1 Introduction
Lithium-ion battery is a new type of rechargeable battery after lead-acid battery, nickel-cadmium battery and nickel-hydrogen battery, which has the characteristics of high energy density and high power density, and is the first choice of batteries for portable electronic products, and is gradually widely used in the fields of electric tools and hybrid/all-electric vehicles
[1]. However, the energy density of the existing lithium-ion battery system cannot fully meet the development requirements of modern power equipment
[2]. Based on this, new batteries with ultra-high specific capacity, such as lithium-sulfur batteries and lithium-air batteries, have attracted wide attention
[3,4]. However, there are many problems that are difficult to overcome, such as the shuttle effect of soluble polysulfides in lithium-sulfur batteries and the corrosion of lithium metal in lithium-air batteries, which are also the problems faced by all lithium metal batteries
[5,6]. Therefore, lithium-ion batteries will remain the mainstream choice in the field of electronic products and power vehicles for quite a long time. In order to better meet the application needs, it is urgent to further improve its energy density and safety performance.
Up to now, the research on lithium-ion batteries has focused on electrode materials. The development of new cathode materials with higher specific capacity and higher working voltage can improve the energy and power density of lithium-ion batteries
[7]. In recent years, new high-voltage cathode materials, such as lithium nickel manganate (LiNi
0.5Mn
1.5O
4), lithium cobalt phosphate (LiCoPO
4) and layered lithium-rich materials xLi
2MnO
3-(1-x)LiMO
2(M=Ni, Co, Mn), have been gradually developed. Their discharge voltage can reach more than 5 V, but they have not yet been commercialized
[8,9][10][11]. The most important reason is that the main component of the current commercial electrolyte is carbonate, which has weak oxidation resistance. When the working voltage of the battery reaches above 4.5 V(vs Li/Li
+), serious oxidation and decomposition will occur, and products such as CO
2 and H
2O will be produced, which will lead to the increase of internal resistance of the battery, and ultimately lead to the rapid reduction of battery capacity and the attenuation of cycle life. Therefore, the development of electrolytes that can match the high-voltage cathode materials is particularly important for the development of high energy density lithium-ion batteries.
Electrolytes for lithium-ion batteries are usually prepared by dissolving one or more conductive lithium salts in a single solvent or a mixture of non-aqueous solvents. Common lithium salts include lithium perchlorate (LiClO
4), lithium hexafluoroarsenate (LiAsF
6), lithium tetrafluoroborate (LiBF
4) and lithium hexafluorophosphate (LiPF
6). Common solvents include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC)
[12]. Generally, the nature of the electrolyte can have a great impact on the high and low temperature performance, cycle performance and safety performance of the battery. Over the past decade, the basic composition of electrolytes has not changed significantly, with some research focusing on the development of new components to improve the characteristics of electrolytes, and others on the exploration of battery interface chemistry and mechanisms. Based on the study of its components, the current methods to improve the working voltage of electrolyte can be roughly divided into the design and development of new high voltage resistant organic solvents and the synthesis and preparation of new high voltage electrolyte additives
[13].
In this paper, the research status of high voltage electrolyte for lithium ion batteries at home and abroad in recent years is reviewed from two aspects of new organic solvents and high voltage resistant additives, and the role of theoretical calculation in the study of high voltage electrolyte is introduced. Finally, the development and prospects of high voltage electrolytes are summarized and prospected.
2 Novel solvents with wide electrochemical window
Organic solvents, as the main components of the electrolyte, have a direct impact on the performance of the electrolyte. Fluorinated, sulfone, nitrile and ionic liquid compounds have a wide electrochemical window and good oxidation stability, which are expected to replace the existing commercial carbonate solvents and be widely used in high-voltage lithium-ion battery electrolytes.
2.1 Fluorinated solvent
Fluorocarbonates, as a kind of fluorine-containing compounds, have higher oxidation stability and lower flammability than traditional carbonate solvents due to the strong electronegativity and electron-withdrawing ability of fluorine atoms. According to the molecular structure, it can be divided into cyclic fluorinated carbonate and linear fluorinated carbonate, and the common fluorinated solvents are shown in Figure 1.
Fluorinated ethylene carbonate (FEC) is a commonly used cyclic fluorinated carbonate co-solvent and film-forming additive, which has been widely used in the electrolyte of high voltage lithium ion battery system due to its excellent oxidation resistance
[14,15]. Hu et al. Used FEC in LiNi
0.5Mn
1.5O
4(LNMO)/ graphite batteries, and the results showed that the electrolyte could significantly improve the cycle performance of batteries at room temperature and 55 ℃
[16]. Similarly, Xia et al. demonstrated that the electrolyte containing FEC exhibited superior oxidation stability, with LNMO/MCMB batteries still maintaining more than 82% capacity after 200 cycles
[17]. However, the use of a large amount of FEC will significantly increase the viscosity and cost of the electrolyte, and FEC is unstable at high temperatures and easy to decompose to produce HF, which will lead to the reduction of battery performance.
