In batteries, 2+2=1. Actually more like 1/2. Well... maybe a bit less.

This is a blog post I've wanted to write for a decade. The reason I haven't (other than the obvious problem that a decade ago, I did not know what a blog was!), is because its a tough post to write. But, folks tell me that I have a gift for explaining things (I use the world "folks" is a generic sense to indicate a number greater than 0), so I shall try.

Everyone wants to make a better battery. What they mean when they say "better" is a battery that has more energy. This is what many (not all) battery researchers are trying to do, and this is what every user wants. If you read my post titled "A Moore's law for batteries? Maybe not", you will know that the game is to find new materials that make up the anode and cathode of a battery.

The idea here is to find a new material that has more capacity than the existing material and/or find one that operates at a higher voltage. Capacity is a measure of the amount of charge (electrons) you can get per gram. More is obviously better. Capacity times the voltage is energy; for real-world applications what matters is the energy. Typical numbers for capacity for lithium-ion batteries would be 140 mAh/g for the cathode and 330 mAh/g for the anode. The typical voltage of a lithium-ion battery is 3.7 V.

A lot of research in lithium-ion batteries is focussed on increasing the capacity. There is also an active area of interest in increasing the voltage to above 3.7 V. Increasing the voltage is going to be hard (very hard), so increasing the capacity appears to be the way batteries will improve, atleast in the short-term.

Simple enough.

Those of you who are paying attention have probably noticed that for every gram of material, you only have ~1/2 the capacity in the cathode compared to the anode. If you want to make a battery with a capacity of, say 330 mAh, then you have to take 1 gram of the anode, but you need 2.35 g of the cathode (330/140). What this means is that you have a total weight of 3.35 g to get a capacity of 330 mAh. So the capacity of your battery is actually 98 mAh/g (330/3.35). So you started with a anode at 330 mAh/g, a cathode at 140 mAh/g and you get 98 mAh/g for the battery. A 2 mAh/g cathode with a 2 mAh/g anode give you a 1 mAh/g battery. 2+2 is actually only 1. Certainly not 4. Not even 2! Welcome to batteries.

If you have a new anode with say 10 times the capacity (so 3300 mAh/g) you can do the same math and you will get a cell capacity of 134 mAh/g (for a battery of capacity 3300 mAh the weight is 24.5 g). You go to all this effort to make something 10 times better and you get to use your iPhone for an extra 30% talk time. A bit disappointing! On the other hand, if you had an cathode that was, say, twice as good, at 280 mAh/g (with an anode at 330 mAh/g), then your cell capacity goes up to 151 mAh/g. Much better. 50% better. This is why most researchers want to find a better cathode. Its more bang for the buck.

All this is pretty simple. All battery folks know this. 2+2=1. End of story.

Or is it? There is another small factor that even battery researchers sometimes miss. This factor is the dead weight in a battery.

If you really want to use the anode and cathode, you need some extra real estate. Things like separators to keep the electrodes apart, current collectors to collect the current, and packaging to make sure you contain it in a neat little package. All these add weight and volume. It doesn't matter if you have 10 times the capacity in a new anode, you still have to carry this dead weight.

This is a lot like a gasoline-powered car. Only the gasoline has any useful energy in the car. But to use the gasoline, you need a tank, an engine, the wheels, the drivetrain.... you get the point.

Obviously, if you can make the weight of the rest of car as light as you can (no seats?), it helps you get more from your tank of gas. Similarly, if you can minimize the amount of unwanted weight, it helps a lot in the battery. What this means is that you try to increase the ratio of the active materials (the anode and cathode) to that of the inactive material (the separators, current collectors etc).

But there is a catch. Turns out that you can't increase the amount of the active material willy-nilly. It has to do with losses in a battery. If you put extra active material in, you have to add a bit of the inactive with it. And increasing the amount of active materials involves making the anode and cathode thicker and there is a limit to how thick these can be made before losses become prohibitive. One needs to account for these factors.

*Geek meter on*

Remember the example above where we calculated 98 mAh/g using a battery of capacity 330 mAh with a weight of 3.35 g? If you do the math on the extra weight for the inactive material, you have to add an extra ~3.35 g. You can do the math to convince yourself of this number or you can trust me. I would suggest doing the latter. So you actually only get a capacity of 49 mAh/g (1/2 of 98)! 2+2=1/2!

For you battery geeks, you can verify these numbers by calculating the theoretical energy of the battery using the 98 mAh/g and multiplying by 3.7 V to get 360 Wh/kg (the theoretical capacity of a graphite/LiCoO2 cell). You can calculate the practical capacity by multiple 49 mAh/g by 3.7 V to get ~180 Wh/kg (A typical value for a 18650 cell using cobalt oxide). Well well well... the math works, does it not?

Here is the rub. Remember the example where we had an anode that has 10x the capacity. We had a cell capacity of 134 mAh/g. If you do the calculation for the extra weight and recalculate the capacity you get only 55 mAh/g.

You have to think about this a little bit, but it turns out that if you have more capacity you will need less of the anode, so now the inactive weight becomes a larger fraction of the total weight of the battery. You could have compensated for this by taking the same weight of the anode and just talking a lot more cathode, but like I was saying, this is impossible because it increases the losses in the battery to a point where it would be useless.

So we have an anode with 10x the capacity and we gain 12% in cell capacity (and don't get me started on the voltage! That is for another post).

If you don't believe me, do the math. If you don't know how to do the math; I guess you have to believe me! It would be embarrassing if someone spots an error; but then again I'm assuming that no one has actually made it this far.

If you do the same math on the battery where instead of the anode being better, the cathode is twice the capacity, where we calculated a cell capacity of 151 mAh/g without the inactive weight you will calculate a cell capacity of 68 mAh/g with the inactive material. So you made a cathode of twice the capacity and your cell capacity actually went up by 38% (remember we calculated this to be 50% better before).

