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

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Progress in Chemistry >

2024 , Vol. 36 >Issue 10: 1456 - 1472

DOI: https://doi.org/10.7536/PC240310

综述

High-Voltage Tolerant Electrolyte for Lithium-Ion Batteries

  • Luoqian Li 1 ,
  • Mumin Rao 2 ,
  • Hong Chen 3 ,
  • Shijun Liao , 1, *
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  • 1 School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China
  • 2 Guangdong Energy Group Science and Technology Research Institute Co., Ltd., Guangzhou 511466, China
  • 3 Shenzhen Xiongtao Power Technology Co., Ltd., Shenzhen 518120, China
*e-mail:

Received date: 2024-03-06

  Revised date: 2024-05-06

  Online published: 2024-07-01

Supported by

National Natural Science Foundation of China(U22A20419)

National Natural Science Foundation of China(51971094)

Guangdong Provincial Key Research and Development Program(2023B0909060003)

Guangdong Provincial Key Research and Development Program(2020B0909040003)

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Abstract

With the rapid development of consumer intelligent electronic devices and electric vehicles, the development of lithium-ion batteries with high energy density has become a very urgent and important issue. Using high-voltage electrode materials and enhancing the work voltage of batteries is an effective pathway to realize the high energy density of battery. However, the conventional carbonate-based electrolyte will undergo oxidation reactions when the voltage is higher than 4.3 V, which will lead to electrolyte decomposition, and finally resulting in the failure of the battery. Actually, it has become one of the main bottlenecks in the development of high-voltage batteries. In order to solve this problem, researchers have carried out a lot of exploration in the design of high-voltage electrolyte in recent years, and made many important research achievements. This review introduces the failure mechanism of batteries under high voltage, and focuses on the strategies and research progress in suppressing high voltage failure from the perspective of electrolytes in recent years, indicates the challenges still existing in the design of high-voltage electrolyte, and finally prospects the future developments of high voltage lithium-ion battery electrolyte.

Contents

1 Introduction

2 Failure mechanism of high-voltage batteries

2.1 Electrolyte decomposition

2.2 Transition metal ion leaching

2.3 HF erosion

3 Progress on high-voltage electrolyte

3.1 Improvement of intrinsic stability of electrolyte

3.2 Construction of stable CEI Layer

3.3 Scavenge H2O and HF

4 Conclusion and outlook

Key words： lithium-ion batteries; high voltage electrolyte; solvent; additive; failure mechanism

Cite this article

Luoqian Li , Mumin Rao , Hong Chen , Shijun Liao . High-Voltage Tolerant Electrolyte for Lithium-Ion Batteries[J]. Progress in Chemistry, 2024 , 36(10) : 1456 -1472 . DOI: 10.7536/PC240310

1 Introduction

Lithium-ion batteries have been the preferred choice for chemical energy storage batteries since their commercialization, due to their high energy density, long cycle life, and environmental friendliness[1,2], occupying a large market share in areas ranging from portable electronic 3C products, new energy electric vehicles to large-scale energy storage power stations. The development of new types of lithium-ion batteries with higher energy density and greater safety has always been a hot research topic in the battery field. However, the rapid development of new energy vehicles, represented by pure electric vehicles in recent years, has further highlighted the urgency and necessity of solving this problem. Therefore, whether it is the research and development of key cathode and anode materials, the design of new electrolytes, or the study of battery structures and advanced battery management systems, all are crucial for the development of next-generation lithium-ion batteries with high energy density, long cycle life, and outstanding safety performance.
Enhancing the operating voltage of lithium-ion batteries is the most effective way to increase their energy density. In fact, the current cathode materials for batteries can basically adapt to the increase in battery voltage, and some high-voltage platform cathode materials have already been developed. For example, the operating voltage of lithium cobalt phosphate (LiCoPO4) can reach as high as 4.8 V[3], and that of lithium nickel manganese oxide (LiNi0.5Mn1.5O4) can reach 4.7 V[4]. However, the carbonate electrolytes currently in use undergo oxidative decomposition and chemical reactions with the cathode material when the voltage exceeds 4.3 V, leading to rapid battery failure. In other words, the biggest obstacle to increasing the operating voltage of lithium-ion batteries lies in the electrolyte. Therefore, revealing the operational mechanisms and failure mechanisms of electrolytes under high voltage, and on this basis, developing new types of lithium-ion battery electrolytes that are tolerant to high voltages and have better safety, is of great significance for promoting the advancement of lithium-ion batteries, as well as the development of new energy vehicles and the electronics industry.
In recent years, researchers at home and abroad have carried out a large amount of research work in the study and development of high-voltage electrolytes, including the optimization and improvement of existing electrolytes as well as the development of completely new electrolytes. These efforts not only designed a series of electrolyte systems with excellent high-voltage tolerance but also greatly enriched people's understanding and cognition of the failure mechanisms of lithium-ion batteries under high voltage. This article first introduces in detail the working principles of electrolytes and their failure mechanisms under high voltage. Then, from three aspects—the optimization of intrinsic stability of electrolytes, the enhancement of interfacial layer stability, and the self-removal of H2O and HF within the electrolyte—it provides an introduction and summary of high-voltage electrolyte research both domestically and internationally. At the same time, it points out the challenges and issues that still need to be addressed in the development of high-voltage electrolytes and offers a prospect and research direction for the future development of high-voltage electrolytes.

2 High Voltage Failure Mechanism of Batteries

At present, the actual working cut-off voltage of commercial layered cathode materials, such as lithium cobalt oxide and ternary cathodes, is usually below 4.3 V. Numerous studies have shown that increasing the upper limit of the cut-off voltage can significantly enhance the depth of discharge (with respect to its theoretical capacity) of the cathode material, alleviating the problem of insufficient energy density in lithium-ion batteries and further improving the range in practical use of power batteries. However, with the increase in the depth of discharge, the intensity of side reactions at the interface between the highly active cathode material and the electrolyte further increases, inducing structural changes in the cathode material and the occurrence of electrolyte decomposition reactions, which deteriorate the cycle stability and safety performance of the battery. Therefore, in response to these issues, researchers have conducted extensive studies and attributed the performance degradation of batteries under high voltage to the following three reasons: electrolyte decomposition, dissolution of transition metal ions from the cathode, and erosion by HF.

2.1 Electrolyte Decomposition

Currently, the widely used lithium-ion battery electrolyte is a mixture with lithium hexafluorophosphate (LiPF6) as the solute, and carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) as the main solvents, along with functional additives. Theoretical calculations and linear sweep voltammetry (LSV) have found that when LSV tests are conducted using an inert metal as the working electrode, carbonate-based electrolytes can withstand voltages above 5 V without decomposition[5,6]. However, when high cutoff voltage and high state of charge (SOC>80%) cathodes coexist with carbonate electrolytes, the oxidative stability of the electrolyte immediately drops below 4.3 V. This is mainly because singlet oxygen on the surface of the cathode easily provides two electrons to form chemical bonds with carbonate solvent molecules, thereby catalyzing the decomposition of the electrolyte; this process is known as nucleophilic attack, and the nucleophilic attacking ability increases with the increase in the electronegativity of the transition metals (Ni>Co>Mn)[7].
The decomposition products of these carbonate electrolytes form a cathode-electrolyte interphase (CEI) on the surface of the positive electrode, but this CEI layer is often uneven in thickness, has high impedance, and its formation is usually accompanied by the generation of CO2, CO, O2, and alkanes (Figure1) as well as the loss of active lithium in the electrolyte. This leads to a decrease in battery capacity and an uneven distribution of current on the electrode surface, making it unable to effectively protect the electrode materials and electrolyte in subsequent long cycles, thus shortening the cycle life of high-voltage lithium-ion batteries[9,10]. Studies have shown that the oxidative decomposition of electrolytes at high cut-off voltages is the primary cause leading to the failure of lithium-ion batteries.
图1 碳酸酯电解液中各组分高电压分解示意图[8]

Fig. 1 Overview of electrolyte decomposition reactions that occur at high voltages [8]

