting point of 2 8C.[87] However, molecular symmetry has nodistinctive influence on the boiling point, which is more closely

Table 2. Summary of the physical properties and electrochemicalwindow of sulfone-based electrolytes.[73, 78–80]

Solvent[a] Tm

[8C]Tb

[8C]s25

[mS cm�1]Window(vs. Li+/Li)

DMS 110 238 / /EMS 36.5 240 6.3 (1 m LiTFSI) 5.9MEMS 15 275 2.8(1 m LiTFSI) 5.6EMES 2 286 2.8 (1 m LiTFSI) 5.6EMEES <0 >290 3.1 (1 m LiTFSI) 5.3ESCP 38 328 3.2 (1 m LiTFSI) ~5.5TMS(SL) 27 285 2.5 (1 m LiPF6) 5.8FPMS 56 180 0.035 (1 m LiTFSI) 5.8/LiPF6 + DMC

[a] DMS = dimethyl sulfone; EMS = ethyl methyl sulfone; MEMS = methox-yethyl methyl sulfone; EMES = ethyl methoxyethyl sulfone; EMEES = ethylmethoxyethoxyethyl sulfone; ESCP = ethyl sulfonyl cyclopentane;TMS(SL) = tetramethylene sulfone or sulfolane; FPMS = 3,3,3-trifluoroprop-yl methyl sulfone.

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connected to the dielectric constant (dipolar intermolecular at-traction) and the molecular weight (van de Waals intermolecu-lar attraction) of the solvent.[80]

Regarding new high-voltage electrolytes, the compatibilityof sulfone or mixed sulfone-carbonate electrolyte systems withcathode and anode materials (excluding graphite) is compara-ble with that of carbonate-based electrolytes. With 0.7 m

LiBOB + (TMS:DMC)(1:1) in the LiNi0.5Mn1.5O4/Li cell,[89] or 1 m

LiPF6 + EMES in LiCr0.015Mn1.985O4/Li,[90] the electrolytes exhibitedseveral advantages, such as a stable cycle performance, lowcell resistance, good rate performance, and a high dischargevoltage plateau. In the case of 1 m LiTFSI + (TMS:EMS)(1:1),LiMn2O4/Li4Ti5O12 full cells showed a reversible capacity ofmore than 80 mA h g�1 at the current density of 33 mA g�1 inthe potential range of 1.5–3 V (with Li4Ti5O12 as counter elec-trode). At the same time, the cells achieved good capacity re-tention of up to 99 % after 100 cycles. In addition, when TMS ismixed with EMC, the LiNi0.5Mn1.5O4/Li4Ti5O12 full cells deliveredan initial capacity of 80 mA h g�1 at the current rate of 2 C (i.e.240 mA g�1), and very little capacity decay after 1000 cycles.[89]

It is obvious that the sulfone solvent successfully providesanodic stability for particular high-voltage lithium-ion batterysystems.

However, special attention should be paid to the compatibil-ity of sulfone with graphite/carbon anode materials, whichhave been widely used in commercial lithium-ion batteries.[80]

As shown in Figure 4, the LiCoO2/graphite full cell in a 1.0 m

LiPF6 + EMES electrolyte system exhibited similar electrochemi-cal performances to those in 1.0 m LiPF6 + (EC:DMC)(1:1), onlywhen an appropriate amount of vinylene carbonate (VC) isadded to the former cell’s electrolyte.[87] This implies that sul-fone-based electrolytes may fail to form an effective SEI filmon the surface of graphite anodes, allowing graphene exfolia-tion due to solvent co-insertion or further solvent reduction.Therefore, the ability to form an effective SEI is critically impor-tant for sulfone-based electrolytes, in order to solve the com-patibility problem with graphite anodes. In addition, since VCand lithium oxalyldifluoroborate (LiDFOB) are able to form

a stable SEI layer on the surface of graphite or mesocarbon mi-crobeads (MCMB),[90, 91] they have been chosen as an additiveor alternative lithium salt to improve the electrochemical per-formance of the graphite anodes in sulfone-based electrolytesystems. Changing the alkyl substituent is another way to im-prove the compatibility of sulfone with anode materials. Forexample, the fluorination of alkyl groups in the sulfone seemsto aid in forming a stable solid electrolyte interface on theanode. The irreversible capacity shown in the first cycle is com-parable to that of carbonate-based electrolytes, and coulombicefficiency close to unity after the subsequent 20 cycles.[80]

However, many sulfone-based high-voltage electrolytessuffer from the serious problem of wettability with commer-cially available separators[89] made of materials such as poly-propylene or polyethylene, which have a relatively higher vis-cosity compared with a system using traditional carbonate-based electrolytes. Some compromise solutions to this prob-lem are to mix sulfones with other carbonate solvents, or touse surface-modified separators.

