Multielectron Conversion: Peculiar Transition Metal Sulfides with Mixed Vulcanized States toward High-Capacity Metal-Ion Storage

Transition metal sulfides with mixed vulcanized states (TMS-mVs) possess tremendous potential to realize highcapacity, superior redox reactions, and structural reversibility for metal-ion (M-n(+)) storage owing to their multielectron reactions caused by the simultaneous participation of transition metal (TM) cations and S-2(2-) anions as well as multifarious TM or S valence states. Here, recent advances are systematically introduced regarding the mainstream TMS-mVs that can be applied to M-n(+) storage. These TMS-mVs can be divided into two categories of TMS, those with mixed sulfur-valence states (TMS-mSs) and those with mixed metal-valence states (TMS-mMs). It is found that TMS-mV anodes mainly experience three reaction mechanisms, inculding insertion-accompanied conversion, insertion, and conversion reactions. During the reversible charge process, Li2S is possibly oxidized into polysulfides or even S. TMS-mVs have the ability to transfer more electrons than other homogeneous TMSs. TMS-mS anodes usually present higher theoretical specific capacities than TMS-mM anodes. In these TMS-mV anodes, Mo-based, V-based, and Co-based TMS-mM anodes exhibit good electrochemical reversibility, Ni-based TMS-mM anodes exhibit moderate electrochemical reversibility, and Fe-based TMS-mM and TMS-mS anodes exhibit poor electrochemical reversibility. The strategies for enhancement of their electrochemical performance are classified into composite, coating, nanostructure, heterointerface, and lattice engineering.