Mg(Al, Fe, Mn, REE)2O4 Spinel Prepared from Pelagic REE-Rich Clays and Application as Magnesium-Ion-Battery Cathodes

The oxidation and lattice distortion of spinel oxides used for magnesium-ion battery (MIB) cathodes lead to poor stability and cycling performance. Herein, the highly inverted spinel oxide Mg(Al, Fe, Mn, REE)2O4 of i = 0.62 with incorporated rare-earth elements (REE) and decent specific surface area was prepared by utilizing leachate of the pelagic rare-earth-rich clays via a foamed sol-gel/calcination method. Measurements of specific capacity, cycling performance, and multiplicity performance showed that the foamed spinel exhibited distinguished electrochemical performance of MIB. At the current density of 100 mA h-1, the initial discharge and charge specific capacity were 125.7 mAh g-1 and 139.7 mAh g-1, and the reversible discharge and charge specific capacities were maintained as 96.7 mAh g-1 and 102.4 mAh g-1 after 200 cycles of charging-discharging. The Mg-ion diffusion rate for MAFMRO was 1.08 × 10-5 cm2 s-1, which was significantly improved, compared to traditional magnesium spinel anodes. This work highlights an approach for modification of spinel-type cathode materials and the high-value utilization of pelagic clay resources.

Organic-Inorganic Coupling Strategy: Clamp Effect to Capture Mg2+ for Aqueous Magnesium Ion Capacitor

The rapid transport kinetics of divalent magnesium ions are crucial for achieving distinguished performance in aqueous magnesium-ion battery-based energy storage capacitors. However, the strong electrostatic interaction between Mg2+ with double charges and the host material significantly restricts Mg2+ diffusivity. In this study, a new composite material, EDA-Mn2O3, with double-energy storage mechanisms comprising an organic phase (ethylenediamine, EDA) and an inorganic phase (manganese sesquioxide) was successfully synthesized via an organic-inorganic coupling strategy. Inorganic-phase Mn2O3 serves as a scaffold structure, enabling the stable and reversible intercalation/deintercalation of magnesium ions. The organic phase EDA adsorbed onto the surface of Mn2O3 as an elastic matrix, works synergistically with Mn2O3, and utilizes bidentate chelating ligands to capture Mg2+. The robust coordination effect of terminal biprotonic amine in EDA enhances the structural diversity and specific capacity characteristics of the composite material, as further corroborated by density functional theory (DFT) calculations, ex situ XRD, XPS, and Raman spectroscopy. As expected, the EDA-Mn2O3 composite achieved an outstanding specific discharge capacity of 188.97 mAh/g at 0.1 A/g. Additionally, an aqueous magnesium ion capacitor with EDA-Mn2O3 serving as the cathode can reach 110.17 Wh/kg, which stands out among the aqueous magnesium ion capacitors that have been reported thus far. The abundant reversible redox sites are ensured by the strategic design concept based on the synergistic structure and composition advantages of organic and inorganic phases. This study aimed to explore the practical application value of organic-inorganic composite electrodes with double-energy storage mechanisms.

Aqueous Multivalent Metal-ion Batteries: Toward 3D-printed Architectures

Energy storage has become increasingly crucial, necessitating alternatives to lithium-ion batteries due to critical supply constraints. Aqueous multivalent metal-ion batteries (AMVIBs) offer significant potential for large-scale energy storage, leveraging the high abundance and environmentally benign nature of elements like zinc, magnesium, calcium, and aluminum in the Earth's crust. However, the slow ion diffusion kinetics and stability issues of cathode materials pose significant technical challenges, raising concerns about the future viability of AMVIB technologies. Recent research has focused on nanoengineering cathodes to address these issues, but practical implementation is limited by low mass-loading. Therefore, developing effective engineering strategies for cathode materials is essential. This review introduces the 3D printing-enabled structural design of cathodes as a transformative strategy for advancing AMVIBs. It begins by summarizing recent developments and common challenges in cathode materials for AMVIBs and then illustrates various 3D-printed cathode structural designs aimed at overcoming the limitations of conventional cathode materials, highlighting pioneering work in this field. Finally, the review discusses the necessary technological advancements in 3D printing processes to further develop advanced 3D-printed AMVIBs. The reader will receive new fresh perspective on multivalent metal-ion batteries and the potential of additive technologies in this field.

