Solid Electrolyte Interphase (SEI) is widely used to describe the new phase formed between the anode and the electrolyte in working batteries. High energy density lithium (Li) metal batteries are severely hampered by dendritic lithium deposition guided by non-uniform SEI. Although it has unique advantages in improving the uniformity of lithium deposition, in practical applications, the effect of anion-derived SEI is not ideal. Recently, Zhang Qiang’s research group from Tsinghua University proposed to use anion receptors to adjust the electrolyte structure to construct a stable anion-derived SEI. The tris(pentafluorophenyl)borane anion receptor (TPFPB) with electron-deficient boron atoms interacts with the bis(fluorosulfonimide) anion (FSI-) to reduce the reduction stability of FSI-. In addition, in the presence of TFPPB, the type of ion clusters (AGG) of FSI- in the electrolyte has changed, and FSI- interacts with more Li+. Therefore, the decomposition of FSI- is promoted to produce Li2S, and the stability of anion-derived SEI is improved.
SEI is composed of the reductive decomposition products of electrolyte. The composition and structure of SEI are mainly controlled by the structure of the electrolyte, that is, the microscopic interaction between the solvent, anion, and Li+. The structure of the electrolyte changes not only with the type of solvent and lithium salt, but also with the concentration of the salt. In recent years, high-concentration electrolyte (HCE) and localized high-concentration electrolyte (LHCE) have shown unique advantages in stabilizing lithium metal anodes by forming a stable SEI. The molar ratio of solvent to lithium salt is low (less than 2) and anions are introduced into the first solvation sheath of Li+, forming contact ion pairs (CIP) and aggregation (AGG) in HCE or LHCE. The composition of SEI is subsequently regulated by anions in HCE and LHCE, which is called anion-derived SEI. Despite its attractive performance in stabilizing lithium metal anodes, current anion-derived SEIs are insufficient in meeting the challenges of practical conditions. Therefore, it is necessary to further improve the stability and uniformity of anion-derived SEI to overcome the challenges under actual conditions.
Anions in the form of CIP and AGG are the main precursors for anion-derived SEI. In general, the electrolyte structure of anions is indirectly regulated by Li+, because the positive charge of solvent and diluent molecules is weakly localized and cannot directly interact with anions. Therefore, new strategies for regulating the structure of anionic electrolytes by directly interacting with anions are highly anticipated.
Post time: Nov-22-2021