In-depth exploration of the effect mechanisms of various lithium salt anions in solid-state and liquid lithium metal batteries

Abstract

Lithium salts exert a great influence on the electrochemical performance of lithium metal batteries. Dissociated Li-ions present rapid transfer dynamics through solvation with a solvent in liquid batteries or complexation–decomplexation with a polymer in solid batteries. However, the effect of the anion on the properties of batteries is still unclear. Herein, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium difluoro(oxalato)borate (LiDFOB) are selected to investigate the working mechanism of anions in solid and liquid systems. LiTFSI demonstrates superior electrochemical performance in solid batteries (stable cycling for 2000 h at 0.1 mA cm⁻²), while LiDFOB exhibits better cycling performance in liquid batteries (the capacity retention reaches 94.8% after 500 cycles at 1C). DFT calculations are conducted to investigate the discrepancy and suggest that TFSI⁻ presents higher binding energy with PEO, indirectly proving the important role of the anion in facilitating the transfer of Li-ions in solid systems. As for liquid batteries, XPS and in situ optical microscope analyses show that DFOB⁻ is conducive to the generation of robust interfaces and inhibits the continuous decomposition of the electrolyte. Therefore, it can be innovatively deduced that the properties of solid batteries are determined by Li-ion transfer adjusted by the anion, while the liquid battery performance is decided by the electrode/electrolyte interface stability regulated by the anion.