Turns out that even if you make a battery with 3300 mAh/g for the anode (in a sense, this is close to the best Li-ion anode we know of) and a cathode of 280 mAh/g (the best Li-ion cathode we know of) we get a cell capacity of 110 mAh/g. 2.2x the present-day battery. But this assumes the voltage of the two are the same. In reality the materials that have this capacity have a lower voltage, which means that the energy is not really that high. Turns out that this best base scenario battery is better by maybe a factor of 1.8 to 1.9. Meaning, this battery will approach 340 Wh/kg.

*Geek meter off*

As a matter of fact, everything else being equal (i.e., amount of inactive material), the best Li-ion battery we can dream of making in the future, based on what we know as of late June 2010, will have a energy density of ~340 Wh/kg. If you want something better, you pretty much have to work on the inactive material. All these are for cell-level numbers. If you go to a battery pack, things get even worse, but that is for another post. If someone tells you that they can make a battery where the energy is greater than this, you better dig.

Most people, including battery researchers, don't think about this extra weight. Its actually a very important factor in a battery. There is such a great focus on new materials that folks forget that reality may be as good as your simple math leads you to believe. In addition to new materials, we have to think about ways to decrease the inactive materials in a battery. There is far too little research on this and a lot to be gained from doing something about it.

So next time you hear about a new material with more capacity, ask not how much more theoretical capacity you can get, instead ask how much practical energy you actually get. Remember that you can't just divide theory by 2 to get practical; it could be less (a lot less). And don't forget the voltage. Its also critical.

To help you, here is a link to a excel spreadsheet that has a battery simulator specifically for a lithium-ion battery. I hope the mac version of excel is compatible with a PC. The sheet that has the calculations is protected. Contact me to unprotect. Let me know if you catch errors. The simulator makes a LOT of assumptions. If you want them all relaxed, contact me.

Venkat

You say potato, I say...battery?

Saw something interesting in the news on a potato battery. Its a Zn-Cu battery with the two rods inserted into a potato. Why is this new? The authors say that they boiled the potato and were able to get less resistance and more power! I swear I'm not making this up.

Check out http://jrse.aip.org/jrsebh/v2/i3/p033103_s1 for the abstract. They say its cheaper than a AA battery and can be used to power a low-power LED light. I have not dug into the numbers to see if the cost claims are right. It would seem that with the low power that one gets from each cell you will need a lot of potatoes; the authors say they need 5.

My dad is visiting us from India and is sitting across from me. He tells me that 5 potatoes in India costs Rupees 15 (~30 cents). He tells me that a AA battery is either a bit cheaper or comparable! And I still need to buy some zinc and copper (and a pot to boil the potatoes, and spend some money heating it)

I get the impression that this is sort of a cute press release of something we already know and I wonder if it solves anything. I have to boil the potato and so need energy; I gain a means of making electricity, but I lose food to do that. And we really need someone from countries in the developing world to tell us if the cost numbers are what my dad tells me they are in India.

Anyway... could not resist.

Venkat

Made in Afghanistan

Many of you have probably heard the criticism against lithium batteries in that we are now dependent on a single resource for our energy storage needs. The trouble being that we substitute from one imported reserve for our transportation needs to another. And lithium is abundant in Bolivia and China (on the Tibetan plateau)- Two countries where US influence is weak. Now comes a report in the New York Times that the US has found vast reserves on lithium in Afghanistan. See http://www.nytimes.com/2010/06/14/world/asia/14minerals.html?hp

They also found copper, cobalt, etc., but its the lithium part that is intriguing to me. Apparently the reports coming out are that there is "potential for lithium deposits as large of those of Bolivia".

Now, in the short-term (i.e., a few lithium-based batteries in cars) there is no reason for any concern or interest in this story. There is more lithium than we need. If (and this is a big if), there is a significant conversion of the automotive fleet to batteries, lithium-based ones make the most sense. In this scenario, we can get lithium limited. But we are not talking about just running out of the metal (which could happen if we convert all our cars), its more the question of: can we mine the metal at the rate we will consume it. Obviously, just because we find deposits, this does not mean we can exploit it.

We shall have to wait and see what this means to the USGS estimates of lithium reserves. And I wonder if proximity of resources will play into which countries will take a lead in battery manufacturing. In the mean time, I'm sure all you conspiracy theorists will have a field day speculating on the cause for the war in Afghanistan.

Venkat

Tesota or maybe Toyola?

Logged onto New York Times and was amazed to see that there is a tie up between Toyota and Tesla.

See link

They plan to produce the sedan EV in the NUMMI plant in Fremont, CA. NUMMI is close to where I live and I'm happy to see something happening to this shuttered plant.

Its good news for Tesla and for Toyota. Hopefully, this will be a fruitful relationship.

I've visited Tesla and sat on the passenger seat of their roadster (I suppose they did not trust me with a $100,000 car). It's an amazing ride. Its easy to fall in love with the car and the concept (the price tag notwithstanding). Despite Toyota's recent troubles, I've always thought they are very good at what they do.

Lets hope that this move will get us closer to an EV I can afford (and trust me, if I can afford it, so can you).

Venkat

John Newman wins the Acheson Award

Professor John Newman has been selected to receive the Acheson Award of The Electrochemical Society. This prestigious award will be given to Professor Newman at the next meeting of the Society, to be held in Las Vegas during the week of October 10-15, 2010. Professor Newman’s greatest contribution to the “objects, purposes or activities of The Electrochemical Society” (i.e., the definition of the Acheson Award, as spelled out below)” has been his seminal approach to the analysis and design of electrochemical systems. Starting in the 1960s and continuing to this day, John has not only clarified the physicochemical laws that govern the behavior of electrochemical systems, he has also demonstrated how to use these laws to correctly formulate and solve problems associated with batteries, fuel cells, electrolyzers, and related technologies. His sophisticated approach to mathematically analyze complex electrochemical problems has been universally accepted by the academic and industrial communities, to the extent that it is now commonly referred to as “The Newman Method.”

