Enabling safe and stable Li metal batteries with protic ionic liquid electrolytes and high voltage cathodes

Here, we present first examples of lithium metal cells stable and safely operating with PYRH4+(TFSI−/FSI−)-based protic ionic liquid (PIL) electrolytes, which is accomplished by encompassing vinylene carbonate (VC) in the PIL-salt solution. VC not only enhances the stability window of PIL electrolytes; it also undergoes electrochemical decomposition during initial cycling, thus creating a protective barrier at the electrolyte/electrode interface. The protective film prevents degradation at the Li metal anode due to hydrogen release, as well as at the cathode side at anodic potential. Materials and related devices are investigated in terms of their main physico-chemical characteristics, ionic conductivity, compatibility with the Li metal electrode, and electrochemical behavior by impedance spectroscopy, cyclic voltammetry and galvanostatic cycling. Newly designed electrolyte formulations enable direct cycling of Li-metal cells with PILs to achieve excellent stability with both standard LFP and 4-V class NMC-based cathodes, almost full capacity (≥160 mAh g−1) and highly reversible operation at ambient temperature and different current rates up to 1C. The PIL-VC based cell outperforms the corresponding bare PIL electrolyte as well as the aprotic PYR14TFSI based cells, thus enlightening a feasible strategy to suppress the high reactivity of PILs towards alkali metals; along with the use of appropriate materials, this may turn high energy density, low-cost PIL-based Li-metal batteries into industrial reality in the coming years.