Solvent-free protic liquid enabling batteries operation at an ultra-wide temperature range

The study addresses the limitations imposed by solvents in conventional liquid electrolytes for rechargeable batteries, including narrow electrochemical stability windows, limited operating temperature ranges due to boiling/freezing points, volatility, flammability, safety concerns, and dissolution-induced degradation of electrode materials. Aqueous electrolytes, while safer, are constrained by water’s stability window, and water-in-salt (WIS) electrolytes still suffer solvent-related drawbacks. Solid-state electrolytes mitigate solvent issues but face interfacial contact and resistance problems; ionic liquids offer safety but often have sluggish ion transport. The authors propose a solvent-free protic liquid electrolyte—polyphosphoric acid (PPA)—where protons (H+) serve as charge carriers without any molecular solvent. The hypothesis is that excluding solvent will broaden the electrochemical stability window, suppress electrode dissolution, and enable stable operation over an ultra-wide temperature range, while maintaining acceptable ionic transport, thereby enabling robust, safe proton batteries.

The paper reviews conventional non-aqueous electrolytes that use flammable/toxic organic solvents and are limited in temperature tolerance and safety. Aqueous electrolytes are safer yet constrained by water’s 1.23 V stability window, limiting achievable cell voltages and energy density. Water-in-salt (WIS) electrolytes extend the stability window versus dilute aqueous systems but still retain water-related issues such as electrode dissolution, narrow temperature windows, and water decomposition as the ultimate limit. The authors note dissolution-related failures in Li–S batteries due to polysulfide shuttling and transition-metal dissolution from layered oxide cathodes in carbonate electrolytes. Solid-state electrolytes face interfacial resistance, poor wettability, and mechanical mismatch with electrodes, while room-temperature ionic liquids have bulky ions limiting kinetics. These shortcomings motivate exploration of solvent-free electrolytes capable of high stability and broad temperature operation.

- Electrolyte design and characterization: Polyphosphoric acid (PPA; Hn+2PnO3n+1, ≥85% P2O5) is employed as a solvent-free protic liquid. Structure and composition were probed by ATR-FTIR, Raman spectroscopy, 1H NMR, and Karl Fischer titration (water content 426.3 ppm). Viscosity (rotary rheometer) and ionic conductivity (EIS between parallel Ti plates, 0.1 Hz–100 kHz) were measured versus temperature; Arrhenius analysis was performed. DSC (NETZSCH DSC 200 F3 Maia) determined thermal stability/boiling behavior up to 400 °C; flammability was tested with PPA-soaked glass fiber membrane. - Electrochemical stability window: LSV on Ti mesh and on 1 cm2 Pt at 60 °C (0.1 mV s−1) assessed anodic/cathodic limits of 1 M, 50 wt%, 85 wt% H3PO4 and PPA. - Electrode materials: MoO3 (anode) and LiVPO4F (LVPF, cathode). Electrodes fabricated by compressing active materials:acetylene black:PTFE = 7:2:1 into membranes, dried at 120 °C; pressed onto Ti mesh current collectors. - Cell configurations: Three-electrode cells with Ag/AgCl (sat. KCl) reference and excess activated carbon counter; electrolyte volume 12 mL. Full cells (CR2016 coin) used glass fiber separator, 100 µL PPA (~206 mg), MoO3 anode areal loading 1.5 mg cm−2, LVPF cathode 2.25 mg cm−2; N/P = 2/3; 1 C for full cell defined as 93 mA g−1 (based on both electrodes’ active mass). - Electrochemical testing: Galvanostatic charge/discharge (Hukuto Denko HJ, Land BT2000, Neware CT-4008) at various temperatures (freezer/oven; 1 h soak for equilibration). CV/LSV with Autolab PGSTAT302N. EIS on full cells to extract Rct at different temperatures. - Dissolution studies: ICP analysis of electrolytes after 5 days of soaking or cycling at 25 °C and 60 °C measured dissolved Mo (from MoO3) and V (from LVPF). XRD and SEM characterized structural changes pre/post exposure/cycling. DSC of MoO3 mixed with electrolyte assessed reactions/dissolution. XPS tracked Li 1s signals for LVPF states to confirm delithiation and H+ insertion. Ex-situ XRD tracked LVPF → VPO4F → HxVPO4F transformations. - Computational: DFT calculations (Gaussian 09; B3LYP/6-311G(d,p); Mo treated with small-core ECP and cc-pVTZ for valence) estimated sizes of solvated proton clusters (Multiwfn), solvation energies of Mo and V species, and steric feasibility of intercalation into MoO3 and VPO4F channels. Pourbaix diagrams from Materials Project evaluated electrochemical stability regions of MoO3 and VPO4F in aqueous media.

