Researchers at The George Washington University are developing an innovative material for use in Li-ion battery anodes. The market for Li-ion battery ANODE materials is about $1 billion per year. The market is expected to grow rapidly in the following decades because of electric vehicles. Current Li-ion batteries use mostly graphite anodes. Silicon (Si) is an attractive active material in Li-ion battery anodes due to its low cost, high abundance, non-toxicity, and high gravimetric and volumetric capacity - Si has 10x more Li-ion storage capacity than graphite. Li-ion batteries will not increase in overall energy density unless Si is widely adopted (next-generation Li-ion cells will use a combination of about 20% Si and 80% graphite). Currently the anodes use only 2-3% Si mixed with graphite due to cost and electrode expansion. Thus, demand for high energy density and longer lasting batteries makes Si very valuable. However, Si must be cost effective and “drop-in” technology.
When used in Li-ion batteries Si undergoes a large volume expansion as it soaks up Li like a sponge, thereby expanding 300% upon lithiation. Si expansion/contraction on lithiation/delithiation causes cracking and fracturing in bulk Si and micron size particles. Si nanoparticle of less than 150nm do not crack upon lithiation/delithiation and are cycling efficiently. However the production of Silicon nanoparticle has many shortcomings: expensive; poorly scalable techniques; gas phase silane thermal decomposition; wide particle size distribution and the large particles cause mechanical failure. In addition the Si nanoparticles form a thick passivation layer/shell (SiO2) on their surface and the excess shell needs to be removed by using Hydrofluoric Acid (HF) which is extremely hazardous. Professor Wagner and his research group at GW have developed a much less expensive and economically feasible method for manufacturing anode grade Nano-Si (i.e. Nano Si from Zintl phase). The method is highly scalable, high yield (97%), high concentration, nontoxic, and solvents are reusable (Nano-Si is purified by safe and simple HCl wash - NO HF !!). The starting reagents are stable in dry air and the reaction occurs at low temperature (125 °C). Nano-Si is stable in moist air environment and will not decompose over time due to ~2% Al surface coating.
Advantages:
The invented Nano-Si synthesis is:
· Scalable,
· High yield
· Conducted at high concentration
· Low temperature
· Non-toxic (does NOT use of HF)
· From earth abundant materials
· One step to complete nano Si/conductive material composite
· Inexpensive
The Nano Si made by our synthesis is:
· Air stable (even in moist air) due to Al coating
· Very low oxide surface coating
· Excellent cycling characteristics in Li-ion anodes
· Potentially as cheap or cheaper than graphite per Ah