2.2 Leaching of Transition Metal Ions

As the battery charging voltage increases, the positive electrode material can dissolve more active lithium, and at the same time, the nucleophilic reaction between the positive electrode material and the electrolyte is also intensified, thereby triggering cation mixing in the positive electrode material, leading to a transition of the surface structure of the positive electrode material from a layered structure to a rock salt structure[5,11]. This change in surface structure induces the dissolution of transition metal ions in the positive electrode material[12] (Figure 2a) and the release of active oxygen (Figure 2b), not only increasing the charge transfer resistance on the positive side but also accelerating the oxidative decomposition of the electrolyte[13⇓~15]. Once transition metal ions diffuse to the negative electrode side under the influence of concentration gradient and electric field, they will be reduced in situ, disrupting the formation of the SEI, thus inducing the formation of dead lithium on the surface of the negative electrode[16], which in turn leads to a severe decline in the electrochemical performance of the battery. Although the dissolution of transition metal ions, oxygen release, and surface phase transformation in the positive electrode material are mainly determined by the intrinsic properties of the material, they are also influenced by the interactions and side reactions between the positive electrode material and the electrolyte[17].
图2 （a）不同阴极电池100圈循环后石墨阳极表面过渡金属离子含量[12]；（b）LiCoO2在不同电压下的滴定质谱和在线电化学质谱[21]；（c）EC的化学氧化机理[8]

Fig. 2 (a) Transition metal ion content on the surface of graphite anode after 100 cycles of different cathode batteries[12]. (b) TMS and OEMS results for LiCoO2[21]. (c) Proposed Mechanism for the Chemical Oxidation of Ethylene Carbonate[8]

2.3 HF Erosion

Acidic substances in the electrolyte (such as hydrogen fluoride) erode the cathode material, the CEI layer on the surface of the cathode, and the aluminum current collector at high voltages, which is one of the important factors leading to the failure of high-voltage batteries[18]. The main sources of HF are as follows: (1) Lithium salt LiPF6 in the electrolyte reacts with trace amounts of water present in the electrolyte to produce acidic substances such as HF[19] (LiPF6 → LiF + PF5; PF5 + H2O → POF3 + 2HF), and this process accelerates with increasing voltage; (2) Carbonate solvents generate water by reacting with singlet oxygen released from the cathode material under high voltage, further participating in the hydrolysis reaction of LiPF6 (Figure 2c[2c])[8]; (3) At high voltage, the PF6- anion can also extract hydrogen atoms from carbonate solvent molecules and oxidize them to produce HF[20]: PF6- + R-H → PF5 + HF + R•.
In the practical application of batteries, the above three high-voltage failure mechanisms do not exist independently but occur simultaneously and are interrelated, ultimately leading to accelerated decomposition of the electrolyte under high voltage and a severely lower battery cycle life than expected. Therefore, to meet the growing market demand for high-energy-density batteries, the development of electrolyte systems with high voltage tolerance is urgent.

3 Advances in High Voltage Electrolyte Research

The electrolyte, as the "blood" of lithium-ion batteries, undertakes the functions of solvation, desolvation, and liquid-phase diffusion of lithium ions at the positive and negative electrode interfaces. However, the solvation and desolvation processes of the electrolyte at the interface are often accompanied by unintended side reactions, and with the increase in voltage, the degree of these side reactions can become significantly more severe. Therefore, high-voltage electrolytes mainly achieve the application of high-voltage lithium-ion batteries by improving the stability of the cathode-electrolyte interface under high-voltage conditions. The main strategies for improvement can be summarized into the following 3 points.
(1) By using solvents with higher electrochemical stability to replace carbonate solvents that easily decompose under high pressure, new lithium salts to replace LiPF6, or by adopting other novel electrolyte systems, the intrinsic stability of the electrolyte components is improved from a thermodynamic perspective. In this process, some of the new lithium salts and antioxidant solvents also participate in the interfacial reactions, which helps to enhance the stability of the CEI interface.
(2) By forming a more effective and uniform SEI/CEI layer on the surface of electrode materials through sacrificial additives, it inhibits harmful side reactions caused by electron exchange between the electrode material and the electrolyte, thereby preventing the dissolution of transition metal ions from the cathode and oxygen evolution.
(3) By adding substances that can react with water and HF, the acidic substances and trace amounts of water generated in the electrolyte circulation are automatically removed, thereby inhibiting the hydrolysis of lithium salts and the erosion of HF on the CEI layer, and indirectly improving the stability of the interfacial layer. We will sequentially introduce the typical research advances in achieving high voltage tolerance through the above strategies in recent years.

3.1 Advances in Enhancing the Intrinsic Stability of Electrolytes

The lithium salt and solvent of the electrolyte account for more than 95% of the mass fraction of the electrolyte system, therefore, the intrinsic stability of the electrolyte can be improved by updating the lithium salt and solvent system, which is of great significance for the development of new high-voltage electrolyte systems.

3.1.1 High Oxidation Stability Solvents

Currently, widely studied high-voltage solvents include fluorinated solvents, nitrile solvents, sulfolane solvents, and ionic liquids, all of which exhibit better high-voltage stability than carbonate solvents.

3.1.1.1 Fluorinated Solvents

Fluorine atoms have strong electronegativity and electron-withdrawing ability[22], which can lower the highest occupied molecular orbital (HOMO) energy level of fluorinated solvent molecules, making fluorinated solvents more electrochemically stable than carbonates. Therefore, replacing or partially replacing carbonates in electrolytes with fluorinated solvents is considered one of the most important approaches to achieving high-voltage electrolytes. Many researchers have conducted extensive studies in this area[23⇓~25]. More importantly, fluorinated solvents have already been industrially produced and practically applied in high-voltage electrolytes. In addition to their excellent resistance to electrochemical oxidation, fluorinated solvents can also form a CEI layer rich in LiF. Theoretical calculations show that compared to other lithiated substances in the CEI, LiF has the widest electrochemical window, the largest Young's modulus, and a wide bandgap[26], which is an important reason why the interface layer rich in LiF can withstand higher operating voltages. Some representative research works are introduced as follows.
Cai et al.[23]improved the Li+solvation structure and thus formed an inorganic-rich CEI layer on the surface of lithium cobalt oxide cathode material by partially replacing carbonate solvents with fluorinated ethylene carbonate (FEC) and trifluoroethyl methyl carbonate (FEMC, molecular structure as shown in Figure 3a). The constant voltage leakage current test results at 4.5 and 4.6 V indicated that the fluorine-containing electrolyte after FEC and FEMC substitution had a smaller leakage current (Figure 4a), proving that the electrolyte has better stability under high voltage. Liao et al.[24]designed a high-voltage electrolyte using FEC, FEMC, and 1,1,2,3,3,3-hexafluoropropyl-2,2,3,3-tetrafluoroethyl ether (HFTFE) as solvents. This fluorinated electrolyte not only improved the average coulombic efficiency of the Li/Cu battery (97.1%) but also significantly inhibited the decomposition of the electrolyte at a high voltage of 5 V.
图3 常用（a）含氟溶剂；（b）腈类溶剂；（c）砜类溶剂；（d）离子液体的分子结构

Fig. 3 The molecular structures of some (a) fluorinated solvents; (b) nitrile-based solvents; (c) sulfone-based solvents; (d) ionic-liquid solvents

图4 (a) 使用不同电解液的Li/LCO电池在4.5和4.6 V恒压下的漏电流[23]；(b) TFP电解液在电池中的作用示意图[42]；(c) 砜类和碳酸酯分子和溶剂化分子的氧化稳定性对比[6]；(d) PP13TFSI离子液体电解液循环稳定性[43]

Fig. 4 Schematic diagram of (a) Leakage currents during 4.5 and 4.6 V constant voltage floating tests of Li/LCO cells using different electrolytes [23], (b) the effect of TFP electrolyte in the cells[42]. (c) Comparison of oxidation stability between sulfone and carbonate molecules and solvated molecules [6]. (d) Cycle performance of ionic liquid electrolyte with PP13TFS[43]

