New Formulations of High-Voltage Cathodes for Li-Ion Batteries with Water-Processable Binders

Abstract

While not electrochemically active, the binder still remains a component of great importance in the electrodes for lithium-ion batteries, as it has a strong influence on their performance. Polyvinilydene fluoride (PVdF) is the most employed binder, thanks to its electrochemical stability and good adhesive properties. However, PVdF is expensive, not readily disposable, and requires the use of organic solvents, such as N-methyl-2-pyrrolidone (NMP), which is also very expensive as well as harmful to health and environment [1]. The use of water-soluble binders could lower production costs and grant an easier and more environment-friendly processing. Although carboxy-methyl-cellulose (CMC) is the most investigated aqueous binder for lithium-ion batteries, many other promising materials are under study [2]. This work investigates the use of two water-soluble binders, namely poly vinyl acetate (PVAc) and sodium alginate, for high-voltage cathode electrodes in Li-ion batteries. PVAc, commercially available under the Vinavil trademark, is an inexpensive synthetic polymer, employed in a wide variety of industrial applications [3]; it was previously investigated by P. Prosini et al. as binder for Li-ion batteries [4, 5], and the published results encouraged further studies. Sodium alginate is a high-modulus, natural polysaccharide commonly extracted from brown algae. It has a peculiar composition since it contains carboxylic groups in each polymer’s monomeric unit. It was also extensively studied as binder for both anodes and cathodes with very promising results [6, 7]. We focused our work on the use of sustainable binders for cathodes based on LiNi0.5Mn1.5O4 (LNMO), a commercially available material with a very high Li+ deinsertion/insertion potential of 4.7-4.75 V versus Li+/Li and a theoretical specific capacity of 147 mAh/g. These properties account for the highest energy density of any material currently available on the market [8]. The present study reports the morphological and structural characterizations carried out on the LNMO-based composite materials and the electrochemical tests at 30°C on the corresponding electrodes assembled in half-cells versus Li/Li+ in conventional electrolyte (EC/DMC – LiPF6). The electrochemical performance of cathodes with PVAc and alginate binders are compared with those obtained with PVdF-based electrodes. While the results obtained for PVAc-based composite material should be improved, the alginate-based material shows very promising electrochemical results, comparable or even better than those of PVdF-based materials. Acknowledgements The authors wish to thank ENEA and Italy’s Ministero dello Sviluppo Economico for financial support under the Program “Electric System Research”, Cathode Materials for High Energy Lithium Ion Batteries.