1. Additives to improve the performance of electrode SEI film
Lithium-ion batteries inevitably react at the interface between the electrode and the electrolyte during the first charge/discharge process, forming a passivation film or protective film on the electrode surface, the thickness of which is determined by the electron tunneling distance. This film is mainly composed of lithium alkyl ester, lithium alkoxide and lithium carbonate. It has the characteristics of a multilayer structure. The side close to the electrolyte is porous and the side close to the electrode is dense. The liquid space acts as an intermediate phase and has the properties of a solid electrolyte, allowing only lithium ions to pass through freely to achieve insertion and extraction, while it is insulated for electrons. Therefore, this film is called "solid electrolyte interphase" (SEI). This membrane prevents the co-intercalation of solvent molecules, avoids direct contact between the electrode and the electrolyte, thereby inhibiting further decomposition of the solvent, and improving the charge and discharge efficiency and cycle life of the lithium ion battery. Therefore, choosing a suitable electrolyte and forming a stable SEI film at the electrode/electrolyte interface is a key factor in achieving electrode/electrolyte compatibility.
Adding some small molecules such as SO2, CO2, NOx to PC electrolyte can promote the formation of SEI film with Li2S, Li2SO3, Li2SO4 and Li2CO3 as the main components. The SEI film is chemically stable, insoluble in organic solvents, has a good ability to conduct lithium ions, and inhibits the co-intercalation and reduction and decomposition of solvent molecules from damaging the electrode. For example, adding some sulfites such as vinyl sulfite (ES) or propylene sulfite (PS) to the PC-based electrolyte can significantly improve the performance of the SEI film of the graphite electrode, and has good compatibility with the cathode material . In addition, adding a certain amount of halogenated organic solvent to the organic electrolyte can form a stable SEI film on the surface of the carbon electrode, improve the cycle performance of the battery, and increase the cycle life of the battery. Adding a small amount of anisole or its halogenated derivatives to the organic electrolyte for lithium ion batteries can improve the cycle performance of the battery and reduce the irreversible capacity loss of the battery. For example, the mechanism that anisole affects battery cycle performance is: anisole and the solvent EC, DEC, the reduction decomposition product ROCO2Li, undergo a transesterification reaction similar to that of LiOCH3, which is conducive to the formation of an efficient and stable SEI film on the electrode surface. There is also a class of compounds containing 1,2 vinylene groups such as vinylene carbonate (VC), vinyl acetate (VA), acrylonitrile (AAN) and so on.
The low temperature performance of 18650 cylindrical commercial lithium-ion battery was investigated, and it was found that the discharge capacity of the battery at -20°C was 67% ~ 88% of the room temperature capacity under 0.2C rate discharge, but at -30°C and -40°C The discharge capacity of the battery decreases rapidly, to 2%~70% and 0~30% of room temperature respectively. From room temperature to -20°C or -30°C, the impedance of the battery at 1kHz generally increases very little, regardless of the battery's discharge capacity. However, the DC resistance of the battery at -30°C is 10 times higher than that at room temperature, and it is increased by 20 times at -40°C. The DC resistance of the battery is related to the battery's room temperature and low-temperature discharge capacity. The SEI film formed on the carbon anode not only affects the ionic conductivity of the electrolyte, but also strongly affects the low-temperature performance of the battery. Therefore, in order to optimize the low-temperature performance of the electrolyte, the intrinsic physical properties of the electrolyte components (including freezing point, viscosity and ionic conductivity) must be compatible with the observed specific battery system (that is, the properties of the SEI film on the electrode). Make a balance. The ionic conductivity of the electrolyte and the solid phase diffusion of lithium ions in the electrode do not limit the low-temperature discharge capacity of the battery. The diffusion of lithium ions in the SEI film on the positive electrode surface is a limiting factor for the low-temperature discharge capacity of the battery.
In order to meet the application of space exploration and weapon systems, the US military and NASA are interested in the development of secondary energy storage devices with improved low temperature (as low as -40°C) performance. By developing a multi-component electrolyte solvent formulation based on a mixture of cyclic and aliphatic alkyl carbonates, low-temperature lithium-ion batteries can work effectively at -30°C. For example, ternary and quaternary carbonate electrolytes can improve the low-temperature performance of the experimental three-electrode battery MCMB/LiNi0.8Co0.2O2. A series of electrochemical test methods were used to characterize the performance of these batteries (including Tafel polarization measurement, linear polarization measurement and electrochemical impedance spectroscopy measurement) and found that the most promising electrolyte formulation is 1.0mol/L LiPF6EC/DEC /DMC/EMC (volume ratio 1:1:1:2) and 1.0mol/L LiPF6EC/DEC/DMC/EMC (volume ratio 1:1:1:3), use these electrolytes in SAFT's 9A·h Performance evaluations of lithium-ion batteries including rate performance at different temperatures, cycle life, and many specific task tests have found that these batteries have good performance in the temperature range of -50~40℃ (in C/10 Under the discharge rate, the specific energy of the battery can reach 95W·h/kg).
