In Operando Synchrotron Diffraction and in Operando X-ray Absorption Spectroscopy Investigations of Orthorhombic V2O5 Nanowires as Cathode Materials for Mg-Ion Batteries

Progress and Challenges of Vanadium Oxide Cathodes for Rechargeable Magnesium Batteries

Among the challenges related to rechargeable magnesium batteries (RMBs) still not resolved are positive electrode materials with sufficient charge storage and rate capability as well as stability and raw material resources. Out of the materials proposed and studied so far, vanadium oxides stand out for these requirements, but significant further improvements are expected and required. They will be based on new materials and an improved understanding of their mode of operation. This report provides a critical review focused on this material, which is embedded in a brief overview on the general subject. It starts with the main strategic ways to design layered vanadium oxides cathodes for RMBs. Taking these examples in more detail, the typical issues and challenges often missed in broader overviews and reviews are discussed. In particular, issues related to the electrochemistry of intercalation processes in layered vanadium oxides; advantageous strategies for the development of vanadium oxide composite cathodes; their mechanism in aqueous, "wet", and dry non-aqueous aprotic systems; and the possibility of co-intercalation processes involving protons and magnesium ions are considered. The perspectives for future development of vanadium oxide-based cathode materials are finally discussed and summarized.

Catalyzing Desolvation at Cathode-Electrolyte Interface Enabling High-Performance Magnesium-Ion Batteries

Magnesium ion batteries (MIBs) are expected to be the promising candidates in the post-lithium-ion era with high safety, low cost and almost dendrite-free nature. However, the sluggish diffusion kinetics and strong solvation capability of the strongly polarized Mg2+ are seriously limiting the specific capacity and lifespan of MIBs. In this work, catalytic desolvation is introduced into MIBs for the first time by modifying vanadium pentoxide (V2O5) with molybdenum disulfide quantum dots (MQDs), and it is demonstrated via density function theory (DFT) calculations that MQDs can effectively lower the desolvation energy barrier of Mg2+, and therefore catalyze the dissociation of Mg2+-1,2-Dimethoxyethane (Mg2+-DME) bonds and release free electrolyte cations, finally contributing to a fast diffusion kinetics within the cathode. Meanwhile, the local interlayer expansion can also increase the layer spacing of V2O5 and speed up the magnesiation/demagnesiation kinetics. Benefiting from the structural configuration, MIBs exhibit superb reversible capacity (≈300 mAh g-1 at 50 mA g-1) and unparalleled cycling stability (15 000 cycles at 2 A g-1 with a capacity of ≈70 mAh g-1). This approach based on catalytic reactions to regulate the desolvation behavior of the whole interface provides a new idea and reference for the development of high-performance MIBs.

Mitigating Interfacial Capacity Fading in Vanadium Pentoxide by Sacrificial Vanadium Sulfide Encapsulation for Rechargeable Mg-Ion Batteries

Rechargeable Mg-ion Batteries (RMB) containing a Mg metal anode offer the promise of higher specific volumetric capacity, energy density, safety, and economic viability than lithium-ion battery technology, but their realization is challenging. The limited availability of suitable inorganic cathodes compatible with electrolytes relevant to Mg metal anode restricts the development of RMBs. Despite the promising capability of some oxides to reversibly intercalate Mg+2 ions at high potential, its lack of stability in chloride-containing ethereal electrolytes, relevant to Mg metal anode hinders the realization of a full practical RMB. Here the successful in situ encapsulation of monodispersed spherical V2O5 (≈200 nm) is demonstrated by a thin layer of VS2 (≈12 nm) through a facile surface reduction route. The VS2 layer protects the surface of V2O5 particles in RMB electrolyte solution (MgCl2 + MgTFSI in DME). Both V2O5 and V2O5@VS2 particles demonstrate high initial discharge capacity. However, only the V2O5@VS2 material demonstrates superior rate performance, Coulombic efficiency (100%), and stability (138 mA h g-1 discharge capacity after 100 cycles), signifying the ability of the thin VS2 layer to protect the V2O5 cathode and facilitate the Mg+2 ion intercalation/deintercalation into V2O5.

