by Simon Nelms, ICP-MS Product Specialist, Trace Elemental Analysis, Thermo Scientific UK
Lithium-ion (Li-ion) batteries are one of the most important power storage materials available today and have enabled the widespread manufacture and adoption of portable electronic devices. The excellent properties of Li-ion batteries are responsible for their success: their superior energy density, high storage capacity, and relatively low cost mean the batteries are long-lasting and economically viable to manufacture.
Even now, Li-ion battery use is growing faster than ever. The green energy revolution is encouraging the transition from traditional vehicles to hybrids and fully electric vehicles (EVs), and Li-ion batteries are the current standard technology for their power source. With the growth of EVs expected to increase 10-fold over the next decade, reaching 300 million electric cars on the road by 2050,1 it is imperative to ensure adequate lithium supply. Yet current lithium sources will be unable to meet this demand, hindering progress toward greener transport. It is, therefore, crucial to explore and make accessible additional natural resources of lithium to meet global initiatives on sustainability and green energy. Currently untapped resources, however, may consist of lower lithium levels and higher levels of impurities, needing better analytical techniques to confirm their viability for extraction and subsequent Li-ion battery production.
Meeting increased lithium demand
Lithium is obtained by extraction from various natural repositories, including mining of geological ores and underground brine reserves (Figure 1). Although several approaches can be taken to extract lithium from brine, ion exchange adsorption is one of the most cost-effective and efficient methods. After extraction and purification steps, the resulting lithium is most commonly traded in the form of two salts: lithium carbonate (Li2CO3) and lithium hydroxide (LiOH). These salts are then used as raw materials to manufacture different Li-ion battery components. Identifying viable lithium sources and subsequent extraction and purification, however, can be challenging.
Figure 1: Simplified schematic of the lithium extraction process, from ores to battery components.
First and foremost, to determine whether a new potential source is suitable for lithium extraction, it needs to be analyzed carefully for economic viability. Most importantly, it should contain enough lithium to be worth undergoing the extraction process, and other impurities present in the material that could affect the quality of the final product must be understood.
Secondly, it is essential that all the Li-based materials used in the production of battery components are of high quality, as impurities in the manufacturing process can affect the performance and safety of the end products. Any source materials therefore need to undergo stringent quality control and be tested for the presence of elemental impurities, including transition metals and alkaline and earth metals.
Finally, the key to ensuring that technology continues to advance — for example EVs with higher mileage between charges — is batteries with greater performance and longer life. Improved batteries need top-quality raw materials, and many manufacturers will require higher purity lithium salts in the coming years. Raw material analysis will therefore undergo increased scrutiny, meaning methods need to be sensitive enough to detect impurities, while also being able to analyze a growing number of elements usually present only at trace or ultra-trace levels.
Portable X-ray fluorescence (XRF) spectroscopy is widely used to measure elements in lithium sources, particularly in the mining industry, as it is a non-destructive approach to determine sample elemental composition in the field. But XRF has limitations to its use, with the most notable being unsuitable detection limits. Another option, atomic absorption spectroscopy (AAS) can also be used to determine the quantity of a given element in a sample. However, AAS is a single element technique, requiring multiple measurements to obtain all necessary data. Additionally, low sensitivity means flame AAS can only measure major elements such as Li itself, so determination of impurities requires additional measurements via graphite furnace AAS. Finally, the method is prone to interference, making sample preparation complex and costly.
Inductively coupled plasma optical emission spectroscopy (ICP-OES) can overcome these drawbacks and is the suggested technique for analyzing elemental impurities described in Chinese standard GB/T-11064.16-2013 and the International Electrotechnical Commission (IEC) 62321 standard. In addition, ICP mass spectrometry (ICP-MS) is a complementary approach offering improved elemental analysis of impurities, equipping scientists with a range of tools to better analyze raw battery materials.
ICP-OES: A robust method for QC requirements
ICP-OES is invaluable for analyzing lithium resources. It has multi-element capability, meaning it can analyze large numbers of trace elements present in the raw materials. In addition, ICP-OES has a high dynamic range, which allows it to simultaneously detect both major and minor elements in a single analysis. Recent studies have highlighted how ICP-OES can be used to determine source viability and analyze minerals and brine solutions involved in the production of Li-ion battery components.
