Lithium–silicon battery
[3] Silicon's large volume change (approximately 400% based on crystallographic densities) when lithium is inserted, along with high reactivity in the charged state, are obstacles to commercializing this type of anode.
[9] In 2016, Stanford University researchers presented a method of encapsulating silicon microparticles in a graphene shell, which confines fractured particles and also acts as a stable solid electrolyte interphase layer.
[15] Group14 Technologies has patented a silicon-carbon composite SCC55, which enables 50% more in fully lithiated volumetric energy density than graphite used in conventional lithium-ion battery anodes.
[16] In May 2022, Porsche AG announced plans to produce lithium-silicon battery cells with Group14's technology in Germany in 2024 to help power their new electric vehicles.
Because the volume expansion and contraction properties of nanoparticles differ greatly from the bulk material, silicon nanostructures have been investigated as a potential solution.
While they have a higher percentage of surface atoms than bulk silicon particles, the increased reactivity may be controlled by encasement, coatings, or other methods that limit surface—electrolyte contact.
However, this geometry does not fully solve the issue of large volume expansion upon lithiation, exposing the battery to increased risk of capacity loss from inaccessible nanoparticles after cycle-induced cracking and stress.
One study examined a three-dimensional conducting polymer and hydrogel network to encase and allow for ionic transport to the electrochemically active silicon nanoparticles.
Other methods to accomplish similar outcomes include utilizing slurry coating techniques, which are inline with presently used electrode creation methodologies.
[36][37] Recent work by Han, et al., has identified an in-situ coating synthesis method that eliminates the redox activity of the surface and limits the reactions that can take place with the solvents.
Although it does not affect the issues associated with volume expansion, it has been seen with Mg cation based coatings to increase the cycle life and capacity significantly[38] in a manner similar to the film forming additive fluoroethylene carbonate (FEC).
[39] 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 interface (SEI).
[43] In a lithium-silicon battery, the SEI plays an especially important role in capacity degradation, due to the large volumetric changes during cycling.
[45] For graphitic anodes in an LiPF6 and ethylene carbonate (EC) electrolyte, Heiskanen et al. identified three distinct phases of SEI formation.
The formation of gases and electrolytically soluble molecules results in the SEI layer becoming more porous, since these species diffuse away from the anode surface.
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.
