Engineering of surface oxygen vacancies in Co-free concentration-gradient Li-rich cathodes for high-capacity batteries

Abstract

Li-rich layered oxides (LLOs) are the most promising high-capacity materials in the Li-ion batteries owing to their cumulative oxygen anionic and cationic redox reactions. However, irreversible oxygen evolution and transition-metal migration aggravate structural rearrangement and capacity fading. Although Co-containing LLO possesses practical capacity and stability for commercial purposes, it suffers from scarcity, toxicity, and high cost of cobalt. To ameliorate these bottlenecks, we improved the reversibility of anionic and cationic redox in Co-free concentration-gradient LLO by integrating oxygen vacancies and high Ni/Mn ratio on LLO's surface. We found that surface oxygen vacancies suppress oxygen release from the surface, reduce the interfacial resistance that arises from electrolyte decomposition, and increase the ionic diffusion; thereby, enhancing the cathode's capacity. Concentration-gradient LLO cathode with an appropriate Mn/Ni ratio improved cation migration's structural stability and reversibility in the layered and spinel-layered Li-rich materials. As a result, the charge transfer and interfacial resistance decreased during the gradual de-lithiation and Ni redox reaction. The Co-free LLO with surface Mn/Ni∼1.5 delivered a reversible capacity of 250 mAh/g, with 99% capacity retention after 85 cycles at 0.1 C. This work provided a practical approach for resolving the capacity decay of the Co-free LLOs by engineering the cathode/electrolyte interface and structure.