Aging generally refers to the completion of battery assembly and liquid injection. After the first charge and discharge, it can be aged at room temperature or high temperature. In the previous article, "Impact of lithium battery aging system on battery performance" mentioned, aging The main purpose of the following are:
1, the battery is placed at high temperature or normal temperature for a period of time, can ensure that the electrolyte can fully infiltrate the pole piece, which is conducive to the stability of the battery performance;
2. After the battery is pre-formed, the graphite negative electrode inside the battery will form a certain amount of SEI film, but the film structure is compact and the pores are small. The battery will be aged at high temperature, which will help the SEI structure reorganization and form loose porous. Membrane.
3. After the formation, the voltage of the battery is in an unstable stage. After the aging of the active material in the positive and negative materials, some side effects can be accelerated, such as gas production, electrolyte decomposition, etc., so that the electrochemical performance of the lithium battery is fast. Stable.
4, Eliminate the self-discharge of the unqualified battery, easy to filter the battery with high consistency.
Among them, the aging process screening internal micro-short-circuit cells is a main purpose. The open circuit voltage will decrease during battery storage, but the amplitude will not be large. If the open circuit voltage drops too fast or the amplitude is too large, it is an abnormal phenomenon. According to the type of reaction, it can be divided into physical self-discharge and chemical self-discharge. From the impact of self-discharge on the battery, self-discharge can be divided into two types: self-discharge and permanent capacity loss that can be reversibly compensated for loss capacity. Self-discharge. In general, the energy loss caused by physical self-discharge is recoverable, and the energy loss caused by chemical self-discharge is basically irreversible. The self-discharge of the battery comes from two aspects: (1) chemical system Self-discharge caused by itself; this part is mainly caused by side reactions inside the battery, including changes in the surface layer of the positive and negative materials; potential changes caused by thermodynamic instability of the electrode; dissolution and precipitation of metal foreign matter impurities; 2) The micro-short circuit inside the battery caused by the diaphragm between the positive and negative electrodes causes self-discharge of the battery.
When the lithium ion battery is aging, the change of 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 indicates that there is a micro short circuit inside, so that the battery can be judged to be a non-conforming product. The K value is a physical quantity used to describe the self-discharge rate of the cell. The calculation method is the open circuit voltage difference of two tests divided by the time interval Δt of the two voltage test. The formula is: K= (OCV2-OCV1) /Δt .
Figure 1 aging unqualified products detected
Particles or traces of metal residues on the pole piece, tiny defects on the diaphragm, dust introduced by the battery during assembly, etc., cause micro-shorts inside the cell. For micro-short cells, screening is impossible only by capacity and primary voltage. Therefore, it is necessary to introduce a K-value test: By accurately calculating the voltage drop rate to determine whether the cell has a micro-short circuit, as shown in Figure 1.
Figure 2 The principle of metal foreign matter causing internal short circuit of the battery
The basic principle of metal foreign matter causing internal short circuit of the battery has two processes, as shown in Figure 2. Larger metal particles directly pierce the diaphragm, causing a short circuit between the positive and negative electrodes. This is a physical short circuit. In addition, when metal foreign matter is mixed in After the positive electrode, the positive electrode potential rises after charging, and the metal foreign matter dissolves at a high potential, diffuses through the electrolyte, and then the metal dissolved at the negative electrode of the negative electrode precipitates on the surface of the negative electrode, eventually piercing the separator to form a short circuit, which is chemical dissolution. Short circuit. The most common metal foreign bodies on the battery factory site are Fe, Cu, Zn, Al, Sn, SUS, etc.
Figure 3 Metal foreign body countermeasures
In the face of such complicated metal foreign objects, the manufacturing site often takes measures to prevent foreign matter from entering the battery product, as shown in Figure 3. If the electrode slurry uses electromagnetic de-ironing equipment to remove metal impurities such as Fe, the pole piece is cut or die-cutting. Wait for the cutting burr, the ear or the edge of the coating is taped, the foreign matter is adsorbed by the dust collector for the process of easily generating metal chips (welding), etc. In the process test, the battery is checked by the withstand voltage test before the injection. Internal short-circuit non-conforming product; aging process through the battery pressure drop ΔV detected non-conforming product.
The voltage drop K value is a function of time t, state of charge, and temperature T. Therefore, the aging process has three main process parameters: (1) aging battery state, (2) aging storage temperature, and (3) aging time.
Under certain temperature conditions, the curve of K versus time is shown in Figure 4. When the temperature is constant, K decreases with the extension of the standing time. This only means that the self-discharge rate of the battery will decrease with time. Small, but the size of self-discharge in a certain period of time is certain, which does not improve self-discharge in essence.
Figure 4K value versus time curve
Under certain conditions of storage time, the K value increases with the increase of temperature. As the temperature increases, the activity of the system increases, the reaction rate increases, the loss of active lithium accelerates, and even some side reactions occur. The dissolution of impurities in the positive electrode and the precipitation in the negative electrode will also increase with the increase of temperature. Since the internal micro-short circuit of the battery takes a long time to be reflected. Therefore, high temperature aging can accelerate the process of selecting the defective product, saving Time and production costs.
Under the condition 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 of SOC will accelerate the self-discharge rate of the battery, the interface of the negative electrode The impedance increases with the increase of the storage SOC. According to the chemical balance, as the Li concentration increases gradually, the interfacial reaction moves toward the consumption of Li, which consumes more active Li.
The general aging procedure is: charging to 4.0-4.2V, storage at normal temperature for 7d, storage at high temperature for 45d for 7d, detecting the voltage difference before and after aging of the battery to reject the unqualified product. Leave the battery open for 7 days or 28 days at high temperature or normal temperature. The self-discharge performance is judged by measuring the discharge capacity of the battery to the cut-off voltage. The method requires the battery to be tested for one month, the time period is long, the influencing factors are large, the accuracy is not high, and it is occupied for a long time. More equipment and venues, poor test safety, is a waste of manpower and financial resources. PierrotS.Attidekou of Newcastle University in the United Kingdom has reduced the self-discharge screening time of lithium-ion batteries from several weeks to 10 minutes through the application of AC impedance. Within, by continuing to optimize, it is expected to continue to reduce the screening time to 1min.
