Multi-Ion Engineering Strategies toward High Performance Aqueous Zinc-Based Batteries

As alternatives to batteries with organic electrolytes, aqueous zinc-based batteries (AZBs) have been intensively studied. However, the sluggish kinetics, side reactions, structural collapse, and dissolution of the cathode severely compromise the commercialization of AZBs. Among various strategies to accelerate their practical applications, multi-ion engineering shows great feasibility to maintain the original structure of the cathode and provide sufficient energy density for high-performance AZBs. Though multi-ion engineering strategies could solve most of the problems encountered by AZBs and show great potential in achieving practical AZBs, the comprehensive summaries of the batteries undergo electrochemical reactions involving more than one charge carrier is still in deficiency. The ambiguous nomenclature and classification are becoming the fountainhead of confusion and chaos. In this circumstance, this review overviews all the battery configurations and the corresponding reaction mechanisms are investigated in the multi-ion engineering of aqueous zinc-based batteries. By combing through all the reported works, this is the first to nomenclate the different configurations according to the reaction mechanisms of the additional ions, laying the foundation for future unified discussions. The performance enhancement, fundamental challenges, and future developing direction of multi-ion strategies are accordingly proposed, aiming to further accelerate the pace to achieve the commercialization of AZBs with high performance.

Dual-Ion Intercalation Chemistry Enabling Hybrid Metal-Ion Batteries

To outline the role of dual-ion intercalation chemistry to reach sustainable energy storage, the present Review aimed to compare two types of batteries: widely accepted dual-ion batteries based on cationic and anionic co-intercalation versus newly emerged hybrid metal-ion batteries using the co-intercalation of cations only. Among different charge carrier cations, the focus was on the materials able to co-intercalate monovalent ions (such Li⁺ and Na⁺ , Li⁺ and K⁺ , Na⁺ and K⁺ , etc.) or couples of mono- and multivalent ions (Li⁺ and Mg²⁺ , Na⁺ and Mg²⁺ , Na⁺ and Zn²⁺ , H⁺ and Zn²⁺ , etc.). Furthermore, the Review was directed on co-intercalation materials composed of environmentally benign and low-cost transition metals (e. g., Mn, Fe, etc.). The effect of the electrolyte on the co-intercalation properties was also discussed. The summarized knowledge on dual-ion energy storage could stimulate further research so that the hybrid metal-ion batteries become feasible in near future.

A Magnesium/Lithium Hybrid-Ion Battery with Modified All-Phenyl-Complex-Based Electrolyte Displaying Ultralong Cycle Life and Ultrahigh Energy Density

Magnesium/lithium hybrid-ion batteries (MLHBs) combine the advantages of high safety and fast ionic kinetics, which enable them to be promising emerging energy-storage systems. Here, a high-performance MLHB using a modified all-phenyl complex with a lithium bis(trifluoromethanesulfonyl)imide electrolyte and a NiCo₂S₄ cathode on a copper current collector is developed. A reversible conversion involving a copper collector with NiCo₂S₄ efficiently avoids the electrolyte dissociation and diffusion difficulties of Mg²⁺ ions, enabling low polarization and fast redox, which is verified by X-ray absorption near edge structure analysis. Such combination affords the best MLHB among all those ever reported, with a reversible capacity of 204.7 mAh g^-1 after 2600 cycles at 2.0 A g^-1, and delivers an ultrahigh full electrode-basis energy density of 708 Wh kg^-1. The developed MLHB also achieves good rate performance and temperature tolerance at -10 and 50 °C with a low electrolyte consumption. The hybrid-ion battery system presented here could inspire a broad set of engineering potentials for high-safety battery technologies and beyond.

Sustainable and cost-effective ternary electrolyte Et3NHCl-AlCl3-Mg(DEP)2 for high-performance rechargeable magnesium batteries

Cost-effective and sustainable battery materials for large-scale batteries are the need of the hour to garner renewable energy with high-performance metal battery technologies. Here, we report the high-performance and long cycle life electrolyte prepared from low-cost triethylamine hydrochloride (Et₃NHCl) and aluminum chloride (AlCl₃) termed as (TA) with different concentrations of magnesium diethylphosphate (Mg(DEP)₂) salt. The optimized ratio of the 0.1 M Mg(DEP)₂ electrolyte has shown a high ionic conductivity of 4.5 × 10^-3 S cm^-1 at ambient temperature and good anodic stability of 2.41 V vs. Mg/Mg²⁺. The dissolution/deposition of magnesium (Mg) on a Pt working electrode was systematically analyzed in this electrolyte. Cyclic voltammetry (CV) of the Mg-graphite battery was used to probe the intercalation/de-intercalation of Mg-AlCl₄^- ions into/from the graphite layer structure. This was confirmed by various analytical techniques, such as energy dispersive X-ray spectroscopy, X-ray diffraction technique and X-ray photoemission spectroscopy. Notably, during the galvanostatic study analysis, the assembled Mg cell delivered a high discharge capacity of 115 mA h g^-1 at a high C/10 rate, with more than 180 cycles at >80% coulombic efficiency. This electrolyte will be helpful in grid-scale power storage systems in future generations.