John is a Professor of Chemical Engineering at the University of California Berkeley campus, a Faculty Senior Scientist and Principal Investigator in the Environmental Energy Technologies Division of the Lawrence Berkeley National Laboratory, and Director of the Department of Energy’s Batteries for Advanced Transportation Technologies Program. He is the author or co-author of more than 390 technical publications, numerous plenary and invited lectures, and the book Electrochemical Systems, which is now in its 3rd edition and is used throughout the world as a monograph and graduate text in electrochemical engineering. Professor Newman has mentored many graduate students, as well as post-doctoral fellows and visiting scientists. Additional details about Professor Newman and his research group can be found at http://www.cchem.berkeley.edu/jsngrp/

The Edward Goodrich Acheson Award of The Electrochemical Society (http://www.electrochem.org/awards/ecs/ecs_awards.htm) was established in 1928 for distinguished contributions to the advancement of any of the objects, purposes or activities of The Electrochemical Society, and it is awarded not more frequently than biennially. It includes a gold medal, a wall plaque, and a prize of $10,000. It is named for Edward Acheson, a U.S. inventor best known for the invention of the highly effective abrasive material Carborundum. Acheson also helped develop the incandescent lamp.

Beside the Acheson Award, John has received 9 other awards from the Electrochemical Society. He also was recognized as a Highly Cited Author, as identified by Thomson ISI; during 2002 he was an Onsager Professor at the Norwegian University of Science and Technology in Trondheim; and in 1999 he was elected to the National Academy of Engineering.

The Acheson is arguably the most prestigious award that an electrochemical scientist could hope to attain, short of a Nobel Prize or a National Medal of Science. The late Professor Charles W. Tobias (in 1972) is among those who have received the Acheson Award, and he the only other member of the Berkeley electrochemical community to be so recognized.

LBNL everywhere...?

ARPA-E announced the results for their latest round of solicitations that they issued a few months ago on transformational energy projects. One of the areas was batteries for vehicles. 10 projects were identified to receive the award (Here is the list). 3 of these (that I can talk about) have an LBNL connection. Its a proud day for what I consider the top battery program in the country. Turns out that the number 3 is not the full story, but I can't talk about any other connections that may (or may not) exist. Welcome to the world of startup's and confidentiality and stealth.

That's the quick summary. Here are the details. ARPA-E stands for Advanced Research Projects Agency-Energy, is the DARPA (Defense Advanced....) of DOE. I guess no one liked EARPA! Their mission is to (I copy and paste here):

- Enhance U.S. economic security by identifying technologies with the potential to substantially reduce energy imports from foreign sources; cut energy-related greenhouse gas emissions; and improve efficiency across the energy spectrum.

- Ensure the U.S. remains a technological and economic leader in developing and deploying advanced energy technologies.

This is the official spiel. In reality, ARPA-E is the agency that is supposed to help identify the next transformative idea related to energy technology. Think, the next transistor, or the next light bulb, but related to energy. You get the picture. These bullets above also do not convey the level of buzz this new agency has generated in the battery community. We have been energized. Getting ARPA-E funding is now considered to be a big deal. A very big deal. There are huge bragging rights for the winners, and those who are finalists.

No officially numbers have been released, but my sources (is that an euphemism for "I pulled it out of you-know-where"?) tell me that 350 concept papers came in the battery area. Of these 70 were down selected for submitting a full proposal. Of these 70, 10 were funded (ARPA-E folks: Please feel free to correct this). Less than 3% of ideas are funded! Now you know why being one of the 75 is a big deal and being one of the 10 is an even bigger deal. So you can see why its a proud day for LBNL when we have 3 funded proposals that we are involved with.

LBNL was involved in 6 submissions in the concept paper stage. All 6 were selected for submitting a full proposal. 2 of these 6 were selected to receive an award. The first of these is a project on a lithium sulfur (Li-S) battery that was led by Sion Power. Li-S is a chemistry that promised very high energy density, but has a lot of problems that hinder it from being commercialized. Sion is in the forefront of developing this technology. They will be working with John Newman, who is, for those of you who don't know, the father of electrochemical engineering (For those who track these things- I consider Charles Tobias to be the grandfather of electrochemical engineering). He wrote the book on the subject, literally. If you don't believe me, check this link. John Newman will be helping Sion with modeling their system. Systems like Li-S, if commercialized, can solve the problem of range for EVs and can help us adopt this technology.

The second is one from Applied Materials. They want to develop a manufacturing process that will lead to low-cost batteries that have high energy. Cost, as I have pointed out again and again, is one of the show-stoppers that prevent large scale adoption of EVs and PHEVs. Addressing this issue is critical. Applied will be working with Gao Liu and Vince Battaglia- both experts in making battery electrodes and cells.

These are the two where LBNL appears in the list. But there is one more that is hidden (that I can talk about). This is Polyplus, which got funded to develop Li-air batteries. Polyplus is a LBNL spinoff started by Steve Visco and Lutgard De Jonghe . Some of the underlying work that led to the formation of Polyplus was conducted under, what is now, the BATT Program. Li-air is the Holy Grail of batteries- a system that has 10x the energy density of today's batteries on a mass basis. Polyplus is pretty much the top company in this area. If they succeed, we can make a big dent on the range issue with EVs.

Three very different projects; but they all promise to change the world if they succeed. The fact that the reviewers have picked so many of the projects that LBNL was involved with is a testimony to the cutting edge research that happens here. Congrats to all the winners. And did I mention that this is not all. There may (or may not) be other connections. Stay tuned.

In a future post, I will delve a bit more into these technologies and provide my thoughts on what the critical challenges are and what (I think) their approach will be.

Venkat

Lithium-lithium everywhere...

Over the last 15 years the whole battery community has slowly starting looking at one single system: the lithium battery. I remember going to an Electrochemical Society meeting in 1996 to present a paper on Ni-MH batteries. My talk was scheduled for Monday AM (read prime time). By 1997, I was down to Wednesday, and by 1999, I was talking Friday PM with 3 people in the room (yours faithfully, the presenter before me, and the one after me. Even the session chair was missing!). If one was looking to read the signs, this was it.

This week I was talking to someone who was interested in electrochemical capacitors (more on this later) and he asked if all the research was only on lithium batteries or do we do anything else? I had to admit that there was very little that goes on other than lithium. Since that conversation, I've been thinking about the effectiveness of having all our eggs in the lithium basket and wondering if this is a good thing.

