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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 7: 1077 - 1096

DOI: https://doi.org/10.7536/PC221132

Review

High Voltage Electrolytes for Lithium Batteries

  • Qimeng Ren 1 ,
  • Qinglei Wang , 2, * ,
  • Yinwen Li 2 ,
  • Xuesheng Song 2 ,
  • Xuehui Shangguan , 2, * ,
  • Faqiang Li , 2, *
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  • 1 School of Chemistry & Chemical Engineering, Linyi University,Linyi 276005, China
  • 2 School of Materials Science and Engineering, Linyi University,Linyi 276003, China
* Corresponding author e-mail: (Xuehui Shangguan);
(Qinglei Wang);
(Faqiang Li)

Received date: 2022-12-01

  Revised date: 2023-03-19

  Online published: 2023-04-30

Supported by

National Natural Science Foundation of China(22209065)

National Natural Science Foundation of China(22172070)

Natural Science Foundation of Shandong Province(ZR2021QE039)

Natural Science Foundation of Shandong Province(ZR2021QE149)

Natural Science Foundation of Shandong Province(ZR2020MB082)

Key R&D Plan of Linyi City(2021019zkt)

2022 Shandong Province Higher Education Youth Innovation Team Development Plan

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Abstract

With the proposal of "peak carbon dioxide emissions" and "carbon neutral" strategic objectives, developing clean energy and promoting the development of new energy industry has become the consensus of the whole society. Lithium battery as the candidate for new generation of energy storage equipment due to its remarkable advantages such as high energy density, high power density, long cycle life and environmental friendliness. Its development plays a significant role in alleviating energy crisis, driving the conversion of old kinetic energy into new and achieving the strategic goal of "carbon peaking and carbon neutrality". In order to further improve the energy density of lithium batteries, the most effective strategy is to use high voltage or high specific capacity cathode materials. However, due to the low oxidation stability and narrow electrochemical window of traditional carbonate ester electrolytes, they are prone to oxidative decomposition when the working voltage exceeds 4.2 V, which cannot be cycled stably at high voltages, so it is particularly important to broaden the electrochemical window of electrolytes. This paper mainly discusses the mechanism of organic solvents and additives in high-voltage electrolytes, explores effective methods to broaden the electrochemical window of new electrolytes, summarizes the characteristics of aqueous electrolytes, solid electrolytes, and polymer gel electrolytes, and finally; summarizes and outlooks the future development and prospects of high-voltage electrolytes to provide scientific basis for the design and development of high-voltage electrolytes for lithium batteries.

Contents

1 Introduction

2 Working mechanism of high voltage electrolyte

3 Research progress on the high-voltage electrolyte for lithium batteries

3.1 New electrolyte organic solvents

3.2 High voltage electrolyte additive

3.3 Aqueous electrolyte

3.4 Solid state electrolyte

3.5 Gel polymer electrolyte

4 Conclusion and outlook

Key words： lithium battery; high voltage; electrochemistry; electrolytes; solvents; additive

Cite this article

Qimeng Ren , Qinglei Wang , Yinwen Li , Xuesheng Song , Xuehui Shangguan , Faqiang Li . High Voltage Electrolytes for Lithium Batteries[J]. Progress in Chemistry, 2023 , 35(7) : 1077 -1096 . DOI: 10.7536/PC221132

1 Introduction

Since the 1990s, with the increasing demand for high energy density batteries for portable electronic devices, new energy vehicles and large-scale grid energy storage devices, lithium batteries have become a new generation of energy storage devices because of their significant advantages such as high energy density, high power density, long cycle life and green environmental protection[1,2][3,4]. According to the Action Plan for Carbon Peak by 2030 issued by the State Council, the proportion of vehicles powered by clean energy will reach 40% by 2030, which will bring huge growth space for the new energy vehicle market. As the development of lithium batteries is highly compatible with the development of new energy industry, the explosive growth potential of new energy market will continue to promote the growth of lithium battery demand. However, the lack of battery life is the biggest obstacle to its large-scale application. Therefore, improving the energy density of lithium batteries is the greatest guarantee to enhance consumers' recognition and purchasing power of new energy vehicles.
According to the theoretical energy density calculation formula (W = C * E), the energy density (W) of a lithium battery is proportional to the capacity (C) and voltage (E) of the battery, so cathode materials with high voltage such as LiNi0.5Mn1.5O4 or high specific capacity such as LiCoO2 and LiNixMnyCo1-x-yO2 should be used[5][6]. Electrolyte, as a medium for transferring anions and cations between positive and negative electrodes, determines the working voltage of the battery by affecting the stability of the electrode/electrolyte interface film, and plays a key role in the performance of the battery[7][8][5]. Under high voltage, the irreversible oxidation and decomposition of traditional electrolyte on the surface of cathode will lead to a series of side reactions, such as gas generation, battery bulging, electrode structure destruction, transition metal dissolution, and increasing polarization voltage, which will lead to the decline of cycle performance of high-voltage cathode materials[9,10][11,12]. Therefore, there is an urgent need to develop new electrolyte systems suitable for high-voltage lithium batteries.
Based on the above background, combined with the frontier molecular orbital theory of free solvent molecules, this paper mainly explores the effective methods to broaden the electrochemical window of new electrolyte solvent system and the mechanism of high voltage electrolyte additives from the working mechanism of high voltage electrolyte, and puts forward the further development direction.

2 Working mechanism of high voltage electrolyte

The frontier molecular orbital theory of free solvent molecules, including the lowest unoccupied molecular orbital theory (LUMO) and the highest occupied molecular orbital (HOMO) theory, is used as the main criterion to describe the stability of electrolyte solvents[13]. According to this theory, the stability of electrolyte depends on the ability of solvent molecules or ions to gain or lose electrons. As shown in Fig. 1, the reduction/oxidation decomposition of traditional commercial electrolyte is inevitable under high voltage[14]. Solvent molecules or ions can remain stable in the electrolyte only if Equation 1 is satisfied.
图1 电池的电极和电解液中相对电子能量和电极/电解液界面膜构成条件示意图[13]

Fig.1 Schematic illustration of open-circuit energy of liquid electrolyte and conditions for electrode-electrolyte interphase formation[13]. Copyright 2010, American Chemical Society