In order to develop lithium-ion battery electrolytes that can be used in a wide operating temperature range, several solvents containing cyclic carbonates (such as EC) and linear carbonates (such as DMC, DEC and EMC) and low freezing points (methyl acetate MA, The ternary electrolyte of ethyl acetate EA, isopropyl acetate IPA, isoamyl acetate IAA or ethylene propionate EP) is the first research object. By studying the cycle life of these electrolyte lithium-ion batteries at various temperatures, it is found that the low freezing point solvent has a much greater impact on the low temperature performance of the electrolyte than the linear carbonate. The electrolyte containing EC/DEC/MA solvent still shows good initial cycle performance at -20°C, but the cycle performance is poor. The overall performance of the electrical materials (EC/DEC/EP and EC/EMC/EP) containing EP and two other solvents (including the initial cycle performance at a low temperature of -20°C, the cycle life and rate performance at room temperature and 50°C) ) The most attractive.
Use 1mol/L LiPF6EC/DMC/EMC (volume ratio 1:1:1) as low temperature electrolyte. This electrolyte not only has good electrical conductivity and electrochemical stability, but also its lithium-ion battery can work at -40℃. The EC-based multi-element composition of various solvents can be measured in a wide temperature range of -40~40℃. Studies on the ionic conductivity of the electrolyte have found that the use of a co-solvent with a high dielectric constant and low viscosity can increase the room temperature ionic conductivity, but only a co-solvent with a low melting point can effectively extend the operating temperature range of the electrolyte. The lithium-ion battery using the optimized electrolyte 1mol/L LiPF6EC/DMC/EMC (8.3:25:66.7) has very good low temperature performance, even under the condition of -40℃ and 0.1C, when it is discharged to 2.0V The battery capacity can still reach 90.3% of the normal capacity.
The low-temperature performance of lithium-ion batteries is mainly affected by the electrolyte solution. The electrolyte not only determines the mobility of ions between the two electrodes, but also strongly affects the properties of the surface film formed on the surface of the carbon negative electrode. The surface film determines the dynamic stability of the electrode relative to the electrolyte, allows the charge transfer between them, and in turn determines the cycle life and rate performance of the battery. In order to improve the low-temperature performance of the battery, some people used various alkyl carbonates, such as EC, DMC, DEC and ester solvents, to prepare electrolytes with different solvent ratios, and studied their conductivity and membrane resistance at different temperatures. , Membrane stability and kinetic properties of lithium intercalation and desorption. Compared with binary solvent electrolytes, electrolytes composed of EC, DEC and DMC tend to have a synergistic effect in terms of electrical conductivity and surface film characteristics, especially at low temperatures. DMC-based electrolytes can show a synergistic high durability, while DEC-based electrolytes can improve low-temperature performance. A clear trend in the stability of the formed surface film was observed. In a solution containing low molecular weight co-solvents (ie, methyl acetate and ethyl acetate), the formed surface film only hinders the movement of ions, and does not have the protective effect of the usual SE1 film, and contains high molecular weight esters. The electrolyte forms more desirable properties.
By optimizing the electrolyte formulation, the preparation process of the gel electrolyte and the electrode coating treatment, the low temperature performance of the gel electrolyte lithium ion battery can be improved. When the concentration of LiPF6 is about 1.0mol/L and the mass ratio of EC:PC is 1:1, in the temperature range of -20~20℃, the conductivity of LiPF6 dissolved in EC/PC in the electrolyte system reaches the maximum. Low-temperature electrochemical performance shows that batteries using low EC content electrolytes can release a limited capacity, while the discharge capacity of batteries using high EC content electrolytes is close to zero. Electrochemical impedance spectroscopy studies have shown that at very low temperatures, the impedance of electrolytes with high EC content is much higher than that of electrolytes with low EC content. The best low-temperature electrochemical performance is obtained when the coated electrode and the gel polymer electrolyte prepared by ultraviolet irradiation are used. The prepared battery has good interface properties at room temperature and at a low temperature of -20°C.