Multi-Site Redox Activities in Chemically Complex Sodium Layered Oxide Cathode Materials

Abstract

Sodium layered oxide cathode materials with high specific capacity, energy density, and long cycle life have been the bottleneck for the widespread market implementation of sodium-ion batteries. Even though sodium layered oxide materials share many similarities with their Li-ion counterpart, there are many dissimilarities between the two which can be utilized to design high performance sodium layered oxide cathode materials. Sodium layered oxide materials can incorporate almost all of the first row transition metals in the transition metal oxide layer. Herein, we have synthesized a sodium layered oxide material with five transition metals to demonstrate such concept. The material is synthesized following a simple co-precipitation method and is a mixture of P2 and P3 type crystal structures (P2/P3=1). The material has good electrochemical performance with excellent stability on long term cycling. It delivers a specific capacity in the range of 95 mAh/g at C/10 with no capacity fading after 100 cycles, 72 mAh/g at 1C with 95% capacity retention after 500 cycles, and 55 mAh/g at 5C with 85% capacity retention after 1000 cycles. We further demonstrate the surprisingly good performance of the material at low temperatures. For example, the material delivers a 60 mAh/g capacity at 1C and 0°C with no capacity fading after 100 cycles. We further studied the structural evolution and redox reaction mechanism of the cathode material at different states of charge. The reversibility of the Na (de)insertion is evident from the reversibility of the XRD pattern upon cycling. Hard XAS study has revealed that most transition metals contribute to some degree towards the redox reaction of the cathode material, with most energy derived from the redox of Ni2+ and Co3+. This study has demonstrated that selecting the ideal transition metal hosts and building up the compositional complexity can be a path towards further improving sodium cathode materials.