Improved ionic conductivity and battery function in a lithium iodide solid electrolyte via particle size modification

Solid electrolytes (SEs) are a promising, safe alternative to liquid electrolytes in high energy density batteries, but low conductivity issues remain a challenge. Herein, a lithium iodide based solid electrolyte (SE) was improved by reducing particle size via several processing methods, i.e. grinding and/or sonication. Particle size was reduced from 5 ± 1 µm to 2.0 ± 0.2 µm, resulting in an order of magnitude increase in ionic conductivity, from 7.7×10⁻⁸ to 6.1×10⁻⁷ 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, 1:2 lithium iodide: 3-hydroxypropionitrile (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. Furthermore, a more stable electrochemical response was achieved when cycled at higher current densities.