Electronic and Ionic Coupled Engineering Strategy of Na<sub>4</sub>Fe<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>(P<sub>2</sub>O<sub>7</sub>) for High-Rate and Long-Cycling Sodium-Ion Batteries
Zhiyu Zhang, Nan Zhang, Han Zhang, Jiaxuan Liu, Huiming Shi, Dmitrii V. Anishchenko, Elena V. Alekseeva, Ruopeng Li, Peixia Yang, Oleg V. Levin, Dianlong Wang, Huan Liu, Shixue Dou, Bo Wang
Abstract
Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (NFPP) has emerged as a promising cathode material for sodium-ion batteries (SIBs) due to its robust structural stability, extensive sodium-ion diffusion pathways, and high safety. However, its practical implementation is constrained by inherent limitations such as poor electronic conductivity and reduced capacity under high-rate conditions. In this study, we engineered a dual electronic-ionic coupling strategy to synergistically enhance the electrochemical dynamic behavior of the NFPP material. The proposed NFPP was synthesized via a sol–gel method, realized strategic Mg-substitution at Fe sites within the NFPP lattice and reduced graphene oxide (rGO) coating to establish a three-dimensional conductive framework. The optimized composite (NFPP/rGO-0.15Mg) demonstrates a reversible capacity of 110.1 mAh·g –1 at 1C with 99% capacity retention over 500 cycles. Remarkably, it maintains 97.0 mAh·g –1 at 20C and retains 94.82% of its initial capacity after 6000 cycles, demonstrating exceptional cycling stability. In situ XRD analysis confirms the minimal volumetric expansion (1.3%) during charge/discharge processes. Theoretical calculation results show that Mg doping reduces the material’s bandgap and sodium-ion migration energy barrier. Furthermore, NFPP/rGO-0.15Mg demonstrates robust electrochemical performance under low-temperature conditions (−15 °C) and full-cell configurations. These findings offer crucial implications for the rational design of advanced polyanionic cathode materials to address the evolving demands of advanced SIBs.