Fast self-healing solid polymer electrolyte with high ionic conductivity for Lithium metal batteries

By introducing multiple molecule/intermolecular dynamic reversible hydrogen bonds into the polydimethylsiloxane elastomer system, the solid polymeric electrolyte with high ion conductivity (2.5×10-4 Scm-1) and stable electrochemical window (> 5 V)...

Rivalry at the Interface: Ion Desolvation and Electrolyte Degradation in Model Ethylene Carbonate Complexes of Li+, Na+, and Mg2+ with PF6 - on the Li4Ti5O12 (111) Surface

Spinel lithium titanate, Li₄Ti₅O₁₂ (LTO), emerges as a "universal" electrode material for Li-ion batteries and hybrid Li/Na-, Li/Mg-, and Na/Mg-ion batteries functioning on the basis of intercalation. Given that LTO operates in a variety of electrolyte solutions, the main challenge is to understand the reactivity of the LTO surface toward single- and dual-cation electrolytes at the molecular level. This study first reports results on ion desolvation and electrolyte solvent/salt degradation on an LTO surface by means of periodic DFT calculations. The desolvation stages are modeled by the adsorption of mono- and binuclear complexes of Li⁺, Na⁺, and Mg²⁺ with a limited number of ethylene carbonate (EC) solvent molecules on the oxygen-terminated LTO (111) surface, taking into account the presence of a PF₆ ^- counterion. Alongside cation adsorption, several degradation reactions are discussed: surface-catalyzed dehydrogenation of EC molecules, simultaneous dehydrogenation and fluorination of EC, and Mg²⁺-induced decay of PF₆ ^- to PF₅ and F^-. Data analysis allows the rationalization of existing experimentally established phenomena such as gassing and fluoride deposition. Among the three investigated cations, Mg²⁺ is adsorbed most tightly and is predicted to form a thicker fluoride-containing film on the LTO surface. Gassing, characteristic for carbonate-based electrolytes with LTO electrodes, is foreseen to be suppressed in dual-cation batteries. The latter bears promise to outperform the single-ion ones in terms of durability and safety.

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.

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.

Silicon as the Anode Material for Multivalent-Ion Batteries: A First-Principles Dynamics Study

Due to its huge capacity, Si is a promising anode material for practical applications in lithium-ion batteries. Here, using first-principles calculations, we study the applicability of the amorphous Si anode in multivalent-ion batteries, which are of great interest as candidates for post-lithium-ion batteries. Of the multivalent Mg²⁺, Ca²⁺, Zn²⁺, and Al³⁺ ions, only Mg²⁺ and Ca²⁺ are able to form Mg_2.3Si and Ca_2.5Si by alloying with Si, delivering very high capacities of 4390 and 4771 mA h g^-1, respectively. Mg_2.3Si has an 8% smaller capacity than Ca_2.5Si, but its volume expansion ratio and ion diffusivity are ∼200% smaller and 3 orders of magnitude higher than those of Ca_2.5Si, respectively. The capacity, volume expansion, and ion diffusion of Mg_2.3Si are excellently high, moderately small, and fairly fast, respectively, when compared to those of Li_3.7Si, Na_0.75Si, and K_1.1Si. The high performance of Mg_2.3Si can be understood in terms of the coordination numbers of Si and the atomic size of Mg. This work suggests that, as a carrier ion for the amorphous Si anode, Mg²⁺ is the most competitive among the multivalent ions and is at least as good as monovalent ions.

Nernstian Li+ intercalation into few-layer graphene and its use for the determination of K+ co-intercalation processes

Alkali ion intercalation is fundamental to battery technologies for a wide spectrum of potential applications that permeate our modern lifestyle, including portable electronics, electric vehicles, and the electric grid. In spite of its importance, the Nernstian nature of the charge transfer process describing lithiation of carbon has not been described previously. Here we use the ultrathin few-layer graphene (FLG) with micron-sized grains as a powerful platform for exploring intercalation and co-intercalation mechanisms of alkali ions with high versatility. Using voltammetric and chronoamperometric methods and bolstered by density functional theory (DFT) calculations, we show the kinetically facile co-intercalation of Li⁺ and K⁺ within an ultrathin FLG electrode. While changes in the solution concentration of Li⁺ lead to a displacement of the staging voltammetric signature with characteristic slopes ca. 54-58 mV per decade, modification of the K⁺/Li⁺ ratio in the electrolyte leads to distinct shifts in the voltammetric peaks for (de)intercalation, with a changing slope as low as ca. 30 mV per decade. Bulk ion diffusion coefficients in the carbon host, as measured using the potentiometric intermittent titration technique (PITT) were similarly sensitive to solution composition. DFT results showed that co-intercalation of Li⁺ and K⁺ within the same layer in FLG can form thermodynamically favorable systems. Calculated binding energies for co-intercalation systems increased with respect to the area of Li⁺-only domains and decreased with respect to the concentration of -K-Li- phases. While previous studies of co-intercalation on a graphitic anode typically focus on co-intercalation of solvents and one particular alkali ion, this is to the best of our knowledge the first study elucidating the intercalation behavior of two monovalent alkali ions. This study establishes ultrathin graphitic electrodes as an enabling electroanalytical platform to uncover thermodynamic and kinetic processes of ion intercalation with high versatility.