Discovering the Order-Disorder Transition in Quinoline Intercalated Vanadium Oxide with Superior Calcium Storage via Polyhedral Distortion

Calcium-ion batteries (CIBs) are considered as potential next-generation energy storage systems due to their abundant reserves and relatively low cost. However, irreversible structural changes and weak conductivity still hinder in current CIBs cathode materials. Herein, an organic molecular intercalation strategy is proposed, in which V2O5 regulated with quinoline, pyridine, and water molecules are studied as cathode material to provide fast ion diffusion channels, large storage host, and high conductivity for Ca ions. Among them, V2O5-quinoline (QVO) owns the largest interplanar spacing of 1.25 nm and the V-O chains are connected with organic molecular by hydrogen bond, which stabilizes the crystal structure. As a result, QVO exhibits a specific capacity of 168 mAh g-1 at 1 A g-1 and capacity retention of 80% after 500 cycles at 5 A g-1 than the other materials. Furthermore, X-Ray diffraction and X-ray absorption spectroscopy results reveal a reversible order-disorder transformation mechanism of Ca2+ for QVO, which can make full use of the abundant active sites for high capacity and simultaneously achieve fast reaction kinetics for excellent rate performance. These results demonstrate that QVO is a promising cathode material for CIBs, providing more choices for the development of high-performance CIBs.

Strategies to alleviate distortive phase transformations in Li-ion intercalation reactions: an example with vanadium pentoxide

Chemical and electrochemical Li-ion insertion in transition metal oxides, either via a phase transformation reaction (ions insert into specific crystallographic sites of the host lattice) or a solid solution insertion (ions distribute uniformly throughout the host lattice), enables high energy density electrochemical energy storage. Many phase transformation cathode materials, that undergo two-phase reactions, exhibit high theoretical capacities arising from multi-electron redox reactions. However, challenges in distortive phase transformations and uncontrolled phase nucleation, propagation, segregation, and co-existence continue to limit the energy density, (dis)charging rate performances, and cycling stability of available phase transformation cathode materials. Vanadium pentoxide (V2O5), a classical layered intercalation host material with high theoretical capacity, undergoes irreversible structural changes and capacity fading when intercalating more than one lithium ion per V2O5 unit in its thermodynamically stable phase. Here, we review recent synthetic strategies to alter the V-O connectivity, thereby alleviating distortive phase transformations and promoting solid solution-based Li-ion insertion in V2O5. We also summarize several widely accessible and classical molecular-based analytical tools that can provide local structural dynamics and phase transformation mechanism information on the lithiation of V2O5, including single-crystal X-ray diffraction, infrared and Raman spectroscopy, electron paramagnetic resonance, and nuclear magnetic resonance spectroscopy.

Organic Molecular Intercalation Enabled Anionic Redox Chemistry with Fast Kinetics for High Performance Magnesium Storage

Rechargeable magnesium-ion batteries possess desirable characteristics in large-scale energy storage applications. However, severe polarization, sluggish kinetics and structural instability caused by high charge density Mg2+ hinder the development of high-performance cathode materials. Herein, the anionic redox chemistry in VS4 is successfully activated by inducing cations reduction and introducing anionic vacancies via polyacrylonitrile (PAN) intercalation. Increased interlayer spacing and structural vacancies can promote the electrolyte ions migration and accelerate the reaction kinetics. Thanks to this "three birds with one stone" strategy, PAN intercalated VS4 exhibits an outstanding electrochemical performance: high discharge specific capacity of 187.2 mAh g-1 at 200 mA g-1 after stabilization and a long lifespan of 5000 cycles at 2 A g-1 are achieved, outperforming other reported VS4-based materials to date for magnesium storage under the APC electrolyte. Theoretical calculations confirm that the intercalated PAN can indeed induce cations reduction and generate anionic vacancies by promoting electron transfer, which can accelerate the electrochemical reaction kinetics and activate the anionic redox chemistry, thus improving the magnesium storage performance. This approach of organic molecular intercalation represents a promising guideline for electrode material design on the development of advanced multivalent-ion batteries.