Improvements in the Magnesium Ion Transport Properties of Graphene/CNT-Wrapped TiO2 -B Nanoflowers by Nickel Doping

Magnesium-ion batteries are widely studied for its environmentally friendly, low-cost, and high volumetric energy density. In this work, the solvothermal method is used to prepare titanium dioxide bronze (TiO2 -B) nanoflowers with different nickel (Ni) doping concentrations for use in magnesium ion batteries as cathode materials. As Ni doping enhances the electrical conductivity of TiO2 -B and promotes magnesium ion diffusion, the band gap of TiO2 -B host material can be significantly reduced, and as Ni content increases, diffusion contributes more to capacity. According to the electrochemical test, TiO2 -B exhibits excellent electrochemical performance when the Ni element doping content is 2 at% and it is coated with reduced graphene oxide@carbon nanotube (RGO@CNT). At a current density of 100 mA g-1 , NT-2/RGO@CNT discharge specific capacity is as high as 167.5 mAh g-1 , which is 2.36 times of the specific discharge capacity of pure TiO2 -B. It is a very valuable research material for magnesium ion battery cathode materials.

Improvements in the Electrochemical Performance of Sodium Manganese Oxides by Ti Doping for Aqueous Mg-Ion Batteries

In recent times, the research on cathode materials for aqueous rechargeable magnesium ion battery has gained significant attention. The focus is on enhancing high-rate performance and cycle stability, which has become the primary research goal. Manganese oxide and its derived Na-Mn-O system have been considered as one of the most promising electrode materials due to its low cost, non-toxicity and stable spatial structure. This work uses hydrothermal method to prepare titanium gradient doped nano sodium manganese oxides, and uses freeze-drying technology to prepare magnesium ion battery cathode materials with high tap density. At the initial current density of 50 mA g-1 , the NMTO-5 material exhibits a high reversible capacity of 231.0 mAh g-1 , even at a current density of 1000 mA g-1 , there is still 122.1 mAh g-1 . It is worth noting that after 180 cycles of charging and discharging at a gradually increasing current density such as 50-1000 mA g-1 , it can still return to the original level after returning to 50 mA g-1 . Excellent electrochemical performance and capacity stability show that NMTO-5 material is a promising electrode material.

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

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

A High-Rate and Long-Life Aqueous Rechargeable Mg-Ion Battery Based on an Organic Anode Integrating Diimide and Triazine

Aqueous Mg-ion batteries (MIBs) lack reliable anode materials. This study concerns the design and synthesis of a new anode material - a π-conjugate of 3D-poly(3,4,9,10-perylenetracarboxylic diimide-1,3,5-triazine-2,4,6-triamine) [3D-P(PDI-T)] - for aqueous MIBs. The increased aromatic structure inhibits solubility in aqueous electrolytes, enhancing its structural stability. The 3D-P(PDI-T) anode exhibits several notable characteristics, including an extremely high rate capacity of 358 mAh g^-1 at 0.05 A g^-1 , A 3D-P(PDI-T)‖Mg₂ MnO₄ full cell exhibits a reversible capacity of 148 mAh g^-1 and a long cycle life of 5000 cycles at 0.5 A g^-1 . The charge storage mechanism reveals a synergistic interaction of Mg²⁺ and H⁺ cations with C-N/C=O groups. The assembled 3D-P(PDI-T)‖Mg₂ MnO₄ full cell exhibits a capacity retention of around 95 % after 5000 cycles at 0.5 A g^-1 . This 3D-P(PDI-T) anode sustained its framework structure during the charge-discharge cycling of Mg-ion batteries. The reported results provide a strong basis for a cutting-edge molecular engineering technique to afford improved organic materials that facilitate efficient charge-storage behavior of aqueous Mg-ion batteries.

Metal-Organic Framework Composites and Their Derivatives as Efficient Electrodes for Energy Storage Applications: Recent Progress and Future Perspectives

Metal-organic frameworks (MOFs) have been important electrochemical energy storage (EES) materials because of their rich species, large specific surface area, high porosity and rich active sites. Nevertheless, the poor conductivity, low mechanical and electrochemical stability of pristine MOFs have hindered their further applications. Although single component MOF derivatives have higher conductivity, self-aggregation often occurs during preparation. Composite design can overcome the shortcomings of MOFs and derivatives and create synergistic effects, resulting in improved electrochemical properties for EES. In this review, recent applications of MOF composites and derivatives as electrodes in different types of batteries and supercapacitors are critically discussed. The advantages, challenges, and future perspectives of MOF composites and derivatives have been given. This review may guide the development of high-performance MOF composites and derivatives in the field of EES.

