An Amorphous Molybdenum Polysulfide Cathode for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) attract research interest owing to the low cost and high reliability, but the design of cathode materials is the major difficulty of their development. The bivalent magnesium cation suffers from a strong interaction with the anion and is difficult to intercalate into traditional magnesium intercalation cathodes. Herein, an amorphous molybdenum polysulfide (a-MoS_x) is synthesized via a simple one-step solvothermal reaction and used as the cathode material for RMBs. The a-MoS_x cathode provides a high capacity (185 mAh g⁻¹) and a good rate performance (50 mAh g⁻¹ at 1000 mA g⁻¹), which are much superior compared with crystalline MoS₂ and demonstrate the privilege of amorphous RMB cathodes. A mechanism study demonstrates both of molybdenum and sulfur undergo redox reactions and contribute to the capacity. Further optimizations indicate low-temperature synthesis would favor the magnesium storage performance of a-MoS_x.     

Multi-Electron Reactions Enabled by Anion-Based Redox Chemistry for High-Energy Multivalent Rechargeable Batteries

The development of multivalent metal (such as Mg and Ca) based battery systems is hindered by lack of suitable cathode chemistry that shows reversible multi-electron redox reactions. Cationic redox centres in the classical cathodes can only afford stepwise single-electron transfer, which are not ideal for multivalent-ion storage. The charge imbalance during multivalent ion insertion might lead to an additional kinetic barrier for ion mobility. Therefore, multivalent battery cathodes only exhibit slope-like voltage profiles with insertion/extraction redox of less than one electron. Taking VS₄ as a model material, reversible two-electron redox with cationic–anionic contributions is verified in both rechargeable Mg batteries (RMBs) and rechargeable Ca batteries (RCBs). The corresponding cells exhibit high capacities of >300 mAh g⁻¹ at a current density of 100 mA g⁻¹ in both RMBs and RCBs, resulting in a high energy density of >300 Wh kg⁻¹ for RMBs and >500 Wh kg⁻¹ for RCBs. Mechanistic studies reveal a unique redox activity mainly at anionic sulfides moieties and fast Mg²⁺ ion diffusion kinetics enabled by the soft structure and flexible electron configuration of VS₄.     

Facile Preparation of CuCo2S4/Cu7.2S4 Nanocomposites as High-Performance Cathode Materials for Rechargeable Magnesium Batteries**

Rechargeable magnesium batteries (RMBs) have been considered a promising energy-storage device due to their high energy density and high safety, but they still suffer from a lack of high-rate performance and cycle performance of the cathode. Nanosized CuCo₂S₄/Cu_7.2S₄ composites have been synthesized for the first time by a facile solvothermal method. Herein, the magnesium ion storage behavior when applied in the cathode for RMBs is discussed. Electrochemical results demonstrated that the CuCo₂S₄/Cu_7.2S₄ composites exhibit a high initial discharge capacity of 256 mAh g⁻¹ at 10 mA g⁻¹ and 123 mAh g⁻¹ at 300 mA g⁻¹ at room temperature and an outstanding long-term cyclic stability over 300 cycles at 300 mA g⁻¹. Furthermore, the electrochemical storage mechanism demonstrated that the storage process of magnesium ion in the CuCo₂S₄/Cu_7.2S₄ cathode is mainly driven by strong pseudocapacitive effects.     

Multi‐Electron Reactions Enabled by Anion‐Based Redox Chemistry for High‐Energy Multivalent Rechargeable Batteries

The development of multivalent metal (such as Mg and Ca) based battery systems is hindered by lack of suitable cathode chemistry that shows reversible multi‐electron redox reactions. Cationic redox centres in the classical cathodes can only afford stepwise single‐electron transfer, which are not ideal for multivalent‐ion storage. The charge imbalance during multivalent ion insertion might lead to an additional kinetic barrier for ion mobility. Therefore, multivalent battery cathodes only exhibit slope‐like voltage profiles with insertion/extraction redox of less than one electron. Taking VS₄ as a model material, reversible two‐electron redox with cationic–anionic contributions is verified in both rechargeable Mg batteries (RMBs) and rechargeable Ca batteries (RCBs). The corresponding cells exhibit high capacities of >300 mAh g^?1 at a current density of 100 mA g^?1 in both RMBs and RCBs, resulting in a high energy density of >300 Wh kg^?1 for RMBs and >500 Wh kg^?1 for RCBs. Mechanistic studies reveal a unique redox activity mainly at anionic sulfides moieties and fast Mg²⁺ ion diffusion kinetics enabled by the soft structure and flexible electron configuration of VS₄.     

