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

Abstract

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-FeO 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-FeO reduction at the nanoscale, and EDS analyzes the composition change of particles at various periods. The surface of α-FeO particles is reduced to an amorphous compound first, and then O ions diffuse from the crystal matrix toward the outside simultaneously causing defects inside the particles. Experiments prove that γ-FeO, FeO, CuO, and BiO have the same rate-limiting step as α-FeO; that is, O ions diffuse inside the oxide particles toward the outside. The diffusion coefficients of O 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 ion diffusion coefficient must be considered when selecting or designing metal oxides as energy material. The conclusion that O 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.

Re-Examination of CuO Reduction Steps and Understanding of the Factors Influencing the Cyclic Voltammetry Profile of CuO

“…The experiment demonstrated that Cu or Cu 2 O is relatively stable in dry air. The overpotential for CuO reduction at −1.65 V is lower than the overpotential at −1.7 V, and the reaction rate of CuO reduction at −1.65 V is slower than that at −1.7 V. In the electrolytic cell, some residual O 2 from air unavoidably remained inside the electrolyte, even though the cell was purged with N 2 gas, 19 and O 2 reduction reaction could consume a part of the charge. The applied voltage was larger than the voltage of H 2 O decomposition (−1.23 V), and H 2 evolution on the cathode side also consumed a part of charge, even thought H 2 suppression additive was added.…”

Diffusionless‐Like Transformation Unlocks Pseudocapacitance with Bulk Utilization: Reinventing Fe2O3 in Alkaline Electrolyte

“…It was found, As shown in Figure 3a, inset charge state d, the charge product hexagonal nanoflakes is identified as Fe(OH) 2 by using HRTEM. [ 34,35 ] The single crystal of Fe(OH) 2 was observed and the interplanar spacing of (100), (010), and (−110) is 0.255 nm. The interplanar spacing of these planes is smaller than 0.282 nm from the standard PDF #13‐0089.…”

Advanced characterization and calculation methods for rechargeable battery materials in multiple scales

“…The application of XEDS mapping has become a feasible and effective way to discover elements' distribution in real space for energy materials. [52,53] In TEM, EELS collects transmission electrons and analyzes their energy distribution. Inelastic transmission electrons also contain electronic structure information inside the loss of energy.…”