Abstract: In application, lithium-ion cells undergo expansion during cycling. The mechanical behavior and the impact of external stress on lithium-ion battery are important in vehicle application. In this work, 18 Ah high power commercial cell with LiNi_0.5Co_0.2Mn_0.3O₂/graphite electrode were adopted. A commercial compress machine was applied to monitor the mechanical characteristics under different stage of charge (SOC), lifetime and initial external force. The dynamic and steady force was obtained and the results show that the dynamic force increases as the SOC increasing, obviously. During the lifetime with high power driving mode, different external force is shown to have a great impact on the long-term cell performance, with higher stresses result in higher capacity decay rates and faster impedance increases. A proper initial external force (900 N) provides lower impedance increasing. Postmortem analysis of the cells with 2000 N initial force suggests a close correlation between electrochemistry and mechanics in which higher initial force leads to higher direct current internal resistance (DCIR) increase rate. In addition, for the cell with higher external force, deformation of the cathode and thicker solid electrolyte interface (SEI) film on the surface of anode and separator are observed. Porosity reduction and closure was also verified after cycles which is an obstacle to the lithium ion transferring. The largest cause of the observed capacity decline was the loss of active lithium through autopsy analysis. In addition, for the cell with higher external force, deformation of the cathode and thicker SEI film on the surface of anode and separator are observed. Porosity reduction and closure was also verified after cycles which is an obstacle to the lithium ion transferring. The largest cause of the observed capacity decline was the loss of active lithium through autopsy analysis.

Key words: Lithium-ion batteries, External mechanical pressure, Capacity fade, Static pressure, Dynamic pressure

摘要： In application, lithium-ion cells undergo expansion during cycling. The mechanical behavior and the impact of external stress on lithium-ion battery are important in vehicle application. In this work, 18 Ah high power commercial cell with LiNi_0.5Co_0.2Mn_0.3O₂/graphite electrode were adopted. A commercial compress machine was applied to monitor the mechanical characteristics under different stage of charge (SOC), lifetime and initial external force. The dynamic and steady force was obtained and the results show that the dynamic force increases as the SOC increasing, obviously. During the lifetime with high power driving mode, different external force is shown to have a great impact on the long-term cell performance, with higher stresses result in higher capacity decay rates and faster impedance increases. A proper initial external force (900 N) provides lower impedance increasing. Postmortem analysis of the cells with 2000 N initial force suggests a close correlation between electrochemistry and mechanics in which higher initial force leads to higher direct current internal resistance (DCIR) increase rate. In addition, for the cell with higher external force, deformation of the cathode and thicker solid electrolyte interface (SEI) film on the surface of anode and separator are observed. Porosity reduction and closure was also verified after cycles which is an obstacle to the lithium ion transferring. The largest cause of the observed capacity decline was the loss of active lithium through autopsy analysis. In addition, for the cell with higher external force, deformation of the cathode and thicker SEI film on the surface of anode and separator are observed. Porosity reduction and closure was also verified after cycles which is an obstacle to the lithium ion transferring. The largest cause of the observed capacity decline was the loss of active lithium through autopsy analysis.

关键词: Lithium-ion batteries, External mechanical pressure, Capacity fade, Static pressure, Dynamic pressure