402030 polymer battery aging mechanism and process

　　Aging generally refers to the completion of battery assembly and liquid injection, and the placement after the first charge and discharge. It can be aged at room temperature or at high temperature. As mentioned in the previous article "The Impact of Lithium Battery Aging System on Battery Performance", aging The main purposes are as follows:

　　1. Placing the battery at high temperature or normal temperature for a period of time can ensure that the electrolyte can fully infiltrate the pole pieces, which is beneficial to the stability of battery performance;

　　2. After the battery undergoes the preformation process, a certain amount of SEI film will be formed on the graphite anode inside the battery. However, this film has a tight structure and small pores. Aging the battery at high temperatures will help the SEI structure reorganize and form a loose and porous film. membrane.

　　3. After formation, the voltage of the battery is in an unstable stage. After aging, the active materials in the positive and negative electrode materials can accelerate some side effects, such as gas production, electrolyte decomposition, etc., making the electrochemical performance of the lithium battery faster. reach stability.

　　4. Eliminate unqualified batteries with serious self-discharge to facilitate the selection of batteries with high consistency.

　　Among them, the aging process to screen internal micro-short-circuit cells is a major purpose. The open circuit voltage will decrease during battery storage, but the amplitude will not be very large. If the open circuit voltage decreases too fast or the amplitude is too large, it is an abnormal phenomenon. Battery self-discharge can be divided into physical self-discharge and chemical self-discharge according to different reaction types. Considering the impact of self-discharge on the battery, self-discharge can be divided into two types: self-discharge in which the loss of capacity can be reversibly compensated and self-discharge in which the capacity is permanently lost. Generally speaking, energy loss caused by physical self-discharge is recoverable, while energy loss caused by chemical self-discharge is basically irreversible. The self-discharge of the battery comes from two aspects:

　　(1) Self-discharge caused by the chemical system itself; this part is mainly caused by side reactions inside the battery, including changes in the surface film layers of positive and negative electrode materials; potential changes caused by electrode thermodynamic instability; metal foreign matter impurities dissolution and precipitation;

　　(2) The micro short circuit inside the battery caused by the separator between the positive and negative electrodes leads to self-discharge of the battery.

　　When a 402030 polymer battery ages, the change in K value (voltage drop) is the formation and stabilization process of the SEI film on the surface of the electrode material. If the voltage drop is too large, it means there is a micro short circuit inside, and the battery can be judged as a defective product. The K value is a physical quantity used to describe the self-discharge rate of the battery core. Its calculation method is the open circuit voltage difference between the two tests divided by the time interval △t between the two voltage tests. The formula is: K=(OCV2-OCV1)/△t .

　　Particles or trace metal residues on the pole pieces, tiny defects on the diaphragm, dust introduced during the assembly process of the battery core, etc. will all cause micro short circuits inside the battery core. For micro-short-circuit cells, it is impossible to complete the screening only by capacity and primary voltage, so the K value test must be introduced: by accurately calculating the voltage drop rate to determine whether there is a micro-short circuit in the cell, as shown in Figure 1.

　　There are two basic processes for metal foreign matter to cause internal short circuit in the battery, as shown in Figure 2. Larger metal particles pierce the separator directly, causing a short circuit between the positive and negative electrodes, which is a physical short circuit. In addition, when metal foreign matter is mixed into the positive electrode, the potential of the positive electrode increases after charging. The metal foreign matter dissolves at the high potential and diffuses through the electrolyte. Then the metal dissolved at the low potential of the negative electrode precipitates and accumulates on the surface of the negative electrode, eventually piercing the separator to form Short circuit, this is a chemically dissolved short circuit. The most common metal foreign objects at battery factory sites include Fe, Cu, Zn, Al, Sn, SUS, etc.

　　Faced with such complex metal foreign objects, manufacturing sites often take measures to prevent foreign objects from being mixed into battery products. For example, electromagnetic iron removal equipment is used to remove Fe and other metal impurities from the electrode slurry. Brushes are used to remove cutting burrs during the pole piece slitting or die-cutting process. Tapes are applied to the tabs or coating edges to protect the electrode slurry. Processes that are prone to generate metal chips (welding) ) Use a dust collector to absorb foreign matter, etc. During the process inspection, the battery was tested to detect internal short-circuit defective products through the withstand voltage test before liquid injection; the aging process was used to detect defective products through the battery voltage drop ΔV.

　　The voltage drop K value is a function of time t, state of charge and temperature T. Therefore, the aging process mainly has three process parameters:

　　(1) Aging battery charge status,

　　(2) Aging storage temperature,

　　(3) Aging time.

　　Under certain temperature conditions, the relationship curve between K and time is shown in Figure 4. When the temperature is constant, K decreases with the extension of standing time. This only means that the self-discharge rate of the battery will decrease over time, but the magnitude of self-discharge within a certain period of time is certain, which does not essentially improve self-discharge.

　　Under the condition of a certain storage time, the K value increases with the increase of temperature. As the temperature increases, the activity of the system increases, the reaction rate accelerates, the loss of active lithium is accelerated, and some side reactions are even produced. The dissolution of metal impurities in the positive electrode and the precipitation in the negative electrode will also accelerate as the temperature increases. Because the internal micro-short circuit of the battery takes a long time to manifest itself. Therefore, high-temperature aging can speed up the process of selecting defective products and save time and production costs.

　　Under certain conditions of storage time and storage temperature, within a certain voltage range (3.8-4.2V), the K value increases with the increase of the state of charge. The increase in SOC will accelerate the self-discharge rate of the battery, and the interface resistance of the negative electrode will increase as the storage SOC increases. According to the chemical balance, as the Li concentration of the negative electrode gradually increases, the interface reaction moves in the direction of consuming Li, which will consume more active Li.

　　The general aging procedure is: charge to 4.0-4.2V, store at room temperature for 7 days, store at high temperature of 45°C for 7 days, detect the voltage difference before and after battery aging and eliminate unqualified products. Leave the battery in open circuit at high temperature or normal temperature for 7 days or 28 days, and judge its self-discharge performance by discharging the battery to the cut-off voltage and measuring its discharge capacity. This method requires the battery to be put on hold for up to a month to test. The time period is long, the influencing factors are large, the accuracy is not high, and it occupies a lot of equipment and space for a long time. The test safety is poor, and it is a waste of human and financial resources. A lot of waste. Pierrot S. Attidekou of Newcastle University in the UK has shortened the self-discharge screening time of lithium-ion batteries from several weeks to less than 10 minutes through the application of AC impedance means. Through continued optimization, it is expected to further shorten the screening time to 1 minute.

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