e V o c = μ A - μ C ≤ E g
Where e refers to the electron charge and Voc refers to the open circuit voltage of the cell; μA and μC represent the electrochemical potentials of the negative and positive electrodes, respectively, and Eg represents the working voltage interval of the electrolyte.
Generally speaking, the lower the HOMO energy level of the electrolyte, the higher the oxidation potential and the better the oxidation resistance; The higher the energy level of LUMO in the electrolyte, the lower the reduction potential and the better the reduction resistance[15]. Therefore, lithium batteries in general require that the LUMO energy level of the electrolyte be higher than the electrochemical potential of the negative electrode, while the HOMO energy level of the electrolyte be lower than the electrochemical potential of the positive electrode[16].
Theoretically, the selection of high-voltage organic solvent should have the following characteristics. First, the new electrolyte system should have a wide electrochemical window, good oxidation/reduction stability, and be compatible with the electrode without side reactions[17]. Second, it should be polar (i.e., dielectric coefficient > 15), which AIDS in the dissolution of salts, and have a relatively low viscosity to provide a high ion migration rate[8,18,19][20]. In addition, it should also have the advantages of wide liquid phase temperature range (low melting point and high boiling point), high ionic conductivity and high flash point[21,22].
Additives are also one of the important components of the electrolyte system. The introduction of electrolyte additives is an economical and effective strategy in order to prevent the decomposition of electrolyte to produce gases and other undesirable by-products[23]. The researchers calculated the HOMO energy level of the additive by density functional theory (DFT), and believed that when the HOMO energy level of the additive was higher than that of the solvent, the additive would be oxidized preferentially to the electrolyte solvent, forming an interfacial film (CEI/SEI) at the interface between the electrode and the electrolyte[24][25]. The interface layer can effectively reduce the side reaction between the electrode and the electrolyte, prevent the dissolution of transition metals, provide a channel for the migration of lithium ions, and reduce the interface impedance[26]. Generally, high-voltage electrolyte additives act in the following ways: (1) prevent electrolyte decomposition (reduction or oxidation) and inhibit the dissolution of transition metals in the positive electrode by enhancing the film-forming properties of the negative and positive electrodes[27][28]; (2) as a wetting agent to improve physical properties such as ionic conductivity, viscosity, and wettability[29]; (3) as an overcharge protectant to enhance safety performance, broaden the electrochemical window of the electrolyte and improve thermal stability, and prevent overcharge of the battery[23,30]. At the same time, the selection of high voltage additives should also meet the following characteristics: (1) small dosage, low cost, generally not more than 5% of the total mass or volume of the electrolyte[31]; (2) no side reaction with other components in the electrolyte[32]; And (3) the effect is quick, and the introduction of the additive can greatly improve the performance of the battery[33].
Lithium salt can provide lithium ions needed by the solvent, and the solvation of lithium salt can increase the ionic conductivity of the electrolyte, so lithium salt is an important component of lithium batteries[34][35]. In general, lithium salts can improve the stability of the electrolyte by directly participating in the formation of a solid electrolyte interfacial film (SEI) to inhibit the redox decomposition of the electrolyte, and determine the ionic conductivity of the electrode surface[36][37].

3 Research Progress of High Voltage Electrolyte

In recent years, a series of high-voltage cathode materials, such as layered lithium-rich materials, LiNixMnyCo1-x-yO2 and LiNi0.5Mn1.5O4, have been applied. However, the instability of traditional electrolytes at high voltage has become a constraint for the development of high energy density lithium batteries. Therefore, it is urgent to study new high-voltage electrolyte systems[17].

3.1 New electrolyte solvent system

In recent years, the research of high voltage electrolyte solvent system has been deepened, and a series of new electrolyte solvents, such as carbonates, sulfones, fluoro, nitriles, ethers and ionic liquids, can meet the application of high voltage lithium batteries to a certain extent. Fig. 2 summarizes the advantages and disadvantages of various high-voltage organic solvents.
图2 各种高电压有机溶剂的优缺点

Fig.2 Advantages and disadvantages of various high-voltage organic solvents.

3.1.1 Carbonate solvent

Traditional commercial carbonate electrolytes are widely favored due to their high ionic conductivity and good solubility of lithium salts[38,39]. However, due to its narrow electrochemical window, it will be oxidized and decomposed on the surface of the cathode with strong catalytic activity at high voltage, resulting in gas generation and the decline of battery coulombic efficiency, which limits its wide application at high voltage[40,41].
The results show that the strategies of adding film-forming additives, using high concentration electrolyte (HCE) and local high concentration electrolyte (LHCE) are effective ways to improve the coulombic efficiency and broaden the voltage window of carbonate-based electrolyte[17,42].
Ming et al. Used 1 wt% ethylene carbonate (VC) and 3 wt% fluoroethylene carbonate (FEC) as functional additives for 1.2 M LiPF6/DMC+EMC,1:9 by wt to form a more stable SEI film with high ionic conductivity to protect the graphite anode through the synergistic effect of VC and FEC, which solved the incompatibility problem between the traditional carbonate electrolyte and graphite anode, and the capacity retention rate of NCM622/graphite battery reached 87.7% after 200 cycles at a voltage of 4.45 V[43].
Recent studies have shown that HCE (> 3 mol/L) can effectively broaden the electrolyte voltage window, and its action principle includes the following aspects:A large number of contact ion pairs (CIP) and ion aggregates (AGG) in high concentration electrolyte indicate that the increase of the number of lithium ions complexed with solvent molecules can enhance the passivation ability of the electrolyte at the electrode and inhibit the oxidation decomposition of the electrolyte at high voltage[44][45][12,46]. Secondly, due to the decrease of the number of free solvent molecules and the change of the solvation structure of the electrolyte, the lithium salt anion is in the lowest unoccupied molecular orbital (LUMO), and the lithium salt can be preferentially reduced and decomposed to form a SEI film, thus inhibiting the decomposition of the electrolyte and effectively enhancing the cycle stability of the electrolyte system[47]. Li et al. Prepared 4 M LiTFSI + 0.5 M LiDFOB/DMC + FEC, 7:3 by vol electrolyte system, and TFSI- and Li+ were fully coordinated at high concentration, forming a unique network structure with a small amount of free solvent[48]. This structure makes the electrolyte have excellent oxidation resistance in a wide potential window and inhibits the corrosion of aluminum foil. The LNMO/Li battery with the high concentration electrolyte has a capacity retention of 88. 5% after 500 charge-discharge cycles at a voltage of 4. 9 V, and has an average coulombic efficiency of 99. 6%, showing good cycle stability.
However, HCEs exhibit high viscosity, poor wettability, low ionic conductivity, low ion migration rate, and high cost, which limit their practical applications[49,50]. The use of diluents to form LHCEs can effectively reduce the viscosity of high-concentration electrolyte while maintaining the high coordination clusters in the electrolyte[45,47,51]. Pu et al. proposed to use fluoromethyl 1,1,1,3,3,3 hexafluoroisopropyl ether (SFE) as the diluent of 1.86 M LiFSI/DMC + SFE, 1:1 by mol system to prepare LHCE to improve the wettability and conductivity of the system, and explained the principle of LHCE forming interfacial film on the electrode by comparing low concentration electrolyte and high concentration electrolyte (Fig. 3)[52]. The optimized LHCE produces dense and smooth lithium deposition and forms a robust and LiF-rich electrode/electrolyte interfacial film at the lithium negative electrode, thereby inhibiting electrode cracking at high voltages. The NMC811/Li battery using this system has a capacity retention of 84% and an average coulombic efficiency of more than 99% after 300 cycles in the voltage range of 2. 7 ~ 4.3 V, which provides a new idea for the further development of long-life and high-energy density LMB.
图3 (a)传统的低浓度电解液(LCE),(b)HCE,以及(c)LHCE中的溶液结构示意图,(d)LHCE在电极上形成稳定和均匀的固态电极/电解液界面膜的示意图[52]

Fig.3 Schematic diagram of the solution structures in (a)conventional low concentrated electrolyte (LCE), (b)highly concentrated electrolyte (HCE), and (c)locally highly concentrated electrolyte (LHCE).(d)Schematic diagram of the formation of stable and uniform solid electrode/electrolyte interphases on the electrode in LHCE electrolyte[52]. Copyright 2022, Royal Society Of Chemistry