Monitoring Structural Changes during Electrochemical Cycling of Solid-Solution Spinel Oxide MgCrVO4

Rechargeable magnesium-ion batteries (MIBs) hold significant promise as an alternative to conventional lithium-ion technology driven by their natural abundance and low-cost, high-energy density, and safety features. Spinel oxides, including MgCrVO4, have emerged as a prospective cathode material for MIBs due to their promising combination of capacity, operating potential, and cation mobility. However, the structural evolution, phase stability, and processes of Mg mobility in MgCrVO4 during electrochemical cycling are poorly understood. In this study, we synthesized a single-phase, solid solution of spinel oxide MgCrVO4 and employed operando X-ray diffraction to couple physical properties with structural changes during cycling. Our results revealed a two-phase reaction mechanism coupled with a solid-solution-like reaction, highlighting the complicated transformation between two distinct phases in the MgCrVO4 lattice during Mg (de)intercalation. Rietveld refinement of the operando data provided valuable insights into the mechanism of the Cr/V-based spinel oxide, shedding light on the transition between the two phases and their roles in Mg-ion (de)intercalation. This study contributes to a deeper understanding of the structural dynamics in multivalent cathode materials and sets the stage for the development of advanced Mg-ion cathodes with enhanced performance and stability.

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.

Photoelectrochemical N2 -to-NH3 Fixation with High Efficiency and Rates via Optimized Si-Based System at Positive Potential versus Li0/

As a widely used commodity chemical, ammonia is critical for producing nitrogen-containing fertilizers and serving as the promising zero-carbon energy carrier. Photoelectrochemical nitrogen reduction reaction (PEC NRR) can provide a solar-powered green and sustainable route for synthesis of ammonia (NH₃ ). Herein, an optimum PEC system is reported with an Si-based hierarchically-structured PdCu/TiO₂ /Si photocathode and well-thought-out trifluoroethanol as the proton source for lithium-mediated PEC NRR, achieving a record high NH₃ yield of 43.09 µg cm^-2 h^-1 and an excellent faradaic efficiency of 46.15% under 0.12 MPa O₂ and 3.88 MPa N₂ at 0.07 V versus lithium(0/+) redox couple (vs Li^0/+ ). PEC measurements coupled with operando characterization reveal that the PdCu/TiO₂ /Si photocathode under N₂ pressures facilitate the reduction of N₂ to form lithium nitride (Li₃ N), which reacts with active protons to produce NH₃ while releasing the Li⁺ to reinitiate the cycle of the PEC NRR. The Li-mediated PEC NRR process is further enhanced by introducing small amount of O₂ or CO₂ under pressure by accelerating the decomposition of Li₃ N. For the first time, this work provides mechanistic understanding of the lithium-mediated PEC NRR process and opens new avenues for efficient solar-powered green conversion of N₂ -to-NH₃ .

A Heterogeneous Single Atom Cobalt Catalyst for Highly Efficient Acceptorless Dehydrogenative Coupling Reactions

A fundamental understanding of metal active sites in single-atom catalysts (SACs) is important and challenging in the development of high-performance catalyst systems. Here, a highly efficient and straightforward molten-salt-assisted approach is reported to create atomically dispersed cobalt atoms supported over vanadium pentoxide layered material, with each cobalt atom coordinated with four neighboring oxygen atoms. The liquid environment and the strong polarizing force of the molten salt at high temperatures potentially favor the weakening of VO bonding and the formation of CoO bonding on the vanadium oxide surface. This cobalt SAC achieves extraordinary catalytic efficiency in acceptorless dehydrogenative coupling of alcohols with amines to give imines, with more than 99% selectivity under almost 100% conversion within 3 h, along with a high turnover frequency (TOF) of 5882 h^-1 , exceeding those of previously reported benchmarking catalysts. Moreover, it delivers excellent recyclability, reaction scalability, and substrate tolerance. Density functional theory (DFT) calculations further confirm that the optimized coordination environment and strong electronic metal-support interaction contribute significantly to the activation of reactants. The findings provide a feasible route to construct SACs at the atomic level for use in organic transformations.

