Cooperative Cationic and Anionic Redox Reactions in Ultrathin Polyvalent Metal Selenide Nanoribbons for High-Performance Electrochemical Magnesium-Ion Storage.

Rechargeable magnesium batteries (RMBs) are considered as potential energy storage devices due to their high volumetric specific capacity, good safety, as well as source abundance. Despite extensive efforts devoted to constructing an efficient magnesium battery system, the sluggish Mg ²⁺ diffusion in conventional cathode materials often leads to slow rate kinetics, low capacity, and poor cycling lifespan. Although transition metal selenides with soft anion frameworks have attracted extensive attention, their Mg ²⁺ storage mechanism still needs to be clarified. Herein, we demonstrate that the ultrathin CoSe ₂ nanoribbons can be used as a robust cathode material for RMBs and reveal a novel Mg ²⁺ storage mechanism based on cooperative cationic (Co) and anionic (Se) redox processes via systematic ex-situ characterizations. Compared to other metal selenide cathodes based on conversion reactions of solely metal cations, the cooperative cationic-anionic redox reactions of the CoSe ₂ cathode contribute to obtaining an enhanced specific capacity and boosted electrochemical kinetics. Moreover, on one hand, the ultrathin nanoribbon structure enables effective contact between the electrode material and electrolyte and on the other hand significantly reduces the length and time consumption of Mg ²⁺ diffusion, leading to dominated surface-driven capacitance-controlled Mg ²⁺ storage behavior and rapid Mg ²⁺ storage kinetics. As a result, the ultrathin CoSe ₂ nanoribbon cathode exhibits a reversible discharge capacity of ∼130 mAh g ^-1 at 100 mA g ^-1 , good rate capability (116 mAh g ^-1 at 300 mA g ^-1 ), and long cyclability over 600 cycles. This finding confirms the development potentiality of polyvalent metal selenide cathode materials based on a cooperative cationic-anionic redox mechanism for the construction of next-generation multivalent secondary batteries.