With the continuous improvement of lithium-ion battery capacity requirements for pure electric vehicles, hybrid electric vehicles and portable energy storage devices, people expect to develop lithium-ion batteries with higher energy density and power density to achieve long-term battery life and energy storage . It can be seen from the following formula that high working voltage is one of the methods to improve the energy density of lithium-ion batteries:
In the formula: E is the energy density; V is the working voltage; q is the battery capacity. Under high working voltage, the electrolyte needs to have good oxidation resistance, and the electrochemical window is stable, so that the lithium-ion battery can maintain a stable cycle at high voltage. This paper introduces the problems existing in the application of traditional electrolytes to high-voltage lithium-ion batteries, their modification methods and new high-voltage electrolytes.
1. Problems with traditional electrolytes
The electrolyte is an important part of the battery. As a bridge between positive and negative materials, it plays an indispensable role in conducting current and other aspects. Commercial lithium-ion battery electrolyte is generally composed of carbonate organic solvent and lithium hexafluorophosphate (LiPF6), EC is an indispensable solvent, due to its high dielectric constant , the ability to dissolve lithium salts Usually, low viscosity DMC, DEC, EMC, etc. are added as co-solvents to improve the lithium ion migration rate.
But the traditional electrolyte usually decomposes when the working voltage is greater than 4.5V. This is due to the commonly used organic carbonate solvents, such as chain carbonate DMC ( dimethyl carbonate ), EMC (ethyl methyl carbonate) ), DEC (diethyl carbonate), and cyclic carbonate PC (propylene carbonate), EC (ethylene carbonate), etc. cannot exist stably under high voltage.Because of their low oxidation potential, oxidative decomposition occurs at high voltages, which degrades the performance of lithium-ion batteries. Conventional electrolytes can no longer meet the needs of high-voltage lithium-ion batteries, so the development of high-voltage electrolytes is crucial.
Second, the improvement method of traditional electrolyte
The traditional carbonate electrolyte is difficult to be used normally in high-voltage lithium-ion batteries due to its inability to withstand high voltage. Therefore, it is particularly important to modify it properly. Generally, increasing the concentration of carbonate-based electrolyte and increasing the number of complexes between lithium ions and solvent molecules can improve the oxidation resistance of the electrolyte. Furthermore, by adding additives to the traditional carbonate electrolyte, it can be preferentially decomposed to form a electrode protective film during battery cycling, which can protect the integrity of high-voltage electrode materials and improve battery performance to a certain extent.
2.1 Increase the concentration
In the high-concentration electrolyte, the lithium salt concentration is high, so the number of solvent molecules complexed with it is large, and the number of uncomplexed solvent molecules is reduced. Under high voltage, the oxidation resistance of the complexed solvent molecules is enhanced, and the stability of the electrolyte is enhanced. In addition, compared with the traditional electrolyte, the high-concentration electrolyte has enhanced flame retardancy, and the safety of the battery has been improved.
Doi et al. applied a high concentration (4.45 mol/kg) of LiPF6-PC to a high-voltage Li/LiNi0.5Mn1.5O4 battery, and calculated by the highest occupied molecular orbital (HOMO) theory that when the PC molecule and lithium ions solvate , the antioxidant stability of PC molecules was significantly increased, and the battery cycle performance was improved.Drozhzhin et al. studied the performance of Li/LiCoPO4 batteries in LiBF4/PC electrolytes with different concentrations. When the molar ratio of the two was 1:12, 1:6, and 1:4, they were cycled at C/10, 2.8-4.9V for 10 The capacity decayed by 40%, 31%, and 21% respectively after the first time. The high-concentration electrolyte improves the cycle efficiency, so the capacity decay is slow, but the cycle performance of the battery needs to be improved.
LiTFSI (lithium bis-trifluoromethanesulfonimide) lithium salt has excellent thermal stability, but usually corrodes aluminum foil . In order to solve this problem, Matsumoto et al. increased the concentration of LiTFSI lithium salt and prepared a 1.8mol/L LiTFSI m(EC):m(DEC)=3:7 electrolyte, and its electrochemical window reached 4.5 when using aluminum working electrode. V. Through analysis, it is found that in the high-concentration electrolyte, a layer of lithium fluoride LiF passivation layer is formed on the surface of the aluminum foil, which successfully inhibits the corrosion of the aluminum foil. Wang et al. studied a high-concentration LiN(SO2F)2(LiFSA)/dimethyl carbonate (DMC) electrolyte system, which can form a three-dimensional network-like structure, thereby effectively preventing the transition metal and aluminum under the condition of 5V voltage. Dissolved, high-voltage graphite C/LiNi0.5Mn1.5O4 batteries exhibit excellent cycling performance. In 10mol/L LiFSI-DMC high-concentration electrolyte, due to its ability to form an interfacial protective layer with high fluorine content, when the charging voltage reaches 4.6V, after 100 cycles, the Li/NMC622 battery maintains 86% of the battery initial discharge capacity. The high-concentration electrolyte has the advantages of high redox resistance, high carrier density, inhibition of aluminum foil corrosion, and good thermal stability, and has the potential to be applied to high-voltage electrolytes. However, it also has shortcomings, such as low conductivity and high cost. How to improve the conductivity and reduce the cost is the key to promoting the practical process of high-concentration electrolyte.
