Tuesday, March 23, 2010

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.


Sunday, March 7, 2010

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!