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.