Its 6:00 p.m. on a Monday evening and the traffic seems horrendous. You decide to whip out your iPhone and log on to Google Maps to check out alternate routes when you see the dreaded warning that your battery is out of �juice.� You curse the guys who work on batteries for not doing their jobs better and wonder if batteries have actually improved in your lifetime. Sound familiar?
The problem of what many consider to be the poor improvement in battery performance is the bane of all professional battery folks�especially in the Bay Area, where (sadly) people know Moore's �law� better than they know Faraday's law (no prizes for guessing which one is a real physical law). So where are the innovative solutions that are the battery equivalents of iPods, iPhones, and [maybe] iPads?
First of, Moore�s law is an observation on the ability to pack more transistors into the same space at optimal cost. In devices where packing more means more performance, this translates to continuous improvement. While there is quite a bit of �packing things into a smaller footprint� that goes on in the battery space, fundamentally, this is not how batteries improve. Let me elaborate.
If you take your cell phone lithium-ion battery, it has a lithium cobalt oxide positive electrode and a graphite negative electrode. The theoretical energy density of this battery (i.e., the best you can hope to achieve) is ~360 watt-hours per kilogram (Wh/kg). The battery you buy is probably ~180 Wh/kg; only one half of the theoretical max.
Where does the other half go? It goes toward making an electrode with the lithium cobalt oxide and the graphite; in putting a current collector in the cell; in the electrolyte that is added to get the reactions to occur; and, in the packaging of these components in a container. Only the lithium cobalt oxide and graphite are �active��the rest is wasted space and weight.
Can we remove this unwanted weight and make things better? Sure, but it�s not easy. Battery companies have actually done quite a bit of this already. Since this chemistry was first commercialized in the early 90�s, the energy density of lithium-ion batteries has doubled. But when you start by being at a factor of four from the ultimate, it�s hard to make dramatic advances. In contrast, for integrated circuits, technology has moved from the 350 nanometer node in 1995 to the 32 nanometer node this year. Dramatic changes in the same time period, but then again, the initial size was far away from the ultimate limit (my cursory reading of the Web suggests that 7-9 nm may be the limit with silicon).
So how do we make a better battery if we cannot depend on packing them tighter? We change the positive and negative electrodes to something that has more energy. This is already happening; many devices are now sold with a different positive electrode, with theoretical energy of ~450 Wh/kg for these cells. And there are some amazing things going on at Berkeley via the Batteries for Advanced Transportation Technologies (BATT) program that will surely make things better in the very near future.
This fundamentally is the problem in the comparison of batteries to semiconductor: in the case of transistors, by using better lithography tools, the industry has made dramatic advances on one material, namely, silicon. Sooner or later, they are going to start butting up against a material problem. I�m told that this is already happening. When this occurs, the industry is going to start moving toward other materials (such as III-V semiconductors, graphene, carbon nanotubes and nanowires).
This will not be an easy change and would have required many years of research. In batteries, changes in materials are pretty much the only way to improve energy; engineering is getting harder and harder to do. To expect that a new material is going to show up every two years is a bit unrealistic considering that in the battery space we are optimizing not just on getting better energy density, but on achieving long life, excellent safety, reasonable power, and low cost.
And there is this tiny little problem that if we actually do succeed in, say, doubling the energy density every two years, we may be violating a few laws (as we know them today) in about eight years! Electrochemists have not yet recovered from the trauma of cold fusion; let�s wait for a bit before we trigger another one of those episodes, shall we? The ultimate energy that one can expect from a battery is an interesting concept. I will address this in the very near future.
So what can we do to make things better? Others have observed that, unlike the semiconductor industry, there is little collaboration among the various battery industries and the same sort of ecosystem does not exist for the adoption of new technology. The Department of Energy national labs could be the key to establishing a new ecosystem where companies can participate on a pre-competitive basis.
In the meantime, every time your battery fails, stop blaming your friendly neighborhood electrochemist, and think back to a few years ago when you could not get on the Web with your mobile device. Now keep repeating to yourself that that was a better time, and before you know it, you will be home.
Venkat
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