Fan et al.[27] synthesized a fluorinated sulfonate solvent: 2,2,2-trifluoroethyl trifluoromethanesulfonate (TTMS) and 2,2,2-trifluoroethyl methanesulfonate (TM) through molecular design. By introducing the -CF3 functional group and maintaining the solvation ability of the O=S=O group, the antioxidation property of the electrolyte was enhanced. The test results of a 1 Ah-NCM811/graphite pouch cell at a high voltage of 4.6 V showed that this fluorinated electrolyte still had 83% capacity after 1000 cycles, and compared to a cutoff voltage of 4.3 V, the energy density of the battery could be increased by 16%.
Fluorinated carboxylate esters, due to their relatively low cost, low melting point, and low viscosity, are often used as co-solvents in high-voltage electrolytes[28,29]. Wang et al.[30] formulated a perfluorinated electrolyte using methyl difluoroacetate (MDFA) and methyl 2,2-difluoro-2-(fluorosulfonyl)acetate (MDFSA) as solvents. Due to the relatively low donor number (DN<10) and high dielectric constant (>5) of fluorinated carboxylate esters, they can minimize the binding energy between lithium ions and the solvent while still dissociating the lithium salt. Therefore, the designed perfluorinated electrolyte can form an LiF-rich interphase at the positive and negative electrode interfaces, and a 4.5 V NCM811/graphite full cell exhibits a capacity retention rate of 80.1% after 400 cycles.
It is worth noting that although fluorinated electrolytes have excellent electrochemical stability, the cost is still too high, which limits the application of such electrolytes in high-voltage batteries. Therefore, developing low-cost fluorinated solvents and high-voltage electrolytes with low fluorine content are important research topics in this field.

3.1.1.2 Nitrile Solvents

The introduction of nitrile substances can significantly improve the electrochemical window of electrolytes, making nitrile substances a very promising class of solvents for high-voltage electrolytes. Common nitrile substances such as succinonitrile (SN, with a molecular structure as shown in Figure 3b)[31,32], adiponitrile (ADN)[33⇓~35], glutaronitrile (GLN)[36], and butyronitrile (BN)[37] have gradually received widespread attention. Moreover, these substances also possess a high dipole moment and dielectric constant, allowing them to dissociate various types of lithium salts just like carbonate solvents[38].
Researchers generally believe that the reason nitrile solvents can widen the electrochemical window of electrolytes is due to the strong coordination ability of the cyano groups in nitriles, which can coordinate with transition metal ions on the cathode, thereby reducing side reactions at the interface[33], and repelling other components from coming into contact with the cathode surface. However, Xing et al.[32] proposed a different mechanism when investigating SN as an electrolyte solvent. The authors found that after a Co3O4 electrode soaked in an SN-containing electrolyte was washed clean and subjected to LSV testing again, it still underwent severe oxidation reactions with the carbonate electrolyte, suggesting that the mechanism may not be due to adsorption and coordination; another Co3O4 electrode, pre-scanned to 4.5 V at a low current in an SN-containing electrolyte, was washed clean and then tested for LSV with a base carbonate electrolyte, resulting in a significant inhibition of oxidative decomposition with the carbonate electrolyte, indicating that a CEI layer derived from the SN-containing electrolyte had formed on its surface, effectively preventing side reactions during subsequent scans.
However, nitrile substances have poor compatibility with current battery anodes, mainly manifested in the reduction and decomposition of the cyano group at low potentials, and the inability to form a stable SEI film[39], which limits the further commercial application of nitrile solvents. Therefore, nitrile substances are currently mainly used as co-solvents (additives) for carbonate solvents rather than being used alone. For example, Abu-Lebdeh et al.[40] explored the changes in voltage windows after mixing a series of dinitrile solvents NC-(CH)n-CN (n=3~8) with carbonate solvents. The electrochemical window of a single dinitrile electrolyte can be as high as 7 V, and after adding carbonate solvents, the electrochemical window decreases slightly but still remains within 6~6.5 V. Ehteshami et al.[41] used adiponitrile (ADN) as a co-solvent for the electrolyte and LiDFOB as the lithium salt; electrochemical test results showed that this electrolyte did not exhibit significant oxidation reactions below 5 V.
Compared to dinitrile solvents, acetonitrile (AN) has a lower cost and is advantageous in practical applications. However, AN has poorer reductive stability and easily undergoes strong side reactions with lithium metal. In recent years, researchers have enabled its application by modulating its structure or altering other components in the electrolyte. Peng et al.[44] addressed the severe side reactions between AN and lithium metal through salt concentration control and the use of VC as a film-forming additive, ultimately forming a stable interfacial film based on cross-linked polycarbonate and lithium fluoride on the surface of lithium metal.
Fan et al.[45]used fluoroacetonitrile (FAN) with low dissolution energy as the electrolyte solvent. Due to its low Li+transfer energy barrier and smaller solvation sheath volume, it can form a fast lithium ion diffusion channel within the electrolyte. The prepared electrolyte has an ultra-high ionic conductivity (25 ℃: 40.3 mS/cm; -70 ℃: 11.9 mS/cm), and also achieved stable cycling of a 1.2 Ah-NCM811/graphite battery under extreme conditions such as 4.5 V and -50 ℃. Zhang et al.[42]reduced the molecular polarization by using methyl substitution for the α-H atom of acetonitrile. As shown in Figure 4b, after methyl substitution, the PN solvent improved the reduction stability of the electrolyte, which is conducive to forming a stable interfacial film at the electrode interface. Lee et al.[46]used SN and AN as electrolyte solvents, achieving stable cycling of a 4.9 V LiNi0.5MN1.5O4/Li battery system, with no significant capacity decay after 100 cycles. From the current research results, it can be seen that nitrile substances are excellent co-solvents (additives) for high-voltage electrolytes. Further in-depth studies on this topic are of great significance for designing high-performance high-voltage electrolytes and understanding the high-voltage tolerance mechanism of nitrile substances.

3.1.1.3 Sulfone Solvents

Compared to the carbonyl group in carbonate solvents, the sulfonyl group has a stronger electronegativity and is less likely to lose electrons; therefore, sulfone solvents have higher electrochemical stability than carbonate solvents[47]. In addition, compared to other solvents, sulfone solvents possess characteristics such as high dielectric constant, low flammability, and good compatibility with cathodes. Xing et al.[6] explored through DFT calculations the oxidation pathways of three carbonate solvent molecules and eleven sulfone solvent molecules in the presence of anions and other solvent molecules in the electrolyte, as shown in Figure 4c. The results indicated that the oxidation potential of a single sulfolane (SL) molecule is lower than that of carbonate solvent molecules. However, in the presence of other active components in the battery system, sulfone molecules exhibit higher stability, which is a key reason why sulfone-based electrolytes have a wider electrochemical window compared to carbonate-based electrolytes.
However, similar to nitrile solvents, the application is limited by high melting points, high viscosity, and poor anode compatibility. This can be addressed by mixing it with other solvents[48]or bridging with other functional groups[49]. For example, Su et al.[49]demonstrated the importance of fluorination position for solvent properties, successfully solving the issue of severe decomposition of sulfone solvents on the graphite surface through the synthesis of β-fluorinated sulfone (TFPMS). The experimental results showed that the electrolyte could form a stable interface on both NCM622 and graphite surfaces, and after 400 cycles at a high voltage of 4.5 V, the full cell could still maintain more than 71% of its capacity retention.
Although obtaining sulfone solvents with specific structures and functions through bridging functional groups is possible, practical applications still face various challenges such as difficult synthesis and high costs, making it more practical to mix them with other solvents[50]. For example, Dai et al.[51] used sulfolane (SL) as the main solvent in the electrolyte. Compared to carbonate electrolytes, this electrolyte can increase the oxidation potential to 5.1 V, and after 150 cycles, the capacity retention rate of the 4.7 V NCM811/Li battery increased from 71.6% to 84.85%. He et al.[48] mixed tetramethylene sulfone (TMS) with FEC. Due to the preferential adsorption of sulfone solvents on different electrodes, this electrolyte has the lowest Li+ bond energy and the highest transition metal adsorption energy reported so far, effectively limiting the dissolution of transition metals under high voltage. The NCM811/Li battery using this electrolyte, when cycled 500 times at a cut-off voltage of 4.4 V and a current density of 0.5 C, showed a capacity decay of only 0.028% per cycle.

