Cross-Linked Ionomer Gel Separators for Polysulfide Shuttle Mitigation in Magnesium–Sulfur Batteries: Elucidation of Structure–Property Relationships

Next-generation magnesium-ion batteries: The quasi-solid-state approach to multivalent metal ion storage

Mg-ion batteries offer a safe, low-cost, and high-energy density alternative to current Li-ion batteries. However, nonaqueous Mg-ion batteries struggle with poor ionic conductivity, while aqueous batteries face a narrow electrochemical window. Our group previously developed a water-in-salt battery with an operating voltage above 2 V yet still lower than its nonaqueous counterpart because of the dominance of proton over Mg-ion insertion in the cathode. We designed a quasi-solid-state magnesium-ion battery (QSMB) that confines the hydrogen bond network for true multivalent metal ion storage. The QSMB demonstrates an energy density of 264 W·hour kg-1, nearly five times higher than aqueous Mg-ion batteries and a voltage plateau (2.6 to 2.0 V), outperforming other Mg-ion batteries. In addition, it retains 90% of its capacity after 900 cycles at subzero temperatures (-22°C). The QSMB leverages the advantages of aqueous and nonaqueous systems, offering an innovative approach to designing high-performing Mg-ion batteries and other multivalent metal ion batteries.

Negative sulfur-based electrodes and their application in battery cells: Dual-ion batteries as an example

In this work, a cell concept comprising of an anion intercalating graphite-based positive electrode (cathode) and an elemental sulfur-based negative electrode (anode) is presented as a transition metal- and in a specific concept even Li-free cell setup using a Li-ion containing electrolyte or a Mg-ion containing electrolyte. The cell achieves discharge capacities of up to 37 mAh g−1 and average discharge cell voltages of up to 1.9 V. With this setup, more than 100 cycles with a high capacity retention (> 90% of the highest achieved value) and Coulombic efficiencies up to 95% could be achieved, which opens a broad new field for energy storage approaches.

Dual Role of Mo6 S8 in Polysulfide Conversion and Shuttle for Mg-S Batteries

Magnesium-Sulfur batteries are one of most appealing options among the post-lithium battery systems due to its potentially high energy density, safe and sustainable electrode materials. The major practical challenges are originated from the soluble magnesium polysulfide intermediates and their shuttling between the electrodes, which cause high overpotentials, low sulfur utilization, and poor Coulombic efficiency. Herein, a functional Mo₆ S₈ modified separator is designed to effectively address these issues. Both the experimental results and density functional theory calculations show that the electrochemically active Mo₆ S₈ layer has a superior adsorption capability of polysulfides and simultaneously acts as a mediator to accelerate the polysulfide conversion kinetics. Remarkably, the magnesium-sulfur cell assembled with the functional separator delivers a high specific energy density (942.9 mA h g^-1 in the 1st cycle) and can be cycled at 0.2 C for 200 cycles with a Coulombic efficiency of 96%. This work demonstrates a new design concept toward high-performance metal-sulfur batteries.

Chemistry-performance relationships of polymer gel-electrolytes for Mg-S and Li-S batteries: Influence of network cation solvation capacity on polymer-polysulfide interactions

Metal-sulfur batteries are a promising next-generation energy storage technology, offering high theoretical energy densities with low cost and good sustainability. An active area of research is the development of electrolytes that address unwanted migration of sulfur and intermediate species known as polysulfides during operation of metal-sulfur batteries, a phenomenon that leads to low energy efficiency and short life-spans. A particular class of electrolytes, gel polymer electrolytes, are especially attractive for their ability to repel polysulfides on the basis of structure, electrostatics, and other polymer properties. Here, within the context of magnesium- and lithium-sulfur batteries, we investigate the impact of gel polymer electrolyte cation solvation capacity, a property related to network dielectric constant and chemistry, on sulfur/polysulfide-polymer interactions, an understudied property-performance relationship. Polymers with lower cation solvation capacity are found to permanently absorb less polysulfide active material, which increases sulfur utilization for Li-S batteries and significantly increases charge efficiency and life-span for Li-S and Mg-S batteries.

Polymer Electrolytes - New Opportunities for the Development of Multivalent Ion Batteries

Batteries, as highly concerned energy conversion system, have a great development prospect in various fields, especially in the field of energy powered vehicles. Multivalent ion batteries are getting more attention due to their low cost, high abundance in earth crust, high capacity and safety compared with Lithium batteries. Despite above advantages, several problems still need to be solved before multivalent ion batteries achieve large-scale application, such as interfacial parasitic reaction, anode passivation, and dendrites. The replacement of liquid electrolytes with gel polymer electrolytes (GPEs) which pose high safety, high mechanical strength and simplified battery system, is an effective strategy to inhibit dendrite growth and improve electrochemical performance. This review mainly discusses the advantages and challenges of multivalent ion batteries including zinc, magnesium, calcium and aluminum batteries. Meanwhile, the major targets of this review are introducing the recent developments and making a summary of the future trends of GPEs in the multivalent ion batteries.

