Effect of Cationic Uniformity in Precursors on Li/Ni Mixing of Ni-Rich Layered Cathodes

Ni-rich layered oxides are promising cathodes to satisfy the long driving range of electric vehicles. However, Li/Ni mixing becomes a critical issue, which affects nearly all of the aspects of electrochemical performance of Ni-rich layered cathodes, such as cycling stability, rate capability, discharge capacity, and thermal stability. Herein, we investigate the effect of cationic uniformity in the precursors on Li/Ni mixing of Ni-rich layered cathodes. The cationic uniformity in precursors is regulated by the sol-gel and solvothermal method. All cations in the precursors obtained by the sol-gel method mix uniformly at a molecular scale, while the solvothermal method with a postaddition of Li salts causes a heterogeneous mixture between Li and transition-metal ions. Refined XRD and TEM results demonstrate that the NCM811 sample synthesized by the solvothermal method shows a higher Li/ Ni mixing. This is because the inhomogeneous distribution of cations leads to local Li-poor regions, in which some Ni2+ ions are produced to keep charge neutrality, resulting in a serious Li/Ni mixing. Part of Ni ions in the Li layer suppresses the H2-H3 transition while it delivers a low capacity for increased electrode resistance. Therefore, advanced Ni-rich layered cathodes of the next-generation high-energy Li-ion batteries should simultaneously possess low Li/Ni mixing and reversible H2-H3 transition.