Tuesday, March 23, 2010

What can nano do for batteries?

First and foremost, I must express my gratitude to Venkat for his introduction and for encouraging me to participate in this new blog-adventure. Now that he cannot hear us, I will tell you that I learned quite a few things myself with his posts. It is a pleasure for me to offer a different perspective to his and I hope you will find it informative. Since he has already introduced me, I’ll move on to the fun stuff.

Nanotechnology is certainly one of the fields of research that has witnessed greatest progress in the last decade (although the concept itself is not all that new; Michael Faraday can be considered one of the first nanoscientists!). When it comes to functional materials, among other things, nanoscience offers the promise of enhanced catalytic properties, controllable band gaps that can have an impact on the efficiency of a solar cell and novel medical diagnostics and treatment tools (not to mention that most of suntan lotions contain nanoparticles nowadays!).

It is easy to wonder whether nanotechnology would also be a helpful tool toward better batteries. There are a few arguments in favor of using nanoparticles in a battery, particularly as part of the electrodes. We will just concentrate on a couple of them for now (and leave the rest for another day if I am still considered a guest after today!). Although the discussion relies on the Li-ion technology for examples, the ideas are pretty much applicable to many battery technologies.

One of the first incentives one can think of is the shorter distances for ions and electrons to travel through. Broadly speaking, think of it as you having to cross your living room as opposed to a football field. The second one has to with the surface area. Let’s imagine that the square in the picture below is made of 1 um edges:

divide each edge ten times and you now have 100 squares with edges of 100 nm:

All those new lines you see are new surfaces (if you want to entertain yourself, expand the exercise to a 3D cube and reduce it to particles of 10 nm). In a battery, all of this surface could ideally be exposed to the electrolyte, which is the media through which ions transfer from electrode to electrode (the key to battery operation).

Now we have (lithium) ions that can access the active material much more extensively (more surface) and have shorter distances to travel through the solid (remember the living room). Diffusion in the solid is typically, but not always, slower than in the liquid, so, in theory, the result is a lower resistance to ion transport and, therefore, better utilization of the material (i.e., higher stored charge/energy) and, potentially, higher rates of charge and discharge (i.e., shorter charging times and higher power).

There are many reports available that show that samples composed of nanoparticles, all things equal, can lead to better performance than (what we call) bulk counterparts. In materials with poor ion conduction, this effect has had an important impact. The now world famous lithium iron phosphate owes some of its glamour to the chemists that were able to nanostructure it. The same happens with an anode material that is getting a lot of exposure as of late: lithium titanate.

So in a sense, nanotechnology has an important role in the field of (Li-ion) batteries, and, in fact, there are many R&D projects that concentrate on exploiting the advantages of nanotechnology to developing advanced materials, including some in our very own Batteries for Advanced Transportation Technologies program (go through our recent scientific reports to see what's up) at the Department of Energy. So, it is settled! Nanotechnology is the way to go to make batteries with five times more energy density!

Hmmm... not so quick... Apart from the fact that geometrically increasing the energy density is much trickier than it seems (we’ll also leave this for another day), there is a key subtlety in the case of lithium iron phosphate and lithium titanate that makes the whole trick work: they are both active within the voltage window of stability of the components of the liquid electrolyte. Venkat has already told you that bad things happen when we fall outside this window. Unfortunately, one of the ways of increasing the energy density is precisely by increasing the voltage (remember, energy = voltage x charge). Graphite electrodes, for instance, ubiquitous as they may be, react outside that window.

If we start using materials that are only slightly outside the thermodynamic window of the electrolyte, the undesired reactions may happen at a rate that is slow enough for us to live with. But, hey, remember that increased surface area of nanoparticles? Yeah, it is increased for every component of the electrolyte, not just the ions. The immediate consequence of reducing the particle size of our electrode materials is that side reactions tend to be exacerbated. And those side reactions produce insoluble products that deposit, just our luck, precisely on the surface of the particles, covering them and producing layers that are resistive to the diffusion of ions. Automatically, the advantage of using nanoparticles is lost. And things can be so bad that we may be better off using slightly less active electrodes!

In conclusion, you can choose to have a lithium iron phosphate/lithium titanate battery that operates extremely well and has a long life thanks to the use of nanoparticles... but what if I told that such battery only has a voltage output of around 2 V (yeah, those lead-acid batteries don't look so bad anymore)? And if I mentioned that, in addition, the lithium titanate has about half the storage capacity of graphite, so the energy density of this great battery is lower than that in your cellphone Li-ion cells? If we are to increase the energy density of current batteries to make them more application friendly, we have to come up with inventive ways of using materials operating at high and low voltages; preferably, not in the form of nanoparticles… unless we find a way of stopping those annoying side reactions without killing the ion transport at the same time.

There are other issues that make nano only a partial answer (sorry, no Moore's law for batteries) to the performance barriers of batteries, among which are higher associated manufacturing or processing costs and lower packing densities. But this is for another post (or two!). If you are left wanting more, you can get even more knitty-gritty details by reading some scientific literature (if you have access to it, of course), which will also offer additional shades of grey to the arguments developed here:

Aricò et al., Nanostructured materials for advanced energy conversion and storage devices, Nature Materials 4, 366 - 377 (2005)

Bruce et al., Nanomaterials for Rechargeable Lithium Batteries, Angewandte Chemie International Edition 47, 2930 – 2946 (2008)

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