Computational Study of Lithium Titanate as a Possible Cathode Material for Solid-State Lithium-Sulfur Batteries

摘要

LiS cells are currently built with metallic lithium as node, a liquid electrolyte, and a cathode composed of a mixture of sulfur, carbon, and binder. While this type of cell produces good capacity during the early cycles, unwanted reactions with the electrolyte degrade the cathode and anode; making the whole cell not competitive with Li-ion batteries. A viable solution to mitigate this problem is the replacement of the carbon, binder, and electrolyte with a ceramic matrix, with high electronic and ionic conductivity. Lithium titanate (Li4Ti5O12) spinel may be a potential candidate for the fabrication of composite cathodes, due to its mechanical robustness and its high electronic and Li-ion conductivity. In this paper, we present an ab initio molecular dynamics study complemented with experimental investigations, offering a novel interpretation for the Li-ion mobility in Li4Ti5O12 and Li7Ti5O12 as well as for the chemical reactivity of these materials with molecular sulfur. Also, we present a model for the passivation of Li4Ti5O12 and Li7Ti5O12 surfaces by lithium carbonate, addressing both Li-ion mobility at the interface and sulfur reactivity. On the basis of our results, the deployment of Li4Ti5O12 and Li7Ti5O12 materials for sulfur-based battery technology is questioned mainly by the lower Li-ion conductivity of the carbonate-passivated surfaces and by the chemical reactivity of Li7Ti5O12 with sulfur molecules, which would lead to self-discharge, with resulting loss of capacity and inferior battery performance.