Cyclohexasilane Derived Doped Silicon Anodes
Suitable silicon anodes have long been sought for use in lithium ion batteries due to their order of magnitude higher charge capacity as compared to traditional graphite anodes (3580 mA/g vs. 372 mA/g). However, initial experiments with silicon anodes revealed severe problems with cycle life as macroscopic sized silicon expands to almost 300% of its original volume upon initial lithiation. Nanostructured silicon (nanowires, nanoparticles, etc.) has somewhat addressed this expansion, but the continual build up and breakdown of the SEI (solid electrolyte interface) layer has made even these structures less than ideal to address cycling life. Cyclohexasilane, by contrast, can be easily deposited as an amorphous thin film (< 300 nm) which has initially shown capacities over 3000 mAh/g and capacity retention above 85%. However, the areal density of this amorphous thin film still does not compare to commercial batteries at ~0.2 mAh/cm2 compared to 2-4 mAh/cm2. Therefore, in order to retain the increased capacity of silicon, minimize the expansion obtained by using nanostructures, AND increase the overall capacity to be on par with commercial solutions, one can utilize n- or p-doping with boron or phosphorus precursors. CHS is ideally suited for this kind of chemistry especially when compared to silane due its superior properties, more favorable chemistry, and higher levels of achievable doping. We believe that CHS shows promise in creating silicon anodes that will enable lithium ion batteries that have the necessary charge density and cycle life to create an inflection point in the market adoption of electric vehicles.