We have written in the past about tips that will help your expensive lithium battery pack last as long as possible. This article will cover some additional information about what happens inside the chemistry of lithium-based batteries, like…why they die when they do, and how to avoid doing the wrong things as much as possible.
Jeff Dahn, and why you should know who he is
In our previous articles, we laid out some handy tips, like…keep the battery cool by charging at a lower rate, and also don’t charge to 100% all the time. However, we didn’t go into much of the details as to why. Up until now, most of it was because we read postings from people we respect, and heard about the “real world” experiences from riders who log a lot of miles on the their packs. Now? we found some info as to why the major players in the industry do the things listed below.
I stumbled across a video lecture by professor Jeff Dahn from the Dalhousie University in Nova Scotia, Canada. You may not have heard of him yet, but you probably heard about the electric car company Tesla. The Tesla car company has enjoyed a rate of success over the last few years that defies logic. Rather than introduce a light economy EV car, they first brought out a wildly successful sports car with an 8-year battery warranty. Then they produced a four-seater (also with high performance), and soon…they will be selling their least expensive model, which already has a back-log of orders.
Their battery has high-performance AND an 8-year warranty, so…how’d they do that? and…whats the connection between Professor Dahn and Tesla?
Dalhousie is the only research University in the world that has a contractual research relationship with Tesla’s battery department, and…Tesla could have partnered with any University they wanted. Professor Dahn can’t really talk about the secret sauce he’s working on right now, but…we can examine the video lecture he gave in 2013, which is about “How do Li-Ion batteries die, and how to improve the situation”…which might help us understand why his top student (Aaron Smith) is now the head of Tesla’s battery longevity program.
The video lecture
This lecture is an hour and 13 minutes, and I was so impressed by it, I transcribed the entire thing into text, just to make it easier to write this article. Of course the text doesn’t show the charts that were projected on the screen for the audience, so I also added the paragraph location times, so readers can go right to any part of the lecture that they might be curious about.
If you want to actually watch the entire hour and 13 minute lecture, click on the center of the pic just below.
If you want to skip the techno-babble, scroll down to the last few paragraphs of this article, and I will post the simple “plain English” tips there.
Negative electrode (anode) plating
Prof. Dahn starts out by mentioning that research has shown that the chemical reactions in the cells during charge and discharge result in a build-up of byproducts on the negative electrode (sometimes called “Anode”). One surprise was that…after a brutal cycling regime to purposefully make lithium cells go bad in the common ways that they die, the positive electrode (sometimes called the “Cathode”) appeared to be as fresh as a new cells’ cathode (when using the common additives, see below). This provided a more specific target to research…the “plating” on the anode. Why does it happen? and possible methods to reduce it as much as is possible.
[9:13] So first thing we gotta do is understand how a lithium-ion battery works. OK, so here’s a picture, showing the positive electrode of a lithium-transition metal-oxide on an aluminum current collector. And the graphite negative electrode on a copper current collector. These two electrodes are separated by an electrolyte, that contains dissolved lithium ions. And each of these electrode materials is layered, and they’re each intercalation compounds, and that means lithium atoms can reside between the layers, and they can be de-intercalated and intercalated when the batteries charge and discharge. And what’s really important to recognize is the intercalation and de-intercalation process is incredibly benign, it causes a structural change of about 3% volume change in the positive, and about 10% in the negative. And, there’s no structural degradation that takes place in these materials, at all. The failure of the lithium-ion battery, really has very little to do with structural degradation of the electrode materials, during the charge / discharge cycling.
[10:34] When the lithium-ion battery is assembled, the negative electrode is graphite, the kind that would be in your pencil. And the positive electrode is a lithium transition metal oxide that’s synthesized in the air at high temperature, it’s stable in air. So both electrode materials are stable in air, you can build a battery in the open air. As soon as you put it together and start to charge the battery, you force electrons in this sense to the right [of the graphic], and the corresponding lithium ion hops out into the electrolyte, and moves to the graphite where it gets intercalated, that charges the lithium-ion cell.
