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http://gm-volt.com/2007/12/21/gm-vol...-breakthrough/
lifepo4 Says:
December 22nd, 2007 at 5:12 am
[quote comment="21019"]On lifespan (cycles) and 10x improvement, one should be careful.[/quote]
Thanks, finally someone else with battery knowledge posted his opinion here.
As you pointed out, the whole rationale behind this is to reduce the thickness of the anode and free the volume for more cathode, thus improving the cell capacity. If a new anode material has a capacity >3000 mAh/g with similar tap density, the derived anode will be very thin and you probably will get a boost of capacity about 50-80%, that’s it. Those who do not know the battery design can remember this: Even if the anode capacity is increased by a factor of 100 or 1000 times, it does NOT mean the battery capacity will increase that much because of the limitation by the cathode and other ingredients in the battery, the theoretical increase can only be about 100-150%, no more.
Now let’s check the reality. The supplementary info accompanied the paper shows anode thickness to be about 60 nm (NANO meter) and I estimate the loading (weight of anode material per square centimeter) to be only 0.1% (OR even less, I assumed a density of 1g/cc for the nano-wire, which is probably an overestimate). It is WELL know when you decrease the loading from a commercial value to a small value, the prototype half cell performs very well, but when you MUST increase that by a factor of several hundred to build a commercial cell, you will definitely lose the performance cited in the paper, that phenomenon is an unfortunate issue virtually can’t be addressed. The declaration of being revolutionary is ridiculous unless he also tried much thicker electrode and still preserve the performance and HE should have tried that and published the result. The reason for the good performance published in the paper is because lithium cation diffusion distance is only 60nm while in reality that distance is about 50-80 micrometer, which is about 1000 times longer. This is like a young guy can carry a 2 lb steel rod and run, he won’t be able to run if he carries a 2000 lb steel bar. You could argue that he can put the steel bar in a heavy truck and drive, but there is no similar mechanism in a battery design.
Other issues: The nano wire is inherently fluffy and has a low density compare to graphite. Because silicon reacts with lithium to have a theoretical volume increase about 300-500% (do not remember the exact value, but something like that range). If the electrode is not compressed during manufacturing, the advantage of capacity increase (per unit weight) is severely compromised, if the anode is compressed (standard process in any commercial cell), the volume expansion can severely damage the cell. One possible solution is to dissipate graphite into the void space of the anode to mitigate the problem, this strategy is used by SONY in their camcorder batteries ( Sn-Co-C anode), but implemented differently.
Still more issues (which I believe potentially can be solved.)
1) Good control of the nano-wire growth, it has to be uniformly coated onto Copper foil and has to be on both sides, not single side. Any spot with lower amount of nano-wire can potentially become a spot where lithium dendrite forms, thus causing safety issues.
2) Cost issues, I am not familiar with nano-wire growth and can’t comment too much on that.
3) Possible side reactions between Silicon nano wire and other ingredients of the battery, SEI formation.
4) Cycle life, it is not clear from the paper, initial results looks interesting, though.
The author should try this with much thicker nano wire deposition on the substrate, from the published data, it is highly unlikely he can achieve even a moderate performance.
As for the battery made by A123, their m1 266520 are hand-made using multiple tabs to boost performance. Yes, they are by far the best LiFePO4 cells made by any company. Now they make the big EV cells, I hope they do not use tools to scratch the electrodes in order to put more tabs (scratching can introduce particles which shorts batteries, like SONY), because the battery consistency will suffer and affect battery life, even though a single cell can perform beautifully. However, considering they were selling m1 in 2006/2007, it will be difficult to switch to automatic product line to weld tabs using a Japanese machine, this kind of transition usually take long time and I hope someone from GM asked them this question. I know there are lots of bashing against Toyota on this forum, but Toyota has far stronger expertise in battery R&D (JV with Panasonic) and still Panasonic had battery recall, I find it funny for Bob Lutz to laugh at Toyota when some small companies provide the Li cells. I am wondering if the PHEV package can last long enough in real tests.
