|Fig. 1: Mechanism by which PR-PAA binders reduce stress on Silicon microparticle anodes. Particles expand during discharge and then return to normal state during charging. (Source: I. Naccarella. After Choi and Auerbach. )|
There has been significant effort put towards improving the energy density of Lithium ion batteries in recent years. Li ion batteries currently represent the forefront of commercial rechargeable battery technology, outperforming lead-acid and nickel-cadmium batteries, but further improvements are necessary for certain mobile applications, such as in electric vehicles. Battery cost is currently a significant component of the cost of an electric vehicle and research has demonstrated that customers are especially sensitive to the driving range and cost of such vehicles.  Thus, improvements to energy density are necessary to further the viability of Li ion batteries. Such improvements are most likely to come from increases to the specific capacities of the anode and cathode as most other battery components have little room for improvement. Currently graphite is used as the anode material, but silicon has been investigated for some time as an alternative anode material due to its high specific capacity of ~4200 mAh/g. This is roughly a 10x improvement over the graphite electrode.  This improvement derives from the fact that the Si anode does not utilize "intercalation" mechanisms in which the Li ions intercalate into crystallographic sites (as in the graphite anode) but rather is lithiated into a LixSiy alloy. However, Si currently has a major issue in that it expands ~400% during discharging which limits its cycle life due to particle pulverization and the formation of an unstable electrode-electrolyte layer. [3,4]
One solution to this issue with silicon anodes is to utilize binders to maintain the electrode structure during repeated volume change. These binders are long molecular chains which bind to the surface of the electrode and help keep the structure "bound" together as the silicon begins to break down due to repeated expansion. However, standard polyvinylidene difluoride (PVDF) binders don't work well with Si due to weak van der Waals interactions between the components. Furthermore, most other binders begin to lose elasticity under large repeated stress. One novel solution has been to utilize a "pulley" system for stress dissipation during expansion.  Such a mechanism was recently developed by Choi and Aurbach through the use of polyrotaxane (PR) integrated with a more conventional linear binder, polyacrylic acid (PAA). Fig. 1 shows the mechanism by which this binder system would work. As can be seen in the figure, the PR consists of polyethylene glycol "threads" and α-cyclodextrin "rings" which covalently bond to the PAA. These rings components can move freely and thus can serve as pulleys by moving closer together when the Si particles expand, reducing stress. This action serves to keep the Si particles together even as they begin to be pulverized. If the particles were not being kept together by this mechanism, a solid- electrolyte interface (SEI) layer would form in between the cracks of the Si particles and eventually lead to electrical disconnection. Furthermore, simply using PAA without the PR leads to significant performance losses over time and a much lower rupture strain (37% vs 390%) due to loss of elasticity in the PAA over time. 
|Table 1: Comparison of specific capacity (mAh/g) of different anode materials for Lithium ion batteries. (Note that the Li-Air battery capacity is based on the mass of the Li2O formed on the structured carbon electrode during discharge). |
Despite the promising advances in Si anode technology, it is not the only technology being explored for improvements to Li ion battery energy density. Table 1 shows the specific capacity of several alternative anode technologies. From this table we see that Si has one of the highest specific capacities of the potential anode materials, but there are several even more promising materials on the horizon. For instance there has been much excitement about potential Li-Sulfur batteries due to their high theoretical capacities. Furthermore, their greater operating voltage would make them even more energy dense that Li-Si batteries and sulfur is both abundant and cheap. However, these batteries suffer from a range of issues regarding the lack of conductivity of sulfur and polysulfide dissolution into the electrolyte among others. Likewise, the Lithium-Air battery is seen as the holy grail of battery technology since the anode material (O2) would simply come from the atmosphere allowing for a massive improvement in energy density.  However, these batteries currently suffer from many problems as well, including poor reversibility and an inability to only allow oxygen into the battery.
Thus, we see that there are many promising ideas in the field of battery technology, but that many issues still need to be resolved for most of the more promising battery technologies. Si anodes are currently being put into production (albeit as SiOx), promising significant improvements in energy density.  While not a complete solution to the expansion problem, the binders discussed here have shown 98% retention over 50 cycles.  While 5000 cycles is the goal for most battery technologies, this is a good start, and other exciting technologies are just on the horizon.
© Ian Naccarella. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J.-W. Choi and D. Aurbach. "Promise and Reality of Post-Lithium-Ion Batteries With High Energy Densities," Nat. Rev. Mater. 1, 16013 (2016).
 J. Xu, Q. Zhang, and Y.-T. Cheng, "High Capacity Silicon Electrodes With Nafion as Binders For Lithium-Ion Batteries." J. Electrochem. Soc. 163, A401 (2016).
 S. Choi et al., "Highly Elastic Binders Integrating Polyrotaxanes For Silicon Microparticle Anodes in Lithium Ion Batteries," Science 357, 279 (2017).
 N. Nitta et al., "Li-Ion Battery Materials: Present and Future," Mater. Today 18, 252 (2015).