(Invited) Manufacturing and Characterization of Advanced High Energy Silicon/Graphite Electrodes

Abstract

Next generation lithium-ion batteries (LIB) with high energy density and high power density have recently become of great interest for electric vehicle and portable devices. With the further upgrade of especially electric vehicles, the next generation LIB with high power and high energy density is urgently required. For this purpose, composite electrode consisting of commercially available graphite active material mixed with silicon nanoparticles is under current development. The main objectives are a significant increase of the practical capacity and energy density of commercial anodes, an overcome of the drawbacks of pure silicon due to large volume changes during electrochemical cycling, and the development of a technology suitable for mass production. In order to reduce the intrinsic mechanical stress of silicon/graphite electrodes and to improve the lithium-ion transport kinetic, free-standing electrode structures were generated by applying ultrafast industrial capable laser material processing. This advanced laser technology is demonstrated to be a flexible and powerful tool for pushing silicon/graphite (Si/C) composite anode materials beyond state of the art electrodes towards application. The electrochemical properties of cells with unstructured and structured electrodes were systematically analyzed by means of cyclic voltammetry, galvanostatic measurements, and electrochemical impedance spectroscopy. The increased active surface enables a significant improvement of lithium-ion diffusion kinetics. Furthermore, it is expected that the increased active surface will also provide additional artificial porosity for active material expansion, which in turn will reduce the mechanical stress within the electrodes during lithiation or delithiation. In this context, in-situ scanning electron microscopy (SEM) was performed in order to analyze the active material volume changes during charging and discharging. A main engineering challenge was to optimize the electrode architecture such as the pitch distance of free-standing structures regarding an enhanced electrochemical performance and a reduced material loss. Furthermore, an alumina (Al2O3) layer with a thickness of 5 nm, which acts as an artificial SEI, was coated on structured silicon/graphite electrodes by applying Atomic Layer Deposition (ALD). Cyclic voltammetry measurements were subsequently performed in order to investigate the fundamental properties of cells with structured and Al2O3- coated silicon/graphite electrodes. Galvanostatic measurements reveal that the cells with structured electrodes exhibit excellent electrochemical properties, i.e., a significantly improved capacity retention. ALD layers can contribute to further improvement of cycle stability and cell lifetime. In addition, advanced full cells with Lithium Nickel Manganese Cobalt Oxide NMC622 as cathode and Si/C as counter electrode were assembled and the electrochemical data will be presented.