Dr. Yi Cui
Many of you have heard about the new breakthrough in lithium-ion battery storage. I had an interview with the scientist who discovered it, and hopes to bring it to market, Dr. Yi Cui, Assistant Professor of Material Science and Engineering at Stanford University.
Can you tell us what you have discovered?
The idea is to use silicon nanowires as the active lithium storage material for the battery electrode. Silicon can store a lot of lihtium but the volume expansion is too high, by using nanowires we are really solving the problem of this big volume expansion and we can achieve extremely high capacity, lithium storage capacity with multiple cycles. this is 10 times more than the current technology. So the implications of this discovery is very, very exciting, it means you can have a better battery.
In current lithium ion batteries carbon or graphite is used as the anode material?
When the battery gets charged then do the lithium ions combine with the anode material?
Yes, when the battery gets charged.
Has silicon been tried as an anode material in the past?
There is some research about that in the past using for example a film of silicon. But that doesn't look as good, because the film peels off right away, because of volume expansion.
Are these nanowires newly created in your lab or have they been around?
People have been making these for a while for transistor applications for example. When I was a graduate student I also worked with silicon nanowires too for making transistors and biosensors. For using silicon nanowires for battery applications, this is the first time. This is a new application for this material.
So nanowires can swell without becoming unstable?
Compared to current A123 or LG Chem cells, if they used your silicon nanowire technology would they be able to store 10 x as much energy?
Yes, that the idea, they would be able to hold 10 x as much energy in the same amount of volume or same amount of weight. Of course the battery consist of two electrodes, the anode and cathode, you would also like to improve the cathode too to make it happen.
If you just changed the anode to nanowire and not the cathode, would the cathode limit the energy potential storage?
If you improve the anode that just means for the same weight or same volume of the batteries you can use less anode materials, you can use the extra weight and volume to hold more cathode materials and you also improve the battery significantly.
If I take a current battery's cathode materials and combine i with silicon nanowire anode, I can significantly improve its performance.
Will that give you a ten-fold improvement?
That will not give you ten-fold but it will give you several fold improvement.
In terms of energy density?
In the study you published, how many cycles did you run on your cells?
In the paper, we showed 10. If you read the supplementary, we also have the data in there up to 30. And in the lab right now we are testing more and more cycles. It looks like it's going up and up and up.
For automotive applications, I'm told you typically need thousands of cycles so that the batteries can last 10 years.
Thats right. That's exactly what we are targeting towards, over 1000 cycles. So since these nanowires are so small they have a way to hold against a big volume expansion, so we will see in the next several months perhaps a year these 1000 cycles, perhaps we can demonstrate that.
So your goal is to show 1000 cycles in as little a 6 months from now?
Do nanowires allow for a lot of surface area in the anode?
I am told then with all this surface area one could then get a significant amount of side reactions that could possibly limit the lifetime of the battery?
This is the argument typically you take from the graphite anode. This is what people find, you have side reactions. On silicon (though) it looks like side reaction is really minimal. It is highly materials-dependent.
So your saying silicon may have less side reactions than graphite (carbon)?
That's what we have seen, yes. It has very little side reaction.
So you wouldn't have to worry about that decaying and thus limiting the battery's performance?
Do you think the strong vibrations of the automotive environment would cause these delicate nanowires to be destroyed?
Actually nanowires are much better than big pixel materials. Because if you think about individual nanowires, the gravity or force is irrelevant. And also due to these vibrations and nanowires, because each individual nanowires mass is so small,it just wouldn't matter at all. Much better than for big materials, for big materials if you vibrate it you might break it. For nanowires, there's no way you can use the usual vibration frequency to break it...it won't break.
Since cost is so relevant to developing batteries for cars, are silicon nanowires more expensive? Would they increase the cost of the cells?
Silicon is the second most abundant element in the world. For battery applications it doesn't have to be high purity silicon. Unlike silicon solar cells which require high purity. the silicon industry is also big, people know everything about silicon. the infrastructure is there, the supply source is there. With the excitement of use of silicon for batteries, the cost will be reduced dramatically.
What about the making of a nanowire?
It can be easily scaled up. I actually did some calculations, the production one could reach with say a factory 3 by 3 miles, you could produce enough batteries for at least 100,000 cars per day easily.
What timeline do you think it would take before your technology could be incorporated into a commercial product?
I am working on it. As a rough timeline, I would say perhaps 5 years.
Are you moving towards your own commercial enterprise to develop this?
I look forward to the opportunity of course. I'm still thinking about what will be the best solution for this technology.
Are you talking to any of the current battery companies, doing something with them or being totally independent?
I am evaluating either way, whether its better to do a start-up company or working with an existing battery company.