The escalating demand for energy storage systems, particularly rechargeable batteries with high safety, wide operating temperatures, and extended cycle life, has spurred significant research. Current rechargeable batteries largely utilize liquid electrolytes, which are solutions of salts dissolved in solvents (aqueous or non-aqueous). These solvents, while enabling ionic conductivity and charge compensation, present drawbacks. Non-aqueous electrolytes often contain flammable and toxic solvents, raising safety concerns. The boiling and freezing points of these solvents restrict the operating temperature range. Aqueous electrolytes, while safer, have narrow electrochemical windows that limit operating voltage and energy density. Water-in-salt (WIS) electrolytes, while improving the stability window, still suffer from limitations due to the presence of water, such as electrode dissolution and a restricted temperature range. Furthermore, solvent-based electrolytes can suffer from active material dissolution, leading to capacity fade. Solid-state electrolytes (SSEs), while offering safety benefits, have poor electrode contact, resulting in high resistance, poor wettability, and mechanical instability. Ionic liquids, a compromise between liquid and solid electrolytes, often have bulky ions that hinder ion transport. Therefore, the development of solvent-free electrolytes is crucial for next-generation high-performance batteries. This research explores polyphosphoric acid (PPA) as a potential solution to these challenges.

The authors review existing literature on various electrolyte types for rechargeable batteries, highlighting the limitations of conventional aqueous and non-aqueous electrolytes. They discuss the safety concerns associated with flammable organic solvents and the narrow electrochemical window of aqueous electrolytes, including water-in-salt approaches. The challenges of solid-state electrolytes, such as poor interfacial contact, are also addressed. Existing proton battery technologies are also discussed, focusing on their typical anode materials and low operating temperatures. The review sets the stage for the introduction of PPA as a novel electrolyte and emphasizes the importance of a solvent-free approach to overcoming the limitations of existing technologies.

The study employed several characterization techniques to analyze the physicochemical properties of polyphosphoric acid (PPA) and compare it with aqueous phosphoric acid solutions. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and Raman spectroscopy were used to investigate the structural differences, specifically focusing on hydrogen bonding and P-O and P=O bond vibrations. ¹H nuclear magnetic resonance (NMR) spectroscopy examined proton chemical shifts. Coulometric Karl Fischer titration determined water content. Shear viscosity and ionic conductivity were measured as a function of temperature, with Arrhenius plots used to analyze the temperature dependence of conductivity. Differential scanning calorimetry (DSC) determined the boiling point and thermal stability. A flammability test assessed the safety of PPA. Linear sweep voltammetry (LSV) determined the electrochemical stability window on both titanium and platinum mesh electrodes. Inductively coupled plasma (ICP) spectroscopy quantified the dissolution of metal oxides (Mo and V) from electrodes after soaking or cycling in different electrolytes. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterized the structural changes in electrodes before and after cycling. Density functional theory (DFT) calculations were used to model the dissolution behavior of the electrodes in different electrolytes. Electrochemical tests using three-electrode cells and full cells (MoO3 anode and LiVPO4F cathode) were conducted to evaluate the electrochemical performance of the batteries at various temperatures and current densities, including high rate cycling and low-temperature performance. Electrochemical impedance spectroscopy (EIS) was used to characterize the charge-transfer resistance.

The solvent-free PPA electrolyte exhibits a significantly wider electrochemical stability window (>2.5 V) compared to aqueous electrolytes. This is attributed to the absence of a solvent that could be electrochemically oxidized or reduced. PPA also shows excellent thermal stability, with no phase transitions observed below 400°C and nonflammability, representing a significant safety improvement over conventional electrolytes. In contrast to aqueous H3PO4, PPA significantly suppresses the dissolution of MoO3 and LiVPO4F electrodes, leading to much improved cycling stability. The MoO3 anode in PPA shows highly stable cycling with a high capacity retention of 89.4% over 200 cycles, while in aqueous H3PO4, rapid capacity decay is observed due to electrode dissolution. The LiVPO4F cathode also exhibits exceptional cycling stability in PPA, with negligible capacity loss over 1000 cycles at a low rate (0.2 A g⁻¹). At higher temperatures (60 °C), even with higher current density, LiVPO4F maintains 82% capacity retention over 200 cycles. The full cell (MoO3||LiVPO4F) demonstrates impressive performance across a remarkably wide temperature range of 0–250 °C, maintaining high capacity and power density even at 200 °C (80 mAh g⁻¹, 2.9 W g⁻¹) and 250 °C (71 mAh g⁻¹, 6.3 W g⁻¹). High rate capability was demonstrated at 100°C with 32% capacity retention at 100C (10.3s discharge). The superior performance is linked to the solvent-free nature of PPA, enabling fast proton conduction and preventing electrode degradation through dissolution. DFT calculations show that the larger size of proton clusters in PPA, compared to solvated protons in aqueous solutions, inhibits co-intercalation of water molecules into the electrode structures, effectively preventing dissolution.

The findings demonstrate the successful development and implementation of a solvent-free protic liquid electrolyte based on PPA for high-performance batteries. The exceptional electrochemical stability window, high thermal stability, nonflammability, and ability to suppress electrode dissolution represent significant advancements over existing liquid electrolytes. The ultra-wide operating temperature range, from 0 °C to 250 °C, and the high power density achieved at high temperatures make this technology particularly suitable for applications requiring high-temperature operation, such as fire rescue robots and space exploration. The observed performance is directly linked to the unique properties of the solvent-free PPA electrolyte, which facilitates faster proton transport and enhances electrode stability. This work highlights the potential of designing solvent-free electrolytes to circumvent the limitations of traditional solvent-based systems.

This study successfully demonstrates polyphosphoric acid (PPA) as a high-performing solvent-free protic liquid electrolyte for batteries. PPA's nonflammability, wide electrochemical window, low volatility, and broad operating temperature range (0–250 °C) surpass conventional electrolytes. The MoO3||LiVPO4F full cell exhibits unprecedented performance, especially at high temperatures and high rates. This solvent-free approach offers a promising pathway to develop safe and stable high-performance batteries for high-temperature applications. Future research could explore other solvent-free electrolytes and investigate the long-term stability of PPA under extreme conditions.

While the study demonstrates excellent performance across a wide temperature range, the relatively low ionic conductivity of PPA at room temperature (0.45 mS cm⁻¹) may limit its applicability at lower temperatures. The high viscosity at low temperatures could hinder ion diffusion. Further research is needed to investigate the long-term stability of PPA under prolonged high-temperature operation and to explore potential strategies to further improve its room-temperature ionic conductivity. The study focused primarily on MoO3 and LiVPO4F as electrode materials; investigating compatibility with other electrode materials is necessary.