Machine Design

Bigger bet on batteries

The current woes of the Boeing Dreamliner and its lithium-ion batteries have put battery technology back in the headlines, but much of the real progress in energy storage is taking place at research facilities and small startups pursuing ideas that are out of the mainstream. Among the most recent developments was the creation by the Dept. of Energy late in 2012 of the Joint Center for Energy Storage Research. The JCESR is supposed to be a research focal point aimed at making batteries five times more powerful than those available today, which are also five times cheaper -- and to do so within five years.

One of the projects likely to have an impact on this quest is work at Argonne National Labs which aims to  develop and scale up the production of advanced electrolytes for batteries. Researchers Gregory Krumdick, Krzysztof Pupek, and Trevor Dzwiniel. The problem, they say, is that the procedures used to make small, research samples of electrolytes don't work for large-scale production. Manufacturing processes for newly discovered advanced materials must be scalable to get new battery chemistries from basic research to commercial application. To that end, the researchers have already developed scalable manufacturing processes for five electrolyte materials. They say they've made kilograms-worth of their electrolytes so far and have sent them off to various entities for industrial evaluation.

Another effort likely to speed battery technology development is called the Materials Project. Based at the Massachusetts Institute of Technology, it aims to provide ways of speeding up the search for materials having specific properties.  Its founders say the average time to move new materials discoveries from lab to market is now about 18 years, largely because materials designers must spend a lot of time painstakingly tweaking new materials in the lab. But computational materials science is now powerful enough to predict many properties of materials before researchers ever synthesize them. Using supercomputing clusters, the Project has computed some properties of over 80,000 materials and screened 25,000 of these for Li-ion batteries. The computations have already predicted several new battery materials which Project creators say were made and tested in the lab and are now being patented. By computing properties of all known materials, the Materials Project aims to speed research by removing guesswork from materials design in a variety of applications.

Novel energy storage techniques are also getting a look from not just researchers but small startups. One in that category is the use of betavoltaic materials. Unlike ordinary batteries which store energy in chemical reactions, betavoltaics store energy in a beta-emitting isotope. Beta particles that the isotope emits interact with a semiconductor p-n junction to create electron-hole pairs that constitute electrical current in a mannor analogous to the way photons create charge carriers in solar cells. The advantage of this technique is longevity. Battery life depends on the half life of the source of beta particles which can extend to decades. And the process works over a super-wide -50 to 150°C range.

Interest in betavoltaics has risen recently because of advances that let less radioactive materials function as beta sources. Tritium, for example, emits weak beta particles that cannot penetrate a piece of paper or travel more than a few millimeters through air. Because elemental tritium is a gas, it usually finds its way into power sources in the form of a metal tritide deposited on a thin layer of semiconductors.

One of the most visible firms working on betavoltaics is a startup called City Labs Inc. located near Miami.  Researchers there say there are good prospects for building tritium-fueled betavoltaic power sources with efficiencies in the 10% range. The theoretical efficiency could someday hit about 70%, they say.

City Labs Inc. researchers compare the energy density and power density of betavoltaics to Lithium chemistry batteries this way.It looks as though tritium betavoltaics could someday take over some roles now filled by Li-ion cells. But they are likely to be niche players rather than a wholesale replacement for Li-ion. The reason is though betavoltaics have a higher energy density than Li-ion, their power density is much lower. That means in the case of the Dreamliner Li-ion batteries, for example, an equivalent betavoltaic would probably weigh too much to be useful.  Instead, City Labs researchers say betavoltaics make sense where power requirements are quite low, as with wireless sensor nodes and implanted medical devices. (Betavoltaics were onced cardiac pacemakers, but the radioisotopes used and shielding they required were too expensive compared to lithium batteries.)

Today, City Labs makes a betavoltaic cell called  NanoTritium which is said to provide a source of continuous nanoWatt-level  power for twenty years or more.

TAGS: Technologies
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