Odyssey of Multivalent Cathode Materials: Open Questions and Future Challenges

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.

Additive-Driven Interfacial Engineering of Aluminum Metal Anode for Ultralong Cycling Life

Rechargeable Al batteries (RAB) are promising candidates for safe and environmentally sustainable battery systems with low-cost investments. However, the currently used aluminum chloride-based electrolytes present a significant challenge to commercialization due to their corrosive nature. Here, we report for the first time, a novel electrolyte combination for RAB based on aluminum trifluoromethanesulfonate (Al(OTf)3) with tetrabutylammonium chloride (TBAC) additive in diglyme. The presence of a mere 0.1 M of TBAC in the Al(OTf)3 electrolyte generates the charge carrying electrochemical species, which forms the basis of reaction at the electrodes. TBAC reduces the charge transfer resistance and the surface activation energy at the anode surface and also augments the dissociation of Al(OTf)3 to generate the solid electrolyte interphase components. Our electrolyte's superiority directly translates into reduced anodic overpotential for cells that ran for 1300 cycles in Al plating/stripping tests, the longest cycling life reported to date. This unique combination of salt and additive is non-corrosive, exhibits a high flash point and is cheaper than traditionally reported RAB electrolyte combinations, which makes it commercially promising. Through this report, we address a major roadblock in the commercialization of RAB and inspire equivalent electrolyte fabrication approaches for other metal anode batteries.

Examining Electrolyte Compatibility on Polymorphic MnO2 Cathodes for Room-Temperature Rechargeable Magnesium Batteries

Rechargeable magnesium batteries are promising candidates for post-lithium-ion batteries, owing to the large source abundance and high theoretical energy density. However, there remain few reports on constructing practical cells with oxide cathodes and Mg anodes at room temperature. In this work, we compare the reaction behavior of various MnO₂ polymorph cathodes in two representative electrolytes: Mg[TFSA]₂/G3 and Mg[Al(hfip)₄]₂/G3. In Mg[TFSA]₂/G3, discharge capacities of the MnO₂ cathodes are well consistent with the changes in Mg composition, where nanorod-like α-MnO₂ and λ-MnO₂ show the capacities of about 100 mA h g^-1 at room temperature. However, this electrolyte has the disadvantage that the Mg anodes are easily passivated. In contrast, Mg[Al(hfip)₄]₂/G3 allows highly reversible deposition/dissolution of Mg anodes, whereas the discharge process of the MnO₂ cathodes involves a large part of side reactions, in which the MnO₂ active material takes part in some reductive reaction together with electrolyte species instead of the expected Mg²⁺ intercalation. Such an unstable electrode/electrolyte interface would lead to continuous degradation on/near the cathode surface. Thus, the interfacial stability between the oxide cathodes and the electrolytes must be improved for practical applications.

Borate-Based Compounds as Mixed Polyanion Cathode Materials for Advanced Batteries

Rational design of new and cost-effective advanced batteries for the intended scale of application is concurrent with cathode materials development. Foundational knowledge of cathode materials’ processing−structure−properties−performance relationship is integral. In this review, we provide an overview of borate-based compounds as possible mixed polyanion cathode materials in organic electrolyte metal-ion batteries. A recapitulation of lithium-ion battery (LIB) cathode materials development provides that rationale. The combined method of data mining and high-throughput ab initio computing was briefly discussed to derive how carbonate-based compounds in sidorenkite structure were suggested. Borate-based compounds, albeit just close to stability (viz., <30 meV at−1), offer tunability and versatility and hence, potential effectivity as polyanion cathodes due to (1) diverse structures which can host alkali metal intercalation; (2) the low weight of borate relative to mature polyanion families which can translate to higher theoretical capacity; and a (3) rich chemistry which can alter the inductive effect on earth-abundant transition metals (e.g., Ni and Fe), potentially improving the open-circuit voltage (OCV) of the cell. This review paper provides a reference on the structures, properties, and synthesis routes of known borate-based compounds [viz., borophosphate (BPO), borosilicate (BSiO), and borosulfate (BSO)], as these borate-based compounds are untapped despite their potential for mixed polyanion cathode materials for advanced batteries.

Bimetallic Rechargeable Al/Zn Hybrid Aqueous Batteries Based on Al-Zn Alloys with Composite Electrolytes

Aluminum is abundant and exhibits a high theoretical capacity and volumetric energy density. Additionally, the high safety of aqueous aluminum-ion batteries makes them strong candidates for large-scale energystorage systems. However, the frequent collapse of the cathode material and passive oxide film results in the difficult development of aqueous aluminum-ion batteries. This work provides a novel battery system, namely, Al-Zn/Al(OTF)₃ +HOTF+Zn(OTF)₂ /Al_x Zn_y MnO₂ ·nH₂ O, with a mixed electrolyte. The cathode applies MnO topology transformation to ensure that the cathode forms Al_x MnO₂ ·nH₂ O. Topology transformation alters the structure of the cathode material so that Zn²⁺ can be intercalated into the Al_x MnO₂ ·nH₂ O spinel structure to provide support for the material structure. Regarding the anode, Zn²⁺ in the electrolyte is deposited onto Al of the anode to produce a regional Al-Zn alloy. Zn²⁺ is reduced to Zn metal during discharging, which adds a platform for secondary discharge beneficial for battery capacity enhancement. This system can provide a 1.6 V discharge platform, while the first cycle discharge can reach 554 mAh g^-1 , thereby maintaining a high capacity of 313 mAh g^-1 after 100 cycles. This study provides a new idea for the further development of aqueous aluminum-ion batteries (AAIBs).

