Structure and electrode reactions of layered rocksalt LiFeO 2 nanoparticles for lithium battery cathode

Abstract

Lithium iron oxide nanoparticles with the layered rocksalt structure ( O3-LiFeO 2) were synthesized by ion exchange from size-controlled α-NaFeO 2 particles. 40-nm-sized O3-LiFeO 2 exhibited intercalation characteristics for cell voltages in the range 2.0–4.5 V. A plateau region around 4 V was observed in the first charge curve. Mechanistic studies using Mössbauer spectroscopy and X-ray diffraction measurements revealed that the oxidation state of Fe did not change and that a phase transition from rhombohedral to cubic symmetry occurred during the initial charging process, indicating oxygen release with lithium deintercalation. The cubic phase showed a reversible charge/discharge capacity. To investigate the detailed mechanism, LiFeO 2− d with oxygen-vacancy defects was directly synthesized using CaH 2 as a reducing agent. The phase with oxygen-vacancy defects contained a larger amount of the cubic phase and had a narrower plateau region during the first charge cycle than LiFeO 2 nanoparticles, which is similar to LiFeO 2 nanoparticles after initial charging. These results demonstrate that the extraction of oxygen leads to the cation-distribution changes in LiFeO 2 during the initial charge process.