How Can Gas Chromatography-Mass Spectrometry Improve Li-ion Battery Performance?

Maintaining battery production quality and improving performance, lifetime, and safety requires application of a variety of analytical techniques. Confirming material composition and purity, ensuring the homogeneity of electrode slurries, detecting defects that can cause short circuits and certifying feedstock materials extracted from recycled batteries are all processes that depend on analytical testing. Investigating the mechanisms of battery aging and degradation in particular requires instruments ranging from electron microscopy imaging systems and X-ray instruments to elemental analysis and chromatographic separation techniques, such as gas chromatography (GC) and gas chromatography – mass spectrometry (GC-MS). The latter two are particularly useful for applications like identifying and quantifying battery swelling gases and deciphering the mechanisms of electrode / electrolyte degradation.

A typical lithium-ion battery consists of four main parts, namely the cathode, separator, anode, and electrolyte (Figure 1).  While the elemental composition of cathode and anode materials can be analysed using inductively coupled plasma optical emission spectroscopy , the focus of this article will be on GC amendable analytes, such as those found in the composition of the electrolyte.

The most common electrolyte used in LiBs is lithium hexfluorophosphate (LiPF6) salt mixed with volatile organic carbonate solvents (i.e., dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate).

Upon the first cycling of the LiB, reduction of the electrolyte at the anode surface produces a conductive film known as the solid electrolyte interphase (SEI), where current (i.e., Li+) can pass through while simultaneously preventing further reduction of the electrolyte. However, the LiPF6 is thermodynamically unstable at elevated operating temperatures (> 60 °C), causing the organic carbonate solvents to be further reduced and limiting the battery’s current flow and charging capacity.

The analysis of the electrolyte composition is therefore important to diagnose the cause of a battery’s failure to perform to designed specifications. For analysis of the major components (present at percent volume concentrations), require dilution factors of at least 10,000. At the same time, samples need to be analyzed almost undiluted for analysis/screening of minor components and electrolyte degradation products. This means that an analysis needs to cover a huge concentration range, to ensure that compounds in trace level concentrations can still be detected, while avoiding detector saturation by the major components.

Another obstacle is the removal of the LiPF6 salt prior to analysis by GC to help protect the column and MS system. In addition, hydrofluoric acid (HF) can be produced when trace amounts of moisture are present, making removal of the PF6– essential prior to analysis to avoid formation of HF in the injection system1. Salt removal can be accomplished using dichloromethane (CH2Cl2), followed by centrifugation. Once this is accomplished, the electrolyte can be analyzed using different GC-MS systems.

Analysis using different Gas Chromatography-Mass Spectrometry systems

Single quadrupole GC-MS instruments can reliably analyse the electrolyte composition and provide information on major components, additives and degradation products.

Triple quadrupole, or GC-MS/MS systems, can provide higher sensitivity and selectivity. Targeted methods screening for specific components can screen for known degradation products in a larger scale experiment.

High resolution accurate mass (HRAM) systems allow for non-targeted analysis and can help with identification of degradation products and/or mechanistic elucidation of processes that lead to performance loss. The sample preparation can be fully automated using robotic sample handling systems can be fitted with all required components, including a vortex mixer and a centrifuge. These automated systems can process relatively large numbers of samples completely unattended, with the analysis being conducted online shortly after the removal of the salt has been accomplished. The process fully overlaps with the subsequent GC-MS analysis, meaning that the next sample is prepared while the current injection is being analysed.

Another emerging application for GC-MS can be found in battery recycling. Batteries having reached the end of their lifespan are often shredded and further processed to obtain valuable materials needed for new battery construction. Due to the complexity of LiBs, efficient separation of the various components is critical to maximize recovery of essential metals. This is particularly important for organic binders, as their presence in LiB shred material can reduce essential metal extraction efficiency during the recycling process.2 This can be accomplished using pyrolysis, either coupled with single quadrupole GC-MS or HRAM.  A common binder material is PVDF, which during the pyrolysis process decomposes to a variety of pyrolyzates, including fluoroalkenes with various chain lengths, subsequently undergoing in-source fragmentation to produce specific molecular markers, such as 1, 1, 3, 3-tetrafluroallylium (C3HF4+, m/z 113.0009).

However, in this mass range, ions formed by other compounds could appear as isobaric interferences, leading to false positive identifications. At a a mass accuracy setting of 500 mmu (typically achieved in single quadrupole GC-MS instrumentation), isobaric interference from the sample matrix is clearly visible (see Figure 2). Using higher mass resolution (60,000 at 200 m/z full width half maximum) and accuracy (5 ppm) settings obtained with HRAM GC-MS technology, this interference is easily removed and simplifying identification with standard material. This highlights the selectivity advantage of high-resolution accurate mass instruments, dramatically simplifying the spectral identification of binder materials.

For more information on the application of gas chromatography mass spectrometry in the field of lithium batteries, and to find out more about battery material analysis generally, take a look here.

References

  1. Kösters, K., Henschel, J., Lürenbaum, C., Diehl, M., Nowak, L., Winter, M., Nowak, S.: Fast sample preparation for organo(fluoro)phosphate quantification approaches in lithium-ion battery electrolytes by means of gas chromatographic techniques; Journal of Chromatography A 2020, 1624, 461258
  2. Petranikova, M.; Naharro, P. M.; Vieceli, N.; Lombardo, G.; Burçak, E. Recovery of critical metals from EV batteries via thermal treatment and leaching with sulphuric acid at ambient temperature. Waste Management 2022, 140, 164–172.

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