Design of an Amphiphilic Anion toward High Loading Solid-State Lithium Metal Battery

To improve the performance of all-solid-state lithium metal batteries (ASSLMBs), it is indispensable to work on lithium salt chemistries beyond the well-known lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), which shows a low lithium-ion transference number (T_Li ⁺, ≈0.2) and induces a poor solid-electrolyte interphase (SEI). Herein, the design and synthesis of a new asymmetric lithium salt are reported in which one trifluoromethyl group of LiTFSI is replaced by a dihexylamino group to obtain lithium (trifluoromethanesulfonyl)(N-N-dihexylsulfamoyl)imide {Li[N(SO₂CF₃)(SO₂N(n-C₆H₁₃)₂)], LiC_6,6TFSI}, seeking for a double effect in the electrolyte: 1) to improve cyclability by tuning the transport properties through the reduction of anion mobility; and 2) to ensure compatibility between the polar and non-polar components of the cathode by developing an anion with amphipathic nature. The solid polymer electrolyte (SPE) based on LiC_6,6TFSI and poly(ethylene oxide) (PEO) offers a reduced anion diffusivity leading to high T_Li ⁺ values (≈0.52). Owing to the high T_Li ⁺ and the amphipathic nature of the salt, the as-obtained SPE empowers the Li||LiFePO₄ cells with good capacity retention under stringent working conditions (e.g., a relatively high cathode areal loading of ≈1.8 mAh cm^-2; high current rates of 1 mA cm^-2).