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 alpha-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 alpha-Fe2O3 reduction at the nanoscale, and EDS analyzes the composition change of particles at various periods. The surface of alpha-Fe2O3 particles is reduced to an amorphous compound first, and then O2- ions diffuse from the crystal matrix toward the outside simultaneously causing defects inside the particles. Experiments prove that gamma-Fe2O3, Fe3O4, CuO, and Bi2O3 have the same rate-limiting step as alpha-Fe2O3; that is, O2- ions diffuse inside the oxide particles toward the outside. The diffusion coefficients of O2- 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 O2- 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.