button battery cr2032.Research progress on lithium-ion battery electrolytes

　　Research progress on lithium-ion battery electrolytes

　　The electrolyte has always been at the center of the performance and stability of lithium-ion batteries. At present, the battery industry continues to conduct in-depth research on new lithium salts and solvents, and has proposed many methods to improve battery performance and safety. However, additives can make up for the deficiencies of the electrolyte in some aspects, especially the protective films on the surfaces of the positive and negative electrodes. The formation of (SEI film) has achieved many results. The electrolyte needs to be compatible with the characteristics of the battery system, so the design and research of electrolyte formulas must focus on different battery systems.

　　Thermal Stability of Electrolyte The current conventional electrolyte system generally includes organic solvents and lithium salts. EC, DMC, EMC, DEC, and pC are several common organic solvents, and the lithium salt is LipF6 [1]. Studies have shown [2] that EMC and H2O in the electrolyte reduce the thermal stability of 1MLipF6 electrolyte. Among them, EMC decomposes into DEC and DMC, and DEC and DMC undergo a series of complex organic chemical reactions with pF5, the decomposition product of LipF6, releasing a large amount of heat and gas. This shows that when EMC is used in batteries under high temperature conditions, or in environments with high thermal safety requirements for batteries, it is necessary to reduce the EMC content in the electrolyte as much as possible. Hu Chuanyue et al. [2] studied the influence of water on the thermal stability of electrolytes and found that the exothermic peak of the electrolyte with a water mass fraction of 5.85×10-3 was at 257°C, and the reaction starting temperature was 240°C; while water The exothermic peak of the electrolyte with a mass fraction of 8×10-6 is at 272°C, and the reaction starting temperature is 255°C. The DSC curves of pure LipF6 and 1mol·L-1LipF6EC-DMC-EMC electrolyte are shown in Figure 1. Figure 2(a) shows the DSC curve of LipF6 at 100~350℃. It can be seen that the first weak endothermic peak appears in the DSC curve at around 195°C, which is the melting peak of LipF6 and is reversible; the second endothermic peak begins to appear at around 250°C, indicating that LipF6 begins to dissolve at 250°C. Thermal decomposition. It can be seen that pure LipF6 is still thermally stable up to 250°C. It can be seen from the above test results that the thermal stability of the electrolyte itself is affected by the thermal stability of protic solvents such as moisture and solvent molecules. The thermal stability of the conventional lithium-ion battery organic electrolyte itself is not bad. The key is that in a real battery, the electrolyte interacts with the positive and negative electrodes in the charge and discharge states. This is the fundamental reason for the safety of lithium-ion batteries. Lithium-ion battery cathode materials in the charged state, such as LixCoO2, LixNiO2, and LixMn2O4, are unstable and decompose, releasing oxygen at high temperatures [3, 4, 5]. The released oxygen reacts with organic solvents (such as EC, DEC, PC, DMC, EMC, etc.) in the electrolyte to generate heat. Under a certain voltage, the solvent and the electrolyte themselves may react and release a large amount of heat, causing safety issues. Due to the low melting point of the electrolyte, it is difficult to measure its thermal stability. Researchers [5, 6, 7, 8] studied the thermal stability of some lithium-ion battery mixed solvent electrolytes in sealed containers by differential scanning calorimetry (DSC). The effects of the added amount of water and metallic lithium on the thermal stability of the electrolyte were studied. The exothermic peak of the LipF6 electrolyte containing DEC appears at 255°C, which is 15 to 20°C lower than the electrolyte containing DMC. DMC has higher reactivity than DEC. Due to the destruction of the solid electrolyte (SEI), the exothermic reaction between 1MLipF6/EC+DEC, 1MLipF6/EC+DMC and 1MLipF6/pC+DMC and metallic lithium begins at the melting point of metallic lithium. This temperature is about 180°C, but a self-heating reaction of 1MLipF6/pC+DEC will occur before. The temperature of the autogenous reaction begins at 140°C. When water is added to the above-mentioned electrolyte containing metallic lithium, the starting temperature of the above-mentioned exothermic reaction is less than 130°C. The reason is probably that HF causes the collapse of the SEI film structure, and HF is the reaction product of LipF6 and water.

　　The interface reaction between electrode and electrolyte cathode material LiCoO2 is unstable under high temperature conditions in conventional electrolytes [7], which greatly limits its application in large-capacity batteries. The electrochemical performance of cathode materials strongly depends on the surface chemistry in the electrolyte and the formation of surface films. Similar to the negative electrode, the cathode materials of many lithium-ion batteries can be considered SEI electrodes [9]. There are many reactions between LixMOy and electrolytes containing carbonate solvents and lithium salts, including irreversible acid-base reactions between LixMOy and trace amounts of HF, O2- in transition metal oxides reacting to solvent molecules with electrophilic properties The nucleophilic attack reaction initiated, the polymerization reaction of cyclic alkyl carbonate on the electrode surface to form polycarbonate, the redox reaction with the electrolyte composition, and the dissolution reaction of transition metal ions into the electrolyte. The above-mentioned interface reactions and compositions can be expressed as follows: the surface of all cathode materials contains - LiF, ROCO2Li, ROCO2M, ROLi, MCO3, Li2CO3, MF2 (M=transition metal), polycarbonate [9]; Li[Mn,Ni ]O4→λ-MnO2; LiCoO2→Co3O4; LixMnO2 (layered material)→spinel LiMn2O4 on the surface. DoronAurbach[10] and others believe that the substances that harm the performance of the positive electrode are mainly acidic electrolytes, and this is the inevitable result of using LipF6 as the electrolyte. When the acidity of the electrolyte is low and the volume ratio of the positive active material to the electrolyte is large, the LiCoO2 electrode can still cycle well even at temperatures above 60°C. When the electrolyte is contaminated with water and has high acidity, the performance of the LiCoO2 electrode deteriorates significantly.

　　High-temperature electrolyte Under high-temperature conditions, there are obvious redox reactions between pF6- anions and solvents, as well as between all electrolyte components and cathode materials. Regarding the mechanism of battery capacity deterioration under high temperature conditions, Doron Aurbach believes that [11] pF6- and its product pF5 generate HF with solvent molecules, and HF will interact with the main components ROLi, ROCO2Li, and Li2O in the solid electrolyte membrane (SEI membrane) on the surface of the negative electrode. Reacts with LiOH to generate LiF and deposits on the surface of the negative electrode. SEI membranes containing LiF will seriously hinder the migration of Li ions. The higher the degree of enrichment, the greater the impact. The high-resistance substances produced at the same time will insulate the graphite particles. With the continuous charging and discharging under high temperature conditions, the electrode interface impedance and the insulation isolation between the active material and the conductive material will continue to lead to the deterioration of the negative electrode performance, and finally reduce the capacity of the lithium-ion battery. Too low and ineffective. Doron Aurbach found [12] that adding organic silicon compounds to conventional electrolytes can significantly improve the high-temperature performance of batteries, while the high-temperature performance of conventional electrolytes without such additives is very poor.

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