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First Work for Wikipedia (Lithium-Silicon Battery article)

Starting from the first cycle of lithium-ion battery operation, the electrolyte decomposes to form lithium compounds on the anode surface, producing a layer called the solid-electrolyte interphase (SEI). For both silicon and graphite anodes, this SEI layer is the result of the reduction potential of the anode. During cycling, electrons flow in and out of the anode through its current collector. Due to the strong voltages present during anode operation, these electrons will decompose the electrolyte molecules at the anode surface. The properties and evolution of the SEI fundamentally affect the overall battery performance through multiple different mechanisms. Since the SEI layer contains numerous lithium compounds, the production of the SEI reduces the total charge capacity of the battery by consuming some of the lithium that would otherwise be used to store charge. This is a degradation mechanism known as Loss of Lithium Inventory (LLI). Furthermore, the SEI’s lithium permeability affects the amount of lithium that the anode can store, while the SEI’s electronic resistivity determines how fast the SEI grows (the more electronically conductive, the more the electrolyte will be reduced and the faster the SEI will grow). When using lithium hexafluorophosphate (LiPF6) salts dissolved in a carbonate solvent, one of the most frequently used electrolyte compositions, SEI formation can also be caused by chemical reactions between the electrolyte and trace amounts of water, producing hydrofluoric acid (HF) that further reduces performance. In a lithium-silicon battery, the SEI plays an especially important role in capacity degradation, due to the large volumetric changes during cycling. Expansion and contraction of the anode material cracks the SEI layer that has formed on top of it, exposing more of the anode material to direct contact with the electrolyte, which results in further SEI production and LLI-based degradation.

Understanding the structure and composition of the SEI layer throughout cycling is critical for improving SEI stability and therefore improving battery performance. However, the composition of the SEI is not fully understood, both for graphitic and silicon-based anodes. For graphitic anodes in an LiPF6 and ethylene carbonate (EC) electrolyte, Heiskanen et al identified three distinct phases of SEI formation. First, the reduction of LiPF6 and EC respectively result in an SEI that is mostly lithium fluoride (LiF) and lithium ethylene dicarbonate (LEDC). Subsequently, the LEDC decomposes into a wide variety of components, which can be solid, gaseous, soluble in the electrolyte, or insoluble. The formation of gases and electrolytically-soluble molecules results in the SEI layer becoming more porous, since these species will diffuse away from the anode surface. This SEI porosity exposes the electrolyte to the anode surface, which results in the formation of more LEDC and LiF on the exterior of the SEI layer. Overall, these mechanisms result in the formation of an inner SEI layer that mostly contains the electrolytically insoluble compounds, and an exterior SEI consisting of the LEDC and LiF that form from electrolyte reduction. In a silicon-anode battery, a similar two-layer SEI structure also results, with inorganic compounds (lithium fluoride, lithium oxide, lithium carbonate, etc) forming an inner layer and organic compounds forming an outer layer.

Since the SEI is formed from the electrolyte, adjusting the electrolyte composition can have large effects on the capacity retention of lithium-silicon batteries. As a result, a wide variety of electrolyte additives have been tested and found to provide capacity improvements, such as silane molecules, succinic anhydride, citric acid, ethers, and additional carbonates (such as fluoroethylene carbonate and vinylene carbonate). These additives have the potential to improve performance through a variety of different mechanisms. For example, vinylene carbonate and fluoroethylene carbonate have both been shown to improve the SEI layer’s ability to block the electrolyte from interacting with the anode surface, potentially by increasing the SEI density. Another potential mechanism is highlighted by silane, which can form Si-O networks on the surface of the anode that stabilizes the organic SEI layer deposited on top of it