Use of Silicon Nano-wires as High Capacity Anodes in Lithium Ion Batteries:

Introduction:
Lithium ion batteries revolutionized the field of electronics by providing an affordable, high energy density power source. The growing need for these batteries has resulted in the research and development of anodes with high charge transfer efficiency as well as high Li carrying and electrical capacities while maintaining the low weight, portable nature associated with the current technology. One such advancement is the application of silicon nano-wires (SiNWs) as an anodic material. The nano-wire provides a high theoretical electrical capacity as well as a limiting of pulverization due to volume expansion from lithium intercalation as compared to previous anodes. However, these materials are not without flaw. SiNWs as anodes require specific growth conditions and extensive preparation and exhibit a substantial drop in capacity after their first electronic cycling. While the technology is a promising one, its use a practical solution to growing demands has yet to be realized.

Previous Lithium Ion Batteries:
A lithium ion battery consists of several parts but for this analysis, an understanding of just a few is needed. Each battery contains two electrodes, a positively charged cathode and negatively charged anode, a current collector, and electrolyte. The composition of the cathode for the batteries being discussed is unimportant but it is the site at which negatively charged ions are oxidized. The electrolyte is a lithium based solution and the current collector is the attachment site for the anode. These two components (anode and current collector) are discussed in more detail later on.
The earliest Li batteries used hard carbon anodes. These anodes were mixtures of various carbon types and had useable capacities of approximately 250 mAh/g. In the late 90s, new electrolytes were being used and allowed for the introduction of graphite as an anodic material.[1]
Current lithium ion batteries still utilize graphite anodes. While these anodes are cost effective and provide an ample amount of energy for the devices they are employed in, they exhibit a low potential for advancement. Carbon based anodes have recently met a new match in the evolution of hybrid and electric vehicles. These devices have a significantly higher power demand than previous lithium battery powered devices [2]. Even at the highest theoretical capacities of carbon anodes, several batteries are needed to provide enough power for an appreciable timeframe to be successful in large scale devices.
Several silicon materials have been studied as applicable anodes for lithium batteries. Bulk films exhibited capacity fading and low cycle lifetimes due to pulverization and loss of contact with the current collector. This is a result of their restrictive geometries. Without ample room for expansion, which is explained in detail later on, the anodic material will undergo high strain and break upon lithiation. Other materials such as Si thin films generated stable capacities but resulted in a lack of sufficient material for a battery to be produced. Simply put, while they create effective electrical properties, Si thin films are simply too small to be used in a battery application[3].

Silicon Nanowires:
A nano-wire is a strand of crystalline material with width of less than 10 nano meters. This is effectively a 1 dimensional structure, with the one dimension being length. In most cases, the nano-wire is far less conductive than the bulk material that produced it. For Si, the nano-wire is actually insulating in theory, but n and p type doped nano-wires are produced in most syntheses[4,5].
Chemical vapor deposition is the most common method used to create SiNWs for Li batteries. In this process, Si containing gasses are flowed over a catalyst which has been applied to an inert plate. This plate in many cases is the current collector of the battery the SiNWs are going to be used in. The plate is heated above the catalyst-Si eutectic melting temperature. As the catalyst melts and more silicon is deposited. The continued addition of Si past the equilibrium saturation causes an anisotropic growth is seen. The catalyst drop rises on top of the crystalline Si and more Si atoms are deposited. This causes the nano-wire growth effect[6]. Figure 1.1 shows the formation mechanism using a silver catalyst as well as some SEM images of the silicon nanowires.

figure_1.1.png

Figure 1.1: (a) Schematic formation of SiNWs via silver-induced etching. (b) Typical cross-sectional SEM image of SiNWs arrays. (c) TEM image of a segment of a single SiNW produced from Si(100) wafer. (d) HRTEM image of a SiNW, revealing the rough
surface reproduced from:[7].

Advantages of SiNWs:
SiNWs provide many advantages over current graphite anodes and other silicon materials. One such advantage is a high lithium intercalation capacity. Current graphite anodes operate with a lithium capacity around 1 lithium atom per 6 carbon atoms, whereas silicon can incorporate 4.4 lithium atoms per atom of silicon.[5] The genesis of this property is the increased surface area between carbon and silicon. With an increased surface area, more lithium ions can arrange themselves about the nano-wire per atom length. This property allows for a greater number of electrons to be transferred to the anode at any given time. This translates to a high electrical capacity for SiNW anodes.
Typical graphite anodes have a theoretical charge storage capacity of 372mAh/g as compared to a theoretical capacity of 4200 mAh/g in the case of a SiNWs. [8] The real-life correlation here is that batteries utilizing SiNW anodic technology would have a higher power output or larger charge lifetime, while remaining portable. SiNWs can also exhibit high capacities with increased current. Even with increased current, the charge and discharge capacities for these anodes have been shown to be 5 times greater than graphite anodes.[3]
Perhaps the most significant advantage of SiNWs results from their geometry. SiNWs are a 1D structure which provides an extremely efficient transfer path from the anode to the current collector in the battery. Not only does this geometry provide an extremely direct route to the current collector, but it also maximizes the available surface area for lithiation. This geometry also allows for a high density of SiNW distribution across the current collector further increasing the amount of lithiation that can occur [4, 5, 9].
A major concern of other silicon materials is pulverization. Pulverization occurs during de-intercalation of Li from the Si structure and is a result of the high Li capacity. The high capacity can force volume changes of up to 400% in many Si structures and if there is insufficient spacing to accommodate for this volume increase, the anode will break9. Figure 1.2 illustrates this point.

figure_1.2.png

Figure 1.2: Structural advantage of nano-wire model as opposed to film or particle based approaches.[9]

SiNWs avoid the problem of pulverization because their geometry allows for a high degree of expansion. This is because they can stretch in both length and width without straining the overall structure to a high degree. SiNWs also form a coating of amorphous silicon after the first charge/discharge cycle which maintains this property [3, 8, 10]. However, the formation of this coating does decrease the overall charge capacity.

