In addition to lithium, silicon is also being investigated as an anode material for future batteries. But what are the advantages of this technology, what are the challenges and what is the current state of commercialization? This article provides the answers.
In the constant search for better batteries for electric cars, home storage systems and cell phones, the search for new materials is at the heart of research. It is striking that there has been little progress in the field of anode materials in recent decades. While the development of cathodes began with LCO and new materials such as NMC, NCA or LFP have been developed over the years, the anode is essentially still the same as it was in the 1990s: the graphite anode. Although extremely cycle-stable cells can be built with this, a particularly high energy density is not achieved. This is due to the fact that six carbon atoms are required to store a single lithium ion, which means that the graphite anode is quite thick and heavy.
But there is an alternative to graphite anodes. It can be better. This is where silicon anodes come into their own. While the lithium in graphite anodes is stored in intermediate layers, the lithium in silicon anodes forms a solid chemical bond. When charging, Li4.4 Si is formed. This means that one silicon atom can already bind more than four lithium ions, which is an enormous difference compared to graphite. This results in a theoretical energy density of which is more than five times what is possible with graphite anodes and is only beaten by pure lithium as an anode [1].
However, a fivefold increase in the anode energy density does not mean that the energy density at cell level will increase just as much. The cathode, housing and separator make up a large proportion of the cell’s mass and current forecasts expect only slight improvements here. Overall, an increase in energy density at cell level of 30-40% is quite realistic, but much greater leaps are unlikely without further leaps in the development of cathodes and separators.
Challenges for silicon anodes
However, developing a stable silicon anode is not easy. The volume of the silicon anode increases by up to 300 % during charging [1]. This is not surprising, as lithium migrates from the cathode to the anode during charging and this additional volume must be accommodated. In today’s graphite anodes, there is also a slight volume expansion (~10 %), but as the lithium is stored in cavities in the intermediate layers, the volume expansion is not usually too much of a problem [2]. With silicon anodes, volume expansion is the biggest problem and leads to damage to the cell. Figure 1 shows the destruction mechanisms as a result of volume expansion.
Figure 1: Damage mechanisms in silicon anodes that cause the cell to age very quickly: As a result of the volume expansion, there is a pulverization of the particles, permanent reformation of the SEI layer and loss of contact between anode and current collector, Own illustration
Initially, the volume expansion causes the SEI protective layer that forms around the particles to crack. This layer is normally intended to prevent the electrolyte from coming into direct contact with the active anode material. If the layer is damaged, parasitic reactions take place, the products of which also act as a passive protective layer, but consume lithium and electrolyte and thus permanently reduce the capacity. The damage and reformation of the protective layer takes place with every cycle, so that the storable energy continues to fall.
The change in volume also causes the particles to break up over time due to the mechanical stress, which is known as pulverization. Unprotected silicon comes into contact with the electrolyte again, so that even more lithium and electrolyte is consumed.
In addition, the volume expansion ensures that the active material detaches from the current collector in the long term and the contacting is significantly impaired, which increases the cell resistance [3].
Another challenge is the poor conductivity of silicon. Here, however, a considerable improvement can be achieved by doping the silicon and adding conductive additives [1].
Solutions for improved cycle stability
Most problems with silicon anodes are due to their volume expansion. There are various approaches to getting to grips with this. As a general rule, large active material particles tend to break up, as the mechanical stresses are greater due to the less favorable volume/surface ratio. On the other hand, small particle sizes mean that the contact surface between the particles and the electrolyte is very large, which is why a lot of active material and electrolyte is used during the first charging cycle in order to form the SEI protective layer. Small particle sizes are also more challenging to manufacture because it is difficult to produce homogeneous surfaces for the active material layers [3]. In development, attempts are therefore being made to find a compromise between the two competing approaches or to try out new approaches with which the SEI layer formation can be optimized. Almost all research attempts to reduce the particle size and then implement measures to limit SEI consumption beyond this.
Figure 2 shows typical measures used to compensate for the disadvantages of the Si anode.
Figure 2: Measures to compensate for volume expansion Figure adapted from [1]
In the simplest case, the structure is as shown in Figure 2(a). The silicon is present as a particle (possibly a hollow particle), whereby the particle size can be in the micro or nano range. The particle is exposed to the electrolyte so that a lot of material is consumed in the SEI layer formation. If lithium is incorporated into the silicon particles during charging, this leads to a sharp increase in volume, even at the macroscopic level, which triggers high mechanical stresses. A first measure to improve the anode is to apply an additional protective layer around the silicon particles (Figure 2 (b)). Although this does not prevent volume expansion, it does ensure that the electrolyte cannot penetrate the porous layers of the active material. The carbon layer also improves conductivity at the interfaces. The volume expansion can be compensated for with additional cavities in the particles. This can be realized as in Figure 2 (c) with a coated hollow particle, or as in Figure 2 (d) with a shell-egg yolk structure. In the latter, the silicon is located in the center and is surrounded by a cavity that is bounded by a shell (usually carbon), which enables electron and lithium transport. The volume expansion then only takes place inside the shell, so that the particles no longer break up[1].
Another approach to limit the volume expansion is to produce the anode as a porous structure (Figure 2 (e)). The active material can then expand in the structure, which is interspersed with cavities, without causing major volume changes. The coating of the porous surface with carbon also acts here as an additional protective layer to prevent the electrolyte from penetrating the active material.
