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Lithium–silicon batteries are a lithium-ion battery technology under development that employ a silicon anode. Silicon has a much larger energy density than currently used anode materials, but the large volume change of silicon when lithium is inserted is the main obstacle to commercializing this type of anode material.
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History
The first laboratory experiments with lithium-silicon batteries took place in the late 1990s. The large volume change of silicon when lithium is inserted is the main obstacle to commercialization this type of anode material.
Silicon-graphite composite electrodes
Test sample production of batches of batteries using a silicon-graphite composite electrode started by the company Amprius in 2014. The same company claims to have sold several hundred thousands 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.
Also in 2014, a company called Enevate presented a battery using an unknown monolithic silicon-composite anode with a low cell resistance. These batteries leave 25% of the capacity unused, most likely to reduce fast degrading of the cell. For this technology it was named an Innovation Award Honoree in three categories at 2016's Consumer Electronics Show (CES). Shortly after CES 2016, it was announced that Sonim Technologies (a company selling rugged mobile phones) will be using Enevate's lithium-silicon batteries in its products.
Specific capacity
A crystalline silicon anode has a theoretical specific capacity of 4200 mAh/g, more than ten times that of anodes such as graphite (372 mAh/g). Each silicon atom can bind up to 4.4 lithium atoms in its fully lithiated state Li
4.4Si, compared to the one lithium atom per 6 carbon atoms for the fully lithiated state of graphite, LiC
6.
Silicon swelling
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, leading to fractures and crumbling of the silicon material and ill-fated 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.
Recent research has pointed to silicon nanostructures as a potential solution. A group at Stanford university created silicon nanowires on a conductive substrate for an anode, and found that not only does the nanowire morphology create direct current pathways to help increase the charge flow of the battery, but it also allows for the decreased disruption from volume change. However, the large volume change of the nanowires can still pose a fading problem for the anode, and active research on increasing anode lifetime and reliability.
Other studies have examined the potential of silicon nanoparticles. Anodes that use the comparatively inexpensive silicon nanoparticles may overcome the price and scale barriers of nanowire batteries, while also having more mechanical stability over cycling compared to typical silicon electrodes. Typically, these anodes also use 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 using conducting polymers as both the binder and the additive for nanoparticle batteries. One study examined a three-dimensional conducting polymer and hydrogel network inside which silicon nanoparticles reside. 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. Researchers at Trinity College in Dublin, Ireland 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 layer
Another factor that prevents commercialization of lithium-silicon batteries is the development of an unstable solid electrolyte interface SEI layer consisting of decomposed electrolyte material.
The SEI layer would normally form a layer impenetratable for electrolyte, which prevents further growth. However, due to the swelling of the silicon, the SEI layer cracks and become porous. Thus, it can grow to into thicker layers. A thick SEI layer results in a higher cell resistance, which decreases the 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. Limited growth of the SEI layer is therefore an important property needed to design commercial lithium-silicon batteries.