Direct Observation of Reversible Magnesium Ion Intercalation into a Spinel Oxide Host

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.

Design Strategies of Spinel Oxide Frameworks Enabling Reversible Mg-Ion Intercalation

ConspectusReversible Mg2+ intercalation in metal oxide frameworks is a key enabler for an operational Mg-ion battery with high energy density needed for the next generation of energy storage technologies. While functional Mg-ion batteries have been achieved in structures with soft anions (e.g., S2- and Se2-), they do not meet energy density requirements to compete with the current rechargeable lithium-ion batteries due to their low insertion potentials. It emphasizes the necessity of finding an oxide-based cathode that operates at high potentials. A leading hypothesis to explain the limited availability of oxide Mg-ion cathodes is the belief that Mg2+ has sluggish diffusion kinetics in oxides due to strong electrostatic interactions between the Mg2+ ions and oxide anions in the lattice. From this assessment, it can be hypothesized that such rate limiting kinetic shortcomings can be mitigated by tailoring an oxide framework through creating less stable Mg2+-O2- coordination.Based on theoretical calculations and preliminary experimental data, oxide spinels have been identified as promising cathode candidates with storage capacity, insertion potential, and cation mobility that meet the requirements for a secondary Mg-ion battery. However, spinels with a single redox metal, such as MgCr2O4 or MgMn2O4, were not found to demonstrate sufficiently reversible Mg-ion intercalation at high redox potentials when coupled with nonaqueous Mg-electrolytes. Therefore, a materials development effort was initiated to design, synthesize, and evaluate a new class of solid-solution oxide spinels that can satisfy the required properties needed to create a sustainable Mg-ion cathode. These were designed by bringing together electrochemically active metals with stable redox potentials and charged states against the electrolyte, for instance, Mn3+, in combination with a structural stabilization component, typically Cr3+. Furthermore, common spinel structural defects that degrade performance, i.e., antisite inversion, were controlled to see correlation between structures and electrochemical overpotentials, thus controlling overall hysteresis. The designed materials were characterized by both short- and long-range structure in both ex situ and in situ modes to confirm the nature of solid-solution and to correlate structural changes and redox activity to electrochemical performance. Consistent and reproducible results were observed for facile bulk Mg2+-ion activity without phase transformations, leading to enhanced energy storage capability based on reversible intercalation of Mg2+, enabled by understanding the variables that control the electrochemical performance of the spinel oxide. Based on these observations, with proper materials design, it is possible to develop an oxide cathode material that has many of the desired properties of a Li-ion intercalation cathode, representing a significant mile marker in the quest for high energy density Mg-ion batteries.This Account describes strategies for the design and development of new spinel oxide intercalation materials for high-energy Mg-ion battery cathodes through a combination of theoretical and experimental approaches. We will review the key factors that govern the kinetics of Mg2+ diffusion in spinel oxides and illustrate how material complexity correlates with the electrochemical Mg2+ activity in oxides and is supported by secondary characterization. The understanding gained from the fundamental studies of cation diffusion in oxide cathodes will be beneficial for chemists and materials scientists who are developing rechargeable batteries.

Unraveling the Phase Transition Behavior of MgMn2O4 Electrodes for Their Use in Rechargeable Magnesium Batteries

Rechargeable magnesium batteries are an attractive alternative to lithium batteries because of their higher safety and lower cost, being spinel-type materials promising candidates for their positive electrode. Herein, MgMn2O4 with a tetragonal structure is synthesized via a simple, low-cost Pechini methodology and tested in aqueous media. Electrochemical measurements combined with in-situ Raman spectroscopy and other ex-situ physicochemical characterization techniques show that, in aqueous media, the charge/discharge process occurs through the co-intercalation of Mg2+ and water molecules. A progressive structure evolution from a well-defined spinel to a birnessite-type arrangement occurs during the first cycles, provoking capacity activation. The concomitant towering morphological change induces poor cycling performance, probably due to partial delamination and loss of electrical contact between the active film and the substrate. Interestingly, both MgMn2O4 capacity retention and cyclability can be increased by doping with nickel. This work provides insights into the positive electrode processes in aqueous media, which is vital for understanding the charge storage mechanism and the correlated performance of spinel-type host materials.

Regulating the kinetics of zinc-ion migration in spinel ZnMn2O4 through iron doping boosted aqueous zinc-ion storage performance

Spinel ZnMn₂O₄ with a three-dimensional channel structure is one of the important cathode materials for aqueous zinc ions batteries (AZIBs). However, like other manganese-based materials, spinel ZnMn₂O₄ also has problems such as poor conductivity, slow reaction kinetics and structural instability under long cycles. Herein, ZnMn₂O₄ mesoporous hollow microspheres with metal ion doping were prepared by a simple spray pyrolysis method and applied to the cathode of aqueous zinc ion battery. Cation doping not only introduces defects, changes the electronic structure of the material, improves its conductivity, structural stability, and reaction kinetics, but also weakens the dissolution of Mn²⁺. The optimized 0.1 % Fe-doped ZnMn₂O₄ (0.1% Fe-ZnMn₂O₄) has a capacity of 186.8 mAh g^-1 after 250 charge-discharge cycles at 0.5 A g^-1 and the discharge specific capacity reaches 121.5 mAh g^-1 after 1200 long cycles at 1.0 A g^-1. The theoretical calculation results show that doping causes the change of electronic state structure, accelerates the electron transfer rate, and improves the electrochemical performance and stability of the material.