Constructing porous TiO2 crystals by an etching process for long-life lithium ion batteries

"Zero strain" materials, which have no volume change when charging and discharging, show ultra-long cycling stabilities when used as lithium-ion battery anodes, making them an area of extreme interest in this decade. For a typical anatase TiO₂ crystal, the volume change is 3-4% during Li insertion/extraction, which is not "zero strain". As the Ti/O packing in the TiO₂ lattice is too tight, there is insufficient void space for Li insertion, leading to volume expansion and structural collapse. Herein, pseudo-"zero-strain" TiO₂ is achieved via designing TiO₂ crystals with abundant inner mesopores, making Ti/O loose-packed via the acid-etching of K₂Ti₈O₁₇, providing sufficient space for Li intercalation. Instead of the traditional cut-off potential of 1 V used for Ti-/Nb-based anodes, we choose 0.01 V as the cut-off to make the best of the extra capacity contributed by the mesopores. As expected, plenty of mesopores could serve as "Li⁺-reservoirs" for fast lithium storage, demonstrating exceptional high-rate performance with an average capacity of 109.6 mA h g^-1 after 30 000 cycles at 60 C and 100 mA h g^-1 at 120 C. Such a strategy of combining a mesoporous structure and cut-off potential regulation may pave a solid pathway for constructing novel high-power anodes.

Highly Efficient Non-Nucleophilic Mg(CF3SO3)2-Based Electrolyte for High-Power Mg/S Battery

The application of high-energy Mg/S batteries was obstructed by the insufficiency of low-cost and non-nucleophilic electrolyte. In this work, a non-nucleophilic electrolyte was prepared by facilely dissolving Mg(CF₃SO₃)₂, MgCl₂, and AlCl₃ in 1,2-dimethoxyethane (DME). The equilibrium species of the MTB electrolyte mainly comprise [Mg₂(μ-Cl)₂(DME)₄]²⁺ and [(CF₃SO₃)AlCl₃]^-, which are generated by dehalodimerization reaction and Lewis acid-base reaction, respectively. The electrolyte exhibits a highly efficient reversible Mg deposition/dissolution, low overpotential of around 250 mV, and good oxidative stability up to 3.5 V after conditioning. The conditioning process that the active [Mg₂(μ-Cl)₂(DME)₄]²⁺ species transforms into [Mg₃(μ₃-Cl)(μ₂-Cl)₂(DME)₇]³⁺ was demonstrated, owing to the irreversible deposition of Al³⁺ on Mg foil. More importantly, this electrolyte exhibited good compatibility and kinetics for reversible Mg/MgS_x redox, resulting in a high specific capacity of 866 mAh g^-1 at 200 mA g^-1 and high power density of 550 W kg^-1. This work offers a new direction for low-cost, non-nucleophilic electrolyte and paves the way to explore high-power Mg/S batteries.

Prelithiated V2 C MXene: A High-Performance Electrode for Hybrid Magnesium/Lithium-Ion Batteries by Ion Cointercalation

The pursuit of high reversible capacity and long cycle life for rechargeable batteries has gained extensive attention in recent years, and the development of applicable electrode materials is the key point. Herein, thanks to the preintercalation of lithium ions, a stable and highly conductive nanostructure of V₂ C MXene is successfully fabricated via a facile self-discharge mechanism, which provides open spaces for rapid ion diffusion and guarantees fast electron transport. Taking the prelithiated V₂ C as electrode, an outstanding initial coulombic efficiency of 80% and an impressive capacity retention of ≈98% after 5000 charge/discharge cycles are achieved for lithium-ion batteries. Especially, it demonstrates a fascinating reversible capacity of up to 230.3 mA h g^-1 at 0.02 A g^-1 and a long cycling life of 82% capacity retention over 480 cycles in the hybrid magnesium/lithium-ion batteries. In addition, the Mg²⁺ and Li⁺ ions cointercalation mechanism of the prelithiated V₂ C is elucidated through ex situ X-ray diffraction and X-ray photoelectron spectroscopy characterizations. This work not only offers an effective approach to compensate the large initial lithium loss of high-capacity anode materials but also opens up a new and viable avenue to develop promising hybrid Mg/Li-storage materials with eminent electrochemical performance.