Unlike cyclic FEC, linear fluorinated carbonates are usually used as solvents to dissolve lithium salts and reduce the viscosity of the system. In 2003, Smart et al synthesized a series of linear fluorinated carbonates and proved that they could significantly improve the low temperature performance of MCMB/Li half cells
[18]. In recent years, many linear fluorinated carbonates have been gradually used as cosolvents for electrolytes, but their working mechanism is usually simply explained as the induction of fluorine atoms or the synergistic effect of several cosolvents
[19,20]. On the other hand, it is not known whether linear fluorinated carbonates can be used alone, so it is necessary to further study the mechanism when they are used as cosolvents or single solvents.
Zheng et al synthesized ethyl (2,2,2-trifluoroethyl) carbonate (ETFEC) and systematically studied its effect on the performance of LNMO materials when used as an electrolyte solvent
[21]. The experimental results show that the self-discharge behavior of LNMO/Li half cell can be effectively inhibited by adding an appropriate amount of ETFEC into the traditional carbonate electrolyte, thereby improving the cycle stability of the cell. After 200 cycles at 60 ℃, the capacity retention of LNMO/Li half cell was improved from 65. 3% to 77. 1%. The main reason is that ETFEC has high oxidation stability, which can prevent the electrolyte from oxidizing and decomposing on the surface of the positive electrode, thus improving the high voltage stability of the electrolyte.
In addition to fluorinated carbonates, fluorinated ethers can also be used as solvents for high voltage electrolytes. It was found that when fluoroethers were added as co-solvents, the electrolyte not only showed excellent cycle stability under high pressure conditions, but also was completely non-flammable in combustion tests
[22][23]. 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) has high boiling point, low cost and good SEI film-forming properties. Xia et al. Used TTE as a cosolvent, which showed high oxidation stability at 5 V and excellent wettability with the separator
[24]. The LNMO/Li half cell has higher discharge capacity and rate capability, and the coulombic efficiency is also obviously improved. The results show that the partially fluorinated ether has excellent cycle stability at high pressure and is a promising co-solvent for high voltage electrolyte.
2.2 Nitrile solvent
Nitrile organic solvents usually have a wide electrochemical window, high thermal stability, boiling point and flash point, and do not produce highly toxic CN-[25] during use. In recent years, aliphatic dinitriles NC-(CH2)n-CN such as glutaronitrile (GLN), adiponitrile (ADN), etc. (The structural formula is shown in fig. 2) have attracted increasing attention as electrolyte solvents.
Ue et al. First reported that organic solvents could be used in electric double layer capacitors, and found that the electrochemical window of dinitrile solvents such as GLN and ADN could reach more than 8 V
[26]. In addition, although the viscosity of ADN is higher than that of EC (6. 10 cP vs 2. 56 cP) and the dielectric constant is lower than that of EC (30 vs 89), the vapor pressure of ADN is lower, and its boiling point and flash point are higher than those of carbonate solvents, so ADN has been widely used as a potential solvent or additive for lithium ion battery electrolytes
[27][28]. However, dinitrile-based electrolytes are difficult to be used alone because of their high viscosity and poor compatibility with negative electrodes and separators, and need to be used in combination with carbonate-based solvents.
Abu-Lebdeh et al. Used ADN and GLN as electrolyte solvents, respectively, and added EC and a small amount of film-forming additives lithium bis oxalate borate (LiBOB) and a proper amount of lithium bis (trifluoromethylsulfonyl) imide (LiTFSI) as lithium salts.It was found that the electrolyte exhibited lower viscosity and higher ionic conductivity, and the addition of EC and LiBOB significantly improved the compatibility of dinitrile solvents with the negative electrode, and the cycle performance of the LiCO
2/ graphite battery was significantly improved
[29,30]. Farhat et al. Used glutaronitrile (GLN) or 2-methylglutaronitrile (MGLN) alone as a solvent and LiTFSI as a lithium salt to prepare an electrolyte for a LiNi
1/3Co
1/3Mn
1/3O
2/Li
4Ti
5O
12(NMC/LTO) cell
[31]. The results show that the electrolyte can keep liquid state in a wide temperature range, and the solubility of lithium salt is improved. After 200 cycles, the capacity retention rate of the battery is still 98%.
In addition to the dinitrile compounds, the mononitrile compounds and other cyano compounds also exhibit high ionic conductivity and cycle stability, and can be used as electrolyte solvents or cosolvents. Hilbig et al. Used butyronitrile (BN) -based electrolyte containing EC and FEC as cosolvent and functional additive respectively in NMC/graphite battery, and evaluated the electrochemical performance of the battery
[32]. The results show that the battery containing BN in the electrolyte can provide higher specific capacity at higher charge-discharge rate, which makes the battery have excellent fast charging ability. After 1000 charge-discharge cycles, the capacity of NMC/graphite battery did not decrease significantly, showing good long-term cycle stability, and the ionic conductivity increased by at least 32% compared with the electrolyte without BN.
