Re: Battery breakthrough

From: Evgenij Barsukov (evgenij_b_no_spam_at_yahoo.com)
Date: 11/04/04 

Date: Thu, 04 Nov 2004 09:32:34 -0600

sanman wrote:
> Here's a PDF of some paper published by Telecordia's Amatucci in
> conjunction with some other researchers:
>
> http://www.arofe.army.mil/Conferences/Recent_Abstract/200th_Meeting/symposia/b1b/0203.pdf
>
> It mentions intercalation of Al+++ into a foamed/aerogel vanadium
> (V2O5) electrode. They also mention that this has only now become
> possible due to use of nanocrystalline oxides. So I guess that means
> they're exploiting the fact of the aerogel having the nifty nano-sized
> cavities.

Interesting work and coming from a respectable group. I agree that more
should be done with Al, because it can eventually work in water rather
then in organic electrolytes as Li-ion battery does. But energy density
(not coulomenric!) will never reach Li-ion because of its much higher
activity (=higher cell voltage).

Now, about nanocrystalline. If I hear that, my first thought are

1) high surface area = high self discharge. This is always true with
batteries involving high energy materials that are only stable because
of insulating layers on the surface.

2) Nanocrystalline matterials have small particles therefeore more
contact area between particles. This means lower overal conductivity
and more problems - because loss of electrical contact between particles
is a huge issue even with relatively large 10-50mkm particles used
in Li-ion. If you go to even smaller particles, accumulation of
decomposition products on their surface will disconnect them even earlier.

3) Finaly, the very need to go to small particle size indicates
that cristalline structure of material itself is not sufficiently
open for intercalation and therefore you are forced to use higher
surface area (with above mentioned problems). So, as a general direction
I would always try to find a cristall with more open structure (like
iron phosphates, for example) rather then going nano - for any
practical battery.

>
> They say that the redox potential of the host material (ie. the foamed
> aerogel) is the limiter on the energy density of the battery.

That basicaly supports my point 2. I would say, voltage drop
over the cathode material is a large problem.
As for redox potential, it is always defined by potential
of both cathode and anode.

>
> If you look at the V2O5 graph (using silver extrapolated to lithium
> ions?), it shows a maximum of 275 mAh/g, which would be ~400 Wh/kg?
>
> So is aerogel or nano-foamed metal now being used to exploit
> Aluminum's energy density?

More likely Al3+ is not able to penetrate V205 cristall and
is deposited mostly on the surface, hence the need for nanocristalline
structure. But as result of this, only a fractioni of full intercalation
capacity of this material is used.

> I guess that the Aluminum would also have to be nano-grain or molten
> to get it intercalated in there, right?
No, Al3+ is getting intercalated.

> How come the Al+++ wouldn't perform better than Li+? Why is the
> aerogel the limiting factor? Do things get better with smaller pore
> size?

Al3+ is expected to perform worse then Li+ because
1) Li+ is much smaller so can more easily intercalate
2) Li->Li+ couple has higher potential then Al

Regards,
Evgenij

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