3.1.2 Sulfone solvent

Compared with the carbonyl group, the sulfone functional group with stronger electron-withdrawing ability can reduce the HOMO energy level and has higher oxidation stability[53]. In addition, sulfone solvents have excellent advantages such as high dielectric constant and high flash point[54]. However, the defects of high melting point, high viscosity and incompatibility with graphite anode limit its wide application in lithium batteries[15,55].
Angell et al. Found that breaking the molecular symmetry can reduce the high melting point problem of sulfones, and synthesized an ether sulfone, methoxyethyl methyl sulfone (MEMS), which reduced the melting point to 2 ℃, while the 1.0 M LiTFSI/MEMS electrolyte maintained the advantage of high oxidation stability of sulfones, showing a high oxidation potential of 5.6 V[56][57][58].
Due to the high viscosity of sulfone electrolyte, it is usually necessary to introduce a cosolvent that can reduce the viscosity[59]. It was found that the viscosity of sulfone-based electrolyte could be reduced by using cyclic carbonate with high dielectric constant as cosolvent, which made the sulfone-based electrolyte show better performance[60].
He et al. Used FEC as a cosolvent to construct a stable electrode/electrolyte interfacial film on the electrode surface while reducing the viscosity of the electrolyte[61]. As shown in fig. 4A, carbonate electrolyte will be oxidized and decomposed on the surface of the cathode under high voltage, resulting in irreversible phase change of the cathode material and dissolution of transition metals. In addition, the severe side reactions of the carbonate electrolyte and the lithium metal anode produce a large amount of dendritic "dead" lithium (Fig. 4C). Fig. 4B shows that the oxidation-resistant sulfolane (TMS) combined with the cathode constructs a CEI film with high adsorption energy, which inhibits the dissolution of transition metals; The film-formed FEC protects the lithium metal anode (Figure 4D), and the NMC811/Li (4.4 V) battery using this system has a capacity retention of 86.1% and a coulombic efficiency of 99.3% after 500 cycles, showing a dual electrode affinity.
图4 使用EC/DMC电解液(a)正极表面示意图(c)负极表面示意图;使用TMS/FEC电解液的(b)正极表面示意图和(d)负极表面示意图[61]

Fig.4 Schematic diagrams of (a)cathode surface,(c)anode surface in EC/DMC electrolyte;(b)cathode surface and(d)anode surface in TMS/FEC electrolyte[61]. Copyright 2022, Elsevier

Incompatibility with graphite anode is the key factor that limits the large-scale application of sulfone electrolytes[62]. Chen et al. Employed 5 vol% p-toluenesulfonyl isocyanate (PTSI) as a co-solvent for TMS, and the hybrid electrolyte exhibited low melting point, high wettability, higher ionic conductivity (10-3S/cm), and wider electrochemical window (vs.Li/Li+>5.0 V) compared with the pure TMS-based electrolyte[63]. The MCMB/Li cell has a high reversible capacity of nearly 360 mAh/G after 50 cycles. The key to the improvement of battery performance is that the solid electrolyte interface layer formed by the reduction and decomposition of PTSI protects the graphite negative electrode.

3.1.3 Fluorinated solvent

Due to the high electronegativity and strong electron-withdrawing ability of F atoms, the HOMO energy level of fluorinated solvents is reduced, and the fluorinated solvents have stronger antioxidant ability[55]. Fluorinated electrolytes are excellent candidates for electrolytes for high voltage lithium batteries. Fluorocarbonates are the most common in fluorinated solvents, which can be divided into cyclic fluorocarbonates and linear fluorocarbonates according to their molecular structures[64].
Fluorinated solvents generally improve the oxidative stability of the system by producing stable fluorinated intermediates[64,65]. Fluoroethylene carbonate (FEC) is a commonly used cyclic fluorocarbonate cosolvent, and Markevich et al. Used FEC as a cosolvent to participate in the formation of CEI membrane, designed a (1 M LiPF6/FEC+DMC 1:4 by wt) electrolyte system, and the LiCoPO4/Li battery showed superior electrochemical performance with a capacity retention of more than 90% after 100 cycles[66].
However, the high viscosity and high cost limit the large-scale application of cyclic fluorocarbonates in practical production[11]. Nowadays, linear fluorocarbonate electrolytes with lower viscosity are gradually developed and used. It can be seen from Table 1 that the newly synthesized fluorocarbonate has a higher oxidation potential and a lower HOMO energy level, proving that it has a higher oxidation stability.
表1 碳酸酯、醚、氟化碳酸酯和氟化醚的氧化电位和HOMO/LUMO能级[67]

Table 1 Oxidation potential and HOMO/LUMO energies of carbonates, ethers, fluorinated carbonates, and fluorinated ethers[67]. Copyright 2013, Royal Society Of Chemistry

Molecule Pox(V theory) HOMO(au) LUMO(au)
EC 6.91(6.83 open) -0.31005 -0.01067
EMC 6.63 -0.29905 0.00251
EPE 5.511 -0.26153 0.00596
F-AEC 6.98 -0.31780 -0.01795
F-EMC 7.01 -0.31946 -0.00363
F-EPE 7.24 -0.35426 -0.00356
Zhang et al. Widened the voltage window of the electrolyte by using linear fluorinated carbonate FEMC as the co-solvent of cyclic fluorinated carbonate FAEC[67]. However, the SEI constructed by FAEC on the graphite negative electrode is not stable, and the cycling stability of the battery is poor. In subsequent work, Zhang et al. Developed a new fluorinated high-voltage electrolyte (1.0 M LiPF6/FEC+FEMC+FEPE,3:5:2 by vol)(HVE)[64]. FT-IR shows that a prominent carbonyl functional group appears in the LNMO cathode after cycling from Gen 2 electrolyte (1.2 M LiPF6/EC+EMC,3:7 by wt), indicating the decomposition of the traditional carbonate electrolyte (Figure 5A), while this phenomenon is not observed in the FT-IR of HVE electrolyte system (Figure 5B); On the graphite negative electrode, the SEI film formed by Gen 2 electrolyte was severely decomposed, while the SEI film formed by HVE electrolyte was relatively stable (Fig. 5C, d). And the LNMO/graphite cell using HVE was able to retain 50% of its initial capacity (at a rate of C/3) after 600 cycles at room temperature and 250 cycles at 55 ° C (Figure 5 e, f). It is proved that this electrolyte has excellent electrochemical stability and is an excellent candidate for matching high-voltage cathode materials.
图5 原始和100圈循环之后的LNMO正极FT-IR光谱(a)Gen 2电解液(b)HVE电解液;原始和100圈循环之后的石墨负极FT-IR光谱(c)Gen 2 电解液(d)HVE 电解液;使用HVE和Gen 2电解液的石墨/LNMO电池在(e)室温(f)55℃下的长循环测试[64]

Fig.5 FT-IR spectra of LNMO cathode pristine and after 100 cycles in(a)Gen 2 electrolyte,(b)HVE electrolyte; graphite anode pristine and after 100 cycles in(c)Gen 2 electrolyte and(d)HVE electrolyte; Cycling performance of graphite/LNMO cells at (e)RT and(f)55℃ with HVE electrolyte and Gen 2 electrolyte[64]. Copyright 2013, Elsevier

3.1.4 Nitrile solvent

According to the number of cyano groups, nitrile solvents can be divided into mono-nitrile and di-nitrile solvents[17].
Dinitrile succinonitrile (SN) has the advantages of high solubility of lithium salt, high flash point, high boiling point and the like[68,69]; SN with high HOMO energy level can be oxidatively decomposed to construct a cathode/electrolyte interface film to protect high-voltage cathode materials, which can be used as a new electrolyte solvent to improve the electrochemical performance of lithium batteries. However, due to the incompatibility between succinonitrile and lithium metal anode, its large-scale application is still a challenge[70][71]. Zhang et al. Added 20 wt% FEC as a co-solvent to the 1 M LiBF4/SN solution to construct a dense and smooth interfacial protective layer on the anode, which promoted the uniform deposition of lithium and inhibited the growth of lithium dendrites[72]. At the same time, the system showed excellent thermal stability, and the capacity retention rate was 77% after cycling at 120 ℃ for 30 min. It has a high coulombic efficiency of 99.5% and a capacity retention of 93% after 100 cycles at room temperature, indicating its excellent electrochemical performance.
In addition to dinitriles, other cyano compounds also exhibit high ionic conductivity, but their large-scale application is limited due to poor reduction stability[73]. Zhang et al. Proposed a fully methylated pivalonitrile (PN), whose active α-H atoms were all replaced by methyl groups, which inhibited the formation of free cyanide and improved the reduction stability of nitrile electrolytes[74]. The 1 M LiTFSI/FEC + PN, 2:3 by vol system has excellent cycle stability, and the capacity retention of NCM622/Li battery is 75. 3% after 300 cycles at 4. 5 V. In addition, it also shows the superior performance of low viscosity (3.47 mPa · s) and high ionic conductivity (11.53 mS/cm2).

