Editor's note The electrolyte is an ionic conductor that conducts electricity between the positive and negative electrodes of the battery. During charging and discharging, lithium ions are transported back and forth between the positive and negative electrodes. The electrolyte has a relatively large influence on the battery's charge and discharge performance (high rate and low temperature), life (cycle storage), and temperature range. Electrolyte concept The electrolyte is an ionic conductor that conducts electricity between the positive and negative electrodes of the battery. During charging and discharging, lithium ions are transported back and forth between the positive and negative electrodes. The electrolyte has a relatively large influence on the battery's charge and discharge performance (high rate and low temperature), life (cycle storage), and temperature range. Suitable solvents require high dielectric constant and low viscosity. Commonly used alkyl carbonates such as PC and EC have strong polarities and high dielectric constant, but the viscosity is large, and the intermolecular forces are large. Lithium ions move in them. Slow. Linear esters such as DMC (dimethyl carbonate) and DEC (diethyl carbonate) have low viscosity but low dielectric constant. Therefore, in order to obtain a solution having high ionic conductivity, it is generally used. PC+DEC, EC+DMC and other mixed solvents. Electrolytes for lithium ion batteries should generally meet the following basic requirements: a. High ionic conductivity, generally should be 1 × 10-3 ~ 2 × 10-2 S / cm; b. High thermal stability and chemical stability, no separation occurs over a wide voltage range; c. a wider electrochemical window that maintains stable electrochemical performance over a wide range of voltages; d. has good compatibility with other parts of the battery such as electrode material, electrode current collector and diaphragm; e. Safe, non-toxic, non-polluting. 2. Electrolyte infiltration effect When a lithium battery is used up to the discarding standard or suddenly fails, it is often disassembled for analysis, and what is the purpose of causing the performance of the battery to decay or plummet. Xiao Bian in the disassembly analysis of lithium batteries, found that the battery performance is not good, often with the electrolyte is not good for the infiltration effect of the pole piece. When the electrolyte infiltration effect is not good, the ion transport path becomes far away, which hinders the shuttle of lithium ions between the positive and negative electrodes. The pole piece that does not contact the electrolyte cannot participate in the electrochemical reaction of the battery, and the resistance of the battery interface increases, affecting lithium. Battery rate performance, discharge capacity and service life. Then, in order to avoid all kinds of drawbacks, we need to find a way to make the electrolyte soak the pole pieces as much as possible. Of course, considering the cost, we need to use the right amount of electrolyte as much as possible. The effect of the amount of electrolyte on the performance of the battery is as follows: [Technical π] What effect does the amount of electrolyte have on battery performance? 3. How to improve the wettability of the electrolyte The infiltration of the electrolyte into the pole piece involves the solid, liquid and gas three-phase contact. When the electrolyte is injected into the battery case, the electrolyte first discharges the air inside the casing, after which the electrolyte adheres to the surface of the positive and negative active materials, and some of the electrolyte enters the positive electrode through the separator of the core. between. As time goes on, there will be a phenomenon in which the electrolyte infiltrates the pole piece and the electrolyte in the diaphragm reversely infiltrates the pole piece. When the standing time is long, the infiltration of the pole piece is achieved under the action of surface tension. A balanced state. In this process, a concept "contact angle" (wetting angle) in physical chemistry is involved. As shown in the figure below, the blue area in the figure represents liquid and the gray area represents solid interface. Then the area in which the blue and gray are in contact is the solid-liquid phase contact interface, and the position where the tangent of the liquid intersects with the solid interface forms an angle θ, wherein the smaller the contact angle θ, the better the infiltration of the electrolyte to the pole piece or the diaphragm. . However, in the actual operation process, it is often impossible to grasp the effect of the electrolyte on the infiltration of the pole piece. According to the principle of the electrolyte infiltration mentioned above, we can find ways to improve the infiltration effect of the electrolyte on the pole piece from the following points. : (1) Improve the liquid injection process Improving the liquid injection process is the most common method, which can effectively improve the infiltration effect of the electrolyte from the aspects of liquid injection efficiency, injection conditions, standing time, and liquid injection method. Injecting liquid under vacuum conditions not only facilitates the discharge of gas in the battery core, but also reduces the resistance of the gas to the electrolyte injection, and helps the electrolyte to infiltrate the pole piece. The principle is that vacuum injection can reduce the gas phase resistance in the solid-gas-liquid three-phase interface, allowing the electrolyte to directly contact the pole piece, reducing the infiltration time. By prolonging the time of standing under vacuum, it is ensured that the electrolyte sufficiently wets the pole piece. After the injection, as the rest time is prolonged, the wetting angle of the electrode liquid and the pole piece gradually decreases, and the wetting radius gradually increases, finally achieving a good wetting effect. In order to avoid the phenomenon that the electrolyte is insufficient to infiltrate the separator and the pole piece, the electrolyte can be injected in batches to facilitate the electrolyte to fully infiltrate the pole piece. This operation mode is to improve the solid-liquid contact probability and expand the contact area. In the case where the amount of electrolyte does not change, the infiltration time can be shortened. (2) Improve the core process The electrolyte infiltration effect is related to the particle properties of the electrode material, the compactness of the pole piece, and the tightness of the core. The positive and negative active materials and conductive agents with different morphologies and particle sizes have different effects on the infiltration of the electrode sheets. The larger the particle size of the raw materials, the closer to the spherical shape, the greater the electrolyte permeation rate, the more the infiltration time short. When the compact density of the pole piece is too large, the porosity in the pole piece is lowered, which is not conducive to the infiltration of the electrolyte into the pole piece, and it is necessary to adjust the appropriate compaction density to satisfy the electrolyte infiltration under the condition of ensuring low impedance of the battery. degree. Similarly, the tightness of the stack or winding of the cells can also affect the infiltration of the electrolyte. When the winding is loose, the pores between the positive electrode-separator and the negative electrode are large, and the amount of electrolyte accumulated is large, which causes partial enrichment and lack of some places, which undoubtedly has a great influence on the performance of the battery. When the winding is tight, it will affect the infiltration speed and efficiency of the electrolyte, which is also insufficient. (3) Adding an electrolyte sizing agent The commonly used electrolyte is an organic solvent, and the pole piece is an inorganic material, and the ability to absorb the electrolyte is weak. The addition of an admixture to the electrolyte also improves the infiltration of the electrolyte. Liu Fangfang et al. used a fluoroether material as an additive for the electrolyte. The test results show that adding a small amount of sizing agent to the electrolyte can effectively shorten the time of battery injection and significantly improve the cycle performance of the battery, but it is necessary to pay attention to the sizing agent. When the amount added reaches 1%, it will have a negative impact on cycle performance. The essence of the sizing agent is a surfactant. The sizing agent has the advantages of high surface activity, high heat stability, low flammability and high chemical stability. The addition of the sizing agent to the electrolyte can reduce the surface tension of the liquid. Improve the wetting ability and penetration ability of the electrolyte to the pole piece, thereby improving the electrochemical performance of the battery. Through the above several methods, the infiltration effect of the electrolyte on the pole piece can be effectively improved. Shortening the infiltration time can save production cost and improve the wetting effect, can reduce the interface impedance of the battery, improve the utilization efficiency of the living material, thereby improving the battery capacity and improving the discharge rate characteristics.
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