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Comment by hwillis | Hacker News Reader

I had a highly upvoted comment on the original submission- the thing I missed was that in the original paper they claim they went from metal lithium on the cathode to metal lithium on the anode after discharge. You can't do that. In order to have a battery the lithium has to move from a high energy state to a low energy state- not from one state to the same state. There is something fundamentally fishy about the experiment, but I'm not sure what. The way it is portrayed in the paper, you could basically just switch the cathode and and suddenly have a fully charged battery.

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If you had a normal electrochemical cell, with a sodium anode and sulfur cathode, and liquid electrolyte, the sodium would release an electron into the circuit and hop off the solid anode into the liquid electrolyte. On the other side, sulfur would grab electrons from the circuit and hop off the solid cathode into the liquid electrolyte.

In the middle, the ions would keep their charges and physically arrange themselves such that the charges are balanced in every direction. This would be an energy minimum, and it would be hard to reverse. Trying would probably cause the electrolyte to decompose instead of redepositing the original reactants onto their electrodes. It would not be rechargeable.

As a physical metaphor, you have two mutually-attracted boulders rolling down from opposite ridges into the same bowl-shaped energy valley. When they collide, they strike with such force they turn to sand.

In the rechargeable battery, you prevent that last step by putting a giant springy foam block in the center of the valley, so that the boulders can't touch. You prevent the sodium and sulfur from coordinating physically to cancel out the physical force from separated charges. Then to recharge, you tow or push the boulders back up the hill.

I think the "same metal at the cathode" might be an artifact of putting the springy foam block in to keep the boulders intact. You couldn't switch the anode and the cathode any more easily than you could swap the elevations of the ridges and the valley. That is, it takes as much energy to do the physical rearrangement as it would to charge the system electrically.

>I think the "same metal at the cathode" might be an artifact of putting the springy foam block in to keep the boulders intact. You couldn't switch the anode and the cathode any more easily than you could swap the elevations of the ridges and the valley. That is, it takes as much energy to do the physical rearrangement as it would to charge the system electrically.

That is the case in normal batteries, but in this battery the start and end product are both just lithium metal- not intercalated into graphite or bound to sulfur, literally just solid lithium. There's no (obvious) place for the energy to be hiding. It would appear that you could just scrape it off and receive free energy.

To follow your boulder/hill metaphor, the lithium boulder rolls down the hill (crosses the electrolyte), settles at the bottom of the valley (bonds sulfur)... And then somehow as more lithium rolls down the hill, it continues to pile up at the bottom until it forms another hill, the same size as the first (lithium plates onto the anode). That's what they claim happens- that the sulfur acts as a redox center and sets the voltage for an unlimited amount of lithium. But a redox center cannot just lower the voltage of an arbitrary amount of metal. The metal is an entirely normal bond and can be cut with an energy proportional to mass^(2/3), while the energy stored increases linearly.

I agree that it is odd, but it isn't categorically impossible from first principles.

For a simple example, consider two plates near each other, one with a positive charge and the other negative close together. That's a capacitor. Now physically pull the plates a bit away. Voila! Every individual piece is in the same state yet you now have more energy stored!

The voltage changes in that case. In this case, a voltage difference exists between two materials with the same voltage relative to reference. It is as if you got pure gasoline out of a cars tailpipe.

I think they start with Li on one side, and a Cu/C/S cathode on the other, and then plate the lithium on the anode. They measure a difference in chemical potentials between cathode and anode, so there is reason for the Li to move, and as the Li moves, I think I remember the V_cell decreasing.

From the paper:

> At the cathode, the build-up of plated metallic lithium changes the morphology of the cathode to increase C_C and create a very slow fade of V_cell with time of discharge

Yes, but you can make the voltage not change.

Take the same capacitor, but make each plate into 2 layers. Pull the outer layers away and attach your electrodes to the inner layers. You can arrange to have more energy, at the same voltage. As the capacitor discharges, bring the outer layers in to keep the voltage across the inner plates constant. Reverse the process as it charges.

Please be clear, I'm not claiming that their battery works this way, or even that it works at all. I'm only making the claim that it is actually possible to design a device where what looks impossible here is not necessarily impossible.

Wait. there's more energy if you pull the capacitor plates apart? What're does it come from?

How much more energy are we talking about?

Since the plates attract each other electrostatically -- one is positively charged, the other negative -- it takes energy to pull them apart. The amount depends on various factors: the size of the plates, how close they are together, how much charge is on them.

Is there a formula for that? I'm curious now.

CV=Q. Pulling plates apart makes the capacitance C decrease, while the charge Q remains constant. This means V must be increasing. Energy in a capacitor is VQ/2, so the energy increased.

(This is somewhat circular reasoning. The point is that the charges on capacitor plates attract each other so it takes work to separate the plates. Integrating the force along the path the plate takes during the separation gives the extra energy stored in the capacitor.)

A similar thing is a static generator made using a pie tin. You put a little charge on the pie tin by some means, like rubbing on styrofoam, and then you separate the tin from the styrofoam, which gives more energy to the tin (increases the voltage), which then you can transfer to a Leyden jar.

I also discovered in high school that you can use a pair of plastic chairs and two willing participants to generate high voltages when there is low humidity. Person one stands up off the chair, touches person two, then sits down. Person two then stands up off the chair, touches person one, then sits down. Repeat. Eventually, you will generate fairly high voltages that sort of hurt. You can hear the excess static charge on the chair squeal into the atmosphere after standing up.

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