Compositional Doping Induced the Suppression of Sluggish Na+ Diffusion for a Robust Pyrophosphate Cathode toward Exceptionally Long Cycle Life

Abstract

Polyanionic compounds are now viewed as attractive cathode materials for sodium-ion batteries (SIBs), which could be utilized broadly in large-scale energy storage equipment, especially for grid application, due to their high safety and outstanding thermal stability upon long-term cycling. Fe-based polyanionic compounds, especially Na₂FeP₂O₇, are considered promising cathode materials due to the benefits of polyanionic compounds and the further advantage of earth-abundant Fe elements, which can lead to a low cost that facilitates the scaling of the sodium-ion industry. However, issues such as low electronic conductivity and poor cyclic durability, especially under high current density, remain substantial barriers that hinder their wide application. Herein, a carbon-coated Mg- and Ti-codoped Na₂Fe_0.95(MgTi)_0.05P₂O₇ cathode material is prepared, with significantly improved rate ability and cycling performance ascribed to the synergistic effect of the compositional doping strategy. Electrochemical tests show that Na₂Fe_0.95(MgTi)_0.05P₂O₇ exhibits exceptionally long cycle life at a high current density of 20C with 80.93% capacity retention after 30,000 cycles. In situ XRD and X-ray absorption fine structure (XAFS) spectra disclose that Mg and Ti codoping could effectively stabilize the structure with mere lattice distortion and the barely changed local structure environment during Na extraction/insertion, thus leading to the noteworthy ultralong cycle life. A theoretical study using the DFT calculation method demonstrates that the remarkable electrochemical property can be attributed to the large improvement of the conductivity and dramatic reduction of Na⁺ diffusion barriers for Na₂Fe_0.95(MgTi)_0.05P₂O₇. This study provides insightful perspectives on material design for polyanionic compounds and also inspires an innovative approach for the improvement of high-performance SIB cathode materials.