Engineering of yolk-shelled FeSe2@nitrogen-doped carbon as advanced cathode for potassium-ion batteries

Potassium-ion batteries (KIBs) have become the most promising alternative to lithium-ion batteries for large-scale energy storage system due to their abundance and low cost. However, previous reports focused on the intercalation-type cathode materials usually showed an inferior capacity, together with a poor cyclic life caused by the repetitive intercalation of large-size K-ions, which hinders their practical application. Here, we combine the strategies of carbon coating, template etching and hydrothermal selenization to prepare yolk-shelled FeSe₂@N-doped carbon nanoboxes (FeSe₂@C NBs), where the inner highly-crystalline FeSe₂ clusters are completely surrounded by the self-supported carbon shell. The integrated and highly conductive carbon shell not only provides a fast electron/ion diffusion channel, but also prevents the agglomeration of FeSe₂ clusters. When evaluated as a conversion-type cathode material for KIBs, the FeSe₂@C NBs electrode delivers a relatively high specific capacity of 257 mAh/g at 100 mA/g and potential platform of about 1.6 V, which endow a high energy density of about 411 Wh/kg. Most importantly, by designing a robust host with large internal void space to accommodate the volumetric variation of the inner FeSe₂ clusters, the battery based on FeSe₂@C NBs exhibits ultra-long cycle stability. Specifically, even after 700 cycles at 100 mA/g, a capacity of 221 mAh/g along with an average fading rate of only 0.02% can be retained, which achieves the optimal balance of high specific capacity and long-cycle stability.     

Highly Concentrated Electrolyte towards Enhanced Energy Density and Cycling Life of Dual-Ion Battery

Dual-ion batteries (DIBs) have attracted much attention owing to their low cost, high voltage, and environmental friendliness. As the source of active ions during the charging/discharging process, the electrolyte plays a critical role in the performance of DIBs, including capacity, energy density, and cycling life. However, most used electrolyte systems based on the LiPF₆ salt demonstrate unsatisfactory performance in DIBs. We have successfully developed a 7.5 mol kg⁻¹ lithium bis(fluorosulfonyl)imide (LiFSI) in a carbonate electrolyte system. Compared with diluted electrolytes, this highly concentrated electrolyte exhibits several advantages: 1) enhanced intercalation capacity and cycling stability of the graphite cathode, 2) optimized structural stability of the Al anode, and 3) significantly increased battery energy density. A proof-of-concept DIB based on this concentrated electrolyte exhibits a discharge capacity of 94.0 mAh g⁻¹ at 200 mA g⁻¹ and 96.8 % capacity retention after 500 cycles. By counting both the electrode materials and electrolyte, the energy density of this DIB reaches up to ≈180 Wh kg⁻¹, which is among the best performances of DIBs reported to date.     

Self-Assembled FeSe2 Microspheres with High-Rate Capability and Long-Term Stability as Anode Material for Sodium- and Potassium-Ion Batteries

Sodium- and potassium-ion batteries have attracted intensive attention recently as low-cost alternatives to lithium-ion batteries with naturally abundant resources. However, the large ionic radii of Na⁺ and K⁺ render their slow mobility, leading to sluggish diffusion in host materials. Herein, hierarchical FeSe₂ microspheres assembled by closely packed nano/microrods are rationally designed and synthesized through a facile solvothermal method. Without carbonaceous material incorporation, the electrode delivers a reversible Na⁺ storage capacity of 559 mA h g⁻¹ at a current rate of 0.1 A g⁻¹ and a remarkable rate performance with a capacity of 525 mA h g⁻¹ at 20 A g⁻¹. As for K⁺ storage, the FeSe₂ anode delivers a high reversible capacity of 393 mA h g⁻¹ at 0.4 A g⁻¹. Even at a high current rate of 5 A g⁻¹, a discharge capacity of 322 mA h g⁻¹ can be achieved, which is among the best high-rate anodes for K⁺ storage. The excellent electrochemical performance can be attributed to the favorable morphological structure and the use of an ether-based electrolyte during cycling. Moreover, quantitative study suggests a strong pseudocapacitive contribution, which boosts fast kinetics and interfacial storage.     