First off, I should note that when it comes to batteries, lithium has a lot more energy per weight or volume compared to the other batteries we have been looking at in the past. Its got 4-5 times the energy per mass as a lead-acid battery and 2-3 times that for a Ni-MH battery. There is still a lot more that can be done with this chemistry and so there is a reason for us to obsess about this. When it comes to batteries for a plug-in or an electric car, we need all the energy we can get and working on lithium does make a lot of sense.

But does this mean that there are no advances that can be made in other energy-storage technologies? And even if we do make advances in these other areas, does it really have any impact in the world? Let's look at the second question first.

Remember that we have argued that batteries for plug-in and EVs are going to very expensive and that they may not last very long either. Its far from given that we will indeed be driving in these battery-powered cars. We may not be cost competitive with gasoline for a while, and we may end up seeing that these cars as a niche market. Maybe hydrogen will take off as a carrier and batteries as a primary energy source will not be the future. The future is far from clear, but it is possible that we may end up finding out that hybrid vehicles (like your out-of-control Prius) may be the most common vehicle on the road for a decade. But even your Prius is not as popular as it could be because its expensive.

But when it comes to hybrids, its not energy that is critical, but the power. Many batteries, including the lead-acid and the Ni-MH batteries have the power capability to satisfy the requirements for a hybrid. The cheapest battery we know of is the lead-acid battery. So if we can get the power with a lead-acid battery and if its the cheapest battery we know of, why don't we use this for hybrids?

Remember the post on battery rules where we asked you to "keep you lead-acid charged"? You may also remember that this was important because if the lead-acid is ever discharged, it sulfates and causes capacity loss. In a hybrid, the battery is always sitting partially discharged. This is important because if you hit the brakes, you need to be able to accept the juice in your battery. Hybrid batteries operate around 50% state of charge because of this. Try using a lead-acid at a partially discharged state and you will have a dead battery long before the new car smell fades.

But something strange happened to the lead-acid battery when the rest of us were obsessing over the lithium battery. The companies working on this started using activated carbon in their negative plates. Lo and behold!, these companies started seeing much better cycling with this new concept. Some companies are doing variations of this by replacing the negative electrode with activated carbon so that its a hybrid between a battery and a capacitor. Companies doing this are promising all the cycle life you need at 1/4 the cost of the Li-ion for hybrids! The catch: they don't yet know if the batteries will last 10 years. So they can get the cycle life, but its not clear they can get the calendar life.

But that is not the point I want to make. What amazes me is that someone can take a 150 year old technology and show that they can make it better by solving a particular problem that stops it from being used for an application. I'll take a bet that 90% of the researchers in the field of batteries don't know of this advancement (all right, that may be a bit of an overstatement, but you get the point). I had worked on a mathematical model for a lead-acid battery a few years ago where I had included some features in the model that make it easy to address this advance. A person in the lead-acid industry had sent me a mail when these findings started coming out asking if I was doing anything to address these new findings. I had to (sadly) tell him that although this was interesting, I had no way of doing anything because all my funding is in the area of lithium batteries.

But what if we find out that the new lead-acid batteries do have a calendar life issue. Should we leave it to companies to figure this out by themselves, or should battery researchers be helping with this effort to see if we can find a solution? It seems to me that the answer has to be the latter, but I fear that this will not be the case. The chemistry is considered too "un-sexy" (for want of a better word). Try presenting a paper on this at the Electrochemical Society meeting and you will asked to present on a Saturday morning (after the conference has ended!). To be fair there has recently been government support for these technologies for trying to commercialize them; but I don't see anything happening at the research stage.

The story of the lead-acid is far from unique. The few (very few) folks who continue to work on the Ni-MH system talk about the use of carbon fiber instead of nickel plaques to decrease the weight and cost of the battery and increase its specific energy. The person working on capacitors was telling me that he had ideas for increasing the energy by the factor of 2 (which, as it turns out, could be huge for a hybrid). There are companies working on Ni-Zn batteries that think they can do something better than Li-ion in some applications. But look at research in the US on batteries and there is pretty much zero effort in these areas. None of these ideas may pan out, but the question remains: should battery researchers be looking at these issues along with companies or should we all focus on one system (the lithium system)?

Part of the problem is the amount of funding that is available for battery research. If there was unlimited funding available, all these problems will be looked at. But with funding being tight, one needs to focus on a few problems and not spread one-selves too thin. Another problem is the community. As in any other area, there is a bit of jumping on the bandwagon that happens. Its tough being the only guy doing something, especially when no one cares for what you do.

At LBNL I wear two hats: one as a researcher looking at mathematically modeling batteries and another as the technical manager of the Batteries or Advanced Transportation Technologies (BATT) program. In my latter role, I have a hand at picking the kind of systems we work on. And I know exactly why we have picked to focus on the lithium battery exclusively. But the advances in these less "sexy" fields makes we wonder if this strategy is right. Weigh in with your views.

Venkat

Nano for batteries: the challenge of volumetric density

In my previous post I discussed some of the pros and cons of using our skills making materials in various nanoarchitectures in Li-ion battery technology, mainly related to the increase in surface area and shortening of diffusion paths. Today I will concentrate in other aspects: volume and density.

Most probably, you have heard how silicon-based electrodes can boost the storage of Li-ion batteries. And you may have even noticed that a lot of the releases announcing breakthroughs in this area have the word nano in them (here's a very recent example). Indeed, one of the most serious problems of silicon electrodes is that they expand and contract enormously upon cycling because they can uptake so much lithium (hence the boost in storage capacity). In brief, what happens with big particles is that they crack into much smaller ones due to these expansion/contraction cycles (think of freeze/thaw cycles in the winter) and lose contact with the electrode additives and the electrolyte, cripling the electrode life. So starting small (i.e., nano) bypasses this issue. With less volume in a particle, it is less likely to break in several domains.