3.1.5 Ether solvent

Although ether solvents have high reductive stability, they cannot match the high-voltage cathode system due to their poor oxidative stability (<4 V vs.Li/Li+)[75]. At present, the electrochemical window of ether electrolytes can be effectively broadened by using high concentration electrolytes, cosolvents and hydrofluoroether (TTE), and the cycle stability of the system at high voltage can be improved[76].
Xu et al. Reported an ether high-concentration electrolyte system, namely 2 M LiTFSI + 2 M LiDFOB/DME, which formed an SEI/CEI film through the preferential oxidation of high-concentration lithium salt, greatly improving the oxidation stability of ether solvents. This system constructed a highly stable SEI layer on the metal lithium anode, inhibited the formation of "dead" lithium, realized the efficient cycle of metal lithium, and improved the average coulombic efficiency[77]. The CEI film was constructed on the positive electrode to inhibit the dissolution of transition metals. The capacity retention of the Ni1/3Co1/3Mn1/3/Li cell was about 80% for 500 cycles at a cut-off voltage of 4.3 V, showing efficient dual electrode affinity.
Coskun et al. Prepared 1.5 M LiFSI-8 TTD-2DME electrolyte using a fluorinated dioxolane (TTD) as a co-solvent[78]. As can be seen from Figure 6a, compared with DME and DOL, the TTD molecule exhibits a lower HOMO energy level (-8.52 eV), proving that the solvent has superior oxidation stability; In addition, the low LUMO energy value of TTD (-0.49 eV) indicates that it can preferentially decompose on the surface of lithium anode to form a robust SEI. The high oxidation stability of the system was also verified by LSV test (Fig. 6B). The NCM811/Li battery using this system still has 75% capacity retention after 160 cycles in the voltage range of 2.8 ~ 4.7 V, indicating that the TTD electrolyte effectively broadens the voltage window of the high nickel cathode. Zheng et al. Prepared the 1 M LiFSI+0.1 M LiPF6/DME+TTE system electrolyte (L-LDT). The introduction of TTE improves the solvation sheath of the electrolyte, makes more DME combine with the Li+ (Li+:DME=1:4), and enhances the Li+ migration rate and oxidation stability of the electrolyte[79]. At the same time, the PF5 produced by the decomposition of LiPF6 can initiate the polymerization of free dimethyl ether, which forms a more stable SEI protective layer on the electrode and inhibits the oxidative decomposition of electrolyte. As far as the L-LDT electrolyte is concerned (Figure 6C), the synergistic effect of polyether and LiF helps to construct a more stable SEI, and meanwhile, the introduction of elements such as S, P, and N enhances the ionic conductivity of the SEI. The NMC90/Li cell employing this system achieved a capacity retention of 93.7% after 250 cycles at a high current density of 4.0 mA/cm2. This work provides a new idea for the stable cycling of ethers at high voltage.
图6 (a)不同溶剂HOMO-LUMO能级(b)使用LSV测试不同电解液的氧化稳定性[78];(c)L-LDT电解液的SEI构成机理图[79]

Fig.6 (a)Comparison of HOMO-LUMO energy levels of different electrolytes.(b)LSV test for oxidation stability of different electrolytes[78]; Copyright 2022, American Chemical Society.(c)Schematic illustration of the SEI structure in the L-LDT electrolyte[79]. Copyright 2022, Elsevier

3.1.6 Ionic liquid

Ionic liquids (ILs) are liquid molten salts composed of anions and cations, and are a new type of electrolyte solvent[80]. Generally, the types of ionic liquids include pyrrolidines, piperidines, ammonium, etc[80⇓⇓⇓~84]. Ionic liquids (ILs) have great application prospects as electrolytes for lithium batteries because of their high thermal stability, low volatility and wide electrochemical window[85]. However, the disadvantages of ionic liquids, such as high viscosity, high cost and low ionic conductivity, need to be solved urgently[17,81].
Chen et al. Reported a pyrrolidine-based ionic liquid (PYR14-TFSI), introducing fluorinated ether solvent (HFPM) to reduce the viscosity of PYR14-TFSI and improve the lithium ion migration rate of the system[7]; And 0.3 wt%LiNO3 was added as an additive to cooperate with HFPM to construct the SEI film containing LiNxOy and LiF, which is beneficial to improve the interfacial stability between the electrode and the electrolyte. A large number of lithium dendrites were observed on the surface of the traditional ionic liquid electrolyte (1 M PYR14-TFSI/EC+DEC+DMC,1:1:1 by vol) anode by SEM. Due to the instability of the CEI film, the transition metal dissolves from the positive electrode at high voltage, resulting in the collapse of the positive electrode structure. The CEI composite membrane constructed by the new ionic liquid system prevents the dissolution of transition metals (Fig. 7A). At the same time, the linear sweep voltammetry test shows that the electrolyte has a wider electrochemical window (5.35 V) than the traditional ionic liquid electrolyte (Fig. 7B). The LiNi0.6Co0.2Mn0.2O2/Li cell based on this system showed 94% capacity retention and 99.9% Coulombic efficiency after 100 cycles (Fig. 7 C, d). This work provides a reference for the design of high voltage ionic liquid electrolyte.
图7 (a)不同电解液对NCM622/Li 电池电极的作用机理示意图,(b)线性扫描伏安法示意图,(c)长循环放电比容量图,(d)长循环库仑效率图[7]

Fig.7 (a)Schematic diagram of mechanism on NCM622/Li battery electrode with different electrolytes.(b)The linear sweep voltammetry. Cycling performance (c)and Coulombic efficiency (d)of the NCM622/Li half cells in the different electrolytes[7]. Copyright 2021, Elsevier

Li et al. Introduced piperidine-based ionic liquid (PP14TFSI) into the electrolyte system and sulfolane (SL) as a cosolvent to design 0.5 M LiDFOB/PP14TFSI+SL,1:1 by wt electrolyte[82].
The system showed high thermal stability and wide electrochemical window, and the capacity retention of LNMO/Li battery was 94. 5% and 92. 7% after 50 cycles at 55 ℃ and 70 ℃, respectively, in the voltage range of 2. 0-4. 6 V. Li et al. Suggested that this system could be used as a candidate material for high-voltage lithium-rich batteries.
Choline chloride is an ammonium ionic liquid, which is favored due to its low production cost and environmentally friendly characteristics[83]. However, the hydroxyl and chloride ions of choline chloride have poor compatibility with highly reductive negative electrode and oxidizing positive electrode, so it can not be directly used as lithium battery electrolyte, and additional modification must be carried out[83]. Zhang et al. Reported a new cholinylammonium-based ionic liquid (SN1IL) prepared by replacing the hydroxyl group with trimethylsilyl, propenyl, and cyanoethyl groups and replacing the chloride ion by TFSI-[83]. It aims to combine the environmental friendliness of organosilicon, the SEI film-forming property of unsaturated double bonds and the positive electrode protection ability of cyano groups. The 4.4 V LiCoO2/ graphite full cell with the mixed electrolyte of 0.6 M LiPF6+0.4 M LiDFOB/SN1IL+DMC,1:1 by vol has a specific discharge capacity of 152 mAh/G after 90 cycles, and has a capacity retention of 72%, showing excellent cycling performance.