Nitrile solvents have a wide electrochemical window, and their poor compatibility with the negative electrode can be improved by mixing them with carbonate solvents or adding film-forming additives, but in practical applications, it is still necessary to further explore how to reduce their cost and viscosity
[33].
2.3 Ulfone solvent
Due to its inherent oxidation stability, high dielectric constant and flash point, sulfone solvents can be used as good solvents for 5 V high-voltage electrolyte, and are expected to replace traditional carbonate solvents
[34,35]. However, when sulfone solvents are used in the electrolyte, they also have the disadvantages of high viscosity and low ionic conductivity in lithium salts (including LiPF
6).
Commercial sulfone compounds, such as dimethyl sulfone (DMS), diethyl sulfone (DES), ethyl methyl sulfone (EMS), and tetramethylene sulphone (TMS), have been widely used in lithium-ion batteries, and their molecular structures are shown in Figure 3. However, these solvents have poor wettability with separators and poor compatibility with graphite negative electrodes, which largely hinder their application in high-voltage electrolytes. Xue et al. Demonstrated that EMS as a cosolvent could significantly improve the high voltage stability and cycle stability of LNMO/Li half cells, with a capacity retention rate of 97% after 100 cycles
[36]. Wu et al. selected DMS, TMS and EMS as solvents and added LiBOB, LiTFSI and LiPF
6 as lithium salts to prepare various electrolytes by combining theoretical quantum chemistry calculation and electrochemical performance experiments. The results showed that all electrolytes had high ionic conductivity and electrochemical window exceeded 5.4 V
[37]. In addition, there is a strong interaction between lithium salt and sulfone group, which plays an important role in improving the ionic conductivity and electrochemical stability of the solvent system, and also proves that sulfone compounds with high electrochemical stability can be used as electrolyte solvents at higher working voltages.
TMS, also known as sulfolane (SL), is a common by-product of petroleum industry. It has high boiling point, high flash point, high dielectric constant and oxidation potential exceeding the 5.8 V(vs Li/Li
+). In recent years, it has gradually attracted wide attention as a solvent for high-voltage electrolyte
[38][39]. Unlike linear alkyl sulfones, SL still shows good SEI film-forming properties for graphite anode at 90 ℃, which makes it more advantageous in the application of lithium ion batteries
[40]. Alvarado et al. Proposed a two-component electrolyte formulation consisting of SL and lithium bis (fluoromethylsulfonyl) imide (LiFSI), which can meet the interface requirements of both graphite anode and LNMO anode, and obtain stable electrochemical performance. The battery can work for a long time at high voltage of 4.8 V, and can still maintain 70% of the discharge capacity after 1000 cycles
[41]. Zhang et al. added lithium difluorooxalate borate (LiDFOB) as a lithium salt to an EC-free SL-based electrolyte to study the effects of the electrolyte on the physical, chemical, and electrochemical properties of the battery
[42]. The results showed that the electrolyte had better electrochemical stability than EC-based electrolyte, and the graphite anode showed stable capacity and high coulombic efficiency. The addition of appropriate amount of additives such as FEC and LiBOB is more conducive to the formation of SEI film on the graphite anode and further improves its performance.
In recent years, it is of great significance to synthesize new sulfone compounds and further explore the possibility of their use as high-pressure solvents. Flamme et al. Synthesized and prepared a variety of bis-functional sulfone compounds containing ether, ester and carbonate groups for the first time, and studied their effects on the physical, chemical and electrochemical properties of lithium-ion batteries
[43]. The experimental results show that the bifunctional sulfone has higher boiling point, ionic conductivity and oxidation potential, and can form a stable passivation layer on the graphite anode, which is expected to be used as an electrolyte for lithium-ion batteries alone or in the presence of co-solvents. Among the different kinds of bifunctional sulfones, 3-methoxythiophene-1,1-dioxide (MESL) has the characteristics of low melting point and high boiling point, which is the most promising for high-voltage lithium-ion batteries. In the presence of LiTFSI, the oxidation potential of MESL on NMC cathode material is as high as 4.9 V(vs Li/Li
+), which has a significant effect on improving the high-voltage stability of the electrolyte.
Due to their high electrochemical stability, sulfone solvents are expected to be widely used as a new generation of solvents for high voltage lithium ion battery electrolytes, but it is still necessary to explore how to further improve the safety performance of sulfone electrolytes and reduce the viscosity of sulfone.
2.4 Ionic liquid
Ionic liquids (ILs) are room-temperature molten salts composed of organic cations and inorganic anions, which have a wide electrochemical stability window, high chemical and thermal stability, non-flammability, non-volatility, and environmental friendliness
[44⇓~46]. As far as battery systems are concerned, ionic liquids can play a variety of roles, such as electrolyte solvents, additives, or plasticizers and binders for gel polymer electrolytes, depending on battery components and actual application needs
[47].
