Design of Vacuum Post‐Drying Procedures for Electrodes of Lithium‐Ion Batteries

Abstract

In order to reduce the residual moisture in lithium‐ion batteries, electrodes and separators need to be post‐dried prior to cell assembly. On an industrial scale, this is often conducted batch‐wise in vacuum ovens for larger electrode and separator coils. Especially for electrodes, the corresponding post‐drying parameters have to be carefully chosen to sufficiently reduce the moisture without damaging the sensitive microstructure. This requires a fundamental understanding of structural limitations as well as heat transfer and water mass transport in coils. The aim of this study is to establish a general understanding of the vacuum post‐drying process of coils. Moreover, the targeted design of efficient, well‐adjusted and application‐oriented vacuum post‐drying procedures for electrode coils on the basis of modelling is employed, while keeping the post‐drying intensity as low as possible, in order to maintain the sensitive microstructure and to save time and costs. In this way, a comparatively short and moderate 2‐phase vacuum post‐drying procedure is successfully designed and practically applied. The results show that the designed procedure is able to significantly reduce the residual moisture of anode and cathode coils, even with greater electrode lengths and coating widths, without deteriorating the sensitive microstructure of the electrodes.

Increased Moisture Uptake of NCM622 Cathodes after Calendering due to Particle Breakage

“…Post-drying procedure.-Post-drying of the coils was conducted in a vacuum oven VD 115 with a pump VAP 1 (final pressure 7 mbar, Binder GmbH & Co. KG, Germany) and a vacuum controller CVC3000 (Vacuubrand GmbH & Co. KG, Germany) which is installed in a dry room with a dew point of ∼−40 °C. The same post-drying procedure as in a previous study 8 was used. It consisted of a 2 h heating phase to 80 °C under atmospheric pressure and a 4.7 h post-drying phase at 80 °C with 14 Argon purging cycles between 10 and 30 mbar.…”

“…The achievement of the target temperature of 80 °C inside the coils was validated with temperature measuring stripes (Reatec-AG © , Switzerland). For further details, see Huttner et al 8 Determination of water content via Karl Fischer Titration.-The moisture content of the cathodes was measured via coulometric Karl Fischer Titration (KFT, Aqua 40.00 with headspace module, ECH Elektrochemie Halle GmbH, Germany). Therefore, the vials with the cathode samples were placed in the KFT oven, which was pre-heated to 120 °C.…”

“…The measuring temperature was determined beforehand with the help of temperature ramps and comparison of moisture measurements at different temperatures and finally set as low as possible to reproduce practical post-drying conditions and to prevent side reactions or the removal of chemically bound water. 5,8,9,34 For each analysis, a threefold measurement was conducted and averaged (for the value of the cathode with a density of 3.0 g cm −3 after post-drying, only one measurement was taken as the other two were measured incorrectly). As the current collector foils only adsorb very small amounts of moisture, the measured moisture was considered to come solely from the coating.…”

“…5 If the moisture is not sufficiently reduced before cell assembly, it can undergo unwanted side reactions with the conducting salt LiPF 6 during cycling, which is commonly used in organic electrolytes for LIBs. 5,[8][9][10] These side reactions cause the formation of gaseous hydrogen fluoride (HF) and further acidic decay products. This poses a high safety risk as it can cause explosion due to increasing cell pressure.…”

“…In contrast, the substantially higher moisture uptake of graphite anodes with carboxymethyl cellulose (CMC)/styrene-butadiene rubber (SBR) binder in comparison to PVDF cathodes is mainly caused by the hygroscopic binder system. 5,8,14 Thus, the sorption behavior of graphite anodes is almost exclusively determined by moisture sorption in the hygroscopic binders, especially in CMC. In comparison, surface adsorption and capillary condensation play a subordinate role.…”