1.5. Ionic-Liquid (IL)-Based Solvent Systems

Salts with a low melting point tend to be liquid at room tem-perature or lower and form a new class of liquids usuallycalled room-temperature ionic liquids (RTILs), or organic-typeroom-temperature molten salts. They are composed of a largecation and a flexible anion.[93] During the last decade, there hasbeen a growing interest in ionic liquids (ILs) as additives or co-solvents for lithium-ion batteries because of their outstandingphysicochemical properties including good thermal stability,non-flammability, exceptionally low vapor pressure, wideliquid-phase temperature, and high oxidation resistance athigh voltages (above 5.3 V vs. Li+/Li).[94–100] In most ILs, the con-stituent cations include organic onium ions such as quaternaryammonium, imidazolium, pyrrolidinium, piperidinium salts,hexyltrimethyl phosphonium, and triethylsulfonium salts,which contain nitrogen, phosphorus, or sulphur. Commonanions include N,N-bis(trifluoromethane)-sulphonamide (TFSI�),bis(fluorosulphonyl)imide (FSI�), tetrafluoroborate (BF4

�) andhexafluorophosphate (PF6

�). Figure 5 shows the structure ofcation and anion components in some general IL systems.[92]

Among the many tested IL systems, those based on quaternaryammonium salts (with pyridine and pyrrole groups) have ex-hibited good electrochemical performance and could suppressthe formation of dendritic Li, which can enhance the safety ofthe battery. In addition, ILs show a broad electrochemical sta-bility window, generally >5 V (vs. Li+/Li), which is the primarybasis of their demand for application in 5 V lithium-ion batter-ies. For example, N-butyl-N-methyl pyrrolidiniumbis(trifluoro-methanesulfonyl)-imide (Py14-TFSI) ILs demonstrates a wideelectrochemical stability window exceeding 5.5 V (vs. Li+/Li).[101]

This interesting property indicates that it is possible to usePy14-TFSI ILs as practical high-voltage electrolytes.[102, 103] Forexample, Zheng et al. were the first to report that they canachieve a good electrochemical performance forLi1.2Mn0.54Ni0.13Co0.13O2 by using a mixture of Py14-TFSI and car-bonate solvents.[104] Later, Lu et al. reported similar results with

Figure 4. Comparison of the cycling performance of button cells (graphi-te j jLiCoO2) using different electrolyte solutions. Ic = Id = 0.115 mA cm�2.[87]

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N-methyl-N-propylpiperidiniumbis(trifluoromethanesulfonyl)-imide(Pp13TFSI)-containing electrolytes.[105]

Although the attractive properties of ILs make them verypromising candidate electrolytes for lithium-ion batteries, thereare still some unsolved issues involving their practical applica-tion. For example, most of the ILs are still quite expensive andalso show unexpected irreversible reactions with carbonanodes.[95] In addition, they usually possess a relatively high vis-cosity, which is associated with a low intrinsic conductivity anda poor rate capability.[98, 107–109] These issues restrict the applica-tion of IL-based electrolytes to the field of high-power-densitybatteries. Therefore, many strategies have been developed toreduce the viscosity and solve the compatibility problem be-tween IL electrolytes and carbon anodes. One of the mostcommon methods to improve the electrochemical per-formance of IL-based electrolytes is using mixed (composite)solvents, electrolyte solutions containing ionic liquid and someother organic solvents.[104, 110–113] Xiang et al. reported that byadding a certain amount (e.g. 20 % or 40 %) of low-viscosity di-ethyl carbonate (DEC) as a co-solvent into 0.4 mol kg�1 LiTFSI +PP13-TFSI electrolyte, the rate capability performance ofa LiCoO2/Li cell was substantially improved (as shown inFigure 6).[106] The improved electrochemical performance (espe-cially the rate capability) of these mixed solvents may bemainly attributed to the reduced viscosity of IL-based electro-lytes, which is caused by introducing the organic carbonatesolvents, compounds beneficial to the conductivity and wetta-bility of the electrolyte separators. In addition, Xu et al.showed that the reversible discharge capacities of the LiFePO4/Li half cell, which contains a 0.5 m LiTFSI + (PP14-TFSI :TMS)(3:2)mixed electrolyte, can reach up to 160 and 150 mA h g�1 at cur-rent rates of 0.05 C and 1 C, respectively. Also, the mixed elec-trolytes show a better thermal stability than conventional car-bonate-based electrolytes when the percentage of organic car-bonate solvents in the mixed electrolytes is carefully con-

trolled.[88] It should be notedthat the mixed-electrolytesystem exhibits a wider electro-chemical window than tradition-al carbonate-based electrolytesbecause of the extraordinaryanodic stability of ILs.[103, 106, 114, 115]

