RedTechNeck
Tech Assistance for Escaping Grid Slaves
- Joined
- Mar 6, 2022
- Messages
- 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.
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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