Surface chemical heterogeneous distribution in over-lithiated Li1+xCoO2 electrodes

In commercial Li-ion batteries, the internal short circuits or over-lithiation often cause structural transformation in electrodes and may lead to safety risks. Herein, we investigate the over-discharged mechanism of LiCoO₂/graphite pouch cells, especially spatially resolving the morphological, surface phase, and local electronic structure of LiCoO₂ electrode. With synchrotron-based X-ray techniques and Raman mapping, together with spectroscopy simulations, we demonstrate that over-lithiation reaction is a surface effect, accompanied by Co reduction and surface structure transformation to Li₂CoO₂/Co₃O₄/CoO/Li₂O-like phases. This surface chemical distribution variation is relevant to the depth and exposed crystalline planes of LiCoO₂ particles, and the distribution of binder/conductive additives. Theoretical calculations confirm that Li₂CoO₂-phase has lower electronic/ionic conductivity than LiCoO₂-phase, further revealing the critical effect of distribution of conductive additives on the surface chemical heterogeneity evolution. Our findings on such surface phenomena are non-trivial and highlight the capability of synchrotron-based X-ray techniques for studying the spatial chemical phase heterogeneity.

Over-lithiation often causes structural transformation in electrodes and may lead to safety issues in Li-ion batteries. Here, authors investigate the over-discharged mechanism of LiCoO2/graphite pouch cells, and spatially resolve the morphological, surface phase, and local electronic structure of LiCoO2 electrode.

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.

Towards a Stable Layered Vanadium Oxide Cathode for High-Capacity Calcium Batteries

Calcium-based batteries have promising advantages over multivalent ion batteries. However, the fabrication of highly efficient calcium batteries is limited by the quality of available cathode materials, which motivates the exploration of electrodes that can enable reversible, stable Ca²⁺ intercalation. Herein, layered vanadium oxide Mg_x V₂ O₅ ·nH₂ O is used as a calcium battery cathode, and it exhibits a high capacity of 195.5 mA h g^-1 at 20 mA g^-1 and an outstanding cycling life (93.6% capacity retention after 2500 cycles at 1 A g^-1 ). Combining theoretical analysis and experimental design, a series of layered oxides (M_x V₂ O₅ ·nH₂ O, M = Mg, Ca, Sr) is selected as a model system to identify the Ca storage mechanism. It is found that the hydrated alkaline earth metal ions in the vanadium-based layered oxide interlayers play a critical role as pillared stabilizers to facilitate Ca²⁺ insertion/extraction. Compared with Ca²⁺ and Sr²⁺ , the presence of Mg²⁺ provides vanadium oxides with a rigid framework that allows for minimized volume fluctuation (a tiny variation of ≈0.15 Å of the interlayer spacing). Such an understanding of the Ca storage mechanism is a key step in the rational design and selection of materials for calcium batteries to achieve a high capacity and long cycle life.

Investigating the role of structural water on the electrochemical properties of α-V2O5 through density functional theory

The α polymorph of V₂O₅ is one of the few known cathodes capable of reversibly intercalating multivalent ions such as Mg, Ca, Zn and Al, but suffers from sluggish diffusion kinetics. The role of H₂O within the electrolyte and between the layers of the structure in the form of a xerogel/aerogel structure, though, has been shown to lower diffusion barriers and lead to other improved electrochemical properties. This density functional theory study systematically investigates how and why the presence of structural H₂O within α-V₂O₅ changes the resulting structure, voltage, and diffusion kinetics for the intercalation of Li, Na, Mg, Ca, Zn, and Al. We found that the coordination of H₂O molecules with the ion leads to an improvement in voltage and energy density for all ions. This voltage increase was attributed to the extra host sites for electrons present with H₂O, thus leading to a stronger ionization of the ion and a higher voltage. We also found that the increase in interlayer distance and a potential "charge shielding" effect drastically changes the electrostatic environment and the resulting diffusion kinetics. For Mg and Ca, this resulted in a decrease in diffusion barrier from 1.3 eV and 2.0 eV to 0.89 eV and 0.4 eV, respectively. We hope that our study motivates similar research regarding the role of water in both V₂O₅ xerogels/aerogels and other layered transition metal oxides.

Rechargeable Batteries for Grid Scale Energy Storage

Ever-increasing global energy consumption has driven the development of renewable energy technologies to reduce greenhouse gas emissions and air pollution. Battery energy storage systems (BESS) with high electrochemical performance are critical for enabling renewable yet intermittent sources of energy such as solar and wind. In recent years, numerous new battery technologies have been achieved and showed great potential for grid scale energy storage (GSES) applications. However, their practical applications have been greatly impeded due to the gap between the breakthroughs achieved in research laboratories and the industrial applications. In addition, various complex applications call for different battery performances. Matching of diverse batteries to various applications is required to promote practical energy storage research achievement. This review provides in-depth discussion and comprehensive consideration in the battery research field for GSES. The overall requirements of battery technologies for practical applications with key parameters are systematically analyzed by generating standards and measures for GSES. We also discuss recent progress and existing challenges for some representative battery technologies with great promise for GSES, including metal-ion batteries, lead-acid batteries, molten-salt batteries, alkaline batteries, redox-flow batteries, metal-air batteries, and hydrogen-gas batteries. Moreover, we emphasize the importance of bringing emerging battery technologies from academia to industry. Our perspectives on the future development of batteries for GSES applications are provided.

