Strain-graded silicon nanoscoops for high power Li-ion battery anodes
02 January 2011
|Nanoscoops for high-power anodes. Credit: ACS, Krishnan et al.Click to enlarge.|
A team at Rensselaer Polytechnic Institute (RPI) has developed a functionally strain-graded carbon-aluminum-silicon anode architecture that overcomes the normally poor performance of Li-ion batteries for high power applications involving ultrafast charging/discharging rates.
The new anode materials consist of an array of nanostructures each comprising an amorphous carbon nanorod with an intermediate layer of aluminum that is finally capped by a silicon nanoscoop on the very top. The gradation in strain arises from graded levels of volumetric expansion in these three materials on alloying with lithium.
A paper on their work is published in the ACS journal Nano Letters.
Batteries designed for electric vehicles should be able to provide high energy and power densities. Lithium (Li)-ion batteries are known to deliver very high energy densities in comparison to other battery systems. However, they suffer from low power densities. In contrast, supercapacitors provide very high power densities due to their surface-based reactions.
To replace a traditional combustion engine, it is highly desirable to combine the advantages of Li-ion batteries and supercapacitors into one single battery system. Earlier reports have shown the development of high rate cathode materials. This has also led to an equal effort in the development of high-rate capable anode architectures. Silicon (Si) has been envisioned as a promising anode material because of its high theoretical capacity of ~4200 mAh/g based on the stoichiometry of the alloy Li22Si5. The main limitation of this high capacity is an accompanying volumetric expansion of ~400% for crystalline Si (or ~280% for amorphous Si) which results in pulverization and delamination of the electrode structure.
Pulverization results in more capacity losses due to increased solid electrolyte interphase (SEI) formation while delamination results in loss of electrical contact with the substrate. At higher charge/discharge rates (C-rates), these failure mechanisms are severely exacerbated and thus it is important to design architectures that perform well at fast C-rates to enable high power Li-ion rechargeable batteries.—Krishnan et al.
By introducing materials between Si and C that have intermediate volumetric strains, the authors note, a natural strain gradation can be developed in a multilayer architecture. In the specific case of the work reported in the paper, the team used aluminum to gradually transition the strain from the least strained material (C) to the most strained material (Si).
This minimizes the mismatch at interfaces between differentially strained materials and enables stable operation of the electrode under high-rate charge/discharge conditions, they explain.
At an accelerated current density of ~51.2 A/g (i.e., charge/discharge rate of ~40C), they found that the strain-graded carbon-aluminum-silicon nanoscoop anode provides average capacities of ~412 mAh/g with a power output of ~100 kW/kgelectrode continuously over 100 charge/discharge cycles.
They also show that the C-Al-Si composite can yield power densities as high as ~250 kW/kgelectrode (current density of ~128 A/g) continuously over 100 cycles with an average capacity of ~90 mAh/g.
The C-Al-Si architecture has the potential for mass scalability by increased deposition time as well as the possibility of stacking C-Al-Si nanostructure films on intermediate carbon thin film supports. When the mechanism of charge generation involves alloying with the host material and the demand for current is high, the electrode architecture is put through large strain rates accompanied by rapid volume changes. In such a situation, a functionally strain-graded structure could potentially undergo rapid volume changes with reduced possibility of interfacial cracking or delamination. By building a strain-graded structure, it is possible to eliminate interfaces between materials that have a large strain difference during lithiation.
Low strain difference between adjacent materials in the composite leads to highly efficient alloy-dealloy reactions preserving the overall structural integrity of the electrode. To further improve the strain gradient, we could potentially insert materials such as Sb (strain of ~147%) or As (strain of ~201%) between Al and Si. This will also help in increasing the area mass density while still improving the performance. Such strain-graded multilayer anode architectures show significant potential for the design of high power and high capacity Li-ion rechargeable batteries.—Krishnan et al.
Rahul Krishnan, Toh-Ming Lu, and Nikhil Koratkar; Functionally Strain-Graded Nanoscoops for High Power Li-Ion Battery Anodes Nano Lett., Article ASAP doi: 10.1021/nl102981d
This technology seems to have the potential for future very high performance batteries for EVs. Wonder how long it will take to fully develop the process and mass produce various size battery modules?
Posted by: HarveyD | 02 January 2011 at 01:21 PM
The future is "nano."
Posted by: ai_vin | 02 January 2011 at 02:12 PM
Sorry, that should have read; "I have seen the future, and it is nano."