Usually, electrodes composed of nanoparticles of silicon cycle better than those composed of bigger particles... provided that we are good at building the electrode structure so that all these new particles are well connected to the current collectors (and, subsequently, the battery leads) through the electrode additives. However, issues arise when using this strategy. Guess what one of the problems is? Yes, our friends the side reactions; silicon reacts outside the voltage of stability of the electrolyte. A second problem, related to the increase in surface, is that silicon nanoparticles are much more reactive with air than micron-size particles, bringing up the need for additional safety controls during handling. In fact, some people have even proposed primary silicon/air batteries.

It is quite likely that silicon will have to be used in nanometric form to make it a viable electrode with very high capacity. But be careful with overstating the numbers. A typical strategy to keep a good electrical contact is to use more conductive additives (carbon, basically) because there are more particles to connect to each other and the current collector. Increasing the amount of polymeric binder is another strategy that is commonly used to keep the particles together. And more carbon and/or binder means less silicon, which means that we are reducing the total capacity of the electrode. Yes, in terms of the silicon only, the capacity is unchanged, but we have to count everything when building the battery! Pay attention to what companies that announce silicon-based batteries show as capacity gains and you'll see they are more modest than would be expected theoretically.

In addition, nanoparticles have a very annoying tendency to form aggregates that is very difficult to control. These aggregates form rather disorderly and, therefore, leave a lot of dead space within, which can be helpful for electrolyte wetting but also be unnecessarily high. Finally, a lot of the strategies to alleviate volume expansions in silicon electrodes rely on placing the particles far from each other, so that they don't crush against each other upon expansion. Obviously, this also helps with the aggregation, but now we have even more inactive space between particles!

The result of all these approaches is always the same: the density (mass per unit of volume) of the electrode is lowered considerably with respect to an electrode made using more traditional methods with bigger particles. In general, it is very difficult to achieve bulk packing densities of materials using nanostructured electrodes, which may, after all, be something we have to live with, in some instances, if we want them to work.

Unfortunately, lower packing density also means lower total volumetric energy density of the electrode/battery. And volumetric energy density is no laughing matter when thinking of batteries for electric vehicles. There is very limited volume available for the battery (especially in a hybrid). Using certain nanostructures as electrodes can lead to very long life, but very modest volumetric densities, so that you still need a bigger battery to power your car. When volume is factored in, the gains of using nanoparticles are systematically much more modest or even totally offset. This is a problem that is still unsolved. Scientists are getting better at synthesizing nanoparticles and we are starting to be able to control aggregation and assembly, so there are possibilities that are being explored. Now, the methods associated may imply an increase in cost of manufacturing. But I'll leave this for another day.

More reading materials for those interested in the science of silicon electrodes:

Larcher et al., Recent findings and prospects in the field of pure metals as negative electrodes for Li-ion batteries, Journal of Materials Chemistry 17, 3759 - 3772 (2007)

What can nano do for batteries?

First and foremost, I must express my gratitude to Venkat for his introduction and for encouraging me to participate in this new blog-adventure. Now that he cannot hear us, I will tell you that I learned quite a few things myself with his posts. It is a pleasure for me to offer a different perspective to his and I hope you will find it informative. Since he has already introduced me, I’ll move on to the fun stuff.

Nanotechnology is certainly one of the fields of research that has witnessed greatest progress in the last decade (although the concept itself is not all that new; Michael Faraday can be considered one of the first nanoscientists!). When it comes to functional materials, among other things, nanoscience offers the promise of enhanced catalytic properties, controllable band gaps that can have an impact on the efficiency of a solar cell and novel medical diagnostics and treatment tools (not to mention that most of suntan lotions contain nanoparticles nowadays!).

It is easy to wonder whether nanotechnology would also be a helpful tool toward better batteries. There are a few arguments in favor of using nanoparticles in a battery, particularly as part of the electrodes. We will just concentrate on a couple of them for now (and leave the rest for another day if I am still considered a guest after today!). Although the discussion relies on the Li-ion technology for examples, the ideas are pretty much applicable to many battery technologies.

One of the first incentives one can think of is the shorter distances for ions and electrons to travel through. Broadly speaking, think of it as you having to cross your living room as opposed to a football field. The second one has to with the surface area. Let’s imagine that the square in the picture below is made of 1 um edges:

divide each edge ten times and you now have 100 squares with edges of 100 nm:

All those new lines you see are new surfaces (if you want to entertain yourself, expand the exercise to a 3D cube and reduce it to particles of 10 nm). In a battery, all of this surface could ideally be exposed to the electrolyte, which is the media through which ions transfer from electrode to electrode (the key to battery operation).

Now we have (lithium) ions that can access the active material much more extensively (more surface) and have shorter distances to travel through the solid (remember the living room). Diffusion in the solid is typically, but not always, slower than in the liquid, so, in theory, the result is a lower resistance to ion transport and, therefore, better utilization of the material (i.e., higher stored charge/energy) and, potentially, higher rates of charge and discharge (i.e., shorter charging times and higher power).

There are many reports available that show that samples composed of nanoparticles, all things equal, can lead to better performance than (what we call) bulk counterparts. In materials with poor ion conduction, this effect has had an important impact. The now world famous lithium iron phosphate owes some of its glamour to the chemists that were able to nanostructure it. The same happens with an anode material that is getting a lot of exposure as of late: lithium titanate.

So in a sense, nanotechnology has an important role in the field of (Li-ion) batteries, and, in fact, there are many R&D projects that concentrate on exploiting the advantages of nanotechnology to developing advanced materials, including some in our very own Batteries for Advanced Transportation Technologies program (go through our recent scientific reports to see what's up) at the Department of Energy. So, it is settled! Nanotechnology is the way to go to make batteries with five times more energy density!

Hmmm... not so quick... Apart from the fact that geometrically increasing the energy density is much trickier than it seems (we’ll also leave this for another day), there is a key subtlety in the case of lithium iron phosphate and lithium titanate that makes the whole trick work: they are both active within the voltage window of stability of the components of the liquid electrolyte. Venkat has already told you that bad things happen when we fall outside this window. Unfortunately, one of the ways of increasing the energy density is precisely by increasing the voltage (remember, energy = voltage x charge). Graphite electrodes, for instance, ubiquitous as they may be, react outside that window.

