Batteries failure depends on the chemistry and can be broadly classified as mechanical failure and chemical failure. When I say mechanical, think cracking, breaking, and shedding of the electrode. Chemical means reactions, like corrosion, that alter the state of the battery for the worse. In lithium batteries both kinds of failure can happen. For example, people have shown that the electrode particles can break, especially when you fast charge the battery.
But the question that was posed regarding failure when you plug-in the battery is specifically chemical in nature.
First some basics. For chemical stability, the battery should be operated within the stability window of the electrolyte. For water-based batteries, the stability window is 1.2V. Go above this window and you split water and make hydrogen gas and oxygen gas. This is about the time you should be wondering how lead acid batteries even work considering that their voltage is ~2 V, but that is outside of the scope of this post.
But getting back to our laptop, the stability window is ~3.2V. Meaning that when you operate the battery above this the electrolyte is oxidized on the positive electrode and reduced on the negative electrode. Remember that we only want to oxidize and reduce the “active” materials and don’t want to do anything else. All these reactions other than the ones we want are called “side reactions” and these are really bad for the battery. The nominal voltage of a laptop battery is 3.7 V which means that something bad wants to happen as we use the battery. Just because things want to happen does not mean that they actually do (for example, I want to buy a Tesla or a Volt, but...).
So long story short, stuff (e.g., passive layers and poor kinetics of reactions) happens and things are not as bad as they seem and you can increase the voltage up to 4.2V without bad things really happening. All chargers for Li-ion cells today cut the battery off when it reaches 4.2V. What you have to realize is that at 4.2V, these side reactions are present in finite amounts and start to chemically kill the battery, but its not that dramatic.
Operating to 4.1V makes things better and extends the life, 4.0 V is even better and so on. So why don’t battery manufacturers cut the voltage off at, say, 4 V to get better battery life? Because every time you cut this voltage down you decrease the capacity of the battery and its run time. The 4.2V cutoff is a compromise between good run time and decent (read “not pathetic”) life.
Were you supposed to understand all that? Not really, I just wanted you to know that I’ve really thought about these things. What you do need to know is that if you keep your laptop plugged in, you force your battery to remain at 4.2V continuously and these side reactions continue to happen and slowly kill the battery.
On the other hand, if you charge the battery and then pull the plug (so to speak), the battery discharges some, the voltage drops, and these reactions become less of a problem and your battery life goes up. So the best things you can do is to charge the laptop (or cell phone, camera etc.) and once its charged, pull the plug. Your battery will thank you for it.
As a matter of fact, if you own a Lenovo Thinkpad, you can actually change the state of charge to which you charge the battery using the “Battery Maintenance” utility. You can change this from charging to 100% state (where the voltage is 4.2V) to 90% so that your voltage is less. You lose some energy is doing that, but atleast you can change it to 100% when you need battery power and put it back down to 90% when you can plug in. I wish my Mac has the same feature.
This problem has implications for PHEVs and EVs. Lets say you have a 15 kWh PHEV pack. You come home after a 40 mile commute and you plug it in at 6:00 PM. Let’s say you have a 120V, 15A outlet, so that you can put out 1.8 kW of power. So the battery is going to charge in 8 hours.
By 2 AM you have a fully charged battery. If you leave your house at 8:00 AM, your battery is going to be sitting at 4.2V for 6 hours in any 24 hour period. This is not going to be good for the battery. It gets worse if you decide to bump the amp/volts on your house to charge it faster. So we need to get these batteries charged faster, but we also want to make sure to have smart chargers that don’t do what I’ve described above. Something to think about.
What does this mean for researchers? If someone can find an electrolyte that has a wide voltage window of stability, then this problem goes away. Or you can try to use materials that work within this window (For example A123 Systems battery does this on the positive side). But this means the battery has a lower voltage, which means it has lower energy and less run time. We don't want that, do we? Finally, we can try to isolate the electrode and the electrolyte and see if we can kinetically hinder these reactions. In the Battery Program at Berkeley we are actively working on this problem so that we can get more energy and better life.
In the mean time, remember to pull the plug.