Silicon could provide lithium-ion batteries with ten times their typical storage capacity, but this ubiquitous semiconductor is notoriously fragile when applied to battery technology. One promising line of research is to apply silicon to a flexible platform, enabling it to expand without cracking. That is the approach taken by a team at North Carolina State University, which has just published the results of its work using sheets of carbon nanotubes as a sort of “scaffolding.”
The project is some time away from commercial development, but when you consider some other silicon battery projects currently under way, it looks like Tesla Motors’ renowned Model S is going to have to share the field for long distance EV batteries, sooner rather than later.
The Elusive Silicon Lithium-ion Battery
Researchers have been eyeballing silicon hungrily for some time now, as a lightweight substitute for the graphite typically used to coat the anodes in lithium-ion batteries. Graphite is cheap and durable, but the energy density of silicon gives it a huge advantage when it comes to engineering the next generation of smaller, lighter, long-range EV batteries.
However, silicon tends to swell every time the battery discharges. Eventually the stress causes it to crack, slip its moorings, and float freely. This phenomenon, called pulverization, obviously does not bode will for battery stability and lifespan.
Some significant progress has already been made in resolving the problem, one recent development being an announcement by the company XG Sciences that it has a market-ready “platelet” solution using our old friend graphene to stabilize silicon, resulting in a storage capacity four times greater.
Another approach is still in the early research stages at the University of Maryland, where a team is growing silicon “beads” on carbon nanotubes coated with organic molecules.
Yet another promising avenue of research is being pursued by Rice University, in which a team has etched pores into silicon to create a flexible “sponge.” The project ran into a hitch last year when it came to translating the labwork into a commercial manufacturing process, but the researchers have found that crushing the sponge into powder form will do the trick.
Our sister site, Gas2.org, has also been following the silicon “nanowire” research going on at Stanford University. The last time we checked, the team had made significant progress but was still pursuing various strategies for bumping up the number of cycles.
Carbon Nanotubes and Silicon Lithium-Ion Batteries
At North Carolina State University, the research team went down the carbon nanotube trail. They aligned carbon nanotubes (CNTs) coated with silicon “like a layer of drinking straws laid end to end,” with the carbon scaffolding helping to control the expansion of the silicon:
The horizontal super-aligned CNT sheets provided high surface area and a porous structure to facilitate both the uniform chemical vapor deposition of silicon during fabrication and the electrochemical kinetics between the silicon and the electrolyte during use…A spring-like deformation behavior of the aligned CNTs helped to explain the electrochemical stability of the crystalline silicon coatings.
Don’t hold your breath for this to hit the market, but in a paper published online at Advanced Materials, the research team claims that their fabrication process is scalable and suitable for commercial production.
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