Radiation Curing and Its Application in Li Cells

Abstract

Li-ion battery composite electrodes consist of three components including active materials, conductive additives and polymer binders. Conventionally, the electrodes are manufactured through a slurry-based process using polymer binder, typically polyvinylidene difluoride (PVDF) that is dissolved in organic solvent, N-Methyl-2-pyrrolidone (NMP). A powder mixture of active materials and carbon black is added to the solution to form a slurry, which is coated onto a current collector (i.e. aluminum foil) and then passed through drying ovens to evaporatethe NMP. Several drawbacks have been identified using the conventional manufacturing method.1–4 These processes require high energy consumption from electrode drying, a large equipment footprint, low production speed and mandatory solvent recovery, which results in a significant portion of battery cell and battery plant cost. It has been calculated that the total electrode manufacturing costs can contribute about 8-9% of the total pack cost.1 Alternative processing methods of electrode coatings are being sought for faster production and lower cost. Radiation-based curing has been developed to replace the solvent-based methods in certain sectors of the coating industry.5 The former offers significant process energy savings, is ultra-high speed, and utilizes much more compact equipment than conventional drying ovens (much less plant floor space required). Radiation curing employs ultraviolet (UV) light or an electron beam (EB) as the energy source to chemically polymerize and crosslink small molecules (i.e. monomers or oligomers) into high molecular weight (MW) crosslinked polymers. Therefore, a minimal amount or even no volatile organic compounds (VOCs) are required during the coating formulation and deposition. Both UV and EB cured Li-ion battery cathodes have been reported to have good performance in lab coin cells.6,7 However, the penetration of UV is limited by its low energy and the black solid powders (active material and carbon black) further decrease the curing efficiency due to blockage of the UV light. The UV cured electrode loading is 6 mg/cm2,6 which is significantly lower than EB cured electrodes.7 The penetration of EB radiation is related to the applied accelerating voltage, which means 25 mg/cm2 (~ 4mAh/cm2) loading can be easily cured with EB equipment operating at 250 keV. Given the Li-ion battery industry is seeking even higher areal loadings to achieve higher energy density,8–10 EB curing is likely more suitable for electrode processing compared to UV curing. Based on decades of development and commercial deployment, self-shielded EB machines routinely operate with high reliability and low maintenance in industrial roll-to-roll production environments. In this study, we report the scale-up of high speed EB curing for processing thick battery electrodes with EB-curable acrylate binders. High speed EB curing at a 500 feet-per-minute (fpm) line speed were successfully applied to thick cathodes (25 mg/cm2). The performance of the EB cured cathodes were evaluated in 1.5 Ah pouch cells.