How Does a Lithium-Ion Battery Work? Charging, Discharging and Wearing Out
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The same kind of battery runs a phone, a laptop, an electric car and, increasingly, the power grid overnight. A lithium-ion cell stores energy by moving lithium ions from one layered material to another and gets it back by letting them return, and almost everything about how it behaves, its capacity, its slow decline, its rules about charging and its occasional fires, follows from that one movement.
Two hosts and a shuttle
A cell has two electrodes separated by a thin porous membrane soaked in a liquid electrolyte. The negative electrode, the anode, is graphite, whose carbon atoms form sheets with gaps between them. The positive electrode, the cathode, is a metal oxide, typically containing cobalt, nickel, manganese or iron phosphate, also built in layers or channels. Both materials can hold lithium ions between their layers without changing their own structure, a property called intercalation.
Lithium ions shuttle between the two through the electrolyte, which conducts ions but not electrons. The electrons have to travel the long way round, through the external circuit, and that flow of electrons is the current that powers the device. The membrane keeps the electrodes from touching while letting ions through.
Discharging and charging
In a charged cell the lithium is stored in the graphite, where it is at high energy and would rather be in the oxide. When the circuit is closed, lithium atoms in the graphite give up an electron, the electron flows out through the device, and the resulting ion crosses the electrolyte to the cathode, where it meets the electron again and settles into the oxide. The cell delivers energy until the graphite is empty of lithium.
Charging reverses it. A charger pushes electrons back into the graphite and pulls them from the cathode, and the ions follow, crossing back and lodging in the carbon sheets. The voltage of a single cell, around 3.6 to 3.7 volts, comes from the difference in how tightly the two materials hold lithium, and it is roughly three times that of the older nickel-based cells, which is the main reason lithium-ion took over. Lithium is also the lightest metal, which is why the cells store so much energy per kilogram.
Why capacity fades
A new phone battery lasts all day and a three-year-old one does not, because the shuttle slowly loses passengers. On the first charge the electrolyte reacts with the graphite surface to form a thin protective film, and that film keeps growing slightly with every cycle, consuming lithium each time. Some lithium plates onto the graphite as metal instead of slipping between the layers, particularly when charging is fast or cold, and that lithium is lost too. The cathode's crystal structure cracks a little each time it swells and shrinks.
Each effect is small, but after several hundred cycles they add up to a cell that holds perhaps 80 percent of its original charge. Heat speeds all of them up, which is why a phone left on a dashboard ages faster, and so does time spent at the extremes of charge. Keeping a battery between about 20 and 80 percent when convenient, and avoiding heat, is the practical advice that follows from the chemistry, and it is what electric cars do automatically with their own packs.
The management system
A lithium-ion cell is unforgiving of abuse. Charged above its limit or drained fully to zero, its electrode materials break down permanently. So every battery ships with a small circuit, the battery management system, that stops charging at the upper limit, cuts off discharge before the lower one, balances the cells in a pack so that none runs ahead of the others, and monitors temperature. When a phone reports 0 percent it is protecting a cell that still has a little charge left; when it charges quickly to 80 and slowly after, the circuit is protecting the graphite from plating.
Why they sometimes burn
The electrolyte is a flammable organic liquid, and the charged cell stores a great deal of energy in a small space. If the membrane is breached, by a manufacturing flaw, physical damage or a spike of plated lithium growing through it, the electrodes short-circuit internally, the cell heats, the heat drives reactions that release more heat and oxygen from the cathode, and the process runs away. Thermal runaway is why damaged or swollen cells should not be charged, why airlines ban loose batteries from the hold, and why the rare fires in phones and scooters are so hard to put out.
Manufacturers reduce the risk with membranes that shut their pores when hot, chemistries such as lithium iron phosphate that release less oxygen, and cooling systems in car packs. The next generation aims to replace the liquid with a solid electrolyte that cannot burn at all. The cell at a glance:
- •Anode: graphite sheets that hold lithium when charged
- •Cathode: a layered metal oxide that holds lithium when discharged
- •Electrolyte: a liquid that carries ions but not electrons
- •Separator: a porous membrane that keeps the electrodes apart
- •Management circuit: limits on voltage, current and temperature
The takeaway
A lithium-ion battery stores energy by packing lithium ions into graphite and releases it by letting them cross to a metal oxide while the electrons take the long way round through the device. It fades because a little lithium is lost on every cycle, it needs a management circuit to stay within safe limits, and it burns when an internal short lets its stored energy out all at once.