Revealing the atomistic origin of the disorder-enhanced Na-storage performance in NaFePO4 battery cathode

Among the various types of cathode materials for sodium ion batteries, NaFePO4 attracts much attention owing to its high theoretical capacity (155 mA h g−1), low cost, high structural stability, and non-toxicity. Nevertheless, the NaFePO4 with maricite structure, thermodynamically stable phase, has been considered as electrochemically inactive for sodium-ion storage. In this work, we succeeded in tuning the degree of disorder in NaFePO4 cathode material by a mechanochemical route to enhance electrochemical performances of Na-ion batteries. The derived NaFePO4 cathodes containing both amorphous and maricite phases exhibit much improved sodium storage performance with an initial capacity of 115 mA h g−1 at 1 C and an excellent cycling stability of capacity retention of 91.3% after 800 cycles. By X-ray absorption near edge and Raman spectroscopy, we revealed the atom-scale structural origin of the enhanced Na-storage performances of the amorphous NaFePO4 electrode. The transformation of edge-sharing FeO6 octahedra into various FeOn polyhedra upon amorphization was found to be a key to attain the superior performances for Na ion batteries.