Transition-Metal Dependence of Anion Redox Reversibility in Amorphous Electrodes

Abstract

Anion redox reactions can considerably enhance battery capacity; however, they face challenges, such as phase separation and slow kinetics due to large structural changes. In crystalline oxide materials, phase separation of the anionic component is governed by the positional relationship between the energy levels of the orbitals of the anionic component and unoccupied orbitals of the constituent transition metals. However, in addition to these elemental parameters, structural constraints are important for crystalline materials. Previously, we reported that the slow kinetics of the anion redox reactions in Na₃FeS₃ can be improved through amorphization, which increases the structural degrees of freedom. In this study, we examined amorphous Na₃CoS₃, in which the Fe in Na₃FeS₃ was replaced by Co, to investigate the transition metal dependence of the anion redox reversibility in amorphous compounds with large structural degrees of freedom. The reversibility was reduced by replacing Fe with Co owing to the phase separation caused by the sulfur multimer formation. First-principles calculations revealed that multimer formation was driven by the transfer of electrons from dimeric sulfur to the unoccupied orbital of Co. The results confirm the transition-metal selection guidelines for the reversibility of anion redox reactions, even for amorphous compounds with few structural constraints.