Alternating Si/C Thin-Films as Model Anode Material for Lithium-Ion Batteries

Abstract

Graphite is currently the state-of-art material for the anode-side of lithium-ion batteries (LIB) with a theoretical capacity of 372 mAh g-1. However, its technological potential is already largely exhausted, shifting research focus to other materials, such as silicon (Si) with its high theoretical capacity (3579 mAh g-1) [1]. However, Si cannot be used on its own because of its low electrical conductivity. Moreover, it suffers from a strong increase in volume and amorphization during lithiation, which is the primary origin of several degradation mechanisms, e.g. as pulverization, delamination and an unstable solid electrolyte interface. There are various approaches to counteract these issues, amongst them the structuring of Si into nanowires or hollow structures [2]. Silicon is most often used with graphite in a composite material, due to decades of experience in graphite synthesis an application [3].In this work we investigate anodes consisting of alternating Si and C thin-films with a layer thickness of a few hundred nanometers each. In general, Si thin-film anodes show a higher performance than its bulk counterparts [4]. The layers are produced by deposition from the gas phase. This process enables good adhesion between the particles and the substrate, which can counteract delamination and provide a longer cycle life. Additionally, it can also be performed in a binder-free fashion, thereby reducing the need for inactive electrode components.The alternation of carbon and silicon layers improves the electrical conductivity and also separates the silicon layers from each other aiding to reduce the mechanical stress caused by volume expansion. The characterization of the layers is done by cross sectional SEM, EDX and AFM. Furthermore, the crystallinity of the deposited layers is determined via XRD. The manufacturing process influences the morphology of the layers, which in turn affects the electrochemical properties. For this reason, galvanostatic cycling and electrochemical impedance spectroscopy are subsequently carried out in a coin cell type Swagelok half-cell setup with 2- and 3-electrode arrangements. Constant current cycling is used to investigate the long-term stability of the cells in order to analyze the change in the respective Si layers as a function of their distance from the surface. In addition, the electrodes are analyzed post-mortem via SEM and XRD to investigate which degradation mechanisms play a role in the observed behavior.[1] Kasavajjula, U. et al., Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells, J. of Power Sources 163 (2), 1003 – 1039, 2007.[2] Wu, H., Cui Y., Designing nanostructured Si anodes for high energy lithium ion batteries, Nano Today 7 (5), 414 – 429, 2012.[3] You, S. et al., Design Strategies of Si/C Composite Anode for Lithium-Ion Batteries, Chem. Eur. J. 27 (48), 12237 – 12256, 2021 [4] Graetz, J. et al., Highly reversible Lithium storage in nanostructures Silicon, Electrochem. Solid-State Lett. 6 (9), A194, 2003.