Although hundreds of ionic liquids have been developed, their relatively high cost and poor compatibility with negative electrodes and separators limit their practical application
[48]. In addition, their high viscosity leads to low ionic conductivity, and most ionic liquids have poor solubility for lithium salts
[49]. Therefore, the development of new ionic liquids with low viscosity, high lithium salt solubility and good compatibility with electrodes has become the core issue in this field.
Bordes et al. Found that the longer the cationic alkyl side chain in the ionic liquid, the worse the wettability between the electrolyte and the graphite electrode
[50]. To solve this problem, an organic carbonate solvent is usually added to the ionic liquid to obtain higher conductivity and good film-forming properties. Le et al prepared a mixed electrolyte of N, N-dimethyl-N-ethyl-N-propylamino bis (trifluoromethyl) sulfonimide (N
1123TFSI) and EC. The results showed that EC could significantly reduce the viscosity of the N
1123TFSI, and the battery had no significant capacity loss after 20 cycles, and the cycle performance was significantly improved
[51]. Wu et Al synthesized N-propyl-N-methylpiperidine bis (trifluoromethylsulfonyl) imide (PYR
13TFSI or PP
13TFSI), and successfully inhibited the dissolution of Al in the electrode and the oxidative decomposition of the electrolyte by adjusting the ratio of IL/EC and the concentration of LiPF
6 in the mixed electrolyte
[52]. After 300 cycles, the capacity retention rate of the battery was as high as 97%, and the battery showed good charge-discharge reversibility even at 55 ℃. The electrolyte has good wettability and low flammability for polyethylene separator, and good compatibility with graphite and silicon anodes, which further confirms that ionic liquids have great potential for application in 5 V high-voltage lithium-ion batteries.
Beside, that performance of the ionic liquid can be improved by introduce other groups or adding functional additives. Ahmed et al introduced a highly flexible ether group into 1-methylpyrrolidine to prepare N-methoxyethyl-N-methylpyrrolidine-bis (trifluoromethyl) sulfonimide (PYNTFSI), which was mixed with an appropriate amount of DMC to prepare an electrolyte
[53]. It was found that the addition of ether group could significantly reduce the viscosity of the electrolyte, and the battery showed good cycle stability and excellent rate capability at room temperature and 60 ℃. Liang et al. Used PP
13TFSI and DMC as cosolvent, LiDFOB as lithium salt and FEC as film forming additive
[54]. The experimental results show that the gadolinium-based electrolyte has high ionic conductivity, low polarizability and high safety performance, stable low-impedance SEI film can be formed on the surface of the positive and negative electrodes, and the Li
1.1Ni
0.25Mn
0.65O
2/ graphite battery shows good cycle stability and capacity retention.
The above solvents all have different effects on the battery system. With the electrolyte oxidation potential and the capacity retention rate of the battery as reference standards, the performance of each solvent is shown in Table 1.
3 High voltage electrolyte additive
The practical application of new solvents has been seriously hindered to a large extent due to their poor compatibility with graphite anode and separator, low ionic conductivity and higher viscosity than conventional carbonate electrolytes. There is no doubt that adding small amounts of additives to conventional carbonate electrolytes to alter some of the targeted characteristics of the electrolyte, rather than completely replacing the main components of existing electrolyte systems, is a more efficient and economical option,Especially when the operating voltage range of the battery is above 4.5 V or the electrode material uses sensitive materials such as graphite, LNMO or other manganese-based materials
[55,56].
Because the design working voltage of the positive electrode material and the negative electrode material of the lithium ion battery far exceeds the thermodynamic stability limit of a conventional carbonate electrolyte component, the initial activation process of the lithium ion battery is always accompanied by sacrificial decomposition of an electrolyte to form a passivation component on the surface of an electrode,Known as the Solid Electrolyte Interface (SEI) on the negative electrode and the Cathode Electrolyte Interface (CEI) on the positive electrode, these interfaces prevent further side reactions between the electrolyte and the electrode while enabling easier Li
+ transport. The electrolyte additive can be preferentially decomposed on the surface of an electrode to participate in the formation of an SEI film or a CEI film, so that H
2O, HF, LiF and the like formed in the decomposition process of the electrolyte are eliminated, the chemical aging of a bulk electrolyte is inhibited, the film-forming performance of the SEI film and the CEI film is obviously improved, and further the performance of a lithium ion battery is improved
[57]. In addition, the addition of functional electrolyte additives can also change the thickness of the interfacial film and the composition of different interfacial film components (as shown in fig. 5)
[58].
There are many kinds of additives, and the action mechanism of each additive is different, so it is difficult to study, analyze and summarize them comprehensively. In addition, different types of electrodes require highly matched additives to provide the best cycling results. Different cut-off voltages will produce different additional effects (such as the formation, decomposition and loss of gases on the battery), and the combination of additives may also lead to unpredictable battery performance and impact
[59]. All of these problems make the search for suitable additives increasingly complex and difficult.
