Rechargeable magnesium batteries (RMBs) are promising candidates for sustainable energy-storage technologies, yet their advancement has been constrained by the scarcity of high-performance cathode materials. While conversion-type cathodes circumvent the structural limitations of Mg-intercalation mechanisms, most existing conversion cathodes depend exclusively on the redox activity of transition metal cations, whose capacity is inherently restricted by the reversibility of metal-ion oxidation states. Herein, we report amorphous CoS₂ hollow nanoleaves (a-CoS₂) derived from cobalt-based metal-organic frameworks, which host a dual redox Mg-storage mechanism wherein sulfur anions and cobalt cations undergo simultaneous and reversible redox reactions. Coupled with the abundant anionic sites, hollow architecture and amorphous structure, the a-CoS₂ cathode delivers a high reversible capacity of 195 mAh g^-1 at 50 mA g^-1, remarkable rate capability of 76.1 mAh g^-1 at 1.0 A g^-1, and outstanding long-term cyclability (74.6% capacity retention after 300 cycles at 200 mA g^-1), which are significantly outperforming amorphous CoS (a-CoS) and crystalline CoS₂ (c-CoS₂). Mechanistic studies confirm the reversible reconstruction of S–S bonds during magnesiation/demagnesiation, along with valence evolution of cobalt ions. This work establishes an anion-activated dual redox chemistry that transcends the limitations of conventional cation-only redox paradigm, providing new design principle for high-capacity cathode materials in multivalent-ion batteries.

Rechargeable Mg batteries (RMBs) are a promising large-scale energy-storage technology with low cost and high safety, but the performance is limited by the inferior kinetics of Mg-intercalation cathodes. In the present study, an octylamine-supporting interlayer expanded molybdenum diselenide (e-MoSe₂) is synthesized and used as cathode for RMBs, in comparison with ordinary crystalline MoSe₂. The octylamine molecules introduced show a strong interaction with the MoSe₂ layers and increase the layer spacing significantly from 6.46 to 11.5 Å. e-MoSe₂ shows a high Mg-storage capacity of 238 mAh g⁻¹ at 50 mA g⁻¹ and a superior rate performance of 39 mAh g⁻¹ at 10 A g⁻¹, far advantageous over crystalline MoSe₂. e-MoSe₂ also shows a considerably high structure stability during repeated magnesiation/demagnesiation, providing an outstanding cycling stability for 1000 cycles. Further electrochemical tests demonstrate the high Mg²⁺ diffusion coefficients in e-MoSe₂. Theoretical computation indicates the interlayer expansion changes the Mg²⁺ diffusion paths from “hollow site → hollow site” to “hollow site → Se atom site → hollow site”, largely decreasing the energy barrier and improving the Mg²⁺ diffusion kinetics. The present work highlights an efficient strategy for the improvement of Mg-storage performance for RMB cathodes.

Electrochemical conversion reactions provide more selections for Na-storage materials, but the reaction suffers from low reversibility and poor cyclability. Introducing an electrochemically inactive component is a common strategy, but the effect is quite limited since it could not stabilize the structure during long-term cycling. In this study, a new approach is developed using an amino group-functioned hyperbranched polymer (AHP) as a template and electrode additive for the design of high-performance FeSe₂-AHP composite with chemical interaction. The assembled FeSe₂-AHP composite nanoneedles were prepared by the selenylation of FeS-AHP composite microflowers and entirely inherit the polymer network from the precursor. The amino groups of AHP in composite coordinate with iron cations to achieve uniform polymer dispersion in the composite, and maintain the molecular level mixed state during the long-term cycling. Moreover, the in-situ constructed uniform 3D elastic polymer network effectively accommodates volume expansion and alleviates nanoparticle aggregation during sodiation/de-sodiation. FeSe₂-AHP composite provides a superior rate capability (584.8 mAh·g^-1 at 20 A·g^-1) and a remarkable cyclability with a capacity retention rate of 93.3% after 2, 000 cycles. FeSe₂-AHP composite shows a high pseudocapacitive behavior for the abundant nanometer interface established by AHP, enhancing the solid-state Na⁺ diffusion. The FeSe₂-AHP anode is also compatible with Na₃V₂(PO₄)₃/C cathode in a full Na-ion battery, which provides a high-power performance (powering 51 LEDs). The work herein highlights an innovative and efficient strategy for conversion-type material design and demonstrates the function of chemical interaction of polymer additive in the enhancement of long-term cyclability for conversion electrode.