Water-Lubricated Aluminum Vanadate for Enhanced Rechargeable Magnesium Ion Storage

Magnesium ion batteries (MIBs) have attracted much attention due to their low cost and high safety properties. However, the intense charge repulsion effect and sluggish diffusion dynamics of Mg²⁺ ions result in unsatisfactory electrochemical performance of conventional cathode materials in MIBs. This work reports water-lubricated aluminum vanadate (HAlVO) as high-performance cathode material for Mg²⁺ ions storage and investigates the capacity fade mechanism of water-free aluminum vanadate (AlVO). The charge density difference based on density functional theory calculation is performed to analyze the charge transfer process of water-lubricated/free aluminum vanadates (HAlVO/AlVO). The different charge transfer phenomena of two materials and the charge shielding effect of water molecule in HAlVO are revealed. Moreover, the single-phase structural evolution process and the Mg²⁺ ions storage mechanism of HAlVO are further investigated deeply by different in situ and ex situ characterization methods. This work proves that HAlVO is a potential candidate cathode material to satisfy the high-performance reversible Mg²⁺ ions storage, and the water-lubricated method is an effective strategy to improve the electrochemical performance of vanadium oxides cathode.

Monodispersed flower-like MXene@VO2 clusters for aqueous zinc ion batteries with superior rate performance

Monoclinic B phase VO₂ with a distinctive tunnel structure is regarded as a viable cathode material for use in aqueous zinc ion batteries (AZIBs). However, the low electron conductivity and poor rate performance prevent it from being used further. Herein, we report 3D flower-like MXene nanosheets loaded with the VO₂ cluster (MXene@VO₂) synthesized via a one-step hydrothermal process, where MXene nanosheets were spontaneously stacked as a skeleton for the growth of VO₂ nanobelts. The synergistic effect between MXene nanosheets with high electronic conductivity and VO₂ nanobelts with a unique tunnel structure benefitted the electron and Zn²⁺ transport; the 3D hybrid structure with a high specific surface area provided an increased contact area with the electrolyte and a shortened distance of the Zn²⁺ transfer path. As a result, this material exhibits a promising Zn²⁺ storage behavior with a superior rate capability (363.2 mA h g^-1 at 0.2C and 169.1 mA h g^-1 at 50C) and outstanding long-cycling performance (206.6 mA h g^-1 and 76% capacity retention over 5000 cycles at 20C). In addition, a self-charging battery could be prepared by using oxygen in air to oxidize vanadium oxide with lower valence states. Our prepared MXene@VO₂ composite with a synergistic effect has been proved to be a promising cathode for AZIBs, offering a progressive paradigm for the development of AZIBs.

Polymorphic Biological and Inorganic Functional Nanomaterials

This perspective involves two types of functional nanomaterials, amyloid fibrils and metal oxide nanowires and nanogrids. Both the protein and the inorganic nanomaterials rely on their polymorphism to exhibit diverse properties that are important to sensing and catalysis. Several examples of novel functionalities are provided from biomarker sensing and filtration applications to smart scaffolds for energy and sustainability applications.

Uneven Stripping Behavior, an Unheeded Killer of Mg Anodes

Uniform magnesium (Mg) plating/stripping under high areal capacity utilization is critical for the practical application of Mg-metal anodes in rechargeable Mg batteries. However, the failure of the Mg-metal anode when cycling under a practical areal capacity (>4 mA h cm^-2 ), is of rising concern. The mechanism behind these failures remains controversial. In this work, it is illustrated that the initial plating Mg can be undoubtedly uniform in a wide range of current densities (≤5 mA cm^-2 ) and under a practical areal capacity (6 mA h cm^-2 ). However, an unusual self-accelerating pit growth is observed in the Mg stripping side under moderate current densities (0.1-1 mA cm^-2 ), which severely deteriorates the anode integrity and subsequent Mg plating uniformity, causing failure of the Mg-metal anode or short circuit of the battery. The stripping morphology depends on the applied current density, as non-uniformity versus the current density displays a volcano plot during the stripping process. Through in situ spectroscopy, it is illustrated that this current-dependent behavior is determined by the evolution of chlorine-containing complex ions near the interface. This research reminds that the plating/stripping process of the Mg-metal anode must be considered comprehensively for practical Mg-metal batteries.

Rapid discovery of stable materials by coordinate-free coarse graining

A fundamental challenge in materials science pertains to elucidating the relationship between stoichiometry, stability, structure, and property. Recent advances have shown that machine learning can be used to learn such relationships, allowing the stability and functional properties of materials to be accurately predicted. However, most of these approaches use atomic coordinates as input and are thus bottlenecked by crystal structure identification when investigating previously unidentified materials. Our approach solves this bottleneck by coarse-graining the infinite search space of atomic coordinates into a combinatorially enumerable search space. The key idea is to use Wyckoff representations, coordinate-free sets of symmetry-related positions in a crystal, as the input to a machine learning model. Our model demonstrates exceptionally high precision in finding unknown theoretically stable materials, identifying 1569 materials that lie below the known convex hull of previously calculated materials from just 5675 ab initio calculations. Our approach opens up fundamental advances in computational materials discovery.

