Finally, we know why the Note 7 exploded.

The answer, according to the New York Times:  “Samsung seems to have packed it with so much innovation it became uncontrollable”.  Apparently phone components are like 4-year olds; put 3-4 of them in a room and they are ok, but pack 10 of them in a small place they will destroy the room with all the energy feeding off of each other.  Of all the theories, this one takes the cake. 

As much as I think the community should not be speculating on the underlying case of the explosion, I think we can’t help ourselves.  For battery types, this is the most interesting event that happened since the-last-time-there-was-a-fire, so we are all salivating.  What can I say: we don’t get out much. 

For non-battery types, I can see their need to know if more phones are going to be exploding.  Only way to know is to understand what happened with the Note 7. 

I have been getting this question steadily the past few weeks, so I thought I would collect all the speculation in one single place.  I also provide my views on this.  So here goes:

1. The battery was overcharged (BMS failure, too high an upper cut off voltage, aliens, whatever) and hence the fire.  I think the eager ones among us, who do not believe in waiting for more information, speculatively stated this.  I think Samsung’s revelation that there was a manufacturing flaw negates this theory.

2. Samsung used a 6 um separator in the battery and this lead to defects when assembling with a thin separator. This in turn led to shorts and the explosions.  As I explain below, this may be part of the story.

3. The battery had a manufacturing defect where the anode and cathode did not line up correctly, leading to edge effects, lithium plating and shorting.  This seemed very likely until the New York Times article came out. 

While these three issues are obvious ones that most battery types would guess, the New York Times articles makes the point that after initially concluding that it was the battery, Samsung realized that it was not that simple.  The article claims that Samsung could not pinpoint the reason!  Hence the wacky statement about “uncontrolled innovation”.

Let us be clear: Samsung makes great batteries. ATL makes great batteries.  If this were an obvious issue, they would have caught it after the first few weeks.  There is something to be said about the argument that this is a more complicated problem.

Since the Times article came out, we have had three more, system level, theories that have popped up.

4. The battery was being fast charged due to a chip design flaw.  Faster than it was rated for. This lead to overheating, thermal runaway, ending with you-know-what.  I’m not so sure about this.  The Note 7 phones were exploding even when not fast charged so…

5. There was so much being packed in a small volume that the battery was getting squeezed and the edges pinched, unintentionally, leading to shorts.  This theory does seem possible, but I like the theory below (which is a variant) the most. 

6. Samsung used higher content of silicon in the graphite-silicon blended anode.  The silicon expands on charge and swells the pouch.  Because the pouch was unable to swell in the phone due to the lack of space, it shorted and exploded.  I really like this theory.  There have been problems with battery swelling even before silicon came to the scene, but this has only gotten worse with silicon-graphite anodes.  Many consumer electronics companies have been worried about this and have a spec. for how much the battery can expand.  According to Mashable, the Note 7 had a 750 Wh/l battery; which is PRETTY energy dense.  Much more so than previous generations of batteries, suggesting higher silicon content than before. 

It is possible that higher silicon content combined with a thinner separator and less space in the phone for volume expansion all came together to lead to shorts and fires. 

I’m sure I’ve missed a few other theories (aliens?), but I think I got the majority of the ones I have heard.  

Now that we have that out of the way, let us talk a bit about what this all means. 

I think the initial speculation that this was a battery-level issue appears too simplistic.  Clearly, there is more to the story.  Batteries all over are safe, have been safe, and will continue to be safe.  Assuming you know what you are doing.

But what if you have “uncontrolled innovation” happen again? What is a poor battery to do if the overall system does not want to treat it kindly?  Li-ion batteries are energy storage devices.  Meaning, if you release the energy very very fast it is not going to be pretty.  So, some TLC is in order. 

But even if the system screws up how it handles the battery, shouldn’t the storage device itself be made to withstand any abuse?  As we move toward wearable technology with things attached to every part of our body, we need to ensure that the battery remains robust even if there is a system-level failure.

There has been a narrative going around that Li-ion batteries today are similar to where crystalline silicon solar cells where a decade ago: meaning, the prices are dropping and one can get installers and system integrators to come in and start to make them ubiquitous.  The Note 7 incident shows the perils of this thinking. 

Batteries are not plug and play devices where Jane-solarinstaller is going to buy something off of Alibaba and dump it in your garage and get you city permit folks to sign off as if they are inspecting your plumbing.  We better be buying from someone who knows how to make them well.  And we better know how to design the system correctly, install them well, and control them through the life of the device.