First-principles prediction of layered MoO2and MoOSe as promising cathode materials for magnesium ion batteries

Magnesium ion battery is one of the promising next-generation energy storage systems. Nevertheless, lack of appropriate cathode materials to ensure massive storage and efficient migration of Mg cations is a big obstacle for development of Mg-ion batteries. Herein, by means of first principles calculations, the geometric structure, electronic structure, Mg intercalation behavior and Mg diffusion behavior of the layered MoO₂and two MoOSe (MoOSe(I) and MoOSe(V)) were systematically investigated. Layered MoO₂shows semiconductor properties, while MoOSe displays metallic characteristics which ensure higher conductivity. The Mg cations tend to intercalate into octahedral sites for both MoO₂and MoOSe. The maximum Mg-storage phases of the layered MoO₂, MoOSe(I) and MoOSe(V) correspond to Mg_0.666MoO₂, Mg_0.666MoOSe(I) and Mg_0.666MoOSe(V), with theoretical specific capacities of 279, 191 and 191 mAh g^-1, respectively. The calculated discharge plateaus of MoO₂and two MoOSe are all about 1 V, which ensure that the layered MoO₂and MoOSe electrodes can act as cathodes for Mg-ion batteries in the early stage. Moreover, comparing with other cathodes, the diffusion barrier of Mg cations and volume expansion during Mg intercalation process are competitive. The results suggest that layered MoO₂and MoOSe are the promising cathode materials for Mg-ion batteries.

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

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

A rechargeable aqueous proton battery based on a dipyridophenazine anode and an indium hexacyanoferrate cathode

A rocking-chair aqueous proton battery is assembled by using dipyridophenazine and indium hexacyanoferrate as the anode and cathode materials, respectively. The reversible amination of redox-active phenazine moieties in dipyridophenazine and fast intercalation-deintercalation of protons in hexacyanoferrate enable the aqueous proton battery to achieve a reversible specific capacity of 37 mA h g-1 at 1 A g-1, good cycling stability with 76.1% capacity retention over 3000 cycles and excellent rate capability.

Challenges and Strategies for High-Energy Aqueous Electrolyte Rechargeable Batteries

Aqueous rechargeable batteries are becoming increasingly important to the development of renewable energy sources, because they promise to meet cost-efficiency, energy and power demands for stationary applications. Over the past decade, efforts have been devoted to the improvement of electrode materials and their use in combination with highly concentrated aqueous electrolytes. Here the latest ground-breaking advances in using such electrolytes to construct aqueous battery systems efficiently storing electrical energy, i.e., offering improved energy density, cyclability and safety, are highlighted. This Review aims to timely provide a summary of the strategies proposed so far to overcome the still existing hurdles limiting the present aqueous batteries technologies employing concentrated electrolytes. Emphasis is placed on aqueous batteries for lithium and post-lithium chemistries, with potentially improved energy density, resulting from the unique advantages of concentrated electrolytes.

Phase engineering of cobalt hydroxide toward cation intercalation

Multi-cation intercalation in aqueous and neutral media is promising for the development of high-safety energy storage devices. However, developing a new host matrix for reversible cation intercalation as well as understanding the relationship between cation intercalation and the interlayer structure is still a challenge. In this work, we demonstrate layered cobalt hydroxides as a promising host for cation interaction, which exhibit high metal ion (Li+, Na+, K+, Mg2+ and Ca2+) storage capacities after phase transformation. Moreover, it is found that α-Co(OH)2 with an intercalated structure is more conducive to phase transition after electrochemical activation than β-Co(OH)2. As a result, the activated α-Co(OH)2 delivers four times higher capacity in multi-cation storage than activated β-Co(OH)2. Meanwhile, the α-Co(OH)2 after activation also shows an ultralong cycle life with capacity retention of 93.9% after 5000 cycles, which is also much superior to that of β-Co(OH)2 (∼74.8%). Thus, this work displays the relationship between cation intercalation and the interlayer structure of layered materials, which is important for designing multi-ion storage materials in aqueous media.

Surface modification of tin oxide through reduced graphene oxide as a highly efficient cathode material for magnesium-ion batteries

Among post-lithium ion technologies, magnesium-ion batteries (MIBs) are receiving great concern in recent years. However, MIBs are mainly restrained by the lack of cathode materials, which may accommodate the fast diffusion kinetics of Mg²⁺ ions. To overcome this problem, herein we attempt to synthesize a reduced graphene oxide (rGO) encapsulated tin oxide (SnO₂) nanoparticles composites through an electrostatic-interaction-induced-self-assembly approach at low temperature. The surface modification of SnO₂ via carbonaceous coating enhanced the electrical conductivity of final composites. The SnO₂-rGO composites with different weight ratios of rGO and SnO₂ are employed as cathode material in magnesium-ion batteries. Experimental results show that MIB exhibits a maximum specific capacity of 222 mAhg^-1 at the current density of 20 mAg^-1 with a good cycle life (capacity retention of 90%). Unlike Li-ion batteries, no SnO₂ nanoparticles expansion is observed during electrochemical cycling in all-phenyl-complex (APC) magnesium electrolytes, which ultimately improves the capacity retention. Furthermore, ex-situ x-ray diffraction and scanning electron microscopy (SEM) studies are used to understand the magnesiation/de-magnesiation mechanisms. At the end, SnO₂-rGO composites are tested for Mg²⁺/Li⁺ hybrid ion batteries and results reveal a specific capacity of 350 mAhg^-1 at the current density of 20 mAg^-1. However, hybrid ion battery exhibited sharp decay in capacity owing to volume expansion of SnO₂ based cathodes. This work will provide a new insight for synthesis of electrode materials for energy storage devices.

