Effect of binder viscosity on the rheology of cathode slurries and performance of lithium-ion battery half cells

Abstract

Reliably benchmarking the performance of new cathode materials is an ongoing challenge in the development of high-capacity lithium-ion batteries. {Small variations in the formulation or processing of a cathode can severely affect the performance of the resulting battery cell.} Understanding and controlling interactions between the binder and other slurry components will enable faster, more rationalised battery cell optimisation. For the first time, this work systematically varies the viscosity of polyvinylidene fluoride (PVDF) binder solutions across a decade (15 to 320 mPa·s) whilst holding constant the solid loading of carbon black (CB) and cathode active material LiNi_0.8Mn_0.1Co_0.1O₂; NMC811). Steric stabilisation, rather than depletion or flocculation, is shown to be the mechanism by which PVDF modulates the structure of CB colloidal particles in a slurry. Gelled CB structures, evident in shear-thinning behaviour, are observed for all slurries, but two distinct CB gel regimes are evident: percolated and dispersed. The percolated regime occurs when the ratio of dissolved PVDF to free CB is reduced, e.g. by the incomplete dissolution of the PVDF. These samples exhibit thixotropic flow behaviour and lead to more porous cathodes. Well-dispersed CB gels exhibit reversible flow behaviour and form denser cathodes. The mass loading of cathodes increases with solution viscosity, and above a viscosity of 128 ± 46 mPa·s, the mass loading across the cathode tape becomes uniform. The first discharge capacity of battery cells increases monotonically with the viscosity of the PVDF solution, but no correlation is evident after 100 cycles. Process models that incorporate these relationships will improve the reproducibility of cathode coatings, enable higher coating weights and reduce materials losses leading to lower manufacturing costs and higher capacity batteries.