Designing Co-Free Medium-Ni Layered Oxide Cathodes via Additional Li Substitution.

High-capacity cathode materials are requisite for constructing high-energy-density Li-ion batteries. Although efforts are being concentrated on exploring ultra-Ni-rich layered oxides, the structural instability of these cathodes remains a hard nut to crack before reaching their practicality. Alternatively, raising the upper cutoff voltage of medium-Ni oxides can equivalently increase the capacity, but it also devastates the structure. Here, a strategy is proposed to circumvent this dilemma by enriching the Li ions in medium-Ni layered oxides, and meanwhile remaining Co-free. Through surveying a range of compositions of Li1+yNi1-3yMn2yO2 (0.03≤ y≤0.15), a threshold for Li richness in this class of compounds is pinpointed, as represented by Li1.12Ni0.64Mn0.24O2, which is crucial to reach optimum capacity and cycling. This is delicately mediated by the proper amount of reversible anionic redox, as evidenced by electrochemistry and spectroscopy, that contributes to the structural stability even cycled to 4.5 V. We also found that the existence of additional Li ions in the lattice can effectively suppress the Li/Ni mixing, thereby weakening the necessity of Co in Ni-based cathodes. As a result, Li1.12Ni0.64Mn0.24O2 shows a capacity of ≈200 mA h g-1 that can be sustained for 100 cycles, which is further validated by an excellent long-life full-cell performance.