Water-assisted increase of ionic conductivity of lithium poly(acrylic acid)-based aqueous polymer electrolyte

We propose a novel aqueous polymer electrolyte (APE) using a strongly hydrophilic poly(acrylic acid) (PAA) matrix containing mobile lithium counterions. The conductivity of this new PAA−Li⁺−water electrolyte increases dramatically (to 10⁻² S/cm at 298 K) with the addition of water. This value is almost 100 times higher than those of nonaqueous electrolytes and solid-state electrolytes. From the molecular dynamics simulations, we find that the increase of ion conductivity originates from the close interplay between ions, water, and the polymers in the molecule level. The structural features (i.e., ion/water distribution around the polymer) and transport properties (i.e., diffusion coefficient and ionic conductivity) are systematically investigated along with the quantifications of the microscopic properties such as the binding index of the ion, hydration numbers, and the equilibrium distance between the ion and PAA monomer at various water-content conditions. In particular, the change in the conductivity according to water content, ϕ_Wt, is divided into the diffusion-dominant regime at the low-water-content condition (ϕ_Wt < 0.7) and the structure-dominant regime at the high-water-content condition (ϕ_Wt ≥ 0.7). In the diffusion-dominant regime, the conductivity increases by diffusion enhancement proportional to the water content, while in the structure-dominant regime, the conductivity varies little due to the considerable reduction of the number density of Li ions. Namely, there exists an optimal water content, above which the effects of additional water become negligible. We believe that our innovative findings would provide significant advances in developing APE-based high-power and long-life lithium-ion batteries. Also, the proposed nontoxic and flexible APE could offer a promising solution for the development of flexible and wearable aqueous rechargeable lithium-ion batteries.