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Influence of Aging and Decline of Lithium Battery Materials

The charge and discharge cycle of lithium ion battery is the process of lithium ion moving back and forth between positive and negative materials through electrolyte. In the process of lithium ion battery cycle, besides redox reaction, there are many side reactions. If the side reactions of lithium-ion batteries can be reduced to a low level, and lithium ions can always smoothly travel between positive and negative materials through electrolyte, the cycle life of lithium-ion batteries can be prolonged.
When lithium ions move from the positive electrode to the negative electrode, they must pass through the SEI film covered on the carbon negative electrode, and the quality of the SEI film directly affects the cycle life of the battery. Foreign scholars have studied the aging degradation of battery materials earlier, especially the SEI film. The main research method is to analyze the stability and decay mechanism of battery materials through the experimental data of battery life combined with electrochemical characterization.
The stability of SEI film has an important influence on the stability of battery. The unstable SEI film is easy to precipitate lithium metal, which will lead to the rapid decline of negative active materials. The lithium battery with stable SEI film can be stored at high temperature for more than 4 years. D. Aurbach et al. dismantled the recycled lithium cobaltate battery, and analyzed the positive and negative plates by SEM, XRD and other experiments. The capacity decline was mainly attributed to the irreversible side reactions such as the continuous consumption of Li+ by the negative SEI film and the LiF interface film formed by LiCoO2 and HF in the positive electrode. P.Ramadass et al. established a capacity decline model by describing the process of lithium ion loss caused by the continuous growth of the negative electrode SEI film during the charge-discharge cycle. S.Sankarasubramanian et al. established a capacity decline model including solvent diffusion and SEI film growth mechanism, and obtained that the capacity decline is linearly related to SEI film thickness and battery aging time.
The aluminum-plastic film lithium-ion battery with 200 charge-discharge cycles was studied. The results showed that the discharge capacity of the battery gradually decreased, while the internal resistance and thickness gradually increased. After disassembling the batteries with different cycle times, the experimental observation shows that after 200 cycles, many cracks appear on the surface of the positive electrode and the average particle size decreases; The negative electrode showed that SEI film became thicker and lithium and lithium compounds precipitated at the end of cycle. The removal and intercalation of lithium ions will lead to the generation of lattice internal stress. Under the fatigue action of this internal stress, LiCoO2 cracks and the final particle size decreases.
In this paper, the aging mechanism of battery internal materials with charge-discharge cycle is analyzed in depth, and the factors such as the stability of crystal structure of electrode materials, the side reaction of interface between active materials and electrolyte, and the degradation of binder performance are summarized, and the reasons and effects of aging of positive and negative electrodes are summarized. As for the negative electrode materials, besides the factors that the contact between anode components is deteriorated due to the formation and growth of SEI film, the main factors are: the C particles are broken due to the gas generated by the solvent embedded in the C electrode, the contact between active material particles is deteriorated due to the change of volume in circulation, and the precipitated lithium metal reacts with the electrolyte to accelerate aging. Causes and effects of aging and decay of cathode materials.
Causes and effects of aging of cathode materials
Common electrolyte consists of solvent (alkyl carbonates such as EC, DEC, DMC, etc.), lithium salt (LiPF6, LiBF4, etc.) and various additives. The process of lithium ion removal and intercalation of positive and negative materials always interacts with electrolyte. Complex redox reactions will occur at the interface because of this action, and even produce gas or solid products, which will cause the loss of electrolyte. Gas will increase the internal pressure of the battery, resulting in deformation of the battery. Solid products will form passivation film on the electrode surface, which will increase the polarization of the battery and reduce the output voltage of the battery. These factors will have a negative impact on the battery capacity and safety, and ultimately affect the cycle life of the battery. Adding additives can effectively improve the cycle performance of lithium ion batteries, such as adding trace additive anisole in EC/DEC solvent system.
The properties of positive and negative current collectors also affect the capacity and cycle life of batteries. The commonly used current collector materials for positive and negative electrodes of lithium ion batteries are aluminum and copper, which are both corrosive metal materials. Passivation film formed after corrosion of current collector, poor adhesion, local corrosion (pitting corrosion) and overall corrosion will increase the internal resistance of battery, resulting in capacity loss and reduction of discharge efficiency. The adhesion and corrosion resistance can be enhanced by pretreatment methods such as acid-alkali etching and conductive coating.