Lithium-silicon battery use lithium ions and silicon-based anode as the charge carriers. A huge specific capacity is generally possessed by silicon-based materials. 

Silicon is possessed in small quantities by the commercial battery anodes, and it slightly boosts their performance. Due to its remarkably high specific capacity, there have been extensive studies on silicon as a material of anode for lithium-ion batteries (LIB).

LIBs have different applications among various industries because of their stable performance, comparatively high energy density, and environmental benignity. LiBs market size can also surpass the market size of the portable electronics in electric vehicles case. There has been a great demand for batteries with enhanced safety, lower cost, higher power, and energy density in this regard. There has been the adoption of lithium-ion batteries (LIBs) for portable electronic devices as the major energy storage technology and they are also being taken into consideration for various markets like grid-scale energy storage.

Introduction

A substrate of lithium-ion battery technology is known by the name lithium-silicon battery and they use lithium ions and silicon-based anode as the charge carriers. A huge specific capacity is generally possessed by silicon-based materials, for instance for pristine silicon, it's 3600 mAh/g, as compared to graphite that has 372 mAh/g as its maximum theoretical capacity for LiC6 the fully lithiated state. When lithium is inserted, the large volume change of silicon (almost 400 percent based on crystallographic densities) is one of the major complications and the high reactivity in the charged state to this type of anode’s commercialization is another major complication.

Silicon is possessed in small quantities by the commercial battery anodes, and it slightly boosts their performance. The nearly held trade secret is the amount, as in 2018 it was limited to 10% of the anode. Cell configurations are also included in the lithium-silicon batteries where Si at low voltage may store lithium through a displacement reaction as Si is in compounds, including silicon nitride, silicon monoxide, or silicon oxy carbide.

Due to its remarkably high specific capacity, there have been extensive studies on silicon as a material of anode for lithium-ion batteries (LIB). Although, during the discharge and charge process, usually the silicon-based anode materials go through a large volume change resulting in continuous side reactions, loss of electric contact, and silicon's subsequent pulverization. The poor life cycle is caused by these transformations and they cause a hindrance in silicon's broad commercialization for LiBs. There have been progressive studies and understanding of the interphase reaction mechanisms, and the lithiation and de-lithiation behaviors.

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In comparison to commercial carbon-based anodes

As compared to commercial carbon-based anodes, there have been reports of numerous nanostructured silicon anodes that possess both excellent cycle life and specific capacity. Although, there are some practical issues regarding nanostructured silicon that can't be ignored and if it is going to be utilized broadly in commercial LIBs then it should be addressed. Major affective work on the silicon-based anodes is outlined in this review along with the latest research directions in this field, particularly, silicon architecture’s engineering, silicon-based composite’s construction, and other performance-improvement studies for instance binders and electrolytes.

There is special stress on the burgeoning research efforts in developing fuel-cell silicon-based LIBs and practical silicon electrodes, which are key to silicon anode's successful commercialization, and next-generation high energy density LIB's large-scale deployment. Recently, there has been high dependence of mankind on non-renewable energy that has resulted in an increase in the number of concerns on human health, climate, and environment.

Significance

Right now, one of the major and interesting topics around the globe is the research and development of clean energy and it is seeking more and more attention still. Over the world, there has been an increase in the market size of clean energy from wind and solar, resulting in an extremely strong requirement for extremely efficient energy conversion and storage devices for clean energies' broad utilization. Huge efforts on the other hand have also been devoted to vehicle electrification for lessening petroleum's reliance on us, while there are the correspondingly appropriate energy storages devices still under probe. There has been the adoption of lithium-ion batteries (LIBs) for portable electronic devices as the major energy storage technology and they are also being taken into consideration for various markets like grid-scale energy storage.

Applications of Silicon-Lithium Batteries

IBs have different applications among various industries because of their stable performance, comparatively high energy density, and environmental benignity. LiBs market size can also surpass the market size of the portable electronics in electric vehicles case. Although due to impractical driving ranges, most of the EVs are not still competent enough by now for replacing traditional vehicle due to impractical driving ranges. The battery's energy density/size determines the range from a single charge. When the integrated battery's size increases, it also increases the whole electric vehicle's mass and EV's cost, lessening the range. An optimization problem is introduced because of this dependence loop between the total vehicle mass, battery size, driving range, and cost, and the EV's system design completely determines it as it strongly depends on it. The presence of EVs with near practical ranges can be seen in the market but the large battery pack's high cost typically makes them too expensive.

