Charge‐Compensation in a Displacement Mg²⁺ Storage Cathode through Polyselenide‐Mediated Anion Redox

Abstract

Chalcogenides have been viewed as important conversion-type Mg 2 + -storage cathodes to fulfill the high volumetric energy density promise of magnesium (Mg) batteries. However, the low initial Columbic efficiency and the rapid capacity degradation remain challenges for the chalcogenide cathodes, as the clear Mg 2 + -storage mechanism has yet to be clarified. Herein, we illustrate that the charge storage mechanism of the Cu 2À x Se cathode is a reversible displacement reaction along with a polyselenide (PSe) mediated solution process of anion-compensation. The unique anion redox improves charge storage, while the dissolution of PSe also leads to performance degradation. To address this issue, we introduce Mo 6 S 8 into the Cu 2À x Se cathode to immobilize PSe, which significantly improves performance, especially the reversible capacity (from 140 mAh g À 1 to 220 mAh g À 1 ). This work provides inspiration for the modification of the Mg 2 + -storage cathode, which is a milestone for high-performance Mg batteries.

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

“…Moreover, Se 2− ions and Mg−Se bonds are formed at stage B (Figure 4e), indicating that some Se−Se (Se 2 2− ) dimers are reduced to Se 2− when discharged to 1.2 V, accompanied by the oxidation of Co 2+ to Co 3+ . 48,49 When further discharged to 0.35 V (stage C), Co 0 species appear, 49,50 accompanied by more Co 2+ and Se 2− ions and Mg−Se bonds, manifesting that Co 3+ , Co 2+ , and Se 2 2− species are reduced together by the interaction of Mg 2+ ions. At stage D, the content of Co 0 and Se 2− species reaches the maximum levels.…”

A Magnesium/Lithium Hybrid-Ion Battery with Modified All-Phenyl-Complex-Based Electrolyte Displaying Ultralong Cycle Life and Ultrahigh Energy Density

“…Moreover, the (111) plane with a lattice spacing of 0.208 nm is assigned to the copper, which is also detected in the NiCo 2 S 4 cathode, indicating a reversible conversion between Cu and Cu + for Mg storage. In addition, Cu + and Mg 2+ undergo a reversible displacement reaction due to the high similarity in the sulfur anion arrangement. , The high Cu + and Li + ion mobility facilitates electron access to the reaction sites on MgS and further enhances the continuous conversion kinetics on the NiCo 2 S 4 cathode. However, the copper from the current collector can only contribute less capacity in hybrid-ion storage, which is restricted by the higher thermodynamic transformation and lower theoretical specific capacity. , The pure Cu 9 S 5 (Figure S24) exhibits a poor cycle reversibility at 0.1 A g –1 (Figure S25a).…”

“…In addition, Cu + and Mg 2+ undergo a reversible displacement reaction due to the high similarity in the sulfur anion arrangement. 51,57 The high Cu + and Li + ion mobility facilitates electron access to the reaction sites on MgS and further enhances the continuous conversion kinetics on the NiCo 2 S 4 cathode. However, the copper from the current collector can only contribute less capacity in hybrid-ion storage, which is restricted by the higher thermodynamic transformation and lower theoretical specific capacity.…”

The Conversion-Type Selenides as Potential High-Energy Cathode Materials for Mg-Based Batteries: A Review

“…Cui et al added a small amount of Mo 6 S 8 into Cu 2−x Se (Cu 2−x Se Figure 9e) to alleviate the shuttle effect of polyselenide(PSe) and realize Mg storage through an intercalation-displacement mechanism. 76 The important role of Mo 6 S 8 could anchor the soft sulfur species, restrain the shuttle effect of polyselenide (PSe), and enhance the reversible capacity. The as-assembled Mg−Mo 6 S 8 /Cu 2−x Se battery had one sloping discharge voltage platform in the voltage range of 0.9−1.2 V and one platform at ∼0.9 V vs Mg 2+ /Mg at a current density of 20 mA/g (Figure 9g).…”

“…(g) The discharge curve consisting of first platform at 20 mA/g. (h) The schematic diagram of the Mg 2+ -storage mechanism of the Cu 2– x Se cathode and the Mo 6 S 8 /Cu 2– x Se cathode . Reprinted with permission from ref .…”