Layered LiTiO2 for the protection of Li2S cathodes against dissolution: mechanisms of the remarkable performance boost

Abstract

Lithium sulfide (Li₂S) cathodes have been viewed as very promising candidates for next-generation lightweight Li and Li-ion batteries. Prior work on the deposition of carbon shells around Li₂S particles showed reduced dissolution of polysulfides and improved cathode stability. However, due to the substantial volume changes during cycling and the low chemical binding energy between carbon and sulfides, defects almost inevitably forming in the carbon shell during battery operation commonly lead to premature cell failure. In this study, we show that conformal coatings of layered LiTiO₂ may offer better protection against polysulfide dissolution and the shuttle effects. Density functional theory (DFT) calculations revealed that LiTiO₂ exhibits a strong affinity for sulfur species (Li₂S_x) and, most importantly, induces a rapid conversion of longer (highly soluble) polysulfides to short polysulfides, which exhibit minimum solubility in electrolytes. Quite remarkably, even the mere presence of the electronically conductive layered oxides (LiMO₂, M = metal) such as LiTiO₂ in the cathodes (e.g., as a component of the mix with Li₂S) enhanced the cell rate and cycling stability dramatically. Advanced material characterization in combination with quantum chemistry calculations provided unique insights into the mechanisms of the incredible performance boost, such as interactions between Li₂S_x and the LiTiO₂ surface, leading to breakage of S–S bonds.