In situ mitigating cation mixing of Ni-rich cathode at high voltage via Li2MnO3 injection

Abstract

Ni-rich layered oxides (LiNixCoyMnzO2, x ≥ 0.8) have been under intense investigation as cathode materials for high-energy rechargeable lithium ion batteries (LIBs) due to their high capacity and relatively low cost. However, Ni/Li cation mixing, in most cases, brings about capacity degradation, structure evolution and poor thermal stability, especially at high cut-off voltage. Herein, a universal strategy with novel mechanism-in situ mitigating cation mixing at 4.55 V via injecting Li2MnO3 has been achieved (label as LD-NCM811), significantly improving the electrochemical property, structural integrity and thermal stability of Ni-rich cathode materials compared with the conventional NCM811. LD-NCM811 maintains a high capacity retention of 93% at 0.3 C after 200 cycles at 25 °C with negligible voltage decay of 40 mV, whereas the NCM811 shows a retention of 68% and large voltage decay of 248 mV, and the corresponding cation mixing has been mitigated from 13.5% to 7.5%. At the temperature of 45 °C, LD-NCM811 still keeps a considerable capacity retention of 93% at 1 C, significantly superior to the NCM811 with 75%. Characterization and calculation reveal that the excellent performances result from the Li2MnO3 phase with unique superlattice providing lithium voids in transition metal (TM) oxide layers when it is charged above 4.5 V, which is favorable for the mixed Ni ions migrating back to TM layers instead of blocking the lithium channel. This new finding establishes a general strategy for mitigating cation mixing of NCM811 to realize its application in high energy density and safety batteries.