Fabrication of PVDF- and NMP-Free Lithium Ion Electrodes By Electrophoretic Deposition

Abstract

Lithium-Ion batteries electrodes are fabricated by mixing the active (Li-ion host) and conductive material along with polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (NMP) which are the binder and solvent respectively. The paste created is then doctor-bladed onto the current collector, convectively heated, and pressed. The problem with this method is that NMP is toxic and expensive and the binder has a limited solubility meaning replacing the solvent has proven difficult. Current attempts to eliminate the solvent have included finding a water soluble binder (such as Styrene Butadiene Rubber, SBR) or developing a new fabrication method. Among the new methods suggested is Electrophoretic Deposition (EPD). EPD is well suited to provide a low cost and green manufacturing technology for LIB battery electrodes thus contributing towards wider acceptance of electric vehicle transportation and renewable energy adoption. Thus EPD research can be very fertile ground in this domain. In particular, EPD systems that can produce thick electrode films using less hazardous solvents can be a major advancement. With this in mind our McGill HydroMET group has successfully used EPD recently to fabricate binder-free nanotitania/carbon anodes (Journal of The Electrochemical Society, 162, D3013-D3018 (2015)). The performance of the EPD-deposited electrodes was further improved via sintering, this was then compared to standard PVDF-based electrodes and shown to have higher capacity retention characteristics. These encouraging results prompted research into carbon-coated lithium titanate spinel (Li4Ti5O12, LTO) electrodes. After screening different solvents (e.g. water, isopropanol, and several aqueous/organic hybrids) an acetonitrile and water (90/10 vol% respectively) medium was selected which made a stable suspension for both the carbon coated LTO and the conductive carbon. To improve adhesion, SBR binder was added. EPD conditions were 60V for three 15-second stages which yielded a deposit thickness of ~20 μm. In this paper an overview of this work will be presented by emphasizing the selection of EPD medium, characterization of the film, and battery performance (evaluated via galvanostatic charging/discharging cycling tests). In addition post-EPD film processing (compression or sintering) are to be discussed as a means to improve the mechanical integrity of the film.