Rate-Limiting Step in Batteries with Metal Oxides as the Energy Materials

Metal oxides are used as the energy materials in some aqueous and nonaqueous batteries. However, a large overpotential and poor rate-performance limit their wide application. Low electrical conductivity of the oxide is commonly considered to be the reason for these limitations. The present study specifically reveals the electrochemical reduction process of α-Fe₂O₃ particles by using a cyclic voltammetry technique combined with an electron spectroscopy technique. SEM and TEM observe the phase and crystal structure transformation process during α-Fe₂O₃ reduction at the nanoscale, and EDS analyzes the composition change of particles at various periods. The surface of α-Fe₂O₃ particles is reduced to an amorphous compound first, and then O^2- ions diffuse from the crystal matrix toward the outside simultaneously causing defects inside the particles. Experiments prove that γ-Fe₂O₃, Fe₃O₄, CuO, and Bi₂O₃ have the same rate-limiting step as α-Fe₂O₃; that is, O^2- ions diffuse inside the oxide particles toward the outside. The diffusion coefficients of O^2- in these metal oxides are also estimated. This study demonstrates that the ionic conductivity of metal oxides is the critical factor which affects the overpotential and rate-performance of the batteries with these oxides as active material, and the O^2- ion diffusion coefficient must be considered when selecting or designing metal oxides as energy material. The conclusion that O^2- diffusion in oxides is the rate-limiting step of their reduction may be applicable to a group of oxides whose reduction reaction is not involved in ion diffusion from an electrolyte into their crystal matrix.