Anion-Incorporated Mg-Ion Solvation Modulation Enables Fast Magnesium Storage Kinetics of Conversion-Type Cathode Materials

High-Capacity and Ultra-Long-Life Mg-Metal Batteries Enabled by Intercalation-Conversion Hybrid Cathode Materials

The advancement of rechargeable Mg-metal batteries (RMBs) is severely impeded by the lack of suitable cathode materials. Despite the good cyclic stability of intercalation-type compounds, their specific capacity is relatively low. Conversely, the conversion-type cathodes can deliver a higher capacity but often suffer from poor cycling reversibility and stability. Herein, a WSe2/Se intercalation-conversion hybrid material with elemental Se uniformly distributed into WSe2 nanosheets is fabricated via a simple solvothermal method for high-performance RMBs. The uniformly introduced Se confined in WSe2 nanosheets can not only efficiently improve the conductivity of the hybrid cathodes, facilitating the fast electron transport and ion diffusion, but also provide additional specific capacity. Besides, the WSe2 can effectively inhibit the detrimental Se dissolution and polyselenide shuttle, thereby activating the activity of Se and improving its utilization. Consequently, the synergy of intercalation and conversion mechanisms endows WSe2/Se hybrids with superior reversible capacity of 252 mAh g-1 at 0.1 A g-1 and ultra-long cyclability of up to 5000 cycles at 2.0 A g-1 with capacity retention of 78.1%. This work demonstrates the feasibility of the strategy by integrating intercalation and conversion mechanisms for developing high-performance cathode materials for RMBs.

Fine nanostructure design of metal chalcogenide conversion-based cathode materials for rechargeable magnesium batteries

Magnesium-ion batteries (MIBs) a strong candidate to set off the second-generation energy storage boom due to their double charge transfer and dendrite-free advantages. However, the strong coulombic force and the huge diffusion energy barrier between Mg2+ and the electrode material have led to need for a cathode material that can enable the rapid and reversible de-insertion of Mg2+. So far, researchers have found that the sulfur-converted cathode materials have a greater application prospect due to the advantages of low price and high specific capacity, etc. Based on these advantages, it is possible to achieve the goal of increasing the magnesium storage capacity and cycling stability by reasonable modification of crystal or morphology. In this review, we focus on the application of a variety of sulfur-converted cathode materials in MIBs in recent years from the perspective of microstructural design, and provide an outlook on current challenges and future development.

Redesigning Solvation Structure toward Passivation-Free Magnesium Metal Batteries

Simple magnesium (Mg) salt solutions are widely considered as promising electrolytes for next-generation rechargeable Mg metal batteries (RMBs) owing to the direct Mg2+ storage mechanism. However, the passivation layer formed on Mg metal anodes in these electrolytes is considered the key challenge that limits its applicability. Numerous complex halogenide additives have been introduced to etch away the passivation layer, nevertheless, at the expense of the electrolyte's anodic stability and cathodes' cyclability. To overcome this dilemma, here, we design an electrolyte with a weakly coordinated solvation structure which enables passivation-free Mg deposition while maintaining a high anodic stability and cathodic compatibility. In detail, we successfully introduce a hexa-fluoroisopropyloxy (HFIP-) anion into the solvation structure of Mg2+, the weakly [Mg-HFIP]+ contact ion pair facilitates Mg2+ transportation across interfaces. As a consequence, our electrolyte shows outstanding compatibility with the RMBs. The Mg||PDI-EDA and Mg||Mo6S8 full cells use this electrolyte demonstrating a decent capacity retention of ∼80% over 400 cycles and 500 cycles, respectively. This represents a leap in cyclability over simple electrolytes in RMBs while the rest can barely cycle. This work offers an electrolyte system compatible with RMBs and brings deeper understanding of modifying the solvation structure toward practical electrolytes.