High-Performance NiS2 Hollow Nanosphere Cathodes in Magnesium-Ion Batteries Enabled by Tunable Redox Chemistry

Two-dimensional metal dichalcogenides have demonstrated outstanding potential as cathodes for magnesium-ion batteries. However, the limited capacity, poor cycling stability, and severe electrode pulverization, resulting from lack of void space for expansion, impede their further development. In this work, we report for the first time, nickel sulfide (NiS₂) hollow nanospheres assembled with nanoparticles for use as cathode materials in magnesium-ion batteries. Notably, the nanospheres were prepared by a one-step solvothermal process in the absence of an additive. The results show that regulating the synergistic effect between the rich anions and hollow structure positively affects its electrochemical performance. Crystallographic and microstructural characterizations reveal the reversible anionic redox of S^2-/(S₂)^2-, consistent with density functional theory results. Consequently, the optimized cathode (8-NiS₂ hollow nanospheres) could deliver a large capacity of 301 mA h g^-1 after 100 cycles at 50 mA g^-1, supporting the promising practical application of NiS₂ hollow nanospheres in magnesium-ion batteries.

Sodium Pre-Intercalated Carbon/V2 O5 Constructed by Sustainable Sodium Lignosulfonate for Stable Cathodes in Zinc-Ion Batteries: A Comprehensive Study

The aqueous zinc-ion battery (AZIB) has been widely investigated in recent years because it has the advantages of being green, safe, and made from abundant raw materials. It is necessary to continue to study how to prepare cathode materials with excellent performance and high cycling stability for future commercialization. In this work, a strategy was proposed that uses sustainable sodium lignosulfonate as both carbon and sodium sources to obtain a sodium pre-intercalated vanadium oxide/carbon (VO/LSC) composite as the cathode of AZIB. The carbon matrix could improve the electronic conductivity of vanadium oxide, while the sodium lignosulfonate could provide sodium ions pre-intercalated into the layered vanadium oxide simultaneously. Through this strategy, vanadium-based cathode materials with high stability and excellent rate capability were obtained. The VO/LSC cathode delivered high capacities of 350 and 112.8 mAh g^-1 at 0.1 and 4.0 A g^-1 , respectively. Zinc sulfate and zinc trifluoromethyl sulfonate were selected as electrolytes, and the influence of electrolytes on the performance of VO/LSC was analyzed. The oxygen in the environment was used to oxidize the low-priced vanadium oxide to achieve a self-charging AZIB. This paper provides a valuable strategy for the design of vanadium-based cathode material for AZIB, which can broaden the research and application of AZIB.

Will Vanadium-Based Electrode Materials Become the Future Choice for Metal-Ion Batteries?

Metal-ion batteries have emerged as promising candidates for energy storage system due to their unlimited resources and competitive price/performance ratio. Vanadium-based compounds have diverse oxidation states rendering various open-frameworks for ions storage. To date, some vanadium-based polyanionic compounds have shown great potential as high-performance electrode materials. However, there has been a growing concern regarding the cost and environmental risk of vanadium. In this Review, all links in the industry chain of vanadium-based electrodes were comprehensively summarized, starting with an analysis of the resources, applications, and price fluctuation of vanadium. The manufacturing processes of the vanadium extraction and recovery technologies were discussed. Moreover, the commercial potentials of some typical electrode materials were critically appraised. Finally, the environmental impact and sustainability of the industry chain were evaluated. This critical Review will provide a clear vision of the prospects and challenges of developing vanadium-based electrode materials.

