High-energy density and long-cycle rechargeable batteries are necessary for the pursuit of a low-carbon society. Lithium metal batteries have revived due to their actual specific energy density exceeding 350 Wh kg−1. However, dendritic lithium deposition seriously hinders the practical application of lithium metal batteries, resulting in short battery cycle life and even potential safety hazards. It is generally believed that the dendritic lithium deposition is caused by the heterogeneous non-uniform solid electrolyte interface (SEI) formed on the lithium metal negative electrode. Therefore, building a stable and uniform SEI on the negative electrode of a lithium battery is a necessary condition for the application of lithium batteries.
SEI is composed of 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 solvent, anion, and lithium ion. 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 stable SEI. The molar ratio of solvent to lithium salt is low (<> anion is absorbed into the first solvent sheath of lithium ion, forming contact ion pair (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.
Although its performance in stabilizing lithium metal anodes is attractive, the current anion-derived SEI is insufficient in meeting the challenges of practical conditions, including ultra-thin lithium anodes (<>
Anions in the form of CIP and AGG are the main precursors of anion-derived SEI. Adjust the ratio of CIP (FSI− coordinated to one lithium ion), AGG-I (FSI− coordinated to two lithium ions) and AGG-II (FSI− coordinated to more than two lithium ions) in the electrolyte by salt concentration The types of solvents and diluents have an important influence on the composition and structure of anion-derived SEI.
Under normal circumstances, the electrolyte structure of anions is indirectly regulated by lithium ions, because solvent and diluent molecules have weak positive charge localization and cannot directly interact with anions. Therefore, new strategies for regulating the structure of anionic electrolytes by directly interacting with anions are highly anticipated.
Tsinghua University Zhang Qiang's research group recently reported in the internationally renowned chemical journal Angew An anion acceptor, tris(pentafluorophenyl)borane (TPFPB), proposed through the lack of electrons The atomic center of boron (B) directly regulates the electrolyte structure of the anion.
Through direct interaction with TFPPB, FSI− in the form of AGG-II is significantly increased. In addition, according to the calculation of density functional theory ( DFT ), the reduction stability of FSI− decreases due to the interaction between TFPPB and FSI−.Therefore, FSI-decomposition is promoted to produce Li2S. In Li | LiNi0.5Co0.2Mn0.3O2 (NCM523) batteries that work under actual conditions, TFPPB-regulated anion-derived SEI can cycle 194 cycles under the baseline of 80% capacity retention, while conventional anion-derived SEI is 98 times cycle.
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