Another approach is the use of silicon nanowires (Figure 2 e). Here, the silicon is produced in tiny nanotubes using chemical vapor deposition (CVD). In this process, the active materials are vapor-deposited onto the metal foil, which achieves very favorable mechanical and electrical properties. The volume expansion can take place radially along the tubes. At the same time, the silicon is conductive along the tube. One disadvantage of this approach, however, is that the manufacturing process of chemical vapor deposition is still very little established and the process is correspondingly expensive [1].
Overview of the silicon anode industry
Figure 3: Overview of large and small companies that have focused on the production of silicon anodes, own illustration
The possibility of using silicon as an anode material for batteries has been known for many years and companies have been founded around the silicon anode for almost two decades. Figure 3 provides an overview of the market. Some of the larger (and smaller) companies are presented below:
Nexeon Ltd
Nexeon was founded in 2006 and has grown strongly since then, making it one of the largest companies in the silicon anode sector. Little information is publicly available on the exact technology used by the company. What is clear is that reducing the particles to nano-size and increasing the porosity are part of the solution to achieve a silicon anode with a good number of cycles. [12].
Patents granted in recent years also indicate that nano-sized silicon particles with a solid core and additional shell similar to those shown in Figure 2(b) are used to construct the anode. The silicon core is surrounded by a silicon oxide layer and then a silicon carbide shell. On the one hand, this double layer prevents parasitic reactions between the particles and the electrolyte. On the other hand, this extra layer absorbs the volume change so that a sufficient service life of the cell is achieved [18].
Nexeon currently offers silicon anodes in two versions: With NSP1TM , a small mass fraction of silicon is added to a normal graphite anode so that a slightly increased energy density is achieved, but the volume expansion remains minimal. The second product, NSP2TM , focuses on an anode made exclusively of silicon, whereby the volume expansion (as far as is known) is compensated for by the measures described above [19].
Amprius
Amprius is another major company in the silicon anode sector and was founded in 2008 in the Stanford University environment. To limit volume expansion, the company uses silicon nanowires, similar to the concept shown in Figure 2 (f). By attaching the nanowires directly to the current collector, there is a good electrical and ionic connection and no additional passive materials are required to ensure the functionality of the cell [4]. Amprius is currently building its first gigafactory in Colorado, which is expected to achieve an annual production of up to 5 GWh in its final anode expansion stage.
Ionic Mineral Technologies
In addition to very large companies, there are also start-ups that have not been on the market for very long but are trying to advance the development of the technology with new ideas. Ionic Mineral Technologies is one of these companies, which focuses on special silicon nanotubes (similar to those shown in Figure 2 (f)) and aims to revolutionize the manufacturing process. The nanotubes are not produced synthetically, but are obtained from halloysite through a reduction process. Halloysite is a naturally occurring mineral that consists primarily of oxygen, silicon and aluminum and already exists in its natural form as a nanotube structure. By adding a metallic reducing agent, the unwanted substances are removed so that only the silicon nanotubes remain. These reach a length of 500 nm with a tube diameter of 50 nm. During the charging process, the lithium reacts with the structure and the thickness of the silicon nanotube increases. The nanotube structure ensures that the volume expansion at macro level is significantly reduced and the mechanical load is improved. Ionic Mineral Technologies has exclusive access to the world’s largest high-purity halloysite production site, ensuring the supply of raw materials [20].
Silicon anodes for solid-state batteries
Silicon is also to be used as an anode material for future solid-state batteries. Solid Power, for example, is planning to start series production of solid-state batteries with a silicon anode and solid sulphur electrolyte by 2026. [21] However, no precise information is known about how the anode is constructed in detail.
But what advantages are expected from a combination of silicon anode and solid electrolyte? On the one hand, increasing the energy density is nowhere as important as with solid-state batteries, which is why either lithium metal or silicon is normally used as the anode material. Both have an extraordinarily high energy density, which is why they are favored. Silicon and solid electrolyte are said to be a particularly good match. One of the main problems of the silicon anode with liquid electrolytes is that the SEI layer at the interfaces is damaged with each cycle due to volume changes during charging and discharging and the active material comes into direct contact with the liquid electrolyte. The contact of the liquid electrolyte with the active material then leads to the SEI layer being rebuilt and lithium, which should actually be stored in the silicon, is passivated and is no longer permanently available. This reduces the capacity. In cells with silicon anodes and solid electrolyte, the interface is structured differently so that parasitic reactions should not take place. Unlike with lithium-metal anodes, dendrites should not be a problem [22].
A look at the literature shows that silicon as an anode for solid-state batteries is not without its disadvantages: in order for the anode to function, relatively high proportions of conductive agent and binder are required for solid electrolytes. A meta-analysis found that the mass fraction of silicon in the anode was less than 40 % in all the studies examined, which reduces the theoretically achievable energy density accordingly [23].
Conclusion
Silicon as an anode material undoubtedly holds promising potential and has a good chance of becoming “the next big thing”. Battery manufacturers are already making great efforts to gradually increase the proportion of silicon in the anode. Concepts that use silicon as the sole anode material are already being tested for niche applications and will be expanded to other areas in the coming years. The biggest problem continues to be volume expansion and the associated damage to the interfaces. However, by reducing the particle size and using porous structures, the mechanical loads can be significantly reduced. Whether this will be sufficient to ultimately produce cells with a long service life that are suitable for use in electric cars or as home storage units will have to be clarified in the coming years.
This also applies to solid-state batteries with silicon anodes. Silicon anodes are easier to control than lithium metal anodes and also enable a high energy density, although this is not quite as high as with lithium metal. Silicon could therefore be a good interim solution for the next few years until the problems with lithium metal have been resolved. If the problems prove to be more serious, there is also a long-term perspective for silicon to be used for solid-state batteries.
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