3.1.1.4 Ionic Liquids

Ionic liquids are a type of room-temperature molten salt entirely composed of cations and anions, with most ionic liquids being in a liquid state below 100 ℃[52,53]. Due to their characteristics such as high dielectric constant, high flash point, and wide electrochemical window[52], there have been some research reports in recent years on the application of ionic liquids as high-voltage electrolytes. For example, Sun et al.[54] found that the electrochemical stability of 1-ethyl-3-methylimidazolium ([emim]+)-based ionic liquids (with molecular structures as shown in Figure 3d) highly depends on the nature of their anions. Hayyan et al.[55] investigated various combinations of anions and cations to determine the specific impact of ionic liquid structure on the electrochemical window. They discovered that ionic liquids with TFSI− anions exhibit higher oxidative stability compared to those with other anions (such as TfO− and TFA−). Chang et al.[56] found that by adjusting the molar ratio of FSI−/TFSI− to 1:3, the electrochemical window of the ionic liquid electrolyte could be expanded to 5 V.
In addition to altering the anion to adjust the electrochemical window of ionic liquids, hybrid ionic liquid electrolytes can also be designed by introducing carbonates, ethers, and other organic solvents. For instance, Wang et al.[43] successfully constructed a solvation sheath dominated by anions, reducing the free solvent molecules in the electrolyte, by introducing the piperidine ionic liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI) into ether-based electrolytes, which extended the upper voltage limit of NCM811/Li batteries to 4.5 V (Figure 4d) and exhibited excellent cycling performance.

3.1.2 Novel Lithium Salts

Apartfromsolvents,thepropertiesoflithiumsaltsanionsarealsocriticaltotheelectrolyte'sphysicochemicalproperties,interfacecomposition,andsolvationstructure.Therefore,moreandmoreresearchersarefocusingondevelopingnewlithiumsaltstoalternativetoliPF6foruseinhigh-voltageelectrolytes.LiFSI(lithiumbis(fluorosulfonyl)imide)andLiTFSI(lithiumbis(trifluoromethanesulfonyl)imide)possesshigherthermalstability,conductivity,andmoistureresistancecomparedtoliPF6 [57].Furthermore,Wangetal.[58]discoveredthattheelectrolyteswithLiFSIasthemainsaltexhibitbettercyclicstabilityandratecapabilityathighvoltages,thusattractingconsiderableresearchinterestfortheirapplicationinhigh-voltageelectrolytes.

3.1.2.1 AI Current Collector Corrosion

However, sulfone-based lithium salts such as LiFSI and LiTFSI, despite their aforementioned advantages, tend to corrode the Al current collector during battery charging and discharging, leading to increased internal resistance and the detachment of the active layer. In lithium-ion batteries, the oxidation potential of pure Al is 1.39 V, but a natural layer of Al2O3 on the surface of the Al current collector can protect the Al from corrosion at higher voltages[59]. Furthermore, in electrolytes with LiPF6 as the lithium salt, trace amounts of water inevitably induce the hydrolysis of LiPF6 to produce HF, which further reacts with Al2O3 to form AlF3, creating a third fluorine-containing passivation layer on the surface of the Al current collector. This layer can prevent further corrosion of the aluminum current collector at high voltages[59,60]. Therefore, the corrosion of the aluminum current collector under high voltage conditions is not very severe when using an electrolyte with LiPF6 as the main salt. However, when the electrolyte uses LiFSI and LiTFSI as the primary lithium salts, FSI− and TFSI− continuously form Al(FSI)3 and Al(TFSI)3 compounds with Al3+ and dissolve in carbonate solvents[61], causing continuous exposure and dissolution of the Al current collector surface (Figure 5a and 5b), ultimately resulting in severe Al corrosion[62,63]. Compared to LiTFSI, LiFSI has a slightly higher Al corrosion potential, possibly because the S—F bond in LiFSI is less stable than the C—S bond in LiTFSI, making it more prone to cleavage and passivation of the Al current collector. This also makes LiFSI more widely used in high-voltage electrolytes compared to LiTFSI. Additionally, studies have reported that residual chloride ion impurities in LiFSI production can also induce corrosion of the Al current collector, so the amount of these impurities needs to be strictly controlled[64].
图5 （a）LiPF6和LiFSI对Al集流体作用示意图；（b）3~4.6 V LiFSI电解液循环100圈后NCM811颗粒和Al集流体的SEM图[63]；（c）传统电解液和LHCE对Al腐蚀的影响示意图[61]

Fig. 5 (a) Schematic diagram of the effect of LiPF6 and LiFSI on Al current collector [65]; (b) SEM of NCM811 particles and Al collector after 100 cycles of 3~4.6V with LiFSI electrolyte [63]; (c) Schematic diagram of the influence of traditional electrolytes and LHCE on Al corrosion[61]

3.1.2.2 Local High-Concentration Electrolyte

As Al corrosion is caused by the side reaction between anions in the electrolyte and the current collector, adjusting the composition of the electrolyte is the most direct and effective way to suppress Al corrosion. In recent years, increasing the concentration of lithium salts and introducing diluents to form localized high-concentration electrolytes (LHCE) have been hot topics in the field of electrolyte research[65⇓~67], and LiFSI, with its high degree of dissociation, is often used as the main lithium salt for LHCE. Compared to traditional carbonate electrolytes, the solvation structure of LHCE can change from the conventional solvent-separated ion pair configuration to contact ion pairs (CIP, where the anion coordinates with one lithium ion) and aggregated ion pairs[66] (AGG, where the anion coordinates with two or more lithium ions). Furthermore, Fan et al.[67] found that this change in coordination structure can improve the oxidative stability of the electrolyte, which is attributed to a reduction in the number of free-state solvent molecules that are prone to oxidation in the electrolyte.
In addition to eliminating uncoordinated solvent molecules in the electrolyte, anions will replace solvent molecules at the electrode interface due to increased coordination with lithium ions, forming a passivation layer rich in inorganic substances[68], thus avoiding side reactions of solvent molecules at the interface and inhibiting the corrosion of the Al current collector (Figure 5c). For example, Choi et al. [69] designed a locally high-concentration electrolyte using 1 mol/L LiFSI as the lithium salt, with glyme (DME) and 2,2-bis(trifluoromethyl)-1,3-dioxolane (BTFMD). The authors tested the LSV of Li/Al cells, finding no oxidation current even at voltages up to 6 V, and the surface of the aluminum current collector remained smooth, indicating significant inhibition of Al current collector corrosion. Ren et al. [70] introduced a diluent, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), into the high-concentration LiFSI-tetramethylene sulfone electrolyte, maintaining an average CE above 99% during cycling, and significantly reducing the corrosion current of aluminum. This is attributed to the participation of diluent molecules in the formation of a protective layer on the Al current collector, effectively preventing the decomposition of the electrolyte and Al corrosion, thereby achieving stable cycling of a 4.9 V LNMO/Li battery.

