The integration of renewable energy sources like solar and wind power into electric grids necessitates reliable large-scale energy storage. Rechargeable multivalent-ion batteries (MIBs) offer a cost-effective alternative to lithium-ion batteries for grid-scale applications. Among multivalent cations, Ca²⁺, Mg²⁺, and Al³⁺ are attractive due to their non-toxicity, stable valence states, small ionic radii, low redox potentials, and abundance. Their ability to transfer multiple electrons per cation also increases specific energy. However, MIB research faces significant challenges. Slow kinetics of multivalent cation insertion/extraction in metal oxide cathodes, commonly used due to their higher potential and capacity compared to other cathode materials, is a major hurdle. Strong electrostatic interactions between multivalent ions and organic solvent molecules and cathode lattices cause sluggish cation diffusion, polarization, and poor cycling stability. Conversion reactions also negatively impact cathode performance. At the anode, multivalent cations struggle to penetrate the passivating interphase layers formed on Ca, Mg, and Al metal anodes due to organic solvent reduction. Only a few organic electrolytes (using flammable ether solvents) prevent these layers, but they suffer from uncontrollable dendrite growth and low oxidation resistance, limiting the use of high-voltage metal oxide cathodes. Alternative anode materials, like alloys, carbonaceous materials, and polymers, provide limited capacity. This necessitates a re-design of MIB chemistry.

Previous research on multivalent-ion batteries has highlighted the limitations of organic-based electrolytes. Studies have shown the slow kinetics of multivalent cation insertion/extraction in metal oxide cathodes due to strong electrostatic interactions. The formation of passivating layers on metal anodes in organic electrolytes also presents a significant challenge. While some progress has been made with alternative anode materials and electrolyte formulations, these approaches often compromise capacity or safety. Research on aqueous lithium-ion batteries has demonstrated the potential of high-concentration aqueous electrolytes for improving performance and safety; however, extending these principles to multivalent-ion systems has remained largely unexplored. This paper builds on these existing findings, seeking to overcome the limitations of current MIB technologies by proposing a novel aqueous system.

This research focuses on developing aqueous multivalent-ion batteries by rationally designing a system with highly concentrated aqueous gel electrolytes, sulfur-containing anodes, and high-voltage metal oxide cathodes. The authors used concentrated aqueous gel electrolytes to replace flammable organic electrolytes, offering low toxicity and non-flammability. These electrolytes significantly expand the voltage window, supporting high-specific energy electrochemical redox couples based on high-voltage metal oxide cathodes. Sulfur was selected as the anode material due to its high theoretical capacity and the potential to avoid irreversible plating/stripping and dendrite growth associated with multivalent metal anodes. The high potential of sulfur in aqueous electrolytes also aids in avoiding the formation of organic-rich interphase layers. In the concentrated aqueous gel electrolyte, a protective inorganic solid electrolyte interphase (SEI) forms on the sulfur anode, allowing for reversible polysulfide conversion. The use of high-voltage metal oxide cathodes provides high output voltages comparable to non-aqueous MIBs, contributing to high specific energy. The study primarily focused on calcium-ion batteries, using a calcium-ion/sulfur battery (ACSB) as a model. This ACSB comprises a sulfur/carbon (S/C) anode, a layered Ca0.4MnO2 cathode, and a gel electrolyte based on 8.37 m Ca(NO3)2 aqueous solution. The electrochemical properties of the electrolytes were investigated using linear sweep voltammetry (LSV), revealing the expanded electrochemical stability window of the gel electrolyte. Molecular dynamics (MD) simulations provided insights into the structure and behavior of the electrolytes at different concentrations. Raman spectroscopy and nuclear magnetic resonance (NMR) were employed to characterize the interactions between ions, water, and polyvinyl alcohol (PVA) in the electrolytes. The conversion mechanism of the sulfur/carbon anode was investigated using cyclic voltammetry (CV), ex situ Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible spectroscopy (UV-vis). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and atomic force microscopy (AFM) were used to analyze the morphology and mechanical properties of the solid-electrolyte interphase (SEI) layer. Density functional theory (DFT) calculations were performed to study the interfacial chemistry. The structure and electrochemical performance of the Ca0.4MnO2 cathode were characterized using HAADF-STEM, selected area electron diffraction (SAED), synchrotron X-ray powder diffraction, XPS, galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS). Finally, the electrochemical performance of the full ACSB cell was evaluated using galvanostatic charge-discharge cycling, CV, and rate capability tests. Safety tests, including water soaking of a pouch cell, were also conducted. The methodology was then extended to magnesium and aluminum-ion battery systems.

