Water Domain Enabled Transport and Enhanced Stability in Aqueous Solid Polymer-in-Salt Electrolytes for Lithium-Ion Batteries

Abstract

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. Aqueous-based solid polymer electrolytes (SPEs) offer a safer approach by using water as an abundant, cheap, and non-toxic solvent in an inherently nonflammable polymer matrix. 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 at room temperature and slower Li+ transport compared to anion transport due to slow polymer chain mobility. The use of water as a solvent also lends to restrictive electrochemical stability windows (ESWs) due to its nonideal redox properties that lead to minimal anodic and cathodic limits, dominated by hydrogen evolution at potentials beyond common anode materials (>2 V vs. Li/Li+).To combat these challenges, our work builds upon the concept of super-concentrated salt systems. In the solid-state regime, highly concentrated “polymer-in-salt” systems can exhibit an ion cluster effect allowing for fast cationic transport decoupled from polymer chain segmental motion. In the aqueous regime, highly concentrated “water-in-salt” systems exhibit the formation of a passivating solid electrolyte interphase (SEI) through reduction of TFSI- to form LiF on the anode surface. The presence of water in the “water-in-salt” system also gives rise to a disproportionation of Li+ solvation environments, namely TFSI-rich and water-rich domains, allowing for fast cationic transport through the water domains. By including a highly concentrated “water-in-salt” system into a polymer matrix, the unique aqueous solid polymer-in-salt electrolyte (ASPE) demonstrates preferential Li+ transport compared to anion transport, high ionic conductivity, and extended ESW. This system also exhibits unique stability in air that eliminates the need for meticulously dry environments and solution processing, which is desirable to manufacturers for substantial savings in productions costs.We demonstrate that the ASPE comprised of poly(ethylene oxide) (PEO), water, and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt can achieve a high room temperature ionic conductivity of >1.75 mS/cm. Pulsed field gradient NMR shows a lithium transference number of 0.66, which was corroborated with electrochemical impedance spectroscopy. Molecular dynamic simulations reveal that the exceptional transport properties of the ASPE are likely due to decoupling of ion conduction from the polymer-assisted transport seen in typical SPEs, instead being dominated by water-assisted vehicular transport. The system exhibits stability to ~1.5 V vs. Li/Li+, enabling access to anode materials like LTO and LVPF previously incompatible with aqueous electrolytes. Cycling performance was assessed using LTO/LMO, a ~2.8 V output, to demonstrate the capability of the ASPE to promote intercalation and deintercalation in these electrodes.Further work demonstrates improvement to the ESW of the ASPE by changing the composition of the “water-in-salt” electrolyte to a highly concentrated hybrid aqueous/ionic liquid electrolyte and changing the polymer matrix. The inclusion of ionic liquid as a second, non-aqueous solvating molecule reduces the amount of water needed in the system and provides an alternative pathway for quality SEI formation. High molecular weight polymer offers a solvating matrix to reduce the overall concentration of water while simultaneously reducing water activity through changes in the hydrogen bonding structure. Additionally, the new polymer matrix exhibits superior thermodynamic stability at 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 new class of ASPE demonstrates a remarkable extended cathodic limit <1 V vs. Li/Li+ and overall ESW >4 V with minimal electrochemical activity up to 5.5 V vs. Li/Li+. Cyclic voltammetry demonstrates compatibility with commercial LTO and TNO anodes, which when paired with appropriate high voltage cathode material can enable fast-charging systems with increased energy density.