3.1.2.3 Multi-Salt Electrolyte

In addition, by adding one or more lithium salts capable of passivating aluminum foil to the electrolyte with LiFSI as the main salt, it not only alleviates the corrosion of LiFSI on the Al current collector at high voltage but also improves the high-voltage stability of the electrolyte. For example, lithium tetrafluoroborate (LiBF4) has been found to react with Al at high voltages to form a protective layer containing AlF3 and Al(BF4)3[71,72]. Based on this, Rummeli et al.[73] designed a high-voltage electrolyte using LiBF4 and LiFSI as the primary lithium salts. In this, the introduction of LiBF4 passivated the Al current collector and inhibited the side reactions of imide anions in LiFSI at high voltages. Moreover, this dual-salt electrolyte facilitated the formation of a uniform and thin inorganic passivation layer on the surface of the Li1.2Mn0.54Ni0.13Co0.13O2 cathode and the lithium metal anode, reducing the voltage decay of the lithium-rich material during cycling. This allowed the battery to maintain a capacity retention rate of 80.4% after 400 cycles at 4.8 V and 1 C. Hu et al.[72] added 2% by weight of lithium difluoro(oxalato)borate (LiDFOB) to the LiFSI-based electrolyte, and the CV test results showed that the oxidation current representing aluminum corrosion in the electrolyte without LiDFOB was ten times that of the electrolyte with LiDFOB, indicating that the introduction of LiDFOB can alleviate the corrosion of Al foil by LiFSI at high potentials.
Similarly, Zhang et al[60] demonstrated that adding only 0.05 mol/L LiPF6 to a LiFSI-LiBOB dual-salt electrolyte can completely inhibit the corrosion of Al current collectors. Ouyang et al[74] used 0.8 mol/L LiFSI, 0.1 mol/L LiTFSI, and 0.6 mol/L LiPF6 as electrolyte lithium salts, resulting in a more stable inorganic component on the cycled NMC811 surface, uniformly distributed with F, N, and S elements, all derived from the decomposition of anions FSI- and TFSI-. The capacity retention rate of a 4.5 V-NCM811/graphite pouch full cell after 200 cycles was improved to 82.1%, which is 8 times higher than that of traditional carbonate electrolytes in terms of cycling stability.
The high conductivity, cost similar to LiPF6, and excellent electrochemical performance of LiFSI make it promising for widespread application in the context of current power batteries that generally require high energy density and ultra-fast charging. However, its corrosion of Al current collectors cannot be overlooked. Among the current mitigation strategies, directly introducing a lithium salt capable of passivating the Al current collector as a secondary main salt in the electrolyte is undoubtedly the simplest and most feasible method. Nevertheless, under different cutoff voltages, the types, proportions, and extent of impact on Al corrosion of the introduced passivating lithium salts, as well as their specific effects on electrochemical cycle life, still need further exploration and clarification.
Table 1 lists several typical research results that can enhance the intrinsic stability of electrolytes. From these research outcomes and electrolyte formulations, it can be observed that many electrolyte systems have good high-voltage tolerance; secondly, changes in the electrolyte system are not limited to a single modification but can involve multiple methods in parallel. For example, in the case of locally concentrated electrolytes, when the lithium salt is LiFSI, the solvent system may include any one of carbonate, fluorinated solvents, nitriles, sulfones, or ionic liquids. The key lies in how to improve the high-voltage stability of the electrolyte without deteriorating other performances (such as rate, temperature, and safety) under all scenarios and throughout the entire lifecycle. However, the new types of electrolyte systems summarized so far more or less suffer from issues such as compatibility with both positive and negative electrodes and deterioration of the physical and chemical properties of the electrolyte. Among them, high-voltage electrolytes using fluorinated solvents have comprehensive performance advantages over other electrolyte systems, and some have already entered industrial production. Nevertheless, compared to carbonate electrolytes, their cost remains significantly higher. Therefore, reducing the cost of electrolytes is still the most critical issue that needs attention. It is believed that with the optimization of synthesis processes and the development of the power battery market, the cost reduction and commercialization of fluorinated electrolytes will be further promoted.
表1 新型电解液体系电化学性能

Table 1 Electrochemical performance of novel electrolyte

Strategy Electrolyte Electrochemical system Cut-off Voltage/V Capacity Retention Ref
Fluorinated 2 mol/L LiPF6-
DMC/FEC/FEMC		 LiCoO2/Graphite 4.5 74.2%(0.5 C,270 cycles) 23
2 mol/L LiPF6-
DMC/FEC/FEMC		 LiNi0.5Mn1.5O4/Li 5 89.9%(0.5 C,300 cycles) 23
1 mol/L LiPF6-FEC/FEMC/HFTFE LiNi0.92Co0.04Mn0.04O2/Li 5 68%(1 C, 500 cycles) 24
1.2 mol/L LiPF6-FEC/DFEC/FEMC NCM811/Li 4.4 90.8%(0.5 C, 200 cycles) 25
1 mol/L LiPF6-FEC/MTFP NCM811/Li 4.5 80%(0.5 C, 250 cycles) 29
1 mol/L LiTFSI-
MDFA/MDFSA/TTE		 NCM811/Graphite 4.5 80.1%(0.5 C, 400 cycles) 30
Nitrile 1 mol/L LiDFOB-ADN/DMC NCM111/graphite 4.5 77%(0.5 C, 40 cycles) 41
10 mol/L LiFSI-AN+VC NCM111/Li 4.5 85%(3.6 mAcm-2, 40 cycles) 44
1 mol/L LiTFSI-FEC/PN NCM622/Li 4.5 75.3%(1 C, 300 cycles) 42
LiFSI-SN/AN LiNi0.5Mn1.5O4/Li 4.9 90.4%(0.1 C,100 cycles) 46
NCM811/Li 4.4 73%(0.1C, 200 cycles) 46
Sulfone 1.2 mol/L LiPF6-FEC/TFPMS NCM622/graphite 4.5 71%(0.5 C, 400 cycles) 49
1 mol/L LiPF6-EMS/DMC LiNi0.5Mn1.5O4/Li 4.9 97%(0.2 C, 100 cycles) 50
1 mol/L LiFSI-SL/FEC/HFE NCM811/Li 4.7 84.95%(0.5 C, 150 cycles) 51
1 mol/L LiTFSI-TMS/FEC NCM811/Li 4.4 86%(0.5 C, 500 cycles) 48
Ionic-Liquid 2.4 mol/L LiTFSI/PMP-FSI LNMO/graphite 5 85%(0.5 C, 200 cycles) 56
1 mol/L LiFSI+0.3 mol/L LiNO3-PP13TFSI/DME NCM811/Li 4.3 ~72%(5 C, 600 cycles) 43
LHCE LiFSI-DME/FEC/PFPN NCM811/ graphite 4.6 89.8%(0.33 C, 300 cycle) 65
2.0 mol/L LiFSI/DME-BTFMD NCM811/Li 4.4 87%(1 C, 250 cycle) 69
1 mol/L LiFSI/TMS-TTE LNMO/Li 4.9 ~93%(1 C, 100 cycles) 70
Dual Salt 1 mol/L LiPF6+0.1 mol/L LiFSI+0.1 mol/L LiBF4 Li1.2Mn0.54Ni0.13Co0.13O2/Li 4.8 80.4%(1 C, 400 cycle) 73
0.8 mol/L LiFSI+0.1 mol/L LiTFSI+
0.6 mol/L LiPF6-EMC		 NCM811/graphite 4.5 82.1%(1 C, 200 cycle) 74

3.2 Advances in Constructing Stable Cathode CEI Layers

By introducing film-forming additives, a uniform passivation layer can be formed in situ at the interface between the electrode material and the electrolyte. This method requires a small amount of additive, is cost-effective, and does not conflict with existing electrolyte systems, making it the simplest and most feasible way to improve the high-voltage stability of electrolytes. Currently, there are many types of additives available, and additives with different functional groups can form CEI layers with specific functions. Therefore, choosing different combinations of additives has a significant impact on inhibiting the dissolution of transition metal ions, enhancing the high-voltage stability, performance under high and low temperatures, high rate capability, and safety of batteries.

3.2.1 Phosphate Additives

Phosphorus-containing compounds can capture highly reactive radicals in chemical reactions and are widely used as flame retardant additives. Later, researchers found that phosphorus-containing additives not only have flame-retardant effects but also participate in the formation of the CEI interfacial film, inhibit the dissolution of transition metal ions, and effectively improve the high-voltage resistance of electrolytes[75].
Zheng et al.[76] designed and synthesized a new class of pentacyclic asymmetric phosphoramides (CPAs, molecular structure as shown in Figure 6a) as electrolyte additives. These phosphoramidic molecules, featuring a pentacyclic ring with an asymmetric amine structure, easily undergo ring-opening polymerization under high voltage to form a polymer cathode-electrolyte interphase with certain mechanical strength, thereby alleviating the dissolution of transition metal ions and the oxidation of the electrolyte at high voltages. Thanks to these advantages, the electrolyte containing the asymmetric EMPA additive significantly improved the cycling stability and Coulombic efficiency of 4.5 V NCM111/Gr pouch lithium-ion batteries.
图6 （a）磷酸盐添加剂；（b）硼酸盐添加剂；（c）含硫添加剂；（d）其他添加剂的分子结构

Fig. 6 The molecular structures of additives (a) phosphorous; (b) boronated; (c) S-containing; (d) others

