Simulations of Lithium-Magnetite Electrodes Incorporating Phase Change

The phase changes occurring in magnetite (Fe₃O₄) during lithiation and voltage recovery experiments are modeled using a model that simulates the electrochemical performance of a Fe₃O₄ electrode by coupling the lithium transport in the agglomerate and nano-crystal length-scales to thermodynamic and kinetic expressions. Phase changes are described using kinetic expressions based on the Avrami theory for nucleation and growth. Simulated results indicate that the slow, linear voltage change observed at long times during the voltage recovery experiments can be attributed to a slow phase change from α-Li_xFe₃O₄ to β-Li₄Fe₃O₄. In addition, the long voltage plateau at ∼1.2 V observed during lithiation of electrodes is attributed to conversion from α-Li_xFe₃O₄ to γ-(4 Li₂O + 3 Fe). Simulations for the lithiation of 6 and 32 nm Fe₃O₄ suggest the rate of conversion to γ-(4 Li₂O + 3 Fe) decreases with decreasing crystal size.