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.

High-Energy Interlayer-Expanded Copper Sulfide Cathode Material in Non-Corrosive Electrolyte for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMB) have been regarded as an alternative to lithium-based batteries because of their abundant elemental resource, high theoretical volumetric capacity, and multi-electron redox reaction without the dendrite formation of magnesium metal anode. However, their development is impeded by their poor electrode/electrolyte compatibility and the strong Coulombic effect of the multivalent Mg²⁺ ions in cathode materials. Herein, copper sulfide material is developed as a high-energy cathode for RMBs with a non-corrosive Mg-ion electrolyte. Given the benefit of its optimized interlayer structure, good compatibility with the electrolyte, and enhanced surface area, the as-prepared copper sulfide cathode exhibits unprecedented electrochemical Mg-ion storage properties, with the highest specific capacity of 477 mAh g^-1 and gravimetric energy density of 415 Wh kg^-1 at 50 mA g^-1 , among the reported cathode materials of metal oxides, metal chalcogenides, and polyanion-type compounds for RMBs. Notably, an impressive long-term cycling performance with a stable capacity of 111 mAh g^-1 at 1 C (560 mA g^-1 ) is achieved over 1000 cycles. The results of the present study offer an avenue for designing high-performance cathode materials for RMBs and other multivalent batteries.

Mechanism of Mg extraction from MgMn2O4 during acid digestion

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

Ultrafast Na intercalation chemistry of Na2Ti3/2Mn1/2(PO4)3 nanodots planted in a carbon matrix as a low cost anode for aqueous sodium-ion batteries

Na2Ti3/2Mn1/2(PO4)3 nanodots uniformly planted in a carbon matrix are reported for the first time as a promising low-cost anode for aqueous sodium-ion batteries, showing ultrafast Na-intercalation chemistry with stable capacities of 78.8 mA h g-1 at 0.5C and 65.1 mA h g-1 at 10C, and a capacity retention of 93% after 200 cycles.

Rapid room-temperature synthesis of ultrasmall cubic Mg–Mn spinel cathode materials for rechargeable Mg-ion batteries

Reducing the particle size of cathode materials is effective to improve the rate capability of Mg-ion batteries. In this study, ultrasmall cubic Mg–Mn spinel oxide nanoparticles approximately 5 nm in size were successfully synthesized via an alcohol reduction process within 30 min at room temperature. Though the particles aggregated to form large secondary particles, the aggregation could be suppressed by covering the particles with graphene. The composite exhibited a specific capacity of 230 mA h g⁻¹, and could be cycled more than 100 times without any large capacity loss even at a moderate current density with the Mg(ClO₄)₂/CH₃CN electrolyte.

An ultrasmall cubic Mg–Mn spinel obtained by an alcohol reduction process is demonstrated as a cathode candidate for rechargeable Mg-ion batteries.

Rapid synthesis of MgCo2O4 and Mg2/3Ni4/3O2 nanocrystals in supercritical fluid for Mg-ion batteries

High-quality spinel MgCo₂O₄ and rocksalt Mg_2/3Ni_4/3O₂ nanocrystal cathodes allow fast extraction/insertion of magnesium ions for high energy-density batteries.

Investigating the Impact of Particle Size on the Performance and Internal Resistance of Aqueous Zinc Ion Batteries with a Manganese Sesquioxide Cathode

Aqueous zinc ion batteries are considered to be one of the most promising battery types for stationary energy storage applications. Due to their aqueous electrolyte, they are inherently safe concerning flammability and environmentally friendly. In this work, the strong influence of the particle size of manganese sesquioxide on the performance of the battery is investigated. Ball milling was used to decrease the particle diameter. The resulting powders were used as active material for the cathodes, which were assembled in coin cells as full cells together with zinc foil anodes and aqueous electrolyte. It was shown that about one third of the original particle size can nearly triple the initial capacity when charged with constant current and constant end-of-charge voltage. Additionally, smaller particles were found to be responsible for the collapse of capacity at high current densities. By means of electrochemical impedance spectroscopy, it was shown that particle size also has a large impact on the internal resistance. Initially, the internal resistance of the cells with small particles was about half that of those with big particles, but became larger during cycling. This reveals accelerated aging processes when the reactive surface of the active material is increased by smaller particles.