Thermo-mechanical characterization and stress engineering of Lipon solid electrolyte

Abstract

A high temperature multibeam-optical-stress sensor (HTMOSS) was used to characterize the coefficient of thermal expansion (CTE) and yield stress of 1-micron thick Lipon films. Fully dense, amorphous films were deposited on glass and sapphire substrates. The films were then annealed at temperatures ranging from 80 to 200 °C for 3 hours. The CTE of Lipon is found to be approximately 4.1 × 10⁻⁶. This value did not vary appreciably with the substrate type, and was similar in tension and compression. With this intermediate CTE value, the films heated on the two different substrates imposed either tension or compression due to the thermal expansion mismatch. We observed further that the yield stress of the film is approximately 60–100 MPa. Using constant-load holds at and beyond the yield point, the stress developed during heating was relaxed via visco-plastic deformation. A permanent residual stress then evolved during cooling, up to 120 MPa in either tension or compression depending on substrate type. This ability to engineer stress into Lipon films also suggests a strategy for creating a protective layer on other solid electrolytes with higher ionic conductivities (e.g., LLZO and sulfides), which is a potentially effective approach for mitigating lithium dendrite penetration. In addition, Lipon lost ductility at an annealing temperature above 140 °C. Thismay be associated with composition changes that were observed in XPS measurements.