3.2 High voltage electrolyte additive

The additive with higher HOMO energy level can construct the electrode/electrolyte interface film prior to the oxidative decomposition of the electrolyte, thereby inhibiting the oxidative decomposition of the electrolyte and the dissolution of transition metals[86]. Commonly used high voltage electrolyte additives include unsaturated carbonate derivatives, boron-based additives, phosphate-based additives, silicon-based additives, fluorinated additives, vulcanized additives, and nitrile additives. Fig. 8 summarizes the advantages and disadvantages of various additives.
图8 各种添加剂的优缺点

Fig.8 Advantages and disadvantages of various additives.

3.2.1 Unsaturated carbonate derivative

The unsaturated carbonate derivative additive is mainly vinyl. Vinyl ethylene carbonate (VEC) was first proposed as a film-forming additive for graphite anode[87]. However, compared with VC, the film forming potential of VEC is relatively high and the kinetic rate is slow, so the use conditions of VEC additives are relatively harsh, and it is usually necessary to combine with other film forming additives to improve its electrochemical performance[88].
Vinylene carbonate (VC) is more widely used[89]. VC can form a polymer protective layer on the surface of the negative electrode and the positive electrode through a free radical polymerization mechanism, which can improve the coulombic efficiency of the battery and reduce the self-discharge by slowing down the oxidative decomposition of the electrolyte on the surface of the electrode[90,91][92]. Dahn et al. Added 3 wt% VC as an additive to the sulfolane (SL)/EMC electrolyte system, which effectively alleviated the oxidative decomposition of the electrolyte[93]. However, high concentration of VC will increase the interfacial resistance and gas generation, so it is necessary to introduce other additives to improve the cycle performance[94]. Dahn et al. Subsequently suppressed the gas generation and the voltage drop during the charging process by combining 2 wt% VC with 2 wt% TAP, but still had a high interfacial impedance[95]. Subsequently, Dahn et al. Studied the effect of the combination of vinyl sulfite (ES) and VC. The combination of 2 wt% VC and 1 wt% ES significantly improved the coulombic efficiency of the battery and reduced the loss of capacity[96]. The system reduces the interface impedance of the NCM/graphite battery and reduces the generation of gas. The 1M LiPF6/EC+EMC,3:7 by wt,2 wt%VC+1 wt%ES system showed only 4.5% capacity loss after 500 cycles in the voltage range of 2.8 – 4.2 V, while the blank electrolyte showed only 78% capacity retention after 500 cycles. A series of research results of Dahn et al. Provide new ideas for the wide application of VC.

3.2.2 Boron based additive

The central B atom of boron compounds is in an electron-deficient state, so it has the ability to combine with anions to form complexes and inhibit the decomposition reaction of anions[97]. Because of the high HOMO energy level of boron-based additives, SEI/CEI membrane protected electrodes can be constructed in preference to electrolyte oxidative decomposition[98]. In addition, boron-based additives can improve the dissociation degree of LiPF6, inhibit its hydrolysis by complexing with PF 6 -, and reduce the deposition of LiF, thereby reducing the battery interface impedance and improving the ionic conductivity, so boron-based additives have good application prospects[37][99].
Wang et al. introduced 2 wt% trimethyl borate (TMB) as an additive for the commercial electrolyte. Due to the high HOMO energy level of TMB, it can build a protective layer on the positive electrode in preference to the oxidative decomposition of the electrolyte, thus inhibiting the decomposition of the commercial electrolyte and the precipitation of HF (Figure 9 a)[100]. The capacity retention of the LiCoO2/Li battery with TMB additive increased from 64% to 81% after 100 cycles in the voltage range of 2.5 – 4.5 V, proving that the development of TMB provides a new prospect for the design of high-voltage electrolytes.
Nan et al. Used Tris (trimethylsilane) borate (TMSB) as an additive for the 1 M LiPF6/EC+EMC system, and its LiNi0.5Co0.2Mn0.3O2/ graphite battery retained 92.3% of the initial capacity after 150 cycles, while the system without the TMSB additive only retained 28.5% of the capacity. It can be seen from Figure 9 B that the system without the additive formed a thick CEI layer of LiF, which was not conducive to the migration of lithium ions, resulting in rapid capacity decay[101]; In contrast, TMSB can promote the dissolution of LiF, form a thinner interfacial film and reduce the interfacial resistance (Fig. 9 C, d), which significantly improves the cycle performance of the electrolyte.
图9 (a)TMB稳定电池正极的作用机理图[100];(b)TMSB提高LIB的高电压性能的作用机理图;LiNi0.5Co0.2Mn0.3O2/石墨电池在(c)第1个循环和(d)第150个循环后的EIS阻抗图[101]

Fig.9 (a)Schematic illustration of the contribution of TMB to stabilizing cathode interface[100]; Copyright 2019, American Chemical Society.(b)Schematic illustration of TMSB to enhance the high voltage performance of LIB. EIS patterns of the LiNi0.5Co0.2Mn0.3O2/graphite cells after (c)the 1st cycle and (d)the 150th cycles[101]. Copyright 2013, Elsevier

3.2.3 Phosphate ester based additive

Phosphorus-containing groups in phosphate ester compounds can capture hydrogen radicals, thus preventing the chain reaction of hydrocarbon combustion or explosion, so they are usually used as flame retardant additives for lithium batteries[102][103]. However, phosphate-based additives are unable to construct a stable interfacial film on the electrode, and it is usually necessary to introduce other groups to improve its electrochemical performance[104].
Choi et al. Synthesized lithium difluorobisoxalatoborate (LiDFBP) by introducing a fluorine group into phosphate as an additive for 1.3 M LiPF6/EC+EMC+DMC,3:4:3 by vol. The SEI film on the lithium-rich cathode without the additive is thin and uneven, which is easily attacked by HF in the decomposition products of the electrolyte, resulting in serious phase transformation of the cathode and dissolution of transition metals[105]. LiDFBP, on the other hand, constructs a stable interface layer with high ionic conductivity, which inhibits the decomposition of the electrolyte and the dissolution of transition metals (Figure 10a). The Li-rich/Li battery shows excellent high voltage cycling performance with almost no capacity loss and a high coulombic efficiency of 99. 5% after 100 cycles in the voltage range of 2. 0 ~ 4.6 V.
图10 (a)LiDFBP在富锂正极构建SEI膜的效果示意图[105];采用TTEP电解液的LiCoO2/Li电池在25℃下的(b)倍率测试,(c)长循环测试;LiCoO2/graphite电池在(d)55℃,(e)25℃下的长循环测试;(f)空白电解液(e)采用TTEP电解液的扫描电镜图像[106]

Fig.10 (a)Schematic diagram of the effects of LiDFBP in constructing SEI film in Li-rich cathode[105]; Copyright 2017, Wiley Online Library. (b)Rate capabilities, (c)cycling performances of LiCoO2/Li cells using base electrolyte and 0.1 wt% TTEP electrolyte at 25℃; LiCoO2/graphite cells cycling performances at(d)55℃,(e)25℃,SEM images of LiCoO2 electrodes after cycled in the (f)base and(g)0.1 wt% TTEP electrolyte[106]. Copyright 2019, Elsevier

Xiang et al. Synthesized phosphate with thiophene group (TTEP) as an additive for commercial electrolyte. Compared with the thick and uneven CEI film constructed by traditional electrolyte, the thiophene group can construct a thin and uniform CEI film to protect the cathode (Fig. 10 f, G)[106]; The long cycle performance of graphite full battery and lithium metal half battery with 0.1 wt% TTEP as additive has been significantly improved, and they also have excellent performance at high temperature and high rate charge and discharge (Fig. 10 B ~ e).

