A universal strategy towards high-energy aqueous multivalent-ion batteries

The study addresses the challenge of enabling practical multivalent-ion batteries (MIBs) using abundant cations such as Ca2+, Mg2+, and Al3+, which can deliver multiple electrons per ion for higher specific energy. Conventional non-aqueous MIBs suffer from sluggish multivalent-ion diffusion in metal oxide cathodes due to strong ion–solvent and ion–lattice interactions, irreversible or dendritic metal anodes caused by organic solvent reduction and unstable interphases, and low-voltage stability of ether-based electrolytes that preclude high-voltage cathodes. The research aims to redesign MIB chemistry using high-voltage aqueous batteries with concentrated aqueous gel electrolytes to expand the electrochemical stability window, accelerate ion kinetics via water/proton co-insertion, and employ sulfur anodes to avoid metal plating/stripping and dendrite formation. The hypothesis is that suppressing water activity and forming an inorganic SEI in a concentrated aqueous gel electrolyte will enable reversible sulfur conversion and stable multivalent-ion intercalation in metal oxides, delivering high energy and safety.

Prior work highlights multivalent-ion systems as cost-effective alternatives to Li-ion for grid-scale storage but identifies key obstacles: (1) slow multivalent cation insertion/extraction in metal oxide cathodes within organic electrolytes due to strong electrostatic interactions, causing polarization and poor cycling; (2) conversion reactions complicating cathode behavior; (3) metal anodes that form passivating organic-rich interphases in most organic electrolytes, impeding ion transport, with only a few ether systems allowing reversible plating/stripping but suffering dendrite growth and very low oxidative stability (<0.9 V vs SHE), which excludes high-voltage oxides; (4) alternative anodes (alloys, carbons, polymers) provide limited capacities (<300 mAh g−1). Aqueous Li-ion systems with water-in-salt electrolytes have expanded voltage windows and shown improved safety, and water/proton co-insertion can enhance multivalent-ion kinetics in oxides. However, aqueous multivalent systems with high-voltage metal oxides and sulfur anodes have not been broadly realized. This study builds on concepts of concentrated electrolytes to suppress water activity and form protective interphases while leveraging sulfur’s high theoretical capacity and avoiding metal dendrites.

Electrolyte design and characterization: Prepared Ca(NO3)2 aqueous electrolytes at 1 m, 2 m, 5 m, and saturated 8.37 m; formulated an aqueous gel by dissolving 10 wt% polyvinyl alcohol (PVA, Mw 50,000) into the saturated solution at 80 °C. Evaluated electrochemical stability windows by linear sweep voltammetry (stainless-steel working, Ag/AgCl reference, Pt counter, 1 mV s−1). Measured ionic conductivity at 25 °C (11.56 mS cm−1). Conducted Raman spectroscopy (anion pairing, water O–H stretching), 1H NMR, DFT, and MD simulations to analyze solvation structures, hydrogen bonding, and water activity. MD probed solvation and diffusion of CaS4 polysulfide in 1 m, 8.37 m, and gel electrolytes; quantified coordinated water fraction and hydrogen bonds per water molecule. Anode preparation and analysis: Synthesized porous carbon from Sterculia lychnophora biomass via KHCO3 activation (mix 1:4, calcine 800 °C 5 h in Ar), acid-wash and dry. Prepared sulfur/carbon (S/C) composite by melt-diffusion (S:carbon 4:6, 155 °C, 12 h). Fabricated S/C anodes with PTFE binder (9:1) on Ti mesh (200 mesh, 0.3 mm thick); areal sulfur loading ~1–2 mg cm−2; electrode area ~0.385 cm2. Investigated electrochemistry by CV (vs Ag/AgCl) in gel and organic electrolytes, Raman during cycling, UV–vis for polysulfides, in-depth XPS for products and SEI composition, HAADF-STEM for SEI thickness/morphology, and AFM for SEI mechanical properties. Cathode synthesis and analysis: Prepared MnO2 precursor by precipitating from MnSO4 (aq) to pH 11 with NH4OH, washing and forming electrodes (MnO2:carbon black:PTFE = 8:1:1) on stainless-steel mesh (200 mesh, 0.2 mm thick; 7–9 mg cm−2 loading; area ~0.385 cm2). In situ electrochemical transformation in 8.37 m Ca(NO3)2 (three-electrode glass cell, Ag/AgCl reference, Pt counter) by cycling −0.5 to 1.0 V at 50 mA g−1 for 10 cycles to form layered birnessite MnO2 then Ca0.4MnO2 via Ca2+ intercalation. Characterized by HAADF-STEM/SAED (layer spacing ~6 Å), elemental mapping, synchrotron XRD (phase evolution from spinel Mn3O4 to birnessite MnO2 to Ca0.4MnO2), and Mn 3s XPS (oxidation state changes). Electrochemical testing: Assembled CR2032 full cells: S/C anode | gel electrolyte-saturated GF/A glass fiber separators (2 pieces, 260 μm each) | Ca0.4MnO2 cathode; Ca0.4MnO2:S/C mass ratio ~1.6:1; gel electrolyte mass ~2–3 mg per mg of total electrodes; electrolyte area ~2.5 cm2. Activation at 0.1–2 V, 0.5 C for one cycle, then rate and cycling tests at various C-rates (1 C = 1675 mA g−1 based on sulfur). CV and EIS performed; GITT on electrodes for diffusion analysis. Conducted safety tests on pouch cells (corner cut and water immersion). Extended methodology to Mg2+ and Al3+ systems by preparing analogous gels with saturated Mg(NO3)2 or Al2(SO4)3 plus 10 wt% PVA, in situ generating MxMnO2 (M = Mg or Al), and pairing with S/C anodes.