3.1 Phosphorus additive
Phosphorus-containing compounds usually have flame retardant properties and are often used as flame retardant additives. In addition, due to its high oxidation stability, it has been gradually used as an additive in high voltage electrolyte systems in recent years. The molecular structure of common phosphorus additives is shown in Figure 6.
Tris (trimethylsilane) phosphite (TMSP) is a common phosphorus-based additive. Because of its ability to stabilize LiPF
6 and scavenge corrosive HF, TMSP is often used in high-voltage battery systems with conventional LiPF
6 as electrolyte, which can significantly improve the high-voltage stability of batteries
[60]. In addition, TMSP preferentially decomposes and participates in the formation of a protective interfacial film on the electrode. Lu et al. Added 5 wt% TMSP to the LNMO/MCMB battery system, and electrochemical experiments showed that the additive could significantly improve the room temperature and high temperature cycle performance of the battery
[61]. After 300 cycles at 0. 5 C at room temperature, the capacity retention increased from 64. 1% to 80. 5%, and from 33. 2% to 81. 6% at 50 ℃. In addition, SEM, EIS and XPS experiments show that the TMSP additive is preferentially decomposed at the electrode interface, which is conducive to the formation of thicker but more conductive SEI layers at the negative and positive electrodes.
Zhao et al. Used tripropargyl phosphate (TPP) as a multifunctional additive for LiNi
0.5Mn
0.3Co
0.2O
2(NMC532)/ graphite batteries
[62]. The results showed that the addition of 1. 0 wt% TPP in the electrolyte not only increased the initial coulombic efficiency of the battery by about 4. 4%, but also significantly improved the cycle stability of the battery at 25 ℃ and 55 ℃. On the positive electrode side, TPP participates in the formation of a firm and uniform CEI layer through oxidative polymerization, which completely covers the surface of NMC532 and enhances the stability of the positive electrode by preventing the dissolution of transition metal ions, inhibiting the oxidative decomposition of electrolyte and eliminating HF. On the negative side, TPP forms a uniform SEI layer on the surface of graphite through reductive polymerization, which prevents the erosion of H
+ and significantly improves the cycle stability of the battery under high pressure.
Beltrop et al. Used triphenylphosphine oxide (TPPO) as an additive for LiNi
0.8Mn
0.1Co
0.1O
2(NMC811)/ graphite batteries
[63]. The addition of 0. 5 wt% TPPO can significantly improve the coulombic efficiency and capacity retention of the battery, and the additive can also be used as a film-forming additive for the positive/negative electrode to participate in the formation of interfacial film, effectively reducing the loss of active lithium during cycling. Yue et al. demonstrated that Tris (pentafluorophenyl) phosphine (TPFPP), as a bifunctional electrolyte additive, can improve the stability of the electrolyte/electrode interface at high voltages
[64]. Due to its special structure (as shown in Figure 6), TPFPP is preferentially oxidized and reduced to form a uniform and stable passivation film on the electrode surface. The capacity retention rate of the Li
1.17Ni
0.25Mn
0.58O
2(LLO)/ graphite battery was increased from 62.6% to 90.6%, and the coulombic efficiency was still 99.8% after 200 cycles.
3.2 Boron additive
In recent years, boric acid esters have been found to be used as additives in the electrolyte of high voltage lithium ion batteries. Because of their good protection of electrode materials and low cost, they have important research significance and commercial application prospects
[65⇓~67]. The molecular structure of common boron additives is shown in Figure 7.
Qin et al. Used triisopropyl borate (TIB) as a film-forming additive and confirmed that it could preferentially oxidize and decompose on the surface of the positive electrode
[68]. After 300 cycles, the capacity retention of the LiNi
0.6Co
0.2Mn
0.2O
2(NMC622)/Li with 1.0 wt% TIB was 82.7%, while the capacity retention of the electrolyte without TIB was only 50.0%. Liu et al. introduce trimethyl borate (TMB) as an additive to improve that cycle performance of the battery
[69]. The results showed that TMB was preferentially oxidized to form a dense protective layer on the cathode surface, which inhibited the decomposition of electrolyte and protected the cathode material from structural damage. The capacity retention of the battery increased from 64% to 81% after 100 cycles at 0. 1 C. In addition, Yue et al. demonstrated that Tris (pentafluorophenyl) boron (TPFPB), as a multifunctional additive, can stabilize both lithium metal anodes and LNMO anodes
[70]. In TPFPB, boron and pentafluorobenzene preferentially form a stable and smooth interface layer on the electrode, which prevents the decomposition of electrolyte and the dissolution of transition metal. After 500 cycles, the battery can obtain 90% capacity retention.