3.2.4 Silicon-based additive

Silicon-based additives have many advantages, such as low flammability, low volatility, high oxidation resistance and environmental friendliness[107,108]. Generally, siloxane (Si-O) or silazane (Si-N) additives can be oxidized prior to the electrolyte, polymerize on the positive electrode to form a CEI layer, inhibit the decomposition of LiPF6 in the traditional electrolyte, thereby reducing the formation of harmful species such as HF and PF 5 -, so that the battery electrode can be protected from the nucleophilic attack of fluoride species, and thus cycle stably at high voltage[109,110].
Usually, siloxane additives can only provide protection for one of the electrodes. Xing et al. Proposed a new trifunctional siloxane additive, trifluoropropane trimethoxysilane (TTS).TTS constructs a highly stable protective film on the surface of both lithium metal anode and high voltage cathode, while TTS has lower binding energy with HF, F-, and H+, indicating that it can effectively capture and eliminate harmful HF (Fig. 11 a – C)[111]. The presence of HF was not detected in the 19F NMR of the electrolyte with TTS as an additive, verifying the excellent performance of 2 wt% TTS in removing HF (Fig. 11e, d) and effectively inhibiting the dissolution of transition metals (Fig. 11f). With the addition of 2 wt% TTS to the 1 M LiPF6/EC+EMC+DEC,3:5:2 by wt system, the LiNi0.5Mn1.5O4/Li battery has a high capacity retention of 92% and a high coulombic efficiency of 99.2% after 500 cycles at 4.9 V, which is 44% higher than that of the system without TTS additive. The development of this new additive provides a new idea for the application of the future generation of high energy density lithium metal batteries.
图11 (a~c)结构式与结合能Eb(Eb,kJ/mol)、(a)A-HF,(b)A-F,(c)A-H+ (A=EC、 EMC、DEC、 TTS);电解液中添加1 wt% HF的19F 核磁共振谱图(d)空白电解液(e)添加2 wt% TTS的电解液,(f)循环500次后的空白电解液和含有2 wt% TTS电解液的LNMO/Li电池中提取的锂负极上过渡金属离子的含量示意图[111];(g)不同电解质的前沿分子轨道能级,(h)Li/Li对称电池的恒电流长循环,(i)初始放电过程中Li/Li对称电池的电压-时间曲线,(j)50 h循环后Li/Li对称电池的Nyquist图[112]

Fig.11 Optimized structures and binding energy (Eb, kJ/mol)of (a)A-HF,(b)A-F and(c)A-H+ (A= EC, EMC, DEC and TTS);19F NMR spectra of (d)base and(e)2 wt% TTS-containing electrolytes after adding 1 wt% HF aqueous solution;(f)content diagram of transition metal ions on lithium electrode extracted from base electrolyte and LNMO/Li battery containing 2 wt% TTS electrolyte after 500 cycles[111]; Copyright 2020, Royal Society Of Chemistry.(g)Frontier molecular orbital energies of different electrolytes. (h)Galvanostatic long-term cycling of the Li/Li symmetrical cell. (i)Voltage-time profiles of Li/Li symmetric cells for initial discharge process.(j)Nyquist plots of Li/Li symmetric cells after 50 h cycles[112]. Copyright 2021, American Chemical Society

Xiang et al. used a multifunctional silazane, bis (trimethylsilyl) trifluoroacetamide (BTA), as an additive for a commercial electrolyte. BTA has a higher HOMO energy level and a lower LUMO energy level (Fig. 11g), which can be decomposed prior to the redox of the electrolyte. A LiF-rich SEI/CEI layer was constructed on both the positive and negative electrodes.The Li/Li battery shows a small polarization voltage, a lower nucleation/deposition overpotential, and a smaller interfacial impedance and charge transfer impedance (Fig. 11h ~ J), and the long cycle performance of the NCM811/Li battery of the system is also significantly improved, with a capacity retention rate of 50% after 750 cycles at a voltage of 4.4 V[112]; The multifunctional BTA additive shows excellent application prospects.

3.2.5 Fluorinated additive

Fluorine-containing additives can generally construct more robust and stable SEI/CEI films in preference to electrolyte redox decomposition, and these films are composed of fluorinated species/polymers, which prevent the side reactions of electrolyte solvents on the electrode surface[113,114]. Therefore, the use of fluorine-containing substances as additives to improve the oxidation resistance of the electrolyte is a practical approach for the lithium battery industry[115]. Among the fluorinated additives, the most common are fluorinated carbonates and fluorinated phosphate esters.
FEC has lower LUMO and HOMO energy levels, and has higher oxidation stability, and can be preferentially reduced by electrolyte to construct a low-impedance SEI film protection electrode, so it has superior application prospects as an additive[116][117]. Zhang et al. Used 5 vol% FEC as an additive in the 1 M LiPF6/EC+DEC system to construct a LiF-rich SEI film on the lithium metal anode, which is beneficial to the plating/stripping of lithium, inhibits the formation of lithium dendrites, and promotes more uniform lithium deposition (Fig. 12A)[118]; It can be seen from the XPS images that FEC constructs a more stable and LiF-rich SEI film and inhibits the reductive decomposition of the electrolyte at the negative electrode (Fig. 12b, C). The capacity retention of NCM/Li battery after 100 cycles has been significantly improved, indicating that FEC as an additive can effectively improve the electrolyte performance.
图12 (a)FEC添加剂对锂金属负极的SEI膜构成影响示意图(b)0%和5 vol% FEC 10次循环后Cu上剥离的锂SEI膜XPS表征图(b)F 1s、(c)Li 1s[118];EC、EMC、DEC、LiPF6和LiPO2F2的氧化电位(V vs. Li/Li+)(d)有及无添加剂的LNCM/Li电池的(e)循环伏安曲线图和(f)计时电流响应图,(g)电化学测试前后电解液的19F NMR谱;有及无添加剂的LNCM/Li电池的(h)长循环图和(i)库仑效率图[119]

Fig.12 (a)Schematic diagram of the effect of FEC additives on SEI layer on a Li metal anode. (b)F 1s and (c)Li 1s XPS characterization spectra of the SEI layer induced by 0% and 5 vol% FEC after lithium stripping on Cu substrate after ten cycles[118]. Copyright 2017, Wiley Online Library. (d)Calculated oxidation potential (V vs. Li/Li+) of EC, EMC, DEC, LiPF6 and LiPO2F2 (e)cyclic voltammogram and (f)chronoamperometric responses of LNCM/Li cells with and without additive; (g)19F NMR spectra of electrolytes before and after electrochemical test;(h)cyclic stability and (i)Coulombic efficiency of LNCM/Li cells with and without additive[119]. Copyright 2018, Elsevier.

Li et al. Used 3 wt%LiPO2F2 as an additive in the 1 M LiPF6/EC+EMC+DEC,3:5:2 by wt system, and the DFT calculation showed that LiPO2F2 had the lowest oxidation potential, indicating that it would be oxidized and decomposed prior to the electrolyte during cycling, which was verified in the cyclic voltammetry curves (Fig. 12d, e)[119]; Chronoamperometry showed that the residual current was lower when the LiPO2F2 additive was used, indicating that the decomposition of the electrolyte was inhibited (Fig. 12f);19F NMR spectrum showed that the introduction of LiPO2F2 inhibited the decomposition of LiPF6 and reduced the generation of HF (Fig. 12g). The LNCM/Li battery had a capacity retention rate of 89% after 250 cycles, which was significantly improved compared with the blank electrolyte (61%), and also showed a higher and more stable coulombic efficiency (Fig. 12h, I). A series of excellent electrochemical properties show that LiPO2F2 have excellent application prospects.

