As lithium-ion batteries (LIBs) continue to grow in use for energy storage applications, concerns over their safety have influenced research for alternative materials, particularly solid-state electrolytes to replace the highly flammable and electrochemically unstable liquid organic electrolytes used in commercial LIBs. Solid polymer electrolytes (SPEs) offer a safer approach due to their inherit nonflammability. This class of electrolyte also offers decreased cost and weight, ease of processibility, and increased long-term chemical and mechanical stability. However, widespread commercial adoption of SPEs for lithium-ion batteries has been hindered by subpar transport properties, namely, ionic conductivities <1 mS/cm and lithium-ion transport numbers <0.5 at room temperature due to slow polymer chain mobility, and reduced oxidative stability that prevents the use of high voltage cathodes (>4 V).

To combat these challenges, our work builds upon the concept of plasticization of polymer networks with strong oxidative stability. Small-molecule “plasticizers” can disrupt inter- and intrachain interactions to reduce the overall crystallinity of the network, impart mobility to polymer chains, and reduce impedance for lithium-ion movement. Furthermore, by using super-concentrated salt systems a “polymer-in-salt” electrolyte can be achieved that exhibits an ion cluster effect allowing for fast cationic transport decoupled from polymer chain segmental motion. This is analogous to the highly concentrated “water-in-salt” systems that give rise to fast cationic transport in water-rich domains. Therefore, by using water as a plasticizer we can create aqueous solid polymer electrolytes (ASPEs) that exhibit preferential Li⁺ transport and high ionic conductivity. These systems also exhibit unique stability in air that eliminates the need for meticulously dry environments and solution processing, which is desirable to manufacturers for substantial savings in production costs.

While the use of water as a plasticizer lends to favorable transport properties, its restrictive electrochemical stability window (ESWs) limits its compatibility with many common electrode couples. The “water-in-salt” system and subsequent work in the field of aqueous electrolytes has demonstrated strategies for reducing the impact of water on stability, such as reducing the activity of “free” water molecules and focusing on the formation of a robust solid electrolyte interphase (SEI). Inclusion of a second, non-aqueous solvating molecule can reduce the amount of water, and thus “free” water, in the system. Furthermore, recent literature also shows that a second, non-aqueous solvent can produce additional SEI species, beyond the typical lithium fluoride, and extend the ESW.

Inspired by these strategies, this work demonstrates how overall performance is influenced by the inclusion of water and non-aqueous plasticizers in ASPEs. Various room-temperature ionic liquids (RTIL) were chosen as the non-aqueous plasticizer because of their high degree of tunability. We demonstrate that ASPEs comprised of polyacrylonitrile (PAN), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), water, and a non-aqueous RTIL plasticizer can achieve a high room-temperature ionic conductivity >2 mS/cm. Spectroscopic techniques are used to determine overall lithium-ion mobility and transference number. Thermogravimetric analysis is used to determine precise compositions and correlations of components to performance, allowing for optimization. Differential scanning calorimetry shows the achievement of low glass transition temperatures and amorphous networks over a large range of temperatures.

We also demonstrate the enhanced stability of these ASPEs via amperometry, including voltammetry. High molecular weight PAN offers a solvating matrix to reduce the overall concentration of water while simultaneously reducing water activity through changes in the hydrogen bonding structure. Additionally, PAN exhibits strong thermodynamic stability and high voltages and aids in decoupling ionic mobility from polymer mobility, making it a strong choice for highly concentrated systems without sacrificing transport properties. This class of ASPE shows a cathodic limit of nearly 0 V vs. Li/Li⁺ and an overall ESW >4 V with minimal electrochemical activity up to 5.5 V vs. Li/Li⁺. Surface characterization techniques are used to determine the SEI species contributing to the enhanced stability. Cyclic voltammetry demonstrates compatibility with LTO and TNO anodes, which when paired with a high voltage cathode material (>4.2 V) can enable increased energy density. Optimized ASPEs are cycled with tailored, composite electrodes designed for better compatibility with solid state electrolytes, including enhanced lithium-ion diffusion and capacity utilization.