A few years ago, when I visited a battery company with a few colleagues, I saw data from the company that was pretty impressive. Slide 1 showed the rate capability of the battery, which was better than the state-of-the-art; Slide 2 was a calculation of the energy density, which was better than the baseline; slide 3 was cycle life and this too looked impressive; slide 4 was…
You get the drill.
Suitably impressed, I came out wondering what I needed to polish in my resume to land a job at the company before they went public… when my travelling companion wondered aloud if each slide was from a different battery!
My colleague had a point. Most (not all) battery chemistries can be made to perform well for a particular metric. The trick is getting all the metrics to work out for the same battery.
Make the electrodes thin to get power; make them thick to increase energy (but at the loss of power). Increase the voltage to increase the energy, but at the loss of cycle life. The endless games one can play. This dependence of battery design, cycling conditions, voltage of operation, etc. on performance is the reason battery companies have been getting away with obfuscation.
But is it really obfuscation? A proof-of-concept can be a useful learning tool. If going to high voltages yields more energy but kills the cycle life, while we know that the voltages being accessed should be possible, then it gives us hope that the problem is not fundamental and that a solution exists. Given time, maybe we will find it.
Then again, we have been looking for a cure for baldness for ages. There is proof that baldness is not fundamental (after all, some are oh so lucky), but that does not mean we will find the answer to that problem either.
But there is merit in learning about what the best possible system is and understanding why it is the best. To this end, I thought I would list out the best-known (at least to me) battery chemistry for each of four metrics of importance: energy, charge time, life, and cost. Each would be a “Hero” battery; a term borrowed from other technology areas to denote a proof-on-concept that something amazing is possible for that metric. While each metric has a different “Hero” battery there are probably lessons we can learn from them. These lessons, I summarize in the end.
The energy-density Hero: All seven readers of my blog know that Li-ion batteries are the highest energy density secondary batteries one can buy, with energy density in the range of 250 Wh/kg. But the Hero is actually the Li-thionyl chloride battery, which has an energy density of 550 Wh/kg and is, more importantly, commercially available. The catch is that it is a primary battery (i.e., not rechargeable).
Typical Li-ion cells have a graphite anode. Moving from graphite to Li metal (which holds ten times the charge and results in a slightly higher voltage) bumps the energy by 50% without changing anything. On top of that, the thionly chloride cathode has a capacity of 450 mAh/g compared to 180 mAh/g for typical Li-ion cathodes. Combine the two and you get something much better than Li-ion.
Imagine a battery with 2x the energy density. Maybe we double the range of a Nissan Leaf, making it a car that is actually useful (joking, joking, all you Leaf lovers) for the same cost. Or cut the cost of the Tesla battery by half without sacrificing range, bringing it tantalizing closer to the point where everyone can continue to not be able to afford it.
Unfortunately, all these will not come to pass because the Li anode does not recharge gracefully while the thionly chloride cathode is not at all rechargeable. If you want to know why, you have to read my blog post titled “A Brief History of Batteries- Part 1” and “A Brief History of Batteries- Part 2”. Frankly, everything you need to know about batteries is probably hidden somewhere in these pages.
To make it rechargeable, you just need to change the anode, the cathode, and the electrolyte (i.e., all the three components that make a battery). When you do that, the battery now cycles a lot better, but at the loss of capacity.
So is 550 Wh/kg the ceiling? No. There is a lab demonstration of a 750 Wh/kg Li-air primary battery. This would be a record for energy density. But as I have alluded to in the past, a one-off lab demonstration does not a product make. So, for now, we shall call 550 Wh/kg the Hero for energy density. As an aside, the Li-air battery also does not cycle.
Reversibility restricts us to certain materials; a constraint when removed, allows high energy density batteries to be made.
An interesting question to ponder: Is it possible to get batteries that are rechargeable and at least as high in energy as these Hero’s?
The fast-charge Hero: One of my funnier blog post was titled “I’ll be back…in 8 hours”. That post had nothing to do with charging times (then again, this blog is like Seinfeld; its about nothing, as far as I can tell) but the title articulates the problem: We tend to want to charge our batteries very slowly.