IBs have different applications among various industries because of their stable performance, comparatively high energy density, and environmental benignity. LiBs market size can also surpass the market size of the portable electronics in electric vehicles case. Although due to impractical driving ranges, most of the EVs are not still competent enough by now for replacing traditional vehicle due to impractical driving ranges. The battery's energy density/size determines the range from a single charge. When the integrated battery's size increases, it also increases the whole electric vehicle's mass and EV's cost, lessening the range. An optimization problem is introduced because of this dependence loop between the total vehicle mass, battery size, driving range, and cost, and the EV's system design completely determines it as it strongly 

There has been a great demand for batteries with enhanced safety, lower cost, higher power, and energy density in this regard. USABC set a long-term goal for addressing this issue. According to that goal, the LiB pack system’s energy density should reach 500 Wh L -1 or 235 Wh kg-1 at a discharge rate of 1/3 C (1/3 C discharge rate refers to a battery that one can completely discharge in 3 hours), along with the need of calendar life of 15 years and up to 1000 cycles.

Silicon-graphite composite electrodes

Yoshio first reported the silicon-carbon composite anodes in 2002. According to the studies of these composite materials, the capacities are the two end member's weighted average (silicon and graphite). On cycling, Silicon particle's electronic isolation takes place with the capacity falling off to the graphite component's capacity. Alternative synthetic morphologies or methodologies are used for tempering this effect and they can be made for helping in maintaining contact with the current collector and there have been identifications of it in studies and they involve grown silicon nanowires and alloy formation is used to chemically bind them to the metal current collector.

In 2014, Amprius used a silicon nanowire-graphite composite electrode to produce the sample production of batteries. In 2014, many hundred thousands of these batteries were claimed to be sold by the same company. A method of the encapsulation of silicon microparticles in a graphene sheet was presented by Stanford University researchers in 2016. The fractured particles are confined by that method and that also functions as a stable solid electrolyte interphase layer. 3,300 mAh/g of energy density was reached by these microparticles. Elon Musk, the founder of Tesla, claimed in 2015 that the range of the car can be increased by 6% by using silicon in Model S batteries.

In 2018, consumer-electronics companies, car companies, and battery manufacturers took products by startups Sila Nanotechnologies, Group14 Technologies, Enevate, Enovix, Global Graphene Group, and others under tests. Battery suppliers, Amperez Technology, and BMW are included in Sila clients for companies including Samsung and Apple. It is planned by BMW to incorporate Sila technology in 2023 to enhance the battery-pack capacity by ten to fifteen percent. Enovix was the first company until now to transport finished silicon anode batteries to the end customers. SCC55™, a silicon-carbon composite is patented by Group 14 technologies that allow 50% more in completely lithiated volumetric energy density as compared to the graphite that's been utilized in the conventional lithium-ion battery anodes.

SK materials, Showa Denko, and Amperex Technology Limited have backed Group 14. Tesla showed on September 22, 2020, about its plans to increase the amounts of silicon gradually in its future batteries, specifically focusing on the anodes. Tesla plans to use an ion-permeable elastic coating for encapsulating silicon particles. The silicon-swelling concern is accommodated in this way, thus enabling the required increase in the battery capacity to be attained. This charge doesn’t cause an impact on the overall battery life expectancy. Confirming stepwise changes and Enabling testing were the reasons for the gradual and not sudden increases in the usage of silicon.

Specific capacity

3600 mAh/g of theoretical specific capacity is possessed by a crystalline silicon anode, which is almost 10 times more than that of the usually utilized graphite anodes (restricted to 372 mAh/g). In its fully lithiated state (Li3.75Si), each atom of silicon can bind up to 3.75 lithium atoms (Li3.75Si), whereas for the fully lithiated graphite (LiC6), it's one lithium atom per 6 carbon atoms.

Silicon swelling

As the silicon atoms go through lithiation (accommodates lithium ions), the lattice distance between the silicon atoms multiplies, reaching 320 percent of the original volume. Large anisotropic stress takes place due to the expansion within the electrode materials, therefore causing a fracture and crumbling in the material of the electrode, and detaching from the current collector.

