I'll be back... in 8 hours

Some of my readers have wondered why I have been off the blogosphere in the last few months. The reason is that we brought a house and the move from the apartment to our new place has been a bit of a time sink.

First we went through the four stages of home buying:

Stage 1: What the &%#@ do you mean they accepted our bid? I thought you said we were lowballing?

Stage 2: When you use words like "downpayment", does this involve us giving you a check?

Stage 3: I assume roof's are like batteries? Meaning, when you say it is at the end of its life, there is still 80% left, right?

Stage 4: Keep repeating after me: "Owning is better than renting" and, please, stop asking "why?"!

Then we realized that owning a house also meant owning things like leaf blowers and lawn mowers! So, when I saw that there was a battery-powered lawn mower, I jumped at the chance to push my favorite technology forward.

I was looking forward to using my expertise in batteries to maintain and extend the life of my lawn mower for many years to come.

I was not particularly looking forward to mowing the lawn, but owning a battery-powered mower seemed to make up for that.

Until I realized that the top-rated battery-powered mower uses lead-acid batteries.

Lead-acid!!! really! How old school can one really get.

My first thought: Start a battery company to make Li-ion batteries for lawn mowers.

Then I started thinking about this some more. There must be a catch here. So I started digging into what it was.

Let us do some math: The battery for this lawn mower cost ~$270/kWh. That is one expensive lead-acid battery. Presumably, it is a deep-discharge battery, and so it is better made than a car battery.

And there is probably a significant markup.

Did I mention that the mower was on sale for $300! More like a mark-way-way-up.

Considering a typical Northern California growing cycle, this mower will probably be used ~50 times a year (once a week). These deep-discharge batteries can probably go a few hundred cycles. So I'm thinking 4-5 years easy.

But the bigger problem is going to be the calendar life. Sulfation can kill these cells.

Having said that, the battery is probably going to be at the top-of-charge pretty much through its life (think about it: Mown for 1 hour. Keep it plugged in all week). And remember our rule for lead-acid batteries: Keep them charged. So I think we can expect to get ~3 years from these cells.

Lets do some math for a Li-ion battery. I bought one a few weeks ago. This battery will probably last me 3 years and get me ~300 cycles or so. So it has similar specs to the lead-acid battery.

The Li-ion battery, on the other hand, cost me a whooping $2300/kWh!! No... really. This is what it cost me.

Now... knowing Apple, a new word has to be coined for their level of markup. But still this is one expensive battery.

At this price, a Li-ion lawn-mower-battery would have cost me $800!

All you MBA-types are probably cringing because you all know that cost and price are very different from each other and that the price is dictated by what the market is willing to pay (my wife is an MBA and she gave me this spiel).

Granted. So let us do a cost-differential comparison. This comes out to be ~$50/kWh more expensive for a Li-ion cell. Using this, the cost of a Li-ion battery for this mower would be higher than the lead-acid battery by... the price of lunch!

Not at Chez Panisse. But at the LBNL cafeteria (same quality, but at a much lower price?)

So why not use a Li-ion instead. After all it is almost 5x the energy density.

The mower that I have been talking about in this blog is a push mower. So the battery does not need to drag itself. All it has to do is turn the cutting blade. And space is not a big constraint. The mower's size is dictated by the size of the blade anyway.

So why bother using a new type of battery when you don't really see much of a benefit?

Frankly, although I have not looked at the life-cycle, I'll take a bet that it is probably better for the environment to use a lead-acid battery considering how much of this lead is recycled. In comparison, all you recycle in a Li-ion is the high-value metals in the cathode. The rest, literally, goes down the drain.

Maybe the math will change for a self-propelled mower. Or if the weight of the mower is an issue. But I don't see any reason to jump to using a Li-ion battery for the mower I was looking at. This business plan does not appear to have much legs.

So what did I do? I got the corded version of this mower. It cost me $100 less.

I guess the price that I'm willing to pay to push my favorite technology forward is less than $100!

In my guilt I decided to do my part for the technology by buying a Roomba.

I'm not sure if you guys know about this amazing robotic vacuum. It is pretty interesting to watch. It has sensors that make it slow down when it approaches objects, detect dirt, and prevent it from falling off of stairs.

It is not particularly good at vacuuming. But it feels like it is cleaning the floors and isn't that what's important!

But here is the kicker. The one I have has a Ni-MH battery.

I suppose I should be thankful it is not a lead-acid, but Ni-MH? COME ON!

The last time I checked, the cost of Li-ion and Ni-MH batteries were pretty comparable. And the energy density of Li-ion is 2-3x greater. And remember that in this machine (unlike the mower) the vacuum has to drag its battery along. And space is a big deal. The smaller the footprint, the smaller the space it can vacuum.

So why use a 20th century battery for a 21st century machine? Strange.

A web search revealed that Li-ion Roomba's have apparently died prematurely. If this is the case, then iRobot (the company that makes the Roomba) needs to change battery suppliers.

The vacuum I got goes through 2 rooms and then runs out of juice. It could have finished my house in one charge if it had a Li-ion of the same size. Or you could have a better vacuum on it so that it actually picks up dirt instead of moving it around and still only vacuum 2 rooms. You get the point.

When I first saw this machine, I thought that all the things on Terminator (the movie) were coming true. Jokes apart, it really is a pretty decent robotic cleaner which does indeed find its way around. You can, pretty much, set it and forget it.

But then I realized that we had nothing to worry from the machines as long as battery technology evolves the way it has been in the past.

When Arnold Schwarzenegger's character said "I'll be back" on T2, he (it) actually meant "I'm running out of battery and I need to go find an outlet and charge for 8 h. I will then come back to look for you for the 1/2 hour my battery lasts. Wish me luck".

So much for the machines taking over.

If you constantly complain about how batteries are not evolving fast, did you ever consider for the second that maybe we are out to save the world in our own way?

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)