2.2 Adding high-voltage additives
Usually, high-voltage electrolyte additives are mainly used to form a film on the surface of the positive electrode. Compared with the electrolyte solvent, the additive has a lower oxidation potential, and can be preferentially decomposed under high pressure to form a positive electrode protective film, reducing The contact between the electrolyte and the electrode is improved (Figure 1), and the oxidative decomposition of the electrolyte and its parasitic reactions are suppressed, thereby improving the electrochemical performance of the lithium-ion battery.
Figure 1 Schematic diagram of electrolyte additives for protecting electrode materials The organic additives are mainly vinylene carbonate , thiophene and its derivatives, imidazoles, acid anhydrides and new organic additives, etc. The main mechanism is that the organic matter preferentially polymerizes or decomposes during the charge and discharge process to form an electrode protective film. Yan et al. used tris(trimethylsilane) phosphate (TMSP) as a novel film-forming additive for LiNi0.5-Co0.2Mn0.3O2 at 1 mol/L LiPF6 m(EC): m(EMC)=3:7 After adding 1% TMSP, the initial discharge capacity and the capacity retention rate were both improved. PFPN (ethoxy pentafluorocyclotriphosphazene) with a mass fraction of 5% was added to the electrolyte of 1 mol/L LiPF6 j(EC):j (DMC)=3:7, Li/LiCoO2 (3.0～4.5 V) The battery discharge capacity is improved.
Inorganic salts can be used as additives for high-voltage electrolytes to improve the performance of lithium-ion batteries, mainly LiBOB (lithium dioxalate borate), LiODFB (lithium difluorooxalate borate) and new additives, which can be decomposed into inorganic protection in small amounts membrane. As an additive in Li/NCM622 (3.0-4.6V) battery, LiODFB can be oxidized and decomposed at 4.15V to form a dense protective film, and the battery impedance is reduced, and the cycle performance is improved.Tris(2,2,2-trifluoroethyl) phosphite (TTFEP) was used as the 4.6V NCM111 cathode material additive to significantly improve the battery cycle performance and rate capability. Li et al. synthesized a new additive lithium bis(2-fluoropropoxy)borate (LiBFMB), and after 100 cycles of Li/LNMO battery (3.0-4.9V), the capacity loss of LiBMFMB with 0.05 mol/L addition was 13.5 %, while the loss without additives reached 42.2%. The LiBMFMB in the electrolyte can decompose on the surface of LNMO to form a thin and dense protective film to protect the electrode structure and effectively improve the battery performance. The additive can form a film on the surface of the positive electrode material to prevent the decomposition of the solvent in the electrolyte under high pressure from destroying the electrode structure. However, there are many kinds of additives, and the film thickness and type of each additive to the positive electrode material are inconsistent, and the reaction mechanism is different. The mechanism of action at high voltage still needs further study.
3. High-voltage electrolyte of the new system
With the continuous development of lithium-ion batteries towards high energy density, the research on high-voltage electrolytes is also becoming more and more in-depth. At present, new high-voltage electrolytes include sulfone, nitrile, ionic liquid and fluorinated electrolytes, etc. These new system electrolytes can meet the needs of high voltage to a certain extent.
3.1 Sulfone-based electrolyte
Sulfone-based electrolyte has low cost and an electrochemical window of over 5V, making it a potential high-voltage electrolyte for lithium-ion batteries. Tan et al. outlined a series of sulfone solvents, among which methyl ethyl sulfone (EMS) has an electrochemical window up to 5.9 V. Abouimrane et al. used 1 mol/L LiTFSI-EMS electrolyte for Li4Ti5O12/LiNi0.5Mn1.5O4 batteries, and the capacity decay was small after 100 cycles at a current density of 33 mA/g.The anti-oxidative stability of the electrolyte with 0.7mol/L LiBOB j(SL):j(DMC)=1:1 is 5.3V, which shows stable cycling performance when applied in Li/LiNi0.5Mn1.5O4 batteries. Lower impedance and excellent rate performance, but its performance at low temperature needs to be further improved.
Xue and other studies have shown that a single sulfone electrolyte has good compatibility with graphite, but poor compatibility with high-voltage cathode materials. After the Li/LiNi0.5Mn1.5O4 half-cell was cycled 10 times when the electrolyte was 1mol/L LiPF6 m(EMS):m(FMS)=1:1, the capacity faded seriously due to the decomposition of the electrolyte. When sulfones (EMS) are mixed with carbonates (DMC) as electrolytes, the Li/LiNi0.5Mn1.5O4 battery has a capacity retention rate of 97% and an efficiency higher than 99% after 100 cycles. It can be seen that when sulfones and carbonates are used as co-solvents, the compatibility of sulfones and cathode materials can be optimized, and it also provides a new idea for the development of high-voltage electrolytes.
