An in-depth understanding of chemomechanics in Ni-rich layered cathodes for lithium-ion batteries

Anisotropic lattice variations in Ni-rich layered oxides of lithium-ion batteries (LIBs) have been investigated extensively to suppress the chemomechanics and achieve high energy density with long-term cycling stability. However, an in-depth understanding of the anisotropy is lacking and is very important in the design of high-performance Ni-rich cathodes. Therefore, we reinvestigate the fundamentals of anisotropic lattice variations in Li [Ni10/12Mn1/12Co1/12]O2 (NCM) to understand the correlation between cycling stability degradation at high rate and intergranular microcrack generation between the primary particles, which is confirmed as follows: first, the capacity retention of the NCM under 4.3 V cutoff voltage (NCM4.3V) is much poorer than that under 3.8 V (NCM3.8V); this is described by various electrochemical analyses showing the multiple phase transitions accompanying anisotropic lattice variations and structural collapse. These structural evolutions are clearly observed in the ex situ X-ray diffraction patterns. Second, the resistance of NCM4.3V increases at a faster rate than that of NCM3.8V upon cycling, which supports the direct evidence regarding intergranular microcracks in the cycled particles of NCM4.3V. Third, the nonlinear lattice change in the c direction plays a critical role in accelerating cycling stability degradation. Fourth, serious lattice changes originate from the cationic repulsions between the Li and Ni ions with the electrostatic repulsion of oxygen ions. This mechanism is universally expected in Ni-rich layered oxides; furthermore, these findings provide insights into design strategies that mitigate chemomechanical degradations caused by long-term cycling stabilities in LIBs.