Electrochemical Actuators with Multicolor Changes and Multidirectional Actuation

Electrochemical (EC) actuators have garnered significant attention in recent years, yet there are still some critical challenges to limit their application range, such as responsive time, multifunctionality, and actuating direction. Herein, an EC actuator with a back-to-back structure is fabricated by stacking two membranes with bilayer V₂ O₅ nanowires/single-walled carbon nanotubes (V₂ O₅ NWs/SWCNTs) networks, and shows a synchronous high actuation amplitude (about ±9.7 mm, ±28.4°) and multiple color changes. In this back-to-back structure, the inactive SWCNTs layer is used as a conductive current collector, and the bilayer network is attached to a porous polymer membrane. The dual-responsive processes of V₂ O₅ nanowires (V₂ O₅ NWs) actuation films and actuators are also deeply investigated through in situ EC X-ray diffraction and Raman spectroscopy. The results show that the EC actuation of the V₂ O₅ NWs/SWCNTs film is highly related to the redox behavior of the pseudocapacitive V₂ O₅ NWs layer. At last, both V₂ O₅ NWs and W₁₈ O₄₉ nanowires (W₁₈ O₄₉ NWs)-based EC actuators are constructed to demonstrate the multicolor changes and multidirectional actuation induced by the opposite lattice changes of V₂ O₅ NWs and W₁₈ O₄₉ NWs during ionic de-/intercalation, guiding the design of multifunctional EC actuators in the future.

In Situ Coupling Applied Voltage and Synchrotron Radiation: Operando Characterization of Transistors

A compact voltage application setup has been developed for in situ electrical testing of organic field effect transistors in combination with X-ray scattering studies at a synchrotron beamlines. Challenges faced during real condition in-operando test of newly developed OFETs originated an idea of creation of a new setup which excludes number of factors that make experiments complicated. The application of the setup is demonstrated on a prototype of an organic transistors based on α,ω-dihexyl-α-quaterthiophene molecules. The new setup allows to monitor material structural changes by X-ray scattering under applied voltage conditions and their direct correlations. The versatile setup eliminates possible shadowing effects and short circuits due to misalignment of the contacts. The electrical stability of the prototypes was characterized by the application of different voltage values. Corresponding structural changes were monitored by grazing X-ray scattering technique before, during and after the voltage was applied. The selected oligothiophene material with proved transistor properties shows high stability and directional anisotropy under applied voltage conditions. Thanks to a compact and flexible design of the setup, different type of small dimension devices could be studied under external voltage conditions at various synchrotron beamlines.

Reconstructing Vanadium Oxide with Anisotropic Pathways for a Durable and Fast Aqueous K-Ion Battery

Aqueous potassium-ion batteries are long-term pursued, due to their excellent performance and intrinsic superiority in safe, low-cost storage for portable and grid-scale applications. However, the notorious issues of K-ion battery chemistry are the inferior cycling stability and poor rate performance, due to the inevitably destabilization of the crystal structure caused by K-ions with pronouncedly large ionic radius. Here, we resolve such issues by reconstructing commercial vanadium oxide (α-V₂O₅) into the bronze form, i.e., δ-K_0.5V₂O₅ (KVO) nanobelts, as cathode materials with layered structure of enlarged space and anisotropic pathways for K-ion storage. Specifically, it can deliver a high capacity as 116 mAh g^-1 at the 1 C-rate, an outstanding rate capacity of 65 mAh g^-1 at 50 C, and a robust cyclic stability with 88.2% capacity retention after 1,000 cycles at 1 C. When coupled with organic anode in a full-cell configuration, the KVO electrodes can output 95 mAh g^-1 at 1 C and cyclic stability with 77.3% capacity retention after 20,000 cycles at 10 C. According to experimental and calculational results, the ultradurable cyclic performance is assigned to the robust structural reversibility of the KVO electrode, and the ultrahigh-rate capability is attributed to the anisotropic pathways with improved electrical conductivity in KVO nanobelts. In addition, applying a 22 M KCF₃SO₃ water-in-salt electrolyte can impede the dissolving issues of the KVO electrode and further stabilize the battery cyclic performance. Lastly, the as-designed AKIBs can operate with superior low-temperature adaptivity even at -30 °C. Overall, the KVO electrode can serve as a paradigm toward developing more suitable electrode materials for high-performance AKIBs.

