Mitigating Lattice Distortion of High-Voltage LiCoO2 via Core-Shell Structure Induced by Cationic Heterogeneous Co-Doping for Lithium-Ion Batteries

Inactive elemental doping is commonly used to improve the structural stability of high-voltage layered transition-metal oxide cathodes. However, the one-step co-doping strategy usually results in small grain size since the low diffusivity ions such as Ti⁴⁺ will be concentrated on grain boundaries, which hinders the grain growth. In order to synthesize large single-crystal layered oxide cathodes, considering the different diffusivities of different dopant ions, we propose a simple two-step multi-element co-doping strategy to fabricate core-shell structured LiCoO₂ (CS-LCO). In the current work, the high-diffusivity Al³⁺/Mg²⁺ ions occupy the core of single-crystal grain while the low diffusivity Ti⁴⁺ ions enrich the shell layer. The Ti⁴⁺-enriched shell layer (~ 12 nm) with Co/Ti substitution and stronger Ti-O bond gives rise to less oxygen ligand holes. In-situ XRD demonstrates the constrained contraction of c-axis lattice parameter and mitigated structural distortion. Under a high upper cut-off voltage of 4.6 V, the single-crystal CS-LCO maintains a reversible capacity of 159.8 mAh g^-1 with a good retention of ~ 89% after 300 cycles, and reaches a high specific capacity of 163.8 mAh g^-1 at 5C. The proposed strategy can be extended to other pairs of low- (Zr⁴⁺, Ta⁵⁺, and W⁶⁺, etc.) and high-diffusivity cations (Zn²⁺, Ni²⁺, and Fe³⁺, etc.) for rational design of advanced layered oxide core-shell structured cathodes for lithium-ion batteries.