But to get the world where we do treat our batteries like we treat our microwave (bang on it to try to get it working?), we need the batteries to be robust inside out.  This requires a separate blog post, hopefully, in the near future.  

In the meantime, with the Note 7 off the streets, time for the battery folks to crawl back to the cave and focus on achieving a few breakthroughs.  Until the next incident… 

Venkat

The Hero with four faces: Part 1

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). 

What gives?

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.

 Venkat

p.s. the title of this post is inspired by the Joseph Campbell book “The hero with a thousand faces.”  Campbell was trying to point out that all the world’s myths, across religions, shared a Hero figure with similar characteristics.  We will see next week that in batteries there is no one Hero.  We basically have four Hero’s with four faces.  

A shout-out to the separator

Anybody who has paid attention to batteries (especially, lithium batteries) and/or read this blog knows that in most batteries the anode and cathode materials are the main players for holding charge. There is a lot of research in trying to find new materials that hold more charge at high voltages. But as I have pointed out in my previous posts, we need a few other materials to ensure that we tap into this charge.

Things like current collectors, separators, and the electrolyte.

All these play as important a role as the electrode materials. In some cases, they are actually more important. I have decided to spend sometime giving them the credit they deserve.

I will start with a shout-out to the separator.

Most electrochemical systems (and, yes, batteries fall in the class of electrochemical systems) require some way to separate the anode and the cathode. One tries to keep these electrodes in very close proximity to decreases resistances for ions to travel between them while preventing shorts. An ideal way to achieve this is via the use of a separator. It’s a (arguably) simple physical barrier between the two electrodes that lets ions go through, but not electrons.

However, in some cases, the separator serves a larger purpose. For example, it also ensures that the anode and cathode reactants/products don’t mix. If you are trying to electrolyze water to make hydrogen and oxygen, it helps to not have them mix together (trust me). Separators help ensure that.

Be it a fuel cell, a flow battery or a containerized battery (like a lithium battery), a lot of effort is spent on the separator to make sure that it does its job. For the flow battery that we are planning to work on with a recent ARPA-E award, the separator will be an integral part of our developmental effort.

In the battery space, the separator has always had its part to play. In a lead-acid battery the absoptive glass mat (AGM) separator helps increase the life of the battery. In batteries that use zinc or lithium metal, the separator may help prevent dendritic shorts by retarding the growth of the dendrite. And in the Ni-MH battery it can help decrease the rate of self discharge.

Which bring me to the first news item that caught my eye.

After coming up with the magical iPhone and the magical iPad, Apple has recently unveiled the Magic Trackpad. Interestingly, Apple also announced that they were selling rechargeable batteries with a (magical?) charger for the trackpad (which operates on Bluetooth). Trust Apple to make a Ni-MH battery with a charger sound cool. Will the magic never stop?

In general, the Ni-MH battery is a terrible battery for Bluetooth applications. This battery has notoriously high self-discharge. A typical Ni-MH battery can discharge by as much as 20% of its capacity in 2 weeks in the SF Bay Area and 50% in balmy India, in summer. Bluetooth devices are used sparingly (hopefully your job does not require you to type 24 h a day), so the self-discharge can be a killer.

Apple, on the other hand, is promising 20% capacity loss in 1 year. Magical you think?

Not really. The answer, my friend, is (partly) a separator blowing in the wind.

There are three reasons why Ni-MH batteries self discharge. The first is oxygen evolution on the nickel electrode, the second is hydrogen evolution on the metal hydride electrode, and the third is a nitrate redox shuttle across the two electrodes. All three are forms of internal leaks that discharge the battery.

Mother Nature dictates the first two. It can be hard to beat Mother Nature, especially for mere mortals like me (and, yes… even Steve Jobs), but there are some things we can do to decrease the rate of these gas evolution reactions.

The third mechanism is what interests me in this post and it involves using a separator that traps the nitrates and prevents the ion from shuttling across. This prevents the battery from slowly leaking and keeps the battery charged. Very simple, yet very effective.

These developments in this mature chemistry are only 5 years old and have resulted in a significant decrease in the rate of self-discharge. Which takes me back to my post on how we tend to ignore older chemistries (read non-lithium) in most, if not all, R&D projects in this country.

So here is a shout-out to the humble separator.