Citrate Improves Biomimetic Mineralization Induced by Polyelectrolyte-Cation Complexes Using PAsp-Ca&Mg Complexes

Magnesium ions are highly enriched in early stage of biological mineralization of hard tissues. Paradoxically, hydroxyapatite (HAp) crystallization is inhibited significantly by high concentration of magnesium ions. The mechanism to regulate magnesium-doped biomimetic mineralization of collagen fibrils has never been fully elucidated. Herein, it is revealed that citrate can bioinspire the magnesium-stabilized mineral precursors to generate magnesium-doped biomimetic mineralization as follows: Citrate can enhance the electronegativity of collagen fibrils by its absorption to fibrils via hydrogen bonds. Afterward, electronegative collagen fibrils can attract highly concentrated electropositive polyaspartic acid-Ca&Mg (PAsp-Ca&Mg) complexes followed by phosphate solution via strong electrostatic attraction. Meanwhile, citrate adsorbed in/on fibrils can eliminate mineralization inhibitory effects of magnesium ions by breaking hydration layer surrounding magnesium ions and thus reduce dehydration energy barrier for rapid fulfillment of biomimetic mineralization. The remineralized demineralized dentin with magnesium-doped HAp possesses antibacterial ability, and the mineralization mediums possess excellent biocompatibility via cytotoxicity and oral mucosa irritation tests. This strategy shall shed light on cationic ions-doped biomimetic mineralization with antibacterial ability via modifying collagen fibrils and eliminating mineralization inhibitory effects of some cationic ions, as well as can excite attention to the neglected multiple regulations of small biomolecules, such as citrate, during biomineralization process.

Bilayered nanostructured V2O5 nH2O xerogel constructed 2D nano-papers for efficient aqueous zinc/magnesium ion storage

Aqueous zinc ion batteries (AZIBs) and aqueous magnesium ion batteries (AMIBs) offer powerful alternatives for large-scale energy storage because of their high safety and low cost. Consequently, the design of high-performance cathode materials is essential. In this paper, we present a simple strategy that combines oxygen defect (Od) engineering with a 2D-on-2D homogeneous nanopape-like bilayer V2O5 nH2O xerogel (BL-HVOd NPS). This strategy employs Od to improve Zn2+/Mg2+insertion/extraction kinetics and reduce irreversible processes for high-performance AZIBs/AMIBs. And interlayer water molecules serve as an effective spacer to stabilize the expanded interlayer gap in BL-HVOd NPS, thereby providing extended diffusion channels for Zn2+/Mg2+ during insertion/extraction. The interlayer water molecules help shield the electrostatic interaction between Zn2+/Mg2+ and BL-HVOd NPS lattice, which improves diffusion kinetics during repeated. In addition, electrochemical characterization results indicate that the BL-HVOd NPS can effectively the surface adsorption and internal diffusion of Zn2+/Mg2+. More importantly, the successfully prepared unique 2D-on-2D homogenous nanopaper structure enhances electrolyte/electrode contact and reduces the migration/diffusion path of electrons/Zn2+ and Mg2+, thus greatly improving rate performance. As a result, the BL-HVOd NPS as AZIBs/AMIBs electrodes offer better reversible capacity of 361.8 and 162.8 mA h g-1 (at 0.2 A g-1), while displaying impressively long cycle lifes. This method provides a way to prepare advanced xerogel cathode materials for AZIBs and AMIBs.

High-efficiency electrodeposition of magnesium alloy-based anodes for ultra-stable rechargeable magnesium-ion batteries

Rechargeable magnesium batteries (RMBs) have attracted much attention because of their high theoretical volumetric capacity and high safety. However, the uneven deposition behavior, harmful corrosion reaction and poor stability of magnesium metal anodes have hindered the practical application of RMBs. Herein, we propose a facile alloy electrodeposition method to construct an artificial layer on an Mg anode. Experimental results show that the polarization of the symmetric magnesium alloy-based (Mg-Sn@Mg and Mg-Bi@Mg) cells is significantly reduced (∼0.05 V) at a current density of 0.1 mA cm-2. The symmetric cells using the prepared Mg alloy anodes exhibited lower voltage hysteresis and ultra-stable cycling performance at a higher density of 1.0 mA cm-2 over 700 h. The in situ optical microscopy study clearly demonstrated that the Mg dendrite formation was successfully retarded by the designed Mg-Sn and Mg-Bi alloy artificial protective layer on Mg anodes. The superiority of Mg-Sn@Mg and Mg-Bi@Mg was further confirmed in full cells using Mo6S8 as the cathode. Compared with the Mo6S8//Mg full cell, the Mo6S8//Mg-Sn@Mg and Mo6S8//Mg-Bi@Mg full cells maintained an ultra-stable electrochemical performance even after 5000 cycles. This proof-of-concept provides a novel scope for the artificial coating layers on Mg anodes prepared by alloy electrodeposition and can be extended to other alloy anodes (i.e. Mg-Cu@Mg and so on). This work provides an avenue for the design of practical and high-performance RMBs and beyond.

