(Invited) A New Family of Lithium-Rich Layered Sulfides (Li[LitTi1-t]S2, 0 < t ≤ 0.33) Belonging to the LiTiS2 – Li2TiS3 System

Abstract

Li-rich layered oxide (Li[LixM1-x]O2, (M = Mn, Ni, Co,...) positive electrode materials are considered as a key component for next generation lithium ion batteries, as they display outstanding specific energy originating from a combination of additional lithium de-intercalation and the unusual oxidation of oxygen anions.Recently we have extended these studies to the homologous titanium sulfides belonging to the LiTiS2 – Li2TiS3 system. Layered Li2TiS3 was obtained for the first time by direct reaction of Li2S on TiS2. Its structure is similar to the Li2MnO3 one, with a small amount of stacking faults. Overlithiated compositions (Li[LitTi1-t]S2, (0 < t ≤ 0.33)) were synthesized by direct synthesis from Li2TiS3 and LiTiS2 and structurally characterized. While LiTiS2 exhibits the O1 type sulfur packing, all compositions with ~0.08 < t ≤ 0.33 exhibit the O3 type sulfur packing like the homologous oxides.High resolution HAADF images recorded for the Li2TiS3 end-member reveal that lithium and titanium are fully ordered within the [Li1/3Ti2/3]S2 slabs of the typical honeycomb lattice usually observed for Li-rich layered oxides. Nevertheless, stacking faults are observed along the hexagonal C axis. In the Li[LitTi1-t]S2, (0.08 < t ≤ 0.33) solid solution the honeycomb ordering is observed only in some domains thank to the departure from the ideal ratio required for the ordering.The electrochemical studies shows that up to 1.1 Li per Li[LitTi1-t]S2 can be reversibly cycled with a negligible irreversibility. The high capacity (≈240 mAh/g) and the shape of the charge curves suggest that Ti3+ ions are oxidized in a first step and the S2- ions in a second step to form S2 2- ions. The very small irreversibility results from the easy oxidation of S2- ions, which does not induce any sulfur release from the particle surface and subsequent structural densification. The XPS study of the deintercalated material confirms the formation of S2 2- species.Contrary to oxides, there is a very small fading and voltage decay upon cycling. This behavior suggests that there is no significant structure modification as it is observed in lithium-rich oxides. In a particular composition range, they exhibit high electrochemical performance with a reversible capacity and an energy density exceeding respectively 265 mAh·g-1 and 600 Wh·kg-1, a very low irreversible capacity in the first cycle, fast activation and a limited voltage decay.Their operation potential within the electrochemical stability window of sulfide-based fast ionic conductors makes them promising cathode materials for all-solid-state lithium and Li-ion batteries.