The solid-state electrochemical reduction process of magnetite in Li batteries: in situ magnetic measurements toward electrochemical magnets

The solid-state electrochemical reduction of magnetite (Fe3O4) nanoparticles was studied using a miniature Li battery that included Fe3O4 as a cathode active material. X-ray diffraction and absorption analyses clearly elucidated the relationship between the battery voltage and the chemical species reduced from Fe3O4. Upon discharging, the Fe3O4 nanoparticles suffer 1.4-electron reduction in the voltage range from 2.9 to 1.3 V, while maintaining the original inverse spinel structure. This process is reversible, so that the LiFe3O4 battery can be rechargeable with a fairly large capacity of 160 A h kg(-1). In the range below 1.3 V, Fe3O4 is irreversibly reduced to alpha-Fe through LixFe(3)O(4), with drastic changes in the structure. This electrochemical process exhibits nanomilling for alpha-Fe. In situ magnetic measurements supported this two-step conversion, and indicated a ferrimagnetic ordering of LixFe(3)O(4) at T-N = ca. 150 K and superparamagnetic behavior of alpha-Fe. Furthermore, the reversible solid-state electrochemical reaction in the range between 1.8 and 1.3 V was associated with a controllable change in magnetization (13%), suggesting that this reaction might be applied to the development of "electrochemical magnets".