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

Abstract

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.

Toward High Performance of Na4Fe3(PO4)2P2O7 Cathode via Constructing a Porous Structure for Sodium-Ion Batteries

“…The current improved strategies include carbon coating, nanosized particles, element doping, defect generating, and phase composite regulation. − The incorporation of rGO generates enhanced rate capacity of the material but also increases the raw material cost, as does the element doping. − The Fe defect exhibits a pure phase with an outstanding cycle stability but a less reversible specific capacity . The introduction of a small amount of Na 2 FeP 2 O 7 is established useful to restrain the generation of maricite NaFePO 4 , but the formation of a low discharge potential (2.5 V vs Na + /Na) of Na 2 FeP 2 O 7 decreases the energy density . The electrospun Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 nanofibers display a pure phase with high electronic conductivity and ultrafast Na + migration but are difficult to scale up .…”