Unravelling Zn<sup>2+</sup> Intercalation Mechanism in TiX<sub>2</sub> (X = S, Se) Anodes for Aqueous Zn-Ion Batteries

Abstract: In recent years, increased attention has been paid to aqueous Zn-ion batteries (ZIBs) owing to their low cost, inherent safety, and environmental benignity, which enable them to become promising electrochemical energy storage systems for grid-scale applications. However, some critical issues in zinc metal anode, like zinc dendrites growth, corrosion, and side reactions, act as obstacles to developing aqueous ZIBs. Exploring zinc-storage anodes to replace zinc-metal anodes is proposed as an effective strategy to promote the practical application of ZIBs. Therefore, several transition metal oxides, sulfides, and conductive polymers have been intensively studied as zinc-metal-free anodes. Two-dimensional metal dichalcogenides (TMDs), especially TiX2 (X=S, Se), are the most appealing candidates because of their sizeable interlayer space and facile 2D ion-transport channels. However, the reaction mechanism of these TiX2 in ZIBs still needs fundamental study. In this work, density functional theory (DFT) calculations are performed to investigate the behavior of zinc intercalation reaction in TiX2 systematically. First, the most stable interlayer configurations of zinc-intercalated TiX2 are characterized by group theory. We define a supercell-dependent group that only involves translation and rotation operation and find that the subgroup of the group that describes the symmetry of the most stable interlayer configurations possesses the highest order. So, the most stable configuration can be fast screened out among thousands of candidates. Calculations based on a series of those stable configurations at different discharge depths reveal the low open circuit voltage (OCV) of <0.5 V for both ZnxTiS2 and ZnxTiSe2. The density of states (DOS) result suggests the good electronic conductivity of TiX2, and the partial density of states (PDOS) result indicates the closed-shell Ti(IV) is reduced to open-shell Ti(III), and the formation of Zn―X bonds as zinc ions intercalate into the interlayer of TiX2. Interestingly, Bader charge analysis demonstrates that X anions also participate in the redox process during charge and discharge because X gains more negative charge than Ti as zinc intercalates into TiX2. The climbing image nudged elastic band (CINEB) calculations reveal the low zinc ion diffusion barriers (0.333 and 0.338 eV for TiS2 and TiSe2, respectively). This study proves that TiX2 is suitable as zinc intercalating anode materials for ZIBs and provides new insights into the DFT investigation of other TMDs as high-performance battery materials.