Pseudocapacitive Lithium Storage of Cauliflower-Like CoFe2O4 for Low-Temperature Battery Operation

Binary transition-metal oxides (BTMOs) with hierarchical micro–nano-structures have attracted great interest as potential anode materials for lithium-ion batteries (LIBs). Herein, we report the fabrication of hierarchical cauliflower-like CoFe₂O₄ (cl-CoFe₂O₄) via a facile room-temperature co-precipitation method followed by post-synthetic annealing. The obtained cauliflower structure is constructed by the assembly of microrods, which themselves are composed of small nanoparticles. Such hierarchical micro–nano-structure can promote fast ion transport and stable electrode–electrolyte interfaces. As a result, the cl-CoFe₂O₄ can deliver a high specific capacity (1019.9 mAh g⁻¹ at 0.1 A g⁻¹), excellent rate capability (626.0 mAh g⁻¹ at 5 A g⁻¹), and good cyclability (675.4 mAh g⁻¹ at 4 A g⁻¹ for over 400 cycles) as an anode material for LIBs. Even at low temperatures of 0 °C and −25 °C, the cl-CoFe₂O₄ anode can deliver high capacities of 907.5 and 664.5 mAh g⁻¹ at 100 mA g⁻¹, respectively, indicating its wide operating temperature. More importantly, the full-cell assembled with a commercial LiFePO₄ cathode exhibits a high rate performance (214.2 mAh g⁻¹ at 5000 mA g⁻¹) and an impressive cycling performance (612.7 mAh g⁻¹ over 140 cycles at 300 mA g⁻¹) in the voltage range of 0.5–3.6 V. Kinetic analysis reveals that the electrochemical performance of cl-CoFe₂O₄ is dominated by pseudocapacitive behavior, leading to fast Li⁺ insertion/extraction and good cycling life.     

Li+/Mg2+ Hybrid‐Ion Batteries with Long Cycle Life and High Rate Capability Employing MoS2 Nano Flowers as the Cathode Material

The demand for large‐scale and safe energy storage is increasing rapidly due to the strong push for smartphones and electric vehicles. As a result, Li⁺/Mg²⁺ hybrid‐ion batteries (LMIBs) combining a dendrite‐free deposition of Mg anode and Li⁺ intercalation cathode have attracted considerable attention. Here, a LMIB with hydrothermal‐prepared MoS₂ nano flowers as cathode material was prepared. The battery showed remarkable electrochemical properties with a large discharge capacity (243 mAh g^?1 at the 0.1 C rate), excellent rate capability (108 mAh g^?1 at the 5 C rate), and long cycle life (87.2 % capacity retention after 2300 cycles). Electrochemical analysis showed that the reactions occurring in the battery cell involved Mg stripping/plating at the anode side and Li⁺ intercalation at the cathode side with a small contribution from Mg²⁺ adsorption. The excellent electrochemical performance and extremely safe cell system show promise for its use in practical applications.     

Effects of Conjugated Structure on the Magnesium Storage Performance of Dianhydrides

Inorganic cathodes of rechargeable Mg batteries suffer from limited selections, while organic materials provide more options. Herein, three conjugated dianhydrides, pyromellitic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride and 3,4,9,10-perylenetetracarboxylic dianhydride are comparatively investigated to elucidate the effects of conjugated structure on the Mg²⁺ storage performances. It is observed that the reversible Mg²⁺ storage capacity is more dependent on the conjugated structure than carbonyl numbers. Ex-situ mechanism study illustrates that the extended conjugated structure delocalizes the electron density, hence enhancing carbonyl enolization and increasing the Mg²⁺ storage capacity. Furthermore, the largely conjugated structure buffers the charge density change during repeated magnesiation/demagnesiation resulting in better cyclability. Prominently, 3,4,9,10-perylenetetracarboxylic dianhydride shows a high Mg²⁺ storage capacity (160 mAh g⁻¹) and a good cycling stability (80 % capacity retention after 100 cycles) with the largest conjugated structure. This work provides a low-cost cathode for rechargeable Mg batteries that can be utilized for designing high-performance organic Mg battery cathodes.     

