Rheological behaviour of Na-CMC and Na-alginate as binders for lithium ion batteries

Lithium ion batteries are the current technology of choice for electric vehicles due to their light weight, high volumetric and gravimetric energy density, low selfdischarge rate, quick charge acceptance, excellent cycle life and wide operating temperature range. However the current manufacturing process requires the use of polyvinylidene fluoride (PVDF), in order to bind the anode components within the electrode which prevents the recycling of the electrode materials. Furthermore PVDF necessitate the application of N-methyl-2-pyrrolidone (NMP), an environmentally toxic material as a solvent during the manufacturing. The aim of this research was to investigate the suitability of using water soluble Na-alginate as an alternative binder for the anodes of lithium ion batteries to reduce the environmental impact from current manufacturing processes and to allow recycling of the active anode components. The main function of the binder is the improvement of the mechanical strength and the adhesion of the electrode on the current collector. The properties of the binder which are most important during manufacturing are the rheological characteristics of the slurries comprising binder and active materials. This was the focus of this investigation. The viscosity of Na-alginate suspensions at 1% or 1.5% loading in water were typically 1 to 8 Pa·s which are similar to PVDF suspensions with loadings of 2.5 to 3.0%. Comparing to the benchmark Na-CMC, the degree of flocculation shows that for the same concentration of binder in water, Na-alginate suspensions are more stable, Na-alginate has a degree of flocculation of 1.17 while for Na-CMC it was 1.90. The rheology measurements show that Na-alginate slurries have a higher viscosity than Na-CMC at a shear rate of 50 s-1 with that for a 1.5% of Na-alginate binder being 1.26 Pa·s while for Na-CMC it was for 0.20 Pa·s. The loss factor was lower for Na-alginate, between 2 and 3 against between 2.9 and 3.3 for Na-CMC, showing a more developed network structure. The casting and drying process, demonstrated that while 1.5% Na-CMC in water wasn´t sufficiently viscous to cast the slurries it was possible to do it with Naalginate