Replacing N-methyl-2-pyrrolidone (NMP) with water
for processing of lithium-ion battery (LIB) electrodes has both cost
and environmental benefits, which include reduced drying time, lower
dryer capital cost, elimination of NMP recovery capital equipment,
and no release of volatile organic compounds (VOCs) into the environment.
However, aqueous-processed thick cathodes (≳4 mAh/cm2) typically exhibit detrimental cracking during drying that is not
observed for the NMP-based counterpart. The reasons for cracking of
these water-based thick electrodes are still not well understood due
to the complex nature of the colloidal dispersions used in the LIB
electrode processing steps. In this work, the contributions of various
factors responsible for cracking are discussed. We show that eliminating
hydrogen evolution due to corrosion of the aluminum current collector
eliminated the majority of the cracks regardless of the coating thickness,
identifying the gas evolution as the primary reason for electrode
cracking. Some secondary cracks and pinhole-type defects remained
after addressing the aluminum current collector corrosion, which are
thought to be caused by an inferior binding network formed by carbon
black and binder in aqueous-processed cathodes compared to those processed
with NMP. The thick aqueous processed cathodes are not able to sufficiently
withstand the drying stresses without crack formation. We demonstrate
reduction of these secondary defects by either improving the binding
network or by reducing the drying stress. The former was achieved
by replacing carbon black with vapor grown graphite tubes (VGGTs)
that caused a more efficient utilization of the emulsion binder. The
latter was achieved by adding a small amount of IPA as a co-solvent
that has been shown to reduce capillary stresses.
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