Selective core-shell doping enabling high performance 4.6 V-LiCoO2

Constructing robust surface and bulk structure is the prerequisite for realizing high performance high voltage LiCoO2 (LCO). Herein, we manage to synthesize a surface Mg-doping and bulk Al-doping core-shell structured LCO, which demonstrates excellent cycling performance. Half-cell shows 94.2% capacity retention after 100 cycles at 3-4.6 V (vs. Li/Li+) cycling, and no capacity decay after 300 cycles for full-cell test (3.0-4.55 V). Based on comprehensive microanalysis and theoretical calculations, the degradation mechanisms and doping effects are systematically revealed. For the undoped LCO, high voltage cycling induces severe interfacial and bulk degradations, where cracks, stripe defects, fatigue H2 phase, and spinel phase are identified in grain bulk. For the doped LCO, Mg-doped surface shell can suppress the interfacial degradations, which not only stabilizes the surface structure by forming a thin rock-salt layer but also significantly improves the electronic conductivity, thus enabling superior rate performance. Bulk Al-doping can suppress the lattice "breathing" effect and the detrimental H3 to H1-3 phase transition, which minimizes the internal strain and defects growth, maintaining the layered structure after prolong cycling. Combining theoretical calculations, this work deepens our understanding of the doping effects of Mg and Al, which is valuable in guiding the future material design of high voltage LCO.