In less than 10 discharge-charge cycles, most of the capacity is lost by the prototypical lithium-silicon batteries. On lithiation, major volume expansion is important for the success of the silicon anodes as it also gives a solution for the stability and capacity issues that are posed. There have been a huge number of investigations on silicon nanostructures as a potential solution because the contraction characteristics and volume expansion of nanoparticles are extremely different from that of the bulk. As compared to the bulk silicon particles, they possess a higher percentage of the surface atoms.

Coatings, encasement, or other methods, are used to control the increased reactivity and limit surface—electrolyte contact. Researchers have identified one method in which they suggested utilizing silicon nanowires on a conductive substrate for an anode and found that direct current pathways are created by the nanowire morphology for helping an increase in the power density and the change in the volume decreases disruption. Although still a fading problem can be posed by nanowires large volume change. The potential of silicon nanoparticles was examined in the other studies. As compared to the other silicon electrodes, anodes utilizing silicon nanoparticles offer much more mechanical stability over cycling and they may overcome the scale and price barriers of the nanowire batteries. These anodes typically add a binder for improved mechanical stability and carbon as a conductive additive. Although even upon lithiation, the issue of large volume expansion is not fully solved by this geometry, therefore exposing the battery to improved risk of capacity loss from the inaccessible nanoparticles after cycle-induced stress and cracking.

Approach of Nanoparticles

Utilizing a conducting polymers matrix as both the polymer electrolyte and the binder for the nanoparticle batteries is another nanoparticle approach. A hydrogel network and a 3-dimensional conducting polymer were examined in one study for encasing and allowing for ionic transport for the electrochemically active silicon nanoparticles. After 5,000 cycles, with over 90 percent of capacity retention, the framework led to a marked enhancement in the stability of the electrode. Slurry coating techniques are utilized and are some of the other methods for accomplishing similar outcomes, which are in line with currently utilized electrode creation methodologies. Zhang et al. used 2-dimensional, covalently bound silicon-carbon hybrids in a recent study for reducing stabilize capacity and volume change.

Charged Silicon Reactivity

Other than the well-recognized problems associated with the large volume expansion, for instance, cracking the SEI layer, there is another issue that involves the charged material's reactivity. Lithium silicide is charged silicon. The salt-like structure of lithium silicide is made from a combination of lithium cations and silicon (-4) Zintl anions. There is a high reduction of these silicide anions and they exhibit high reactivity with the electrolyte components, solvent’s reduction locally compensated the charge.

An in-situ coating synthesis method has been identified by Han et al. in his recent work and that method eliminates the surface’s redox activity and restricts the reactions that can occur with the solvents. However there is no influence of it on the issues that are associated with the volume expansion, Mg cation based coating are shown to improve the capacity and cycle life considerably in a manner same as the film producing additive fluoroethylene carbonate (FEC).

Solid Electrolyte Interphase Layer

Lithium compounds are formed on the surface of the anode by the decomposition of the electrolyte starting from the lithium-ion battery operation's first cycle, therefore forming a layer that is known as the solid-electrolyte interphase (SEI). The SEI layer is due to the anode's reduction potential for both silicon and graphite anodes. During cycling, the current collector is used for the in and out-flow of the electrons from an anode. The electrolyte molecules will be decomposed by these electrons at the anode surface because of the presence of strong voltages during anode operation.

Multiple various mechanisms are used by the evolution and characteristics of the SEI to influence the total battery performance fundamentally. The battery's total charge capacity is reduced by SEI's production through the consumption of some lithium that would be utilized for storing the charge, as various lithium compounds are contained by the SEI layer. This mechanism is a degradation mechanism which is called Loss of Lithium Inventory (LLI). In addition, lithium's amount stored by the anode can be affected by the lithium permeability of the SEI's, whereas the electron resistivity of the SEI can determine the growth rate of the SEI. SEI formation is one of the most usually utilized electrolyte compositions when lithium hexafluorophosphate (LiPF6) salts dissolved in carbonate solvent is being used.

Chemical reactions between the trace amounts of water and electrolytes can cause the formation of SEI, forming hydrofluoric acid (HF), which then decreases the performance. A very major part is played by the SEI in capacity degradation in a lithium-silicon battery because of the large volumetric changes during the cycling. The SEI layer on top of it is cracked by the anode material's expansion and contraction, exposing direct contact of more of the anode material with the electrolyte, and that leads to further LLl-based degradation and SEI production. It is important to understand the SEI layer’s composition and structure throughout cycling if we want to enhance the stability of SEI and thus enhance the battery performance. Although, the composition of the SEI for both silicon-based and graphitic anodes is not completely understood.

