Layered Ti_1-xFe_xS₂ Cathode Materials with Anionic Redox Chemistry for Mg Storage

Rechargeable magnesium batteries (RMBs) are a promising alternative to lithium-ion batteries because of the high capacity and crustal abundance of magnesium. However, RMBs are primarily plagued by low-energy-density of traditional cathode materials, which utilize transition-metal cationic redox chemistry. Herein, a new family of layered Ti_1-xFe_xS₂ (0 < x < 1) materials is proposed to assess anionic redox and boost the energy density of RMBs. The structural stability and redox activity are systematically investigated using first-principles calculations and operando experimental techniques. It is found that layered Mg_nTi_1-xFe_xS₂ materials are stable below x = 0.5, forming the solid-solution sulfides. The Mg_nTi_0.75Fe_0.25S₂ sample is experimentally shown to offer a high operating voltage of 1.8 V, a competitive energy density of 103 mAh g⁻¹, and better cycling stability compared to prototype TiS₂ material. Both cations (Ti and Fe) and anion (S) are revealed to take part in the redox reaction. The new family of layered Ti_1-xFe_xS₂ cathodes with joint cationic and anionic redox chemistry proposed in this work are promising high-energy cathode candidates for RMBs.