If we start using materials that are only slightly outside the thermodynamic window of the electrolyte, the undesired reactions may happen at a rate that is slow enough for us to live with. But, hey, remember that increased surface area of nanoparticles? Yeah, it is increased for every component of the electrolyte, not just the ions. The immediate consequence of reducing the particle size of our electrode materials is that side reactions tend to be exacerbated. And those side reactions produce insoluble products that deposit, just our luck, precisely on the surface of the particles, covering them and producing layers that are resistive to the diffusion of ions. Automatically, the advantage of using nanoparticles is lost. And things can be so bad that we may be better off using slightly less active electrodes!

In conclusion, you can choose to have a lithium iron phosphate/lithium titanate battery that operates extremely well and has a long life thanks to the use of nanoparticles... but what if I told that such battery only has a voltage output of around 2 V (yeah, those lead-acid batteries don't look so bad anymore)? And if I mentioned that, in addition, the lithium titanate has about half the storage capacity of graphite, so the energy density of this great battery is lower than that in your cellphone Li-ion cells? If we are to increase the energy density of current batteries to make them more application friendly, we have to come up with inventive ways of using materials operating at high and low voltages; preferably, not in the form of nanoparticles… unless we find a way of stopping those annoying side reactions without killing the ion transport at the same time.

There are other issues that make nano only a partial answer (sorry, no Moore's law for batteries) to the performance barriers of batteries, among which are higher associated manufacturing or processing costs and lower packing densities. But this is for another post (or two!). If you are left wanting more, you can get even more knitty-gritty details by reading some scientific literature (if you have access to it, of course), which will also offer additional shades of grey to the arguments developed here:

Aricò et al., Nanostructured materials for advanced energy conversion and storage devices, Nature Materials 4, 366 - 377 (2005)

Bruce et al., Nanomaterials for Rechargeable Lithium Batteries, Angewandte Chemie International Edition 47, 2930 – 2946 (2008)

Welcome to Jordi Cabana

Dear Blog readers- Over the past month or so, I have introduced various topics related to batteries and their use in, primarily, vehicle applications. As we move forward, I wanted to get different perspectives on this subject matter from other experts and broaden the discussion. This may be hard to believe, but I actually don’t know everything. (my family is going to save this post, because this would be the first time I’ve admitted this!)

The thought is that we will periodically bring in guest bloggers to write about something that interests them. We begin this week with a post by Jordi Cabana. Jordi is a scientist colleague at Lawrence Berkeley Lab and is an expert on synthesizing new electrode materials and characterizing them. He has a great understanding on many different issues related to this topic. Check out his website at http://berc.lbl.gov/jcabana/

Jordi will start by telling us a bit about nano as it applies to batteries. This is a hot topic and one that has a lot of complexity to it. Jordi’s post will be up shortly.

Stay tuned for more.

Venkat

Is there an electric vehicle in your future?

You may remember the post on my pathetic financial state that does not allow me to buy a plug-in hybrid (PHEV). I had expected that a public cry for (financial) help would have gotten my bosses to do something. A month has gone by and I don’t see any action, so I have decided to try the plea again couched in the form of a blog post describing the challenges that prevent the widespread penetration of electric vehicles (EVs) on the road. No prizes for guessing that cost would be one of them and a big one at that.

Other than being expensive, batteries tend to blow up every once in a while, they tend to not have great energy density, they seem to fade and die a lot sooner than they should, and take forever to charge back up. Low temperature performance is also an issue with these batteries, but I think this can be solved (by using a second lead-acid battery, for example), so I’m not going to talk about it here.

Turns out all these are connected. For example, if you want to increase the life, then you can do that by cutting the depth of discharge you cycle the battery, but this cuts the energy down, and lessens the driving range, which means you have to have a larger battery, which costs more. So as you read this, remember that if you have the solution to one metric, you can’t make another metric worse. Let’s look at these in more detail.

For the scientifically minded, on my website, you will see a presentation and a paper that gets into details on this topic and describes the detailed science behind what we are trying to do to solve these problems. I’m going to stick to a general description in this post.

As you can imagine, if you want to drive 300 miles on batteries you probably need quite a few. A rule of thumb you hear is you can drive 1 miles on 300 Wh. This is for a typical family sedan. This rule of thumb is a bit misleading in that a lightweight car (like the Fisker Karma) would allow one to go further for the same battery, but let us use this rule for the time being.

For a 300 mile range you need ~90 kWh of energy and at $1000/kWh this will cost you ~$90,000! You can do what Tesla does and use laptop batteries that last ~4 years and cost $450/kWh and get the cost down to $27,000 for 200 miles range and $40,000 for 300 miles, but it’s still expensive. I’m not sure Lawrence Berkeley can afford this kind of pay hike. Suffice to say that cost is going to be a big challenge.

One could argue that we need to change our lifestyle and get a car that goes, say 100 miles, like the Nissan Leaf. Now you only need a third of the battery and so the costs can come down significantly. But this does require a change in our way of thinking.

So what can we do about this? Mass manufacturing will help. Different ways of making batteries that are either cheaper or promise more energy will help. Remember that more energy for the same cost means you need less battery for the system, leading to less total cost. And new materials that promise higher energy will help. None of these are trivial, but there is real hope in increasing the energy of batteries by a factor of two. If another factor of two can be achieved by lower manufacturing costs, then EVs start to look pretty attractive.

But let us go beyond cost and ask what else we need to do in order to make EVs a reality. Let’s talk safety for a second. A laptop on fire can be a fun thing to watch on YouTube. But a car burning on the freeway will be the death of the whole battery industry. Lithium batteries go up in flames when something goes wrong with the charging circuitry or if there is an internal short. So if you charging circuit does not cut out and the battery starts to overcharge, some (not all) of the cathode materials that we use have a tendency to react and release a lot of heat. The electrolytes are also flammable. Combine these two and you get a violent thermal event. Some call this rapid disassembly (read explosion).

