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Strategy of Cation and Anion Dual Doping for Potential Elevating of Titanium Redox for High-Power Sodium-Ion Batteries.

Titanium-based polyanions have been intensively investigated for sodium-ion batteries due to their superior structural stability and thermal safety. However, their low working potentials hindered the further applications. Here, we introduce a cation and anion dual doping strategy to boost the redox potential of Ti-based cathodes of Na 3 Ti 0.5 V 0.5 (PO 3 ) 3 N as a new cathode material for sodium ion batteries. Both the Ti 3+ /Ti 4+ and V 3+ /V 4+ redox couples are reversibly accessed, leading to two distinctive voltage platforms at ~ 3.8 V and ~3.3 V, respectively. The remarkably improved cycling stability (86.3%, 3000 cycles) can be ascribed to the near-zero volume strain in this unusual cubic symmetry, which has been demonstrated by the in-situ synchrotron-based X-ray diffraction. First-principles calculations reveal its well-interconnected three-dimensional Na diffusion pathways with low energy barriers, and the two-sodium-extracted intermediate NaTi 0.5 V 0.5 (PO 3 ) 3 N is also a stable phase according to the formation energy calculations. The nitridophosphate family will provide insights for the development of high-energy sodium batteries, with breakthroughs on wide-voltage-window solid-solid electrolytes expected in the near future.