Achieving superior energy density and high Ionic conductivity for electric vehicles and electronic devices

Lithium-ion batteries used in electric vehicles and numerous electronic devices continue to face safety concerns related to the flammability of organic liquid electrolytes and their potential for leaking. Inorganic solid-state electrolytes (SSEs) show promise to conquer such safety issues. Among existing SSEs, sulfide-based electrolytes possess highly favorable traits, such as high ionic conductivity, good electrode contact, and processing capability at low-temperatures. However, sulfide-based SSEs also suffer from poor stability when exposed to air and may generate toxic hydrogen sulfide gas during the manufacturing process. These drawbacks impede large-scale production of desirable sulfide-based SSEs.

Two innovations developed by Pacific Northwest National Laboratory (PNNL) overcome such drawbacks.

First, PNNL’s new, reversible surface protection agent for SSEs repels water. This attribute greatly reduces the reaction potential between sulfide and water in the air, improving SSE stability under ambient processing conditions. The air-stable coating could be simply removed—through evaporation or washing with other solvents, for example—from the SSE particle surface without sacrificing ionic conductivity. In laboratory processing tests with the protection agents, SSEs were stable with oxygen, carbon dioxide, and nitrogen in the air, and generated less than 10-ppm hydrogen sulfide over two hours at relative humidity around 15 percent. These benefits are in addition to superior lithium-ion conductivity for battery performance.

Second, sulfide-based SSEs, with or without the protective coating, can pair with a new, chemically compatible polymeric binder to form an ultra-thin (< 60 microns), flexible film, about the width of a human hair. The binder assures uniform distribution and stability of the SSEs throughout the battery electrodes, and the ultra-thin form provides the high structural integrity and flexibility needed for the SSE layers to adapt to any deformations during battery cell operation. The key is to achieve just enough contact between the binder and SSE particles to maintain both a flexible film and high ionic conductivity.

A competing patented process for flexible polymer-based SSE films using lithium salts suffers from low ionic conductivity at room temperature and flammability issues associated with the binding polymers. PNNL’s air-stable sulfide SSE coating and film significantly improve material handling for the cell manufacturer and greatly reduce associated processing costs. These improvements support the scalable synthesis, storage, transfer, and processing of solid electrolytes and fabrication of solid-state lithium batteries under ambient conditions.