Selectivity of Electrochemical Ion Insertion into Manganese Dioxide Polymorphs

The ion insertion redox chemistry of manganese dioxide has diverse applications in energy storage, catalysis, and chemical separations. Unique properties derive from the assembly of Mn-O octahedra into polymorphic structures that can host protons and nonprotonic cations in interstitial sites. Despite many reports on individual ion-polymorph couples, much less is known about the selectivity of electrochemical ion insertion in MnO₂. In this work, we use density functional theory to holistically compare the electrochemistry of A_xMnO₂ (where A = H⁺, Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, Al³⁺) in aqueous and nonaqueous electrolytes. We develop an efficient computational scheme demonstrating that Hubbard-U correction has a greater impact on calculating accurate redox energetics than choice of exchange-correlation functional. Using PBE+U, we find that for nonprotonic cations, ion selectivity depends on the oxygen coordination environments inside a polymorph. When H⁺ is present, however, the driving force to form hydroxyl bonds is usually stronger. In aqueous electrolytes, only three ion-polymorph pairs are thermodynamically stable within water's voltage stability window (Na⁺ and K⁺ in α-MnO₂, and Li⁺ in λ-MnO₂), with all other ion insertion being metastable. We find Al³⁺ may insert into the δ, R, and λ polymorphs across the full 2-electron redox of MnO₂ at high voltage; however, electrolytes for multivalent ions must be designed to impede the formation of insoluble precipitates and facilitate cation desolvation. We also show that small ions coinsert with water in α-MnO₂ to achieve greater coordination by oxygen, while solvation energies and kinetic effects dictate water coinsertion in δ-MnO₂. Taken together, these findings explain reports of mixed ion insertion mechanisms in aqueous electrolytes and highlight promising design strategies for safe, high energy density electrochemical energy storage, desalination batteries, and electrocatalysts.

Formation and impact of nanoscopic oriented phase domains in electrochemical crystalline electrodes

Electrochemical phase transformation in ion-insertion crystalline electrodes is accompanied by compositional and structural changes, including the microstructural development of oriented phase domains. Previous studies have identified prevailingly transformation heterogeneities associated with diffusion- or reaction-limited mechanisms. In comparison, transformation-induced domains and their microstructure resulting from the loss of symmetry elements remain unexplored, despite their general importance in alloys and ceramics. Here, we map the formation of oriented phase domains and the development of strain gradient quantitatively during the electrochemical ion-insertion process. A collocated four-dimensional scanning transmission electron microscopy and electron energy loss spectroscopy approach, coupled with data mining, enables the study. Results show that in our model system of cubic spinel MnO₂ nanoparticles their phase transformation upon Mg²⁺ insertion leads to the formation of domains of similar chemical identity but different orientations at nanometre length scale, following the nucleation, growth and coalescence process. Electrolytes have a substantial impact on the transformation microstructure ('island' versus 'archipelago'). Further, large strain gradients build up from the development of phase domains across their boundaries with high impact on the chemical diffusion coefficient by a factor of ten or more. Our findings thus provide critical insights into the microstructure formation mechanism and its impact on the ion-insertion process, suggesting new rules of transformation structure control for energy storage materials.

Expanding the Material Search Space for Multivalent Cathodes

Multivalent batteries are an energy storage technology with the potential to surpass lithium-ion batteries; however, their performance have been limited by the low voltages and poor solid-state ionic mobility of available cathodes. A computational screening approach to identify high-performance multivalent intercalation cathodes among materials that do not contain the working ion of interest has been developed, which greatly expands the search space that can be considered for material discovery. This approach has been applied to magnesium cathodes as a proof of concept, and four resulting candidate materials [NASICON V₂(PO₄)₃, birnessite NaMn₄O₈, tavorite MnPO₄F, and spinel MnO₂] are discussed in further detail. In examining the ion migration environment and associated Mg²⁺ migration energy in these materials, local energy maxima are found to correspond with pathway positions where Mg²⁺ passes through a plane of anion atoms. While previous studies have established the influence of local coordination on multivalent ion mobility, these results suggest that considering both the type of the local bonding environment and available free volume for the mobile ion along its migration pathway can be significant for improving solid-state mobility.

Material Design and Energy Storage Mechanism of Mn-Based Cathodes for Aqueous Zinc-Ion Batteries

Mn-based cathodes have been widely explored for aqueous zinc-ion batteries (ZIBs), by virtue of their high theoretical capacity and low cost. However, Mn-based cathodes suffer from poor rate capability and cycling performance. Researchers have presented various approaches to address these issues. Therefore, these endeavors scattered in various directions (e. g., designing electrode structures, defect engineering and optimizing electrolytes) are necessary to be connected through a systematic review. Hence, we comprehensively overview Mn-based cathode materials for ZIBs from the aspects of phase compositions, electrochemical behaviors and energy storage mechanisms, and try to build internal relations between these factors. Modification strategies of Mn-based cathodes are then introduced. Furthermore, this review also provides some new perspectives on future efforts toward high-energy and long-life Mn-based cathodes for ZIBs.

