(Invited) In Situ Imaging of Interphase Evolution and Degradation Processes in Solid-State Batteries

Abstract

Understanding the electrochemical interface between alkali metals and solid-state electrolytes is key for developing stable, high-energy density solid-state batteries. The vast majority of solid-state electrolyte (SSE) materials are unstable in contact with lithium metal, and (electro)chemical reactions between SSEs and lithium result in the formation of an interphase region. Understanding the growth kinetics and chemo-mechanical consequences of interphase formation is key for controlling solid-state interfaces, and this knowledge may enable the use of a wider variety of SSE materials within lithium metal batteries. Here, we investigate NASICON-structured L1+xAlxGe2-x(PO4)3 (LAGP), as well as sulfide-based SSEs. For LAGP, electrochemical experiments combined with multi-modal in situ investigation of interfacial reactions reveal how the formation of the interphase is linked to cell failure. In situ transmission electron microscopy (TEM) shows that the reaction of LAGP with lithium is similar to a conversion reaction, in which lithium insertion causes amorphization and volume expansion of ~130%. The interphase is a mixed ionic-electronic conductor, resulting in continuous growth. In situ X-ray tomography experiments of operating LAGP-based cells reveal that the growth of the interphase causes fracture of the SSE, and quantification of the crack network shows that the extent of fracture with time is directly correlated to impedance increases within the cell. Finite-element analysis is used to model stress evolution during interphase formation, and the initial fracture locations predicted from modeling correspond well to experimental observations. Operation of cells at different currents between 100 and 500 mA/cm2 causes the interphase to grow in different ways: higher currents result in highly non-uniform interphase growth deep into the SSE pellet, resulting in rapid fracture and cell degradation. Interestingly, we have found that interphase growth trajectories can be modulated through the deposition of interfacial protection layers. Controlling the morphology of the interphase with protection layers results in the ability to extend cycling stability of symmetric cells from ~30 hours with unprotected SSEs to >1000 with protected materials. Overall, these results provide fundamental insight into interfacial transformations in solid-state batteries, and they show that control over interfacial transformation processes could enable a wider variety of materials to be used in solid-state lithium metal batteries.