Structuring of ultra-thick high-energy silicon/graphite anodes by multilayer coating

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Lithium-ion battery cells with high specific capacity and energy density are one of the current research priorities in industry and science. The challenges of these so-called „thick“ electrodes are transport limitations within the electrode: lithium ions cannot reach the deeper layers of the electrode coating, which leads to a drop in performance during charging and discharging. This phenomenon is a major hurdle in the further development of future high-capacity electrodes. One possible solution is to use multilayer electrodes with different layer properties. In this way, pathways for the Li-ions can be created that reduce the ionic resistance of the electrode and ensure an increase in fast charging capability.

The focus of this work is on two-layer anodes with different electrode layer configurations, whereby the influence of a varying active material is specifically taken into account. For this purpose, an active material mixture of graphite, hard carbon and silicon is used (≈ 575 mAh g- 1). Looking at the electrical conductivity of the multilayer anodes, it can be observed that the conductivity of the multilayer anodes increases significantly compared to the single-layer reference, which can be explained by the good conductivity of the hard carbon active material. A special focus was applied to the characterization by means of impedance spectroscopy. Here, the electrodes were examined for their ionic conductivity and thus conclusions were drawn about the pore network and the tortuosity of the electrodes. It was found that the two-layer anodes with an active material mixture of all three materials in the upper layer show a significant increase in their ion conductivity compared to a single-layer reference anode. One suggestion is that the addition of hard carbon could create a larger number of pores for the lithium ions, leading to improved ion transport and a reduction in transport limitations. In addition, promising multilayer anodes are being tested for their electrochemical performance in full cells. The electrochemical characterization includes in particular a comprehensive evaluation of the charging performance (fast charging capability, Li-plating) in order to evaluate the different layer configurations. It was found that the two-layer anodes with an active material mixture of all three materials in the upper layer show a significant increase in charging capacity, especially at low C rates, compared to a single-layer reference anode. Last but not least, the occurrence of lithium plating was also considered. No lithium plating could be detected at C-rates up to 5C.

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