Size-Controllable Nickel Sulfide Nanoparticles Embedded in Carbon Nanofibers as High-Rate Conversion Cathodes for Hybrid Mg-Based Battery

The integration of highly-safe Mg anode and fast Li+ kinetics endows hybrid Mg2+ /Li+ batteries (MLIBs) a promising future, but the practical application is circumvented by the lack of appropriate cathodes that enable the realization of an enough participation of Mg2+ in the reactions, resulting in a high dependence on Li+ . Herein, the authors develop a series of size-controllable nickel sulfide nanoparticles embedded in carbon nanofibers (NiS@C) with synergistic effect of particle diameter and carbon content as the cathode material for MLIBs. The optimized particle size is designed to maximize the utilization of the active material and remit internal stress, and appropriate carbon encapsulation efficiently inhibiting the pulverization of particles and accelerates the ability of conducting ions and electrons. In consequence, the representative NiS@C delivers superior electrochemical performance with a highest discharge capacity of 435 mAh g-1 at 50 mA g-1 . Such conversion cathode also exhibits excellent rate performance and remarkable cycle life. Significantly, the conversion mechanism of NiS in MLIBs is unambiguously demonstrated for the first time, affirming the corporate involvement of both Mg2+ and Li+ at the cathodic side. This work underlines a guide for developing conversion-type materials with high rate capability and cyclic performance for energy storage applications.

Ether-Water Hybrid Electrolyte Contributing to Excellent Mg Ion Storage in Layered Sodium Vanadate

Magnesium ion batteries have potential for large-scale energy storage. However, the high charge density of Mg²⁺ ions establishes a strong intercalation energy barrier in host materials, causing sluggish diffusion kinetics and structural degradation. Here, we report that the kinetic and dissolution issues connected to cathode materials can be resolved simultaneously using a tetraethylene glycol dimethyl ether (TEGDME)-water hybrid electrolyte. The lubricating and shielding effect of water solvent could boost the swift transport of Mg²⁺, contributing to a high diffusion coefficient within the sodium vanadate (NaV₈O₂₀·nH₂O) cathode. Meanwhile, the organic TEGDME component can coordinate with water to diminish its activity, thus providing the hybrid electrolyte with a broad electrochemical window of 3.9 V. More importantly, the TEGDME preferentially amassed at the interface, leading to a robust cathode electrolyte interface layer that suppresses the dissolution of vanadium species. Consequently, the NaV₈O₂₀·nH₂O cathode achieved a specific capacity of 351 mAh g^-1 at 0.3 A g^-1 and a long cycle life of 1000 cycles in this hybrid electrolyte. A mechanism study revealed the reversible interaction of Mg²⁺ during cycles. This organic water hybrid electrolyte is effective for overcoming the difficulty of multivalent ion storage.

Nanoscale interface engineering of inorganic Solid-State electrolytes for High-Performance alkali metal batteries

All-solid-state metal batteries (ASSMBs) have been regarded as the ideal candidate for the next-generation high-energy storage system due to their ultrahigh specific capacity and the lowest redox potential. However, the uncontrollable chemical reactivity during cycling which directly determines the growth behaviour of metal dendrites, the low coulombic efficiency and the safety concerns severely limit their real-world applications.. Crystallographic optimization based on solid-state electrolytes (SSEs) provides an atomic-scale and fundamental solution for the inhibition of dendrite growth in metal anodes, which has attracted widespread attentions. From this perspective, we summarize the recent advance of the crystallographic optimization for various classes of solid-state electrolytes. We highlight the recent experimental findings of crystallographic optimization for a new generation of all-solid-state batteries, including lithium-ion batteries, sodium-ion batteries, magnesium-ion batteries, with the aim of providing a deeper understanding of the crystallographic reactions in ASSMBs. The challenges and prospects for the future design and engineering of crystallographic optimization of SSEs are discussed, providing ideas for further research into crystallographic optimization to improve the performance of rechargeable batteries.

Enhancing Surface Chemical Stability of LiMn2 O4 Cathode by Strontium Enrichment at Grain Boundaries

LiMn₂ O₄ (LMO) cathodes suffer from limited cycle life, resulting from Mn dissolution and side reactions between electrode and electrolyte. In this study, Sr-modified LMO is prepared by using a simple strategy. The nature and position of large-radius Sr ions are investigated, alongside their influence on the structural stability of the bulk. SrMnO₃ (SMO) is found to be enriched at grain boundaries of LMO, with Mn-O-Sr bonds forming at the SMO/LMO interface. Furthermore, stable SMO alleviates the migration of Mn ions in LMO associated with structural integrity and suppresses side reactions between the electrode and electrolyte. The modified LMO cathodes maintain their structural integrity and display improved rate performance and cycling stability under harsh conditions. Remarkably, the discharge capacity of a Sr-modified LMO||Li half-cell maintains 94.8 % at 25 °C and 79.6 % at 55 °C after 500 cycles. Consequently, enrichment of strontium at grain boundaries presents a promising strategy for developing cathodes for long-term use.

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.

Phenylphosphonate surface functionalisation of MgMn2O4 with 3D open-channel nanostructures for composite slurry-coated cathodes of rechargeable magnesium batteries operated at room temperature

Spinel-type MgMn₂O₄, prepared by a propylene-oxide-driven sol–gel method, has a high surface area and structured bimodal macro- and mesopores, and exhibits good electrochemical properties as a cathode active material for rechargeable magnesium batteries. However, because of its hydrophilicity and significant water adsorption properties, macroscopic aggregates are formed in composite slurry-coated cathodes when 1-methyl-2-pyrrolidone (NMP) is used as a non-aqueous solvent. Functionalising the surface with phenylphosphonate groups was found to be an easy and effective technique to render the structured MgMn₂O₄ hydrophobic and suppress aggregate formation in NMP-based slurries. This surface functionalisation also reduced side reactions during charging, while maintaining the discharge capacity, and significantly improved the coulombic efficiency. Uniform slurry-coated cathodes with active material fractions as high as 93 wt% can be produced on Al foils by this technique employing carbon nanotubes as an electrically conductive support. A coin-type full cell consisting of this slurry-coated cathode and a magnesium alloy anode delivered an initial discharge capacity of ∼100 mA h g⁻¹ at 25 °C.