- Electrolyte stability and structure: The gel electrolyte (8.37 m Ca(NO3)2 + 10 wt% PVA) expanded the electrochemical stability window to ~2.6 V (vs Ag/AgCl), compared to ~1.95 V at 1 m. HER onset shifted from −0.8 V (1 m) to −1.1 V (saturated) and −1.2 V (gel); OER onset increased from 1.15 V (1 m) to 1.4 V (gel). MD showed coordinated water fraction increased from ~10.9% (1 m; 6.2 H2O per Ca2+) to ~63.1% (8.37 m; with ~3 NO3− in Ca2+ solvation), reducing water activity. Hydrogen bonds per water: ~1.35 (1 m), ~1.20 (8.37 m), ~1.25 (gel due to PVA–water interactions). Ionic conductivity was 11.56 mS cm−1 at 25 °C. - Polysulfide suppression and SEI: MD and visualization tests showed greatly suppressed CaS4 diffusion in saturated and especially gel electrolytes; clear phase segregation persisted ≥5 days in gel, indicating negligible dissolution. In gel, a robust amorphous inorganic SEI (~10 nm) formed on S/C, with AFM Young’s modulus ~445 MPa versus ~165 MPa in 1 m electrolyte. DFT indicated Ca2+(NO3)3(H2O)6 aggregates reduce below −0.5 V vs Ag/AgCl (vs −0.84 V for isolated NO3−), favoring NO3−-derived SEI before HER. XPS depth profiling revealed SEI inner layers enriched in CaO and Ca3N2, with CaCO3 mostly in outer layers; Ca–S species present. - Sulfur redox reversibility in gel: CV showed reversible sulfur conversion with cathodic formation of long- then short-chain polysulfides and final CaS/Ca(HS)2; anodic oxidation back to S and intermediates (Raman/XPS/UV–vis corroborated). In organic electrolytes (PC, TEGDME-based), sulfur conversion was largely irreversible. - Cathode performance: Ca0.4MnO2 exhibited reversible Ca2+ intercalation/deintercalation with redox around ~0.2 V vs Ag/AgCl. Delivered 210, 170, 135, 116 mAh g−1 at 10, 50, 100, 200 mA g−1, respectively, with fast kinetics attributed to reversible pre-insertion of protons/water facilitating Ca2+ diffusion. - Full-cell performance (ACSB): S/C | gel | Ca0.4MnO2 achieved specific capacities (based on total mass of S/C + Ca0.4MnO2) of 86, 66, 46, 35 mAh g−1 at 0.1 C, 0.2 C, 0.5 C, 1 C, respectively (corresponding to 560, 431, 302, 228 mAh g−1 based on sulfur). Specific energy reached 110 Wh kg−1 at an average discharge voltage of 1.29 V. Cycling at 0.2 C showed 83% capacity retention after 150 cycles with ~93% average Coulombic efficiency, outperforming saturated electrolyte (65% retention, 86% CE). - Safety: A fully charged pouch cell with gel electrolyte did not short or burn after a corner cut and maintained voltage when immersed in water, demonstrating intrinsic safety. - Generality: Analogous gel electrolytes (PVA + saturated Mg(NO3)2 or Al2(SO4)3) provided stability windows >2.2 V. In situ formed MxMnO2 (M = Mg, Al) cathodes and S/C anodes showed reversible multivalent-ion intercalation and sulfur conversion with stable cycling and low interfacial resistance.

The concentrated aqueous gel electrolyte strategy directly addresses the central limitations of multivalent systems by suppressing water activity, thereby expanding the electrochemical stability window and shifting HER to more negative potentials while improving OER tolerance. The NO3−-rich Ca2+ solvation structure and polymer-induced molecular crowding promote preferential anion reduction, forming a robust inorganic SEI on sulfur that inhibits polysulfide dissolution and side reactions. This enables a reversible sulfur conversion mechanism in water, solving the typical irreversibility observed in organic multivalent systems. On the cathode side, the aqueous medium allows water/proton co-insertion that lowers Ca2+ diffusion barriers in layered MnO2, facilitating room-temperature kinetics previously unattainable in organic electrolytes. Together, these effects deliver a practical aqueous Ca-ion sulfur|metal-oxide full cell with competitive energy (110 Wh kg−1), good rate capability, stable cycling, and excellent safety. The same principles extend to Mg2+ and Al3+ chemistries, suggesting a universal pathway to safe, low-cost, high-energy aqueous multivalent-ion batteries.

This work establishes a universal aqueous multivalent-ion/sulfur battery chemistry combining a concentrated Ca(NO3)2–PVA gel electrolyte, a sulfur/carbon anode, and an in situ formed layered Ca0.4MnO2 cathode. The gel electrolyte’s reduced water activity and NO3−-driven inorganic SEI formation suppress side reactions and polysulfide shuttling, while enabling reversible sulfur conversion. The Ca0.4MnO2 cathode supports stable Ca2+ intercalation with fast kinetics. The resulting aqueous Ca-ion full cell achieves 110 Wh kg−1 specific energy, robust cycling stability, and outstanding safety. The approach generalizes to Mg and Al aqueous systems with similarly extended stability windows and reversible operation. Future work should develop higher-potential/higher-capacity cathodes to more fully utilize the expanded voltage window and design next-generation aqueous electrolytes to improve Coulombic efficiency by stabilizing the SEI and mitigating residual side reactions.

The current system does not fully exploit the expanded electrochemical window; higher-potential, higher-capacity cathodes could raise energy density further. Coulombic efficiency remains limited (~93%), likely due to SEI breakdown/reformation and other side reactions. Further electrolyte optimization to enhance electrode compatibility and SEI stability is needed to improve efficiency and long-term durability.