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

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

Voltage issue of aqueous rechargeable metal-ion batteries

Over the past two decades, a series of aqueous rechargeable metal-ion batteries (ARMBs) have been developed, aiming at improving safety, environmental friendliness and cost-efficiency in fields of consumer electronics, electric vehicles and grid-scale energy storage. However, the notable gap between ARMBs and their organic counterparts in energy density directly hinders their practical applications, making it difficult to replace current widely-used organic lithium-ion batteries. Basically, this huge gap in energy density originates from cell voltage, as the narrow electrochemical stability window of aqueous electrolytes substantially confines the choice of electrode materials. This review highlights various ARMBs with focuses on their voltage characteristics and strategies that can effectively raise battery voltage. It begins with the discussion on the fundamental factor that limits the voltage of ARMBs, i.e., electrochemical stability window of aqueous electrolytes, which decides the maximum-allowed potential difference between cathode and anode. The following section introduces various ARMB systems and compares their voltage characteristics in midpoint voltage and plateau voltage, in relation to respective electrode materials. Subsequently, various strategies paving the way to high-voltage ARMBs are summarized, with corresponding advancements highlighted. The final section presents potential directions for further improvements and future perspectives of this thriving field.

Eco-compatible oxides enabling energy storage via Li+/Mg2+ co-intercalation

The storage of energy by means of reversible intercalation of bivalent magnesium ions represents, nowadays, the shortest route for doubling the energy density of conventional lithium ion batteries. Contrary to the intercalation of monovalent lithium ions, the intercalation of Mg²⁺ is a kinetically limited process. Herein we demonstrate a new approach for improving Mg²⁺ intercalation, which is based on dual intercalation of Li⁺ and Mg²⁺ ions with a synergic effect. The concept is proved on the basis of eco-compatible oxides such as magnesium manganate spinels, MgMn₂O₄, and lithium titanates Li₂TiO₃ and Li₄Ti₅O₁₂ with a monoclinic and spinel structure. These two types of oxides are selected since they exhibit high and low potentials of ion intercalation due to the redox couples Mn^2,3+/Mn^3,4+ and Ti³⁺/Ti⁴⁺, respectively. Through a newly developed method of synthesis, we succeeded in the preparation of well-crystallized nanosized spinels with a specific cationic distribution. The intercalation properties of MgMn₂O₄, Li₂TiO₃ and Li₄Ti₅O₁₂ are first examined in model cells versus the metallic Li anode. The Li⁺ and Mg²⁺ intercalation is directed by the kind of the used electrolyte: a lithium electrolyte consisting of 1 M LiPF₆ solution in EC : DMC and a magnesium electrolyte consisting of 0.5 Mg(TFSI)₂ solution in diglyme. The mechanism of Li⁺ and Mg²⁺ co-intercalation is assessed by ex situ X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM). Finally, a new type of hybrid Li-Mg ion cell combining MgMn₂O₄ and Li₄Ti₅O₁₂ oxides as electrodes is constructed. The cell configuration allows reaching an operating voltage of around 1.7 V by using an electrolyte containing 0.5 M LiTFSI in diglyme.

Two-dimensional Nb2O5 holey nanosheets prepared by a graphene sacrificial template method for high performance Mg2+/Li+ hybrid ion batteries

Mg²⁺/Li⁺ hybrid ion batteries (MLIBs) are regarded as promising candidates for electrical energy devices due to their combination of a fast lithium intercalation cathode and a safe magnesium anode. Herein, two-dimensional Nb₂O₅ holey nanosheets are synthesized using a graphene oxide sacrificial template method and these are then used as the cathode of a MLIB for the first time. It exhibits a high discharge capacity (195.9 mA h g^-1 at 100 mA g^-1), excellent rate performance (72.1 mA h g^-1 at 2000 mA g^-1) and a long cycling life (exhibiting a capacity fading rate of 0.032% per cycle at 2000 mA g^-1 over 1000 cycles). The remarkable electrochemical performance can be attributed to the unique two-dimensional holey nanosheet structure, which is beneficial for improving electronic conductivity and lithium ion conductivity.