Charge them too fast and we have unwanted side reactions (lithium plating and electrolyte breakdown) that can degrade the battery. 1h charge is doable, 30 mins makes it degrade a bit; 10 mins a lot more; 1 min would basically kill the battery (and kill you from the fire that is creates).
This is true for most batteries, but it is not fundamental. Meaning, there is no law that says that we cannot charge a battery fast. One can design the battery for fast charge. But the compromise is loss of energy.
How fast do we really need to charge? For an electric car, a really smart person (who shall not be named) told me that we needed to charge within the time frame of a restroom break. Another smart person told me that we needed the battery to charge and discharge fully within the time frame of clouds covering a solar panel. After a few experiments timing myself on restroom breaks and watching clouds move (I plan to watch grass grow next) we can approximate the charging time needs as 5 minutes. What can I say: I was having an Austin-Powers-just-got-out-of-hibernation moment.
The question of the grid actually handling this kind of electricity load is a whole other area of debate, but let us focus on the battery for a second.
Electrochemical capacitors can easily charge this fast. But they have no energy. Question is: Is there a battery chemistry that can mimic a capacitor’s charge rate?
There was one system that was kind-of-sort of commercialized that I would consider the Hero in this regard. This was the Toshiba Super Charge battery, which is rated to charge to 80% capacity in 6 minutes and more than 95% in 10 mins!
Here the anode (lithium titanate) operates at a higher voltage than the typically used graphite anode. This helps because the potential of the anode is far away from the lithium plating potential. This makes it much easier to charge fast and not worry about plating lithium, shorting, and the ensuing degradation and possible fires.
But the downside is that the higher anode voltage decreases the overall cell voltage, which in-turn deceases the energy. The battery has a third of the energy density of a typical Li-ion cell (so a third the driving range). At 90 Wh/kg and 177 Wh/l at the cell level, it is far, far lower than most Li-ion batteries. Cost of these devices scales with the energy: this battery will probably cost three times a typical Li-ion cell!
So… an ideal EV battery for the super-rich-with-overactive-bladder demographic?
Question is: can we get both high energy and fast charge?
If we want to get there I believe we cannot use thick porous electrodes. While they are great to spread the current, it seems impossible to get away from the electrolyte losses of these highly-resistive organic electrolytes. Which means that we cannot have thick bulky current collectors and separators. But then how do we collect the current (especially if the currents are high, which seems likely with fast charge batteries)?
Should we move to electrodes that are not porous? This limits the useable energy, unless we use electrodes that undergo deposition. One can continue to deposit the metal on top of itself and so “build” capacity, without the added losses from the porous structure. This may be an avenue.
My suspicion is that even if we find ways for the electrodes to accept the charge at that rate, the electrolytes in lithium-based batteries will not have the ability to move ions from one side to the other. Unless we move toward much thinner separators. This topic requires some careful thinking.
But let us revisit the question: how fast do we really need to charge?
If we can ensure we have a 400-mile range battery, this should translate to an approximately 5-6 hour driving time (at 70-80 mph). Then we may be willing to wait a half hour to charge the battery as we make a beeline to our favorite artery-clogging fast food joint.
For the grid, if we can use the battery to not just take care of the intermittency, but also to time-shift from peak to off-peak, we can size the battery for the time shift and use the (big) battery for handling the small 5-min intermittency. Each 5-min charge and discharge would only require the battery to swing by a few percent: easily possible with most systems.
The catch: both of these would require us to pay for the bigger battery! But hey, batteries are getting so cheap, and companies are going to give us money when they hand us the battery anyway.
The cycle/calendar-life Hero: I know what you are thinking: there is no such thing as a cycle-life Hero! After all, every battery we own seems to last all of 1-2 years before they crap out. You must be thinking that our Hero must be a 3-year life battery.
What if I told you there are batteries that last 20, even 30+ years and they cycle 20,000 times (no error there, really meant to have four zero’s)? And what if I told you that these batteries are not the hybrid car batteries that cycle 3-5% per cycle but are cycled deep, greater than 50% of the capacity per cycle? And that they are (at least they were) being used day in and day out?
Imagine batteries that last as long as a solar panel. Imagine being able to cycle them once a day and make them last those full 20 years with no maintenance.
Intrigued? Come back next week and you shall learn more.