Phenylphosphonate functionalisation is an easy, highly effective strategy to fabricate slurry-coated nanostructured MgMn₂O₄ cathodes for rechargeable magnesium batteries at active material fractions up to 93 wt% for rechargeable magnesium batteries cycled at 25 °C.

Reshaping two-dimensional MoS2 for superior magnesium-ion battery anodes

Few-atom-thick two-dimensional (2D) molybdenum disulfide (MoS₂) monolayers possess numerous crucial applications in energy storage. Usually, the strategy of activating interfacial electron transfer was employed to promote their performance. Herein, we reshape the structure of materials to excite their subinterfacial and interfacial electron transfer for superior metal-ion batteries. As an example, we rationally design and reconfigure the structure of 2D MoS₂ and propose a new stable structure, B-MoS₂, which has an S-Mo-S sandwich structure with a buckled square lattice. The B-MoS₂ monolayer is a promising anode material for magnesium-ion batteries (MgIBs) with a high capacity (921.3 mA h g^-1) and a low averaged open circuit voltage (0.154 V). Multiscale underlying mechanisms for the storage of Mg and Li ions in MoS₂ are provided. Based on the electronic level, the high capacity is ascribed to the occurrence of interfacial and subinterfacial electron transfer between metal ions and B-MoS₂. Based on the atomic level, the insertion-adsorption mechanism or adsorption-insertion mechanism is determined for different ion storage at B-MoS₂. The intrinsic metallic property of B-MoS₂ and the enhanced electronic conductivity of Mg/B-MoS₂ systems as well as low migration barriers (∼0.604 eV) of Mg ions at MoS₂ suggest that the B-MoS₂ anode has fast charge/discharge rates. This work offers novel concepts (i.e. subinterfacial electron transfer and its activation) for superior energy storage materials, and proposes new multiscale underlying mechanisms for ion storage in the MoS₂ family.

Structure Design of Long-Life Spinel-Oxide Cathode Materials for Magnesium Rechargeable Batteries

Development of metal-anode rechargeable batteries is a challenging issue. Especially, magnesium rechargeable batteries are promising in that Mg metal can be free from dendrite formation upon charging. However, in case of oxide cathode materials, inserted magnesium tends to form MgO-like rocksalt clusters in a parent phase even with another structure, which causes poor cyclability. Here, a design concept of high-performance cathode materials is shown, based on: i) selecting an element to destabilize the rocksalt-type structure and ii) utilizing the defect-spinel-type structure both to avoid the spinel-to-rocksalt reaction and to secure the migration path of Mg cations. This theoretical and experimental work substantiates that a defect-spinel-type ZnMnO₃ meets the above criteria and shows excellent cycle performance exceeding 100 cycles upon Mg insertion/extraction with high potential (≈2.5 V vs Mg²⁺ /Mg) and capacity (≈100 mAh g^-1 ). Thus, this work would provide a design guideline of cathode materials for various multivalent rechargeable batteries.

Testing the reversible insertion of magnesium in a cation-deficient manganese oxy-spinel through a concentration cell

A new electrochemical procedure to explore the intercalation of magnesium using a concentration cell with two cubic spinels Mg_1−xMn₂O₄.

Recent Progress and Challenges in the Optimization of Electrode Materials for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) have been regarded as one of the promising electrochemical energy storage systems to complement Li-ion batteries owing to the low-cost and high safety characteristics. However, the various challenges including the sluggish solid-state diffusion of highly polarizing Mg²⁺ ions in hosts, and the formation of blocking layers on Mg metal surface have seriously impeded the development of high-performance RMBs. In order to solve these problems toward practical applications of RMBs, a tremendous amount of work on electrodes and electrolytes has been conducted in the last few decades. Creative optimization strategies including the modification of cathodes and anodes such as shielding the charges of divalent Mg²⁺ , expanding the layers of host materials, and optimizing the interface of electrode-electrolyte are raised to promote the technology. In this review, the detailed description of innovative approaches, representative examples, and facing challenges for developing high-performance electrodes are presented. Based on the review of these strategies, guidelines are provided for future research directions on improving the overall battery performance, especially on the electrodes.

Promises and Challenges of Next-Generation "Beyond Li-ion" Batteries for Electric Vehicles and Grid Decarbonization

The tremendous improvement in performance and cost of lithium-ion batteries (LIBs) have made them the technology of choice for electrical energy storage. While established battery chemistries and cell architectures for Li-ion batteries achieve good power and energy density, LIBs are unlikely to meet all the performance, cost, and scaling targets required for energy storage, in particular, in large-scale applications such as electrified transportation and grids. The demand to further reduce cost and/or increase energy density, as well as the growing concern related to natural resource needs for Li-ion have accelerated the investigation of so-called "beyond Li-ion" technologies. In this review, we will discuss the recent achievements, challenges, and opportunities of four important "beyond Li-ion" technologies: Na-ion batteries, K-ion batteries, all-solid-state batteries, and multivalent batteries. The fundamental science behind the challenges, and potential solutions toward the goals of a low-cost and/or high-energy-density future, are discussed in detail for each technology. While it is unlikely that any given new technology will fully replace Li-ion in the near future, "beyond Li-ion" technologies should be thought of as opportunities for energy storage to grow into mid/large-scale applications.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Studies of FeSe2 Cathode Materials for Mg–Li Hybrid Batteries

