Quality control of cathode materials

The IEA (International Energy Agency) expects demand for electric vehicle batteries to increase between four and a half and seven times by 2030 compared to 2023 [1]. The biggest part of the battery production cost is the materials, and cathode production is the most expensive part of the material costs [2]. A good quality control program for cathode production is important to avoid high scrap rates and achieve high production efficiency. This article presents several key analytical parameters throughout the cathode production process. 

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Analysis of lithium salts for cathode production

• Assay of lithium salts
• Ionic impurities in lithium salts

Composition analysis of cathode active materials by titration

• Main component analysis
• Residual lithium hydroxide and lithium carbonate content

Determining water content in cathodes and raw materials

Measuring fluorine content in cathode black mass for recycling

Analysis of lithium salts for cathode production

Lithium hydroxide (LiOH) and lithium carbonate (Li₂CO₃) are the main lithium salts used in the production of cathode active materials (CAM) [2]. Lithium hydroxide is preferred because lithium hydroxide-based CAMs have better storage capacity and longer life cycles [3].

Therefore, it is important to assess the quality of lithium salts. This includes determining the content of the main lithium salts (assay) as well as ionic impurities to ensure this raw material does not exceed nor fall short of specific production requirements.

Titration for the assay of lithium salts

Titration with hydrochloric acid is ideal to analyze the content of lithium hydroxide and lithium carbonate. This simple method can distinguish between both salts and thus detect carbonate impurities in lithium hydroxide. The International Organization for Standardization (ISO) proposes titration to analyze lithium carbonate as well as lithium hydroxide and its carbonate impurities, respectively [4,5]. 

To analyze lithium hydroxide, it is crucial to protect the sample from exposure to CO₂. Otherwise, carbonate impurities will form. Figure 1 shows the results of a fully automated analysis of lithium hydroxide. One sample series ran covered with a lid to prevent exposure to CO₂, while the other series ran uncovered. The uncovered series showed a clear increase in carbonate impurities.
 

For more information about the assay of lithium hydroxide and lithium carbonate, take a look at our Application Note.

Application Note: Assay of lithium hydroxide and lithium carbonate – Precise and reliable determination by potentiometric titration

Ion chromatography for ionic impurities

Battery-grade lithium salts must be extremely pure, since ionic impurities can negatively affect the finished battery. One challenge when processing lithium brine is the removal of magnesium [7,8]. Ion chromatography (IC) is ideal for determining the efficiency of the magnesium removal process. In addition, other ionic impurities such as potassium, sodium, or calcium can be analyzed simultaneously.

In contrast to other techniques, such as spectroscopic methods, ion chromatography is a very easy and economical way to determine ionic impurities. An additional benefit of using IC is its robustness when analyzing samples with complex matrices—for example, high loads of salts.
 

Take a look at our related Application Notes to learn more about the analysis of lithium brines and ores using ion chromatography.

Application Note: Online determination of lithium in brine streams with ion chromatography

Application Note: Cations in lithium ore

Composition analysis of cathode active materials by titration

Main component analysis in precursor cathode active materials production by titration

The proper composition of the starting solutions is essential for producing CAMs as errors cannot be corrected [9], resulting in high scrap rates. Potentiometric titration can be used to analyze the solution used to produce the precursor cathode active material (pCAM).

Titration can handle much higher metal concentrations than other methods such as ICP-OES (inductively coupled plasma - optical emission spectrometry). Therefore, there is no need to dilute the sample, reducing potential measuring errors.

Analysis of layered oxides is straightforward with a single titration. Ternary metal oxides require more than one titration to differentiate between the metals. Table 1 summarizes the titration of the different metals in cathode active materials.

Our free Application Note below describes the fully automated analysis of the nickel, cobalt, and manganese (NCM) content in an NCM pCAM starting solution.

Application Note: Analysis of Li-ion battery cathode materials made from Co, Ni, and Mn – Fully automated determination including sample preparation using the OMNIS pipetting equipment

Residual alkali content 

Determining water content in cathodes and raw materials

Lithium-ion batteries should be virtually water-free as traces of water can also negatively impact the performance of these batteries. More than 1000 µg/L (ppm) water can cause capacity loss and swelling of the battery cell [12]. Furthermore, water will react with lithium hexafluorophosphate (LiPF₆) in the electrolyte, forming toxic hydrofluoric acid (HF). Therefore, it is important to monitor the water content during the entire production process of the cathode active material.

One consideration is to ensure that the workshop environment is as dry as possible during cathode production [13]. Others include measuring the water content in the purchased raw materials and during production of the cathode. Coulometric Karl Fischer titration is an established method for determining the water content in battery materials [12].

Solid samples cannot be added directly to the coulometric titration cell, therefore an indirect method with an oven is used [12]. The sample is weighed and sealed in an air-tight vial. The vial is then placed in the oven and the evaporated water is transferred to the titration cell. There the water content is determined.
 

Learn more about the oven method in our blog article.

Oven method for sample preparation in Karl Fischer titration
 

For more information on the analysis of cathode samples, download our Application Bulletin below.

Application Bulletin: Water in lithium ion battery materials

Measuring fluorine content in cathode black mass for recycling

As demand for electric vehicles (EVs) and thus lithium-ion batteries increases, recycling of spent batteries becomes more important. The recycling process typically targets nickel, cobalt and copper, but now there is increased emphasis on lithium recovery [14]. 

The lithium recovery process is hampered because the PVDF binder releases fluoride during calcination of the black mass. The fluoride reacts with the lithium, resulting in lithium fluoride, which is insoluble [15]. Fixation of fluoride can help with recovering lithium. To determine the amount of fixing agent required, combustion ion chromatography (CIC) can be used to measure the fluorine content in the black mass.

During combustion in chromatography, the sample (cathode black mass) undergoes pyrohydrolysis. The PVDF decomposes and the released fluorine is absorbed in ultrapure water. The resulting fluoride content is then measured by ion chromatography. Figure 5 shows the chromatogram for the analysis of a cathode material.

Learn more about combustion ion chromatography in our blog post.

History of Metrohm IC – Part 6

Conclusion

Monitoring the quality of the incoming raw materials and other key quality parameters during the production process, such as water content or CAM composition, can reduce the risk of quality failure of the finished battery. As recycling of batteries becomes more important, it is essential to implement analytical methods to ensure efficient and effective recycling processes.

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