[11:17] And once the lithium-ion cell is charged, NOW…the lithiated graphite, graphite with lithium inside? is very reactive. Will react with like, lithium-metal. And the lithium-transition metal oxide with missing lithium is also very reactive. And what happens is, both of those electrodes actually react with the electrolyte solution, that they’re in contact with. And you would say, well…then, how do you make a battery that has any life-time at all if the electrolyte reacts with the electrodes? But, by luck and by chance, when the reaction occurs, the reaction products turn out to be solid on the negative electrode, and they form a passivated film that slows down and limits for the reaction. And on the positive electrode, a similar thing happens. So by luck, these reactions don’t destroy the battery. In fact they form passivating surfaces that allow the thing to operate for many many many months.
[41:53] So how do you design a cell, to make it do this?…Well if the reason that the cells show this dramatic roll over, or catastrophic failure, is because electrolyte oxidation products migrate to the negative where they’re reduced, and eventually shut down the negative electrode. What would happen if you really highly compact the graphite particles in the negative electrode…OK, so here, it’s kind of a cartoon, this is a negative electrode it’s made of graphite particles, it’s been highly compacted so the porosity is pretty small…You’re cycling the cell, you’re getting electrolyte oxidation products that come across, and they get over there, they see a low potential surface at the front, of the electrode, and they get reduced in form, some solidified junk there. And the cell capacity is not going down very much. And you cycle some more…and the layer of junk gets thicker and the pore openings start to get closed off. And you cycle some more, and the pores ultimately become filled. And now it’s very hard for the lithium-ions to penetrate into the back of the electrode because it’s blocked. And then, lithium plating begins on the surface, and the capacity dies.
[45:30] And if you look at the positive electrode by SEM, I just picked one of the cells with [the additives] VC, VEC, and FEC, after 420 cycles it failed. Before the cycling, and after the cycling, the positive electrode looks exactly the same…and if you look at the negative electrode before, and after, you can see the build-up of this film of reaction products on the surface of the negative, so this square region has been expanded over here, and you see all this gunk on the surface of the negative, and that’s what’s leading to the, the failure of the cell…
Temperature…HOT is BAD
The first paragraph below shows that the bad chemical reactions are worse at higher temperatures, and the second paragraph details how a cell implanted in the human body (for a medical device) can last a very long time, which is partially explained by the human flesh around it acting as a heat-sponge to stabilize temps.
[20:02] So these reactions between the electrode materials and the electrolyte, they’re bad. OK, It’s bad, and temperature aggravates those reactions…So, just to remind you…these parasitic reactions, that are going on in the cell, they’re bad, and by measuring the coulombic efficiency of the cell, you can quantify the amount of parasitic reactions that are going on in the cell.
[34:00] Here’s some data from Medtronic…for cells that are implanted in the human body, to run a pain…a pain relieval system. And, they have cells in the lab, that are eight years of testing accumulated at 37 degrees C [98.6 degrees F]. So, here you can see six to eight cycles a day, twenty thousand cycles, eight years of testing at 37 degrees C [98.6 F]. That’s pretty impressive. Cells like this, with nickel cobalt aluminum [NCA] are in the Tesla Motors vehicle, OK? So Tesla Motors uses technology that’s at least this good.
Amount of TIME when hot is BAD
[24:24] And if you…instead, plot the coulombic IN-efficiency, just one minus the CE, so just flip the data over, and then take a look carefully, you’ll notice that…the coulombic in-efficiencies scale one to two to four…just like the cycle times do. And if you divide…one minus the coulombic efficiency by the time of a cycle, all the data falls on a universal curve. OK, so it’s telling you that, time of exposure, is really the bad actor here, in the failure of these cells at elevated temperature. And now these measurements can be used to rank all lithium-ion technologies.
Amount of TIME at HIGHER VOLTS is BAD
[35:51] Now here comes the ultimate challenge for this method. And that is that sometimes…lithium-ion cells show this kind of failure that’s incredibly insidious here. So these are nickel manganese cobalt positive electrodes with graphite negatives. Cells are cycling to an upper cutoff of 4.25 volts, and they look really really good. And if you change the upper cutoff voltage to 4.35 volts, they start out looking really really good. Imagine this was in your car, you’d say oh everythings great, I love it, I love it, then…all of a sudden you can’t even get out of the driveway.