The study achieved several key findings: 1. **Electrolyte Development:** Highly concentrated aqueous gel electrolytes, based on Ca(NO3)2 and PVA, were developed. These electrolytes exhibited an expanded electrochemical stability window (2.6 V) compared to dilute aqueous electrolytes. MD simulations revealed that the high salt concentration and PVA significantly reduced water activity, suppressing side reactions and polysulfide dissolution. The gel electrolyte also showed high ionic conductivity. 2. **Sulfur Anode Mechanism:** The sulfur/carbon composite anode exhibited a reversible sulfur conversion chemistry in the aqueous gel electrolyte. Unlike in organic electrolytes where irreversible reactions dominate, a reversible transformation between elemental sulfur and calcium polysulfides was observed. This reversibility was attributed to the formation of a protective inorganic SEI layer, characterized by HAADF-STEM and AFM, which prevented polysulfide dissolution and provided structural integrity. DFT calculations and XPS analysis revealed the composition of this SEI layer, composed primarily of CaO, Ca3N2, and Ca-S species. 3. **Cathode Synthesis and Characterization:** The Ca0.4MnO2 cathode was successfully synthesized via in situ electrochemical transformation from a Mn3O4 precursor. Characterizations using HAADF-STEM, SAED, and synchrotron X-ray diffraction confirmed the successful synthesis and layered structure of the cathode. The Ca0.4MnO2 exhibited fast Ca²⁺ diffusion kinetics, attributed to proton and water co-insertion. 4. **High-Performance Aqueous Calcium-ion Battery:** The assembled aqueous calcium-ion/sulfur full cell (ACSB) delivered a high specific energy of 110 Wh kg⁻¹ based on the total mass of the active materials. It exhibited excellent cycling stability (83% capacity retention after 150 cycles at 0.2 C) and superior safety, even under abusive conditions (water immersion after corner cutting). 5. **Universality of the Approach:** The strategy of using highly concentrated aqueous gel electrolytes and sulfur anodes was demonstrated for magnesium and aluminum-ion batteries, showcasing its potential for broader applications. These batteries also exhibited promising performance and stability.

The findings of this study address the long-standing challenges of developing high-energy, reversible, and safe multivalent-ion batteries. The innovative use of highly concentrated aqueous gel electrolytes, combined with a sulfur-based anode and carefully selected metal oxide cathodes, offers a promising pathway to overcome the limitations of organic-based systems. The exceptional cycling stability and safety demonstrated by the aqueous calcium-ion battery highlight the efficacy of this approach. The expanded electrochemical window enabled by the gel electrolyte is critical for achieving high specific energy and reversibility, while the protective SEI layer effectively mitigates polysulfide dissolution and enhances the overall performance. The successful demonstration of this strategy in magnesium and aluminum-ion batteries signifies the broad applicability of the approach and its potential to revolutionize multivalent-ion battery technology. This work opens new avenues for research in aqueous multivalent-ion batteries, driving the development of low-cost and high-performance energy storage solutions.

This research demonstrates a highly effective strategy for developing high-energy, reversible, and safe aqueous multivalent-ion batteries. The use of concentrated aqueous gel electrolytes, sulfur anodes, and tailored metal oxide cathodes successfully addresses critical limitations in existing multivalent-ion battery technologies. The development of a high-performance aqueous calcium-ion battery with significant energy density and remarkable safety characteristics showcases the potential of this approach. Future research should focus on exploring new cathode materials with even higher capacity and redox potential to further enhance specific energy, optimizing electrolyte formulations for enhanced Coulombic efficiency, and extending this strategy to other multivalent metal ions.

While this study presents significant advancements in aqueous multivalent-ion batteries, certain limitations exist. The Coulombic efficiency, though improved compared to prior work, could be further enhanced. The long-term stability of the SEI layer under extended cycling needs more extensive investigation. Furthermore, the scalability and cost-effectiveness of the manufacturing process for the gel electrolyte and the Ca0.4MnO2 cathode require further study to ensure practical implementation.