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

Abstract

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 Li–Fe₃O₄ 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 α-Fe through Li_xFe₃O₄, with drastic changes in the structure. This electrochemical process exhibits nanomilling for α-Fe. In situ magnetic measurements supported this two-step conversion, and indicated a ferrimagnetic ordering of Li_xFe₃O₄ at T_N = ca. 150 K and superparamagnetic behavior of α-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”.