Deng et al[77]reported a currently available tripropargyl phosphate (TPP) additive with the highest degree of unsaturation. Due to the high reactivity of TPP, it undergoes redox decomposition at both the positive and negative electrode surfaces under a high voltage of 4.6 V, forming an excellent interfacial layer. Battery test results show that adding only 1% by mass significantly improves the cycling performance of the NCM523/AG battery system, with the capacity retention rate increasing from 2.9% to 88.2%. In-situ electrochemical mass spectrometry (OEMS) and scanning electron microscopy (SEM) results further indicate that TPP can also reduce the release of reactive oxygen species during cycling, thereby preventing the decomposition of the electrolyte.
Another important phosphorus-containing additive is the lithium salt additive containing phosphorus, such as the classic lithium difluorophosphate (LiDFP) additive[78,79], which can reduce the battery impedance and improve the cycle life of the battery during the cycling process. Later, Choi et al.[80] designed a new type of lithium difluorobis(oxalato)phosphate (LiDFBP) additive based on this, which was applied in a 4.6 V lithium-rich battery system. The CEI film derived from LiDFBP (Figure 7b) can inhibit the voltage decay and surface structural phase transition of lithium-rich materials, allowing the battery to maintain a capacity retention rate as high as 90% after 100 charge-discharge cycles at 0.5 C.
图7 （a）CPA作用机理示意图[76]；（b）LiDFBP衍生的SEI作用原理图[80]；（c）LiBOB作用原理图[88]

Fig. 7 Schematic diagrams of (a) the electrolyte with CPA[76]; (b) the beneficial effects of the LiDFBP-derived SEI layer on a Li-rich cathode[80]; (c) the electrolyte with LiBOB[88]

3.2.2 Borate Additives

The B atoms in boron-containing additives are in an electron-deficient state[81,82], which can complex with the anions in the electrolyte, thereby increasing the dissociation degree and conductivity of the lithium salt. In addition, boron-containing additives can form B—O bonds with highly reactive oxygen free radicals, effectively anchoring TM ions, and have a high HOMO energy level to form a stable interfacial layer[83]. Among these boron-containing additives, lithium tetrafluoroborate (LiBF4)[84], lithium bis(oxalate)borate (LiBOB)[85], and lithium difluoro(oxalate)borate (LiDFOB, molecular structure as shown in Figure 6b)[86,87] have received significant attention from researchers and have been successfully commercialized. For example, Meng et al.[85] found that when LiBOB is used as an additive, it can form a uniform interface on the surface of the cathode and inhibit the dissolution of transition metal ions, thus avoiding the H2→H3 phase transition of the electrode material during cycling and reducing the dissolution and redeposition of transition metal ions on graphite.
Stefanod et al.[88] further explored the decomposition mechanism and interfacial action of LiBOB additives under high voltage. As shown in Figure 7c, the BOB− anion in the LiBOB additive first loses electrons under high voltage, leading to the breaking of B—O bonds, that is, ring-opening reactions. Then, the unstable C2O4 group undergoes further oxidative decomposition to produce CO2 gas. Another part of the boron-containing decomposition products (1OB) complexes with EC to form a 1OB-EC complex, eventually forming a protective layer containing boron polymers on the electrode surface. Huang et al.[89] found that after introducing LiDFOB and LiPO2F2 into the electrolyte, the boron-containing lithium salts also help to construct solvation sheaths rich in multiple anions. The inorganic-rich interfacial layer formed can resist intense interfacial reactions under high voltage, thus significantly improving the stability of the 4.7 V NCM811 battery system.
Ma et al[90] developed a new boron-containing additive, pentafluorophenylboronic acid (PFPBA). The electrolyte containing this additive decomposes at high voltage to form a CEI layer with an inner layer of LiF and an outer layer of LiBxOy. Among them, LiBxOy not only has a high Li+ ionic conductivity but also has a low solubility in carbonates, thus improving the stability of the CEI. The electrolyte containing 5% by mass of PFPBA achieved a capacity retention rate as high as 91.2% after 400 cycles in a 4.6 V NCM622/Li high-voltage battery system.

3.2.3 Sulfur-Containing Additives

Sulfur-containing additives, due to their lower cost and the interfacial layer formed after decomposition, which contains ROSO2Li, Li2SO3, and Li2SO4and has high ionic conductivity, are commonly used as electrolyte film-forming additives[91]. Common sulfur-containing additives such as 1,3-propanesultone (PS)[92,93], 1,3-propenesultone (PST)[94], and ethylene sulfate (DTD)[95]have also been widely used in commercial electrolytes, Figure 6cshows the molecular structures of these additives.
Sun et al.[93] found that PS can suppress the structural changes and interfacial side reactions of high-nickel cathodes at high voltages by constructing an interface layer rich in Li2SO3 on the electrode material interface. The capacity retention rate of NCM811/Li batteries with 1 wt% PS electrolyte increased from 58% to 80.8% after 400 cycles at 0.5 C. Deng et al.[96] systematically investigated the impact of structural changes on DTD and electrolyte stability, obtaining two DTD derivatives, M-DTD and P-DTD, through methyl substitution and propyl substitution. Comparative results of the three additives showed that electrolytes containing M-DTD and P-DTD have higher oxidation stability and high-temperature cycling stability than those containing DTD, with the electrochemical window widened from 5.5 V to 5.75 V, and the capacity retention rate after 900 cycles at high temperature could be improved to 85%. Chou et al.[97] discovered that the methyl methylenedisulfonate (MMDS)-PF6− cluster exhibits stronger interactions and a lower oxidation potential compared to the PF6−-solvent cluster, indicating that MMDS can preferentially decompose to form an interfacial layer through strong interaction with PF6− anions (Figure 8a).
图8 （a）MMDS作用机理[97]；（b）NTSAS作用机理[99]；（c）含有EVS+FEC添加剂的电解液性能[100]；

Fig. 8 Schematic diagrams of the electrolyte (a) with MMDS [98]; (b) with NTSA [99]. (c) Performance of electrolytes containing EVS+FEC additives [100]

In addition to sulfates and thiophene substances, sulfur-containing sulfones[98] and sulfonyls are also commonly used as additives in high-pressure electrolytes. For example, Hu et al.[99] reported a multifunctional electrolyte additive N-tert-butyl-2-thiophenesulfonamide (NTSA) with solvation structure regulation and electrode/electrolyte interface regulation. Molecular dynamics simulation (MD) results showed that the Li+ coordination number with the solvent decreased in the electrolyte containing NTSA, thus forming an electrode interface rich in LiF, Li3N, and Li-S compounds, where Li3N has a high adsorption energy and low ion diffusion barrier, which is conducive to the rapid transmission of lithium ions (Figure 8b). Zhang et al.[100] formed a CEI layer mainly composed of −SO2 and LiF at the interface by adding two highly adsorptive additives, ethyl vinyl sulfone (EVS) and FEC, widening the electrochemical window of the electrolyte to 5.8 V; after 300 cycles at 1 C, the 4.8 V Li1.170Ni0.265Co0.047Mn0.517 O2/Li high-voltage battery still maintained a high capacity retention rate of 97% (Figure 8c). Fan et al.[101] found that bis-tetrafluorobenzene sulfone (BFS) continuously reduced/oxidized to alleviate the release of active oxygen during charging/discharging by forming a reversible SO42−/S2O32− redox pair, thereby inhibiting the decomposition of the electrolyte.

3.2.4 Other Additives

Lai et al.[102] designed a series of CHHIs materials to be used as electrolyte additives by extending the length of the terminal alkyl groups of cyclohexane-1,2,3,4,5,6-hexaimine (CHHI). Among these additives, hexabutylcyclohexane-1,2,3,4,5,6-hexaimine (HBCHHI) forms an LixN-rich CEI layer at the interface due to the synergistic effect of n-butyl and imino groups, which has high interfacial energy and low Li diffusion barrier, capable of inhibiting transition metal dissolution and promoting uniform deposition of lithium ions at the battery interface (Figure 9a and 9b). Meanwhile, LSV results show that the electrochemical window of carbonate electrolytes can be broadened to 4.7 V after introducing the additive.
图9 （a）空白电解质和（b）含HBCHHI的电解质中SEI和CEI的保护机制示意图[102]；（c）ADMF添加剂作用机理[103]；（d）在0.5C、3.0~4.55 V含或不含ADMF的LCO/MCMB全电池的循环性能和库仑效率[103]

Fig. 9 Schematic illustration of the protection mechanism of the SEI and CEI in (a) blank electrolyte and (b) HBCHHI-contained electrolyte [102]. (c) Schematic diagram of the mechanism of ADMF additive [103]. (d) Cycling performance and Coulombic efficiency versus cycle number of LCO/MCMB full-cells with or without ADMF at 0.5 C at 3.0~4.55 V [103].