Lithium bis (oxalato) borate (LiBOB) is considered to be the most effective additive in the electrolyte of lithium ion batteries, which can passivate the electrode surface, prevent solvent diffusion and protect the electrode materials
[71,72]. Ha et al. Used LiBOB in high voltage LNMO positive electrode, and confirmed that the additive could form a stable CEI layer on the surface of the positive electrode and inhibit the oxidative decomposition of the electrolyte
[73]. Xu et al. Confirmed that the CEI layer formed by the electrolyte containing LiBOB can effectively inhibit the Mn
2+ of the cathode and the dissolution of the Ni
+[74]. In addition, the LiBOB additive can also be preferentially reduced on the surface of the anode material to form a strong SEI layer, thereby improving the stability of the anode material
[75,76].
Zhang first proposed that lithium difluoro oxalate borate (LiDFOB) can be used as a lithium salt, which is similar to LiBF
4 and LiBOB in structure and has the advantages of both. Its solubility in linear carbonate is better than that of LiBOB and its viscosity is smaller, so that the battery has better low temperature performance and rate performance
[77]. It was found that LiDFOB was helpful to form a stable SEI film on the graphite anode, which improved the electrochemical performance and high temperature cycle performance of the battery
[78][79,80][81]. In addition, Hu et al. confirmed that LiDFOB also helps to form a stable interfacial film on the positive electrode and passivate the surface of the positive electrode, thereby inhibiting the decomposition of the electrolyte, and the battery shows higher reversible charge/discharge capacity and better cycle stability
[82]. Similarly, Xia et al. demonstrated that the addition of 0.5 wt% LiDFOB significantly improved the cycle stability and rate capability of the MCMB/Li half cell, further confirming that LiDFOB can be used as a potential multifunctional additive for 5 V high-voltage lithium-ion battery electrolytes
[83].
3.3 Benzene and heterocyclic additive
Benzene and its derivatives can be used as additives of high voltage electrolyte because of their inherent conjugated system and strong electron-withdrawing ability, which are easy to polymerize on the surface of electrodes to form conductive polymer films, inhibit the decomposition of electrolyte and improve the working voltage of batteries. The structural formula of common benzene ring additives is shown in Figure 8.
Zhuang et al. Used 4-aminobenzoic acid (4-ABA) as an additive for the Li
1.2Ni
0.2Mn
0.6O
2(LLCs) of lithium-rich oxide cathode materials
[84]. Electrochemical tests showed that the addition of 0. 25 wt% 4-ABA in the electrolyte could significantly improve the cycle stability of the positive electrode, and the capacity retention increased from 61% to 94. 4% after 100 cycles. In addition, 4-ABA tends to adsorb on the surface of LLCs and oxidize prior to the 4.36 V(vs Li/Li
+), and forms a polymer film on the surface of LLCs by in-situ electrochemical polymerization, which effectively inhibits the direct contact between LLCs electrode surface and electrolyte, thus inhibiting the decomposition of electrolyte.
Hamenu et al. Demonstrated that the introduction of benzotriazole (BzTz) as an additive could improve the electrochemical performance of the 1 mol/L LiPF
6-EC/DMC electrolyte
[85]. By adde that 0.1 wt%BzTz,LiCoO
2/, the graphite battery shows more excellent electrochemical stability and good ionic conductivity; Compared with the original electrolyte, the electrolyte can obtain higher discharge capacity at a higher current density, and a stable SEI film is formed in the cycle process, thereby prolonging the cycle life of the lithium ion battery. BzTz can reduce the formation of PF
5 (catalyzing electrolyte decomposition), thereby stabilizing the electrolyte and expanding its potential energy range, making the electrolyte more suitable for high-power and high-voltage applications.
Imilarly, heterocyclic compound have that same advantages as benzene ring and can be used as high pressure additives. Tu et al. Added 0.25 wt% terthiophene (3THP) to the LiPF
6 based electrolyte, and after 350 cycles at 1 C, the capacity retention of the LNMO/Li battery increased from 50% to 91%
[86]. This phenomenon can be attributed to the preferential oxidation of 3THP, which forms a protective CEI film on the surface of the cathode, inhibits the decomposition of the electrolyte and protects the cathode material, and significantly improves the cycle stability of LNMO.
3.4 Other additives
Sulfur-based additives, such as 1,3-propane sultone (PS), are often used as negative electrode film-forming additives, which can significantly improve the thermal stability and cycle stability of lithium ion batteries
[87,88]. Pires et al. Found that adding 1 wt% PS to NMC/Li and NMC/graphite batteries can significantly improve battery performance and increase battery operating voltage
[89]. This phenomenon is attributed to the fact that PS preferentially decomposes on the surface of the electrode material to form a protective layer, which ensures the integrity of the electrode material and reduces the dissolution of Mn
2+ in the electrolyte, so that the battery shows higher reversible capacity, coulombic efficiency and better capacity retention than that without additives.
Hekmatfa et al compared the effects of several different electrolyte additives on the performance of NMC532/graphite batteries
[90]. The results show that both vinylene carbonate (VC) and PS can form an effective passivation layer on the surface of the electrode, which can protect the electrolyte from further decomposition under high pressure and can be used as effective additives to improve the performance of the battery. In addition, the SEI formed on the graphite electrode was studied and compared with the CEI layer on the positive electrode, and it was found that the reduction products of the SEI layer could also be detected on the surface of the positive electrode, which confirmed that there was some crosstalk between the two electrodes.
