Heat generation and degradation mechanisms studied on Na3V2(PO4)3/C positive electrode material in full pouch / coin cell assembly

Na3V2(PO4)3/C cathode and commercial hard carbon anode materials are coated with a roll-to-roll coater on an aluminum current collector. Sodium-ion pouch and coin cells are assembled. The degradation mechanism, cycle stability, morphology of cycle-aged material, and aging influence on the heat generation during cycling are thoroughly investigated. Electrochemical impedance spectroscopy permits resistance assignment to the specific electrodes by using the distribution of relaxation times. Comparison of pristine and cycle-aged cells allows separating the degradation contributions of the particular electrodes. The generated heat during charging and discharging is determined by means of an advanced highly sensitive MS80 3D Tian-Calvet calorimeter under isothermal conditions at 25 °C and the heat generation mechanisms are elucidated. It is observed that less heat is generated during charging than discharging process. Furthermore, the cell balancing reveals a pronounced effect (approximately 50% in charging and 20% in discharging) on the amount of generated heat, which is valuable feedback for the material developers and cell designers. These analyses represent a fingerprint for the underlying electrochemical processes at the electrodes and are very essential for state of health, aging prediction as well as the subsequent tackling of safety-related issues in sodium-ion batteries.