Polyurethane Binder for Aqueous Processing of Li-Ion Battery Electrodes

Abstract

In this manuscript the investigation of the heat-curable polyurethane (PU)/carboxymethyl cellulose (CMC) mixture as binder for Li-ion battery electrodes is reported. The experimental results demonstrate the outstanding thermal and electrochemical stability of PU, as well as the good electrochemical performance of cathode (NMC) and anode (graphite) electrodes in which PU is used as binder. The most important finding is the ability of PU to prevent current collector corrosion usually occurring when an aqueous dispersion (slurry) of NMC is coated on aluminum foils. SEM investigation shows that PU encapsulates the positive active material particles preventing a pH raise in the slurry. Finally, the cycling performance of PU/CMC based anodes and cathodes are tested in half as well as full lithium-ion cell setups. The awareness of the finite nature of our earth´s natural resources as well as growing concerns for increased greenhouse gas emissions are leading to a strong demand for a more environmentally-friendly generation of energy. Considering the importance of sustainable en-ergy solutions, strong efforts are made to develop improved systems for large and small scale energy storage. 1,2 Currently, Li-ion batteries seem to be suitable energy storage devices to fulfil the requirements of small energy storage due to their high energy density and long lifetime. 3 Additionally, their lightweight fulfils very well the rising demand for use in smartphones, computers and other portable devices, as well as for electromobility solutions, facilitating the development toward a modern mobile society. Lithium-ion batteries are composed of several materials including metals and metal oxides, graphite and other carbonaceous materi-als, organic solvents, lithium salts and polymers. These latter are present in the porous separator, laying in between the electrodes (usually a polyolefin), and as binding components in the compos-ite electrodes. Binders are counted among the so-called inactive components since they do not directly contribute to the capacity of the cells. However, their key role in the electrode processing and their dramatic influence on the electrochemical performance of elec-trodes has been extensively outlined. 4–6 Relevant physical and chem-ical properties for binder materials are (i) thermal stability, chemical and electrochemical stabilities, (ii) tensile strength (strong adhesion and cohesion), (iii) flexibility, and (iv) viscosity (of the slurries). 4,7–9