Employing S(-I)/S(-II) Redox Chemistry in Quasi-1D Niobium Trisulfide-Graphene Cathodes for High-Capacity Magnesium–Lithium Hybrid Ion Batteries

Magnesium-lithium hybrid ion batteries (MLIBs) offer a compelling alternative to traditional lithium-ion batteries (LIBs) due to their enhanced safety, abundant magnesium resources, and high theoretical capacities. However, their practical deployment is constrained by the scarcity of high-capacity cathode materials. Herein, we report a quasi-one-dimensional (1D) pseudolayered niobium trisulfide (NbS₃), incorporated with graphene, as a novel cathode that leverages disulfide (S₂^2-) redox chemistry to achieve remarkable electrochemical performance. The electrode exhibits excellent cycling performance, retaining high lithium/magnesium cointercalation capacities of 251 mA h g^-1 after 150 cycles at 100 mA g^-1 and 181 mA h g^-1 over 1000 cycles at 1000 mA g^-1. Mechanistic investigations using <i>in operando</i> PXRD and <i>ex situ</i> XPS/XAS reveal a two-step phase evolution during initial cycling, involving sequential sulfur anion (S₂^2-/S^2-) and niobium cation (Nb⁴⁺/Nb³⁺) redox processes. Over extended cycling, the Nb⁴⁺/Nb³⁺ redox fades, leaving the S₂^2-/S^2- redox associated with a single reversible phase as the dominant contributor to the performance. The primary cause of capacity fade is attributed to the irreversible structural and redox changes related to the Nb⁴⁺/Nb³⁺ process, along with exfoliation-induced detachment of active material. This study provides comprehensive insights into the energy storage mechanisms of NbS₃-based cathodes and underscores the potential of disulfide anion-containing trisulfides as promising candidates for next-generation, high-capacity magnesium-based batteries.