This can be seen in the mixedelectrolytes 1 m LiNTf2 + (N1116-NTf2:EC: DEC)(10:6:9),[116] 1 m

LiBF4 + (IMI-BF4 :BL)(2:3),[117] andso on.

In addition to their use as co-solvents, Zhang et al. recentlycarried out studies on ionic liq-uids as additives, with their re-sults confirming that 3 wt %1-allyl-3-vinylimidazolium bis(tri-fluoromethanesulphonyl)imide([AVIm][TFSI]) in 1.2 m LiPF6 +

(EC:EMC)(3:7) can improve thecyclic stability and rate proper-

ties of the LiNi0.5Mn1.5O4/Li high-voltage cell. This is due toa compact and stable polymer film that is formed by electro-initiated polymerization of imidazolium cations with vinyl andalkyl groups.[118]

Although ionic liquids or mixed ionic-liquid systems are veryattractive, most IL-based electrolytes have a compatibilityproblem with carbon electrodes. Therefore, some strategieshave been introduced to solve this problem; for example, itwas found that by adding an appropriate amount of film-form-ing additive, such as VC, vinyl ethylene carbonate (VEC), or EC,into IL-based electrolytes or by introducing an organic func-tional group such as -CN, -OR, or -CO2R onto the side alkylchain of the cations, the compatibility between ILs and graph-ite electrodes can be geratly improved.[110, 115, 119–121] Weng et al.investigated the disiloxane-functionalized phosphonium-based

Figure 5. Molecular structure of cations and anions in some IL systems.[92]

Figure 6. Rate capability of LiCoO2/Li half cells in RTIL-based electrolytes:0.4 mol kg�1 LiTFSI + RTIL, 0.4 mol kg�1 LiTFSI + (RTIL:DEC)(4:1), and0.4 mol kg�1 LiTFSI + (RTIL:DEC)(3:2). All the cells were charged to 4.2 V at1/10 C and then discharged to 2.8 V at different rates.[106]

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ionic liquid P222Si-TSFI, which demonstrated superior compati-bility with a carbonaceous anode, as shown in Figure 7. TheMCMC/Li half-cell in a LiTFSI + P222Si-TFSI based electrolytedisplayed an excellent capacity retention in the first 30 cycleswhile the coulombic efficiency increased from 60 % to 95 % inthe first three cycles.[115] Also, the addition of 5 wt % EC +

5 wt % VEC to 1 m LiTFSI + EMI-TFSI can improve the interfacialcompatibility between graphite electrode and electrolyte, dueto the formation of a stable SEI film on the surface of theanode.[119]

IL-based electrolytes have been intensively investigated be-cause of their natural properties, and tremendous progress hasbeen achieved. However, despite this fact, the remaining draw-backs and shortages still greatly limit their applications to lithi-um-ion batteries, including a high viscosity and melting point,which cause poor rate capability and low-temperature per-formance of the batteries.

1.6. Nitriles as Co-Solvents orAdditives

According to the number ofcyano groups, the nitrile solventfamily can be classified intomononitrile-[122–124] and dinitrile-[125–129] based solvents. Up tonow, in most cases, the contentof nitriles in electrolytes hasbeen about 10 % v/v or less,making it better to think that ni-triles are co-solvents or additivesin the electrolytes. For example,Wang et al. evaluated the physi-

cal and electrochemical characteristics of lithium electrolytesolutions based on mononitrile solvents such as 3-methoxypro-pionitrile (MPN), 3-ethoxypropionitrile (EPN) and 3-(2,2,2-tri-fluoro)ethoxypropionitrile (FEPN), and their performance innanocrystalline Li4Ti5O12-based high power lithium ion batter-ies. All these mononitrile-based electrolytes showed a superiorrate compared to that of carbonate-based electrolytes.[122, 123]