There is quite a bit of stuff that the research community has done and is doing on this topic. These include developing nonflammable electrolytes, new cathodes that don't have this reaction, and new ways of letting the battery not overcharge. I think we will see some exciting new technological advances in this area. We at Berkeley are actively looking at this topic.

Lets talk energy density. If you look up packaged energy density of batteries for EVs on the web you will come across numbers in the range of ~110 Wh/kg and 160 Wh/l. Remember that we need 90 kWh for our hypothetical car that goes 300 miles. So the batteries are going to weight 1950 lbs and 150 gallons! That’s one really large tank. So we have to get the energy density of the batteries up so that we decrease the weight and volume. This is already happening and we are starting to see some progress in this area.

One should not expect a 10 fold increase in energy to a point where the batteries occupy only 15 gallons, but I think more than a doubling from these numbers is realistic. Remember the laptop battery? A packaged laptop battery can give you as much as 160 Wh/kg and some of the newer materials are going to get these up to 180 Wh/kg. Only problem: they don’t last very long.

If you’ve read my posts from before you know all that I know about life of lithium batteries (and please. No snide remarks on how little I know). Remember: Don’t charge them too high. Don’t swing them too wide. Keep the temperature low to extend their life.

Unfortunately, for an EV, we have to charge them all the way. If we limit the charge, then we decrease the energy and pay a range (or cost) penalty. And we have to discharge them for us to get all that energy out. And it does hot here in California every once in a while and some of you do live in Arizona (sorry about that). And did you not hear? There is global warming coming very soon! Needless to say, life is going to be a worry.

Let’s level-set ourselves first. My laptop lasts a few hundred cycles (my older Dell lasted, probably, 50 cycles, but I’m going to assume that this was an anomaly). But good EV batteries can last quite a bit longer (hence the higher cost). Battery companies are now routinely reporting 2000+ cycles. How many cycles do we really need? If you have a 300-mile EV, 2000 cycles is 600,000 miles! All we probably need is a battery that can go 600-700 cycles. Sounds like life is a solved problem, doesn't it?

Turns out life (the real one and the battery one) is not that easy. There are two kinds of life we have to think about. The one above is cycle life. The other is calendar life (i.e., how many years will my battery last?). Any idea how long it takes to do 2000 cycles in the lab? If you charge and discharge the battery in 6 hours (which is a typical test), you get 4 cycles a day, 120 cycles in a month and 2000 cycles in 16 months! A far cry from the 10 years the car has to last. Remember our 1st rule for long life (don't charge them too high). The side reactions that gave us this rule do not care that you can cycle 2000 cycles in 16 months, they will beat their relentless drumbeat to kill the battery month after month, year after year. Getting those 600 cycles to happen over 10 years is going to be a real challenge.

There is quite a bit of work being done to get better life. Probably the biggest scientific challenge is controlling the interface between the electrode and the electrolyte, where these side reactions occur. There have been some impressive innovations in modifying this interface that, if successful, should make a dent on this issue.

Finally, lets talk charging time. One of the big hit against batteries is that they take as much as 8 hours to charge (hence the business case for companies like Better Place). A lot has been said about the ability of our grid to be able to take the staggering amount of electricity that we will need if all of us charge our car batteries at 6 PM and expect them to charge in say, 5 mins (think 1 MW to charge a 90 kWh battery- that’s a single car!). But let us set aside this question for a minute and ask if batteries, fundamentally, can charge in, say, 5 mins.

The reason why batteries typically don’t charge fast is related to the rate of the various processes in a battery. For you to get the energy into the battery (or get it out), a number of steps have to take place. All you need is one step that is slow and the whole process is gated. Turns out that if the process you want is gated, and if you keep pushing charge into the battery, you pay an energy penalty via the generation of heat. And in some cases, the charge goes into some other (detrimental) process.

In the case of lithium batteries, this detrimental reaction on charge is the plating of lithium and happens on the negative electrode. If you plate lithium in a lithium-ion battery, bad things happen (decrease in capacity at best, a fire at worst). Remember that this is for a lithium-ion battery. There is a class of batteries that people are trying to develop called lithium-metal (or lithium-polymer) batteries, which are designed to not have problems when lithium metal plates; but that is for another post.

So how fast can we charge before we get these problem reactions? This depends on the exact chemistry that we are talking about, but in a typical lithium battery where we use a graphite negative electrode, we can probably charge in 12-20 mins! That’s pretty good, but the problem is going to be the heat generation in the cell that would be bordering on the dangerous. Moreover, people have shown that when you charge these negative electrodes at these sort of rates, the electrode particles tend to crack because of the stress generated within them. Not a good thing for life.

There are some companies that are moving away from using graphite as a negative to other materials (lithium titanate is a favorite among many). This material does appear to not suffer from the problems I told you about, but the voltage of a battery made with this material is ~1.4 V lower than traditional lithium-ion batteries. Lower voltage means lower energy, so its going to be one huge battery if you want the range. But then again, you can argue that if you can charge in 5 mins, maybe 100 miles is all the range you need. Lets see how much you’ll like this when you are on your next road trip. There are many ways to skin a cat (although in full disclosure, I don’t know of even one)

So lets take a step back and ask what all this means. These are all the things I believe: I believe these batteries will be safe. I also believe that we can lick the life issue (i.e., they will last 10 years). I believe we are getting better and better at getting the energy up and car companies will package these better and get the range up. And some of you (I’m being careful and not saying “some of us”) will not need the range.

I don’t believe charge time of batteries are going to be 5 mins anytime soon- this is a lost cause at least for most mainstream batteries. But maybe we don’t need it to be 5 mins. Maybe we will move to being a two car family, with one being an EV and one being a PHEV. Or maybe Better Place will substitute all the gas stations in the world.

What I worry about is cost. As long as gas remains what it is today ($3.02 per galleon as of today), there appears to be little hope for cost parity. Some (including me) believe that the charge time and cost issue of an EV can be solved if we stick to a PHEV.

So is there an electric vehicle in your future? Bill Gates (I saw a comment to my previous post from a Bill, who I have to assume is Gates)- you have no problems. As for me, I’ll stick to a PHEV. Oh wait... I forgot. I can’t afford that either!