Li-rich minerals
Table 1: Linearity (coefficient of determination, R²) and limit of detection (LOD) (in mg·L-1) data for the twelve elements measured.
Mineral ores remain a vital lithium source, particularly in regions where it is not possible to exploit lithium from underground brine reserves. However, Li-containing minerals can vary significantly in composition and lithium content.2 Quantifying the composition of a given mineral is therefore crucial to determine whether it is worth extracting.
ICP-OES was recently used to analyze zinnwaldite, a Li-containing mineral. Four different samples (and therefore compositions) were investigated and underwent three different acid digestion methods prior to analysis. Eleven target analytes were analyzed alongside lithium, including common elements such as sodium and potassium (Table 1).
The method gave high accuracy in results, and LODs as low as 0.0001 mg·L-1 were demonstrated. In addition, ICP-OES obtained high quality data for long sequences without requiring any maintenance.
Brine solutions
The other major lithium source is brine solutions, which are abundant in underground reserves across the globe. Like ores, lithium content in brine solution varies from source to source, and typically ranges from < 10 mg·L-1 – 4,000 mg·L-1. Before establishing an extraction process, the quality of the brine solution needs to be determined for viability. In addition, elements in waste brine must also be quantified as they can have negative environmental impacts.
Table 2: List of analytes, wavelengths, measurement mode, correlation coefficients, and method detection limits (mg L-1).
A recent study demonstrated that ICP-OES is an accurate and robust method to analyze brine solutions. Nineteen analytes, including Li, were determined using the technique. The data obtained was high quality, with a linear range up to 5,000 mg·L-1 established for lithium. The approach was also very sensitive, giving method detection limits (MDLs) of 0.0001 mg·L-1 (Table 2). Furthermore, system modifications described in the study meant samples with high dissolved solid content could be analyzed. Alongside these strengths, a quick sample turnaround means that ICP-OES can increase the productivity of analytical laboratories for brine solution analysis, especially when compared to single element techniques such as AAS.
Improved sensitivity for salt feedstocks with ICP-MS
Although ICP-OES is a powerful technique, ICP-MS can provide complementary analysis of some raw materials. While ICP-OES benefits from higher tolerance of total dissolved solids in samples and so is better for brine solutions, ICP-MS can detect metal elements at parts per trillion levels, making it ideal for analyzing lithium feedstocks with low regulatory limits. As more stringent raw material testing will be required, ICP-MS can bridge the gap and ensure analytical laboratories can meet these demands.
In a recent study, ICP-MS was used to analyze three Li2CO3 and LiOH salts. Sixty analytes were determined, including alkaline and alkaline earth metals, transition metals, heavy metals, and lanthanides. ICP-MS offers 3-4 orders of magnitude lower detection limits compared to ICP-OES, which is highly important for detecting lanthanides —typically existing in µg kg-1 amounts or lower — in geological samples.
Overall, ICP-MS gave reproducible and accurate results for lithium salt analysis. Undergoing QC checks, the method gave excellent internal standard recoveries between 75% and 120% (Figure 2). Continuous acquisition of 280 samples was obtained over 12 hours, showing its robustness and capability to meet increasing testing demand with growing Li-ion battery use.
Figure 2: Internal standard performance from the analysis of about 280 samples over 12 hours showing recoveries between 75 and 120%.
Better lithium sources, better batteries
As demand for Li-ion batteries increases, accurate and sensitive methods to determine the quality of new and existing lithium sources are becoming ever more important. Employing the right analytical instrument for your needs is vital to confirm that lithium sources are high-quality, leading to improved battery components. ICP-OES is a valuable tool that can analyze the quality of lithium resources and raw materials by determining both lithium and impurities present. And as quality requirements grow more stringent, ICP-MS offers the improved sensitivity required for present and future lithium salt feedstock determination.
Both ICP-OES and ICP-MS meet thorough QC checks required for the analytical laboratory, and in conjunction with their reliability and high-throughput capability, they are vital tools to facilitate the larger-scale development of better and safer batteries.
References
1. Electric Vehicles, IEA Report, November 2021, https://www.iea.org/reports/electric-vehicles
2. British Geological Survey: Lithium, June 2016.