Separators for lithium-ion batteries are more crucial in that they can be the difference between an iPhone that is plagued by dropped calls because of antenna issues and one that is burning your pant pocket.

Separators have a checkered history when it comes to lithium batteries. Remember that the volume occupied by this layer is excess space that is wasted. There have been many moves to try to decrease the thickness of the separator, but attempts at making this layer less than 20 microns result in the electrode shorting during a winding process that is part of battery assembly. Shorting a battery is typically not a good idea! Most separators today are 20-25 microns in thickness.

Moreover, these separators have what is called a “shut down” layer. This layer, made of a polymer that melts and shuts the pores if the temperature increases too much, is a mechanism by which reactions are stopped if a battery goes into thermal runaway (or “spontaneous disassembly”, as the industry calls it). However, there are some that say that when this melting occurs, the structural integrity of the separator decreases and the electrodes end up shorting with each other. Statements regarding shorting being a bad idea apply. This issue is still being played out.

As an aside, a couple of researchers at LBNL are doing something interesting with the separator. Tom Richardson and Guoying Chen incorporated a conducting polymer that prevents the battery from going to overcharge. Overcharge causes the thermal runaway in lithium batteries. The idea is to prevent the overcharge and hence make the battery safer.

But lets get back to the separator we use today.

You may remember that YouTube video’s of burning laptops. You may also remember that the cause was attributed to metal particles falling into the battery during assembly and leading to shorting. A way of dealing with this issue is to make a stronger separator; one that will prevent shorting even if particles fall into the battery. Some manufacturers are testing ceramic coatings on the (presently-used) polymer separators to see if this will increase the puncture resistance. This issue is also still being played out.

Obviously all these problems will go away if the electrodes were not kept so close to each other by using a thicker separator. But this decreases the energy density of the battery and decreases the power. Obviously, no one wants that!

Other than being crucial from a safety perspective, separators are also one of the culprits in making lithium batteries expensive.

Battery costs are impossible to find with any clarity (The US military can learn from battery companies on how to keep secrets and prevent incidents like the one with Wikileaks). But, estimates suggest that material costs can range from 50-80% of battery costs. And 25% of the material cost is the cost of the separator!

Think about this. This simple polymer layer, very similar to the polymer used to make grocery bags, can be as much as 20% of the cost of the battery! At the sake of repeating myself, batteries are expensive and we have to decrease the cost significantly to get any widespread penetration of EVs and PHEVs.

Part of the reason why separators are expensive is because of a process that creates the pores. And it appears that the market for separators does not have enough competition to drive down costs.

Which brings me to second news item.

Dupont just announced that they would be getting into the battery separator game by manufacturing their line of lithium battery separators. Information is scarce, but they appear to be using a different process than what their competitors use and promise higher power, and a higher operating temperature. No word on cost, but now there is one more player in this game bringing some competition. That can only be good.

Now if someone can come up with a way to make a really strong separator that is say, 5 microns thick, has a open path for ions, can withstand the winding process, does not puncture even when there are metal particles in the battery, and costs less than $1/square meter, then we should be all set.

For the uninitiated, the paragraph above is like hoping that Microsoft comes up with an operating system that does not crash all the time. It seems doable, but for some reason it never seems to happen!

Until then, let’s thank the separator that we do have.

Venkat

You say potato, I say...battery?

Saw something interesting in the news on a potato battery. Its a Zn-Cu battery with the two rods inserted into a potato. Why is this new? The authors say that they boiled the potato and were able to get less resistance and more power! I swear I'm not making this up.

Check out http://jrse.aip.org/jrsebh/v2/i3/p033103_s1 for the abstract. They say its cheaper than a AA battery and can be used to power a low-power LED light. I have not dug into the numbers to see if the cost claims are right. It would seem that with the low power that one gets from each cell you will need a lot of potatoes; the authors say they need 5.

My dad is visiting us from India and is sitting across from me. He tells me that 5 potatoes in India costs Rupees 15 (~30 cents). He tells me that a AA battery is either a bit cheaper or comparable! And I still need to buy some zinc and copper (and a pot to boil the potatoes, and spend some money heating it)

I get the impression that this is sort of a cute press release of something we already know and I wonder if it solves anything. I have to boil the potato and so need energy; I gain a means of making electricity, but I lose food to do that. And we really need someone from countries in the developing world to tell us if the cost numbers are what my dad tells me they are in India.

Anyway... could not resist.

Venkat