Electrochemical Degradation Mechanism and Thermal Behaviors of the Stored LiNi0.5Co0.2Mn0.3O2 Cathode Materials.

The degradation mechanism of the stored LiNi0.5Co0.2Mn0.3O2 (NCM523) electrode has been systematically investigated by combining physical and electrochemical tests. After stored at 55 °C and 80% relative humidity for 4 weeks, the NCM523 materials are coated with a layer of impurities containing adsorbed species, Li2CO3 and LiOH, resulting in both the weight gains of the materials and the electrochemical performance deterioration of the electrode. The impurities generated in air will react with the electrolyte and instantly turn into Li xPO yF z and other species containing the decomposition products of electrolyte when the stored NCM523 materials are soaked into the electrolyte, causing the charge potential plateau and the impedance to ascend. For the stored NCM523 electrodes, the huge and changeable impedance deteriorates the discharge capacity in the first 10 cycles and the discharge capacity will slowly recover and stabilize within 10 cycles when charging/discharging in 0.1 or 0.2 C. The thermal stability of the stored NCM523 materials get slightly better due to the relatively lower delithiated state after charged to 4.3 V.