Lithium-ion batteries demonstrate a noticeable volume swell before thermal runaway occurs. Therefore, studying the changing law of power battery swelling force is significant for improving battery performance and early warning safety. This study explores the changing law of the swelling force for ternary/graphite batteries when charging at various temperatures and rates. The increase in the swelling force of the power battery is minimum when charged at 25 ℃. Consequently, the low ambient temperature causes lithium ions to accumulate on the surface of the graphite layer, thereby increasing the battery thickness and swelling force. Moreover, the higher the ambient temperature, the larger the volume of the power battery; an SEI film is produced during charging, leading to a greater expansion force. When the power battery is charged at different rates, the swelling force gradually rises with an increase in charging rate, possibly because the high rate charging increases the battery temperature; the SEI film is formed on the material's surface. The CT scan results accurately quantify and analyze the thickness variations of the battery during the charging process. The capacity differential curve (dQ/dV) during the charging process reveals that the variation in the growth trend of the swelling battery force is primarily due to the various phase transitions of the positive and negative materials in the different charging stages. Then, the change characteristics of the expansion force in the charging process of the power battery are revealed from the mechanism level.

Keywords： lithium-ion battery ; swelling force ; phase transition