Role of intermediate phase for stable cycling of Na 7 V 4 (P 2 O 7 ) 4 PO 4 in sodium ion battery

Significance Utilizing low-cost materials, sodium ion batteries (SIBs) are beginning to attract considerable attention, particularly for large-scale utility grid applications. However, electrochemical performance of most SIB active materials is still insufficient for various practical applications. In the current study, we discovered a vanadium-based ortho-diphosphate, Na 7 V 4 (P 2 O 7 ) 4 PO 4 , or VODP, that holds exceptional electrochemical properties represented by well-defined high voltage profiles at 3.88 V (vs. Na/Na + ) and substantial capacity retention over 1,000 cycles. A theoretical analysis suggests that an intermediate phase encountered during phase transformation of VODP is crucial for better kinetics during battery operations, which can be expanded as a general principle in understanding diverse battery materials.