Synthesis and Characterization of Earth-Abundant Silicate Cathodes for Critical-Element-Free Sodium Ion Batteries

Sodium-ion batteries are expected to become commercially competitive with lithium-ion batteries in the near future, providing a more sustainable battery chemistry for stationary energy storage. Although sodium can be extracted from abundant minerals and brine, the sustainability of sodium-ion batteries is still hindered by the availability of high-performance cathodes, that do not contain critical elements such as cobalt and nickel. Sodium orthosilicates based on iron and manganese as redox active centres are a class of polyanionic compounds that can be synthesized from globally accessible raw materials. Although theoretical calculations suggest that sodium orthosilicate cathodes possess promising features for sodium-ion batteries, the electrochemical properties of these materials has not yet been extensively investigated. In particular, previous reports indicate that manganese-based silicates can achieve high energy densities in the stability window of common liquid electrolytes, but their crystal structure is unstable during the extraction and insertion of sodium ions. The purpose of this research is therefore to investigate the stabilization of the local chemical environment of manganese in sodium orthosilicates by substitution with iron. Mixed iron and manganese orthosilicates with different degrees of iron substitution will be synthesized via a sol-gel method and their electrochemical properties will be characterized with in-situ techniques. In detail, the structural evolution of mixed orthosilicates at different states of charge (and discharge) will be monitored with in-situ Raman, in-situ X-Ray Diffraction, and in-situ dilatometry. The practical application of orthosilicates in sodium-ion batteries will be further assessed assembling full cells with hard carbon anodes and characterising their electrochemical performance and ageing mechanism.