[This is a note sent by my friend, Ian. It’s a pretty interesting and comprehensive look at battery technology and trends.]
Looking at the electricity storage I discovered about batteries over the last 6 months. It follows the usual rule that any time you look into an apparently boring technology, it gets fascinating!
There are many dimensions of a battery and some of them are as follow:
1. Weight : If the battery is to be portable or in a car it needs to hold a lot of charge for its weight. This leads to lithium batteries. Various approaches are being tried to reduce the weight. The other electrodes and the supporting stuff that maintain the electrode shape when the lithium has been dissolved. There is an outside chance that a multiple charge (Li has a single charge, thus atomic weight 7/1=7), would have possibilities. Beryllium is the obvious one (9/2=5.5) except for its toxicity. By the time you get to magnesium, things are too heavy. For non-portable battery weight is less important and there are a huge number of potential single and multi-charge options in principle anyway. Moving away with one electrode as in lithium/air is the Holy Grail. IBM has been working on it for years, but obviously letting air into lithium is a recipe for a fire or explosion and the presence of water vapor is a killer. Solid electrolyte batteries (Applied Materials) which use pure lithium as an electrode and a solid electrolyte. It has been pretty expensive in the past, but may be about to drop dramatically in cost, roughly doubling the charge per weight for lithium. One can get more power into a battery by using two electrodes with a larger difference in their desire or hatred for electrons (same number of electrons but each has more oomph)
2. Running temperature : It’s nice to do everything at room temperature, however, higher temperature makes ions zippier, thus speeding up the delivery of the charge. Also, at higher temperature many things are in liquid state which creates interesting options such as completely liquid batteries. Ambri is championing this approach which makes its manufacture really easy. You just need to pour a pile of mixed powders of the electrodes and electrolytes into the cell and then warming it up till they melt. They will get separated into three layers and you will have a battery. Think of lava lamp (I’m not quite sure of the components that Ambri is using as they seems to have been changed for the current version. They are also running a bit late for the delivery, but that’s expected for an innovative product.)
3. Cost or more precisely cost watt/hour stored: This has led to the attempts to use common substances such as iron/salt and sodium /sulphur. Aquion offers batteries using manganese, carbon, and a sodium salt. They are big and slow to charge /discharge but they are effectively half the cost (due to lasting twice as long as lead/acid). They are thus good for some distributed generation uses.
4. Discharge curve : You obviously want a battery whose voltage stays constant as it delivers charge until its empty. This creates problem since you have no way of knowing, how much charge is left but it does make it’s use easier. Most batteries hold steady down to some discharge proportion and then either decays suddenly or slowly. You may have noticed that your electric bike charge meter appears to be fully charged for most of its life then suddenly starts dropping.
5. Discharge/charge rate: This depends mainly on the surface area of the electrodes and the mobility of the ions. The hope is that nano-tech and surface texturing will effectively make the surface fractal and thus gives huge charge/discharge rates. This is important for sudden demands as most of the batteries suddenly lose voltage after sudden demands as the ions haven’t had time to clear away from the electrodes and make room for the next lot.
6. Charge behavior : Typically, a battery will charge upto 80% or so, as quick as one can use higher than equilibrium voltage to push it along, the last 20% of the charge must be charged slowly to avoid bad things happening and the voltage must be reduced slightly to the equilibrium voltage and definitely below the electrolyte breakdown voltage (the reason for hydrogen explosions while recharging is lead acid batteries). A charged meter, will typically show the battery as fully charged during this period. This is why your fully charged electric bike may run better if you charge it for a few more hours with a pulsed charger.
7. Size per watt/hour : We do not hear much about this. It seems to be of a less important dimension in most portable applications and thus is potentially exploitable.