Interchain-Expanded Vanadium Tetrasulfide with Fast Kinetics for Rechargeable Magnesium Batteries

Magnesium batteries are promising energy storage systems because of the advantages of low raw material cost, high theoretical capacity, and high operational safety properties. However, the divalent Mg²⁺ has a sluggish kinetic in the cathode materials which resulted in poor electrochemical performance. Many strategies were adopted to improve the mobility of Mg²⁺ in the host structures. In this paper, we report on the optimization of chain-like structure VS₄@reduced graphene oxide (VS₄@rGO) through expanding interchain distance to increase the ion diffusivity. By combining theoretical calculations and experimental investigations, the expansion of interchain distance and reversible intercalation of MgCl⁺ are revealed. With the fast kinetics of MgCl⁺ (instead of Mg²⁺) intercalation into expanded VS₄@rGO, higher capacity of 268.3 mA h g^-1 at 50 mA g^-1 and better rate capability of 85.9 mA h g^-1 at 2000 mA g^-1 have been obtained. In addition, the expanded VS₄@rGO framework shows a high specific capacity of 147.2 mA h g^-1 after 100 cycles and a very wide operating temperature range (-35 to 55 °C). The high discharge capacity, excellent rate capability, and broad temperature adaptability demonstrate promising application of VS₄@rGO in magnesium batteries.

Effect of Ball Milling Process on Electrochemical Properties of Copper Hexacyanoferrate Active Material for Calcium-Ion Batteries

This study investigates the electrochemical properties of ball-milled copper hexacyanoferrate (CuHCF), a Prussian blue analogue, as a cathode material in aqueous calcium-ion batteries (CIBs). X-ray diffraction analysis confirmed that the ball milling process did not destroy the crystal structure of the CuHCF active material. The general grain size and crystal surface of the synthesized CuHCF active materials were confirmed from the scanning electron microscopy (SEM) images. The electrochemical test results revealed that prolonged ball milling improved the charge/discharge capacity in the initial cycle. After 200 cycles, structural collapse of the CuHCF electrode occurred, as observed by SEM.

On the Beneficial Effect of MgCl₂ as Electrolyte Additive to Improve the Electrochemical Performance of Li₄Ti₅O12 as Cathode in Mg Batteries

Magnesium batteries are a promising technology for a new generation of energy storage for portable devices. Attention should be paid to electrolyte and electrode material development in order to develop rechargeable Mg batteries. In this study, we report the use of the spinel lithium titanate or Li₄Ti₅O₁₂ (LTO) as an active electrode for Mg²⁺-ion batteries. The theoretical capacity of LTO is 175 mA h g⁻¹, which is equivalent to an insertion reaction with 1.5 Mg²⁺ ions. The ability to enhance the specific capacity of LTO is of practical importance. We have observed that it is possible to increase the capacity up to 290 mA h g⁻¹ in first discharge, which corresponds to the reaction with 2.5 Mg²⁺ ions. The addition of MgCl₂·6H₂O to the electrolyte solutions significantly improves their electrochemical performance and enables reversible Mg deposition. Ex-situ X-ray diffraction (XRD) patterns reveal little structural changes, while X-ray photoelectron spectrometer (XPS) (XPS) measurements suggest Mg reacts with LTO. The Ti³⁺/Ti⁴⁺ ratio increases with the amount of inserted magnesium. The impedance spectra show the presence of a semicircle at medium-low frequencies, ascribable to Mg²⁺ ion diffusion between the surface film and LTO. Further experimental improvements with exhaustive control of electrodes and electrolytes are necessary to develop the Mg battery with practical application.

High Active Magnesium Trifluoromethanesulfonate-Based Electrolytes for Magnesium-Sulfur Batteries

The shortage of high-performance and easily prepared electrolyte has hindered the progress of rechargeable magnesium-sulfur (Mg-S) batteries. In this paper, we develop a new electrolyte based on Mg(CF₃SO₃)₂-AlCl₃ dissolved in tetrahydrofuran and tetraglyme mixed solvents. Mg(SO₃CF₃)₂ as an Mg²⁺ source is nonnucleophilic, easy to handle, and much cheaper than Mg(TFSI)₂ (TFSI = bis(trifluoromethanesulfonyl)imide). After modification with anthracene (π stabilizing agent) as a coordinating ligand to stabilize the Mg²⁺ ions and MgCl₂ to improve the interface properties by accelerating the reaction of Mg(CF₃SO₃)₂ with AlCl₃, the electrolyte exhibits a low overpotential for overall Mg deposition and dissolution, moderate anodic stability (3.25 V on Pt, 2.5 V on SS, 2.0 V on Cu, and 1.85 V on Al, respectively), and a suitable ionic conductivity (1.88 mS cm^-1). More importantly, this electrolyte modulated by Li-salt additives exhibits good compatibility with S cathode and can be applicable for Mg-S batteries. The rational formulation of the new electrolyte could provide a new avenue for simply prepared Mg electrolytes of Mg-S and rechargeable magnesium batteries.