An amphoteric betaine electrolyte additive enabling a stable Zn metal anode for aqueous batteries

The corrosion and dendritic growth of the Zn anode limit its electrochemical performance in aqueous Zn batteries. Here, we present an amphoteric betaine additive for 5 m ZnCl₂ aqueous electrolyte. The carboxyl group on betaine forms hydrogen bonds with water and reduces the water activity. The molecule also experiences preferential adsorption on the Zn surface and separates the interactions between Zn and water. Side reactions at the Zn electrode are thus inhibited. The regulated interface also ensures uniform Zn deposition. As a result, the electrolyte with betaine additive allows reversible Zn plating/stripping for over 1400 h at 0.5 mA cm^-2. A capacity retention of 94% is obtained after 3000 cycles for a VO₂ cathode.

Unconventional Charge Transport in MgCr2O4 and Implications for Battery Intercalation Hosts

Ion transport in solid-state cathode materials prescribes a fundamental limit to the rates batteries can operate; therefore, an accurate understanding of ion transport is a critical missing piece to enable new battery technologies, such as magnesium batteries. Based on our conventional understanding of lithium-ion materials, MgCr₂O₄ is a promising magnesium-ion cathode material given its high capacity, high voltage against an Mg anode, and acceptable computed diffusion barriers. Electrochemical examinations of MgCr₂O₄, however, reveal significant energetic limitations. Motivated by these disparate observations; herein, we examine long-range ion transport by electrically polarizing dense pellets of MgCr₂O₄. Our conventional understanding of ion transport in battery cathode materials, e.g., Nernst-Einstein conduction, cannot explain the measured response since it neglects frictional interactions between mobile species and their nonideal free energies. We propose an extended theory that incorporates these interactions and reduces to the Nernst-Einstein conduction under dilute conditions. This theory describes the measured response, and we report the first study of long-range ion transport behavior in MgCr₂O₄. We conclusively show that the Mg chemical diffusivity is comparable to lithium-ion electrode materials, whereas the total conductivity is rate-limiting. Given these differences, energy storage in MgCr₂O₄ is limited by particle-scale voltage drops, unlike lithium-ion particles that are limited by concentration gradients. Future materials design efforts should consider the interspecies interactions described in this extended theory, particularly with respect to multivalent-ion systems and their resultant effects on continuum transport properties.

Zn3V4(PO4)6: A New Rocking-Chair-Type Cathode Material with High Specific Capacity Derived from Zn2+/H+ Cointercalation for Aqueous Zn-Ion Batteries

Phosphate cathode materials with a stable and open framework structure are expected to be one of the favorable cathode materials for aqueous zinc-ion batteries (AZIBs). However, the slow migration rate of Zn²⁺ and complex mechanism in aqueous electrolyte are serious problems that limit their application at the present. Here, a new rocking-chair-type cathode material Zn₃V₄(PO₄)₆@C (ZVP@C) for AZIBs is synthesized for the first time and evaluated using a composite carbon coating to improve the electronic conductivity. Benefiting from the two-electron reaction of vanadium and the cointercalation of Zn²⁺/H⁺, ZVP@C/30%BP delivers a specific capacity as high as 120 mAh·g^-1 at 0.04 A·g^-1. A good capacity retention of 80% after 400 cycles at 1 A·g^-1 is also obtained, which is attributed to the stable crystal structure and the cointercalation reaction of Zn²⁺/H⁺. The reaction mechanism is investigated by in situ X-ray diffraction (XRD), ex situ XRD, ex situ X-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS). This work not only provides a new phosphate cathode material for AZIBs but also gives a new strategy for improving the specific capacity of phosphate cathode material.

A Rechargeable Mg|O2 Battery

Rechargeable Mg|O₂ batteries (RMOBs) offer several advantages over alkali metal-based battery systems owing to Mg’s ease of transport/storage in ambient environment, low cost originating from its high abundance, as well as the high theoretical specific energy of RMOBs. However, research on RMOBs has been stagnant for the past decade, largely owing to unacceptably poor electrochemical performance. Here, we present a RMOB that employs Mg anode, Mg((CF₃SO₂)₂N)₂-MgCl₂ in diglyme (G2) electrolyte, and commercial Pt/C on carbon fiber paper (Pt/C@CFP) oxygen cathode. This battery demonstrates unparalleled improvement over existing RMOBs by rendering a discharge capacity over 1.6 mAh cm⁻², achieving cycle lives up to 35 cycles with a cumulative energy density of ∼3.2 mWh cm⁻² at room temperature. This RMOB system seeks to reignite the pursuit of novel electrochemical systems based on Mg-O₂ chemistries.

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A rechargeable Mg|O₂ battery with prolonged cycle life (∼35 cycles) is demonstrated

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Mg((CF₃SO₂)₂N)₂-MgCl₂ in G2 enables reversible battery cycling O₂ environment

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A multistep discharge product formation pathway is proposed

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MgO is identified as the main discharge product

Rational Design Strategy of Novel Energy Storage Systems: Toward High-Performance Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) are promising candidates to replace currently commercialized lithium-ion batteries (LIBs) in large-scale energy storage applications owing to their merits of abundant resources, low cost, high theoretical volumetric capacity, etc. However, the development of RMBs is still facing great challenges including the incompatibility of the electrolyte and the lack of suitable cathode materials with high reversible capacity and fast kinetics of Mg²⁺ . While tremendous efforts have been made to explore compatible electrolytes and appropriate electrode materials, the rational design of unconventional Mg-based battery systems is another effective strategy for achieving high electrochemical performance. This review specifically discusses the recent research progress of various Mg-based battery systems. First, the optimization of electrolyte and electrode materials for conventional RMBs is briefly discussed. Furthermore, various Mg-based battery systems, including Mg-chalcogen (S, Se, Te) batteries, Mg-halogen (Br₂ , I₂ ) batteries, hybrid-ion batteries, and Mg-based dual-ion batteries are systematically summarized. This review aims to provide a comprehensive understanding of different Mg-based battery systems, which can inspire latecomers to explore new strategies for the development of high-performance and practically available RMBs.

