Synergistic integration of medium-entropy sulfide in MOF-derived TiO₂/C nanocubes for enhanced sodium storage

Although transition metal sulfides possess high theoretical capacity as anode materials for sodium-ion batteries (SIBs), their practical application is still hindered by significant volume expansion and sluggish reaction kinetics during cycling. To address these challenges, we synthesised well-defined (NiCoFeMn)S₂@TiO₂/C nanocubes (MES@TiO₂/CNCs) as advanced anode materials for SIBs using a titanium-based metal‒organic framework (Ti-MOF) assisted strategy. The optimal MES@TiO₂/CNCs-500 composite achieves its enhanced properties through a synergistic multi-level design where the robust TiO₂/C cubic framework ensures structural integrity and facilitates efficient electron transport, while the confined medium-entropy sulfide nanoparticles with mixed valence states provide abundant redox-active sites. This composite architecture, combined with an optimized pore structure, enables fast Na⁺ diffusion kinetics and a dominant surface-controlled pseudocapacitive storage mechanism with 71.9% contribution. The composite demonstrates improved electrochemical properties including a high initial Coulombic efficiency of 97.5%, stable cycling stability maintaining 592.1 mAh g^-1 after 150 cycles with 92.9% retention, and good rate capability delivering 309.0 mAh g^-1 at 2000 mA g^-1. This study establishes an integrated active-phase/host-matrix design for advanced sodium-ion storage, unlike previously reported studies that have primarily focused on entropy-engineered sulfides or MOF-derived sodium-storage hosts as separate optimisation strategies. In this composite, (NiCoFeMn)S₂ serves as the main redox-active sodium-storage phase, whereas the anatase /rutile TiO₂/C framework mainly provides structural stabilization and facilitates ion/electron transport, thereby leading to the improved sodium-storage performance.