Posted by: ai_vin | 02 January 2011 at 02:15 PM
It they keep making these improvements, we may actually end up with better performing, more affordable batteries that are easier to manufacture.
Posted by: SJC | 02 January 2011 at 05:08 PM
Sounds like a comprehensive approach. To answer the questions above, it is really difficult to predict from announcement like this one, how much this is promising. hundred of configurations will have to be tested and then only a very few will be picked at the end. Si is very promising for its ability to suck Li but the challenge are very steep. In the case above it seems that the energy density is limited, but the graded strain approach is quite interesting and has proven to work in other areas (growth of Ge on Si substrates) so let see.
Posted by: Treehugger | 03 January 2011 at 05:17 AM
Um how likely is it this can be scaled up cheaply enough for mass production? Ive seen alot of papers over the years yet few actual products.
Posted by: wintermane2000 | 03 January 2011 at 12:29 PM
With all research being done, somebody will find ways to increase batteries performance (by 2x to 4x) and lower mass production cost (by 1/2 to 1/4) by 2020 or shortly thereafter. That is what is needed for practical highway capable e-cars. It will come.
Meanwhile, incremental improvement steps will certainly make current technologies better and cheaper. Lower cost limited e-range PHEVs and BEVs will be produced by all car manufacturers by 2015.
Posted by: HarveyD | 04 January 2011 at 08:25 AM
A co z pracą w niskich temperaturach? Dzisiaj (-9 C) samochód elektryczny o zasięgu ok. 150km ma zasięg max 20-30 km!
Posted by: Janusz Głazik | 05 January 2011 at 08:26 AM
And what about the work at low temperatures? Today (-9 C) electric car with a range of about 150km has a range up to max. 20-30 km!
Posted by: Janusz Głazik | 05 January 2011 at 08:27 AM
Short answer - insulated battery boxes.
Ever have your ICEV not start because of a frozen gas-line? The simple fact is car owners (whether they drive ICEVs or BEVs) who know they have to deal with cold weather learn to take some precautions: Up here in Canada ICEVs are built with blockheaters and parking spaces have electrical outlets so they can be plugged in. And these blockheaters use more energy to get an ICEV to start than a BEV will need to completely recharge.
Now do SOME electric cars have a range problem in cold weather?
Yes some do, but these are examples of poor conversions or the designers not thinking ahead; http://gm-volt.com/2009/12/28/mini-e-electric-car-performance-in-cold-and-ice-is-not-good/
Posted by: ai_vin | 05 January 2011 at 09:07 AM
Long answer: "All batteries deliver their power via a chemical reaction inside the battery that releases electrons. When the temperature drops the chemical reactions happen more slowly and the battery cannot produce the same current that it can at room temperature. A change of ten degrees can sap 50% of a battery's output. In some situations the chemical reactions will happen so slowly and give so little power that the battery will appear to be dead when in fact if it is warmed up it will go right back to normal output."
There's no great trick to getting the battery warmed up either, the same chemical reaction inside the battery that releases electrons also releases waste heat so if you keep your BEV plugged in, as you would to charge it, the waste heat from the chemical reactions will keep the battery warm inside an insulated battery box. The only real impact on range would be if the driver is trying to keep himself warm with an electric cabin heater. But there are better answers for that problem too.
Posted by: ai_vin | 05 January 2011 at 09:26 AM
Zgoda. Ale stacji ciągłego ładowania jeszcze nie ma. Mój supermarket jest ode mnie 50km. Samochodem elektrycznym nie dojadę! Teraz w Polsce jest ok. -10 do -5 st. C.
Agreed. But the constant charging station does not yet exist. My supermarket is 50km away from me. Electric car can not arrive! Now in Poland is about -10 to -5 deg C.
Posted by: Janusz Głazik | 05 January 2011 at 09:35 AM
Remember what I first said? "Short answer - insulated battery boxes."
With good insulation the batteries should stay warm for hours even if no chemistry is going on. If you are only shopping at this supermarket you should be out and on the road before the batteries have a chance to cool down but if you are working there, well that may be too long.
Posted by: ai_vin | 06 January 2011 at 09:27 AM
Since when are current batteries unable to provide high discharge rates? Tesla does 0-60 in under 4 seconds, electric drag vehicles do even better, and most batteries can charge as fast as any reasonable power source will allow. I'm all for advancements but the power density of today's battery is quite good, what would be more useful is even better energy density. What's the big complaint about EV's? Not enough range.
Posted by: JRP3 | 07 January 2011 at 06:37 AM