Anion π-π Stacking for Improved Lithium Transport in Polymer Electrolytes.

Polymer electrolytes (PEs) with excellent flexibility, processability, and good contact with lithium metal (Li°) anodes have attracted substantial attention in both academic and industrial settings. However, conventional poly(ethylene oxide) (PEO)-based PEs suffer from a low lithium-ion transference number (TLi+), leading to a notorious concentration gradient and internal cell polarization. Here, we report two kinds of highly lithium-ion conductive and solvent-free PEs using the benzene-based lithium salts, lithium (benzenesulfonyl)(trifluoromethanesulfonyl)imide (LiBTFSI) and lithium (2,4,6-triisopropylbenzenesulfonyl)(trifluoromethanesulfonyl)imide (LiTPBTFSI), which show significantly improved TLi+ and selective lithium-ion conductivity. Using molecular dynamics simulations, we pinpoint the strong π-π stacking interaction between pairs of benzene-based anions as the cause of this improvement. In addition, we show that Li°âˆ¥Li° and Li°âˆ¥LiFePO4 cells with the LiBTFSI/PEO electrolytes present enhanced cycling performance. By considering π-π stacking interactions as a new molecular-level design route of salts for electrolyte, this work provides an efficient and facile novel strategy for attaining highly selective lithium-ion conductive PEs.

Stable non-corrosive sulfonimide salt for 4-V-class lithium metal batteries.

Rechargeable lithium metal (Li0) batteries (RLMBs) are considered attractive for improving Li-ion batteries. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) has been extensively used as a conducting salt for RLMBs due to its advantageous stability and innocuity. However, LiTFSI-based electrolytes are corrosive towards aluminium (Al0) current collectors at low potentials (>3.8 V versus Li/Li+), thereby excluding their application in 4-V-class RLMBs. Herein, we report on a non-corrosive sulfonimide salt, lithium (difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (LiDFTFSI), that remarkably suppresses the anodic dissolution of the Al0 current collector at high potentials (>4.2 V versus Li/Li+) and significantly improves the cycling performance of Li(Ni1/3Mn1/3Co1/3)O2 (NMC111) cells. In addition, this sulfonimide salt results in the growth of an advantageous solid electrolyte interphase on the Li0 electrode. The replacement of either LiTFSI or LiPF6 with LiDFTFSI endows a Li0||NMC111 cell with superior cycling stability and capacity retention (87% at cycle 200), demonstrating the decisive role of the salt anion in dictating the electrochemical performance of RLMBs.