However, for electrolyte systems intended for use in high-volt-age lithium-ion batteries, more attention should be paid to thedinitrile solvent. It has been confirmed that dinitrile-containingelectrolyte systems show a wide liquid temperature range,a high thermal stability and a high anodic potential. Further-more, an effective protective layer, which helpfully suppressescorrosion of the Al current collector which is caused by theLiTFSI salt at a high potential, can be formed on the Al surfaceby the nitrile group in LiTFSI + nitrile electrolytes.[124, 126, 130–133]

Table 3 compares the physical properties and electrochemical

Figure 7. a) Cycling performance and b) first three discharge/charge curves for a Li/MCMB half-cell containinga LiTFSI P222Si-TFSI electrolyte, cycling at C/10 rate and 55 8C.[115]

Table 3. List of physical properties and electrochemical windows of dinitrile solvents and common carbon solvents used in lithium electrolytes.[19]

Solvent Structure e[a] h(cp)[b] Tm [8C][c] Tb [8C][d] Tf [8C][e] Window (vs. Li+/Li)

EC 89 2 35 244 150 5.5

DMC 3 0.7 3 90 18 5.3

DEC 3 0.8 �43 127 25 5.15

dinitriles CN(CH2)nCNn

malononitrile (MAN) 1 48 solid 31 220 86 –succinonitrile (SCN) 2 55 2.7 54 266 113 –glutaronitrile (GLN) 3 37 5.3 �29 287 113 7.3adiponitrile (ADN) 4 30 6.1 1 295 163 6.9pimelonitrile (PMN) 5 28 7.6 �31 175 112 7.0suberonitrile (SUN) 6 25 8.2 �4 325 110 6.8azelanitrile (AZN) 7 23 8.7 �18 209 >110 –sebaconitrile (SEN) 8 22 10.7 8 200 >113 7.2

[a] e is the dielectric constant. [b] h is the viscosity. [c] Tm is the melting point. [d] Tb is the boiling point. [e] Tf is the flash temperature; Tauto is the auto-igni-tion temperature.

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windows of dinitrile solvents with n = 1 to 8 with those ofcommon carbonate solvents such as EC, DMC and DEC.[19]

As shown in Table 3, the dinitriles have intermediate dielectricconstant values, which are sufficient to sustain high ionic con-ductivities. In addition, dinitrile solvents have very good ther-mal properties such as a high flash point that can mitigate theflammability of lithium battery electrolyte solutions. The mostnotable property of this class of compounds is that the electro-chemical window is about 7 V for a single nitrile solvent, andcan be maintained at 6-6.5 V for binary- and ternary-solventelectrolytes. Therefore, the excellent electrochemical stabilityof dinitrile-containing solvents is much higher than that ofcommercial carbonate family solvents and even better thanthat of recently reported sulfone-based solvents and ionicliquid electrolytes. It is a very promising candidate for applica-tion in 5 V high voltage lithium ion battery systems.

The lithium salt of the TFSI� ion was found to be moreeasily dissolved in adiponitrile (ADN) than that of the BF4

� ion,while PF6

� was practically insoluble. With the growth of thecarbon chain, the same trend was found but with slight varia-tions such as reduced solubility.[19] Mixing common carbonatesolvents was accepted to improve the solubility. Based on thehigh resistance to electrochemical oxidation of ADN, ourgroup recently examined the possibility of using dinitrile asa co-solvent in a high-voltage electrolyte containing 1 m

LiPF6 + (EC:DMC:ADN)(9:9:2) together with the 5 V-class cath-ode material Li2CoPO4F. As shown in Figure 8,[134] it was foundthat the anodic stability of carbonate-based electrolytes is sig-nificantly improved by the addition of ADN. Figure 9 showedthat an improved cycling performance of Li2CoPO4F is achievedwith ADN co-solvent at 1 C and 2 C cycling rates. Besides con-tributing to the outstanding anodic stability, ADN also hasa high boiling point of 295 8C and a flash point of 163 8C,which is beneficial to the thermal properties and safety factorof lithium-ion batteries.[126, 135, 136] Another key point of ADN isits predictable low cost as it is a precursor in the industrial

manufacture of nylon.[137] This dinitrile solvent is produced in-dustrially with more than a billion kilograms annually.[138]