Microwave-Assisted Synthesis of CuS Hierarchical Nanosheets as the Cathode Material for High-Capacity Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (rMBs) have been recognized as one of most promising next-generation energy storage devices with high energy and power density. However, the development of rMBs has been hampered by the lack of usable cathode materials with high capacity and cycling stability. Herein, we report an ultra-rapid, cost-effective, and scalable synthesis of ultrathin CuS hierarchical nanosheets by a one-step microwave-assisted preparation. Benefiting from the exceptional structural configuration, when used as the cathode material for rMBs at room temperature, the CuS hierarchical nanosheets deliver a high reversible discharge capacity of 300 mA h g^-1 at 20 mA g^-1, remarkable rate capability (256.5 mA h g^-1 at 50 mA g^-1 and 237.5 mA h g^-1 at 100 mA g^-1), and excellent cycling stability (135 mA h g^-1 at 200 mA g^-1 over 200 cycles). To date, the obtained excellent electrochemical performances are superior to most results ever reported for cathode materials of rMBs.

Magnesium-Sodium Hybrid Battery With High Voltage, Capacity and Cyclability

Rechargeable magnesium battery has been widely considered as a potential alternative to current Li-ion technology. However, the lack of appropriate cathode with high-energy density and good sustainability hinders the realization of competitive magnesium cells. Recently, a new concept of hybrid battery coupling metal magnesium anode with a cathode undergoing the electrochemical cycling of a secondary ion has received increased attention. Mg-Na hybrid battery, for example, utilizes the dendritic-free deposition of magnesium at the anode and fast Na⁺-intercalation at the cathode to reversibly store and harvest energy. In the current work, the principles that take the full advantage of metal Mg anode and Na-battery cathode to construct high-performance Mg-Na hybrid battery are described. By rationally applying such design principle, we constructed a Mg-NaCrO₂ hybrid battery using metal Mg anode, NaCrO₂ cathode and a mixture of all-phenyl complex (PhMgCl-AlCl₃, Mg-APC) and sodium carba-closo-dodecaborate (NaCB₁₁H₁₂) as dual-salt electrolyte. The Mg-NaCrO₂ cell delivered an energy density of 183 Wh kg⁻¹ at the voltage of 2.3 V averaged in 50 cycles. We found that the amount of electrolyte can be reduced by using solid MgCl₂ as additional magnesium reservoir while maintaining comparable electrochemical performance. A hypothetical MgCl₂-NaCrO₂ hybrid battery is therefore proposed with energy density estimated to be 215 Wh kg⁻¹ and the output voltage over 2 V.

Lithium- and Magnesium-Storage Mechanisms of Novel Hexagonal NbSe2

As a novel and potential transition metal dichalcogenide (TMDC), NbSe₂ has low ion diffusion barrier when applied in energy-storage systems, such as traditional lithium-ion batteries and novel magnesium-ion batteries (MIBs). In this work, we have developed a novel hexagonal NbSe₂ material with a nanosized surface via a facile microwave-hydrothermal method. The Li⁺-storage mechanism of NbSe₂ with surface conversion and internal intercalation is thoroughly revealed by in situ X-ray diffraction (XRD), ex situ high-resolution transmission electron microscopy, and ex situ scanning electron microscopy. Besides, Mg²⁺ intercalation mechanism is confirmed via ex situ XRD and ex situ X-ray photoelectron spectroscopy for the first time. In addition, as the cathode for MIBs, NbSe₂ with a nanosized surface exhibits a high rate capacity of 101 mA h g^-1 at 200 mA g^-1 with a high discharge plateau at 1.30 V. Our work builds a deep understanding of ion-storage mechanisms in TMDCs and provides guidance for designing new electrode materials with high electrochemical performances.

A Novel Calcium-Ion Battery Based on Dual-Carbon Configuration with High Working Voltage and Long Cycling Life

Rechargeable batteries based on multivalent cations (e.g., Mg2+ and Al3+) have attracted increased interest in recent years because of the merits of natural abundance, low cost, good chemical safety, and larger capacity. Among these batteries, the Ca-ion battery (CIB) shows attractive priority because Ca2+ has the closest reduction potential (-2.87 V vs standard hydrogen electrode (SHE)), to that of Li (-3.04 V vs SHE), enabling a wide voltage window for the full battery. However, most Ca-ion batteries have low working voltage (below 2 V), as well as poor cycling stability (less than 50 cycles). Here, a high-performance Ca-ion full battery with a novel dual-carbon configuration design with low-cost and environmentally friendly mesocarbon microbeads and expanded graphite as the anode and cathode, respectively, is reported. This Ca-ion-based dual-carbon battery (Ca-DCB) can work successfully in conventional carbonate electrolyte dissolving Ca(PF6)2, with a reversible discharge capacity of 66 mAh g-1 at a current rate of 2 C and a high working voltage of 4.6 V. Moreover, the Ca-DCB exhibits good cycling stability with a discharge capacity of 62 mAh g-1 after 300 cycles with a high capacity retention of 94%, which is the best performance of the reported CIBs, suggesting it is a promising candidate for next-generation energy storage devices.