Hierarchical 3D Cuprous Sulfide Nanoporous Cluster Arrays Self-Assembled on Copper Foam as a Binder-Free Cathode for Hybrid Magnesium-Based Batteries

On account of easy accessibility, high theoretical volumetric capacity and dendrite-free magnesium (Mg) anode, Mg battery has a great promise to be next generation rechargeable batteries, yet still remains a challenging task in acquiring fast Mg²⁺ kinetics and effective cathode materials. Herein, hierarchical 3D cuprous sulfide porous nanosheet decorated nanowire cluster arrays with robust adhesion on copper foam (Cu₂ S HP/CF), which is employed as a binder-free conversion cathode material for magnesium/lithium hybrid battery, delivering impressively initial and reversible specific capacity of 383 and 311 mAh g^-1 at 100 mA g^-1 , respectively, which are obviously outperformed corresponding powder cathode in a traditional method by using polymer binder, is reported. Intriguingly, benefiting from the hierarchical nanoporous array architecture and self-assembly feature, Cu₂ S HP/CF cathode shows a remarkable cycling stability with a high capacity of 129 mAh g^-1 at 300 mA g^-1 over 500 cycles. This work not only highlights a guide for designing hierarchical nanoporous materials derived from metal-organic frameworks, but also provides a novel strategy of in situ formation to fabricate binder-free cathodes.

Atomic-scale unveiling of multiphase evolution during hydrated Zn-ion insertion in vanadium oxide

An initial crystalline phase can transform into another phases as cations are electrochemically inserted into its lattice. Precise identification of phase evolution at an atomic level during transformation is thus the very first step to comprehensively understand the cation insertion behavior and subsequently achieve much higher storage capacity in rechargeable cells, although it is sometimes challenging. By intensively using atomic-column-resolved scanning transmission electron microscopy, we directly visualize the simultaneous intercalation of both H₂O and Zn during discharge of Zn ions into a V₂O₅ cathode with an aqueous electrolyte. In particular, when further Zn insertion proceeds, multiple intermediate phases, which are not identified by a macroscopic powder diffraction method, are clearly imaged at an atomic scale, showing structurally topotactic correlation between the phases. The findings in this work suggest that smooth multiphase evolution with a low transition barrier is significantly related to the high capacity of oxide cathodes for aqueous rechargeable cells, where the crystal structure of cathode materials after discharge differs from the initial crystalline state in general.

The detailed understanding of the structural variations during cycling in cathodes for Zn-ion aqueous rechargeable batteries is still limited. Here, the authors utilize atomic-column-resolved scanning transmission electron microscopy to elucidate multiphase evolution during hydrated Zn-Ion insertion in vanadium oxide.

Operando X-ray Diffraction Studies of the Mg-Ion Migration Mechanisms in Spinel Cathodes for Rechargeable Mg-Ion Batteries

A promising high-voltage spinel oxide cathode material MgCrMnO₄ with 18% Mg/Mn inversion was synthesized successfully. A new custom operando battery device was designed to study the cation migration mechanisms of the MgCrMnO₄ cathode using 0.1 M Mg(TPFA)₂ electrolyte dissolved in triglyme and activated carbon as the anode. For the first time in multivalent batteries, high-quality operando diffraction data enabled the accurate quantification of cation contents in the host structure. Besides the exceptional reversibility of 12% Mg²⁺ insertion in Mg_1-xCrMnO₄ (x ≤ 1), a partially reversible insertion of excess Mg²⁺ during overdischarging was also observed. Moreover, the insertion/extraction reaction was experimentally shown to be accompanied by a series of cation redistributions in the spinel framework, which were further supported by density functional theory calculations. The inverted Mn is believed to be directly involved in the cation migrations, which would cause voltage hysteresis and irreversible structural evolution after overdischarging. Tuning the Mg/Mn inversion rate could provide a direct path to further optimize spinel oxide cathodes for Mg-ion batteries, and more generally, the operando techniques developed in this work should play a key role in understanding the complex mechanisms involved in multivalent ion insertion systems.

