The Liquid-Mediated Synthesis and Performance Evaluation of Li-Zr-F Composite for Ion-Conduction

Abstract

Crystalline lithium fluoride (LiF) has been intensively pursued as potential alternative solid electrolytes (SEs) owing to its excellent chemical and electrochemical oxidation stability, and good deformability. However, due to its low ion conductivity, LiF is still challenging for practical SE applications. Herein, Li-Zr-F composite-based SE by liquid-mediated synthesis is proposed to be studied. methanol (CH<sub>3</sub>OH) was mainly evaluated as a liquid-mediated precursor for synthesizing Li-Zr-F composites under the stoichiometric proportion of LiF and ZrF4 (2:1 and 2:0.8) and a subsequent annealing process at 25°C/150°C, 50°C/150°C, and 70°C/150°C, respectively. X-ray diffraction results revealed that the Li-Zr-F composites could be crystallized in the three main types of phase formations, including Li<sub>2</sub>ZrF<sub>6</sub> ( ), Li<sub>2</sub>ZrF<sub>6</sub> ( ), and Li<sub>4</sub>ZrF<sub>8</sub> ( ) octahedron structures. In addition, the effect of cation stack sublattice synthesized by methanol mediator on the ion conduction of Li-Zr-F composites was investigated by using electrochemical impedance spectroscopy (EIS). Through the Zr<sup>4+</sup>-substitution, Li<sub>2</sub>ZrF<sub>6</sub> ( )-based SE exhibited the highest ion conduction which was increased to 2.40 × 10<sup>-8</sup> S/cm and 3.89 × 10<sup>-8</sup> S/cm under the stoichiometric proportion of LiF and ZrF<sub>4</sub> 2:0.8 at a dried temperature of 50°C/150°C with, respectively. A 0.21 eV activation energy ( ) was achieved for a battery with Li<sub>2</sub>ZrF<sub>6</sub> ( )-based SE. Meanwhile, LiF exhibited up to 0.78 eV leading to a low kinetic rate for ion diffusion. These results implied that Li<sub>2</sub>ZrF<sub>6 </sub>( )-based SE was successfully synthesized under the optimal condition of CH<sub>3</sub>OH-50°C/150°C which could improve the ion-conductivity of LiF.