Investigation of the oxygen ligand hole of Ni-rich layered cathodes: a new organic coating for enhancing battery performance

Abstract

Ni-rich (Ni content > 60%) layered cathodes are currently the most promising materials for lithium-ion batteries due to their high capacity and elevated voltage plateau compared to LiFePO₄. However, Ni-rich cathodes face significant challenges, such as Jahn–Teller distortion, cation mixing, electrolyte deprotonation, and the presence of residual lithium compounds. These issues impede the widespread use of high-energy-density lithium-ion batteries employing Ni-rich cathodes. Ni-rich cathodes, containing a high concentration of Ni³⁺, encounter another problem known as the oxygen ligand hole effect, which affects the hybridization of O 2p and Ni 3d orbitals. Anionic redox occurs at the oxygen site with Ni³⁺, leading to a decrease in electron density, making the formation of Ni⁴⁺ at high states of charge (SOC) difficult. Consequently, battery capacity is primarily derived from anionic redox reactions. This study presents an organic coating (OC) designed to enhance the stability of the oxygen ligand hole, enabling greater capacity through enhanced Li⁺ interaction. Additionally, Ni-rich cathodes often suffer from gas evolution when charged to a high SOC, primarily due to the instability of the Ni–O bond. The OC is hypothesized to support the chemical reduction of Ni 2p⁵3dⁿ⁺² back to Ni 2p⁶3dⁿ⁺¹, where the represents O 1s¹2p^x+1 → O 1s²2p^x on the surface of NMC811, thereby strengthening the oxygen ligand hole and stabilizing covalent Ni³⁺. This improvement results in the OC-modified NMC811 exhibiting outstanding cycle performance under high-rate tests and excellent stability at high temperatures.