Recent Progress on Layered Cathode Materials for Nonaqueous Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) are promising candidates for next-generation energy storage systems owing to their high safety and the low cost of magnesium resources. One of the main challenges for RMBs is to develop suitable high-performance cathode materials. Layered materials are one of the most promising cathode materials for RMBs due to their relatively high specific capacity and facile synthesis process. This review focuses on recent progress on layered cathode materials for RMBs, including layered oxides, sulfides, selenides, and other layered materials. In addition, effective strategies to improve the electrochemical performance of layered cathode materials are summarized. Moreover, future perspectives about the application of layered materials in RMBs are also discussed. This review provides some significant guidance for the further development of layered materials for RMBs.

In Operando Synchrotron Studies of NH4+ Preintercalated V2O5·nH2O Nanobelts as the Cathode Material for Aqueous Rechargeable Zinc Batteries

NH₄⁺ preintercalated V₂O₅·nH₂O nanobelts with a large interlayer distance of 10.9 Å were prepared by the hydrothermal method. The material showed a large specific capacity of 391 mA·h·g^-1 at the 500 mA·g^-1 current density in aqueous rechargeable zinc batteries. In operando synchrotron X-ray diffraction demonstrated that the material experienced reversible solid-solution reaction and two-phase transition during charge-discharge cycling, accompanied by the reversible formation/decomposition of a ZnSO₄Zn₃(OH)₆·5H₂O byproduct. In operando X-ray absorption spectroscopy confirmed the reversible reduction/oxidation of V, together with small changes in the VO₆ local structure. The formation of byproduct was attributed to the dehydration of [Zn(H₂O)₆]²⁺, which concurrently improved the desolvation of [Zn(H₂O)₆]²⁺ into Zn²⁺. Bond valence sum map analysis and electrochemical impedance spectroscopy demonstrated that the byproduct improved the charge transfer kinetics of the electrode. Cyclic voltammetry and galvanostatic intermittent titration technique showed that the electrode reaction was dominated by ionic intercalation where the discharge capacity in the voltage window of 1.4-0.85 V was attributed to the intercalation of [Zn(H₂O)₆]²⁺, followed by the intercalation of Zn²⁺ at 0.85-0.4 V.

An Atomic View of Cation Diffusion Pathways from Single-Crystal Topochemical Transformations

The diffusion pathways of Li-ions as they traverse cathode structures in the course of insertion reactions underpin many questions fundamental to the functionality of Li-ion batteries. Much current knowledge derives from computational models or the imaging of lithiation behavior at larger length scales; however, it remains difficult to experimentally image Li-ion diffusion at the atomistic level. Here, by using topochemical Li-ion insertion and extraction to induce single-crystal-to-single-crystal transformations in a tunnel-structured V₂ O₅ polymorph, coupled with operando powder X-ray diffraction, we leverage single-crystal X-ray diffraction to identify the sequence of lattice interstitial sites preferred by Li-ions to high depths of discharge, and use electron density maps to create a snapshot of ion diffusion in a metastable phase. Our methods enable the atomistic imaging of Li-ions in this cathode material in kinetic states and provide an experimentally validated angstrom-level 3D picture of atomic pathways thus far only conjectured through DFT calculations.