- Solvent-free electrolyte properties: PPA is an anhydrous protic liquid (Karl Fischer: 0.0426 wt% H2O). It shows no endo/exotherms up to 400 °C by DSC, indicating thermal stability and low volatility; it is nonflammable. ATR-FTIR and 1H NMR confirm absence of free water and distinct bonding environment versus aqueous H3PO4. - Transport and temperature dependence: Viscosity decreases from 98.6 Pa·s at 25 °C to 0.37 Pa·s at 140 °C. Ionic conductivity increases from 0.45 mS cm−1 at 25 °C to 63.8 mS cm−1 at 200 °C, following Arrhenius behavior. - Electrochemical window: PPA exhibits a wide stability window >2.5 V. The anodic limit exceeds 2.0 V vs Ag/AgCl on Ti, and >2.5 V vs Ag/AgCl on Pt at 60 °C, surpassing aqueous electrolytes. - Suppressed dissolution (MoO3): ICP after 5 days cycling: dissolved Mo in PPA is 0.45 mg L−1 (25 °C) and 2.13 mg L−1 (60 °C) vs 52.4 and 97.4 mg L−1 in 1 M H3PO4. DSC of MoO3 with 1 M H3PO4 shows endotherm <100 °C (dissolution), absent with PPA. XRD and visuals corroborate stabilization by PPA. - MoO3 anode performance: In PPA at 25 °C and 0.2 A g−1: first discharge 360.9 mAh g−1 (partially reversible), subsequent reversible capacity 198.9 mAh g−1 with 89.4% capacity retention over 200 cycles and CE >99%. At 60 °C: 0.5 A g−1 initial 175 mAh g−1, 142 mAh g−1 after 200 cycles, CE >99.5%. - Suppressed dissolution (LVPF): In 1 M H3PO4, LVPF shows 204 mAh g−1 first charge (over-delithiation via water oxidation), but only 70 mAh g−1 discharge and rapid fading to 8 mAh g−1 after 10 cycles. ICP after cycling in 1 M H3PO4: dissolved V 32.7 mg L−1; in PPA: undetectable. Soaked LVPF in 1 M H3PO4 for 5 days: 3.2 mg L−1 V without cycling; after cycling electrolyte turns blue (V(IV)). - LVPF cathode performance in PPA: At 25 °C shows excellent cycling stability (negligible fade over 1000 cycles; capacity limited by viscosity/conductivity). At 60 °C and 0.5 A g−1: 121 mAh g−1 initial capacity, 82% retention over 200 cycles. XPS confirms complete delithiation (loss of Li 1s after first charge) and exclusively proton insertion thereafter; ex-situ XRD shows reversible VPO4F ↔ HxVPO4F with no reappearance of LVPF peaks. - DFT insights: Solvated proton sizes: H3O+ (3.04 Å) vs H+(H3PO4) (5.59 Å) and H+(H4P2O7) (5.16 Å). Channel sizes: MoO3 3.93 Å; VPO4F 5.37 Å. In aqueous electrolyte, small H3O+ allows water co-intercalation causing internal dissolution; in PPA, bulky proton clusters prevent co-intercalation, requiring desolvation at the interface and limiting dissolution to the surface. - Full-cell MoO3||LVPF performance (rocking-chair proton battery, N/P=2/3; 1 C=93 mA g−1): - Room temperature: ~40 mAh g−1 at 0.5 C with negligible capacity fade over 1000 cycles. - Low temperature: After charging at 60 °C to 1.3 V (78 mAh g−1), discharge capacities: 60 mAh g−1 (25 °C), 49 mAh g−1 (10 °C), 46 mAh g−1 (0 °C). At 0.1 C and 1.5 V cutoff: 26.2 mAh g−1 (0 °C), 10.1 mAh g−1 (−10 °C). - High temperature: Discharge capacities at 0.5 C: 51 (40 °C), 77 (60 °C), 84 (80 °C), 85 (100 °C), 80 (200 °C, 2.9 W g−1), 71 (250 °C, 6.3 W g−1). At 100 °C and 1 C: 84 mAh g−1; rate capability retains 32% capacity from 1 C to 100 C, discharging within 10.3 s, achieving max power density 4975 W kg−1. Stable cycling at 20 °C and at 100 °C over 1000 cycles without capacity fading. - Demonstration: Two beaker cells in series lit an LED under alcohol lamp flame (~500 °C), showing exceptional thermal robustness and safety. - Overall: PPA enables safe, wide-temperature (0–250 °C) operation with broadened stability window (>2.5 V), suppressed electrode dissolution, and high-power performance.

