Graphene Ultracapacitor now the same $/lifecycle as LiFePO4?

[svetz]

UPDATE: This is probably a bogus product as described. Possibly some sort of hybrid mix of a supercapitor and lithium cell.

Looks like supercapacitors have made a big graphene jump forward:

Stored Energy: 75 Wh Cost ($/Wh): 1.00 to $0.69 (bulk buy) (LiFePO4 is ~$.15/Wh) Leak current: < 20 mA Self Discharge: 3%/mo (LiFePO4 is 2-5%/mo) ESR: < 1 milliOhm Voltage: 4.2v Capacitance: 21000F -20 to +60℃ > 20,000 cycles Max continuous current: 40A Size: 220*128*7.5mm Weight: 350 G (4.6g/Wh, compared to LiFePO4 @ 7.3,g/Wh) Power volume (kW/m^3): 352 (compared to LiFePO4 prismatic @ 352.5) Ref: https://www.alibaba.com/product-detail/Graphene-Ultracapacitor-4-2V-21000F-221_62575851069.html

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If you look at the $/lifecycle, it's almost the same cost as LiFePO4 and these are just the first generation of market-ready components.

Similarly, if you compare the kW/m^3, its the same as a prismatic cell. This is particularly important for EVs, it means in the same amount of space you can put similar energy storage at 63% the weight. Also for EVs, imagine being able to completely recharge in about the same amount of time as it does to fill a tank of gas.

It won't help folks like me who use energy storage for emergency backup and only consume a few cycles per year. But for those who cycle the batteries a lot (or want to as they have no/poor net-metering) ... oh yeah, this product really shines for off-grid solar because of the $/life-cycle and lower round-trip energy losses than LiFePO4. Ditto for public utilities for energy storage. The self-discharge is roughly the same as LiFePO4, but if you're cycling a lot that's nothing compared to the round-trip energy losses incurred by LiFePO4.

 

[svetz]

Curiouser and curiouser...

In looking at how to calculate the watt-hours from the voltage and capacitors farad rating noticed some stuff...

Let's use a reference to check some values. Looking at a Maxwell datasheet for a 2.7V 3400F with a 3.44 minimum Wh stored energy.

The datasheet notes has the formula as 1/2 CV^2 / 3600. Google shows a number of sources using the same formulae for calculating energy in a capacitor. Possibly because the voltage isn't steady like a battery?

1/2 * 3400 x 2.7^2 / 3600 = 3.44 Wh ... so that does match the datasheet.... but Maxwell also says the typical is 3.66 Wh.​

What about for the capacitor in the OP?

1/2 * 21000 x 4.2^2 / 3600 = 51.45 watt-hours, that's a lot less than the 75 watt hours.​

There's a couple of other funny things about the capacitor in the OP:

• It's only rated for > 20K cycles. A regular supercapacitor is typically rated for > 500,000 cycles (the Maxwell is 1M cycles).
• It says the quick recharge is 30 to 60 minutes, capacitors usually charge much faster.

That manufacturer has several capacitors, the 100,000F 2.7 is rated for 60 Wh, but using the formulae above that would be 101 Wh. So that doesn't match up either and the rated watt hours are far less than advertised.

I wonder what's going on with that? The advertised capacitor has an energy density is about 30x the Maxwell super capacitor; is it that the Wh formulae just doesn't hold up? Or are the datasheet numbers misleading?

 

[svetz]

An Eaton datasheet for a 48.6V 166F uses the same 1/2 F * V^2 / 3600 formula and the numbers match, so not sure what's up with the Alibaba datasheet and it looks like the formulae should be good for high voltages.

Something to do with graphene??

This 4/2020 article says the highest ever reported energy density of graphene is 88.1 wh/L and the capacitor in the OP is about 4x that... so it's starting to sound pretty bogus.

 

[gnubie]

These capacitors were posted on the forum quite a while ago. Has anyone posted a voltage vs charge (ie stored energy) chart for them yet?

 

[svetz]

...These capacitors were posted on the forum quite a while ago...

Have a link by any chance? Couldn't find anything in these forums (other than this thread) to this model using the advanced search for supercapacitor, super capacitor, ultra capacitor, ultracapacitor, or 21000.

 

[svetz]

While googling (hoping to find something in any forum anywhere) I ran across this Russian article published on May 10th similar to the OP (basically says how cool they are); and of course one comment said what I did in post #3: Where does 75W * h come from? Capacitor Energy: E = C × U² / 2 = 21,000 × 4.2² / 2 = 185 220 J = 51.45 W * h The article also included a link to ready-made 48V 1500Wh battery made from these cells for $5,250 shown to the right. Tried searching on the company "gonghe electronics", but other than the two entries in Alibaba (one stating the company formed in 1992 and another saying it was formed in 2008) couldn't find anything useful. So, it's still a mystery to me!

Just to be clear, this is probably a bogus product. My guess is it's some sort of hybrid supercapcitor and lithium cell

 

[svetz]

So, are capacitors ready for EVs that is are they lighter and close to the same volume?

Moving away from the possibly bogus device and using the original paper for the 4/2020 article that reported the highest ever energy density of a graphene supercapacitor is 88.1 wh/L; there are some interesting things.

They say it compares favorably with lead-acid of 50-90 Wh/L. Given E = C V^2 / 2 by choosing a substrate that allows higher voltages you get more energy storage. What they realized is that they needed to balance density and porosity rather than the prior approach of increasing porosity. The trick seems to be to minimizing the surplus spaces, whilst adapting the pore sizes to that of the electrolyte ions. So the work they did was to fabricate a graphene laminate film with pores that could be tuned.

From the paper it looks like the density was between 1.51 to .94 g per cc, so a liter would be 1.51 to .94 kg/l, or an energy density of 88.1/1.51 = 58 Wh/kg. LiFePO4 is 90–160 Wh/kg, so it would take two to three times the weight in an EV. So, doesn't seem quite ready yet; but 10x closer than before!

 

[gnubie]

There's more to it than just raw watt hours. It has to be at a voltage that the device can use. If a device requires 12v and the capacitor is 14v, similar to a LA battery, the typical linear relationship of volts and charge means that at 50% charge remaining we are down to 7v. Far below the point at which a 12v device will work, so we need a better power supply. But as we go down further in charge that linear relationship really bites with current going through the roof in order to get all those tasty watt hours. Perhaps these capacitors have a different volts / charge relationship but the manufacturer does not publish any data whatsoever so even if they are real, you have to assume linear.

I posted similar the last time these things were mentioned and in other threads where people touted high capacity capacitors as battery replacements, but it didn't look like anyone understood.

 

[svetz]

...There's more to it than just raw watt hours. It has to be at a voltage that the device can use...

Very true, batteries try to hold the voltage steady, but capacitors (AFAIK) can't. You need another component to regulate the output voltage. But just like a DoD on a battery or a sleep voltage on Li ion there's some bottom limit to V. The reputable vendors (not necessarily the device in the OP) seem to account for it by separating "usable power" from total storage.