Disadvantages of Silicon Nano-Wires:
While SiNWs solve or improve upon many of the faults in current lithium ion battery technology, there are of course disadvantages to their application. One major disadvantage stems from the formation of the amorphous LixSi coating along the nano-wire surface. This coating is generated during the first discharge cycle. As intercalated lithium ions leave the nano-wire surface, some undergo reactions that leave them bonded to the anodic silicon in a metastable state. [8,11] This results in the destruction of a portion of crystalline silicon and greatly reduces the lithium insertion and, in turn, charge-carrying capacity of the nano-wire. Experiments have shown that after the first charge cycle, capacitance drops from 4200mAh/g to approximately 3000 mAh/g are expected. This drop correlates to a 73% charge capacity[3].Despite this initial drop, cycles 2-20 have been shown to maintain capacities of around 3000 mAh/g as little amorphous LixSi is formed in later cycles[12, 13].
Another disadvantage of SiNWs arises from their high rate of lithiation. The expansions that occur place great strain on the SiNW connections to the current collector and can cause the wires to lose contact. This renders the nano-wire virtually inoperable and hurts the overall efficiency of the cell. [3, 7]
In addition to the initial capacity drop, the volume change (ca 300% to 400%) exhibited by SiNWs is another potential disadvantage. SiNWs undergo several complex formations during lithiation, the ideal case being Li22Si5 (4.4 ratio14), all of which exhibit different volume changes. The most prevalent of these complexes is Li12Si7. This complex exhibits a volume which is 2.17 times as large as the crystalline Si host. [15] The presence of this complex would require any vessel to accommodate for the size increase of its anode and could place limitations on potential sizes for batteries utilizing this technology.

Potential Improvements and Applications:
One solution to the SiNW issues is the utilization of carbon reinforced SiNWs. The reinforced SiNWs would generate far less amorphous LixSi and maintain high performance across charge cycles. However, the SiNWs would show a slightly decreased tensile strength due to carbon limiting the structure’s ability to expand during lithiathion.[2, 16]
SiNWs are also being imbedded into carbon nano-tubes. Simulations have demonstrated Li ions can permeate carbon nano-tubes through the tube end freely. The expansion of the nano-structure due to lithiation was also analyzed and applied. The overall result was that as nano-wire size decreased, the reinforcing effect increased. This is due to a change in the crystalline structure that creates a more ductile nano-wire during cycling. The presence of the nanotube prevents the ductile structure from losing contact with the current collector and could potentially decrease the amount of capacity loss after several charging cycles that is typically observed.[16]
Other researchers are working on carbon coated Si nano-tubes. The geometry provides a less direct route to the current collector but has an extremely high active surface area due to the presence of the inner channel.[16] Researchers working with Jaephil Cho at Ulsan National Institute of Science & Technology in Korea built a lithium-ion cell including the silicon nanotubes and found it had a ten times higher capacity than cells with graphite anodes and retained its high performance even after 200 cycles of charging and discharging. An SEM image of the nano-tubes and a rendering of the design are shown below.

Figure 1.3:

figure_1.3.png

Figure 1.3: SEM image and nano-tube renderings based on research done by Jaephil Cho et al. [11, 14].
Another focus for improving lithium ion batteries is the reduction of overall mass. The component being addressed in this case is the current collector. Current collectors take up more weight than active materials in most lithium ion batteries while an ideal collector’s weight should be minimal. Utilization of nano-materials such as carbon matrices or thin films could generate approximately 25% mass reduction n in this component.[11] Such collectors have been attempted by Cho. In order to deposit the Silicon nano-materials on the carbon matrix, a layer of gold is first layered on the matrix to generate the growth of the Si structure. [11].
Silicon nano-structures are applicable to far more devices than just batteries. Devices that could benefit from the application of these structures include fuel cells, in-body biosensors, pacemakers, micro-computers and many others. All of these devices are benefitted by the high efficiency of the nano-structures as well as their stability over many charge cycles.

Discussion:
Overall, the application of Si nano-materials, and SiNWs in particular, is a promising development in the field of lithium powered electronic cells. SiNWs present a number of advantages over current cells including a durable material resistant to pulverization and a high theoretical lithium insertion capacity. The technology has been shown to provide a maintained capacity around 3000 mAh/g , compared to capacities of less than 400 mAh/g for conventional graphite anodic cells. Downfalls of the technology including high volume changes and initial loss of capacity are being addressed in full through applications of combined carbon and silicon nano-materials and may lead to further advancements to the technology.

References
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