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!