Safety remains a lingering issue with rechargeable lithium batteries due to flammability hazards associated with traditional liquid organic electrolytes. ln recent years, sulfide solid electrolytes (SSEs) have been shown to reduce this hazard while meeting the high bar for electrochemical performance. However, current production processes for SSEs involve costly heating and mixing steps that do not fully dissolve the precursors, yielding undesirable large-particle powders. The large size of the particles reduces contact area at the interface between the SSE and the battery cathode, which impedes conductivity. This “interfacial resistance” issue presents a challenge for commercialization of all-solid-state batteries.

PNNL researchers have identified a low-cost, manufacturing-friendly synthesis method that is easy to scale up and that can uniformly control particle size and produce SSEs exhibiting high conductivity. The wet-chemical method involves making a solution of lithium and phosphate precursors in a commercially available solvent, which is subsequently evaporated. The concentration and drying temperature are easy-to-control parameters for manipulating the electrolyte particle size, even down to tens of nanometers—about 1,000 times smaller than the width of a human hair. The small-sized SSEs create tight contact areas with active materials in the cathode, prompting the formation of thin SSE layers to achieve high energy density in the battery. High energy density translates into longer-range vehicles for the same battery weight.

For large-scale production of electrolytes, wet-chemical methods are cheaper, more flexible, and particle sizes are easier to control than existing ball-milling, quench, hot-press, and microwave heating methods. PNNL’s method uses an inexpensive, commercially available solvent, which can quickly and fully dissolve lithium and phosphate precursors. By tuning the experimental conditions, PNNL’s method allows for production of electrolyte particles having high lithium conductivity and an average size of about one hundred nanometers in a uniform size distribution. The wet synthesis method is a general recipe that can be used for large-scale synthesis of important lithium and phosphate-based SSEs for all solid-state lithium-sulfur batteries.