Rechargeable magnesium (Mg)-based energy storage has attracted extensive attention in electrochemical storage systems with high theoretical energy densities. The Mg metal is earth-abundant and dendrite-free for the anode. However, there is a strong Coulombic interaction between Mg2+ and host materials that often inhibits solid-state diffusion, resulting in a large polarization and poor electrochemical performances. Herein, we develop a Mg–Li hybrid battery using a Mg-metal anode, an FeSe2 powder with uniform size and a morphology utilizing a simple solution-phase method as the counter electrode and all-phenyl-complex/tetrahydrofuran (APC)-LiCl dual-ion electrolyte. In the Li+-containing electrolyte, at a current density of 15 mA g−1, the Mg–Li hybrid battery (MLIB) delivered a satisfying initial discharge capacity of 525 mAh g−1. Moreover, the capacity was absent in the FeSe2|APC|Mg cell. The working mechanism proposed is the “Li+-only intercalation” at the FeSe2 and the “Mg2+ dissolved or deposited” at the Mg foil in the FeSe2|Mg2+/Li+|Mg cell. Furthermore, ex situ XRD was used to investigate the structural evolution in different charging and discharging states.

Anion Association Strength as a Unifying Descriptor for the Reversibility of Divalent Metal Deposition in Nonaqueous Electrolytes

Developing next-generation battery chemistries that move beyond traditional Li-ion systems is critical to enabling transformative advances in electrified transportation and grid-level energy storage. In this work, we provide the first evidence for common descriptors for improved reversibility of metal plating/stripping in nonaqueous electrolytes for multivalent ion batteries. Focusing first on the specific role of chloride (Cl^-) in promoting electrochemical reversibility in multivalent systems, rotating disk (RDE) and ring-disk electrode (RRDE) investigations were performed utilizing a variety of divalent cations (Mg²⁺, Zn²⁺, and Cu²⁺) and the bis-(trifluoromethane sulfonyl) imide (TFSI^-) anion. By introducing varying concentrations of Cl^-, a cooperative effect is observed between TFSI^- and Cl^- that yields the more reversible behavior of mixed electrolytes relative to electrolytes containing only TFSI^-. This effect is shown to be general for Mg, Zn, and Cu electrodeposition, and mechanistic understanding of the role of Cl^- in improving reversibility of TFSI-based electrolytes is obtained through the combination of R(R)DE experimental results and density functional theory (DFT) evaluation of the redox activity and thermodynamic stability of various TFSI- and Cl-based solution complexes of metal ions. The cooperative anion effect is further generalized to other mixed-anion systems, where systematic variations in anion association strength predicted from DFT (i.e., Cl^- > OTf^- ≈ TFSI^- > BF₄^- > PF₆^-) yield corresponding trends in redox potentials and improvements of ≥200 mV in the reversibility of metal deposition/dissolution. These results identify anion association strength as a common descriptor for the reversibility of divalent metal anodes and suggest a set of general design principles for developing new electrolytes with improved activity and stability.

Understanding the Behaviors of λ-MnO2 in Electrochemical Lithium Recovery: Key Limiting Factors and a Route to the Enhanced Performance

Recently developed electrochemical lithium recovery systems, whose operation principle mimics that of lithium-ion battery, enable selective recovery of lithium from source waters with a wide range of lithium ions (Li⁺) concentrations; however, physicochemical behaviors of the key component-Li⁺-selective electrode-in realistic operation conditions have been poorly understood. Herein, we report an investigation on a λ-MnO₂ electrode during the electrochemical lithium recovery process with regards to the Li⁺ concentration in source water and operation rate of the system. Three distinctive stages of λ-MnO₂ originating from different limiting factors for lithium recovery are defined with regard to the rate of Li⁺ supply from the electrolyte: depleted, transition, and saturated regions. By characterization of λ-MnO₂ at different stages using diverse X-ray techniques, the importance of Li⁺ concentration in the vicinity of the electrode surface is revealed. On the basis of this understanding, increasing the density of the electrode/electrolyte interface is suggested as a realistic and general route to enhance the overall lithium recovery performance and is experimentally corroborated at a wide range of operation environments.

Probing Mg Intercalation in the Tetragonal Tungsten Bronze Framework V4Nb18O55

While commercial Li-ion batteries offer the highest energy densities of current rechargeable battery technologies, their energy storage limit has almost been achieved. Therefore, there is considerable interest in Mg batteries, which could offer increased energy densities in comparison to Li-ion batteries if a high-voltage electrode material, such as a transition-metal oxide, can be developed. However, there are currently very few oxide materials which have demonstrated reversible and efficient Mg²⁺ insertion and extraction at high voltages; this is thought to be due to poor Mg²⁺ diffusion kinetics within the oxide structural framework. Herein, the authors provide conclusive evidence of electrochemical insertion of Mg²⁺ into the tetragonal tungsten bronze V₄Nb₁₈O₅₅, with a maximum reversible electrochemical capacity of 75 mA h g^-1, which corresponds to a magnesiated composition of Mg₄V₄Nb₁₈O₅₅. Experimental electrochemical magnesiation/demagnesiation revealed a large voltage hysteresis with charge/discharge (1.12 V vs Mg/Mg²⁺); when magnesiation is limited to a composition of Mg₂V₄Nb₁₈O₅₅, this hysteresis can be reduced to only 0.5 V. Hybrid-exchange density functional theory (DFT) calculations suggest that a limited number of Mg sites are accessible via low-energy diffusion pathways, but that larger kinetic barriers need to be overcome to access the entire structure. The reversible Mg²⁺ intercalation involved concurrent V and Nb redox activity and changes in crystal structure, as confirmed by an array of complementary methods, including powder X-ray diffraction, X-ray absorption spectroscopy, and energy-dispersive X-ray spectroscopy. Consequently, it can be concluded that the tetragonal tungsten bronzes show promise as intercalation electrode materials for Mg batteries.

