Influence of Residual Contaminants in Li-Ion Battery Electrolytes Investigated Via Operando Total and Partial Pressure Analysis
Authors: Lundström Robin , Berg Erik J. 
Published: July 2021

Gas evolution in Li-ion batteries (LIB) is triggered by an intricate mix of potential-induced electrochemical and thermally activated reactions. A small percentage of these reactions lead to the formation of interphases capable of stabilizing the electrodes when operating outside of a stable potential window of the electrolyte. The most well-known example thereof is the solid electrolyte interphase (SEI), but the formation process is still not fully understood. What makes interphase reactions complicated to understand is that many compounds that form are metastable and will over time decompose or further react with other species at the interphase.1

Advanced operando and/or in situ characterisation techniques are highly advantageous when monitoring the formation of metastable compounds during charge/discharge of the battery. Online electrochemical gas analysis (OEMS) is a technique where volatile species evolving in LIBs can be monitored by utilizing mass spectrometry. The use of online gas analysis for LIBs has been around for 20 years,2 where adaptations have continuously improved the characterisation of interphase and decomposition processes in LIBs.3

In this presentation our newest improvements to online gas analysis will be presented, where simultaneous partial and total pressure measurements for Li-ion cells are possible. By combining the two techniques in the same experiment and utilizing improved gas connections throughout the OEMS system we gain refined calibration protocols, possibility to map out unidentified volatile species and selective detection of volatile compounds evolving on a ~0.1 nmol/min scale.4

The detailed gas profile of the classical LiCoO2/Graphite (LCO/G) system with 1 M LiPF6 ethylene dicarbonate:diethyl carbonate (1:1 volume ratio) during overcharge and elevated temperature (50°C) will be used as the basis to discuss interphase and decomposition reactions in LIBs (Figure 1). The gas profiles for LiCoO2/LiFePO4 and Graphite/LiFePO4 cells will be presented along with the LCO/G cell to further derive where gases originate in a Li-ion cell and what chemical and electrochemical processes play major roles in non-reversible interphase reactions in LIB systems. Finally, the influence of residual contamination in electrolytes will be discussed as it makes a major impact on the gas evolution in LIBs, which will be showcased in a glassy carbon/LiFePO4 model system with varying volumes of electrolyte. The influence of contamination needs to be put in the spotlight to further our understanding of metastable processes in complex systems such as LIBs and to improve their overall performance.
[[Figure 1 goes here]]
Figure 1. Stacked gas and total pressure profiles in a LiCoO2/Graphite cell with 1M LiPF6 ethylene dicarbonate:diethyl carbonate electrolyte during cyclic voltammetry to overcharge potentials at elevated temperature (50°C).

References

(1) Heiskanen, S. K.; Kim, J.; Lucht, B. L. Generation and Evolution of the Solid Electrolyte Interphase of Lithium-Ion Batteries. Joule
2019, 3 (10), 2322–2333.
(2) Imhof, R.; Novák, P. In Situ Investigation of the Electrochemical Reduction of Carbonate Electrolyte Solutions at Graphite Electrodes. J. Electrochem. Soc.
1998, 145 (4), 1081–1087.
(3) Tripathi, A. M.; Su, W. N.; Hwang, B. J. In Situ Analytical Techniques for Battery Interface Analysis. Chem. Soc. Rev.
2018, 47 (3), 736–751.
(4) Lundström, R; Berg; EJ; Submitted to J. Power.
Sources.

Figure 1