Ni-rich layered cathode materials are important candidates for lithium-ion batteries used for high energy density applications, such as electric vehicles. However, its capacity fades during cycling, especially when a high cut off charge voltage is applied (>4.4 V). To understand the origin of this capacity fade, comprehensive diagnostic tools are applied to understand their structure, chemical and morphological properties during cycling. X-ray diffraction results show that while the LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) can mostly recover its original structure at fully discharged state, even after 0.95 Li is extracted (charged to 5.2 V), it does undergo slab sliding that leads to stacking fault (O1 type) which does not go away during cycling. In addition, transmission X-ray imaging (TXM) results indicate that cycling can induces Li content inhomogeneity within the particle. As a result, the strain builds up and is directly responsible for the cracking of primary and secondary particles. Hard X-ray absorption spectroscopy (XAS) results indicate that both Ni and Co contribute to the capacity by going through the Ni³⁺/Ni⁴⁺ and Co³⁺/Co⁴⁺ redox couples. A comparison is made between Ni-rich and the Li-rich layered materials, which is well known for the problem of oxygen release. It is found that for the Ni-rich system, the oxidization state of transition metals (bulk) are well maintained during cycling, whereas for the Li-rich system the average oxidization states keep decreasing (lower oxidation states at both fully charged and fully discharged states after cycling than the pristine sample), suggesting at least the bulk of Ni-rich material is robust against oxygen release. Soft XAS that differentiates the surface chemical information from the bulk indicates that inactive rock salt phase is formed during cycling. These results show that cycling induced strain, stacking fault and surface reconstruction are the main issues to be addressed for further improvement of this material.

Acknowledgement

This project was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies through Advanced Battery Material Research (BMR) program (Battery500 consortium) under Contract No. DESC0012704. This research used resources 8-ID and 28-ID-2 of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract no. DE-SC0012704. The authors would also like to acknowledge the use of beamline 6-2c of Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-76SF00515.