Revealing the correlation between structural evolution and Li+ diffusion kinetics of Nickel-rich cathode materials in Li-ion batteries.

Nickel-rich cathode material such as LiNi0.8Co0.1Mn0.1O2 (NCM811) is attracting wide attention due to its high specific capacity and low cost. However, the low initial coulomb efficiency (CE) and poor rate performance of the material have hindered the maximum ultilization of its capacity. Here, we systematically investigated the causes of the low CE and correlation between structural evolution and Li+ diffusion kinetics during the first and followed charge/discharge cycles. Combined with GITT, in situ EIS and in situ XRD, we disclose that the diffusion of lithium ions is mainly affected by the structural evolution or phase changes during charge-discharge cycles. The lithium diffusion kinetics of H1 phase at high lithium content is rather slow, and the decrease of lithium vacancies together contribute to the sharp decrease of lithium diffusion coefficient at the end of discharge. The slow kinetics of H1 phase is even more conspicuous at low temperature as evidenced by in situ XRD and GITT experiments at low temperature. In addition, a new charge/discharge mode is proposed and demonstrated for increasing the deliverable discharge capacity and CE at the first cycle for NCM811 cathode materials.