Lithium–silicon electrodes are a lithium-ion battery technology that employ a silicon anode and lithium ions as the charge carriers. Silicon has a much larger energy density (25 times as many lithium ions) than graphite. Silicon's large volume change when lithium is inserted is the main obstacle to commercializing this device. Commercial battery anodes may have small amounts of silicon, boosting their performance slightly. The amounts are closely held trade secrets, limited as of 2018 to at most 10% of the anode.
The first laboratory experiments with lithium-silicon materials took place in the early to mid 1970s.
Silicon-graphite composite electrodesEdit
Silicon carbon composite anodes were first reported in 2002 by Yoshio  Studies of these composite materials has shown that the capacities are a weighted average of the two end members (graphite and silicon). On cycling electronic isolation of the silicon particles tends to occur with the capacity falling off to the capacity of the graphite component. This effect has been tempered using alternative synthetic methodologies or morphologies that can be created to help maintain contact with the current collector. This has been identified in studies involving grown silicon nanowires that are chemical bonded to the metal current collector by alloy formation. Sample production of batteries using a silicon NW -graphite composite electrode were produced by Amprius in 2014. The same company claims to have sold several hundred thousand of these batteries as of 2014. 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 interface layer. These microparticles reached an energy density of 3,300 mAh/g.
As of 2018, products by startups Sila Nanotechnologies, Angstron Materials, Enovix, Enevate, and others were undergoing tests by the battery manufacturers, car companies, and consumer-electronics companies. Sila clients include BMW and Amperex Technology, battery supplier to companies including Apple and Samsung. BMW plans to incorporate Sila technology by 2023 and increase battery-pack capacity by 10-15%.
|Anode material||Specific capacity (mAh/g)||Volume change|
A crystalline silicon anode has a theoretical specific capacity of 3600 mAh/g, approximately ten times that of anodes such as graphite (372 mAh/g). Each silicon atom can bind up to 3.75 lithium atoms in its fully lithiated state (Li
3.75Si), compared to one lithium atom per 6 carbon atoms for the fully lithiated graphite (LiC
The lattice distance between silicon atoms multiplies as it accommodates lithium ions (lithiation), reaching 320% of the original volume. The expansion causes large anisotropic stresses to occur within the electrode material, fracturing and crumbling the silicon material and detachment from the current collector. Prototypical lithium-silicon batteries lose most of their capacity in as little as 10 charge-discharge cycles. A solution to the capacity and stability issues posed by the significant volume expansion upon lithiation is critical to the success of silicon anodes.
Silicon nanostructures are one potential solution. Researchers created silicon nanowires on a conductive substrate for an anode, and found that the nanowire morphology creates direct current pathways to help increase power density and decreases disruption from volume change. However, the large volume change of the nanowires can still pose a fading problem.
Other studies examined the potential of silicon nanoparticles. Anodes that use silicon nanoparticles may overcome the price and scale barriers of nanowire batteries, while offering more mechanical stability over cycling compared to other silicon electrodes. Typically, these anodes add carbon as a conductive additive and a binder for increased mechanical stability. 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.
Another nanoparticle approach is to use conducting polymers as both the binder and the additive for nanoparticle batteries. One study examined a three-dimensional conducting polymer and hydrogel network to carry silicon nanoparticles. The framework resulted in a marked improvement in electrode stability, with over 90% capacity retention after 5,000 cycles. However, the potential for inexpensive scale up has not been thoroughly investigated. Other researchers offered another potential solution, utilizing slurry coating techniques – which are currently employed at large scales for electrode production – with a conducting polymer binder. In general, the conducting polymer additive provides both mechanical stabilization and an avenue for conduction, replacing the conventional two-material system of a polymer stabilizer and carbon black particles. The substitution allows both better stabilization and better conduction.
Solid electrolyte interface layerEdit
Another issue is the destabilization of the solid electrolyte interface (SEI) layer consisting of decomposed electrolyte material.
The SEI layer normally forms a layer impenetratable to the electrolyte, which prevents further growth. However, due to the swelling of the silicon, the SEI layer cracks and become porous. Thus, it can thicken. A thick SEI layer results in a higher cell resistance, which decreases cell efficiency.
The SEI layer on silicon is composed of reduced electrolyte and lithium. At the operating voltage of the battery, the electrolyte is unstable and decomposes. The consumption of lithium in the formation of the SEI layer further decreases the battery capacity. Limiting growth of the SEI layer is therefore critical for commercial lithium-silicon batteries.
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