Recently, researchers have also developed a sulfone-based high-concentration electrolyte 3.25 mol/LLiFSI-SL, which can form a protective film on the positive and negative surfaces at the same time. The battery, after 1000 cycles, retains 70% of its first discharge capacity. Sulfone solvents have problems such as high melting point, most sulfones are solid at room temperature, and poor compatibility with positive electrode materials. Solving these problems will lead to wider application of sulfone electrolytes.
3.2 Nitrile electrolyte
Nitrile has a series of advantages, such as: high thermal stability, good anode stability, wide liquid temperature range and so on. The most prominent feature is that the electrochemical window is wide, and the anti-oxidative stability of single nitrile can reach 7V, and it is difficult to decompose in the usual 5V high-voltage lithium-ion battery.Abu-Lebdeh et al. found that the oxidative decomposition resistance window of 1mol/L LiTFSI-GLN (glutaronitrile) can reach 6.5V, but its matching with high-voltage electrode materials needs further exploration. When adiponitrile is used as solvent and LiTFSI is lithium salt, the electrochemical window of electrolyte exceeds 6V, but when single adiponitrile is used as solvent, it is incompatible with graphite.
In order to solve the problem of compatibility with negative electrodes, researchers mix nitriles and carbonates, such as adiponitrile and dimethyl carbonate as co-solvents, which have good compatibility with graphite and can be used at high voltages. application below. Nitrile-based solvents are more stable at high voltages than carbonate-based solvents and have better performance at low temperatures. However, it has poor compatibility with negative electrodes such as graphite or metal lithium , and will polymerize at the negative electrode, and the resulting polymer will hinder the deintercalation of lithium ions. Therefore, how to solve its compatibility with the negative electrode material and make use of its strengths and avoid its weaknesses is the only way to apply it to the high-voltage electrolyte of lithium-ion batteries.
3.3 Fluorinated electrolytes
The electronegativity of fluorine atoms is relatively strong, the polarity is weak, the chemical stability of fluorinated solvents is excellent, and it has great potential in the application of high-voltage electrolytes. The high-performance fluorinated electrolyte is the goal of scientific researchers.
Xia et al. used density functional theory to study the oxidative decomposition mechanism of fluoroethylene carbonate (FEC) as a high-voltage electrolyte. The study showed that it can form SEI film on the surface of nickel lithium manganate material, which can inhibit the decomposition of electrolyte . Fan et al. developed a perfluorinated electrolyte [1mol/L LiPF6 m(FEC):m(FEMC):m(HFE)=2:6:2], which can form a nano-scale fluoride protective layer, and It can effectively prevent the decomposition of the electrolyte and the dissolution of transition metal elements, and the capacity retention rate of Li/LiCoPO4 batteries (5V) is as high as 93% after 1000 cycles.In addition, in the 7mol/L LiFSI-FEC high-concentration electrolyte, since both LiFSI and FEC contain fluorine atoms, a LiF protective layer can be formed on the negative electrode, the pores of the metal lithium negative electrode are reduced, and the reversibility is improved. In a 5V Li/LiNi0.5Mn1.5O4 battery, the capacity retention rate was 78% after 130 cycles at a charge-discharge rate of 0.36C.
3.4 Ionic Liquids
Ionic liquids have the characteristics of low volatility, excellent flame retardancy, wide electrochemical window, etc. Recently, their research has been extensive, and they can improve the stability of lithium-ion batteries under high voltage.
Borgel et al. studied the performance of lithium nickel manganate half-cell (Li/LiNi0.5Mn1.5O4) in TFSI (bis-trifluoromethanesulfonimide)-based ionic liquid. Compared with conventional electrolyte, the irreversible capacity of the battery is greatly reduced . However, when these ionic liquids are applied in high-rate and low-temperature environments, their properties still need to be further optimized. 1mol/LLiNTf2-C4mpyrNTf2 (lithium bistrifluoromethanesulfonimide/1-butyl-1-methylpyrrolidinium bistrifluoromethanesulfonimide) electrolyte for Li/LiNi0.5Mn1.5O4 batteries, Compared with the electrolyte [1mol/L LiPF6 j(EC)∶j(DEC)=1∶2], the battery discharge capacity is similar, but the coulombic efficiency is significantly improved, and the ionic liquid has better flame retardancy and safety. . The disadvantage is that the coulombic efficiency of the battery after using the ionic liquid is only about 95%, and the capacity decays rapidly, so the coulombic efficiency needs to be improved to truly achieve high efficiency and high capacity retention. To improve its shortcomings, ionic liquids and conventional solvents can be used as co-solvents to improve the performance of lithium-ion batteries at high voltages.
Although ionic liquids can be used in high-voltage lithium-ion batteries, their high viscosity and low electrical conductivity lead to reduced battery cycle and rate performance; secondly, their wettability is not good, resulting in poor compatibility with electrodes; Furthermore, the high melting point of ionic liquids degrades the performance at low temperatures.More research is needed for ionic liquids to be truly applied.
Reference: Dai Wenhui et al. "Research Progress of High Voltage Electrolyte for Lithium Ion Batteries"
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