[36:35] There is no way for a lithium-ion battery manufacturer to learn about when they get this rapid catastrophic failure, except to cycle it, until they get there. And if this happens after three years or five years…you gotta, ya gotta go there to find it…We believe that this roll over, or catastrophic failure comes about, because of electrolyte oxidation at the positive side. There’s no capacity fade significantly here at all. So lithium is not getting consumed in the SEI at the negative. By contrast what’s happening in it is the electrolytes are getting oxidized. Oxidation products moved to the negative and they coat across the surface of the negative, and eventually they shut the cell down…Now, if you charge to higher and higher voltage, you accelerate electrolyte oxidation, and this catastrophic failure moves to lower and lower cycle numbers.
[55:07] And I’ve added a data-set for half-percent VC now as well, OK, so at low voltage 3.9 volts, all the four cells are about the same. Then there’s a big entropy change due to an order / disorder transition in lithium cobalt oxide, but after that, between 4.1 and 4.2 volts, look…here’s no vinylene carbonate, half percent less heat, two and four percent, less heat again, and the difference gets greater with voltage. So the vinylene carbonate is suppressing parasitic heat due to electrolyte oxidation, we saw that early on from our charge slippage measurements, and you can see that at voltages above 4.1 volts or so
[1:03:07] Um, lithium titanate is amazing [Well, that’s probably because it’s] I’m just, it’s because the voltage is high, I think I might have, I don’t have a lithium titanate slide here…So, sort of the best coulombic efficiency numbers we get for C over 20 cycling at 40 degrees C for graphite negatives in lithium-ion cells is about 999…2? Lithium titanate in a lithium-ion cell, we would get 99998…Amazing. Lithium cobalt oxide lithium titanate cells with going only to 4.1 volts, amazing. They’re gonna last, you implant them [in a medical device] in Linda they’re gonna last till she’s dead…no doubt
[1:07:05] The biggest, the biggest issue is the time spent at highest voltage. The longer you spend at the highest voltage the worse it is…right? Well you could see from the calorimeter experiment, things got worse, if you went up above 4.1 volts. So if you go to 4.2, it’s worse than 4.1, if you go to 4.1, it’s worse than four. So the GM Volt for example, it charges to eighty percent, just 4.03 for that cell, which is decent. Not too much parasitic reactions going on there…OK? But if it charged to 100%?…would be worse, OK?
“Secret Sauce ” additives
[28:50] Typical lithium-ion cell might have five additives in it, for various purposes…So here, I’ll just show you the impact of vinylene carbonate, so here are lithium cobalt oxide graphite lithium-ion cells. They’re being cycled with the high-precision charging equipment. So as you charge and discharge, you can see the voltage capacity curve, again, shifting to the right. And this is caused by parasitic reactions taking place, causing electrolyte oxidation at the positive electrode, things slipping to the right. So a cell measured at 40 degrees C, but if you add vinylene carbonate to the electrolyte, bang…just stops, OK? So you really impact that parasitic reaction a lot with 2% by weight, of an electrolyte additive. And the same thing happens at 60 degrees C, there’s a huge, huge improvement in the…in the rate of electrolyte oxidation at the positive electrode side.
[46:09] OK…so…what about their [other] additives? Where do they fall on the graph? I showed you the additives that we specified, now I’ve changed the scale because, the battery manufacturers are a lot smarter than university professors…and that’s where theirs fall…So, you would take a look at some of these things. Their coulombic efficiencies are similar to these guys, but BANG, the life cycle is way better…This 4UA, and 5UA [anonymous composition labels], differ only by the addition of one additive, which is additive number five. This has 1, 2, 3, 4, this has 1, 2, 3, 4, 5. Look at that, incredible. How does it work?…From 5UA to the control with no additives, there’s a 20-fold increase in cycle life. Just with a few percent of a few magic ingredients. That’s amazing…How does it work? Why are these points off our line, while our model assumed that any oxidation products go over to the negative, and form solid products that block the surface of the negative. Maybe when these guys go over, they don’t form solid reaction products.
Long term storage, use low volts at low temps
[1:07:05] The biggest, the biggest issue is the time spent at highest voltage. The longer you spend at the highest voltage the worse it is…right? Well you could see from the calorimeter experiment, things got worse, if you went up above 4.1 volts. So if you go to 4.2, it’s worse than 4.1, if you go to 4.1, it’s worse than four. So the GM Volt for example, it charges to eighty percent, just 4.03 for that cell, which is decent. Not too much parasitic reactions going on there…OK? But if it charged to 100%?…would be worse, OK? So all cycles are not created equal, to answer your question. The more time you spend at higher voltage, the worse. So, you know, I have cells…from 1999, that were stored at about 20% state of charge so maybe 3.5 volts for those cells, put them on in 2013…like new. Because the positive electrode side is not doing anything bad, at low voltage, but, if they had been stored at high voltage, it wouldn’t have been nearly as good.
