Development of a sodium negative electrode based on sol–gel derived amorphous TiO2

Abstract

Sodium-ion batteries (SIBs) have emerged as promising alternatives to lithium-ion batteries (LIBs) for long-duration electrochemical energy storage. Although SIBs have certain electrochemical characteristics similar to those of LIBs, the larger ionic radius of sodium leads to sluggish kinetics as well as strain, irreversible (de)insertion reactions, and degradation. Amorphous materials are able to overcome some of these issues because of greater free volume compared to crystalline materials and the ability of some amorphous structures to withstand greater levels of strain. The research presented in this paper shows that a predominantly amorphous TiO₂ structure is able to achieve a high Na⁺ capacity of 250 mAh g⁻¹ with good kinetic reversibility. This research takes advantage of the established ability of sol–gel chemistry to synthesize amorphous materials, especially transition metal oxides. A suite of electrochemical, chemical, and structural methods is used to detail key features such as the reversibility of Ti^3+/4+ reactions (from X-ray photoelectron spectroscopy) and the evolution of the short-range structure of the sol–gel derived TiO₂ upon (de)sodiation (from X-ray absorption spectroscopy). Synchrotron X-ray diffraction was used to characterize the relatively small content of rhombohedral crystallites embedded in the amorphous matrix, and transmission X-ray microscopy was used to directly follow the reversible volume change that occurs upon (de)sodiation in these materials. The use of sol–gel chemistry to synthesize an amorphous phase which exhibits very stable, high capacity Na⁺ electrode behavior is a positive new direction for creating electrode materials for SIBs.