Abstract: Driven by increasing demand for long range of electric vehicles, Ni-rich cathode materials have attracted lots of attention for the development of high energy density EV batteries. As the life span of EV batteries needs to be more than 10 years and the product development time is limited, an accelerated life span testing is often used to assess the long-term performance of the batteries. In this work, we prepared NCM811 cathode material through co-precipitation and high-temperature calcination and stored NCM811/Graphite pouch-type full cells at 60 ℃ in a fully charged state for the investigation of their storage performance. It was found that the storage capacity of the cells decreased to 80% after 180-day storage. XRD, SEM, ICP-AES, XPS and HRTEM techniques were used to investigate the differences between freshly made and the degraded electrodes. The results demonstrated the formation of by-products on the surface of cathode, and cathode materials exhibited layered-spinel-rock salt phase transformation after storage, both greatly increased the cell impedance. In addition, transition metal ions dissolved from the cathode were found to accumulate on the anode, which may have destroyed the SEI, leading to the consumption of the active lithium. Surface coating and bulk doping could resolve the problem through stabilizing the surface and bulk structure of the cathode materials.

Key words: Ni-rich cathode, high temperature storage, failure mechanism