Electroanalytical study of the viability of conversion reactions as energy storage mechanisms

Abstract

Storing electrochemical energy by means of fully reducing transition metal compounds to their metallic state is attractive due to the large amounts of charge that are produced. However, while more reversible than originally envisioned, electrochemical conversion reactions are accompanied by large inefficiencies. These inefficiencies are associated with hysteresis between the potentials of reduction and re-oxidation, as well as the loss of capacity when reversing back to the initial state. This work presents a series of results collected from different kinds of electroanalytical experiments. The use of data from conventional powder electrodes as well as thin films offered the opportunity to compare measurements of common occurrence in the literature with more sophisticated experiments carried out at higher temperature, which provided a complete picture of the kinetic nature of the processes involved. This picture is supportive of the thermodynamic nature of potential hysteresis, and indicates that coulombic inefficiencies stem from the low reversibility of the conversion reaction. Taken together, the results cast a clear light on the significant challenges that lie ahead if this kind of reactivity is to become technologically relevant in devices such as Li-ion batteries.