Despite the associated cathodic stability and suitable physi-cal and thermal properties of some mononitriles and dinitriles,their use in lithium-ion batteries has been long perceived as in-feasible due to their thermodynamic instability at low poten-tials, that is, the reduction of nitrile solvents cannot forma stable SEI on carbonaceous anodes, especially on graph-ite.[127, 129] For example, a LiCoO2/MCMB cell using1 M LITFSI +ADN alone only delivered an initial discharge capacity of43 mA h g�1, and deteriorated gradually with failure in 40cycles.[126] Recently, it was found that this shortcoming of ni-triles can be partially overcome in the case of carbonaceousanodes by the addition of additives like fluoroethylene carbon-ate (FEC) and VC,[129] EC, or even salts like LiBOB, which areknown as SEI promoters.[126]

1.7. Derivatives of Carbonate-Based High-VoltageElectrolytes

In general, binary solvent mixtures including cyclic and linearcarbonates have several distinctive advantages, such as lowviscosity, high conductivity, excellent solubility with lithium

Figure 8. Linear sweep voltammetry (LSV) curves of three-electrode cells inan electrolyte containing 10 % ADN in the potential range of OCP–6.5 V. Pt isthe working electrode, the scan rate was 1 mV s�1, and Li metal was used asboth the counter and reference electrodes.[134]

Figure 9. Cycling performance of Li/Li2CoPO4F half-cells in a 1 m LiPF6

+ (EC:DMC:ADN)(9:9:2) electrolyte in the potential range of 3–5.4 V and ata current rate of : a) 1 C and b) 2 C.[134]

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salts, and an ability to form an effective SEI layer. They are stillconsidered to be the most competitive candidates for use inhigh-voltage lithium-ion-battery electrolyte systems.[15, 33]

Because of this, it is important to consider modifying the mo-lecular structures of both cyclic and acyclic carbonate-basedsolvents with the introduction of a highly electronegative mo-lecular group like F� , -CN, -SO2-, and so on. According torecent reports, these modified carbonate compounds show animproved oxidation stability.[139–144] This strategy of structuralmodification is regarded as a simple and feasible way to widenthe electrochemical window of conventional carbonate-basedelectrolytes. Here, fluorinated carbonates have significant dif-ferences in their physical and chemical properties compared toconventional electrolytes. In particular, the viscosity, meltingand boiling points of fluorinated solvents are significantlylower than their hydrogenated counterparts. Also, fluorinatedsolvents are much less flammable in general. More importantly,the oxidation stability of partly fluorinated solvents is quitegood, due to the stability of the carbon–fluorine bond.[145] Forexample, an FEC-based high-voltage electrolyte forLiNi0.5Mn1.5O4/graphite was evaluated in comparison to conven-tional 1 m LiPF6 + (EC:EMC)(3:7) electrolytes. An improved cy-cling performance was demonstrated in the FEC-based electro-lytes either at room temperature or at 55 8C.[146] A similar obser-vation of FEC-based electrolytes was also given by the cyclingresults of LiNi0.5Mn1.5O4/Si cells, which obtained a 74.2 % capaci-ty retention after 500 cycles in 1 m LiPF6 + (FEC:DMC)(1:4)mixed electrolyte.[147] Zhang et al. synthesized a series of par-tially and fully fluorinated cyclic and linear carbonates to inves-tigate the effect of the fluorination of carbonate solvents onthe oxidation stability.[148] It was found that the oxidation po-tential of the electrolytes distinctively increased by the intro-duction of fluorine substitutes into carbonate molecules, a con-clusion supported by DFT calculations as well as electrochemi-cal evaluations using LiNi0.5Mn1.5O4/Li half cells andLiNi0.5Mn1.5O4/Li4Ti5O12 full cells. In addition, Abraham et al. syn-thesized a series of polyfluoroalkyl (PFA) compounds with sub-stituted ethylene carbonates (PFA-EC) and studied them aselectrolyte additives for lithium-ion cells. Among the variousPFA-EC compounds studied, they showed that perfluorooctyl-substituted ethylene carbonate (PFO-EC) significantly improvedthe long-term cycling performance of full cells, due to PFO-ECbeing oxidized on the cathode and forming surface filmswhich suppressed the increase in cell impedance. The authorsalso proposed two synergistic mechanisms.[149]

Table 4 shows the chemical structure, HOMO (highest occu-pied molecular orbital, which corresponds to the oxidative de-composition potential), and LUMO (lowest unoccupied molec-ular orbital, correlated to the reductive decomposition poten-tial) energies of a conventional carbonate solvent (DMC) andderivatives of dimethyl carbonate (F-DMC, CN-DMC, CN-F-DMC). The B3LYP/6-311 + G(d, p) basis set was chosen for thecalculation of HOMO and LUMO energies.[151, 152] Based on theDFT calculation results in Table 4, it can be seen that both theHOMO and LUMO energies of carbonate derivatives are de-creased by the addition of F� and -CN functional groups,which indicate a stronger oxidation resistance at high poten-

tials but a poorer reduction resistance at low potentials. Thisincreased reduction potential can be attributed to the en-hanced electron-accepting ability of the central atom upon theintroduction of an electronegative molecular group.