Mo3S13 Cluster-Based Cathodes for Rechargeable Magnesium Batteries: Reversible Magnesium Association/Dissociation at the Bridging Disulfur along with Sulfur-Sulfur Bond Break/Formation

Multivalent cation batteries are attracting increasing attention in energy-storage applications, but reversible storage of highly polarizing multivalent cations is a major difficulty for the electrode materials. In the present study, charge-delocalizing Mo3S13 cluster-based materials (crystalline (NH4)2Mo3S13 and amorphous MoSx) are designed and investigated as cathodes for rechargeable magnesium batteries. Both of the cathodes show high magnesium storage capacities (296 and 302 mAh g-1 at 100 mA g-1) and superior rate performances (76 and 80 mAh g-1 at 15 A g-1). A high area loading of 3.0 mg cm-2 could be achieved. These performances are of the highest level compared with those of reported magnesium storage materials. Further mechanism study and theoretical computation demonstrate the magnesium storage active sites are the bridging disulfur groups of the Mo3S13 cluster. The valence state of bridging disulfur decreases/increases largely during magnesiation/demagnesiation along with breaking/formation of the sulfur-sulfur bond, which makes the Mg-association/dissociation highly reversible. The sulfur-sulfur bond breaking and formation provides high reversible capacities. Prominently, the valence state increase and sulfur-sulfur bond formation of the bridging disulfur during charge weakens the bonding with Mg2+, significantly assisting the magnesium dissociation. The present study not only develops high-performance magnesium storage cathode materials but also demonstrates the importance of constructing favorable magnesium storage active sites in the high-performance cathode materials design. The findings presented herein are of great significance for the development of electrode materials for the storage of multivalent cations.

Nanostructured Design Cathode Materials for Magnesium-Ion Batteries

Energy is undeniably one of the most fundamental requirements of the current generation. Solar and wind energy are sustainable and renewable energy sources; however, their unpredictability points to the development of energy storage systems (ESSs). There has been a substantial increase in the use of batteries, particularly lithium-ion batteries (LIBs), as ESSs. However, low rate capability and degradation due to electric load in long-range electric vehicles are pushing LIBs to their limits. As alternative ESSs, magnesium-ion batteries (MIBs) possess promising properties and advantages. Cathode materials play a crucial role in MIBs. In this regard, a variety of cathode materials, including Mn-based, Se-based, vanadium- and vanadium oxide-based, S-based, and Mg2+-containing cathodes, have been investigated by experimental and theoretical techniques. Results reveal that the discharge capacity, capacity retention, and cycle life of cathode materials need improvement. Nevertheless, maintaining the long-term stability of the electrode-electrolyte interface during high-voltage operation continues to be a hurdle in the execution of MIBs, despite the continuous research in this field. The current Review mainly focuses on the most recent nanostructured-design cathode materials in an attempt to draw attention to MIBs and promote the investigation of suitable cathode materials for this promising energy storage device.

Bismuth Nanoparticle-Embedded Carbon Microrod for High-Rate Electrochemical Magnesium Storage

Bismuth metal is regarded as a promising magnesium storage anode material for magnesium-ion batteries due to its high theoretical volumetric capacity and a low alloying potential versus magnesium metal. However, the design of highly dispersed bismuth-based composite nanoparticles is always used to achieve efficient magnesium storage, which is adverse to the development of high-density storage. Herein, a bismuth nanoparticle-embedded carbon microrod (Bi⊂CM), which is prepared via annealing of the bismuth metal-organic framework (Bi-MOF), is developed for high-rate magnesium storage. The use of the Bi-MOF precursor synthesized at an optimized solvothermal temperature of 120 °C benefits the formation of the Bi⊂CM-120 composite with a robust structure and a high carbon content. As a result, the as-prepared Bi⊂CM-120 anode compared to pure Bi and other Bi⊂CM anodes exhibits the best rate performance of magnesium storage at various current densities from 0.05 to 3 A g^-1. For example, the reversible capacity of the Bi⊂CM-120 anode at 3 A g^-1 is ∼17 times higher than that of the pure Bi anode. This performance is also competitive among those of the previously reported Bi-based anodes. Importantly, the microrod structure of the Bi⊂CM-120 anode material remained upon cycling, indicative of good cycling stability.