Alloying Materials: The pathway to a higher capacity lithium-ion battery?

• Most alloying anodes, such as silicon, tin, and germanium, have higher gravimetric and volumetric capacities than graphite.
• Alloying anodes face challenges to adoption, however, due to the large volume changes that typically accompany the alloying mechanism, changes that have negative consequences at multiple levels of the battery.
• Most of the current solutions focus on trying to solve the challenges stemming from volume expansion that include pulverization and repeated SEI destruction and growth.
• Integration of graphite and silicon anodes is currently the closest commercial solution to a pure silicon anode.

Figure 1 Annual battery demands in GWh from electromobility, stationary battery energy storage, and consumer electronics per year. [1]

Policymakers in the European Union have recently unveiled the RePowerEU plan, which sets a goal of 40% of energy coming from renewable sources in 2030. This rapid transition will only be possible with the continual improvement of energy storage systems. The demand for lithium-ion batteries is set to exponentially increase in other segments as well, as seen in Figure 1, although electromobility is expected to retain the highest share (83% of the global demand for lithium-ion batteries).

In order to advance LIBs, improvement and innovation are critical. One of the pathways to improving current lithium-ion batteries is replacing graphite with materials that have a higher capacity density than graphite’s specific capacity of 372 mAh/g and volumetric capacity of 719 Ah/L.

Graphite as an anode material and the intercalation process

As an anode material, graphite offers low volume change and high electronic conductivity. Graphite is composed of sp2 hybridized graphene layers. The weak van der Waals forces between these layers facilitate the intercalation (insertion into 2D layered structures) of lithium into graphite by allowing the layers of graphene to move apart slightly and restack [2].

Graphite replacements

Alloying anodes, like silicon, tin, and germanium, have higher gravimetric and volumetric capacities, as shown in Figure 2 [3]. This difference in capacity stems from the lithiation mechanism [4].

Figure 2 (a) Volumetric and (b) gravimetric capacities of different anode materials at the state of full lithiation [3].

Alloying anodes tend to contain a much higher fraction of lithium than intercalating anodes. For example, silicon reacts with lithium to form Li3.75Si and with tin to form Li4.4Sn. For graphite, 1 lithium atom reacts with 6 carbon atoms forming (LiC6) while, for Si, 3.75 lithium atoms react with 1 silicon atom. With 22.5 times the lithium content per host atom, silicon anodes have a specific capacity of 3579 mAh/g and a volumetric capacity of 2194 Ah/L and the equivalent silicon anode would take up one-third as much space as graphite in the battery. If this space was used for additional electrode content, it would lead to a 34% increase in the battery’s energy density [4]. In addition, tin has a specific capacity of 993 mAh/g and a volumetric capacity of 2111 Ah/L [5].

Negative consequences of the alloying mechanism

For practical applications, nonetheless, a high gravimetric capacity does not on its own guarantee the usefulness of the electrode material in question. In particular, the alloying mechanism of lithiation often leads to problems due to large volume changes. These volume changes have negative consequences on every level of the battery. At the anode particle level, repeated volume change can lead to the destruction and subsequent reformation of the SEI layer, which consumes both electrolyte and lithium [3]. At the anode level, volume change can lead to the loss of electrical contact and pulverization of the active material (Figure 3), which results in capacity fade in the battery. At the cell level, volume change can warp the shape of the current collector, crumble the separator, and cause the cell to bulge.

Figure 3 Schematic diagram of the cracking of silicon particles during charging and discharging [6].

Potential Solutions

Most of the solutions focus on solving the challenges stemming from volume expansion. Pulverization of the anode particles, for example, can be mitigated by using nano-sized silicon instead of macro-sized silicon. The same strategy has been effective for tin-based anodes. Nanosized tin helps mitigate the effects of volume change by avoiding the propagation of cracks and thereby improving the structural integrity of the electrode [8]. Nanostructures can also promote homogenous lithiation and delithiation in a single particle, which reduces the propagation of cracks and improves structural stability. Another advantage of using nanomaterials in lithium-ion batteries is the shortening of the diffusion length for ions and electrons. Another strategy used to mitigate the effect of volume change is the employment of an active/inactive alloy matrix, where the host element is embedded in an electrochemically inactive material. The inactive material has the role of alleviating the volume changes and absorbing the resulting stresses [3,9]. The choice of the electrolyte can also play a role in stabilizing the SEI of alloying anodes. Both silicon and tin show improved cycling stability with the addition of fluoroethylene carbonate (FEC) to the electrolyte, possibly due to the production of a more stable SEI [7, 8,10]. The addition of FEC results in better capacity retention and lower electrochemical impedance. Nguyen et al. showed that adding FEC and VC (vinylene carbonate) reduces cracking in silicon anodes [11]. Figure 4 shows the combination of nanosizing and electrolyte additives for tin anodes.

Figure 4 The effect of nanosizing and electrolyte additives for tin anodes [8].

Outlook

Taking parameters like cycle life, coulombic efficiency, and rate capability into consideration, graphite anodes currently remain superior to alloying type anodes. The initial application of alloying anodes will likely be in energy cells for small consumer devices where the size of the device is of primary importance [3]. On Tesla’s Battery Day in 2020, Tesla announced that they would be working on a novel silicon anode material where the silicon is stabilized by an elastic ion-conducting polymer coating [12]. This announcement brings us one step closer to a full silicon anode. Companies other than Tesla working on silicon anodes include Sila and Enovix [13,14]. Moreover, there are companies that try to solve the problems of silicon with other solutions like OneD’s SINANODE [15].

The closest silicon anodes have come to production so far is integrating low amounts of silicon into graphite anodes, with the specific capacity of the composite anode determined by the ratio of silicon to graphite. The combination of both materials allows for improved volumetric capacity compared to graphite anodes while maintaining cycle life that is competitive with commercial graphite, with low anode swelling compared to pure silicon anodes [15].

About the author

Mahmoud Reda is a PhD researcher at the Department of Functional Materials and Catalysis at the University of Vienna. Mahmoud’s background includes corrosion engineering and materials engineering. His current interests are next-generation LIB electrode materials and the thermal properties of battery components. He keeps an open mind for new ideas and is always eager to learn and grow.

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Disclaimer: The views expressed in this article are those of the author and do not necessarily reflect those at University of Vienna.

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