Unveiling the Interfacial Composition of Li-Metal | Polymer Electrolytes during Lithium Plating-Stripping Employing Operando ATR-FTIR Spectroscopy
Authors: Wang Jian Fen , Weiling Matthias , Pfeiffer Felix , Baghernejad Masoud 
Published: December 2024

The Li-metal interphases formed in contact with solid-state (polymer) electrolytes (SSE) during electrochemical plating and stripping influences remarkably the lifespan and safety of solid-state batteries (SSBs). Effective 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 challenge, necessitating development of advanced techniques and methodologies. Moreover, highly sensitive instruments are demanded to detect the extremely thin interphases. Furthermore, real-time monitoring experimentations are needed to capture the interfacial dynamics. Addressing the interphase in solid-state battery under real working conditions is another major complication regarding the signal acquisition of the buried interfacial products between non-transparent electrodes. As a result, there are very limited studies of such a kind, which make our state-of-the-art understanding of interfacial process in SSBs very poor.
Towards addressing this, we have developed and operando spectroscopy technique based-on Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) for real time monitoring of interphase composition between Li-metal and solid polymer electrolytes. ATR-FTIR spectroscopy is a powerful analytical technique that offers several significant advantages, 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 the characterization under real working conditions. Moreover, it is rather straightforward to combine the ATR-FTIR measurements with the electrochemical system by utilizing specifically designed 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 electrode1. Based on the successful implementation of this technique, we further developed the setup for the analysis the interphases on solid-state electrolytes with lithium metal in anode-free configuration. In our investigation, a well-known system comprising polyethylene oxide (PEO) polymer, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and benzophenone as photoinitiated crosslinker, was chosen for use as a proof of concept. PEO is one of the most extensively used polymer for SSE, its conductivity at room temperature is however relatively low, as its optimated usage temperature is above 60oC. To ensure that our experimental setup remains comparable to contemporary research on PEO electrolytes, a temperature control system was implemented in the design of our spectro-electrochemical cell, besides, pressure controlling is also incorporated to enhance internal material adhesion. Furthermore, the ATR-FTIR instrument is equipped with adjustable incident angles for the IR beam, enabling the characterization of the interphases at different penetration depths. In terms of cell internal configuration, an anode-free setup was opted, which not only facilitates penetration of infrared beam, but is also regarded as a promising configuration for next-generation SSBs.
Following the acquisition of IR spectrum during the Li plating-stripping process, it was observed that certain bands exhibited intensity variations, as a result of dynamicity in interfacial composition. Notably, some prominent variations were observed in the IR spectrum around 1500-1750 cm-1, primarily originated from bands associated with benzophenone. The reactions of the crosslinker may undergo within the electrolyte system during electrochemical process are often overlooked, which is due to the lack of techniques with the ability to monitor these reactions. The presented study outlines the need for advanced operando methods to increase the understanding of interphases formed by solid-electrolytes.
In future perspectives, efforts will be directed towards further optimizing the cell setups and with the aim of applying this technique to investigate interphases and aging mechanisms in various solid electrolyte systems.
Reference:

Weiling, Matthias, et al. "Mechanistic Understanding of Additive Reductive Degradation and SEI Formation in High‐Voltage NMC811|| SiOx‐Containing Cells via Operando ATR‐FTIR Spectroscopy." Advanced Energy Materials5 (2024): 2303568.