Dual Dopamine Derived Polydopamine Coated N-Doped Porous Carbon Spheres as a Sulfur Host for High-Performance Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are considered to be one of the most promising energy storage systems owing to their high energy density and low cost. However, their wide application is still limited by the rapid capacity fading. Herein, polydopamine (PDA)-coated N-doped hierarchical porous carbon spheres (NPC@PDA) are reported as sulfur hosts for high-performance Li-S batteries. The NPC core with abundant and interconnected pores provides fast electron/ion transport pathways and strong trapping ability towards lithium polysulfide intermediates. The PDA shell could further suppress the loss of lithium polysulfide intermediates through polar–polar interactions. Benefiting from the dual function design, the NPC/S@PDA composite cathode exhibits an initial capacity of 1331 mAh g⁻¹ and remains at 720 mAh g⁻¹ after 200 cycles at 0.5 C. At the pouch cell level with a high sulfur mass loading, the NPC/S@PDA composite cathode still exhibits a high capacity of 1062 mAh g⁻¹ at a current density of 0.4 mA cm⁻².     

Electrochemically Driven Transformation of Amorphous Carbons to Crystalline Graphite Nanoflakes: A Facile and Mild Graphitization Method

Although, in the carbon family, graphite is the most thermodynamically stable allotrope, conversion of other carbon allotropes, even amorphous carbons, into graphite is extremely hard. We report a simple electrochemical route for the graphitization of amorphous carbons through cathodic polarization in molten CaCl₂ at temperatures of about 1100 K, which generates porous graphite comprising petaloid nanoflakes. This nanostructured graphite allows fast and reversible intercalation/deintercalation of anions, promising a superior cathode material for batteries. In a Pyr₁₄TFSI ionic liquid, it exhibits a specific discharge capacity of 65 and 116 mAh g⁻¹ at a rate of 1800 mA g⁻¹ when charged to 5.0 and 5.25 V vs. Li/Li⁺, respectively. The capacity remains fairly stable during cycling and decreases by only about 8 % when the charge/discharge rate is increased to 10000 mA g⁻¹ during cycling between 2.25 and 5.0 V.     

Magnesium hexakis(methanol)-dinitrate complex electrolyte for use in rechargeable magnesium batteries

Functional compatible electrolyte with Mg²⁺ intercalation cathodes represents one of the largest obstacles in the development of practical Mg batteries MBs. In current work, we report for the first time magnesium hexakis(methanol)-dinitrate complex (MHMD) electrolyte product reaction of 2,2-dimethoxypropane with magnesium nitrate hexahydrate via ‘Solvent-in-Salt’ method. 2,2-Dimethoxypropane as a water scavenger can capture reducible molecules like H₂O and dehydrate Mg(NO₃)₂.6H₂O to form magnesium hexakis(methanol)-dinitrate complex. Meanwhile, Mg cloud bonds will become weak—something which frees up the mobility of Mg²⁺. This electrolyte exhibits high ionic conductivity with low activation energy ~ 0.18 eV. The general aim of the investigation was to demonstrate a potential application of MHMD electrolyte in Mg-ion cell. Mg cells were analyzed with the use of cyclic voltammetry (CV), galvanostatic charging/discharging tests, and electrochemical impedance spectroscopy. A comparative study between different cathodes like V₂O₅, GeO₂, TiO₂, and S using MHMD electrolyte was performed. The S cathode has an initial discharge capacity of 370 mAh g^?1 and retained a reversible capacity at 60 mAh g^?1 after 20 cycles exhibiting better electrochemical performances than those of V₂O₅, GeO₂, and TiO₂ cathodes. This work opens up a new pathway to explore new electrolytic materials for MBs with high performance.     

