Nanoporous silicon a promising material for next-generation lithium-ion batteries

Tiny batteries for your cell phone or camera that can hold days of charge?  Although it may seem like fantasy, this is the goal for researchers looking into new materials for lithium (Li) – ion batteries.  Current state-of-the-art technology uses a graphite anode, which is the electrode in the battery where the Li is stored during charging of the battery and discharged during use.  The capacity of the anode is a crucial quantity, as it relates to how much charge can be stored in the battery and used before needing to plug it in to recharge.

Capacity depends mainly on the chemistry and binding between the anode and the Lithium.  Theroetically, silicon should have about ten times the capacity of graphite, however it’s been difficult for researchers to find a way to develop cheap silicon electrodes that don’t expand and break when charged with lithium.  Fortunately, a recent study in Nano Letters has found a method to create anodes using nanoporous silicon very cheaply that limits volume expansion while providing greater capacity!

The researchers chose to use a metallurgical Si because of its cost ($1000/ton compared to $50,000/ton for nanoporous Si that is pyrolized at high temperatures.  What’s metallurgical Si (I had to look it up too :))?  Basically, silicon is naturally found on Earth with a significant portion of other metals.  Metallurgical grade silicon is >98% pure Si, usually made by heating  SiO2 to about 2000 C with some carbon material such as charcoal or coal.

Figure courtesy of [1]

Figure courtesy of [1]

The key process the researchers discovered was a method to create porous Si from the metallurgical Si as shown above.  They did this as follows.  They happened to have a metallurgical Si sample that was made of about one percent iron (Fe) and aluminum (Al) impurities (basically, this means Si atoms substituted by either Fe or Al about one percent of the time).   They first ground the silicon into a fine powder, then soaked the powder in ferric etchant made of  (Fe(NO3)3) and HF.  This etchant alone made a porous Si precipitant that could be used as the anode material!

Figure courtesy of [1]

Figure courtesy of [1]

This all seems very simple, but the researchers describe the process through some basic physics.  The etchant acts as a reservoir full of holes (C above) which are attracted to the Fe impurities in the Si, which contain localized electrons (the yellow blob in D, above).  These electrons and holes react and the iron leaves, creating a void in the silicon lattice and its resulting porous nature.  This is definitely a great place where future research could help identify the best etchant and Si dopants to create porous Si with the highest capacity!

Figure courtesy of [1]

Figure courtesy of [1]

The end result if a battery anode with much better cycling capability.   The cycle number above refers to the number of charge/discharge cycles tested using a given anode in batteries the researchers created in-house.  In blue, the non-etched anode shows poor cycling, as its capacity along the y-axis decreases significantly with the number of cycles.  This demonstrates probably large volume expansion of plain old silicon with charging that leads to pulverisation and destruction of the anode.  With the etched anodes using the process described above, the capacity maintains high for large numbers of cycles, especially using the Fe(NO3)3, compard to another type of etchant used, FeCl3.  This shows the importance of using the best type of etchant that will react with the impurities in the silicon to create voids in the lattice.

These results should inspire lots of more related studies examining similarly cheap methods to create porous materials for anodes.  In particular, it will be important to find an optimal doping level of the silicon that provides the best porous structure with the most surface area for Li sites without creating too fragile a crystal structure.

References

 

ResearchBlogging.org

Ge, M., Lu, Y., Ercius, P., Rong, J., Fang, X., Mecklenburg, M., & Zhou, C. (2014). Large-Scale Fabrication, 3D Tomography, and Lithium-Ion Battery Application of Porous Silicon Nano Letters, 14 (1), 261-268 DOI: 10.1021/nl403923s

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