Lattice Water for the Enhanced Performance of Amorphous Iron Phosphate in Sodium-Ion Batteries

We report amorphous iron phosphate with lattice water, namely FePO 4 · x H 2 O ( x ∼ 2.39), as a promising sodium-ion battery (SIB) cathode. After carbon coating, micrometer-sized FePO 4 · x H 2 O exhibits a reversible capacity that is higher than that of its counterpart without lattice water (130.0 vs 50.6 mAh g –1 at 0.15 C rate) along with clearly enhanced rate capability and cyclability. The superior electrochemical performance of FePO 4 · x H 2 O is attributed to the lattice water that facilitates sodium-ion diffusion via enlarged channel dimensions and the screening of the electrostatic interactions between sodium ions and host anions. The amorphous phase is also advantageous in accommodating the stress created in the host framework during sodium-ion (de)intercalation. The presence of lattice water also protects the oxidation state of Fe from reductive surface carbon coating and slightly lowers the operation voltage via reduced inductive effect. The current study provides a useful insight into how to design SIB electrode materials particularly focusing on facile sodium-ion diffusion.