In summary, it appears that fluorinated carbonate solventsand carbonates with additives are a more practical solutionwhen the working potential is about 4.5 V, but if a batterywork at potentials higher than 5 V is desired, it is likely thata choice of solvent stable at high voltages, such as nitriles, sul-fones, and ionic liquids, is required. However, since the easy re-duction of these solvents is always a threat within their broadelectrochemical windows, how to solve this problem is stilla challenging work to face. Some new strategies such as theutilization of new additives that are able to suppress the re-duction of these solvents on graphite-type anodes should beconsidered in future studies.

2. Summary and Outlook

As the most remarkable technological innovation of modernelectrochemistry in the past 20 years, lithium-ion batteries nowpower most of the portable electronic devices in our daily life.However, insufficient energy and power densities are criticaltechnical bottlenecks for the wide usage of lithium-ion batter-ies in the transportation and industrial fields, in applicationssuch as energy storage stations for “smart” grids. These limita-tions are caused by a shortage of high-performance electrodematerials and electrolytes. Electrolytes, a vital component oflithium-ion batteries, have been proven to have a large impacton their energy density, power density, and working tempera-ture range, as well as their cycling stability, safety, and cost.Recently, a new generation of 5 V lithium-ion batteries has cre-ated a demand for a corresponding set of novel electrolyteswith a wider electrochemical window. In response to thisdemand, several feasible solutions have been proposed and in-vestigated, including the development of high-voltage sol-vents, such as sulfones and nitriles, the creation of carbonatederivatives, such as fluorinated carbonates, and the utilizationof high-voltage electrolyte additives. However, as far as we

Table 4. The chemical structure, HOMO and LUMO energies of DMC-based solvents. From Ref. [151].

Organic solvent Structure E(HOMO)[eV]

E(LUMO)[eV]

DMC �8.179 �0.076

F-DMC �8.660 �0.103

CN-DMC �9.026 �0.859

CN-F-DMC �9.458 �1.252

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know, no excellent high-voltage electrolyte recipe has been re-ported yet, although certain composite electrolyte recipes maywork acceptably well for some charge–discharge cycles ina practical battery system. Some main issues include the short-age of very stable solvents that can work in a wide electro-chemical window (0~5 V vs. Li) along with some water residue,compatibility problems with high-voltage solvents, and cath-ode and/or carbon anodes, and the high cost of production ofhigh-purity electrolytes. Sulfones, nitriles, ionic liquids, as wellas carbonate derivatives such as fluorinated carbonates, arepromising and may serve as alternatives to standard carbonatesolvents as electrolytes in a commercialized 5 V lithium-ion-battery electrolyte. Electrolyte additives are likely to consistent-ly be a sometimes overlooked but vital part of the develop-ment of high-voltage electrolyte systems, even if a more stablesolvent is found. In addition, another feasible way forward is tosearch for some stable solid-state electrolytes, either inorganicor polymer in structure. A configuration of solid electrolytescomposed of anode- and cathode-compatible electrolytelayers should be considered in future studies. Finally, the com-bination of quantum chemical methods and experimental veri-fication, especially high-throughput screening with combinato-rial methods, should be a very important research strategy inthe exploration of new electrolytes.[153] In fact, many studies ofelectrolytes have shown that theoretical calculations of the oxi-dative and reductive ability of the molecules examined so farare very helpful in evaluating and searching for new solvents,additives, and even some novel additions of functional groupsinto primary molecules. This greatly saves research time andexpense by checking new molecules at the initial stage. There-after high-throughput screening researches can efficiently findappropriate recipes. Although the search for new, effectivehigh-voltage electrolytes is not easy, we expect that excitingprogress could be made in the next few years.

Acknowledgements

Financial support from the National Natural Science Foundationof China (Grants No. 21233004 and 21021002) and the NationalBasic Research Program of China (973 program, Grant No.2011CB935903) are gratefully acknowledged. The authors alsothank Matthew McDonald for the English revision of themanuscript.

Keywords: carbonate-based solvents · electrochemistry ·electrolytes · Li-ion batteries · power sources

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