(Student Battery Slam Best Presentation Award Winner) “Water-in-Salt” Polymer Electrolyte for Li-Ion Batteries

Abstract

Lithium ion batteries (LIBs) have been widely applied in multitudinous energy storage systems due to their high energy density, long cycle stability and high energy efficiency. While the public concern over the safety of LIBs increases rapidly with our reliance on this technology in our daily life, for which the nonaqueous and highly flammable electrolyte should bear most of the responsibility. Aqeuous rechargeable lithium ion batteries resolve several challenges of conventional LIBs. However, the electrochemical stability window of aqueous electrolytes is less than 2.0 V, beyond which the electrolysis of H2O with the hydrogen evolution reaction at anode (H2 generated) and oxygen evolution reaction at cathode (O2 generated). Recently, the ground-breaking of “water-in-salt” electrolyte (WiSE) successfully expanded the ESW of water to 3.0 V. To further enhance the energy density of WiSE based batteries, the cathodic limitation of 1.9 V needs to further reduced, and a high Coulombic efficiency (CE) of > 99.9% at a matched areal capacities of cathode/anode is required for a long-term cycling. Here, we designed a solid-state aqueous polymer electrolyte (SAPE) by incorporating WiSE with UV-curable methacrylic polymer. The abundant hydrophilic groups of the polymer stabilized water molecules due to the sluggish water mobility in SAPE and formed a water-less thin passivation interphase between anode and electrolyte. In addition, by pre-coating a strongly basic water-free solid polymer electrolyte on the surface of anode, an artificial passivation interphase was formed. A robust SEI was formed on the surface of anode and the water reduction reaction is suppressed in our electrolyte system. Also, an extended electrochemical stability window of 3.86 V is achieved at 12 mol kg-1 aqueous polymer electrolyte, enabling the 3 V full cells cycled with an unprecedented high initial CE of 90.5% and average CE of 99.97%. The SAPE exhibits intrinsic safety and tolerance of drastic mechanical abuse, the flexible full cell with supper robustness can be widely used for low-cost and high-safety flexible electronic devices. Moreover, the UV-cured polymerization process possesses the merit of facile and high efficiency, and the whole process have the potential to be practical used in a large scale. Overall, this newly developed reduced salt concentration WiSE not only reduces the cost of aqueous electrolyte, but also opens up another perspective on future directions and guidance for the design of aqueous electrolyte for high-energy-density Li-ion batteries in practical applications.