Mechanism of Mg extraction from MgMn2O4 during acid digestion

The Mg extraction process of MgMn₂O₄ was studied by chemical digestion in HNO₃ at different concentrations and reaction times. Mg extraction from MgMn₂O₄ is a two-step two-phase reaction, via the intermediate Mg_0.5Mn₂O₄ to the fully oxidised Mn₂O₄.

High-Energy-Density Rechargeable Mg Battery Enabled by a Displacement Reaction

Because of its high theoretical volumetric capacity and dendrite-free stripping/plating of Mg, rechargeable magnesium batteries (RMBs) hold great promise for high energy density in consumer electronics. However, the lack of high-energy-density cathodes severely constrains their practical applications. Herein, for the first time, we report that a CuS cathode can fully reversibly work through a displacement reaction in CuS/Mg pouch cells at room temperature and provide a high capacity of ∼400 mA h/g in a MACC electrolyte, corresponding to the gravimetric and volumetric energy density of 608 W h/kg and1042 W h/L, respectively. Even after 80 cycles, CuS/Mg pouch cells can maintain a high capacity of 335 mA h/g. Detailed mechanistic studies reveal that CuS undergoes a displacement reaction route rather than a typical conversion mechanism. This work will provide a guide for more discovery of high-performance cathode candidates for RMBs.

Microstructure Characteristics of Cathode Materials for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) that use pure Mg or Mg alloy as anode and materials allowing Mg ions to insert/extract as cathode have many advantages such as high energy density, environmental friendliness, low cost, and safety of handling. RMBs are regarded as a promising candidate for portable power sources and heavy load energy devices. However, there are still some technological issues impeding their commercial application. The most important issue is the absence of applicable cathode materials because of the high charge density, strong polarization effect, and very slow insertion/extraction speed of Mg²⁺ ions. In recent years, the research reports on the cathode materials of RMBs have increased significantly. Here, an extensive number of research papers are reviewed in terms of the microstructure characteristics of cathode materials for RMBs. The status and issues of cathode materials are analyzed and discussed in detail. The future development directions and perspectives are prospected for providing an understanding of the related research activities on RMBs.

Effects of Particle Size on Mg2+ Ion Intercalation into λ-MnO2 Cathode Materials

An emergent theme in mono- and multivalent ion batteries is to utilize nanoparticles (NPs) as electrode materials based on the phenomenological observations that their short ion diffusion length and large electrode-electrolyte interface can lead to improved ion insertion kinetics compared to their bulk counterparts. However, the understanding of how the NP size fundamentally relates to their electrochemical behaviors (e.g., charge storage mechanism, phase transition associated with ion insertion) is still primitive. Here, we employ spinel λ-MnO₂ particles as a model cathode material, which have effective Mg²⁺ ion intercalation but with their size effect poorly understood to investigate their operating mechanism via a suite of electrochemical and structural characterizations. We prepare two differently sized samples, the small nanoscopic λ-MnO₂ particles (81 ± 25 nm) and big micron-sized ones (814 ± 207 nm) via postsynthesis size-selection. Analysis of the charge storage mechanisms shows that the stored charge from Mg²⁺ ion intercalation dominates in both systems and is ∼10 times higher in small particles than that in the big ones. From both X-ray diffraction and atomic-resolution scanning transmission electron microscopy imaging, we reveal a fundamental difference in phase transition of the differently sized particles during Mg²⁺ ion intercalation: the small NPs undergo a solid-solution-like phase transition which minimizes lattice mismatch and energy penalty for accommodating new phases, whereas the big particles follow conventional multiphase transformation. We show that this pathway difference is related to the improved electrochemical performance (e.g., rate capability, cycling performance) of small particles over the big ones which provides important insights in encoding within the particle dimension, that is, the single-phase transition pathway in high-performance electrode materials for multivalent ion batteries.

MgSc2 Se4 -A Magnesium Solid Ionic Conductor for All-Solid-State Mg Batteries?

Recently, the ternary spinel selenide MgSc₂ Se₄ was proposed to have high magnesium ion mobility and is therefore an interesting potential candidate as a solid electrolyte in magnesium secondary batteries. To test the properties of the material, a modified solid-state reaction was used to synthesize pure MgSc₂ Se₄ . Electrochemical characterizations identified detrimental high electronic conductivities, which limit its application as a Mg-conducting solid electrolyte. Two methods were attempted to lower electronic conductivities, including the implementation of Se-rich phases and aliovalent doping, however, with no sufficient improvement. Based on the mixed conducting properties of MgSc₂ Se₄ , a reversible insertion/extraction of Mg²⁺ into the spinel structure could be demonstrated.