Phases of SEI’s Formation

Three different phases of the formation of SEI were identified by Heiskanen et al. for graphitic anodes in ethylene carbonate (EC) electrolyte and LiPF6. EC and LiPF6 reduction first leads to an SEI that is usually lithium ethylene dicarbonate (LEDC) and lithium fluoride (LiF). LEDC then subsequently decomposes into a broad number of components, which can be insoluble, gaseous, solid, or soluble in the electrolyte. The SEI layer becomes more porous because of the production of electrolytically-soluble molecules and gases, as these species will diffuse away from the surface of the anode. Electrolytes are exposed to the surface of the anode by this SEI porosity, which leads to the production of more LiF and LEDC on the SEI layer’s exterior. These mechanisms overall lead to the production of an inner SEI layer, which usually possesses an exterior SEI made up of the LiF and LEDC, and the electrolytically insoluble compounds.

Electrolyte reduction forms LiF and LEDC. The same 2-layer SEI structure appears as a result of a silicon-anode battery, in which organic compounds form an outer layer whereas inorganic compounds produce an inner layer. As the electrolyte makes up the SEI, the electrolyte composition can be adjusted and can have huge effects on lithium-silicon batteries' capacity retention. In conclusion, there have been tests of a broad number of electrolyte additives and they provided capacity enhancements, like additional carbonates (like vinylene carbonate and fluoroethylene carbonate), ethers, citric acid, succinic anhydride, and silane molecules. These additives have the potential to use various mechanisms for making performance improvements. Silane highlighted another potential mechanism and it can produce Si-O networks on the anode’s surface which stabilizes the deposited organic SEI layer on its top.

Silicon Full-cell Designs

Silicon anodes were usually tested in a Si-half-cell or as a cathode versus Li metal for early-stage research purposes. Fuel cell performance was evaluated by pairing the as-formed Si anodes with the commercial cathodes due to the constant advancement in Si anode research. The replacement for graphite anodes is Si. Li-rich cathodes, LiMn2O4, LiFePO4, LiCoO2, etc are the corresponding cathode of choice.

A more practical evaluation of the cells is displayed by the production of a fuel-cell Si battery and it brought the research and its application in practical life one step closer. Now total energy densities can be more specifically evaluated with a full cell. In 2009, Cui et al. in their early work displayed a 1 mg cm-2 loading of Si NW anode paired with LiCoO2 of 10 mg cm-2 for delivering capacity retention of 80%, same as the cell with Li metal as anode. Moreover, when Si NW anode was used, 4 mAh cm-2 of commercially comparable areal capacity was obtained with the full cell.

Fabrication of Fuel-cell

A fuel cell was fabricated by Son et al. in a 18650 type battery with 3.8 mAh cm-2 of a little low specific areal capacity but considered 972 Wh L-1 of volumetric energy density even after accounting for the thickness of the current collector, electrodes, and separator to be approximately 105 micrometers. An increase of 80% in volumetric energy density is possessed by this Si LIB as compared to the commercial batteries, and after 200 cycles, it has 72% cycle retention too. Typically, the electrode that is utilized for a Si-anode half-cell is also utilized in Li metal oxide fuel cells and Si anode. Pope et al. recently gave an interesting analysis on the volumetric and gravimetric energy density of the lithium-sulfur battery for sulfur's given areal loading.

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Conclusion

Due to silicon’s remarkably high specific capacity, there have been extensive studies on silicon as a material of anode for lithium-ion batteries (LIB). LIBs have different applications among various industries because of their stable performance, comparatively high energy density, and environmental benignity. There is a great demand for batteries with enhanced safety, lower cost, higher power, and energy density in this regard. 

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References

https://onlinelibrary.wiley.com/doi/am-pdf/10.1002/smll.201702737

https://cutt.ly/8HBnuWw

https://www.engadget.com/new-solid-state-battery-tech-promises-longer-life-and-faster-charging-rates-091158105.html

http://www.civil.northwestern.edu/people/qu/Qu%20Papers/Cui%20-%20JMPS%20-%20Interface-reaction%20controlled%20diffusion%20in%20binary%20solids%20with.pdf

https://cutt.ly/DHBnocG