Electrochemical Modification of Negative Thermal Expansion Materials in the Ta xNb1- xVO5 Series.

Abstract

Electrochemical processes transfer charge carriers to and from electrodes, e.g., Li+ ions are inserted into anodes during discharge and extracted during charge in a Li half-cell. Using an electrode that features negative thermal expansion (NTE) properties in an electrochemical cell allows a means to study the interaction of the charge carrier with an NTE material and potentially modify or tune its NTE properties. This work examines the NTE properties of Ta xNb1- xVO5 ( x = 1, 0.9, 0.75, 0.5, 0.25) and the effect of Li+/Na+/K+ electrochemical discharge of TaVO5-based electrodes. Sodium discharge was found to drastically alter NTE properties with 25% Na+ discharged electrodes exhibiting a linear volumetric coefficient of thermal expansion of -5.75 ± 0.20 × 10-5 Å3/°C between 350 and 500 °C, one of the largest reported for any NTE system. Furthermore, at higher temperatures, the Na+- and K+-discharged and heated electrodes generate new phases, suggesting that a combination of electrochemical discharge and thermal treatment can be used to synthesize new compounds. This work lays the foundation for the concept of using electrochemical discharge followed by subsequent thermal treatments to modify the physical properties of a compound.