Shi et al. Used succinic anhydride (SA) as a functional additive in NMC811/Li battery, which is simple in structure, low in cost and harmless to the environment
[91]. The addition of 3 wt% SA can significantly improve the cycle performance and rate capability of the battery, and the capacity retention rate is as high as 93. 8% after 400 cycles at 1 C. TEM and XPS characterization showed that the CEI film was uniformly distributed on the surface of NMC811 particles. The CEI film was composed of organic polymer and had good Li
+ diffusivity, so the battery had higher rate capability and capacity retention. The results of EIS and TEM further show that SA contributes to the formation of CEI film during cycling, which significantly reduces the surface activity of NMC811 and protects the electrode material from the side reactions of electrolyte and the corrosion of harmful compounds.
Park et al. Used the sulfate-based compound BDTT (molecular structure as shown in Figure 9) as a positive stabilizing additive, and added VC as a negative film-forming additive to improve the electrochemical performance of NMC532/graphite batteries
[92]. The results showed that the use of dual additives had a synergistic effect on improving the cycle performance of NMC532/graphite battery, which could significantly improve its initial capacity and capacity retention, and further confirmed that the development of new additives and the use of combined additives played an important role in improving the performance of electrolyte for high voltage lithium ion batteries.
The difference of additive type, dosage and electrolyte composition will cause the difference of battery performance. Table 2 lists the comparison of the performance of different high-voltage additives.
4 The Role of Theoretical Calculation in the Preparation of High Voltage Electrolyte
The novel solvent generally has the characteristics of poor compatibility with the graphite negative electrode and higher viscosity than the conventional carbonate electrolyte,Usually, the possible characteristics and action mechanism of a substance can be preliminarily judged through some simple theoretical calculations, so as to select solvents or additives suitable for high-voltage lithium-ion battery systems more pertinently.
Density functional theory (DFT) calculations are commonly used to measure the electron affinities, charge densities, and frontier orbitals of molecules. According to the frontier molecular orbital theory, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are nucleophilic and electrophilic, respectively
[93]. HOMO and LUMO are closely related to the chemical reactions of substances
[94]. The HOMO energy level represents the ability to lose electrons, and the lower the energy level, the stronger the oxidation resistance and the higher the oxidation potential. The LUMO energy level represents the electronic capacity, and the higher the energy level, the stronger the resistance to reduction and the lower the reduction potential
[95]. The ideal high pressure electrolyte solvent should have strong oxidation stability, and the additive should be more easily oxidized, so the substance with lower HOMO energy level can be selected as the solvent to improve the oxidation stability of the electrolyte under high pressure through DFT calculation.Substances with higher HOMO energy levels are selected as additives to form a suitable interfacial film and improve the stability of the electrolyte.
Zheng et al studied and compared the oxidative stability of several solvents by DFT calculation
[21]. The HOMO energy levels and LUMO energy levels of different solvents were calculated, and the results are shown in Table 3. It is clear that the HOMO energy level is significantly reduced when the hydrogen atoms in the carbonate molecule are replaced by fluorine atoms. It is shown that the fluorocarbonate is in a low-energy state and is difficult to lose electrons, and the oxidation potential is higher. According to theoretical calculation, the order of oxidation resistance of various solvents is DEC < EC < ETFEC < FEC < DTFEC. In other words, the fluorinated solvent is more resistant to oxidation than the conventional solvents DEC and EC, and is more suitable for a high voltage electrolyte system.
Yang et al. Used lithium oxalate difluorophosphate (LiBODFP) as an electrolyte additive, calculated the HOMO and LUMO energy levels of several solvents and additive molecules in advance, and designed the optimal electrolyte composition based on this
[96]. As shown in fig. 10 a, LiBODFP has a higher HOMO energy level compared to EC and EMC solvents, indicating that it can be oxidized prior to the solvent molecules. Considering the solvation between the organic molecules and the Li
+, the HOMO energy level of the organic molecules dissolved by the Li
+ was calculated, and it was confirmed that there was a great difference between them. Although the HOMO energy level (-9.75 e V) of the organic molecule dissolved by the Li
+ decreased, it was still higher than that of the solvated EC-Li
+(-14.67eV) and EMC-Li
+ molecules (-14.34 e V), which further proved that the additive could be oxidized preferentially.
On the other hand, due to the interaction between BODFP and the solvent molecule EC, considering the different combination positions of EC and BODFP, four structures with the lowest energy levels of BODFP: nEC (n = 1 ~ 4) are obtained, as shown in Fig. 10B. Compared with the BODFP-Li+, the HOMO energy level of BODFP-EC complex is higher, indicating that BODFP is more easily oxidized in the presence of EC. The BODFP: 4EC complex shows the lowest value for the LUMO energy level, indicating that the additive molecule can be reduced before the solvent in the presence of EC. According to the above theoretical calculation, it is predicted that LiBODFP can be used as a multifunctional additive in the high voltage electrolyte system, which preferentially oxidizes and reduces, and protects the positive and negative electrode materials at the same time. The above inference has been further confirmed in the subsequent electrochemical experiments.
