Conventional lithium-ion batteries with graphite anodes are approaching their theoretical maximum in terms of energy density. To overcome this, replacing the standard graphite anodes with high-capacity silicon-based anodes is one of the most promising ways to greatly boost the energy density of lithium-ion batteries. However, the enormous volume expansion that silicon-based materials naturally undergo after lithiation and the ensuing series of unresolvable problems, including the production of an unstable Solid Electrolyte Interphase (SEI) layer, electrode cracking, and particularly the rapid capacity degradation of cells, severely restrict the practical use of silicon anodes.
Since there are more surface atoms in nanometre-sized active silicon particles than in bulk silicon particles, there is a higher degree of reactivity and ultimately reduced ‘swelling’ with silicon nanoparticles. The nanosized particles can therefore withstand higher stress levels without cracking, which can be further reduced by either encasing, coating, or adopting other techniques that restrict surface-electrolyte interaction. In order to increase battery integrity, recent research has focussed on adding carbon or polymer coatings to silicon nanoparticles. However, some of the fundamental problems with using silicon are still present, despite reducing the size of the silicon particles and incorporating structural rigidity.

Key factors that affect the Lithiation Kinetics of Silicon

Electronic conductivity:

It has been discovered that several parameters, including elemental doping, composites with conductive polymers, and carbon coatings, have a significant impact on the lithiation performance of silicon anodes. The introduction of phosphor doping has been found to have a significant impact on the lithiation rate of silicon anodes regardless of any applied coating with reaction speeds increasing by several orders of magnitude due to the enhanced electrical conductivity. In order to achieve optimum loading efficiency, high energy density, robustness, and fully utilise the available active silicon, efficient doping to increase the conductivity of the silicon anode is very important.

Native oxide layers:

Amorphous silicon oxide layers are typically present on the surface of pure silicon nanoparticles. Since it is an insulating layer, it sometimes prevents the electron conduction required for lithiation. Recently, some researchers extensively examined the impact of the native silicon oxide layer on battery performance. As lithiation causes silicon nanoparticles to grow, they transform the surface silicon oxide layers into lithium oxide at low biasing voltage. Large gaps then appear in the lithium oxide crystalline layer after lithiation and provide routes for electron conductivity. The lithiation properties are likely linked to the system’s higher impedance, which results from the formation of lithium oxide. The reduced conductivity caused by oxide layers results in similar behaviour to in-situ open cell tests utilising a lower biasing voltage than a real battery.Researchers commonly found a porous structure in the silicon nanoparticles after delithiation, a phenomenon observed in other research as well.

Alucone coatings:

Some researchers have investigated the functional mechanisms associated with alucone coated silicon nanoparticles upon electrochemical cycling. Chemically etching into the thin silicon oxide layer generates Alucone coatings, forming a mechanically flexible metal-carbon-oxygen network. During lithiation, the alucone coating improves conductivity. The fast electron conductivity of lithium and the mechanical toughness of the alucone covering cause the silicon nanoparticles coated with the alucone to exhibit ‘balloon-like’, rapid, and highly reversible lithiation behaviour. Overall, the electrical and mechanical benefits provided by the alucone coating improve the cyclic performance and coulombic efficiency of the battery.

Metallic coatings:

A further method to improve the electronic conductivity of nanostructured silicon-based batteries are metallic coatings. Although the metallic coating must transport lithium ions, it should not react with the lithium or the electrolyte. A partial coating of the nanostructure as opposed to a complete coating is one approach to achieving improved conductivity and battery robustness during lithiation. Some researchers have suggested that copper coatings are highly advantageous and can mitigate volume expansion during lithiation. They deposit the copper coating on only one side of the nanoparticles to prevent enclosing the lithium transportation pathway. During lithiation, the length and width of the silicon nanoparticles expand whilst the copper support structure remains intact, providing structural rigidity.

Conductive polymer binders:

Over the past ten years, numerous studies have demonstrated the need for polymer binders for stabilising cycling, preventing volume expansion, and maintaining the integrity of silicon-based anodes during lithiation. People can use both natural and artificial materials to create polymer binders, and each material type has its own set of advantages and disadvantages. Synthetic polymer binders, for example, have several advantages over natural polymer binders, such as the ability to create structure-optimised designs and functionality-tuned capabilities for producing high-performance silicon electrodes. Researchers can modify the conductive and self-healing synthetic polymer binders they have created to enhance the mechanical and electrical integrity of silicon electrodes.

Advanced electrolyte additives:

Electrolyte additives can influence the performance of the silicon electrode during cycling. One of the most effective, cost-effective, and practical methods for resolving the problems associated with significant volume changes and early capacity loss of silicon electrodes during lithiation is the use of a modest quantity of functional additives. Researchers still widely use Fluoroethylene Carbonate (FEC) and Vinylene Carbonate (VC) as additives in electrolyte solutions for silicon electrodes.

The scientific community generally recognizes FEC as the most effective electrolyte additive for forming a passivating SEI layer on the silicon surface. Researchers acknowledge LiF, polymerized FEC, and ROCO2Li as the primary by-products of FEC decomposition on the electrode. These decomposition products can contribute to the formation of a uniform and stable SEI layer on the silicon surface, thus preventing further decomposition of LiPF6 salts and FEC at lower potentials. Researchers also identified VC as an effective additive for silicon thin-film anodes, with suggestions that VC additives outperformed FEC additives concerning lifetime and efficiency, particularly for nano-silicon anodes.

Prelithiation:

One of the methods to reduce the irreversible capacity loss during the first cycle of a silicon nanoparticle lithium-ion battery is prelithiation. Several prelithiation methods, such as electrochemical prelithiation, chemical prelithiation, additive prelithiation, and lithium metal prelithiation, have been studied. Some researchers have introduced stabilised lithium metal powder into the anode prelithiation in lithium-ion batteries. One of the most popular methods for doing this is spreading the stabilised lithium metal powder on the electrode surface followed by a pressing process to activate this powder. Instead of using lithium metal powder, some other researchers have utilised other lithiated active materials such as dry-air-stable lithium-silicide oxide to prelithiate the silicone anode to improve robustness.

The Future of Nanoparticle Silicon Anode-based Lithium-ion Batteries

Producing incremental improvements in lithium-ion battery chemistry is the most practical way to produce better batteries, with the specifications of these batteries varying depending on their usage. The simplest method to improve battery performance is to gradually replace graphite with silicon and introduce some of the characteristics discussed in the previous section. Stakeholders should carry out a cost-benefit exercise on the mentioned key factors to enable the attainment of maximum benefits with minimal costs. This will ultimately lead to the development of cost-effective, high-performance batteries.
Despite active development of new production methods and particle structures, the commercialization of silicon nanoparticle-based batteries is lagging. Therefore, new ways of commercialising silicon anode batteries should be implemented to ensure the proliferation of these energy-dense energy storage devices.