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Record Battery Energy Density in Context [Updated]

Envia Batteries

A tech company called Envia Systems has announced that it is able to produce rechargeable lithium-ion batteries (Li-ion, i.e., the standard kind of rechargeable batteries that go in everything from phones to electric cars) with a world-record energy density of 400 Watt-hours per kilogram! (Gigaom has lots of info, and useful background material.) Cool, right?

Yes? No?

Energy density is one of those really important concepts that not many people know about; it's not an exaggeration to say that a viable renewable energy future depends upon boosting energy density of batteries.

But it's hard to evaluate the importance of an announcement like this if you don't have context, so here you go:

Okay, 400 Watt-hours per kilogram (henceforth Wh/kg) means that one kilogram of battery material will be able to pump out electricity at a level of 400 Watts for one hour.

According to Envia, the best commercially-available Li-ion battery has an energy density of around 245 Wh/kg, so this new technology almost doubles that. This is good. Moreover, most Li-ion batteries operate at about 100-150 Wh/kg. The batteries in the Nissan Leaf, for example, have an energy density of about 120 Wh/kg (24 KWh/200kg). Tripling that density would, in principle, triple the range of the Leaf, taking it from around 100 miles to around 300 miles, a range close to a typical gasoline-powered car. This is very good.

But it's not revolutionary -- it's a (significant) incremental improvement.

That's because, even at 400Wh/kg, batteries still don't have an energy density anywhere close to fossil fuels.

Gasoline offers somewhere around 12,000 Wh/kg, 30x the energy density of the Envia battery technology. A Nissan Leaf with the same mass of gasoline-equivalent superbatteries would have a range of around 9,000 miles. Alternatively, to get the same 300 mile range as with the Envia batteries, the Nissan SuperLeaf would only need around 3kg of batteries.

I'm not discounting the importance of this breakthrough, not by any means, but it's important to keep this in context. There's a good reason why petroleum has such a hold on the world of transportation, and it's going to take a lot more than a tripling of battery energy density to beat it. Or, more to the point, moving beyond the gasoline automobile is going to take more than simply chipping away at energy density comparisons -- it's going to take a complete re-thinking of what we mean by transportation.

As has been pointed out to me, in comments and in direct communication (and with varying degrees of politeness), this isn't an entirely fair comparison. It would be more accurate to compare the combination of energy density + drive efficiency.

Most standard automobiles have an average internal combustion engine efficiency of around 20% -- that is, of the energy available in the fuel, about 20% is eventually translated into motive force. So that 12,000 Wh/kg is effectively "only" 2,400 Wh/kg.

Electric motors, conversely, are extremely efficient at translating available energy into motive force; at 90%, that 400 Wh/kg Envia battery is still effectively 360 Wh/kg.

So a gasoline engine system 6.67x better than the Envia, not 30x better. The difference isn't as gobsmacking, but it's still significant, and remains a reminder of just how far battery technology has yet to evolve.


The comparison with gasoline energy density completely ignores the efficiency of ICE vs. electrical motors. The first is below 20%, the second is over 90%, so you can divide the factor 30 stated in the article by 5. Since we are comparing the range at equal total weight, we should include the fact that the IC engine is likely far heavier than the electrical motors. Additionally there is a heavy exhaust system including particle filters and catalyzer.
It would be interesting to make a proper comparison, but this article utterly fails at doing so.

It would be most useful to compare apples to apples here. 12,000 Wh/kg for gasoline represents the raw chemical energy. How about the *delivered* energy? IC engines are only about 20% efficient. NEMA Design B Electrical Motors over 125 hp are required to have minimum nominal efficiency of 92.4%

At only 20% efficiency, gasoline has a delivered energy density of only 2,400 Wh/kg. Okay, this is still a factor of six over these batteries, but much closer than you made it appear.

Building on Greg and Kristian's comments your estimation that the battery has ~3% of the energy density of gasoline is too low for the reasons they state. I would think the battery has ~15-20% of the effective energy density of gasoline.

For electic bikes and scooters this new battery doubles the range for the same cost while weighing a third less.

I agree that this new battery by itself is not going to transform the transportation system. Robot cars and trucks are the way to transform the transportation system.

Thanks for doing the math for me. I was about to go do it myself.

Ignoring the weight of the rest of the system still leaves the numbers almost useless.

The weight of the engine and transmission should be substantially greater than the electric motors (and optional transmission).

To be fair, that makes the equation non-linear, but when we're talking about 300-600 miles of range it makes sense.

I just spent 15 minutes googling and I can't find anything that looks half reasonable to use as starting point to estimate this (nor anyone who's just calculated it) but I'm pretty sure that after everything there should be another divide by 2 or 3.

With optimistic estimates, that means the effective energy density of an EV with Envia batteries would be a bit less than 1/2 that of a conventional ICE. Which is actually incredibly impressive.

But when you add in insane cost of the batteries and risks associated with lifetime cycle dammage (Tesla bricks) I don't see how the EV's will ever compete with ICEs for cost vs effectiveness.

I do really want to be wrong though....

It seems like the only overall efficiency contribution would be the reduced weight allowed by an Envia-equipped electric vehicle. (Most of a car's energy is used to move the car, not the people/things in it.)

That is a more difficult calculation than what's done here, which seems to have nothing directly to do with efficiency in terms of $s per mile, or in terms of wellhead-to-walmart energy usage.

That would be the interesting calculation -- will it make buying an Envia electric car an attractive purchase compared to and ICE car?

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