Tailoring the electrochemical activity of magnesium chromium oxide towards Mg batteries through control of size and crystal structure

Chromium oxides with the spinel structure have been predicted to be promising high voltage cathode materials in magnesium batteries. Perennial challenges involving the mobility of Mg2+ and reaction kinetics can be circumvented by nano-sizing the materials in order to reduce diffusion distances, and by using elevated temperatures to overcome activation energy barriers. Herein, ordered 7 nm crystals of spinel-type MgCr2O4 were synthesized by a conventional batch hydrothermal method. In comparison, the relatively underexplored Continuous Hydrothermal Flow Synthesis (CHFS) method was used to make highly defective sub-5 nm MgCr2O4 crystals. When these materials were made into electrodes, they were shown to possess markedly different electrochemical behavior in a Mg2+ ionic liquid electrolyte, at moderate temperature (110 °C). The anodic activity of the ordered nanocrystals was attributed to surface reactions, most likely involving the electrolyte. In contrast, evidence was gathered regarding the reversible bulk deintercalation of Mg2+ from the nanocrystals made by CHFS. This work highlights the impact on electrochemical behavior of a precise control of size and crystal structure of MgCr2O4. It advances the understanding and design of new cathode materials for Mg-based batteries.

Modifications in coordination structure of Mg[TFSA]2-based supporting salts for high-voltage magnesium rechargeable batteries

Systematic structural and electrochemical studies on the Mg[TFSA]₂-based electrolytes revealed that the coordination state of [TFSA]⁻ predominates the electrochemical magnesium deposition/dissolution activity.

Electrochemical phase transformation accompanied with Mg extraction and insertion in a spinel MgMn2O4 cathode material

Electrochemical Mg extraction from spinel MgMn₂O₄ active material was firmly proven experimentally by various techniques such as STEM, XPS, XRD, etc.

Atomic Substitution Enabled Synthesis of Vacancy-Rich Two-Dimensional Black TiO2- x Nanoflakes for High-Performance Rechargeable Magnesium Batteries

Rechargeable magnesium (Mg) batteries assembled with dendrite-free, safe, and earth-abundant metal Mg anodes potentially have the advantages of high theoretical specific capacity and energy density. Nevertheless, owing to the large polarity of divalent Mg²⁺ ions, the insertion of Mg²⁺ into electrode materials suffers from sluggish kinetics, which seriously limit the performance of Mg batteries. Herein, we demonstrate an atomic substitution strategy for the controlled preparation of ultrathin black TiO_{2- x} (B-TiO_{2- x}) nanoflakes with rich oxygen vacancies (OVs) and porosity by utilizing ultrathin 2D TiS₂ nanoflakes as precursors. We find out that the presence of OVs in B-TiO_{2- x} electrode material can greatly improve the electrochemical performances of rechargeable Mg batteries. Both experimental results and density functional theory simulations confirm that the introduction of OVs can remarkably enhance the electrical conductivity and increase the number of active sites for Mg²⁺ ion storage. The vacancy-rich B-TiO_{2- x} nanoflakes exhibit high reversible capacity and good capacity retention after long-term cycling at large current densities. It is hoped that this work can provide valuable insights and inspirations on the defect engineering of electrode materials for rechargeable magnesium batteries.

Opportunities and Challenges in the Development of Cathode Materials for Rechargeable Mg Batteries

Recent years have seen an intense and renewed interest in the Mg battery research, naming Mg-S the ≫Holy Grail≪ battery, and expectations that Mg battery system will be able to compete and surpass Li-ion batteries in a matter of years. Considerable progress has been achieved in the field of Mg electrolytes, where several new electrolytes with improved electrochemical performance and favorable chemical properties (non-corrosive, non-nucleophilic) were synthesized. Development in the field of cathodes remains a bit more elusive, with inorganic, sulfur, and organic cathodes all showing their upsides and downsides. This review highlights the recent progress in the field of Mg battery cathodes, paying a special attention to the performance and comparison of the different types of the cathodes. It also aims to define advantages and key challenges in the development of each type of cathodes and finally specific questions that should be addressed in the future research.

A critical review of cathodes for rechargeable Mg batteries

Benefiting from a higher volumetric capacity (3833 mA h cm-3 for Mg vs. 2046 mA h cm-3 for Li) and dendrite-free Mg metal anode, reversible Mg batteries (RMBs) are a promising chemistry for applications beyond Li ion batteries. However, RMBs are still severely restricted by the absence of high performance cathodes for any practical application. In this review, we provide a critical and rigorous review of Mg battery cathode materials, mainly reported since 2013, focusing on the impact of structure and composition on magnesiation kinetics. We discuss cathode materials, including intercalation compounds, conversion materials (O2, S, organic compounds), water co-intercalation cathodes (V2O5, MnO2etc.), as well as hybrid systems using Mg metal anode. Among them, intercalation cathodes are further categorized by 3D (Chevrel phase, spinel structure etc.), 2D (layered structure), and 1D materials (polyanion: phosphate and silicate), according to the diffusion pathway of Mg2+ in the framework. Instead of discussing every published work in detail, this review selects the most representative works and highlights the merits and challenges of each class of cathodes. Advances in theoretical analysis are also reviewed and compared with experimental results. This critical review will provide comprehensive knowledge of Mg cathodes and guidelines for exploring new cathodes for rechargeable magnesium batteries.

Manganese-Oxide-Based Electrode Materials for Energy Storage Applications: How Close Are We to the Theoretical Capacitance?