Eliminating molecular solvents by using PPA as a protic liquid fundamentally changes the electrolyte–electrode interactions and stability. The absence of water removes its decomposition limits and prevents solvated water co-intercalation that drives internal dissolution of oxide and polyanion electrodes. The broadened electrochemical window (>2.5 V) enables higher-voltage cathode operation, while the proton-only cation supports fast kinetics, especially at elevated temperatures where PPA’s viscosity drops and conductivity rises. DFT supports the mechanistic picture: large proton clusters in PPA cannot co-intercalate, so protons must desolvate before insertion, limiting dissolution to interfaces and preserving structures (MoO3, VPO4F). Consequently, both electrodes show enhanced cyclability in PPA compared to aqueous acid, and the full cell operates over an ultra-wide temperature range with strong rate performance and safety. These findings directly address the challenges of safety, temperature tolerance, and electrode stability that plague conventional liquid and even some solid-state systems, indicating solvent-free, proton-conducting liquids as a promising direction for robust energy storage under harsh conditions.

The work introduces polyphosphoric acid (PPA) as a solvent-free protic liquid electrolyte that combines advantages of liquids (conformal contact) and solids (safety, nonflammability) while mitigating their respective drawbacks. PPA delivers a wide electrochemical stability window (>2.5 V), thermal stability beyond 400 °C, nonflammability, and acceptable proton conductivity that improves strongly with temperature. It suppresses dissolution of MoO3 and LVPF, enabling a MoO3||LiVPO4F rocking-chair proton battery to operate from 0 to 250 °C, with high power density (up to 4975 W kg−1 at 100 °C), durable cycling (up to 1000 cycles) and robust safety. Future work could focus on reducing low-temperature viscosity to enhance ionic conductivity and capacity at sub-ambient temperatures, exploring compatibility with broader electrode chemistries, optimizing interfacial transport via surface engineering, and scaling to practical cell formats while maintaining ultra-wide temperature performance.

- PPA exhibits high viscosity and relatively low ionic conductivity at room and sub-ambient temperatures, limiting rate capability and capacity at low T (e.g., only 10.1 mAh g−1 at −10 °C at 0.1 C). - At 25 °C, full-cell capacity is modest (~40 mAh g−1 at 0.5 C) compared to performance at elevated temperatures, reflecting transport limitations. - PPA is deliquescent; although cell architecture limits water uptake, long-term environmental exposure risks and moisture management in practical systems require further study. - The study focuses on MoO3 and LVPF; broader electrode compatibility, long-term chemical stability at extreme temperatures, and scale-up in larger formats need validation. - Detailed SEI/interphase chemistry and gas evolution at extremes were not fully explored; optimizing interfacial kinetics at low temperature remains an open challenge.