Off-Planar, Two-Dimensional Polymer Cathode for High-Rate, Durable Rechargeable Magnesium Batteries

Rechargeable magnesium batteries are of considerable interest due to their high theoretical capacity, and they are projected as good alternates for stationary energy storage and electric vehicles. Sluggish Mg2+ kinetics and scarce availability of suitable cathode materials are major issues hindering the progress of rechargeable magnesium batteries. Herein, a conjugated, off-planar, two-dimensional (2D) polymer is explored for reversible magnesium storage. The polymer cathode reveals high capacity and high cycling stability with high rate capability. Replacing the Mg metal anode with the Mg alloy, AZ31 further enhances the ion storage performance. At a high current density of 2 A g-1, stable capacity is shown for almost 5000 cycles with 99% Coulombic efficiency. A composite of carbon nanotube with the polymer delivers capacity values higher (>1.5 times) than that of a pristine polymer at a current density of 2 A g-1 and shows cycling up to 5 A g-1. Electrokinetic studies reveal a contribution of pseudocapacitive nature, and the mechanism is investigated by ex situ X-ray photoelectron spectroscopy and infrared spectroscopy. The use of 2D polymer electrodes opens up opportunities for developing high-rate, high-capacity, and stable rechargeable magnesium ion batteries.

Aqueous zinc batteries: Design principles toward organic cathodes for grid applications

The development of low-cost and sustainable grid energy storage is urgently needed to accommodate the growing proportion of intermittent renewables in the global energy mix. Aqueous zinc-ion batteries are promising candidates to provide grid storage due to their inherent safety, scalability, and economic viability. Organic cathode materials are especially advantageous for use in zinc-ion batteries as they can be synthesized using scalable processes from inexpensive starting materials and have potential for biodegradation at their end of life. Strategies for designing organic cathode materials for rechargeable zinc-ion batteries targeting grid applications will be discussed in detail. Specifically, we emphasize the importance of cost analysis, synthetic simplicity, end-of-life scenarios, areal loading of active material, and long-term stability to materials design. We highlight the strengths and challenges of present zinc-organic research in the context of our design principles, and provide opportunities and considerations for future electrode design.

Theory-guided experimental design in battery materials research

A reliable energy storage ecosystem is imperative for a renewable energy future, and continued research is needed to develop promising rechargeable battery chemistries. To this end, better theoretical and experimental understanding of electrochemical mechanisms and structure-property relationships will allow us to accelerate the development of safer batteries with higher energy densities and longer lifetimes. This Review discusses the interplay between theory and experiment in battery materials research, enabling us to not only uncover hitherto unknown mechanisms but also rationally design more promising electrode and electrolyte materials. We examine specific case studies of theory-guided experimental design in lithium-ion, lithium-metal, sodium-metal, and all-solid-state batteries. We also offer insights into how this framework can be extended to multivalent batteries. To close the loop, we outline recent efforts in coupling machine learning with high-throughput computations and experiments. Last, recommendations for effective collaboration between theorists and experimentalists are provided.

Flexible and Safe Additives-Based Zinc-Binder-Free-Hierarchical MnO2 -Solid Alkaline Polymer Battery for Potential Wearable Applications

The next-generation flexible wearable electronics are among the most rapidly growing industries due to their extended use in everyday applications resulting in an increased demand for cheaper, safer, and flexible energy storage devices. This study aims to investigate and enhance the overall performance of a Zn-MnO₂ alkaline battery and make it suitable for safe and flexible wearable applications. To achieve high cyclability and performance of the cathode, issues of low active-material availability for redox reactions and inactive-phase formations are overcome by fabricating a binder-free hierarchical (increased surface area) additives (enabled reversible compound formation) based MnO₂ cathode. Furthermore, zinc/stainless steel composite anode (to reduce anode shape changes) and calcium hydroxide coated polymer electrolyte (to stop zincate ion transfer) are used to improve cyclability. By assembling the above mentioned layers, excellent rate capabilities, high-capacity utilization (487 mAh g^-1 ), long cycling stabilities (1000 cycles with 70% retention), and high energy density (400 Wh kg^-1 ) are achieved. Moreover, bending, hammering, puncturing, and lighting up an light emitting diode are conducted (under flat, bent, and cut) to demonstrate the cells' safety, flexibility, and robustness. The successful findings in this study can chart new pathways to the development of safe, flexible, and cost-effective next-generation energy storage sources for wearables.

Eutectic Electrolytes Chemistry for Rechargeable Zn Batteries

Rechargeable zinc batteries (RZBs) have proved to be promising candidates as an alternative to lithium-ion batteries due to their low cost, inherent safety, and environmentally benign features. While designing cost-effective electrolyte systems with excellent compatibility with electrode materials, high energy/power density as well as long life-span challenge their further application as grid-scale energy storage devices. Eutectic electrolytes as a novel class of electrolytes have been extensively reported and explored taking advantage of their feasible preparation and high tunability. Recently, some perspectives have summarized the development and application of eutectic electrolytes in metal-based batteries, but their infancy requires further attention and discussion. This review systematically presents the fundamentals and definitions of eutectic electrolytes. Besides, a specific classification of eutectic electrolytes and their recent progress and performance on RZB fields are introduced as well. Significantly, the impacts of various composing eutectic systems are disserted for critical RZB chemistries including attractive features at electrolyte/electrode interfaces and ions/charges transport kinetics. The remaining challenges and proposed perspectives are ultimately induced, which deliver opportunities and offer practical guidance for the novel design of advanced eutectic electrolytes for superior RZB scenarios.

