Structure and Electrochemical Properties of α-NaFeO2 Obtained under Various Hydrothermal Conditions

High-crystallinity α-NaFeO2 samples can be obtained from α-FeOOH in highly concentrated NaOH solution under various hydrothermal conditions. The electrochemical performance of the process depends on the hydrothermal condition: selecting a higher treatment temperature engenders better cycle performance. The cycle performance can also be changed by adding small amounts of KOH or LiOH·H2O to the NaOH solution. Better cycle performance is obtainable for a sample with added KOH. X-ray Rietveld analysis revealed a metastable structure: Na and Fe ion sites (3a and 3b sites) were not fully occupied in a hexagonal layered rock salt structure (Rm). The electrochemical cycle performance depends strongly on the crystal structure: conversion to nearly ideal α-NaFeO2 structure with small lattice parameters is effective to attain better cycle performance. Growing-up primary particle size was also important strategy to obtain the sample with better cycle performance.