Magnesium/Lithium-Ion Hybrid Battery with High Reversibility by Employing NaV3O8·1.69H2O Nanobelts as a Positive Electrode

Recently, magnesium-ion batteries (MIBs) have been under remarkable research focus owing to their appealingly high energy density and natural abundance of magnesium. Nevertheless, MIBs exhibit a very limited performance because of sluggish solid-state Mg²⁺ ion diffusion and high polarizability, which hinder their progress toward commercialization. Herein, we report a Mg²⁺/Li⁺ hybrid-ion battery (MLIB) with NaV₃O₈·1.69H₂O (NVO) nanobelts synthesized at room temperature working as the positive electrode. In the hybrid-ion system, Li⁺ intercalates/deintercalates along with a small amount of Mg²⁺ adsorption at the NVO cathode, whereas the anode side of the cell is dominated by Mg²⁺ deposition/dissolution. As a result, the MLIB exhibits a much higher rate capability (i.e., 446 mA h g^-1 at 20 mA g^-1) than the previously reported MLIBs. MLIB maintains a high specific capacity of 200 mA h g^-1 at 80 mA g^-1 for 150 cycles, showing excellent stability. Moreover, the effect of different Li-ion concentrations (i.e., 0.5-2.0 M) in the electrolyte and cutoff voltage (ranging from 2 to 2.6 V) on the specific capacities are investigated. The current study highlights a strategy to exploit the Mg²⁺/Li⁺ hybrid electrolyte system with various electrode materials for high-performance MIBs.

Integrating Desalination and Energy Storage using a Saltwater-based Hybrid Sodium-ion Supercapacitor

Ever-increasing freshwater scarcity and energy crisis problems require efficient seawater desalination and energy storage technologies; however, each target is generally considered separately. Herein, a hybrid sodium-ion supercapacitor, involving a carbon-coated nano-NaTi₂ (PO₄ )₃ -based battery anode and an activated-carbon-based capacitive cathode, is developed to combine desalination and energy storage in one device. On charge, the supercapacitor removes salt in a flowing saltwater electrolyte through Cl^- electrochemical adsorption at the cathode and Na⁺ intercalation at the anode. Discharge delivers useful electric energy and regenerates the electrodes. This supercapacitor can be used not only for energy storage with promising electrochemical performance (i.e., high power, high efficiency, and long cycle life), but also as a desalination device with desalination capacity of 146.8 mg g^-1 , much higher than most reported capacitive and battery desalination devices. Finally, we demonstrate renewables to usable electric energy and desalted water through combining commercial photovoltaics and this hybrid supercapacitor.

High-Capacity Mg-Organic Batteries Based on Nanostructured Rhodizonate Salts Activated by Mg-Li Dual-Salt Electrolyte

A magnesium battery is a promising candidate for large-scale transportation and stationary energy storage due to the security, low cost, abundance, and high volumetric energy density of a Mg anode. But there are still some obstacles retarding the wide application of Mg batteries, including poor kinetics of Mg-ion transport in lattices and low theoretical capacity in inorganic frameworks. A Mg-Li dual-salt electrolyte enables kinetic activation by dominant intercalation of Li-ions instead of Mg-ions in cathode lattices without the compromise of a stable Mg anode process. Here we propose a Mg-organic battery based on a renewable rhodizonate salt ( e. g., Na₂C₆O₆) activated by a Mg-Li dual-salt electrolyte. The nanostructured organic system can achieve a high reversible capacity of 350-400 mAh/g due to the existence of high-density carbonyl groups (C═O) as redox sites. Nanocrystalline Na₂C₆O₆ wired by reduced graphene oxide enables a high-rate performance of 200 and 175 mAh/g at 2.5 (5 C) and 5 A/g (10 C), respectively, which also benefits from a high intrinsic diffusion coefficient (10^-12-10^-11 cm²/s) and pesudocapacitance contribution (>60%) of Na₂C₆O₆ for Li-Mg co-intercalation. The suppressed exfoliation of C₆O₆ layers by a firmer non-Li pinning via Na-O-C or Mg-O-C and a dendrite-resistive Mg anode lead to a long-term cycling for at least 600 cycles. Such an extraordinary capacity/rate performance endows the Mg-Na₂C₆O₆ system with high energy and power densities up to 525 Wh/kg and 4490 W/kg (based on active cathode material), respectively, exceeding the level of high-voltage insertion cathodes with typical inorganic structures.