To clarify those issues brought by me, please ask the following battery scientists:
1) Dr. Khali Amine phone: (630) 252-3838
2) Dr. Michael Thackeray: (630) 252-9184
3) Dr. John Goodenough: (512) 471-1646
4) Dr. Yet-ming Chiang: (617) 253-6471
5) Dr. Jai Prakash: (312) 567-3639
6) Dr. Glen Amatucci: (732) 932-6856
PS: It’s good to be modest to other people and be aggressive inside, well, Bob Lutz and GM do not, that may explains why GM’s fortune fluctuates like the currency of poor African country while Toyota has NEVER lost money in the past 50 years. DO NOT underestimate Toyota, they had problems (even fire), but they are highly competent and will NOT let GM take the lead
lifepo4 Says:
December 22nd, 2007 at 3:48 pm
I am someone who is actually designing Li batteries and have actual experiences in watching how batteries are made, so please do not say st***d things like that.
One thing I feel about GM (based on my expertise) is that they underestimate the difficulty. Yes, A123 battery performs very well, if you check their data, it has something called BSF, which means they test a single cell, but simply magnify the data by a factor of several hundred and show the test, this is fine in discussion, but when you put 200 cells together, ensuring the uniformity is paramountly difficult, and any small inconsistency over extended period may lead to premature aging for one particular cell over another. Since A123 USED manual process to make m1, I hope they DO not make 32 series in the same way. Even one company can make the cell 99.99% consistent, when you put the 200 into a pack, you must ensure that their thermal environment is the same. Put it this way, if you put 3 cells like ABC, B will always be 2-3C higher than the other two, after several years, difference starts to show up and BMS has to work hard to ensure consistent SOC, this is because when one particular cell has low SOC, it may go to charge reversal and fail or even explode. This is a less issue for HEV, but serious challenge for PHEV due to much higher SOC swing range. To tell you the truth, first generation of Prius is not particularly good in this respect, but fortunately NiMH is a little less sensitive to temperature than Li-ion. One reason why A123 performs well is they have more tabs. The Japanese do not use multiple tabs because the machine can’t be designed to do this, so they choose single tab and reduce the coating thickness to achieve performance (energy density compromised). With recent recalls, they will be even more careful. The battery coating room in Japan is like clean-room in semiconductor industry, virtually power-free, and occasionally problem still shows up. I do not know what it look like in A123 facilities. One thing funny is A123 has a Canadian division to make battery packs, yet they choose to work with Continental on battery pack. As for the CPI/LG, they use LiMn2O4/graphite, this battery is also safer, but performance degrades a lot under higher temperature and PHEV battery will often experience higher-than-normal temperature.
Toyota is working aggressively on their development, they are not testing Li ion on a vehicle simply because the cell reliability does not meet their demand, AND in fact, I believe there is a small chance that graphite based Li battery may never will. GM has no real battery experts in battery manufacturing and engineering. You know that American CEOs often boast about certain thiings due to short-term pressure by media and Wall-street, but they often under-deliver.
Battery in EV/PHEV is far more complicated and difficult than you might think. Americans have a tendency to be optimistic about everything, but I advise you to be a little cautious on this front. When it comes to some quick business (youtube, google, Apple iPhone), they are doing good, but certain things need dedication and long-term planning and mouth shut-up, and the Japanese excels on this. GM might get Volt out earlier, but who knows. If both GM and toyota have their PHEV out, who will have a better reliability? Remember, when 1 out of one million fail (DELL), SONY recalled battery. You need to ensure 10000 better reliability on PHEV battery, because if Toyota have one milion PHEVs, they will need 200 million cells. If one of them fails and blows up, it will be headline on CNN. Three-mile island incident does not kill a single person, but kills the nuclear power business in this country. For something like PHEV, it is better to be cautious than sorry later. And Tesla will learn this hard way in the future!
