(Invited) Developing Titanate Anodes for Sodium Ion Batteries

Abstract

Sodium titanates are among the most promising anode materials for sodium ion batteries, due to their low cost, possibility of high tap density, and relatively higher operation voltage compared to commonly used hard carbons that prevents metallic sodium plating. 1 However, these oxides have a finite number of sites for ion insertion, which limits the Na+storage capacity, and thereby the achievable energy density. The sluggish Na+ (de)intercalation kinetics and poor electronic conductivity are other impediments to the practical capacity and rate capability. The presence of mobile cations (e.g., Li+) instead of less mobile ones (e.g., Mg2+) in the metal oxide layers was shown to provide additional diffusional pathways for Na+ and improves the practical capacities.2 This observation motivated us to design and synthesize lepidocrocite-structured titanates with vacancies in the metal oxide layers, on the assumption that these vacancies will have similar beneficial effects as mobile cations.3 These non-stoichiometric titanates do indeed show improved capacities, due to the additional sites in the metal oxide layers, and, possibly, some surface contributions to the redox behavior. To overcome kinetic limitations, large amounts of carbon additives are often used to improve electrical conductivity in the electrode composites. However, it is possible to manipulate the titanate structure instead, by incorporating conductive carbon between the metal oxide layers to overcome the electronic limitations. To this end, we have designed and synthesized heterostructures of lepidocrocite titanates.4 Carbon-free composite electrodes containing these heterostructures outperform conventional titanate electrodes containing carbon, illustrating the principle.