Nanoscale Thermomechanical Coupling Study on the Aging of NASICON-Type Solid Electrolytes

Previous investigations on the aging mechanism of solid electrolytes (SEs) predominantly focused on electrochemical and mechanical properties, often overlooking their thermal characteristics. In this study, we introduced scanning thermal microscopy and scanning thermo-ionic microscopy to delve into the microscopic aging process of the Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP) SE assembled with Li metal. Our findings revealed the formation of a Li₃Al_0.4Ti_1.6(PO₄)₃ secondary phase during cycling. Compared with the unreacted LATP, this secondary phase exhibits a remarkable decrease in thermal conductivity (from 1.72 to 0.63 W m^-1 K^-1), Young's modulus (from 117.4 to 31.3 GPa), and ionic activity, while demonstrating an increase in thermal expansion response. These mismatches worsen the interfacial contact, contributing to the formation of cracks and a sharp increase in impedance. To the best of our knowledge, this is the first time that the thermomechanical coupling of SEs has been studied at the nanoscale, underscoring the pivotal impact of the thermal properties on SE degradation.