Precision Silicon Surface Chemistry for Energy Storage Applications

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Solar and wind energy, while abundant sources of energy in Canada (NRCan statistics), are by their very nature intermittent. For personalized energy applications such as mobile electronics, as well as stationary microgrid- and large grid-scale considerations, energy storage is absolutely necessary. Lithium ion batteries (LIBs) are widespread for portable electronics applications, and we are seeing the first examples of grid-scale (gigawatt scale) battery storage, worldwide. These commercial LIBs use what are called 'conventional' graphite anodes that have a theoretical gravimetric capacity of 372 mAhg-1. Silicon, on the other hand, is of great interest due to its theoretical gravimetric capacity that is an order of magnitude larger, ~4200 mAhg-1. Imagine for a moment a world where batteries last ten times longer, and yet are the same weight. The keys to accessing high capacity silicon-based anodes are multifold, and several critical aspects depend exquisitely upon the interfacial chemistry of the silicon surface. Very high surface area nanostructured silicon (e.g. amorphous nanoparticulate silicon, crystalline silicon nanoparticles) is needed to enable the lithium ions to cycle in and out of the material with accompanying volume expansion and contraction without damage. Bulk silicon cannot accommodate the stress of these volume changes, leading to pulverization and exposure of freshly cleaved and highly reactive surfaces to the electrolyte; these interfaces then react in-situ with the electrolyte to form surface species that have profound effects on battery performance. Our group has been working on silicon surface chemistry for over two decades, and we will apply our expertise to the study and application of nanostructured silicon anodes, to produce surface coatings that are modular, and are designed to transport ions selectively, maintain charge transport, and protect the silicon during expansion and contraction of electrochemical cycling. These coatings will be prepared before the silicon is integrated within the anode, and via in-situ chemical reactivity during cycling. To balance the high capacity silicon anode, a high capacity partner cathode is needed in a full cell configuration, since the overall battery performance is restricted by the electrode with the lowest capacity. A standard commercial metal oxide cathode would only result in a small increase in capacity over a regular graphite anode. To overcome this limitation, and take advantage of the silicon anode, chalcogenide cathodes, for instance sulfur and selenium, will be used in a full cell configuration. Again, control over the silicon surface chemistry is critical, and thus functionalization will be the key to the development of highly reversible, long-lived high capacity silicon batteries. The final goal is the development of silicon surface chemistry that leads to the production of high capacity, stable silicon electrodes for large-scale batter applications.