Space Charge Layer Evolution in All-Solid-State Batteries Probed via Operando Kelvin Probe Force Microscopy and Nuclear Reaction Analysis

The current controversies about the role of space charge layers hinder the development of better solid-solid interfaces and, thus, the improvement of solid-state batteries (ASSBs). To overcome this, we have combined high spatial resolution and nondestructive techniques, operando heterodyne Kelvin probe force microscopy (KPFM), and operando nuclear reaction analysis (NRA) to conduct a study of space charge layers in ASSBs. A model thin-film ASSB was fabricated from lithium (Li)|Li₃PO₄ (LPO)|LiCoO₂ (LCO) for this study. This battery excels due to negligible interfacial defects and side reactions. For a working battery voltage range from 3.0 to 4.3 V vs Li/Li⁺, a space charge layer mainly exists at the LPO|LCO interface. This space charge layer with a width <50 nm arises from the redistribution of Li-ions at the interface. We clarified controversial views on the role of space charge layers in ASSBs by quantitatively determining the interfacial space charge layer resistance and found a maximum value between 18.4 and 19.1 Ω cm² at 4.3 V vs Li/Li⁺. The absolute value of interfacial resistance from space charge layer formation is much smaller compared with the bulk solid electrolyte resistance in the fabricated thin-film ASSB. By employing KPFM and NRA techniques in ASSB research, our knowledge of space charge layer evolution at the solid electrolyte electrode interface is more comprehensive, even beyond the investigation of space charge layers.