Rechargeable lithium-ion batteries power many of today’s electric vehicles and nearly all portable electronic gadgets and power tools. These devices boast extreme reliability, but they depend on a flammable liquid organic electrolyte solution to shuttle ions between the electrodes, and those liquids pose a tiny but potentially serious fire hazard. So researchers have been searching for nonflammable solid electrolytes as replacements. Although some are nearing commercialization, scientists still do not know some basics, for example, how grain boundaries—interfaces between crystallites of the electrolyte—affect lithium-ion conduction, which controls battery current. So James A. Dawson and M. Saiful Islam of the University of Bath and coworkers carried out molecular dynamics simulations to study how readily lithium ions can hop across grain boundaries in polycrystalline Li3OCl, a promising solid electrolyte candidate. The team stresses that grain boundaries may enhance or impede ion conduction: The effect cannot be determined a priori. It turns out they don’t help matters in Li3OCl. Crystal interfaces lower ion conductivity by about a factor of 10 (J. Am. Chem. Soc. 2018 DOI: 10.1021/jacs.7b10593). The team used the results to develop a model that quantifies the effect of grain boundaries on conductivity as a function of crystallite size and suggests how tailoring the microstructure can optimize the performance of a variety of solid electrolyte materials.