Lithium fluorinated sulfonimide-based solid polymer electrolytes for Li || LiFePO4 cell: The impact of anionic structure

Abstract

Solid polymer electrolytes (SPEs), comprising lithium fluorinated sulfonimide including Li[(FSO2)(RFSO2)N] (RF = n-CmF2m+1, m = 0 (LiFSI), 1 (LiFTFSI), 2 (LiFPFSI), and 4 (LiFNFSI)) and Li[(CF3SO2)2N] (LiTFSI) as conducting salt and poly(ethylene oxide) (PEO) as polymer matrix, are utilized for investigating the impact of anionic structure of lithium salt on the performances of rechargeable lithium metal (Li) batteries (RLMBs), through comparing their fundamental physical properties, including ionic conductivity and anodic stability, and electrochemical performances for Li || Li and Li || LiFePO4 cells. It is found that the cycling stabilities of both the Li anode and LiFePO4 cathode are highly dependent on the structure of fluorinated imide anion. The cycling stabilities for both the Li || Li and Li || LiFePO4 cells with Li[(FSO2)(RFSO2)N] systematically outperfom those with LiTFSI at 0.2 mA cm−2 and 80 °C (i.e., being increased in the order of LiTFSI < LiFTFSI, LiFPFSI < LiFSI < LiFNFSI for Li || Li cells, and in the order of LiTFSI < LiFSI, LiFTFSI < LiFPFSI < LiFNFSI for Li || LiFePO4 cells). Specifically, LiFNFSI exhibits the best compatibility toward Li anode and the strongest anodic stability, thus making the Li || LiFePO4 cell display the outstanding cycling stability (e.g., 80.8% capacity retention after 570 cycles at 1.0C and 80 °C). All the above results suggest that the structure of sulfonimide anion of lithium salt tailored for SPE-based RLMBs should have both the ability to form robust SEI films on the Li anode and strong anodic stability against the oxidation on the cathode.