Water – the most important QC parameter for battery manufacturers

Lithium-ion batteries exist in various forms for different applications. From mm-scale button cells for med tech applications to big battery packs in electric vehicles. In simple terms, manufacturers successively stack cathode and anode sheets, aluminum (cathode) or copper (anode) foil with active material on both sides, and separate them with a polymer separator. These are brought together into a housing and the liquid electrolyte is added to produce the final battery. The water content of each component needs to be as low as possible.

Quality Control of Cathode and Anode The best way to measure the water content of electrode sheets is the gas-extraction technique with coulometric Karl Fischer (KF) detection. The electrode sheets are cut into small flakes, filled into vials, placed onto the InMotion KF™ oven rack (see Figure 2) and the method is started with OneClick™. The oven heats the electrode sample to 160 °C and a pre-dried stream of nitrogen extracts and carries the vaporized water to the coulometric titration cell, where it is detected. In the application note M808 the detailed procedure is explained. The analysis of anode and cathode sheets revealed water contents of 106 ppm and 152 ppm respectively with relative standard deviations of 2.6 and 5.2%  

Quality Control of the Electrolyte The measurement of the electrolyte is more challenging. The water content is very low (<50 ppm), it is highly hydrophilic, and commonly used organic carbonates like dimethyl, ethyl methyl and vinylene carbonate show side reactions with conventional methanol-based KF reagents. Low water content samples are most accurately measured using coulometric KF titration. Small amounts of iodine are generated in-situ by a generator electrode. The hydrophilicity of the electrolyte requires very careful sample handling. The sample container should be closed in a dry argon atmosphere, protected from air humidity by a rubber septum and the syringes must be rinsed with dry electrolyte prior to the injection. Using methanol-free KF reagents prevents side reactions and enables the White Paper METTLER TOLEDO 5 solvent to be reused for several measurements. Application note M820 describes the water content determination for liquid electrolytes. Along with water, hydrofluoric acid (HF) – one of the detrimental degradation products of LiPF6 – can be tested using an acid-base titration with sodium hydroxide as the titrant. Application note M813 describes the method in more detail. As the density of a liquid depends on its molecular composition, a quick density check can help to reveal electrolyte contaminations with water or other impurities.