Hierarchical Doping Engineering with Active/Inert Dual Elements Stabilizes LiCoO₂ to 4.6 V

It is highly desirable to raise the charge cutoff voltage to realize the potential of LiCoO₂ (LCO) with its ultra-high theoretical capacity of 275 mAh g^-1. However, rapid fading due to structure collapse caused by the formation of the H1-3 metastable phase and the release of surface lattice oxygen has largely hindered the operation of LCO under voltages of higher than 4.55 V. Here, stable cycling of LCO at 4.6 V through hierarchical doping engineering with inert P-outside and active Ni-inside dual doping is achieved. This ingenious outside-in structure design enables Ni²⁺ occupation in the Li layer in the bulk layered phase and P gradient doping at the superficial lattice. Compared with the conventional inert element substitution strategy, the doped active Ni²⁺ can not only serve as a “pillar” to restrain the formation of the metastable H1-3 phase, but also regulate the electronic structure of LCO and trigger the superexchange interaction of Ni²⁺-O-Co⁴⁺, together with strong P–O coordination to substantially suppress the lattice oxygen escape from the surface. Therefore, it considerably reduces the risk of layer structure collapse and consequently achieves stable and high-capacity operation over 4.6 V. This hierarchical outside-in doping strategy may serve as inspiration for stabilizing high energy electrode materials working under high voltages.