A method of increasing the energy density of layered Ni-rich Li[Ni1−2xCoxMnx]O2 cathodes (x = 0.05, 0.1, 0.2)

Abstract

Lithium-ion batteries with high energy density, long cycle life, and appropriate safety levels are necessary to facilitate the penetration of electrified transportation systems into the automobile market. Currently, Ni-rich layered Li[Ni_1−2xCo_xMn_x]O₂ (NCM, x ≤ 0.2) cathodes show high capability for increasing the energy densities of cells. However, the poor thermal stability of this type of cathode is retarding their commercialization. In this study, it is demonstrated that operating Ni-rich cathodes at higher cut-off potentials (>4.3 V) rather than progressing to highly nickel enriched compositions can be a better method of enhancing their energy densities and maintaining adequate thermal stability. It is shown that a Li[Ni_0.6Co_0.2Mn_0.2]O₂ (NCM-622) cathode cycled up to 4.5 V exhibits a discharge capacity of 200 mA h g⁻¹ and a capacity retention of 93% after 100 cycles, which are similar to those of Li[Ni_0.8Co_0.1Mn_0.1]O₂ (NCM-811) cycled up to 4.3 V. A similar volume change during cycling and comparable NiO-like rocksalt impurity layer after 100 cycles in both of the cathodes may be the reason for their similar cycle lives despite operating at different charge cut-off potentials. In spite of the comparable capacity and retention, the NCM-622 cathode exhibits superior thermal stability, in which the occurrence of the exothermic reaction is delayed by 50 °C, to NCM-811. In addition, analogous trends are observed in the cathodes with higher nickel compositions, i.e., NCM-811 and Li[Ni_0.9Co_0.05Mn_0.05]O₂ cycled up to 4.5 V and 4.3 V, respectively.