Guo et al.[104]used succinonitrile (SN) and cyclohexylbenzene (CHB) as co-additives. After adding 1 wt% SN and 0.1 wt% CHB, the 4.6 and 4.7 V NCM811/Li batteries cycled for 500 and 600 cycles, respectively, with remaining capacities as high as 77.4% and 72.3%. This is due to the strong coordination between the cyano groups in SN and transition metal ions, and because both SN and CHB can adsorb on the surface of the cathode at high voltages, forming a CEI film with a Young's modulus up to 30 GPa. This indicates that the CEI film not only anchors the transition metal ions, inhibiting their dissolution, but also has certain mechanical strength, which can alleviate the volume changes and phase transition processes of the electrode.
Fan et al[103]reported a multifunctional additive, 7-anilino-3-diethylamino-6-methylfluoran (ADMF). Firstly, the carbonyl group in ADMF can preferentially adsorb on the electrode surface at the beginning of charging. Secondly, the N—H groups can capture reactive oxygen radicals (1O2and O2•−) in the electrolyte, avoiding the oxidative decomposition of the electrolyte under high voltage (Figure 9c). Additionally, the benzene ring in the additive molecule will undergo further electro-polymerization to form a thin CEI layer under high pressure. Therefore, the addition of ADMF significantly enhanced the cycling stability of 4.4 V NCM811/MCMB and 4.55 V LiCoO2/ MCMB full cells. Lu et al[105], by adding electron-deficient tris(pentafluorophenyl)borane (TPFPB), disrupted the LiNO3clusters and manipulated the center of the solvation structure, allowing the NO3 -in the solvation structure to be reduced on the lithium metal anode, forming a robust SEI containing Li2O externally. Meanwhile, TPFPB could decompose on the cathode to form a CEI interfacial layer containing fluorine and boron, alleviating issues such as cathode surface structural reconstruction, active lithium loss, and electrolyte decomposition at high voltages.
Table 2 summarizes the performance of electrolytes containing different additives under high voltage. Additives regulate the composition of the CEI through preferential oxidation or other means at the electrode interface, avoiding side reactions between the electrodes and the electrolyte, thereby improving the cycle life of high-voltage batteries. However, currently, a single high-voltage additive cannot meet the multifunctional requirements of electrolytes. Therefore, exploring combinations of different additives and evaluating additives in practical applications is crucial for the future development of additives.
表2 高压电解液成膜添加剂电化学性能

Table 2 Electrochemical performance of typical additive in high-voltage electrolyte

Additive Electrolyte Electrochemical system Cut-off
Voltage/V		 Capacity Retention Ref
Phosphorous additive 1 mol/L LiPF6-EC/DEC+0.5wt%EMPA NCM523/Gr 4.5 63% (0.5 C, 400 cycles) 76
NCM111/Gr 4.5 200 cycles)
4.6 300 cycles)
1 mol/L LiPF6-EC/DEC+1%TPP NCM523/AG 4.6 88.2%) 77
1.2 mol/L LiPF6-EC/EMC+1%TTFP NCM523/graphite 4.6 99.7% (C/3, 50 cycles) 75
1.3 mol/L LiPF6-EC/EMC/DMC+1%LiDFBP Li1.17Ni0.17Mn0.5Co0.17O2/Li 4.6 90% (0.5 C, 100 cycles) 80
Boronated additive 1 mol/L LiPF6-EC/EMC+0.1%TEAB NCM811/Li 4.3 63.2% (100 cycles) 81
1.3 mol/L LiPF6-EC/EMC/DMC+1%LiDFOB Li1.17Ni0.17Mn0.5Co0.17O2/graphite 4.7 82.7% (0.5 C, 100 cycles) 82
1 mol/L LiPF6-EC/DMC+2%LiBOB Li1.18Ni0.18Mn0.55Co0.09O2/graphite 4.7 89.5% (0.2 C, 200 cycles) 85
1.0 mol/L LiPF6-EC/DMC+1%LiBOB LiNi0.83Co0.11Mn0.05B0.01O2/Li 4.6 73.1% (1 C, 200 cycles) 88
LiDFOB+LiPO2F2+LiPF6-EMC/DMC NCM811/Li 4.7 80% (1 C, 500 cycles) 89
1 mol/L LiPF6-EC/EMC+5%PFPBA NCM622/Li 4.6 91.2% (1 C, 400 cycles) 90
S-containing additive 1 mol/L LiPF6-EC/EMC +1%PS NCM811/Li 4.45 80% (0.5 C, 200 cycle) 93
1 mol/L LiPF6-EC/EMC +1%P-DTD NCM811/graphite 4.4 >90% (1 C, 500 cycles) 96
1 mol/L LiPF6-EC/EMC +1%MMDS NCM523/graphite 4.5 92.78% (1 C, 800 cycle) 97
1 mol/L LiPF6-EC/EMC +1%MMDS NCM523/graphite 4.6 800 cycle)
1 mol/L LiPF6-FEC/EMC/DMC +1% NTSA LiCoO2/ω-LVO 4.35 94.5% (200 cycle) 99
1 mol/L LiPF6-EC/EMC/DMC +FEC+EVS Li1.17Ni0.265Co0.047Mn0.517O2/Li 4.8 97% (1 C, 300 cycles) 100
1 mol/L LiPF6-EC/DMC +0.5%BFS LiCoO2/Li 4.6 88% (1 C, 300 cycles) 101
Other additive 1 mol/L LiPF6-EC/EMC/DEC +5m mol/L HBCHHI NCM811/Li 4.7 63.6% (5 C, 500 cycles) 102
1 mol/L LiPF6-EC/DMC/DEC +1%SN+0.1%CHB LiCoO2/Li 4.6 77.4% (1 C, 500 cycles) 104
NCM811/Li 4.7 600 cycles)
1 mol/L LiPF6-EC/DMC +1%ADMF LiCoO2/MCMB 4.55 87% (0.5 C, 750 cycles) 103
NCM811/MCMB 4.4 750 cycles)
1 mol/L LiPF6-FEC/EMC +1%TPFPB+3wt%LiNO3 LiCoO2/Li 4.6 89.8% (0.2 C, 160 cycles) 105

3.3 Study on Self-Cleaning of O and HF2O和HF自清除的研究

Water and acids can cause the hydrolysis of lithium hexafluorophosphate under high voltage, and react with components in the CEI layer such as lithium alkyl carbonates[19], damaging the integrity of the CEI layer. Therefore, it is necessary to eliminate H2O and HF in the electrolyte for the stable operation of batteries under high voltage.