8. Normally, a battery’s charge and discharge rates are related to each other. However, for some of the applications (e.g. grid peaking) you need a greater charge storage but would not need much of the discharge rate. In that case, the principle is that the electrodes are in liquid state and are held in four tanks: two for the charged state electrodes and two for the discharged state. The electrodes are then pumped past a polymer film that acts as an electrolyte to deliver the charge. The advantage of this is that it doubles the storage capacity as you just build bigger tanks. At present, Vanadium salts are being used but they are too expensive so, other options in particular are Quinone’s(organic compounds that unusually can hold charge because they blur it over a number of atoms and bonds and are pretty stable) which are being looked at.
9. Number of charge /discharge cycles : Obviously, the effective cost is essentially the upfront cost divided by the number of charge/discharge cycles, leaving a good space for engineering optimization. For any battery technology, the relevant curve is published and usually shows a slow decay of the maximum charge level stored over time. Some claim to have practically no aging effect over 10,000 cycles.
10. Maintenance and support : Batteries that contain pumps or where the components decay over time, as all do to some extent, eventually needs refurbishment or replacement. The choice of technology and to what extent the battery components and electrodes can be re-used are a big areas of innovation.
11. Electrode stability : A battery works by dissolving its electrodes and then during recharging by replacing them with the original electrodes. Obviously, they may not exactly replicate the shape and size of the original electrodes. This is called aging and reduces the performance and life of the battery. An electrode may have a matrix as well. I believe that charcoal from coffee or cocoa bean husks has been found to have good properties. Carbon nano rod structures are being tried out as they have nano silicon structures.
12. Total cycle efficiency : It is is critical in many applications, Total cycle efficiency is how much electricity you put in and how much you get reverted that is useful. There is obviously a thermodynamic limit(pushing the charge back into the cell takes slightly more energy than letting it out) and various technologies that offers 70% up into the 90s. For portable devices, efficiency is less important than weight, size and speed. For grid instantaneous, balancing in the distribution system efficiency is not important but maintenance is. For grid load, delivery and efficiency is pretty important. For windfarm, load extension efficiency largely affects coiling issues and cost, as the wind farm can store spare electricity and deliver it when the grid is a bit oversupplied. Each application will involve a different optimization.
13. Few battery technologies can deliver very high short bursts of power than needed. For example, to get a car moving or a plane to take off. Ultra capacitors may fill this performance gap. Instead of storing power by ionizing /de-ionizing metals, it is stored as sort of spring, pushing the electrons onto a metal surface that gets a higher and higher charge and is increasingly reluctant to accept more. New nanotechnology approaching to fractal surfaces can provide an immense surface area in a tiny chunk of material. This may allow the capacitor/battery combination to hit a wider range of applications than either alone.
If you delve into battery technology, it is fascinating as to how two century plus old technology can still be such a craft rather than a science, and also how to move it on seems to require really advanced tools, such as atomic force microscopes to find out what is actually happening. Some of the reactions happens so fast that only with recent technology, we can watch it happening rather than to guess. It is also remarkable that how many different materials can be used to make a battery (even paper!). And to me as an organic chemist, I never thought that useful batteries could be made out of organic materials.
In addition to batteries, mechanical methods of storage are being developed. Flywheels, compressed air, repurposing hydro stations used as pumped storage, big movable weights in deep wells or mines, are my favorite, although to my knowledge it hasn’t been proposed yet to use old gas fields at sea. Open the lower drill pipe to the sea, and pump air, CO2 or a suitable incompressible fluid in the upper drill pipe. When you need power, let the fluid get out through turbines (good use for redundant oil and gas platforms at sea.)
Since this is a critical issue for moving to H3 renewables, as well being valuable anyway for maintaining the integrity of the H1 grid system, there is a huge potential for development and for money and also a huge number of different ways to move forward. It thus looks to be unpredictable what will happen i.e. an H2 situation with high confidence or H3 will happen as it is in the interests of major H1, H2, and H3 actors.
I don’t claim to be an expert on battery technologies, so there will be lot of ideas floating around!
P.S. : In view of the way that mass market solar cells performance has stuck around 14-20% for some years, it’s nice to see Semprius is achieving 40% with a mass production approaching to stack multi-junction cells ( the nano tech V shaped crevices with multiple junctions at different critical absorption frequencies is still in research heaven).