Towards High-Performance Sodium-Ion Batteries via the Phase Regulation Strategy

The iron-based mixed-polyanionic cathode Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (referred to as N4FP) has gained significant attention as an ideal candidate for commercial sodium-ion batteries (SIBs). Its advantages, such as cost-effectiveness, environmental friendliness, and excellent structural stability, make it highly attractive. However, the practical specific capacity of N4FP (approximately 100 mA h g –1 ) tends to fall short of its theoretical specific capacity (129 mA h g –1 ), resulting in a relatively low energy density at the cell level. This study aims to investigate the capacity limitations of the N4FP cathode and identify the formation of inactive maricite-type sodium iron phosphate, NaFePO 4 (termed NFP), as the primary cause for these limitations. To address this issue, a small amount of ferric sodium pyrophosphate (Na 2 FeP 2 O 7, referred to as N2FP) is introduced into the N4FP cathode to eliminate the formation of inactive maricite NFP. The N4FP cathode modified with 5% N2FP (referred to as N4FP-5%N2FP) has a remarkable reversible specific capacity of 125.6 mA h g –1 at 0.1 C, equivalent to 97.4% of the theoretical specific capacity of N4FP. Additionally, the N4FP-5%N2FP cathode demonstrates excellent long cycle life and rate properties during testing.