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.

Investigation of Ca Insertion into α-MoO3 Nanoparticles for High Capacity Ca-Ion Cathodes

Calcium-ion batteries (CIBs) are a promising alternative to lithium-ion batteries (LIBs) due to the low redox potential of calcium metal and high abundance of calcium compounds. Due to its layered structure, α-MoO₃ is regarded as a promising cathode host lattice. While studies have reported that α-MoO₃ can reversibly intercalate Ca ions, limited electrochemical activity has been noted, and its reaction mechanism remains unclear. Here, we re-examine Ca insertion into α-MoO₃ nanoparticles with a goal to improve reaction kinetics and clarify the storage mechanism. The α-MoO₃ electrodes demonstrated a specific capacity of 165 mA h g^-1 centered near 2.7 V vs Ca²⁺/Ca, stable long-term cycling, and good rate performance at room temperature. This work demonstrates that, under the correct conditions, layered oxides can be a promising host material for CIBs and renews prospects for CIBs.

Foldable batteries: from materials to devices

Wearable electronics is a growing field that has important applications in advanced human-integrated systems with high performance and mechanical deformability, especially foldable characteristics. Although foldable electronics such as rollable TVs (LG signature OLED R) or foldable smartphones (Samsung Galaxy Z fold/flip series) have been successfully established in the market, these devices are still powered by rigid and stiff batteries. Therefore, to realize fully wearable devices, it is necessary to develop state-of-the-art foldable batteries with high performance and safety in dynamic deformation states. In this review, we cover the recent progress in developing materials and system designs for foldable batteries. The Materials section is divided into three sections aimed at helping researchers choose suitable materials for their systems. Several foldable battery systems are discussed and the combination of innovative materials and system design that yields successful devices is considered. Furthermore, the basic analysis process of electrochemical and mechanical properties is provided as a guide for researchers interested in the evaluation of foldable battery systems. The current challenges facing the practical application of foldable batteries are briefly discussed. This review will help researchers to understand various aspects (from material preparation to battery configuration) of foldable batteries and provide a brief guideline for evaluating the performance of these batteries.

Cationic Vacancies in Anatase (TiO2): Synthesis, Defect Characterization, and Ion-Intercalation Properties

As one of the most studied materials, research on titanium dioxide (TiO₂) has flourished over the years owing to technological interest ranging from energy conversion and storage to medical implants and sensors, to name a few. Within this scope, the development of synthesis routes enabling the stabilization of reactive surface structure has been frequently investigated. Among these routes, solution-based synthesis has been utilized to tailor the material's properties spanning its atomic structural arrangement, or morphological aspects. One of the most investigated methods of stabilizing crystals with tailored facets relies on the use of fluoride-based precursors. Fluoride ions not only provide a driving force for the stabilization of metastable/reactive surface structures but also alter the reactivity of titanium molecular precursors and in turn the structural features of the stabilized crystals. Here, we review recent progress in the solution-based synthesis of anatase (one of the polymorphs of TiO₂) employing a fluoride precursor, with an emphasis on how cationic vacancies are stabilized by a charge-compensating mechanism and the resulting structural features associated with these defects. Finally, we will discuss the ion-intercalation properties of these sites with respect to lithium and polyvalent ions such as Mg²⁺ and Al³⁺. We will discuss in more detail the relevant parameters of the synthesis that allow controlling the phase composition with the coexistence of oxide, fluoride, and hydroxide ions within the anatase framework. The mechanism of formation of defective anatase nanocrystals has highlighted a solid-state transformation mostly implying an oxolation reaction (the condensation of hydroxide ions) that results in a decrease in the vacancy content, which can be synthetically controlled. The investigation of local fluorine environments probed by solid-state ¹⁹F NMR revealed up to three coordination modes with different numbers of coordinated Ti⁴⁺ and vacancies. It further revealed the occurrence of single and adjacent pairs of vacancies. These different host sites including native interstitial (and single/paired vacancies) display different ion-intercalation properties. We notably discussed the influence of the local anionic environments of vacancies on the thermodynamics of intercalation properties. The selective intercalation of polyvalent cations such as Mg²⁺ and Al³⁺ further supports the beneficial uses of defect chemistry for developing post-lithium-ion batteries. It is expected that the ability to characterize the local structure of defects is key to the design of unique, tailored-made materials.