Hybrid Superlattice-Triggered Selective Proton Grotthuss Intercalation in δ-MnO2 for High-Performance Zinc-Ion Battery

A great deal of attention has been paid on layered manganese dioxide (δ−MnO₂) as promising cathode candidate for aqueous zinc-ion battery (ZIB) due to the excellent theoretical capacity, high working voltage and Zn²⁺/H⁺ co-intercalation mechanism. However, caused by the insertion of Zn²⁺, the strong coulomb interaction and sluggish diffusion kinetics have resulted in significant structure deformation, insufficient cycle stability and limited rate capability. And it is still far from satisfactory to accurately modulate H⁺ intercalation for superior electrochemical kinetics. Herein, the terrace-shape δ−MnO₂ hybrid superlattice by polyvinylpyrrolidone (PVP) pre-intercalation (PVP−MnO₂) was proposed with the state-of-the-art ZIBs performance. Local atomic structure characterization and theoretical calculations have been pioneering in confirming the hybrid superlattice-triggered synergy of electron entropy stimulation and selective H⁺ Grotthuss intercalation. Accordingly, PVP−MnO₂ hybrid superlattice exhibits prominent specific capacity (317.2 mAh g⁻¹ at 0.125 A g⁻¹), significant rate performance (106.1 mAh g⁻¹ at 12.5 A g⁻¹), and remarkable cycle stability at high rate (≈100 % capacity retention after 20,000 cycles at 10 A g⁻¹). Therefore, rational design of interlayer configuration paves the pathways to the development of MnO₂ superlattice for advanced Zn−MnO₂ batteries.     

Flexible Li-CO2 Batteries with Liquid-Free Electrolyte

Developing flexible Li-CO₂ batteries is a promising approach to reuse CO₂ and simultaneously supply energy to wearable electronics. However, all reported Li-CO₂ batteries use liquid electrolyte and lack robust electrolyte/electrodes structure, not providing the safety and flexibility required. Herein we demonstrate flexible liquid-free Li-CO₂ batteries based on poly(methacrylate)/poly(ethylene glycol)-LiClO₄-3 wt %SiO₂ composite polymer electrolyte (CPE) and multiwall carbon nanotubes (CNTs) cathodes. The CPE (7.14×10⁻² mS cm⁻¹) incorporates with porous CNTs cathodes, displaying stable structure and small interface resistance. The batteries run for 100 cycles with controlled capacity of 1000 mAh g⁻¹. Moreover, pouch-type flexible batteries exhibit large reversible capacity of 993.3 mAh, high energy density of 521 Wh kg⁻¹, and long operation time of 220 h at different degrees of bending (0–360°) at 55 °C.     

Facile Synthesis of Ultra-Small Few-Layer Nanostructured MoSe2 Embedded on N,P Co-Doped Bio-Carbon for High-Performance Half/Full Sodium-Ion and Potassium-Ion Batteries

Sodium/potassium-ion batteries (SIBs/PIBs) arouse intensive interest on account of the natural abundance of sodium/potassium resources, the competitive cost and appropriate redox potential. Nevertheless, the huge challenge for SIBs/PIBs lies in the scarcity of an anode material with high capacity and stable structure, which are capable of accommodating large-size ions during cycling. Furthermore, using sustainable natural biomass to fabricate electrodes for energy storage applications is a hot topic. Herein, an ultra-small few-layer nanostructured MoSe₂ embedded on N, P co-doped bio-carbon is reported, which is synthesized by using chlorella as the adsorbent and precursor. As a consequence, the MoSe₂/NP-C-2 composite represents exceedingly impressive electrochemical performance for both sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). It displays a promising reversible capacity (523 mAh g⁻¹ at 100 mA g⁻¹ after 100 cycles) and impressive long-term cycling performance (192 mAh g⁻¹ at 5 A g⁻¹ even after 1000 cycles) in SIBs, which are some of the best properties of MoSe₂-based anode materials for SIBs to date. To further probe the great potential applications, full SIBs pairing the MoSe₂/NP-C-2 composite anode with a Na₃V₂(PO₄)₃ cathode also exhibits a satisfactory capacity of 215 mAh g⁻¹ at 500 mA g⁻¹ after 100 cycles. Moreover, it also delivers a decent reversible capacity of 131 mAh g⁻¹ at 1 A g⁻¹ even after 250 cycles for PIBs.     

