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posted by Fnord666 on Wednesday May 03 2017, @09:08AM   Printer-friendly
from the more-flammable-metals dept.

Arndt Remhof's team has developed a solid electrolyte that facilitates good mobility of sodium ions at 20 degrees. This last point is crucial: ions require a source of heat in order to move, and inducing a reaction at room temperature poses a technical challenge. The electrolyte is also non-flammable and is chemically stable up to 300 degrees, which addresses the various safety concerns associated with lithium-ion batteries. Hans Hagemann's team at the University of Geneva has been working in parallel to develop cheaper technology for the production of this new solid electrolyte.

Unlike lithium, there are huge reserves of sodium: it's one of the two components of table salt. "Availability is our key argument", says Léo Duchêne of Empa and first author of the research paper. "However, it stores less energy than the equivalent mass of lithium and thus could prove to be a good solution if the size of the battery isn't a factor for its application."

Magnesium: the perfect but complex material

The same team has also developed a solid magnesium-based electrolyte. Until now, very little research had been done in this field. The fact that it is much more difficult to set this element in motion doesn't mean that it is any less attractive: it's available in abundance, it's light, and there's no risk of it exploding. But more importantly, a magnesium ion has two positive charges, whereas lithium only has one. Essentially, this means that it stores almost twice as much energy in the same volume.

Some experimental electrolytes have already been used to stimulate magnesium ions to move, but at temperatures in excess of 400 degrees. The electrolytes used by the Swiss scientists have already recorded similar conductivities at 70 degrees. "This is pioneering research and a proof of concept," says Elsa Roedern of Empa, who led the experiments. "We are still a long way from having a complete and functional prototype, but we have taken the first important step towards achieving our goal."

The energy density of a magnesium electrolyte would solve the EV range problem, if it is double lithium's.


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  • (Score: 4, Interesting) by VLM on Wednesday May 03 2017, @12:57PM (5 children)

    by VLM (445) Subscriber Badge on Wednesday May 03 2017, @12:57PM (#503595)

    I wonder why all these battery technologies use such dangerous elements

    The optimist sees an atom with a loosely attached electron and says he can control that loosely attached electron quite easily and make a hell of a battery.

    The pessimist sees an atom with a loosely attached electron and says being loosely attached if that thing gets out of control its going to do crazy stuff like snap water molecules in half to bond with the oxygen making a hell of a hydrogen fire, etc.

    The above is a gross simplification.

    Nuclear fission is the same deal in a hand wavy sense. The stuff thats easy to split is quite unstable even making a large pure pile of it would be very dangerous.

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  • (Score: 2) by Immerman on Wednesday May 03 2017, @05:09PM (4 children)

    by Immerman (3985) on Wednesday May 03 2017, @05:09PM (#503770)

    I agree that fission has it's own hand-wavy double-edged sword, but that has little to with RTGs. Radioisotope Thermoelectric Generators generate energy by capturing the heat generated by nuclear decay, not fission. There is no splitting of atomic nuclei going on, no possibility of chain reactions, just unstable nuclei spontaneously ejecting particles as they decay to a more stable state.

    Of course you do still have a bunch of radioactive material in a sealed container, and it's likely to be an issue if it breaks open, but that's a completely different risk more closely related to managing nuclear waste than operating a reactor. In fact, I believe nuclear "waste" is a primary source of fuel for most RTGs, though it can be produced in other ways as well. Pretty much by definition any material radioactive enough to generate a useful amount of heat will have long since decayed to nothing over the several billion years since it was created. Which is why you can eat uranium ore without risk of radiation poisoning (though I don't recommend it - the heavy metal poisoning will still get you)

    • (Score: 2) by VLM on Wednesday May 03 2017, @06:46PM (3 children)

      by VLM (445) Subscriber Badge on Wednesday May 03 2017, @06:46PM (#503840)

      the heavy metal poisoning will

      Yeah don't forget bioavailability or biological half life. A little bottle of warm to the touch iodine that's permanently liquid sounds like a cool cabinet of curiosities item but thats just not cool.

      And every sword having double edges sure there's an isotope of Argon if I remember correctly with a delicious tasty couple hundred year life weak beta (electron) emitter. So a pure source of electrons and argon's biological activity is nil. Sound great other than the breakdown product is potassium which is going to be conductive thus shorting the blasted thing out from the inside AND your bananas (and innards) are going to glow in the dark because its delicious potassium. So both the source and all the byproducts have to be safe. Its harder than it looks for direct conversion!

      There's a plutonium isotope thats perfect for thermal RTGs other than the whole "kaboom" thing.

      • (Score: 2) by Immerman on Wednesday May 03 2017, @08:41PM (2 children)

        by Immerman (3985) on Wednesday May 03 2017, @08:41PM (#503929)

        Bio-availability is certainly a concern, though only for significantly radioactive isotopes, and we would be wise to select isotopes (and decay chains) that avoid it as much as possible.