Density functional theory calculations for evaluation of phosphorene as a potential anode material for magnesium batteries

We have systematically investigated black phosphorus and its derivative - a novel 2D nanomaterial, phosphorene - as an anode material for magnesium-ion batteries. We first performed Density Functional Theory (DFT) simulations to calculate the Mg adsorption energy, specific capacity, and diffusion barriers on monolayer phosphorene. Using these results, we evaluated the main trends in binding energy and voltage as a function of Mg concentration. Our studies revealed the following findings: (1) Mg bonds strongly with the phosphorus atoms and exists in the cationic state; (2) Mg diffusion on phosphorene is fast and anisotropic with an energy barrier of only 0.09 eV along the zigzag direction; (3) the theoretical specific capacity is 865 mA h g-1 with an average voltage of 0.833 V (vs. Mg/Mg2+), ideal for use as an anode. Given these results, we conclude that phosphorene is a very promising anode material for Mg-ion batteries. We then expand our simulations to the case of bulk black phosphorus, where we again find favorable binding energies. We also find that bulk black phosphorous must overcome a structural stress of 0.062 eV per atom due to a volumetric expansion of 33% during magnesiation. We found that the decrease in particle size is good to increase its specific capacity.

Low-Cost Room-Temperature Synthesis of NaV3O8·1.69H2O Nanobelts for Mg Batteries

Potentially safe and economically feasible magnesium batteries (MBs) have attracted tremendous research attention as an alternative to high-cost and unsafe lithium ion batteries. In the current work, for the first time, we report a novel room-temperature approach to dope the atomic species sodium between the vanadium oxide crystal lattice to obtain NaV₃O₈·1.69H₂O (NVO) nanobelts. The synthesized NVO nanobelts are used as electrode materials for MBs. The MB cells demonstrate stable discharge specific capacity of 110 mA h g^-1 at a current density of 10 mA g^-1 and a high cyclic stability, that is 80% capacity retention after 100 cycles, at a current density of 50 mA g^-1. Moreover, the effects of cutoff voltages (ranging from 2 to 2.6 V) on their electrochemical performance were investigated. The reason for the limited specific capacity of MBs is attributed to the trapping of Mg ions inside the NVO lattices. This work opens up a new pathway to explore different electrode materials for MBs with improved electrochemical performance.

A novel bismuth-based anode material with a stable alloying process by the space confinement of an in situ conversion reaction for a rechargeable magnesium ion battery

Due to the space confinement of in situ conversion, the BiOF electrode shows superior magnesium storage performance.

Exploration of Spinel LiCrTiO4 as Cathode Material for Rechargeable Mg-Li Hybrid Batteries

Mg-Li hybrid batteries have attracted wide interest in recent years because of their potential safety as well as their cost benefit and high volumetric capacity. However, slow kinetic properties strongly hinder their commercial application. In this study, we have prepared spinel LiCrTiO₄ by a solid-state reaction and have conducted a comprehensive study aimed at improving the performance of Mg-Li hybrid batteries by optimizing the dual-salt electrolyte. LiCrTiO₄ has been found to show reversible discharge/charge capacities of 178 and 169 mA h g^-1 in electrolytes of 1 m LiCl and 0.3 m APC (all-phenyl-complex), respectively. When the concentration of APC was increased to 0.4 m, LiCrTiO₄ showed a high capacity retention of 95 % after 30 cycles. In addition, no phase transition could be observed for an LiCrTiO₄ electrode in a dual-salt system, suggesting high electrochemical reversibility. Ex situ EDX and SEM studies have indicated that only Li⁺ ions are inserted into the cathode side, while Mg²⁺ ions are reversibly deposited on the surface of Mg metal without dendrite-like growth, indicative of good safety of the Mg-Li hybrid batteries.

Prelithiation Activates Fe2(MoO4)3 Cathode for Rechargeable Hybrid Mg2+/Li+ Batteries

The development of rechargeable Mg-based batteries with a high energy density is restricted by the high-voltage cathodes and the parasitic side reactions between the battery components and electrolytes operating at relatively high potentials. Here, we develop a hybrid Mg²⁺/Li⁺ cell using a monoclinic or orthorhombic Fe₂(MoO₄)₃ cathode, a Mg anode, and a simple (PhMgCl)₂-AlCl₃ + LiCl electrolyte. Hastelloy-C alloy is proposed as a current collector of high-voltage cathode for the hybrid Mg²⁺/Li⁺ battery within a Swagelok-type cell. The application of the Hastelloy-C alloy current collector breaks the crucial bottleneck of incompatibility between the currently available current collectors and electrolytes. The hybrid cell features a low voltage polarization between the discharge and charge profiles, which is in favor of practical applications. On the other hand, because all Li⁺ ions are supplied by the electrolyte in a hybrid Mg²⁺/Li⁺ battery, a high Li⁺ concentration is required to operate at high capacities for the hybrid battery. We further show the first-hand evidence about the compensation of Li⁺ ions by simply soaking the cathode in the hybrid electrolyte. The prelithiation of Li⁺ ions into monoclinic Fe₂(MoO₄)₃ significantly enhances the cycling stability and reversibility.