Microwave drying introduces key advantages over radiant and convective cooling for electrode drying (figure 1). Microwaves efficiently transfer energy through selective absorption by polar solvent (e.g. NMP) or water molecules. Microwave energy is also uniformly deposited through the bulk of the coated electrode. Electrode drying, however, introduces two barriers to use of microwave energy: avoiding arcing with the presence of metal foil current collectors, and supporting roll-to-roll coating without leakage of the microwave field through the coating web entry and exit ports. Navitas worked with a microwave processing company Lambda Technologies to resolve these barriers through the use of variable frequency microwaves (VFM).
VFM-dried electrodes were validated by Navitas through accelerated life testing and failure analysis of prototype cells. Navitas worked closely with Lambda to design a VFM module able to support continuous web drying operation – a key challenge.
Navitas installed a VFM module onto our Toyo WI pilot coating line and performed pilot scale electrode coating validation for several established commercial anode and cathode formulations to demonstrate accelerated drying with the absence of any deleterious effects on the electrode. The work by Navitas also involved the production of state of the art commercial baseline (i.e. non-VFM dried) benchmark electrodes. The VFM process showed significant advantages, with drying acceleration of >2X for NMP and >5X for water borne slurries. Thorough evaluation of adhesion, cohesion, conductivity, binder distribution etc. has shown the VFM electrode physical properties are as good as or better than the baseline (identical electrode coating passed through coater at lower speed with VFM module toggled off). The Navitas pilot coating trials validated potential cost savings and provided data for presentation to prospective commercial partners to establish confidence that existing coating lines can be retrofitted with VFM.
In the course of the validation runs the Navitas team observed that VFM electrodes showed significantly less binder segregation than conventionally dried electrodes. This is attributed to the fact that VFM deposits the drying energy more uniformly in the bulk of the electrode. Uniform energy distribution enables transport of the solvent to the surface as vapor phase rather than as a liquid that drags dissolved or suspended binder to the surface where radiant/convective drying energy transfer predominates. Our observation of reduced binder segregation led to the realization that the critical-to-quality features that limit the ability to coat cathodes at loadings >5mAh/cm2 in high volume production are greatly improved in the VFM process. These limitations include poor adhesion to the current collector (binder deprivation), cracking due to residual stress (binder deprivation), and defects or performance limitations associated with excess binder at surface. Data will be presented comparing VFM electrodes to commercial state of the art electrodes for electrode loading ranging from 3 to 8 mAh/cm2