High-Performance Aqueous Zinc-Ion Battery Based on an Al0.35 Mn2.52 O4 Cathode: A Design Strategy from Defect Engineering and Atomic Composition Tuning

Rechargeable zinc-ion batteries (ZIBs), which adopt mild aqueous electrolytes with high power density and safety, have received significant interest. As the most widely used cathode material for ZIBs, manganese-based oxide has poor rate performance owing to its low electronic conductivity and slow ion diffusion kinetics. In this study, using the synergistic regulation strategy of defect engineering and atomic composition tuning, a mesoporous Al_0.35 Mn_2.52 O₄ with an ultrahigh surface area (up to 82 m² g^-1 ) is fabricated through Al substitution in the Mn₃ O₄ , followed by an Al-selective leaching process. During the entire process, numerous defects are obtained in the spinel structure by removing ≈30% of the Al cations. Al substitution can improve the material conductivity, while cation defects can weaken the electrostatic effect and promote ion diffusion ability. Therefore, the Al_0.35 Mn_2.52 O₄ cathode of ZIBs exhibits a high reversible capacity of 302 mAh g^-1 at a current density of 100 mA g^-1 . Furthermore, the reversible capacity remains at 147 mAh g^-1 after 1000 cycles at a current density of 1500 mA g^-1 . This synergistic regulation of atomic composition tuning and defect engineering provides a new perspective for improving the performance of electrode materials in ZIBs.

Bioinspired Catechol-Grafting PEDOT Cathode for an All-Polymer Aqueous Proton Battery with High Voltage and Outstanding Rate Capacity

Aqueous all-polymer proton batteries (APPBs) consisting of redox-active polymer electrodes are considered safe and clean renewable energy storage sources. However, there remain formidable challenges for APPBs to withstand a high current rate while maximizing high cell output voltage within a narrow electrochemical window of aqueous electrolytes. Here, a capacitive-type polymer cathode material is designed by grafting poly(3,4-ethylenedioxythiophene) (PEDOT) with bioinspired redox-active catechol pendants, which delivers high redox potential (0.60 V vs Ag/AgCl) and remarkable rate capability. The pseudocapacitive-dominated proton storage mechanism illustrated by the density functional theory (DFT) calculation and electrochemical kinetics analysis is favorable for delivering fast charge/discharge rates. Coupled with a diffusion-type anthraquinone-based polymer anode, the APPB offers a high cell voltage of 0.72 V, outstanding rate capability (64.8% capacity retention from 0.5 to 25 A g-1 ), and cycling stability (80% capacity retention over 1000 cycles at 2 A g-1 ), which is superior to the state-of-the-art all-organic proton batteries. This strategy and insight provided by DFT and ex situ characterizations offer a new perspective on the delicate design of polymer electrode patterns for high-performance APPBs.

Influence of MnO2-Birnessite Microstructure on the Electrochemical Performance of Aqueous Zinc Ion Batteries

KxMnO2 materials with birnessite-type structure are synthetized by two different methods which make it possible to obtain manganese oxides with different degrees of crystallinity. The XPS results indicate that the sample obtained at high temperature (KMn8) exhibits a lower oxidation state for manganese ions as well as a denser morphology. Both characteristics could explain the lower capacity value obtained for this electrode. In contrast, the sample obtained at low temperature (KMn4) or by hydrothermal method presents a manganese oxidation state close to 4 and a more porous morphology. Indeed, in this case higher capacity values are obtained. At current density of 30 mA g−1, the KMn8, KMn4, and HKMn samples display a capacity retention of 88, 82, and 68%, respectively. The higher capacity loss obtained for the HKMn compound could be explained considering that the incorporation of Zn2+ in the structure gives rise to the stabilization of a ZnMn2O4 spinel-type phase. This compound is obtained in the discharge process but remains in the charge stage. Thus, when this spinel-type phase is obtained the capacity loss increases. Moreover, the stabilization of this phase is more favorable at low current rates where 100% of retention for all samples, before 50 cycles, was observed.

Progress and Prospects of Electrolyte Chemistry of Calcium Batteries

The increasing energy storage demand of portable devices, electric vehicles, and scalable energy storage has been driving extensive research for more affordable, more energy dense battery technologies than Li ion batteries. The alkaline earth metal, calcium (Ca), has been considered an attractive anode material to develop the next generation of rechargeable batteries. Herein, the chemical designs, electrochemical performance, and solution and interfacial chemistry of Ca²⁺ electrolytes are comprehensively reviewed and discussed. In addition, a few recommendations are presented to guide the development and evaluation of Ca²⁺ electrolytes in future.

Chemical designs, electrochemical performance, and solution and interfacial chemistry of calcium battery electrolytes are comprehensively reviewed and discussed.

Rapid Electron/Ion Transport in CNT/LiTi2(PO4)3@C-N Electrodes for Aqueous Lithium-ion Batteries with High Stability, Flexibility and Safety

Self-supporting and flexible carbon nanotube/Carbon-coated Nitrogen-doped LiTi2(PO4)3 (CNT/LTP@C-N) hybrid films are manufactured via vacuum filtration. LTP@C-N is entangled into CNT networks to build a 3D conductive network. Such hybrid electrodes...

Old Materials for New Functions: Recent Progress on Metal Cyanide Based Porous Materials

Cyanide is the simplest ligand with strong basicity to construct open frameworks including some of the oldest compounds reported in the history of coordination chemistry. Cyanide can form numerous cyanometallates with different transition metal ions showing diverse geometries. Rational design of robust extended networks is enabled by the strong bonding nature and high directionality of cyanide ligand. By virtue of a combination of cyanometallates and/or organic linkers, multifunctional framework materials can be targeted and readily synthesized for various applications, ranging from molecular adsorptions/separations to energy conversion and storage, and spin-crossover materials. External guest- and stimuli-responsive behaviors in cyanide-based materials are also highlighted for the development of the next-generation smart materials. In this review, an overview of the recent progress of cyanide-based multifunctional materials is presented to demonstrate the great potential of cyanide ligands in the development of modern coordination chemistry and material science.

