Lithium-oxygen (Li-O₂) batteries are widely studied for their potential to improve the energy density for electric vehicles; however, the high reactivity of cell components limits extended cyclability of the batteries. Some of this degradation can be attributed to the formation of O2-, which reacts with the electrolyte and Li+ to form irreversible biproducts. These complexes have been detected on the cathode of Li-O₂ cells by Raman spectroscopy and FTIR after cycling in TEGDME, a popular electrolyte solvent for Lithium-Air batteries [1, 2].

FTIR has potential to quantify electrolyte speciation as a function of battery operation, but there has been little direct FTIR study of the TEGDME-LiTFSI electrolyte. In the past FTIR of electrolytes has allowed insightful operando analysis, where the ratio of peaks impacted by lithium and those which are not can be used to track the lithium concentration during battery operation [3]. Most FTIR studies of LiO2 focus on the cathode. The present work characterizes well-defined standard systems, providing a similar quantitative basis for understanding dynamic electrolyte evolution in operating Li-O₂ batteries. This presentation will describe how the FTIR can track the electrolyte species composition.

Figure 1 shows the FTIR spectra of TEGDME-LiTFSI and TEGDME-LiNO₃ for different concentrations. By comparing these two systems, the TEGDME peaks associated with Li⁺ and TFSI^- can be independently identified, enabling precise quantification of the electrolyte ion concentration. The partnered peaks at 1080 cm^-1 and 1100 cm^-1 show evidence influence of Li⁺, as the peak at 1080 cm-1 increases while the other decreases. A variety of peaks associated with TFSI- can be seen at 740, 786, 1180, 1327, and 1352 cm^-1. At high molarities there is a wavelength shift near the 1327 cm^-1 wavelength which indicates a configuration change in S=O bond [4, 5]. The examination of control electrolyte systems lays groundwork for future FTIR studies of lithium-air system and operando experiments can used to inform Lithium-Air cell design.

[1]	A. Chamaani, M. Safa, N. Chawla, M. Herndon and B. El-Zahab, "Stabilizing effect of ion complex formation in lithium–oxygen battery electrolytes," J. Electroanal. Chem., vol. 815, pp. 143-150, 2018.
[2]	S. Fruenberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Barde and P. G. Bruce, "The Lithium–Oxygen Battery with Ether-Based Electrolytes," Angew. Chem. Int. Ed., vol. 2011, pp. 8609-8613, 2011.
[3]	L. Meyer, D. Curran, B. Ryan, S. Shriram and J. Porter, "Operando Measurements of Electrolyte Li-ion Concentration during fast charging with FTIR/ATR," J. Electrochem. Soc., vol. 169, no. 9, p. 090502, 2021.
[4]	M. Herstedt, M. Smirnov, P. Johansson, M. Chami, J. Grondin, L. Servant and J. C. Lasseguess, "Spectroscopic characterization of the conformational states of the bis(trifluoromethanesulfonyl)imide anion (TFSI−)," J. Raman Spectrosc., vol. 36, pp. 762-770, 2005.
[5]	L. Aguilera, S. Xiong, J. Scheers and M. Aleksandar, "A structural study of LiTFSI–tetraglyme mixtures: From diluted solutions to solvated ionic liquids," J. Mol. Liq, vol. 210, pp. 238-242, 2015.