3.3.1 Clearance of H by Si—O/Si—N Functional Groups2O和HF

Some silicon-based additives containing Si—O and Si—N bonds not only form a CEI passivation layer on the cathode under high voltage conditions but also can clear HF and PF5 [106]from the electrolyte through Si—O and Si—N functional groups. For example, Fan et al.[107]used 2-cyanoethyltriethoxysilane (TEOSCN, molecular structure as shown in Figure 10a) as a single solvent for the electrolyte; even after exposing the electrolyte to air for 1 h, the NCM811/MCMB battery did not show capacity decay after 200 cycles, thanks to the H+trapping ability of the TEOSCN solvent, which inhibited the formation of HF (Figure 11a). Cui et al.[108]also introduced cyano groups into siloxanes, synthesizing a new type of multifunctional cyano-siloxane additive (TDSTCN). First, the Si—O groups in TDSTCN can inhibit the hydrolysis of lithium hexafluorophosphate and remove HF, and the cyano groups in the additive can suppress the dissolution of transition metal ions at the cathode-electrolyte interface by adsorption/coordination. The electrolyte containing TDSTCN significantly improved the stability of a 4.5 V LiNi0.9Co0.05Mn0.05O2/graphite full cell under high voltage, with the capacity retention rate increasing from 50.1% to 83.2% after 200 cycles at 1 C. Wu et al.[109]investigated the role of tetramethyl divinyl disiloxane (DTMS) additive in clearing HF and improving battery stability. By adding 1000 ppm HF (1 ppm=1×10-6) to both an electrolyte containing 2% vol. DTMS and a blank electrolyte, the discharge capacity of the blank electrolyte decreased from 246.4 mAh/g to 60.3 mAh/g after 40 cycles; however, the discharge capacity of the electrolyte containing 2% vol. DTMS only decreased from 240.3 mAh/g to 190.6 mAh/g.
图10 （a）含有Si—O和Si—N官能团的化合物；（b）含有—NCO官能团的化合物；（c）含有酸酐官能团化合物的结构式

Fig. 10 The molecular structures of (a) Compounds containing Si—O and Si—N functional groups. (b) Compounds containing —NCO functional groups. (c) Compounds containing acid anhydride functional groups

图11 （a）TDSTCN组成和作用原理图[108]；（b）在加入2000 ppm H2O后基础电解液和含1.0wt% BA电解液的19F和31P NMR光谱[114]

Fig. 11 (a) Schematic illustration of TDSTCN and its composition [108]. (b) 19F and 31P NMR spectra of baseline and 1.0 wt % BA-containing electrolytes after adding 2000 ppm H2O [114]

3.3.2 Isocyanate Scavenging by H2O和HF

Isocyanate refers to substances with the —NCO group, where the C atom in the C=N bond acts as an electrophilic center due to its lower electron density and can be easily attacked by the lone pair of electrons on the O atom in water molecules[110], thus, it can serve as a scavenger for trace amounts of water in the electrolyte, enhancing the cathode-electrolyte interfacial layer.
Lu et al.[110] studied two additives with the same structure, tert-butyl isocyanate (C-NCO) and trimethylsilyl isocyanate (Si-NCO). By adding 500 ppm (1 ppm=1×10-6) of water to the electrolyte, the infrared spectrum of the electrolyte without additives showed a broad peak between 3328~3670 cm−1 , which comes from the coordinated water in BF3-2H2O. However, in the electrolytes containing the additives, the intensity of the water peak significantly decreased, indicating that C-NCO and Si-NCO have a strong water-removal effect. Ma et al.[111] proposed a functional additive, 3-(trifluoromethyl)phenyl isocyanate (TPIC), with trifluoromethyl and isocyanate functional groups. Fourier transform infrared spectroscopy results showed that the CEI formed by the electrolyte containing the additive exhibited characteristic peaks of N—H, —C=O, and C—N, indicating the presence of polar amide groups in the CEI. The binding energy between Li+ and the amide group (-2.5 eV) is greater than that between Li+ and EC (-2.2 eV) or Li+ and DMC (-1.9 eV), which greatly reduces the desolvation energy barrier of Li+ and improves the transport rate of Li+.

3.3.3 Anhydride Clearance of H2O和HF

Anhydride substances such as succinic anhydride (SA)[112], glutaric anhydride (GA)[113] can inhibit the severe polarization and local overcharging of the electrode during the cycling process, thereby improving the stability of the electrolyte under high voltage conditions, and have been widely used in lithium-ion battery electrolytes.
Recently, Jiang et al.[114]found that anhydride additives such as benzoic anhydride (BA) also have the effect of capturing H2O and HF in the electrolyte (see Figure 11b). Similarly, Yang et al.[115]significantly improved the interfacial stability of LiCoO2cathodes by adding 2,3-dimethylmaleic anhydride (DMMA) to the electrolyte. X-ray photoelectron spectroscopy (XPS) results showed that the electrode surface of batteries without DMMA contained more hydrolysis products of LiPF6, specifically Li xPO yF z(34%), but this content decreased to 22% after the addition of DMMA. Furthermore, storage experiments were conducted by adding 500 ppm of H2O to electrolytes with and without DMMA. The electrolyte containing DMMA almost did not detect any HF component, further indicating the role of DMMA additive in removing trace water from the electrolyte and inhibiting the hydrolysis of lithium hexafluorophosphate.
Ye et al.[116] studied an anhydride electrolyte additive, phenyl maleic anhydride (PMA), for use in a 4.6 V LiCoO2 high-voltage battery system. The anhydride structure in PMA is prone to decompose, forming active intermediate species that induce the re-polymerization of the electrolyte solvent, while the unsaturated C=C double bond is conducive to the formation of an elastic interfacial film on the surface of the positive electrode. The results showed that after 300 cycles at room temperature, the capacity retention rate of the battery with an electrolyte containing 0.5% mass fraction of PMA increased from 55.3% in the reference group to 78.6%. Meanwhile, the capacity retention rate of the LiCoO2/graphite pouch full cell cycled at a cut-off voltage of 4.6 V was over 80% after 500 cycles.
Currently, the improvement of electrolyte stability under high voltage by adding trace amounts of water-removal and acid-suppression additives to the electrolyte is mostly indirectly indicated by the reduction in decomposition products of LiPF6 on the electrode surface or changes in the composition of the CEI layer, lacking direct evidence. It is difficult to discern whether this improvement stems from the additive's ability to remove HF and H2O from the bulk electrolyte or its suppression of electrolyte decomposition through the repair of the interfacial layer. Therefore, future research still needs to focus on distinguishing the contributions of these two mechanisms to the high-voltage stability of the electrolyte and identifying the true source of performance enhancement.

4 Conclusions and Prospects

Insummary,we can seethatsignificantprogresshasbeenmadeintheresearchanddevelopmentofhigh-voltageelectrolytes,andsomehigh-voltageelectrolyteproducts haveenteredthemarket.However,therearestillmanyissueswithhigh-voltageelectrolytesthatneedtobeaddressed.
(1) In recent years, newly developed electrolyte systems, such as locally concentrated electrolytes, multi-salt electrolytes, and electrolytes using solvents like nitriles, sulfones, and ionic liquids, have demonstrated a wider electrochemical window compared to carbonate electrolytes, fundamentally altering the physicochemical properties of the electrolytes. However, these electrolyte systems often suffer from poor anode compatibility, high viscosity, and high costs. Therefore, it is possible to consider combining these electrolyte systems with traditional carbonate electrolytes or film-forming additives to reduce usage risks and improve the oxidative stability of the electrolytes.
(2) In addition, high-voltage electrolytes using fluorinated solvents have shown comprehensive advantages over other novel systems, as they not only possess excellent oxidative stability and thermal stability but also exhibit good compatibility with both positive and negative electrodes and flame-retardant properties. However, their high cost has limited their development. Therefore, by analyzing the impact of fluorine substitution at different positions in fluorinated solvent molecules on the formation of the interfacial LiF layer, the solvation structure of Li+ in the electrolyte, and the electrochemical performance of the battery, this can guide the synthesis of fluorinated solvents with lower fluorination degree and lower cost, thereby promoting their further application.
(3) Using a small amount of additives to improve the high-voltage stability of batteries is currently the simplest approach. However, a single additive often cannot meet the multifunctional requirements of electrolytes (overcharge protection, flame retardancy, low-temperature performance), and the combination of multiple additives can easily lead to incompatibility, high costs, and negative effects. Therefore, in future research, exploring the synergistic mechanisms of additives and developing multifunctional additives are key areas of study.
(4) So far, the evaluation of high-voltage performance of electrolytes in the literature has mostly been conducted using coin cells, while there are few instances of using Ah-level pouch cells to verify the high-voltage performance of electrolytes. This often leads us to overlook the impact of electrolyte components on current collectors, gas generation in pouch cells at high temperatures, and calendar life, as well as the challenges brought by high-loading electrode materials. Therefore, more attention needs to be paid to the practical application assessment of high-voltage electrolytes.
It is believed that with the efforts of global scientific and technological workers, these issues are expected to be well resolved, ushering in a springtime for lithium-ion batteries with high output voltage, high energy density, and high safety.

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