Venkat

Battery Rules.

I’ve been reading Michael Pollan’s new book titled “Food Rules” and thinking about his Haiku on food which goes: “Eat food. Not too much. Mostly plants.”

Succinct, but loaded with a lot of complexity. Inspired by this, I decided to try my hand at a (kind of) poem for battery health. But first, the way to extend the life of a battery depends on the battery type. Lets start with the battery we use everyday in our laptops and cell phone- the lithium-ion battery. The rules read,

Don’t charge them too high

Don’t swing them too wide

Keep the temperature low to extend their life.

Lithium ion batteries are not all equal. A battery from A123 will be different from one from Panasonic. So use these rules only for your cell phone and laptop batteries.

In the previous blog post, I told you why you don’t want to charge them too high in voltage (answer: side reactions). Higher the voltage, higher the fade. Turns out that if you swing the state of charge too much (i.e., charge and discharge the battery completely each time) the life decreases significantly.

This is because the battery materials expand and contract on charge/discharge (by as much as 10%). This constant “breathing” results in the particles cracking. As a matter of fact there there is data that shows that if you swing the battery to, say 3% (like in a HEV) you can get 300,000 cycles (yes, you read that right). But if you swing them all the way, you only get 300-1000 cycles. So you can charge and discharge them a lot, but you cant let them swing too wide.

Turns out that while a small swing in the state of charge is good for life, its wrecks havoc when trying to estimate the state of the battery. So as a good rule of thumb, every so often (say 2 months), discharge the batteries completely and then recharge them back up to make sure the software can reset the battery capacity and predict run-time better.

Finally temperature. Temperature is a boon if you want to make things faster (reaction rates increase with temperature). But remember those side reactions? The rate of these reactions also increases with temperature and they accelerate the capacity fade. Hence the recommendation to keep the temperature low.

As a matter of fact, I’ve seen recommendations that ask you not to keep your cell phone in your pants such that the battery is close to your skin (which would be, assuming you are normal, at 37 C). The 15 C higher temperature compared to the ambient (if you are in California) will kill the battery fast. Folks around the equator- sorry.

Turns out that if you go down to freezing temperatures, you get other problems with lithium batteries, but that for another blog post.

This is it. Three simple rules for a long lasting lithium-ion battery. Forget all the rest of the stuff that you hear about keeping it on the top of charge, get the juices flowing e.t.c.

Question is, where do these myths come from? Like I said before, the rules change when you change the battery chemistry. So rules from one battery chemistry get applied to another and what you have is confusion.

So here is a second (kind of) Haiku that tries to capture the different batteries we typically encounter or have encountered. This reads,

Keep your lead-acid’s charged

Let the Ni-Cad’s completely discharge

Lithium-ion and Ni-MH? Somewhere between

Lets get to the first rule. For a lead-acid car battery, the failure mechanism is called sulfation, where the discharged material undergoes a phase transformation after which it can’t recharge. Remember the time you left the glove compartment light on, the battery died, you got it jumped a couple of days later, and was told to buy a new battery? Yup, that was sulfation. Happens everytime the lead-acid battery discharges. Hence the rule that we should keep the lead-acid battery charged.

But, keeping a lead acid completely charged also leads to other problems (like grid corrosion) which would be lessened if you let the voltage decrease a bit. Moreover sulfation takes a few days. So one could do something complicated like let the battery charge, and then let it discharge a bit, but come back the next day and charge it back up before sulfation kicks in.

But let’s not make life very complicated, shall we. Just keep the battery charged, it will last 6 years, then get a new one. If you are an enthusiast, contact me and we can talk about an optimal charging scheme. And if you are one of those using the lead-acid battery for deep discharge-cycling then you are in real trouble. This rule is not going to help you. Some companies are now claiming to have solved this problem. Maybe they can help you.

On to the next rule.

Remember the mythical memory effect? Picture this (true) scenario: father calls his electrochemist son asking why the cordless phone battery was not holding charge; son admonishes father to get off his cheap lifestyle and buy a new battery already; father is convinced that son doesn't know what he’s talking about; father contacts second son who decided to use google and discoveries memory effect; second son asks father to discharge battery completely and try using it; father reports success and decides to disown electrochemist son. Amateur battery enthusiasts are the bane of my life (especially if they are family).

That was a Ni-Cad battery. if you don’t discharge these batteries completely and charged them back up, they seem to not remember that there was actually some capacity left. I have never experienced this myself, but there are many reports on this subject. Hence the oft repeated mantra- let the battery discharge completely before you recharge. Its specific to Ni-Cad batteries. See how different batteries are different?

Remember when the nickel-metal hydride (Ni-MH) battery came out and everyone said there was no memory effect? Turns out they jumped the gun. Ni-MH also appears to have a memory, albeit not as bad as Ni-Cd. There is some controversy as to why it actually happens and there is a thinking that overcharging these batteries causes memory. I used to work on this system and have overcharged these batteries quite a bit, and have not seen the memory effect, but... maybe I have a golden touch. Without getting into details, its best to pull the plug in these batteries when they are charged. Even if you don’t believe in memory, this also helps prevent drying up of the battery because of hydrogen evolution and venting.

As for discharge: completely charging and discharging a Ni-MH battery is not a good idea (the metal hydride particles also expand/contract and can crack if you do that), but then again, there is some data that suggests that there can be a memory-type effect if you don't discharge them completely.

So what should you do? I’m going to recommend that you don't worry about memory and try not to swing the state of charge too much. Every so often (say 2 months) discharge the batteries completely. May be a bit hard to do in practice, but one can try. Remember that the Toyota Prius has Ni-MH batteries and you never completely discharge them. They work fine for 10 years without any problems from the memory effect.

Hence the rule to keep the state somewhere between the lead-acid and the Ni-Cad for the Ni-MH battery. In many ways this is the same as the lithium-ion battery.

Lets hope this post does not end up adding to the myths that are already out there!

So here’s to Haiku-ing your way to better battery life.