Unraveling the conversion mechanism toward spinel sulfides as cathode materials for Mg-ion batteries

Rechargeable Mg batteries are promising candidates for achieving considerable high-energy-density. Enhancing the energy density can be achieved by integrating metallic Mg anodes with conversion-type cathode materials, which are characterized by multi-electron transfer process and elevated specific capacities in contrast to intercalation-type materials. Despite these advantages, the conversion-type cathodes still have some challenges of substantial volume expansion, sluggish diffusion kinetics and intricate mesophase evolution during repeated electrochemical reactions. Herein, first-principles calculations were performed to probe into the electronic properties, Mg²⁺ dynamical properties, Bader charge and electrochemical mechanism of spinel-type sulfides (M₃S₄, M = Co and Ni). The band gap values of Co₃S₄ and Ni₃S₄ are 0.28 and 0 eV, respectively, showing their superior electrical conductivity. The preferential order of Mg intercalation sites is 16c > 48f > 8b. Computational predictions of the formation energy and discharge voltage indicate that spinel Ni₃S₄ can exhibit a relatively high specific discharge capacity of 220.8 mA h g^-1 and an average voltage of ∼1.6 V vs. Mg²⁺/Mg with an energy density of 353.3 W h kg^-1 at a Mg intercalation concentration of x2+Mg = 1.25, surpassing those of Co₃S₄ and Mo₆S₈. According to the principle of the lowest barrier, the diffusion pathway "oct → tet → oct" of spinel sulfides Co₃S₄ and Ni₃S₄ has low Mg migration barrier values of 1.10 and 0.67 eV, respectively. The Bader charge and AIMD results revealed that the spinel M₃S₄ (M = Co and Ni) underwent conversion reactions to the rock-salt phase especially at deep discharge. These insights significantly advance the rational design of spinel sulfides with a conversion reaction mechanism, providing great potential for the development of Mg batteries with high energy density.