EV drive battery packs comprise lithium-ion cells to store electrical energy. Currently, the active material used in anodes today is graphite. However, beginning in 2019, EV battery makers have added a small amount of silicon to the graphite. 

Why Silicon? Theoretically using silicon in the anode can store ten times more energy than graphite, reducing cell weight and the thickness of the anode electrodes. Silicon can additionally bring benefits like faster charging and discharging plus more energy storage capacity over a greater range of temperatures. 

Limitations? The alloying of Li with Si expands its volume up to three times, creating mechanical stress. To prevent micron-size silicon particles from breaking apart, oxygen atoms are used to bond with silicon atoms and form silicon mono-oxide (SiO). However, oxygen atoms add costs and decrease the efficiency as only hald the number of silicon bonds can pair with lithium, resulting in using only a small amount of SiO mixed with graphite for EV cells. 

Another option would be to use nano-instead of micro-sized silicon. By reducing the size of the silicon by two orders of magnitude, more Si atoms are near the surface than inside the core, reducing mechanical stress. However, nano-sized silicon particles have a much greater surface, and this results in the chemical decomposition of too much of the liquid electrolyte in the cells. To solve this, a second generation of silicon technologies have been developed, combining nano-sized silicon with porous carbon substrate to host the nano-silicon. 

One option for this is to burn polymers like phenolic resin to make porous “hard carbon” particles and then decompose silane gas inside very small pores. However, this technology results in high costs and carbon footprint, and is likely better suited for more expensive EVs. 

Another option is to combine nano silicon with EV-grade natural graphite already produced in large quantities. Natural graphite is less expensive and has a lower carbon footprint than synthetic or hard carbons and contains natural pores. Nano silicon can be grown inside the pores by using nano-catalyst particles to quickly react and fully decompose the silane gas, resulting in uniformly-sized silicon nanowires. The nanowires are mechanically and electronically connected to the graphite an have a much lower surface area. This lowers carbon footprint and battery costs, making the technology attractive in building affordable EVs. 

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