How to improve the infiltration effect of the electrolyte on the pole piece?

1. Electrolyte concept

The electrolyte is an ionic conductor that conducts between the positive and negative electrodes of the battery. During charging and discharging, lithium ions are transferred back and forth between the positive and negative electrodes. The electrolyte has a relatively large impact on the battery's charge and discharge performance (high and low rate), life (cyclic storage), and temperature application range.

Suitable solvents require high dielectric constant and low viscosity. Commonly used alkyl carbonates such as PC and EC have strong polarity and high dielectric constant, but the viscosity is large and the intermolecular force is large, and lithium ions move in it. The speed is slow. Linear esters, such as DMC (dimethyl carbonate) and DEC (diethyl carbonate), have low viscosity but low dielectric constant. Therefore, in order to obtain solutions with high ionic conductivity, they are generally used PC+DEC, EC+DMC and other mixed solvents.

The electrolyte used in lithium ion batteries should generally meet the following basic requirements:

a. High ionic conductivity, generally should reach 1×10-3~2×10-2 S/cm;

b. High thermal and chemical stability, no separation occurs in a wide voltage range;

c. A wide electrochemical window, to maintain the stability of electrochemical performance in a wide voltage range;

d. It has good compatibility with other parts of the battery such as electrode materials, electrode current collectors and separators;

e. Safe, non-toxic and non-polluting.

2. Electrolyte infiltration effect

When the lithium battery reaches the discard standard or suddenly fails, it is often disassembled to analyze the purpose of the battery's performance degradation or drop. When the editor disassembled and analyzed the lithium battery, it was found that the battery with poor cycle performance is often related to the poor infiltration effect of the electrolyte on the pole piece. When the electrolyte infiltration effect is not good, the ion transmission path becomes farther, which hinders the shuttle of lithium ions between the positive and negative electrodes. The pole pieces that are not in contact with the electrolyte cannot participate in the electrochemical reaction of the battery, and the interface resistance of the battery increases, which affects lithium The rate performance, discharge capacity and service life of the battery.

So, in order to avoid all kinds of drawbacks, we need to find a way to make the electrolyte as much as possible soak the pole piece. Of course, considering the cost issue, we need to use the appropriate amount of electrolyte as possible. The amount of electrolyte affects the performance of the battery.

3. How to improve the wettability of electrolyte

The electrolyte infiltration of the pole piece involves the three-phase contact of solid, liquid and gas. When the electrolyte is injected into the battery case, the electrolyte must first discharge the air in the case, and then the electrolyte will adhere to the surface of the positive and negative active materials, and some electrolyte will enter the positive electrode-diaphragm-negative electrode through the diaphragm of the winding core. between. As time goes on, the electrolyte will infiltrate the pole pieces, and the electrolyte in the diaphragm will infiltrate the pole pieces in the reverse direction. When the standing time is long to a certain extent, under the action of surface tension, the infiltration of the pole pieces will reach A state of balance.

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 the liquid, and the gray area represents the solid interface. Then the blue and gray contact area is the solid-liquid contact interface. The position where the tangent of the liquid intersects the solid interface forms an angle θ. The smaller the contact angle θ, the better the wettability of the electrolyte to the pole piece or diaphragm. .

However, in the actual operation process, it is often impossible to grasp the infiltration effect of the electrolyte on the pole piece. According to the principle of 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 injection process

Improving the liquid injection process is the most conventional method, which can effectively improve the infiltration effect of the electrolyte in terms of liquid injection efficiency, liquid injection conditions, standing time, and liquid injection methods.

Liquid injection under vacuum conditions not only facilitates the discharge of gas in the cell, but also reduces the resistance of the gas to the electrolyte injection, and helps the electrolyte to infiltrate the pole pieces. The principle is that vacuum injection can reduce the existence of gas resistance at the solid-gas-liquid three-phase interface, allowing the electrolyte to directly contact the pole piece, reducing the infiltration time.

By extending the time of standing under vacuum, it can ensure that the electrolyte is fully infiltrated into the pole piece. After liquid injection, as the standing time is prolonged, the wetting angle between the electrode solution and the pole piece gradually decreases, and the wetting radius gradually increases, and finally a good wetting effect is achieved.

In order to avoid the phenomenon of insufficient electrolyte infiltration of the diaphragm and pole pieces, the electrolyte can be injected in batches to facilitate the electrolyte to fully infiltrate the pole pieces. In principle, this operation method is to increase the probability of solid-liquid contact and expand the contact area. , In the case of the same amount of electrolyte, the infiltration time can be shortened.

(2) Improve the core process

The electrolyte infiltration effect is not unrelated to the properties of the electrode material particles, the compaction density of the pole pieces, and the tightness of the core. Different morphology and particle size of the positive and negative active materials, conductive agents, and electrolyte have different infiltration effects on the pole pieces. The larger the particle size of the raw material, the closer to the spherical shape, the greater the electrolyte penetration rate and the longer the infiltration time. short. When the compaction density of the pole piece is too large, the porosity in the pole piece will decrease, which is not conducive to the electrolyte infiltration of the pole piece. It is necessary to adjust the appropriate compaction density to meet the electrolyte infiltration under the condition of ensuring low battery impedance. degree. Similarly, the tightness of the cell stack or winding will also affect the electrolyte infiltration.

When the winding is looser, the pores between the positive electrode-the separator-the negative electrode are larger, and the amount of accumulated electrolyte is larger, resulting in enrichment in some places and lack in some places, which undoubtedly has a great impact on the performance of the battery. When the winding is tight, it will affect the infiltration speed and efficiency of the electrolyte, which is not desirable.

(3) Add electrolyte infiltration agent

The electrolyte usually used is an organic solvent, and the pole piece is an inorganic material, so the ability to absorb the electrolyte is weak. Adding additives to the electrolyte can also improve the infiltration of the electrolyte. Liu Fangfang and others used a fluoroether material as an additive to the electrolyte. The test results show that adding a small amount of infiltrant to the electrolyte can effectively shorten the battery injection time and significantly improve the cycle performance of the battery, but it should be noted that it is used as an infiltrant. When the addition amount reaches 1%, it will have a negative impact on the cycle performance.

The essence of the sizing agent is a surfactant. This type of sizing agent has the advantages of high surface activity, high heat stability, low flammability, and high chemical stability. Adding 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 costs, improve the infiltration effect, reduce the battery interface impedance, and improve the utilization efficiency of active materials, thereby increasing the battery capacity and improving the discharge rate characteristics.