Improving electrochemical performance of Li-ion batteries based on a holistic binder concept for water-borne electrode slurries
Energy Process Engineering
Final Report Abstract
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. This research project focused on understanding the fundamentals controlling the functionality of polymeric binders 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. 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 styrenebutadiene rubber (SBR) as reference binder system. First, we evidenced, that the addition of SBR enhances the cohesive strength of anode layers, which plays a key role in with-standing external stresses during electrode and cell fabrication, particularly at high mass loadings. However, augmented SBR concentrations lead to the deterioration of electrochemical cell performance, primarily attributed to elevated internal resistance and SBR degradation. Aiming at a reduction of the disturbing SBR content we investigated how the adsorbing CMC binder indirectly disturbs adhesion and cohesion of the electrode layer. An increase in CMC molecular weight, corresponding to greater uptake of the polymer on the active material surface resulted in a reduced adhesion of the graphite and graphite–micro-Si composite anodes on the current collector. In addition, the adhesion of electrodes can be affected by the structure of the active materials. We showed, that SBR particles can trapped inside nano-silicon agglomerates and consequently, graphite–nano-silicon composite anodes exhibit markedly low adhesion even though the nano-silicon particles adsorb only a very low amount of CMC. Driven by these insights we evaluated a binder concept based on a highly efficient, non-adsorbing crosslinked acrylate thickener, which enabled us to reduce the binder consumption per cell by about 40% and achieving an overall better rate performance than the reference anodes, particularly at high charge/discharge rates. Due to low cohesion, however, these anodes degraded somewhat faster than the CMC/SBR reference anodes. With further refinements regarding amount, molecular weight, and crosslinking of the polymer it should be possible to achieve better overall battery performance compared to the state of the art at a lower binder content.
Publications
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Effect of mechanical properties on processing behavior and electrochemical performance of aqueous processed graphite anodes for lithium-ion batteries. Journal of Power Sources, 593, 233996.
Hofmann, Katarzyna; Hegde, Akshay Dattatraya; Liu-Theato, Xinyang; Gordon, Ronald; Smith, Anna & Willenbacher, Norbert
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How carboxymethylcellulose adsorption and porous active material particles diminish the adhesion of graphite-silicon anodes in lithium-ion batteries. Energy Materials, 5(8).
Hofmann, Katarzyna & Willenbacher, Norbert
