The interphases between Li-metal and the solid electrolyte are pivotal factors influencing the efficiency and safety of solid-state batteries (SSBs). Well-established interphases play a crucial role in impeding the continuous decomposition of electrolytes, the growth of dendrites, and consequently, enhancing the overall battery performance. However, the investigation of interphases of solid electrolytes remains a challenging, necessitating specialized equipment and control systems to achieve the high-temperature conditions required for the operation of SSBs. Moreover, highly sensitive instruments are demanded for detecting extremely thin interphases, and also for real-time monitoring experiments to capture the electrochemical variations within the battery. Additionally, the opacity of solid materials further complicates optical experiments due to the transparency of the beam.

In light of the limited exploration of interphases in solid-state batteries (SSBs), there is an urgent demand to acquire a comprehensive understanding of the underlying processes. To address this, we have chosen to employ the Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) technique, complemented by an operando operating system, as our experimental instrumentation.

ATR-FTIR spectroscopy is a powerful analytical technique that offers several significant features, including rapid data acquisition, applicability for various sample forms, and high sensitivity to trace even small amounts of substances non-destructively. The preservation of the original interphase composition is crucial for their characterization under real working conditions to gain deeper insights into (electro-)chemical and structural processes and mechanisms going on during formation. Moreover, it is rather straightforward to combine the ATR-FTIR measurements with the electrochemical system by utilizing specifically engineered spectro-electrochemical cells.

Our team has been developing the advanced ATR-FTIR set-up for operando interphase analysis, which has already worked successfully for characterizing the interphases of liquid electrolyte system on Si and Li-metal electrode. Based on the successful implementation of this technique, we further developed the setup for the analysis of interphases on lithium-metal with polymer electrolytes. As a case-study, the interphase formed with the commonly used PEO-based polymer electrolyte is investigated. The chemical and structural interphase information obtained from the IR spectra provide targeted insights into the molecular mechanisms of the interphase formation.