Lithium-ion batteries (LIBs) are at the forefront of energy storage solutions thanks to their high storage capacity, superior energy density, and relatively low cost. Over the past three decades, demand for LIBs has grown at an astonishing rate, driven by a combination of ground-breaking scientific advances and consumer product power needs. The rapid and accelerating rise of electric vehicles over the past decade is fuelling even greater demand: by 2050, more than 300 million electric vehicles (EVs) are expected to be on the road, massively reducing gas emissions.1
Despite the popularity of LIBs, their development and production face numerous challenges. Most notably, all components of LIBs require rigorous analysis and quality control: this includes lithium salts, electrodes, and electrolytes. Without these thorough checks, safety and quality can be compromised.
Inductively coupled plasma optical emission spectroscopy (ICP-OES) is the gold standard for LIB material analysis, delivering accurate results with fast sample turnaround. Gas chromatography-mass spectrometry (GC-MS), another powerful technique, enables effective electrolyte composition analysis. When used together, these two highly complementary tools provide a multidimensional approach to support the stringent analysis and quality controls in LIB development. Here we explore how these techniques can improve LIB component examination by delivering detailed analysis of lithium salts and electrolytes.
Identifying and quantifying impurities in lithium salts
Lithium is most commonly traded in the form of two salts, Li2CO3 and LiOH, which are used to manufacture the cathode materials in LIBs. These starting lithium salts must be analysed for any elemental impurities, including transition metals, and alkaline and alkaline earth metals. This insight is vital to ensure the performance of the final cathode products, which themselves also need to be analysed to validate their composition and purity.
Two approved standards exist for lithium salt analysis: Chinese Standard GB/T-11064.16-2013, and International Electrotechnical Commission (IEC) 62321. In both these methods, ICP-OES is the suggested technique: not only can it perform multi-elemental analysis in a short time, but it also handles a variety of sample types.
A recent study demonstrating the power of ICP-OES looked at 45 trace elements present in three samples, including two Li2CO3 and one hydrated LiOH salt, with compelling results. The method used had the following advantages:
Fast: sample run time was 144 seconds.
Sensitive: of the 45 elements, 43 had method detection limits well below 1 mg/L.
Robust: After 12 hours continuous measurements, no significant drift or QC failures were found throughout the duration of the experiment.
The internal standard recovery remained within 90-100%.
The results clearly demonstrate the suitability of ICP-OES for lithium salt analysis. Not only does it show the high accuracy and sensitivity needed for thorough characterisation, but it is robust enough to withstand the continuous testing demands of the analytical laboratory.
Impurity and composition analysis of electrolytes
The LIB electrolyte significantly affects the battery’s charging and discharging performance and must also be well understood. Lithium hexafluorophosphate (LiPF6) is the most commonly used salt in electrolyte solutions. To make the electrolyte, LiPF6 is dissolved in an organic carbonate solvent mixture, containing, for example, dimethyl carbonate, ethyl methyl carbonate, and/or diethyl carbonate.
Several factors influence the performance of an electrolyte and it therefore needs careful analysis. Crucially, any elemental or organic impurities must be determined. What’s more, the composition of the electrolyte needs to be characterised and validated to enable the required battery performance to be achieved. For these analyses, ICP-OES and GC-MS can be used as complementary techniques to gather the detailed insights needed.
Electrolyte impurity analysis with ICP-OES
Analysing impurities in electrolytes provides a wealth of information. Significantly, impurities offer insight into ageing processes as degradation products accumulate in the electrolyte. This knowledge supports the development of better batteries in the future. It is also essential to understand components at their end of life, not only for recycling and potential environmental contamination concerns, but also to minimise risks to personnel disassembling the batteries.
The standard method for electrolyte impurity analysis is detailed in Chinese Standard HGT/ 4067-2015. It is demanding in that it requires method detection limits of 1 mg/L in the final LiPF6 electrolyte samples. Furthermore, analysing LiPF6 in organic solvents over long measurement periods can be challenging, as the high carbon content in the samples may lead to high background signals, injector blockage, and high plasma load.
ICP-OES helps overcome such challenges to deliver effective electrolyte impurity analysis, offering detection limit performance well within the required range for analysis. In addition, its robust set-up enables effective characterisation of the materials involved.
A recent study showed the strength of ICP-OES analysis, investigating three different samples of fresh, unused LiPF6-containing electrolyte solutions.
The linearity of the method was tested up to concentrations of 1000 mg/L for all elements, with the calibration curves giving an R2 value of between 0.9991 and >0.9999. The method also showed excellent accuracy and precision: recovery was 80 to 120% for all elements, with most being >90%. What’s more, the method is robust. Internal standard recovery of between 85 and 90% was found for measurements obtained over 6 hours, demonstrating its suitability for large-scale applications.
Electrolyte composition analysis with GC-MS
LiPF6 is thermodynamically unstable at higher temperatures (above 60°C) and it degrades. This degradation leads to a thicker and more resistive solid-electrolyte interface (SEI), thereby limiting current flow and the LIB’s charging capacity. Additives can reduce degradation, but their structures affect the properties of the SEI. Understanding electrolyte composition and the by-products formed is essential to develop more efficient and higher performing LIBs.
Complete composition analysis can be challenging, particularly as electrolytes contain many different components over wide concentration ranges. Organic carbonate solvents are present at high concentration, but the concentrations of additives and side reaction products are much lower, making thorough analysis difficult.
GC-MS can measure all components of the electrolyte over a wide range of concentrations. One study investigated LIB electrolytes for 16 different electrolyte solvent, additive and degradation compounds. The method showed high linear correlation from 0.1 – 200 mg/L for all compounds except fluoroethylene carbonate and triethyl phosphate. The GC-MS method is also highly sensitive, giving an instrument detection level (IDL) and instrument quantification level (IQL) of 0.021 and 0.07 mg/L respectively, or lower, for all compounds.
Powerful analytical techniques to power better EVs
Rigorous quality control and detailed analysis of LIB components not only ensures safety, quality, and value, but is also vital in driving the development of increasingly effective batteries. ICP-OES and GC-MS are indispensable techniques to enable faster and more accurate analysis of lithium salts and electrolytes. With these in the toolkit, researchers and developers have the power in their hands to unleash the full potential of LIBs for next generation EVs.
1 Electric Vehicles, IEA Report, November 2021,
https://www.iea.org/reports/electric-vehicles