        As for your argon complaints, they felt really off intuitively, so I did a little research starting here (https://en.wikipedia.org/wiki/Isotopes_of_argon) to knock a few points
        Firstly - Argon is a noble gas, so you'd have to have a really large or highly pressurized RTG to collect enough of it in one place to be useful. And a pressurized container filled with radioactive gas is probably a bad idea to begin with. But maybe you could stabilize it as a solid somehow - let's run with that.

        Secondly, something with a half-life of centuries (like Argon 49 at 269 years) probably isn't a good choice for an RTG in most applications outside of interstellar probes. The longer the half-life, the less radioactive it is, and the less energy you can extract from the decay of a given sample size. But hey, there's also Argon 42 with a half-life of only 32.9 years, that's going to be more usefully radioactive. (Most other known Ar isotopes have half-lives measured in milliseconds, with a few in seconds and one of several days - all too short for a useful RTG) Both undergo beta decay into a potassium atom of the same mass.

        Potassium 49 would be a wonderful byproduct, as it's stable and thus presents no radiation hazard at all - radioactivity isn't "contagious" unless you're emitting neutrons that can enrich the surrounding material. Potassium 42 on the other hand is pretty highly radioactive, with a half-life of only 12 hours. But that same half-life means it won't accumulate in the device. Instead it will rapidly decay into stable calcium 42.

        So basically, an Argon RTG would be pretty frigging safe in terms of radioactivity. As for the conductivity of Potassium or Calcium being a problem - I don't see why. There's no reason you couldn't surround the RTG core with an electrically insulating layer - after all it's just a convenient heat source, there's no need for any electrical components that could be shorted out.

        As for Plutonium 238, the isotope often chosen for RTGs because of it's decent power output (~0.54W/gram) and low shielding requirements. It's an alpha emitter, aka helium nuclei, which don't penetrate much. And it's daughter atom is U234, which is pretty stable with a half-life of 246,000 years. There's really not a whole lot of accidental "boom" potential - it doesn't emit neutrons when decaying, so it can't start a chain reaction - on it's own there is no critical mass.

        Of course it is still a fissionable material, so once you assemble a nice concentrated chunk of it you make it easy for someone else to stick it in a bomb that contains the requisite neutron-emitting "trigger" and neutron mirrors to sustain the reaction. Though they still have to build such a bomb, which is arguably almost as difficult as creating the Pu-238 in the first place.

        • (Score: 2) by VLM on Wednesday May 03 2017, @09:24PM (1 child)

          by VLM (445) Subscriber Badge on Wednesday May 03 2017, @09:24PM (#503962)

          Its just so nice to have something biologically irrelevant other than mere proximity like argon. Agreed the engineering would be a pain. But noble gasses are just so biologically uninteresting. Its not like inhaling tobacco smoke or something LOL.

          Yeah 42K is really the question isn't it where K is bioavailable and its going to be generated for decades/centuries if theres 42Ar contamination that isn't blown away physically but the radiation exposure ends in a couple days regardless of biological half life. Thats what I like about 39Ar sure the activity is lower but who cares Ar is not biologically relevant and 39K as the result is just delicious stable. It boils down to what happens when there's a leak, I would not be happy working next to a 42Ar tank with a possible theoretical slow leak 8 hours per day. 39Ar, sure, no worries.

          As far as electrical conductivity there were 60s experiments with direct conversion. A capacitor with a beta emitter treated plate slowly charges itself like a constant current source is plugged into it. Cool. I was thinking radio-battery not technically RTG as you mention. If you're going thermal thats less efficient than direct conversion and you gotta get rid of a lot of heat.

          By "boom" the problem with a Pu source is the lung cancer if a literal hand grenade or car bomb goes off. I guess it matters where you intend to use these batteries. Probably a bad choice for a residential TV remote control or car battery. Good for spacecraft, sure. I just don't see a Pu source like that leaving military / NASA type hands. Where are you putting these batteries?

          • (Score: 2) by Immerman on Wednesday May 03 2017, @11:20PM

            by Immerman (3985) on Wednesday May 03 2017, @11:20PM (#504042)

            Oops, right, Ar39, not 49.

            Fair point on leakage, though I have no idea how bioavailable atomic K is - in fact a quick search suggests not a whole lot is known about potassium bioavailablity in general. Still, most people probably aren't going to want to sit next to a substantially radioactive source for many hours a day regardless. In fact breathing radioactive argon 8 hours a day would likely be rather cancer-inducing in it's own right - no need for it to stick around if you keep breathing a fresh supply.

            And if size or mass are issues at all, then you have to factor in that to get the same energy from Ar39 you're going to need something like 16x as much as you would of Ar42 - roughly 8x the half life, and only 1 beta decay per atom instead of two in rapid succession.

            As for lung cancer from plutonium - it does sound nasty, but I'm uncertain it would be substantially worse than breathing radioactive argon - I suppose it would boil down to the relative length of time the two substances remained in the lungs. Argon is denser than air, but less than CO2, so it would *probably* get flushed out fairly fast, while plutonium, like other dust, would tend to be swept out and disposed of as mucous in otherwise healthy lungs (though unsurprisingly it sounds like smoking makes the cancer risks much worse).