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Some LiFePO4 science and future prospects

RedTechNeck

Tech Assistance for Escaping Grid Slaves
Joined
Mar 6, 2022
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16
My goal here is to have some discussion on the physics of LiFePO4 battery function, structure and design. In my opinion the development of LiFePO4 technology is in a very young stage of developement at present and major improvements are still yet to come as the future proceeds with this technology. It is entirely possible that LiFePO4 will entirely replace the more than a century old lead acid technology for energy storage in the future.

A lot of people look at the requirements for maintaining and regulating LiFePO4 as being more complex than lead acid and mistakenly estimate the old lead acid technology to be more forgiving and easy to operate. In reality it is what people are the most accustomed to and familiar with in their lifetime. Taking a closer look and you find that while LiFePO4 does require constant voltage monitoring in use, this is done by electronic circuits. Lead acid requires monitoring too, while a lot of systems use electronic regulated full time charging systems, some battery banks are quite often only maintained by "human" monitoring. If you let a lead acid battery remain below 50% discharge for more than 24 hours, some irreversible damage occurs internally and the battery will never have exactly the same capacity after that. Overcharging lead acid batteries for extended time also causes internal damage that reduces capacity, although it is sometime intentionally done to "equalize" cells. This is cell balancing on lead acid batteries! Cell balancing is also done on LiFePO4 by electronic means and not by the electro-chemical action that naturally occurs in the lead acid cells when the max charging voltage is exceeded. It is true that there are some lead acid cells that **CAN** last 20 or more years in service. But what is required for this to happen is a gamble against human error that is for all practical matters impossible. One mistake is all it takes to reduce that 20 years down to less than 10 years on that huge investment that must be made to get adequate electrical capacity when you consider only 25% allowable maximum depth of discharge. Since the largest capacity cells I.E. over 2000 amp hour 2 volt cells, are usually flooded cells, continuous watering management must be done and very pure water used over the battery lifetime.

Lead acid technology actually reached its peak at about the end of the Diesel electric submarine era, over 60 years ago, with only minor "consumer targeted" improvements ever since. Mostly adding catalysts in the battery case that convert H2 and O2 back to water for maintenance free batteries (which usually die from drying out after a few years), some changes in the plate grid alloys, and the use of glass mat and gel technologies that reduce the capacity per same battery volume. Even today, flooded cells still rule when it comes to maximum capacity for the same size.

Now LiFePO4 is a semiconductor and in the early stages the lower conductivity was a challenging issue for battery development. This was overcome by reducing particle size and thus increasing surface area. Now days this is in the low nano meter range. At first LiFePO4 was used in both anode and cathode, being laminated on copper foil. Now LiFePO4 coated copper foil is used on the cathode and either aluminum or graphite on the anode. Note that LiFePO4 is only used on the cathode which doubles conductivity over LiFePO4 being used on both electrodes. The nano LiFePO4 particles are being coated in conductive carbon to increase conductivity before being laminated onto the copper foil, but it also retards agglomeration of the particles which is one of the cycle life limitations in the present state of the art. The other limiting factor is the actual irreversible loss of lithium somewhere internally. Probably "plating out" somewhere internally (on the inside of the case?) or becoming sealed off somehow in the pores of the graphite electrode. The copper is the electrochemical source of the voltage limitations of the cell and the formation of dendrite shorts that ruin the cell when the voltage tolerances are exceeded. Someday they may be able to use conductive graphite or carbon fiber cloth in place of the copper foil or aluminum foil in both electrodes, and this would eliminate the dendrite shorting. I also predict that agglomeration and lithium loss will be nearly eliminated sometime in the future.

Ten years from now we may have LiFePO4 cells that last 20,000 cycles or more, and that require no BMS monitoring to prevent dendrite shorting!

I would also remark that for those out there who intentionally cut cells open for curiosity, that lithium is a very reactive material when exposed to air. With moisture present a self igniting source of hydrogen gas could occur. Elemental lithium can be present in cells at various stages of charge/discharge although "theoretically" tucked away within the pores and structures of internal materials. The crystal structure of LiFePO4 is very close to a quasi-chelate of elemental lithium within the FePO4 structure. Any exposed lithium (I.E. internally plated out) will also become lithium hydroxide when exposed to humidity in air. LiOH is a very caustic material just as dangerous as the lye used to dissolve clogs in drains. Lithium can also be a poison type of health hazard too. The electrolytes are also hazardous materials. Always use proper PPE and containment anytime any of these activities are done even by experienced science technicians.
 
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Actually the physics are well-known and have been for decades.

Many of the improvements are not actually necessary, but improved manufacturing and distributing is. Many of these so-called improvements are actually nothing more than "investor-bait" for advanced LFP that never see the light of day in actual production, or simply turn out to be patent-trolls.

Still, we know there are small improvements like what additives are put into the electrolyte to enhance performance for differing application scenarios. Some are published. Some additives are proprietary-secrets.