3.2.6 Sulfur-containing additive

The sulfur-containing additive can be reduced prior to the electrolyte, and the reduction product can construct a SEI film with high ionic conductivity at the negative electrode, thereby further improving the performance of the electrolyte[120,121]. According to the different chemical structural formulas, the common vulcanization additives are mainly sulfonic acid, sulfone and sulfuric acid.
Among the sulfonic acid additives, 1,3-propane sultone (PS) and propenyl-1,3-sultone (PES) are two promising candidates.
Anouti et al. Used 1 wt% PS as an additive in the 1 M LiPF6/EC+DMC,1:1 by wt system, and the cathode in the blank electrolyte was attacked by the electrolyte, resulting in the dissolution of a large amount of transition metal manganese[122]. PS can form a stable CEI protective layer on the surface of the cathode and promote the transformation of the electrode surface structure to the spinel phase, thus preventing the side reactions on the electrode surface and inhibiting the dissolution of transition metal ions, thus improving the cycle stability of the system (Fig. 13A). The Li-rich-NCM/Li cell has a capacity retention of 88. 4% and a high coulombic efficiency of 99% after 240 cycles in the voltage range of 2. 0 ~ 5.0 V, showing excellent cycling stability.
图13 (a)循环中PS在富锂NCM正极的保护机制和相变过程示意图[122];(b)MPS在LNMO正极的作用机理示意图[123]

Fig.13 (a)Schematic diagram of the PS protection mechanism on the Li-rich-NMC cathode during cycling and gradual transformation from the layered to the spinel structure[122]. Copyright 2015, Royal Society Of Chemistry;(b)Schematic diagram of the role of MPS additive on the surface of the LNMO cathode[123].Copyright 2022, American Chemical Society

Compared with PS, the C = C double bond adjacent to the sulfonic acid (SO3) molecule in PES has a stronger electrochemical reduction ability, which is easier to be preferentially reduced at the negative electrode to form an interfacial protective layer. PES is now more frequently selected instead of PS as an additive for the formation of SEI films[14]. Li et al. Formed an SEI interfacial film on the surface of the cathode by using 1 wt% propenyl-1,3-sulfonate (PES) as an additive for 1.0 M LiPF6/EC+EMC,1:2 by wt, which inhibited the decomposition of the electrolyte[124]. The system possessed 90% capacity retention after 400 cycles in the voltage range of 3.5 – 4.95 V, while the blank electrolyte system had only 49%, which successfully improved the cycling stability of LiNi0.5Mn1.5O4/Li batteries at high voltages.
Tan et al. Used 0.1 wt% methyl phenyl sulfone (MPS) as an additive for commercial electrolyte. By studying the reaction mechanism, it can be seen that LiPF6 can react to form Lewis acid at room temperature, while MPS can decompose to form Lewis base at room temperature. The two can further combine to inhibit the decomposition of LiPF6 and the occurrence of side reactions, and form a thin and uniform interfacial film on the cathode to inhibit the dissolution of transition metals (Figure 13B)[123]. The long cycle results show that the capacity retention is as high as 89. 8% after 400 cycles at a high cut-off voltage of 4. 9 V with 0. 1 wt% MPS electrolyte, and the cycle stability of LNMO/Li battery is obviously improved.
Due to the existence of lone pair electrons on the sulfur atom in sulfite compounds, their oxidation stability is very poor (3 V vs.Li+/Li), resulting in the formation of CEI or SEI film instability, so sulfite additives have not been widely used[125]. In contrast, the sulfate ester additive vinyl sulfate (DTD) has excellent film-forming properties and strong complexing ability with cobalt ions, which can inhibit the dissolution of transition metals[126][127]. Li et al. Used 2 wt% vinyl sulfate (DTD) as a cathode film forming additive for 1 M LiPF6/EC+EMC,3:7 by vol system[128]. It can be observed by TEM that the surface of the uncycled positive electrode is flat and smooth without an interfacial film (fig. 14 a). However, the blank electrolyte without DTD had severe electrolyte oxidative decomposition on the surface of the positive electrode after cycling, resulting in cracking of the electrode and the formation of an uneven interface layer (Fig. 14C). The introduction of DTD forms a uniform and dense protective layer at the cathode/electrolyte interface, which inhibits the oxidative decomposition of the electrolyte on the electrode surface and hinders the dissolution of transition metals (Fig. 14 d). The Ni0.5Mn0.3Co0.2O2/Li battery employing this system possessed 84% capacity retention after 100 cycles at high voltage (4.5 V vs.Li/Li+).
图14 (a)有及无DTD添加剂的NCM/Li电池正极的形貌和作用机理示意图,(b)原始正极(c)未使用DTD添加剂(d)使用DTD添加剂的TEM图像[128]

Fig.14 (a)Schematic illustration of NCM/Li cells cycling with and without DTD, TEM images of the (b)fresh cathode, and cycled cathode with(c)baseline and(d)DTD containing electrolytes[128].Copyright 2017, The Electrochemical Society

3.2.7 Nitrile additive

In general, the nitrile functional group (— C ≡ N:) has a lone pair of electrons, which can capture the dissolved transition metal ions in the cathode material cycle, thereby inhibiting/reducing the side reactions between the cathode and the electrolyte[129]. Nitrile additives are mainly mononitriles and dinitriles according to the number of cyano groups.
Acetonitrile can inhibit the oxidative decomposition of carbonate electrolyte by preferential coordination with lithium ions[130]. However, it is difficult to cycle stably at high voltage due to its poor compatibility with lithium metal anode[131]. Therefore, mononitrile additives are not widely used.
Han et al. Used 0.5 wt% dinitrile 1,4-dicyanobutane (ADN) as an additive for 1 M LiPF6/EC+EMC,1:2 by wt. Due to its high oxidation stability, ADN further increased the oxidation potential of the electrolyte, inhibited the oxidative decomposition of the electrolyte and the dissolution of positive transition metal ions. At the same time, ADN participated in the formation of a passive film on the graphite electrode, introduced inorganic components, and stabilized the electrode-electrolyte interface[132]. The capacity retention of the LiNi0.5Co0.2Mn0.3O2/ graphite full cell was 84% after 100 cycles, while that of the blank electrolyte was 68%, showing exceptional high voltage (4.5 V) cycling performance.

3.3 Aqueous electrolyte

Aqueous electrolytes are popular because of their environmental friendliness, non-flammability, low cost, high ionic conductivity and many other advantages. However, their narrow electrochemical stability window (< 1.23 V) and high reactivity of metal electrodes hinder the wide application of aqueous electrolytes in lithium batteries[44][133]. Current studies have shown that the "water-in-salt" strategy can be used, that is, to prepare a high concentration electrolyte and build a passivation layer on the surface of the electrode to improve the electrochemical window of the aqueous electrolyte[134][135].
Suo et al. Used ultra-high concentration of LiTFSI (21 mol) to form a Li+- anion solvation sheath and significantly reduce the content of free water molecules. In addition, the reduction of N(SO2CF3)2-(TFSI-) constructed a protective layer of LiF on the negative electrode, which inhibited the decomposition of water molecules, thereby reducing the precipitation of H2 and O2, and pioneered to broaden the electrochemical window of water-based electrolytes to 3.0 V[136]. The lithium-ion battery using the aqueous electrolyte can be stably cycled for 1000 times, and has a high coulombic efficiency of nearly 100% at both low (0. 15 C) and high (4. 5 C) charge-discharge rates.
Then Yamada et al. Explored the optimized eutectic system of "water-in-double-salt" strategy by using LiTFSI and LiBETI, and showed that the hydrate melt of lithium salt can be used as a stable aqueous electrolyte at room temperature, all water molecules participate in the solvation sheath of Li+, and the LiNi0.5Mn1.5O4/Li4Ti5O12 battery using this system can cycle stably in the voltage range of 3.0 – 3.3 V[137].

