Deterioration and Acoustic Emission Characteristics of Sodium-ion Batteries Induced by Overcharge

Sodium-ion batteries possess broad application potential in large-scale energy storage due to the abundance of sodium resources. However, the inconsistency of cells within modules makes overcharging a potentially harmful condition that can accelerate battery aging and induce thermal runaway accidents. This paper investigated the performance degradation characteristics of overcharged sodium - ion batteries with layered oxides as the cathode material, and studied the active acoustic events occurring in sodium-ion batteries during overcharging cycles through acoustic emission (AE) detection technology. Firstly, experiments combining overcharging and acoustic emission of sodium-ion batteries at different charging rates and cut-off voltages were conducted. The results indicated that before overcharging to 4.2 V (108% SOC), the battery's capacity did not undergo accelerated decay. As the battery continued to be overcharged, it experienced a long voltage plateau, which caused the battery voltage to only reach 4.3 V when overcharged to 136% SOC. The capacity of the sodium-ion battery overcharged to 4.3 V had suffered severe degradation, and the degradation rate was positively correlated with the charging rate. In addition, the analysis results of differential voltage curves and electrochemical impedance spectroscopy indicated that there was a large amount of irreversible sodium loss in the sodium-ion battery overcharged to 4.3 V. Meanwhile, the morphological and elemental characterization results of the disassembled electrodes showed that the cathode of the battery had undergone decomposition, while the anode had experienced severe sodium plating. Finally, the acoustic emission test results showed that there was a strong correlation between the acoustic emission signal and the overcharging process of the battery. Before the sodium-ion battery was overcharged to 4.2 V, the battery generated very few acoustic emission events with low energy. In the subsequent overcharging process, the battery concentrated on generating a large number of high-energy acoustic emission signals. These acoustic emission signals could mainly be divided into two categories. The first category of acoustic signals had a low frequency and a long rise time, which might come from the gas-producing side reaction inside the battery. The second category of acoustic signals had a high frequency and an extremely short rise time, which might come from the growth and diffusion behavior of particle cracks during the decomposition of the positive electrode. According to the acoustic emission analysis, it was possible to accurately locate the critical points of overcharging-induced side reactions at the positive and negative electrodes.