22Sep 2015

Pause for a second and wonder why electric vehicles have frustratingly limited driving ranges? or why your smartphone lasts only for a few limited hours instead of an entire month? Yet, a good ol’ combustion-engine car can go for hundreds of miles without a problem. This is the manifestation of energy density. Let’s talk about it in more detail and hopefully give the reader a bit of more intuitive sense on the importance of this metric.

Energy density, as the title implies, is a measure of how much energy is stored in a certain volume. A battery or a gas tank has a certain limited volume, therefore it is important to have a metric that relates to how much energy can be stored in that volume. Obviously, energy is what powers our smartphones or vehicles, therefore energy density is a metric that describes how far one can drive  or use a device given the limited amount of energy stored in the “tank.”

The following table compares a select number of energy-storing materials or mechanisms, all the way from the traditional lead-acid battery (the type that you will find under your hood) to much more sophisticated energy sources such as nuclear fission. So what is this table telling us?

EnergyDensity3

The first nine rows are all batteries, or devices that can store electrical energy. Batteries are either primary (i.e., non-rechargeable) or secondary (fancy term for rechargeable). The last three rows are widely used energy sources in our society today and are used here for comparison purposes: Ethanol and gasoline are examples of carbon-based fuels, and the last row, well, we all know what nuclear power can do, both the good and the evil.

The first observation: Even the best battery, the absolute best, has 10X lower energy density that carbon-based fuels. That means the same tank (or equivalently sized battery) will let you drive 10X more miles using carbon-based fuels. I cheated a little here — electric systems are more efficient than carbon-fuel systems, so the difference is more like 3X rather than 10X, but that will be left to another discussion.

The second observation: The difference between the best battery and worst battery (in terms of energy density) is substantial, also about a factor of 10X. Lead-acid batteries, discovered over a 150 years ago, don’t provide a lot of energy density. NiCd batteries also leave a lot to desire. Do you remember the bulky batteries in the early cell phones back in the 1990s? Or just google the GM EV1, the first electric vehicle from GM that used lead acid batteries.

But…there is always a but: While NiCd have for the most part disappeared, lead acids are incredibly inexpensive, and they survive. Until the day comes when the price point of alternative batteries drops radically, lead acids will continue to be the king of batteries in applications where energy density is not critical — i.e., where it is ok to occupy a larger volume, for example backup systems for cell phone towers.

The third observation: Energy density increased by a factor of 10X over 150 years! That’s not terribly promising unless the future brings forth some serious breakthroughs in materials. Is there anything on the horizon? There is a lot of promising good material research, but when one takes into account cost, cycle life, and other constraints such as manufacturing and capital, it is very hard to point to one particular technology that is likely to be commercialized in the next 5 years. So the wait and the hope continue.

The fourth observation: Lithium-ion technologies, first commercialized by Sony in 1991, encompass a wide range of energy densities depending on the particular choice of material for the electrodes. Lithium-ion batteries using nickel-cobalt-aluminum oxide (NCA) electrodes — the type used in the Tesla Model S — have over 3X the energy density of lithium-ion batteries with lithium iron-phosphate  (LFP) electrodes. So why is anyone considering LFP lithium-ion batteries: Cycle life! Welcome to the world of compromises.

So by now, you are probably disappointed about the future of batteries! It is true that the progress of batteries over the last 150 years has been slow, and it is true that batteries can’t yet compete with carbon-based fuels…but that does not mean that the incremental progress in batteries is insufficient to meet many needs of our society. Yes, they can be better, but present batteries boasting 700 Wh/l can and are sufficient to provide an electric vehicle with a range of 300 miles. In other words, don’t expect miracles in batteries, but do expect that incremental technologies from materials to algorithms and electronics will be sufficient to address a wide range of energy storage needs, including smartphones that can last an honest day to electric vehicles with a range of 200 – 300 miles.

There is plenty to look forward to here, just be careful about wild claims of amazing discoveries. If there are too good to be true, then there is a probably a good reason to be skeptical.