Of the transition metals, Mn has the greatest number of different oxides, most of which have a special tunnel structure that enables bulk redox reactions. The high theoretical capacitance and capacity results from a greater number of accessible oxidation states than other transition metals, wide potential window, and the high natural abundance make MnO_x species promising electrode materials for energy storage applications. Although MnO_x electrode materials have been intensely studied over the past decade, their electrochemical performance is still insufficient for practical applications. Currently, there is a trade-off between specific capacitance and loading mass. MnO_x species have intrinsically poor electrical conductivity, and current structural designs are not sophisticated enough to accommodate enough redox-active sites. Recent studies have certainly made progress in increasing capacitance through making use of electrically conductive components and controlling the morphology of the MnO_x species to expose more surface area. To increase the capacitance of MnO_x electrodes to the largest extent without limiting loading mass, further structural design at the nanoscale and manipulation of the electrically conductive component are required. An ideal nanostructure is proposed to guide future research toward closing the gap between achieved and theoretical capacitance, without limiting the loading mass.

Water-Activated VOPO4 for Magnesium Ion Batteries

Rechargeable Mg batteries, using high capacity and dendrite-free Mg metal anodes, are promising energy storage devices for large scale smart grid due to low cost and high safety. However, the performance of Mg batteries is still plagued by the slow reaction kinetics of their cathode materials. Recent discoveries demonstrate that water in cathode can significantly enhance the Mg-ion diffusion in cathode by an unknown mechanism. Here, we propose the water-activated layered-structure VOPO₄ as a novel cathode material and examine the impact of water in electrode or organic electrolyte on the thermodynamics and kinetics of Mg-ion intercalation/deintercalation in cathodes. Electrochemical measurements verify that water in both VOPO₄ lattice and organic electrolyte can largely activate VOPO₄ cathode. Thermodynamic analysis demonstrates that the water in the electrolyte will equilibrate with the structural water in VOPO₄ lattice, and the water activity in the electrolyte alerts the mechanism and kinetics for electrochemical Mg-ion intercalation in VOPO₄. Theoretical calculations and experimental results demonstrate that water reduces both the solid-state diffusion barrier in the VOPO₄ electrode and the desolvation penalty at the interface. To achieve fast reaction kinetics, the water activity in the electrolyte should be larger than 10^-2. The proposed activation mechanism provides guidance for screening and designing novel chemistry for high performance multivalent-ion batteries.

Colloidal Bismuth Nanocrystals as a Model Anode Material for Rechargeable Mg-Ion Batteries: Atomistic and Mesoscale Insights

At present, the technical progress of secondary batteries employing metallic magnesium as the anode material has been severely hindered due to the low oxidation stability of state-of-the-art Mg electrolytes, which cannot be used to explore high-voltage (>3 V versus Mg²⁺/Mg) cathode materials. All known electrolytes based on oxidatively stable solvents and salts, such as Mg(ClO₄)₂ and Mg bis(trifluoromethanesulfonimide), react with the metallic magnesium anode, forming a passivating layer at its surface and preventing the reversible plating and stripping of Mg. Therefore, in a near-term effort to extend the upper voltage limit in the exploration of future candidate Mg-ion battery cathode materials, bismuth anodes have attracted considerable attention due to their efficient magnesiation and demagnesiation alloying reaction in such electrolytes. In this context, we present colloidal Bi nanocrystals (NCs) as a model anode material for the exploration of cathode materials for rechargeable Mg-ion batteries. Bi NCs demonstrate a stable capacity of 325 mAh g^-1 over at least 150 cycles at a current density of 770 mA g^-1, which is among the most-stable performance of Mg-ion battery anode materials. First-principles crystal structure prediction methodologies and ex situ X-ray diffraction measurements reveal that the magnesiation of Bi NCs leads to the simultaneous formation of the low-temperature trigonal structure, α-Mg₃Bi₂, and the high-temperature cubic structure, β-Mg₃Bi₂, which sheds insight into the high stability of this reversible alloying reaction. Furthermore, small-angle X-ray scattering measurements indicate that although the monodispersed, crystalline nature of the Bi NCs is indeed disturbed during the first discharge step, no notable morphological or structural changes occur in the following electrochemical cycles. The cost-effective and facile synthesis of colloidal Bi NCs and their remarkably high electrochemical stability upon magnesiation make them an excellent model anode material with which to accelerate progress in the field of Mg-ion secondary batteries.

Water-Lubricated Intercalation in V2 O5 ·nH2 O for High-Capacity and High-Rate Aqueous Rechargeable Zinc Batteries

Low-cost, environment-friendly aqueous Zn batteries have great potential for large-scale energy storage, but the intercalation of zinc ions in the cathode materials is challenging and complex. Herein, the critical role of structural H₂ O on Zn²⁺ intercalation into bilayer V₂ O₅ ·nH₂ O is demonstrated. The results suggest that the H₂ O-solvated Zn²⁺ possesses largely reduced effective charge and thus reduced electrostatic interactions with the V₂ O₅ framework, effectively promoting its diffusion. Benefited from the "lubricating" effect, the aqueous Zn battery shows a specific energy of ≈144 Wh kg^-1 at 0.3 A g^-1 . Meanwhile, it can maintain an energy density of 90 Wh kg^-1 at a high power density of 6.4 kW kg^-1 (based on the cathode and 200% Zn anode), making it a promising candidate for high-performance, low-cost, safe, and environment-friendly energy-storage devices.