Improved Ionic Conductivity and Battery Function in a Lithium Iodide Solid Electrolyte Via Particle Size Modification

Abstract

Solid electrolytes (SEs) are a promising, safe alternative to liquid electrolytes in high energy density batteries, but challenges, including low ionic conductivity and dendrite formation, remain. In a recent work, it was determined that dendrite formation within representative SEs, Li7La3Zr2O12 and Li3PS4, is due to their non-negligible electronic conductivity.[1] Notably, LiI has negligible electronic conductivity and thus the formation of lithium dendrites may be lessened, making LiI a good candidate for SE development. Our previous work on LiI SE, utilized 1:2 lithium iodide:3-hydroxypropionitrile (LiI(HPN)2) as a conductive additive, creating an 80/20 LiI/LiI(HPN)2 composite as a self-forming, rechargeable battery.[3] Additional work has demonstrated interfacial modification can effectively lower the cell impedance and improve coulombic efficiency (CE).[2, 4]In this work, a LiI SE was improved by reducing particle size via several processing methods; grinding (G), sonicating (C-S), and grinding with sonicating (G-S), and compared to a control (C) sample of LiI which underwent no processing. Partial hydration of LiI results in increased conductivity via increased defect density, and thus this parameter was controlled to ca. 34 mol% LiI monohydrate between samples. With further processing, particle size was reduced from 5 ± 1 µm to 2.0 ± 0.2 µm for the C and G-S samples. Utilized in 80/20 LiI/LiI(HPN)2 composites, reducing particle size resulted in an order of magnitude increase in ionic conductivity, from 7.7 x 10-8 to 6.1 x 10-7 S cm-1, at room temperature. Improved conductivity is attributed to an increased number of grain boundaries and defects, enabling ion transport and better mixing with the electrolyte additive, LiI(HPN)2. 3D confocal Raman spectroscopy in conjunction with non-negative matrix factorization (NMF) analysis determined the degree of HPN aggregation was lessened in the sample with smallest particle size. This LiI SE was utilized in a self-forming Li/I2 battery, where reduced particle size (improved conductivity) led to significantly reduced overpotential, allowing the coulombic efficiency to reach 100% in the first cycle. The G-S cells also exhibited improved electrochemical function when cycling at higher rates compared with the other electrolyte types. This work demonstrates the effect of particle size on solid electrolyte in a self-forming, self-healing, all solid state battery with a lithium metal negative electrode.