Current Design Strategies for Rechargeable Magnesium-Based Batteries

As a next-generation electrochemical energy storage technology, rechargeable magnesium (Mg)-based batteries have attracted wide attention because they possess a high volumetric energy density, low safety concern, and abundant sources in the earth's crust. While a few reviews have summarized and discussed the advances in both cathode and anode materials, a comprehensive and profound review focusing on the material design strategies that are both representative of and peculiar to the performance improvement of rechargeable Mg-based batteries is rare. In this mini-review, all nine of the material design strategies and approaches to improve Mg-ion storage properties of cathode materials have been comprehensively examined from both internal and external aspects. Material design concepts are especially highlighted, focusing on designing "soft" anion-based materials, intercalating solvated or complex ions, expanding the interlayer of layered cathode materials, doping heteroatoms into crystal lattice, size tailoring, designing metastable-phase materials, and developing organic materials. To achieve a better anode, strategies based on the artificial interlayer design, efficient electrolyte screening, and alternative anodes exploration are also accumulated and analyzed. The strategy advances toward Mg-S and Mg-Se batteries are summarized. The advantages and disadvantages of all-collected material design strategies and approaches are critically discussed from practical application perspectives. This mini-review is expected to provide a clear research clue on how to rationally improve the reliability and feasibility of rechargeable Mg-based batteries and give some insights for the future research of Mg-based batteries as well as other multivalent-ion battery chemistries.

Mg Anode Passivation Caused by the Reaction of Dissolved Sulfur in Mg-S Batteries

As Li-ion battery optimization approaches theoretical limits, interest has grown in designing next-generation batteries from low-cost earth-abundant materials. Mg-S batteries are promising candidates, exhibiting widespread abundance of elemental precursors and a relatively large theoretical energy density albeit at lower cell voltage. However, Mg-S batteries exhibit poor reversibility, in part due to interactions between dissolved polysulfides and the Mg anode. Herein, we employ electrochemical experiments using Ag₂S quasi-reference electrodes to probe the interactions between Mg anodes and dissolved polysulfides. We show that Mg²⁺ reduction (charging) is impeded in the presence of polysulfides, while Mg metal oxidation (discharging) remains facile. Large reduction overpotentials arise due to the formation of a passivation layer on the anode surface, likely composed primarily of MgS. The passivation layer is removed under oxidative conditions but quickly reforms during reduction. We discover that dissolved S₈ influences the rate of MgS formation by shifting the polysulfide disproportionation equilibria. Shorter-chain polysulfides react more readily than longer-chain polysulfides at the Mg electrode, and thus, film formation is mediated by the electrochemical generation of shorter-chain polysulfide species.

Critical Role of Ion Exchange Conditions on the Properties of Network Ionic Polymers

Ionic polymers are important in a wide range of applications and can exhibit widely different properties depending on the ionic species. In the case of single ion conducting polymers, where one charge is attached to the backbone or as a side group, ion exchange is performed to control the mobile species. While the conditions are often specified, the final ion content is not always quantified, and there are no clear criteria for what concentration of salt is needed in the exchange. A series of ammonium network ionic polymers with different precise carbon spacers (C4-C7) between ionic junctions were synthesized as model systems to understand how the ion exchange conditions impact the resultant polymer properties. The initial networks with free bromide anions were exchanged with 1.5, 3, or 10 equiv of lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt in solution. For networks with seven carbons between cross-links, increasing the LiTFSI concentration led to an increase in ion exchange efficiency from 83.6 to 97.6 mol %. At the highest conversion, the C7 network showed a 4 °C decrease in glass transition temperature (T_g), a 50 °C increase in degradation temperature, 12-fold lower water uptake from air, and a greater than 10-fold increase in conductivity at 90 °C. These results illustrate that properties such as T_g are less sensitive to residual ion impurities, whereas the conductivity is highly dependent on the final exchange conversion.

Effect of Network Architecture and Linker Polarity on Ion Aggregation and Conductivity in Precise Polymerized Ionic Liquids

Four polymerized ionic liquids (PILs) were systematically designed to study the effect of polymer architecture and linker polarity on ion aggregation and transport. Specifically, linear and network PILs with the same ammonium cations (Am) and bis(trifluoromethane)sulfonimide (TFSI) anions were prepared by step-growth polymerization, and polarity was tuned by incorporating two precise linkers, either polar tetra(ethylene oxide) (4EO) linker or nonpolar undecyl (C11) linker. The glass transition temperature (T_g) substantially increased with the nonpolar C11 linker or upon cross-linking to form a network. The low wave-vector (q) ion aggregation peak from wide-angle X-ray scattering (WAXS) was not observable in the linear 4EO PIL, while it was most pronounced in the network C11 PIL. The network C11 PIL exhibited the strongest decoupling, where the ionic conductivity at T_g is greater than 1 order of magnitude higher than the other PILs. This systematic comparison suggests that network structure and nonpolar linkers can promote both ion aggregation and ionic conductivity close to T_g.