Lithium-ion batteries (LIBs) have revolutionized energy storage, but their use is limited by operating temperature and safety concerns. Current research demands batteries functioning across a wider temperature range (below 0°C and above 60°C). High-temperature operation, however, risks thermal runaway due to the inherent instability of electrodes, electrolytes, and separators. Organic liquid electrolytes (LEs), while possessing high ionic conductivity and good interfacial wettability, are highly flammable and volatile, posing significant safety risks at elevated temperatures. Solid-state electrolytes, particularly SPEs, offer improved safety, but their low room-temperature ionic conductivity and poor interfacial contact remain significant challenges. To overcome these limitations, quasi-solid or gel polymer electrolytes (QSPEs or GPEs) incorporating organic solvents or plasticizers are often employed, compromising the inherent safety advantages. This research introduces a new approach: solvent-free LPEs. This concept leverages the flowability of a molten or viscous polymer to act as the sole solvent for lithium salts, aiming to combine the advantages of LEs and SPEs while eliminating their drawbacks. Brush-like polymers, with their reduced chain entanglement, are identified as suitable candidates for this approach due to their low glass transition temperatures and abundance of ion-conducting groups. The design focuses on a polymer with a low glass transition temperature, weak intermolecular forces, and low chain entanglement to facilitate ion transport and good interfacial contact, leading to safer and higher-performing batteries.

The literature extensively covers the challenges and limitations of existing electrolytes for lithium metal batteries. Flammability and volatility of conventional organic liquid electrolytes are well-documented, along with their susceptibility to thermal runaway at high temperatures. Existing research highlights the safety benefits of solid-state electrolytes, particularly solid polymer electrolytes (SPEs). However, these SPEs typically suffer from low ionic conductivity at room temperature and poor interfacial contact with the electrodes, leading to issues like lithium dendrite formation. To address these limitations, researchers have explored the use of quasi-solid or gel polymer electrolytes (QSPEs or GPEs), often incorporating organic solvents or plasticizers to improve ionic conductivity and interfacial contact. However, this addition compromises the safety features of the solid electrolyte. This study builds upon this existing body of research, proposing a new type of electrolyte that combines the advantages of LEs and SPEs, without the associated drawbacks, by using a liquid polymer as a solvent for lithium salts.

This study involved the synthesis of poly[bis-(methoxytriethoxy)phosphazene] (PPZ), a room-temperature liquid-state brush-like polymer, via melt polymerization and subsequent side-group substitution. The molecular structure and properties of PPZ were characterized using NMR, GPC, FTIR, Raman spectroscopy, rheological measurements, DSC, and TGA. Liquid polymer electrolytes (LPEs) were prepared by dissolving lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) into PPZ at varying molar ratios (O:Li⁺). The non-flammability of the LPEs was evaluated through combustion and self-extinguishing time (SET) tests. FTIR and Raman spectroscopy were used to investigate the interactions between LiTFSI and PPZ, determining the degree of LiTFSI dissociation and the coordination environment of Li⁺. Molecular dynamics (MD) simulations were employed to investigate the coordination structures and diffusion coefficients of Li⁺ in the LPEs. Electrochemical impedance spectroscopy (EIS) was used to measure the ionic conductivity of the LPEs at various temperatures, and the activation energy was calculated using the Vogel-Tamman-Fulcher (VTF) equation. The lithium-ion transference number (tLi⁺) was determined by chronoamperometry. Lithium-metal plating/stripping behavior was studied using Li||Li symmetrical cells. LiFePO₄ (LFP) and NCM811 full cells were assembled using glass fiber (GF), cellulose membrane (CM), and polypropylene (PP) separators impregnated with LPEs, and their electrochemical performance was evaluated at various temperatures and current densities. The morphology and chemistry of the electrode-electrolyte interface were investigated using SEM, EDS, and XPS. Accelerating rate calorimetry (ARC) was used to assess the thermal runaway behavior of coin cells. Finally, pouch cells were fabricated to demonstrate the safety and performance of the LPEs under various conditions, including thermal abuse, vacuum, and mechanical abuse.

The synthesized PPZ polymer exhibited a low viscosity at room temperature, enabling its use as a solvent for lithium salts. The resulting LPEs were non-flammable, showing superior fire safety compared to conventional liquid electrolytes. FTIR and Raman spectroscopy confirmed the coordination of Li⁺ with the polar N and O atoms in PPZ, promoting LiTFSI dissociation and facilitating ion transport. MD simulations showed good Li⁺ diffusion in the LPE. The LPEs exhibited high ionic conductivity (1.09 × 10⁻⁴ S cm⁻¹ at 25 °C), significantly higher than that of conventional PEO-based SPEs. Li||Li symmetrical cells with the LPE showed stable cycling for over 2200 h at 90 °C without lithium dendrite growth. LiFePO₄ and NCM811 full cells demonstrated excellent cycling stability (over 1000 cycles) at temperatures ranging from 60 to 120 °C, with high coulombic efficiency and minimal capacity fading. The LPE-based pouch cells demonstrated resistance to thermal abuse, vacuum conditions, and mechanical stress, indicating enhanced safety compared to conventional LEs. Analysis of the electrode-electrolyte interface revealed the formation of a stable, dense SEI layer containing LiF, Li₃N, and Li₃PO₄, contributing to the suppression of lithium dendrite growth. The high adhesion energy between the LPE and the electrodes further contributed to the stability of the interface.

The findings demonstrate the successful development of a high-performance, non-flammable, solvent-free liquid polymer electrolyte for lithium metal batteries. This addresses the key challenges associated with both liquid and solid polymer electrolytes, combining the high ionic conductivity of liquid electrolytes with the enhanced safety of solid electrolytes. The use of a brush-like polymer as the sole solvent for lithium salts, as well as the formation of a stable, high-performance SEI layer, contributes to the superior performance and safety characteristics observed. The wide operating temperature range and long-term cycling stability make this electrolyte a promising candidate for next-generation lithium metal batteries. These findings have significant implications for the development of safer and more efficient energy storage technologies.

This study successfully demonstrates the feasibility of a non-flammable, solvent-free liquid polymer electrolyte for high-performance and safe lithium metal batteries. The unique combination of high ionic conductivity, superior thermal stability, and enhanced safety offers a significant advancement in battery technology. Future research could explore the optimization of the polymer structure for even higher ionic conductivity and broader electrochemical stability windows. Investigating the compatibility of this electrolyte with other cathode materials and exploring its scalability for industrial applications are also important next steps.

While the study demonstrates significant improvements in battery performance and safety, some limitations exist. The synthesis of the PPZ polymer involves multiple steps, potentially increasing the cost and complexity of production. The relatively high viscosity of the LPE, though manageable with solvent-assisted loading, could present challenges for high-throughput manufacturing processes. Further optimization of the electrolyte composition and the manufacturing process will be crucial for broader practical applications.