Structural evolution from layered Na2Ti3O7 to Na2Ti6O13 nanowires enabling a highly reversible anode for Mg-ion batteries

The development of suitable host materials for the reversible storage of divalent ions such as Mg²⁺ is still a big challenge and its progress to date has been slow compared to that of monovalent Li⁺ or Na⁺. Herein, we present the study of layered sodium trititanate (Na₂Ti₃O₇) and sodium hexatitanate (Na₂Ti₆O₁₃) nanowires as anode materials for rechargeable Mg-ion batteries. It is found for the first time that the structural evolution from layered Na₂Ti₃O₇ to Na₂Ti₆O₁₃ with a more condensate three-dimensional microporous structure enables remarkably enhanced Mg-ion storage performance. The Na₂Ti₆O₁₃ electrode can achieve a large initial discharge and charge capacity of 165.8 and 147.7 mA h g^-1 at 10 mA g^-1 with a record high initial coulombic efficiency up to 89.1%. Ex situ XRD, Raman measurements and EDX mapping were used to investigate the electrochemical reaction mechanism. It is suggested that the irreversible structure change and the formation of insoluble NaCl with high yield and large particles when Na⁺ is replaced by inserted Mg²⁺ for the Na₂Ti₃O₇ electrode could be ascribed to the rapid decline in capacity. By contrast, the Na₂Ti₆O₁₃ electrode exhibits good structure stability during the Mg-ion insertion/extraction process, leading to good rate performance and cycling stability.

Anionic Se-Substitution toward High-Performance CuS1- x Sex Nanosheet Cathode for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (rMBs) are promising as the most ideal further energy storage systems but lack competent cathode materials due to sluggish redox reaction kinetics. Herein, developed is an anionic Se-substitution strategy to improve the rate capability and the cycling stability of 2D CuS_{1- x} Se_x nanosheet cathodes through an efficient microwave-induced heating method. The optimized CuS_{1- x} Se_x (X = 0.2) nanosheet cathode can exhibit high reversible capacity of 268.5 mAh g^-1 at 20 mA g^-1 and good cycling stability (140.4 mAh g^-1 at 300 mA g^-1 upon 100 cycles). Moreover, the CuS_{1- x} Se_x (X = 0.2) nanosheet cathode can deliver remarkable rate capability with a reversible capacity of 119.2 mAh g^-1 at 500 mA g^-1 , much higher than the 21.7 mAh g^-1 of pristine CuS nanosheets. The superior electrochemical performance can be ascribed to the enhanced reaction kinetics, enriched cation storage active sites, and shortened ion diffusion pathway of the CuS_{1- x} Se_x nanosheet. Therefore, tuning anionic chemical composition demonstrates an effective strategy to develop novel cathode materials for rMBs.

3D Network V2O5 Electrodes in a Gel Electrolyte for High-Voltage Wearable Symmetric Pseudocapacitors

A 3D network composed of V₂O₅ nanofibers was manufactured on a novel conductive printing paper [urea-LiClO₄-PVA (ULP) deep eutectic solvent gel-doped graphite/printing paper, U-paper] for use as electrodes linked with a ULP neutral gel electrolyte for 3D network V₂O₅ wearable symmetric pseudocapacitors (WSSCs). The function of the ULP gel is not only that it can be doped into the conductive ink to decrease the resistance of the conductive printing paper but also that it increases the stability of V₂O₅-based electrodes. Moreover, 3D network V₂O₅ WSSCs containing the ULP gel can support high operating voltages of 4.0 V with great specific capacitance (160 F/g) and offer a high energy density (355 W h/kg at 0.2 kW/kg). The 3D network V₂O₅ WSSCs exhibit a superior cycling stability/durability after 5000 cycles (capacitance retention of ∼91%). Operando X-ray absorption spectroscopy experiments show the reversibility and pseudocapacitive properties of V₂O₅ from the ULP gel and offer the information of the oxidation states of vanadium during charge-discharge cycles. The 3D network V₂O₅ WSSCs with the ULP gel electrolyte show great potential prospective candidates for smarter 3D wearable energy-storage devices and Internet-of-Things applications.