Abstract
We investigated the effect of carboxymethyl cellulose (CMC) and the particulate fluorine/acrylate hybrid polymer (FAHP) on the flow behavior of LiFePO4-based cathode slurries as well as on electrical and mechanical properties of the corresponding dry layers. CMC dissolves in water and partly adsorbs on the active particles. Thus, it has a strong impact on particle dispersion and a critical CMC concentration distinguished by a minimum in yield stress and high shear viscosity is found, indicating an optimum state of particle dispersion. In contrast, the nanoparticulate FAHP binder has no effect on slurry rheology. The electrical conductivity of the dry layer exhibits a maximum at a CMC concentration corresponding to the minimum in slurry viscosity but monotonically decreases with increasing FAHP concentration. Adhesion to the current collector is provided by FAHP, and the line load in peel tests strongly increases with FAHP concentration, whereas CMC does not contribute to adhesion. The electrical conductivity and adhesion values obtained here excel reported values for similar aqueous LiFePO4-based cathode layers using alternative polymeric binders. Both CMC and FAHP contribute to the cohesive strength of the layers; the contribution of CMC, however, is stronger than that of FAHP despite its lower intrinsic mechanical strength. We attribute this to its impact on the cathode microstructure since high CMC concentrations result in a strong alignment of LiFePO4 particles, which yields superior cohesive strength.
Highlights
Lithium iron phosphate (LiFePO4) has been extensively investigated for over two decades since it was reported as a potential cathode material for lithium-ion batteries (LiB).[1]
fluorine/acrylate hybrid polymer (FAHP) as polymeric binders for Li-ion battery cathodes made from water-based slurries of LiFePO4 and carbon black (CB)
The volume fraction of active materials was kept constant and the concentration of Carboxymethyl cellulose (CMC) and FAHP was systematically varied in a wide range
Summary
Lithium iron phosphate (LiFePO4) has been extensively investigated for over two decades since it was reported as a potential cathode material for lithium-ion batteries (LiB).[1] Its high theoretical capacity (170 mA h g−1), stability during charge/discharge, thermal stability, low cost and toxicity, and its environmental compatibility as well as its safety make it a suitable cathode active material for large cell applications.[1−6] poor electrical conductivity (10−9 S cm−1) and a low Li+-ion diffusion coefficient (1.8 × 10−18 m2 s−1) at room tbeamttpereyractuatrheordeeprmesaetnertiailn.7t−ri9nsTichedrreafworbea, cckosnfsoidr eLraibFleePrOes4eaarscha work dealt with the improvement of ionic and electronic conductivity through decrease of particle size, addition of carbon, or ion doping.[10−17] the former concept has improved cell performance, agglomeration of LiFePO4 fine particles still constitutes a problem during processing of cathode slurries and limits electrochemical performance of corresponding electrodes Organic solvents, such as N-methyl-2pyrrolidone (NMP), are used for dispersing active materials and dissolving polymeric additives for control of processing behavior of the slurry during cathode manufacturing as well as to improve mechanical properties of the dry layer.[18−20] These solvents are environmentally harmful and toxic, flammable, and expensive. The effect of the polymeric binders on the cathode microstructure is studied and linked to the cohesive strength
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