Phase regulation of polymorphic copper-tungsten bimetallic sulfide toward stable dual-ion batteries

Transition metal sulfides have attracted considerable attention as cathodes for Mg-Li hybrid batteries (MLHBs) due to their open frameworks and multielectron redox capability. However, most of them suffer from sluggish kinetics and limited cycling stability. Herein, two polymorphs of Cu 2 WS 4 were controllably synthesized, namely the I-phase with the space group I ̅ 4 2m, featuring an aba-stacking sequence and the P-phase with the space group P ̅ 4 2m, characterized by a compact aa-stacking configuration with no significant interlayer spaces. The layered I-Cu 2 WS 4 was fabricated via an optimized hydrothermal route. Its typical layered structure provides abundant ion transport channels and sufficient interstitial space for the co-insertion of Mg 2+ and Li + . Kinetic analyses further reveal that the I-Cu 2 WS 4 exhibits fast charge transfer kinetics and consistently high ion diffusion coefficients. Finite element simulations demonstrate that the I-Cu 2 WS 4 effectively disperses the stress induced by ion insertion, preventing localized stress concentration and mechanical fracture. Besides, the I-phase possesses superior wettability toward the electrolyte, which accelerates ion migration deep into the electrode and ensures uniform ion distribution throughout the particle. As expected, the I-Cu 2 WS 4 electrode delivers promising electrochemical performance with relatively good reversibility, excellent cycling stability and remarkable rate capability. Ex-situ characterizations further confirm the highly reversible charge storage process. Our findings provide a new paradigm for designing high-performance polymorphic cathode materials for multivalent-ion batteries.