Sulfur–carbon nano-composite as cathode for rechargeable lithium battery based on gel electrolyte

Sulfur–carbon nano-composite with elemental sulfur incorporated in porous carbon was prepared by thermal treatment of a mixture of sulfur and active carbon. The new material was characterized by X-ray diffraction, BET and scanning electron microscopy. The nano-composite, tested at room temperature as cathode in a nonaqueous lithium cell based on PVDF gel electrolyte, exhibited a reversible capacity of 440 mAh g⁻¹ at a current density of 0.3 mA cm⁻². The utilization of electrochemically active sulfur was about 90% assuming a complete reaction to the product of Li₂S during cycling.     

Lithium Nitrate Regulated Sulfone Electrolytes for Lithium Metal Batteries

The electrolytes in lithium metal batteries have to be compatible with both lithium metal anodes and high voltage cathodes, and can be regulated by manipulating the solvation structure. Herein, to enhance the electrolyte stability, lithium nitrate (LiNO₃) and 1,1,2,2-tetrafuoroethyl-2′,2′,2′-trifuoroethyl(HFE) are introduced into the high-concentration sulfolane electrolyte to suppress Li dendrite growth and achieve a high Coulombic efficiency of >99 % for both the Li anode and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathodes. Molecular dynamics simulations show that NO₃⁻ participates in the solvation sheath of lithium ions enabling more bis(trifluoromethanesulfonyl)imide anion (TFSI⁻) to coordinate with Li⁺ ions. Therefore, a robust LiN_xO_y−LiF-rich solid electrolyte interface (SEI) is formed on the Li surface, suppressing Li dendrite growth. The LiNO₃-containing sulfolane electrolyte can also support the highly aggressive LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathode, delivering a discharge capacity of 190.4 mAh g⁻¹ at 0.5 C for 200 cycles with a capacity retention rate of 99.5 %.     

Engineering Na−Mo−O/Graphene Oxide Composites with Enhanced Electrochemical Performance for Lithium Ion Batteries

Sodium molybdate (Na−Mo−O) wrapped by graphene oxide (GO) composites have been prepared via a simple in-situ precipitation method at room temperature. The composites are mainly constructed with one dimension (1D) ultra-long sodium molybdate nanorods, which are wrapped by the flexible GO. The introduction of GO is expected to not merely provide more active sites for lithium-ions storage, but also improve the charge transfer rate of the electrode. The testing electrochemical performances corroborated the standpoint: The Na−Mo−O/GO composites delivers specific capacities of 718 mAh g⁻¹ after 100 cycles at 100 mA g⁻¹, and 570 mAh g⁻¹ after 500 cycles at a high rate of 500 mA g⁻¹; for comparison, the bare Na−Mo−O nanorod shows a severe capacity decay, which deliver only 332 mAh g⁻¹ after 100 cycles at 100 mA g⁻¹. In view of the cost-efficient and less time-consuming in synthesis, and one-step preparation without further treatment, these Na−Mo−O nanorods/GO composites present potential and prospective anodes for LIBs.     

Carbon Nitride Pillared Vanadate Via Chemical Pre-Intercalation Towards High-Performance Aqueous Zinc-Ion Batteries

Vanadium based compounds are promising cathode materials for aqueous zinc (Zn)-ion batteries (AZIBs) due to their high specific capacity. However, the narrow interlayer spacing, low intrinsic conductivity and the vanadium dissolution still restrict their further application. Herein, we present an oxygen-deficient vanadate pillared by carbon nitride (C₃N₄) as the cathode for AZIBs through a facile self-engaged hydrothermal strategy. Of note, C₃N₄ nanosheets can act as both the nitrogen source and pre-intercalation species to transform the orthorhombic V₂O₅ into layered NH₄V₄O₁₀ with expanded interlayer spacing. Owing to the pillared structure and abundant oxygen vacancies, both the Zn²⁺ ion (de)intercalation kinetics and the ionic conductivity in the NH₄V₄O₁₀ cathode are promoted. As a result, the NH₄V₄O₁₀ cathode delivers exceptional Zn-ion storage ability with a high specific capacity of about 370 mAh g⁻¹ at 0.5 A g⁻¹, a high-rate capability of 194.7 mAh g⁻¹ at 20 A g⁻¹ and a stable cycling performance of 10 000 cycles.