Recent Developments and Challenges of Vanadium Oxides (Vx Oy ) Cathodes for Aqueous Zinc-Ion Batteries

The rapid depletion of lithium resources and the increasing demand for electrical energy storage have stimulated the pursuit of emerging electrochemical energy storage. Aqueous zinc ion batteries (ZIBs) are highly sought after for their low cost, high safety, and increased environmental compatibility. However, the search for suitable cathode materials is still tricky for a wide range of researchers. Vanadium oxides (V_x O_y ), with their abundant vanadium valence, easily deformable V-O polyhedrons, and tunable chemical compositions, are of significant advantage in developing emerging materials. This work provides a detailed review of different V_x O_y for the application in aqueous ZIBs. The current problems and optimization strategies of V_x O_y cathode materials are systematically discussed. Finally, the current challenges and possible directions for future research of V_x O_y cathode materials in aqueous ZIBs are presented.

Concentration-dependent ion correlations impact the electrochemical behavior of calcium battery electrolytes

Ion interactions strongly determine the solvation environments of multivalent electrolytes even at concentrations below that required for practical battery-based energy storage. This statement is particularly true of electrolytes utilizing ethereal solvents due to their low dielectric constants. These solvents are among the most commonly used for multivalent batteries based on reactive metals (Mg, Ca) due to their reductive stability. Recent developments in multivalent electrolyte design have produced a variety of new salts for Mg²⁺ and Ca²⁺ that test the limits of weak coordination strength and oxidative stability. Such electrolytes have great potential for enabling full-cell cycling of batteries based on these working ions. However, the ion interactions in these electrolytes exhibit significant and non-intuitive concentration relationships. In this work, we investigate a promising exemplar, calcium tetrakis(hexafluoroisopropoxy)borate (Ca(BHFIP)₂), in the ethereal solvents 1,2-dimethoxyethane (DME) and tetrahydrofuran (THF) across a concentration range of several orders of magnitude. Surprisingly, we find that effective salt dissociation is lower at relatively dilute concentrations (e.g. 0.01 M) than at higher concentrations (e.g. 0.2 M). Combined experimental and computational dielectric and X-ray spectroscopic analyses of the changes occurring in the Ca²⁺ solvation environment across these concentration regimes reveals a progressive transition from well-defined solvent-separated ion pairs to de-correlated free ions. This transition in ion correlation results in improvements in both conductivity and calcium cycling stability with increased salt concentration. Comparison with previous findings involving more strongly associating salts highlights the generality of this phenomenon, leading to important insight into controlling ion interactions in ether-based multivalent battery electrolytes.

MOF-derived porous carbon inlaid with MnO2 nanoparticles as stable aqueous Zn-ion battery cathodes

Cathodes derived from metal-organic framework materials offer unique advantages in terms of improved structural reversibility and electron conduction efficiency. Nevertheless, the capacity contribution of cathodes based on the carbon framework system has not been clearly discussed or is controversial in aqueous batteries. In this essay, we have uncovered the capacity contribution arising from the adsorption of anions/cations onto the carbon surface by examining the bonds of the carbon and the details of unsteady voltage in the CV/GITT during the discharge. Benefiting from the synergistic contribution of the double-layer capacitance and pseudocapacitance, Zn/C-MnO₂ exhibits excellent long-cycling stability and fast kinetics. To the best of our knowledge, this is the first report on the ion adsorption-based double layer effect in aqueous zinc ion batteries. Such a capacity contribution mechanism, and a renewed knowledge of the discharge mechanism, will contribute to the development of high-performance aqueous zinc ion batteries.

Advancing towards a Practical Magnesium Ion Battery

A post-lithium battery era is envisaged, and it is urgent to find new and sustainable systems for energy storage. Multivalent metals, such as magnesium, are very promising to replace lithium, but the low mobility of magnesium ion and the lack of suitable electrolytes are serious concerns. This review mainly discusses the advantages and shortcomings of the new rechargeable magnesium batteries, the future directions and the possibility of using solid electrolytes. Special emphasis is put on the diversity of structures, and on the theoretical calculations about voltage and structures. A critical issue is to select the combination of the positive and negative electrode materials to achieve an optimum battery voltage. The theoretical calculations of the structure, intercalation voltage and diffusion path can be very useful for evaluating the materials and for comparison with the experimental results of the magnesium batteries which are not hassle-free.

Synthesis and Characterization of Magnesium Vanadates as Potential Mg-ion Cathode Materials Through an Ab Initio Guided Carbothermal Reduction Approach

Many technologically relevant transition metal oxides for advanced energy storage and catalysis feature reduced transition metal (TM) oxides and are often nontrivial to prepare because of the need to control the reducing nature of the atmosphere in which they are synthesized. In this work, we show that an ab initio predictive synthesis strategy can be used to produce multiple gram-scale products of various MgV x O y -type phases (δ-MgV 2 O 5 , spinel MgV 2 O 4 , and MgVO 3 ) containing V 3+ or V 4+ relevant for Mg-ion battery cathodes. Characterization of these phases using 25 Mg solid-state NMR spectroscopy illustrates the potential of 25 Mg NMR for studying reversible magnesiation and local charge distributions. Rotor-Assisted Population Transfer is used as a much needed signal-to-noise enhancement technique. The ab initio guided synthesis approach is seen as a step forward towards a predictive synthesis strategy for targeting specific complex TM oxides with variable oxidation states of technological importance beyond Mg-ion (and indeed Li-ion) chemistry.