3.4 Solid electrolyte

Solid-state electrolytes are considered to be one of the most promising candidates for high-performance electrolytes for lithium batteries because of their high temperature resistance, non-flammability, and non-leakage[138]. However, the interface instability of solid electrolyte under high voltage limits its development and application[139].
Solid state electrolytes are generally divided into inorganic solid state electrolytes, polymer solid state electrolytes, and composite solid state electrolytes.
Inorganic solid electrolytes can be divided into oxides and sulfides, among which oxide-based inorganic solid electrolytes can be adapted to high-voltage cathode systems due to their advantages of wide electrochemical stability window, high mechanical strength and high oxidation stability.However, its high production cost and side reaction in humid air have hindered its production development[140][141][142].
Sulfide-based inorganic solid electrolyte has attracted much attention because of its high ionic conductivity, good mechanical strength and low interfacial resistance[141,143]. However, its low oxidation stability, poor compatibility with electrodes, and easy side reaction with humid air limit its large-scale commercial application[144].
Therefore, the choice of inorganic solid-state electrolyte should take advantage of its advantages and take into account its limitations, and guarantee strict sealing conditions when used. In order to improve its comprehensive performance, magnetron sputtering technology, interface doping technology or layered deposition technology can be used to introduce other elements (Ta, Nb, Al, etc.) Into the inorganic solid electrolyte to establish a new interface layer to increase the active sites and reduce the interface resistance[145][146][147].
Solid polymer electrolytes (SPEs) have also attracted much attention in the past decade, mainly including polyethylene oxide (PEO), polyacrylonitrile (PAN), and polyvinylidene fluoride (PVDF). SPE has the advantages of high thermal stability, high mechanical strength, and low cost, but its low ionic conductivity at room temperature and the instability of the interface at high voltage hinder its application[148][149,150]. In practical application, the ionic conductivity can be improved by doping nano-scale inorganic metal particles, selecting lithium salts with high ionic conductivity or blending with other polymers to reduce the phase transition temperature[151][152,153]; In addition, the molecular structure can be changed by introducing other polymer chains to improve the interfacial stability, and supramolecular copolymers, random copolymers, block copolymers and the like can be constructed[154].
Inorganic-polymer composite solid state electrolytes also have excellent research prospects, aiming to combine the advantages of both and complement each other. Inspired by the similarity of H bond and Li bond, Zhang et al. Used the in situ coupling technique, using commercial 3-chloropropyltrimethoxysilane (CTMS) as a coupling agent to facilitate the chemical bond interaction between LGPS and polyethylene glycol (PEG), and then added PEO and LiTFSI to obtain a homogeneous composite solid electrolyte[155]. The composite solid-state electrolyte exhibits high ionic conductivity and high Li+ transference number, and suppresses the growth of lithium dendrites. This work provides a new idea for the preparation of composite solid electrolyte.

3.5 Gel polymer electrolyte

Gel polymer electrolyte (GPE) is usually based on polyethylene oxide (PEO), polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF).It is in the intermediate state of liquid and solid electrolyte, which not only has good ionic conductivity, but also has high mechanical strength, while solving the problems of liquid electrolyte which is not resistant to high temperature and easy to leak, and has excellent application prospects. However, there are still some defects that can not be ignored to be solved, such as poor interface compatibility under high voltage, the growth of lithium dendrites and the ionic conductivity is still lower than that of liquid electrolyte. Because the polymer substrate is roughly the same type as the solid polymer electrolyte, the corresponding improvement technology can be used to improve its comprehensive performance. For example, some inorganic nano-metal particles are introduced to improve the ionic conductivity and interface stability.Such as :Al2O3, SiO2, TiO2,Li3N, LiAg4I5, γ-LiAlO2, Li7La3Zr2O12, etc.Li et al. Used the pouring method to firmly combine the abundant hydroxyl groups in laponite (LAP) with the fluorine in PVDF to reduce the crystallinity of PVDF, thereby improving the ionic conductivity[156]. Benefiting from this, LiFePO4/Li batteries exhibit high capacity retention (> 97%) and long cycling stability (> 1000 cycles).
Li et al. Combined PEO and PAN by interpenetrating network (IPN) method, which significantly improved the oxidation stability of gel electrolyte from 4. 1 V to 5. 1 V, and showed good electrode compatibility, providing a new idea for the development of gel electrolyte for high-voltage lithium metal batteries[157].

4 Conclusion and prospect

The voltage window of traditional carbonate electrolyte is relatively narrow, and decomposition and side reactions will occur at high voltage, resulting in the destruction of cathode structure, the continuous corrosion of aluminum current collector and the growth of lithium dendrite. It causes the capacity attenuation and cycle stability decline of lithium batteries, and even has the potential safety hazards of short circuit and combustion. Therefore, it is particularly important to develop new high-voltage electrolyte solvent systems. This paper mainly summarizes the action mechanism, advantages and disadvantages of various organic electrolyte solvents and additives (Figure 2, Figure 8), and discusses the modification strategies of high voltage electrolyte solvents and the practical application of high voltage additives. In addition, the advantages and limitations of aqueous electrolyte, solid electrolyte and gel polymer electrolyte were summarized, and the corresponding improvement technologies were proposed.
For high voltage organic electrolyte system, high concentration electrolyte and local high concentration electrolyte are still popular development directions. However, high concentration electrolyte shows high viscosity, high cost and other shortcomings, which limits its large-scale production and application. Its essence is to build a strong and reliable SEI layer protection electrode through solvation control. Therefore, the traditional carbonate electrolyte can be modified by structural modification strategies, or new electrolyte solvents (such as fluorinated solvents, nitrile solvents, sulfone solvents, etc.) Can be used as co-solvents to form a new system, and the introduction of highly electronegative molecular groups such as F-, — CN, —SO2, etc. Can greatly improve the oxidative stability of the electrolyte and promote the formation of a stable SEI film. In addition, it is particularly important to study the morphology and mechanism of SEI film, which should be characterized by scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy and other means. However, due to the fragility of SEI film, its analysis and testing are easily affected by external factors, so more sophisticated and high-end characterization methods are needed for the analysis of SEI film, such as in-situ analysis methods, including in-situ infrared spectroscopy, mass spectrometry or micro-electrochemical technology, etc.
The construction of electrode/electrolyte interface layer with various high voltage electrolyte additives is still the main research method to improve the performance of electrolyte. However, the introduction of multiple additives at the same time may be incompatible with each other or even have negative effects, so it is particularly critical to study new multi-functional additives matched with the electrolyte. New multifunctional additives can be synthesized by studying the internal mechanism of action, combining additives with synergistic effect or introducing other functional groups. In addition to this, it is recommended that the number and amount of additives should be as small as possible, because the presence of additives may cause some unpredictable and complicated side reactions and add additional costs.
Studies have shown that the oxidation/reduction stability of electrolyte systems can be evaluated by density functional theory (DFT) frontier molecular orbital energy level analysis, which is very helpful to predict the basic characteristics of specific molecular structures and to find new solvents, additives, or to introduce some new functional groups into primary molecules. In addition, molecular dynamics simulation (MD) (1) can be used to predict the bulk phase and interface structure, for example, the ratio of solvent and anion in the cation solvation sheath and the formation of solvent separated ion pairs (SSIPs), contact ion pairs (CIPs) and aggregates (AGGs) can be used to screen suitable lithium salts to predict their coordination for more reasonable solvation control. (2) Statistical analysis of macroscopic properties, such as the diffusion coefficient of ions or solvents in the electrolyte, dielectric constant and viscosity; (3) to predict interfacial reactions, such as whether solvent and anions can decompose on the electrode surface and produce SEI or CEI. Therefore, it can be predicted that the theoretical calculation will also play a vital role in the future research.

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