[1:12:03] [Q: How do you extend the life of your battery in your phone or computer] OK [Q: do you charge to 80% and don’t discharge completely, or what do you do?] Keep it as cool as possible at all times, put it in the fridge at night…Then it won’t bother you while you’re sleeping, too…added advantage. No, but I’m serious. If you keep any battery as cold as possible, it will last longer. So any, any lithium-ion cell, keep it in the fridge when you’re not using it, it will last longer. If you, if you don’t charge 100%, that will help, but you know, temperature is a bad actor.
For long-term storage (over winter?)
Drain your battery pack to 3.5V per cell, and store it in a cool area. Not necessarily a refrigerator (although that would actually be OK), but at the minimum…someplace that is NOT warm. You might not want to put it in a freezer, because it would have very bad voltage sag until the battery warms to room temperature. Also, definitely disconnect it from the controller, and also disconnect it from the charger.
Whether charging, or during a ride…avoid as much heat as possible in the battery. If you are drawing max amps often (in order to have max performance from a small and light battery?)…it may “work”, but…you will NOT be getting the maximum possible life-cycle from that pack, due to the resulting heat.
There are two ways I know of that you can reduce heat in a battery pack system design. First, use a larger battery pack than you need. You might only “need” a very short amount of range on your particular commute, but a bigger pack will run cooler, since each cell is less stressed at a lower amp-draw per cell.
The next way…is to specify a cell that has a higher amp-rating than you need. If you have a small battery pack that can provide your max amps needed (and it is also the biggest pack that will fit on your frame), but…it’s getting hot? You can buy a pack of the same physical size, but…with a higher amp-rated cell, and it would run cooler.
The Tesla cars and the Chevy Volt both have an on-board battery pack cooling system to help stabilize pack temps at a reasonable level.
Avoid Heat (Part 2)
There are some days when you want to recharge your battery as fast as possible. For example, the Luna Advanced charger has the ability to charge at 5A, or…also at 3A. If you charge at 5A, the battery will charge faster, but…if you charge at the lower 3A rate, it will take longer, however…the 3A charging rate will also leave the battery pack less warm, and…the cooler the battery is, the better. The Grin Tech Satiator also has the ability to adjust the charging amp-rate.
Don’t charge to 100% (4.20V per cell)
The amount of extra charge you get from charging to 4.2V per cell, instead of 4.1V? it is very small, and has very little effect on your range. You may think that charging to 100% will give you the maximum range possible, but it trades a significant amount of pack life for a very small amount of extra range. The Luna Advanced charger, and also the Grin Tech Satiator both allow users to charge their packs to 80% (4.05V per cell), 90% (4.1V), and also 100% (4.2V).
In the graph above, there are 17 graph squares (of range) between 4.2V and the cutoff at 3.3V. You only have to give up 1/17th of the charge to gain double or triple the battery pack life in months. I have not charged any of my ebike battery packs to 4.2V since I found this out.
Also, If you ride only on the weekends, it would be more ideal to leave your pack half-empty during the week, and only charge it an hour or two before the ride. However, I know this is not practical. If you feel you need to charge the battery pack immediately after a ride, charge it to only 80%, and then let it cool off before you ride, or before you charge it more. Then, just before you want to ride, charge it up to 90% to get a little extra range.
Of course, if your battery pack is large enough? you never NEED to charge it to more than 80%. Plus, if your ebike commute is short and you also have a large battery pack…don’t feel as if you need to charge the pack every day. If you can take several rides throughout the week, but only need to actually charge once a week? This would mean your pack is spending less time at the highest voltage [lead-acid batteries need to be topped off completely, and as often as possible…lithium does not]
For long lithium battery life? big packs rule…and try to keep ’em cool.
Dis-assembly of a Tesla 18650 Cell from Panasonic
Here’s a one-minute video that I found very interesting.
Written by Ron/spinningmagnets, March 2017