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

Abstract 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(SO ₂ CF ₃ )(SO ₂ N( n ‐C ₆ H ₁₃ ) ₂ )], LiC _6,6 TFSI}, 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 _6,6 TFSI 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||LiFePO ₄ 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 ).