Still, to this day, we can observe secondary reactions, and are still baffled by the electrolyte interphase layer. We observe its initial formation to protect istelf from immediate degradation, and then later how it can grow get clogged to the point of non-functioning and blocking. We have to have it, but too much growth is a bad thing, so we try to minimize it with proper care and operation, and the special blend of spices (electrolyte additives).

The point here is that the REAL discovery to becoming solid-state, is to somehow prevent the IE layer from growing and eliminate secondary reactions from things and people that use them out of the specification window.

When that is discovered - a lifetime battery - be prepared for it to cost more than gold. :)

Just saying - further research that gets published is usually along the lines of investor-bait. What can be done simply now, is to put more effort into a quality build and distribution process. Then again, that costs the manufacturer money and time in resources and materials. Costs go up. When they don't, you get bloaty cells since some of the cheesier manufacturers actually skip some vital steps in production and leave it up to the consumer to perform (ie, initial aging, etc)
 
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Precision manufacturing makes a difference between vendors!

1). OFFSET. Dendrites first form at the edges of anodes and cathodes, especially if they are of the exact same physical dimension. This is where dendrites like to cross first.

Quality manufacturers make the anodes and cathodes of ever-so-slightly different dimensions, so that edge boundaries are longer and not as natural to cross for dendrites.

2) INITIAL FORMING. Not to be confused with lead-acid forming. When say the pouch cells (either bare or paralleled inside a plastic prismatic case) are initially made, a slight gas forms inside the pouch(es). This small initial gassing should be let out by a low pressure mechanism before the final vacuum-seal is made.

If that is not done, and the pouch allowed to have breathing room by sealing immediately, you run the risk of delamination later - which is simply that the anodes and cathodes are not in tight physical proximity. Thus some areas of the material are more active than the other and not evenly spread in performance. Ie, the center of the anodes can be more efficient than say the near the edges. Hot spots under high current. Also, delamination can give rise to offsets being mis-aligned by movement (breathing to different positioning) defeating the purpose of (#1) above.

3) TESTING
Manufacturers can go through a time-consuming process of initial charge / categorizing / sorting. Or not.

Those that do ensure that the cells are fully charged at least once to make sure that all of the material becomes "active" and activity is spread evenly across the anodes and cathodes. If they don't, then you run the risk of hot-spots - some of the material doing more of the work than the rest. Hot spots / tracks under high-current.

In ye olden diy EV days, this meant taking LFP up to higher voltages temporarily and under a VERY watchful eye so as not to exceed 3.9 to 4v. But ONLY like ONE TIME to make sure the entire surfaces are equally active, and then dropping back to 3.6v for normal use. Again, sub-c or low current users wouldn't likely notice. It was only after this one-time event that manual balancing procedures or automatic balancing would be applied.

These days, the hope is that manufacturers do this properly, or the material used today doesn't need it. But how do you know? Your faith is with the manufacturer.

AGEING:
After cells are manufactured, they *should* go through an ageing process where after initial formation, a cycle or two is applied to ensure that the initial SEI layer is large enough to protect the cell from immediate or quick discharge. Internal resistance should be noted. These cells should be measured after a week or two, or better yet a MONTH of idle time. Then categorized / sorted / discarded.

This isn't everything, but it points out that there are many cost-saving shortcuts that a manufacturer can take before it gets to you. Those running high-current EV environments will cull out the poor manufacturers quickly. Unfortunately, in Sub-C or relatively low-current solar applications, shoddy manufacturing and other faults can be hidden by the application. Notice that we aren't even talking bms issues here. Just bare cells.

Distribution:
Even the most precision manufacturer can have their product degraded by sitting in a hot rail-car or ship hold for months - even at a low state of charge if the temps are high enough. Distributor warehouses with super high-temps can also be a factor.

Just a long way of saying that precision manufacturing and distribution can make a difference *now*, rather than waiting for pie-in-the-sky improvements. Which they too could be degraded by improper distribution. :)
 
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As for now, I won't buy a power station that doesn't have LiFePO4 (lithium-iron) cells. OTOH, there are some very expensive lithium-polymer and lithium-ion stations on the market. I just recently decided to buy a few, so I began researching them. Wow! It didn't take me long to conclude that LiFePO4 cells were the way to go. In fact, I had a Jackery station in my shopping cart all ready to purchase, recommended by a friend, but as I did my homework, it didn't take me long to see the advantages of lithium-iron.

So I don't know whether there are some advantages to other cells, or whether some companies are resting on old tech. Five hundred cycles to 80 percent isn't bad, but if someone does a lot of camping and outdoor activities, they can hit 500 times in just a couple of years. Three thousand times, though, is far better. And if it does hit 3,000-3,500 times, the 85 percent level in these devices is still usable (80 percent also is usable after 500 times, but anyone can see the advantages of 3,000-3,500 vice 500 times). You also can leave your LiFePO4 power station in cubby holes you couldn't leave your other lithium cells for fear of fire. There have been cases of spontaneous combustion occurring with such cells that have burned down homes.

This has not been an issue with LiFePO4 cells. If I had one criticism, its that you can't pull the old cells out and put new ones in. With a modular design, this would be a great feature.
 
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