In addition, the different reaction characteristics and action mechanisms of solvents or additives can be judged in advance by thermodynamic calculation. Han et al. Calculated the self-decomposition reaction energy of the additive and its reactivity with LiF by first principles
[97]. The spontaneous decomposition reaction energy ΔG of TMSB and its cation (TMSB
+) is positive, which is thermodynamically unfavorable, that is, the additive itself is difficult to decompose even in the cationic state. Comparing the solvent phase reaction energy of TMSB and TMSB
+ with LiF (Figure 11), ΔG is negative, indicating that the reaction is very favorable thermodynamically, that is, TMSB and TMSB
+ can effectively remove LiF on the surface of the positive electrode through chemical reaction, thus inhibiting the dissolution of transition metal ions in the positive electrode, which has a positive effect on improving the interface stability of high-performance lithium ion batteries.
Similarly, Zhao et al. confirmed that the reaction was thermodynamically favorable (ΔG < 0) by calculating the reaction energy of TPP and HF, and deduced the formation mechanism of the electrode passivation layer (as shown in Figure 12)
[62]. In the process of battery charging, TPP is easy to oxidize, and F with strong electronegativity attacks the carbon core of TPP, resulting in the breakage of π bond in carbon-carbon triple bond, thus forming C-F covalent bond, and further chain addition reaction occurs at the positive electrode. When TPP gains electrons, H protons in the electrolyte approach the p orbital electron cloud of alkyl due to their strong electrophilicity, forming C — H covalent bonds, and further chain addition reaction occurs at the negative electrode. Therefore, the working mechanism of TPP is inferred, that is, TPP tends to react with harmful HF, and can form a protective interface film on both positive and negative electrodes, thus inhibiting the decomposition of electrolyte.
Theoretical calculation can provide a preliminary judgment basis for the selection of materials suitable for high voltage lithium ion battery system, and provide an effective strategy for the development of functional electrolytes. At the same time, it should be noted that although HOMO and LUMO values or reaction energies are easy to calculate, due to the different mechanisms and effects of various solvents and additives, these calculations can only provide preliminary estimates and can not replace actual experiments
[98]. The accuracy and rationality of the theoretical calculation need to be further verified by experiments to verify whether the solvent can match the negative electrode and whether the additive can form an effective CEI film after oxidative decomposition.
5 Conclusion and prospect
As an important component of lithium-ion batteries, electrolyte is the carrier of lithium ion conduction between the positive and negative electrodes, and its properties can directly determine many performances of batteries. With the gradual application of lithium-ion batteries in the field of power vehicles, higher requirements are put forward for their energy density and safety performance, which also makes the traditional electrolyte face more challenges. In the past research, the basic composition of electrolyte has not changed significantly, and the development of new high-pressure solvents and additives has injected new vitality into improving the working voltage of lithium-ion battery electrolyte, but the research is not yet perfect, and more in-depth research should be focused on the following aspects in the future:
(1) High-pressure solvents have a wide electrochemical window and good oxidation stability, but in practical applications, electrolytes generally have the disadvantages of high viscosity and poor compatibility with negative electrodes and separators (the advantages and disadvantages of various solvents are shown in Table 4), so they are difficult to be used as a single solvent, and usually need to be combined with traditional carbonate solvents or other functional additives. Therefore, it is very important to develop high pressure solvents with low viscosity and good compatibility with electrodes and functional additives for the study of high pressure electrolytes.
(2) The amount of the additive is small and the effect is remarkable, and adding the additive into the electrolyte is an economical and practical method for improving the performance of the lithium ion battery. However, at present, the mechanism of additives is not fully understood, and the function is relatively single, which is difficult to meet the various performance requirements of batteries, and the use of multiple additives may also be incompatible or even produce negative effects. Therefore, the synthesis of new multifunctional additives and the clarification of the mechanism of additives will be the key research direction of high-voltage electrolyte in the future.
(3) As an advanced research method, theoretical calculation can provide theoretical guidance for the development of high-voltage electrolyte, predict the basic characteristics of specific molecular structure, analyze the mechanism of additives, and select solvents and additives that are more in line with the working voltage range and application requirements. However, theoretical calculation can not completely replace the actual experiment, and we should further study how to combine the two organically. With the development of research methods, the research of high voltage electrolyte for lithium ion batteries will also be developed, and the existing problems will be better solved.
National Natural Science Foundation of China(51834008)
Beijing Municipal Natural Science Foundation(2202047)
Opening Project of State Key Laboratory of Advanced Chemical Power Sources(SKL-ACPS-C-20)
Science Foundation of China University of Petroleum, Beijing(ZX20180416)
PDF
