Improving electrochemical performance of Li-ion batteries based on a holistic binder concept for water-borne electrode slurries

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Electrochemical energy storage plays an important role for reliable future energy supply and individual mobility. Today Li-ion batteries (LiB) dominate this technology field. Battery efficiency strongly depends on electrode engineering. Here we want to develop a binder formulation concept yielding electrode layers for LiB cells with improved electrochemical performance, high cyclic stability and long battery lifetime, exploiting the full capacity of the active material based on technical scale slurry formulations. The concept is based on a holistic approach including all aspects of binder functionality. We investigate binder specifications required to achieve slurry flow properties yielding uniform, defect-free electrode layers in industrial coating processes. Then, requirements for strong cohesion within the dry electrode layer and strong adhesion to the current collector are explored in order to avoid mechanical failure during Li-ion intercalation, resulting in high cyclic stability and long battery lifetime. Beyond that, we study the role of the binder as dispersing agent in order to control the microstructure of the electrode layer, i.e. the spatial distribution and orientation of active material and conductivity agent, to obtain high electrical conductivity and Li-ion mobility. This significantly affects the electrochemical cell performance although the binder is not involved in the electrochemical process. Anticipating a strongly growing demand of LiB we focus on waterborne slurries for electrode fabrication including state-of-the-art active materials and a combination of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) as binder system. We vary CMC molecular weight and degree of substitution systematically and hypothesize that an appropriate blend of different CMC types is needed to meet the diverse, partially conflicting interests at a minimum binder consumption. Corresponding slurries with promising binder composition are used to fabricate cells for electrochemical testing enabling us to validate and further improve the binder concept. Slurries with tailored binder composition are used to fabricate staged electrodes consisting of a thin layer including SBR next to the current collector thus providing strong adhesion and a SBR-free top layer with strong cohesion and CMC-controlled microstructure providing superior electrochemical properties. In this approach we also want to take advantage of a capillary force driven self-assembly process developed in our lab for an independent control of layer porosity aiming at a further improvement of cell performance. Our fundamental approach focusing on a tailored binder system to improve electrochemical performance of battery cells is demonstrated here using Li-NMC and graphite as active materials. The concept can be transferred to other systems including different active materials or binders in a straight forward manner, thus offering a generic approach for battery slurry formulation