Are capacitors the superior alternative to lithium ion batteries?

[NikolaiSimonov]

TL;DR Summary

Why didn't we already switch from lithium ion batteries to large capacitance capacitors with big resistors?

I'm an undergrad physics student, so, I don't yet know much about the technicalities behind using chemical batteries vs capacitors. However, from what I've learned so far, capacitors seem to be a far better option than lithium ion batteries - they're not hazardous (at least when discharged), do not require rare or expensive materials, do not lose maximum capacity over time, and, most of all, do not explode when disconnected (I think). Also, we can control a discharge rate of a capacitor by adjusting the resistance. So, the question is why aren't we all using capacitors everywhere and what are the downsides/obstacles with using them?
On the second note, would using stuff like graphene or any other monoathomic layer of something improve the capacitors. It would seem to make them much more compact.

 

[berkeman]

Welcome to the PF!

capacitors seem to be a far better option than lithium ion batteries

The biggest issue is energy density. You can use a Google search to compare the energy density of capacitors to that of batteries (versus fossile fuels).

Also, we can control a discharge rate of a capacitor by adjusting the resistance.

Using a resistor to control the discharge of a capacitor energy storage element wastes a lot of that energy (lost as heat in the resistor). Instead, you would control the discharge using a DC-DC converter circuit, which has minimal energy loss/waste in the conversion. (Another term for you to do a Google search on)

 

[sysprog]

Welcome aboard, @NikolaiSimonov --
Here's an article describing a graphene supercapacitor made with graphite oxide on a LightScribe DVD: https://www.cornellcollege.edu/physics-and-engineering/pdfs/phy-312/Theint-Aung.pdf

 

[Baluncore]

The fundamental difference between a capacitor and a chemical cell is how the voltage changes during the charge-discharge cycle.

A chemical cell has a fixed voltage which makes voltage regulation less of a problem. A capacitor has a varying voltage with energy, E = ½·C·V². A capacitor therefor often needs some form of high efficiency switching voltage converter.

For some bulk applications, maximum energy density per kg, or per cubic metre, will be a more important selection criteria than voltage variation.

 

[sophiecentaur]

from what I've learned so far, capacitors seem to be a far better option than lithium ion batteries

That entirely depends on the application. I think it will require an enormous leap in super capacitor technology and in the necessary circuitry it needs. before it seriously challenges the best Lion cells for most applications. But never say never...

 

[essenmein]

The main reason has already been mentioned, energy density, either volumetric or gravimetric, is a lot lower, somewhere between one and two orders of magnitude lower for supercaps.

Second, supercaps do have a life time and like most capacitors their capacitance does degrade over time.

Third, supercaps have quite poor self discharge, Li-ion is somewhere around 1% per month (from memory), where as supercaps are down to about 50% after a month.

Interestingly supercaps don't function entirely like "normal" capacitors with plates and a dielectric, they rely heavily on "pseudocapacitance", which is actually an electrochemical energy storage mechanism.

 

[anorlunda]

A complete answer depends on the application. I've already seen a demo of supercap to start a car. In most cases, it takes only a quick 1 second burst of power to start modern cars. The supercaps were very good for that.

Of course if the temperature is -40 and the engine needs to crank for 90 seconds before starting, batteries are much better.

So it may be that we start seeing supercaps start picking off niche applications where batteries were the only choice before.

 

[Comeback City]

Of course if the temperature is -40 and the engine needs to crank for 90 seconds before starting, batteries are much better.

In terms of the inefficiency of the supercapacitor at -40, will the low temperature of the wire/material carrying the current be as important as the electrolyte being close to freezing? I would imagine the electrolyte being close to freezing would be much more important, yet current decreases with temperature.

 

[essenmein]

In terms of the inefficiency of the supercapacitor at -40, will the low temperature of the wire/material carrying the current be as important as the electrolyte being close to freezing? I would imagine the electrolyte being close to freezing would be much more important, yet current decreases with temperature.

Losses due to current flowing in conductors (eg copper) will be lower, however the current requirement to crank an engine is proportional to torque, cranking torque is much higher with a cold engine (oil viscosity for example).

Generally for automotive applications -40C is a "normal" operating point so any capacitors are required to meet data sheet values at this temp. We find that even at lower than -40C the capacitor electrolyte is only "frozen" per se for a very short time, the electrolyte at these cold temps exhibits higher effective ESR, which results in fairly rapid self heating from large ripple current, which means off course the electrolyte is no longer cold.

 

[Baluncore]

I think the point anorlanda was making in post #7 was that a super capacitor might start a car once, but only under ideal conditions. Super capacitors do not yet have sufficient energy storage to reliably start a car under adverse conditions.

 

[essenmein]

Absolutely, I was responding to post #8 where he talks about freezing electrolytes and current being reduced at lower temp.