Asymmetric Trihalogenated Aromatic Lithium Salt Induced Lithium Halide Rich Interface for Stable Cycling of All-Solid-State Lithium Batteries

Abstract

Solid polymer electrolytes (SPEs) are the key components for all-solid-state lithium metal batteries with high energy density and intrinsic safety. However, the low lithium ion transference number (t + ) of a conventional SPE and its unstable electrolyte/electrode interface cannot guarantee longterm stable operation. Herein, asymmetric trihalogenated a r o m a t i c l i t h i u m s a l t s , i . e . , l i t h i u m ( 3 , 4 , 5trifluorobenzenesulfonyl)(trifluoromethanesulfonyl)imide (LiFFF) and lithium (4-bromo-3,5-difluorobenzenesulfonyl)-(trifluoromethanesulfonyl)imide (LiFBF), are synthesized for polymer electrolytes. They exhibit higher t + values and better compatibility with Li metal than conventional lithium bis-(trifluoromethanesulfonyl) imide (LiTFSI). Due to the trihalogenated aromatic anions, LiFFF-and LiFBF-based electrolytes are prone to generate an LiF-and LiBr-rich solid electrolyte interphase (SEI), therefore increasing the stability of the solid electrolyte/anode interface. Particularly, LiFBF could induce a LiF/LiBr hybrid SEI, where LiF shows a high Young's modulus and high surface energy for homogenizing Li ion flux and LiBr exhibits an extremely low Li ion diffusion barrier in the SEI layer. As a result, the Li/Li symmetric cells could remain stable for more than 1200 h without a short circuit and the LiFePO 4 / Li batteries showed superb electrochemical performance over 1200 cycles at 1 C. This work provides valuable insights from the perspective of lithium salt molecular structures for high-performance all-solid-state lithium metal batteries.

Solid Electrolyte Interphase on Lithium Metal Anodes

“…Kai Liu's team designed asymmetric trihaloaromatic lithium salts for solid polymer electrolytes to promote the formation of SEI film enriched with LiF and LiBr, thereby improving interface stability (Figure 6 b). [120] The incorporation of LiF and LiBr in the SEI film was found to significantly enhance Young's modulus and surface energy, thereby promoting the homogenization of Li + flux. Moreover, the presence of LiBr effectively reduces the diffusion barrier for Li + within the SEI film.…”

Revealing the synergistic effect of LiF and Li3N in solid electrolyte interphases for stable lithium metal batteries using in situ electrochemical atomic force microscopy

“…21) to enhance the mechanical strength, (ii) extremely low electronic conductivity of 10 −31 S cm −1 to reduce SEI thickness, 23 and (iii) high surface energy towards Li metal to suppress the growth of Li dendrites. 24 When used for Li–S batteries, the LiF-rich SEIs can efficiently suppress the polysulfide attack against the Li metal anode. 25–27 Despite these promising merits, the concern is the poor ionic conductivity of the inorganic LiF that will increase the interface impedance at the Li metal anode.…”

In-depth exploration of the effect mechanisms of various lithium salt anions in solid-state and liquid lithium metal batteries

“…7–9 As one of the brightest jewels in LMBs, lithium salts not only provide Li-ions that travel back and forth across the electrodes but also regulate the formation of a solid electrolyte interface at the electrodes, which greatly affects the cycling performance. 10–12 Conventional lithium salts, such as LiPF 6 , show good solubility in carbonate-based electrolytes, high ionic conductivity at room temperature (for example, 8.88 × 10 −3 S cm −1 at 25 °C) 13 and excellent stability with Al 0 current collectors. 14 However, LiPF 6 exhibits poor thermal stability and is prone to hydrolyze to produce hydrogen fluoride (HF), which destroys the electrode/electrolyte interface and thus affects the cycling performance of the battery.…”