subject: The Three fundamental Laws of Batteries: The Second Law [print this page] The Three fundamental Laws of Batteries: The Second Law
Welcome to a laptop battery specialist of the IBM Laptop Battery First post by: www.best-battery-online.com On to the second law, which states: Any battery that is widely commercialized will operate at a voltage higher than its thermodynamic stability window.Anyone who has worked on batteries will have, at some point in their career, experienced what alcoholics refer to as a moment of clarity (to quote Samuel L. Jackson in Pulp Fiction). The epiphany is that every battery we know of exists because of a freak of nature. For me, the realization came when I was thinking about the Ni-MH battery, but a more glaring example is the lead-acid battery such as IBM ThinkPad R60 Battery.First, some background. Water electrolysis is a process by which water is converted to hydrogen and oxygen in an electrochemical cell. Remember the hydrogen economy? The one that was supposed to power our world in 2005 (or was it 2010? I can't remember, but it was sometime in the past).The environmentalist's dream of the hydrogen economy involved using solar panels to make electricity; the electricity was then to be used to split water into hydrogen and oxygen via electrolysis, then the hydrogen from this was to be used in a fuel cell to get electricity again, which was then to be used to power our cars.This water electrolysis process occurs when the voltage of an electrochemical cell goes beyond 1.23 V in a water-based electrolyte.In other words, anytime a water-based electrochemical cell operates above 1.23 V, there is a very distinct possibility of water electrolysis.The voltage of a lead-acid battery is ~ 2 V. The electrolyte in this battery is water-based.How do you have a battery operating at 2 V when at 1.23 V water starts to split?It also turns out that nature gives us a break, because the rate of the reaction that splits water to make oxygen is very poor, so this reaction isn't that dominant. (Incidentally, the inability to get the oxygen reaction to go in reverse has been one of the many issues that have prevented fuel cells from taking off).What this means is it's more favorable to oxidize lead sulfate than to oxidize the water. Voila: We have a lead-acid battery instead of a water electrolysis cell.Turns out that the lead-acid battery actually does split water into oxygen pretty much continuously, but in small amounts. Those of you who are older will remember when you had to add deionized (or distilled) water into your car battery to "top it off". This was essential, because any oxygen you made increases the pressure, then a vent opened and you lost, in effect, water from the cell. "Topping it off" got this water back into the system.Li-ion cells are so much better than water-based systems because they have no water. This means the voltage window can be expanded dramatically from 1.23 V without the risk of electrolysis. Remember: The higher the voltage, the higher the energy of the battery.But every electrolyte has a voltage limit after which you'll destroy it. The window in a Li-ion cell is anywhere from 2.6 V to 3.3 V (depending on who you talk to).A typical Li-ion battery operates at 3.7 V with the maximum voltage hovering around 4.2 V. So something is happening to the electrolyte. That "something" is the side reaction I have alluded to in the past. These reactions lead to fading of the capacity and increase in the resistance in these cells. Turns out the electrolyte pretty much continuously decomposes during the life of the battery.So can't we find materials that operate within the voltage window?We can. For example, if one were to take a lithium titanate anode and a lithium iron phosphate cathode, then you'd have a cell that could, for the most part, stay within the voltage window of the electrolyte. This system should (at least on paper) give you good cycle/calendar life.But the voltage of this system is around 1.9 V. Compare that to a typical Li-ion cell that has a voltage of 3.7 V, and you begin to see that this system, while being within the voltage window, will have less energy: almost half the energy of a typical Li-ion cell.And in batteries, energy is king.Hence, the second law is essentially a commentary on our expectations from our energy-storage devices. If we're satisfied with a laptop with two hours of run time, then we can stay within the voltage window. But, we overwhelmingly prefer one that operates for four hours, so we operate outside the voltage window.Even in applications where space isn't a constraint (e.g., a stationary battery), we prefer a higher voltage. Without getting into details, this is because it's hard to get high energy efficiency from a low voltage system.You can obviously try to make a commercial success out of a system that doesn't follow this law, but history suggests this may not be a good idea. Hence, the caveat that this law applies to widely commercialized batteries.This law, while not being a fundamental law, is so true that it may as well be one. http://www.articlesbase.com/intra-net-articles/the-three-fundamental-laws-of-batteries-the-second-law-4497795.html
