A primer on lithium-ion batteries: how they work and how they are changing
(If you don’t want to read, you can listen. Just click play above.)
Greetings! Welcome back to Battery Week here at Volts. In my last post, I went over why lithium-ion batteries (LIBs) are so important to decarbonizing both transportation and the electricity sector.
Next week, we’re going to get into the nuts and bolts of different kinds of LIBs, to see how different chemistries offer different kinds of performance and are competing for different market niches.
Before that, though, it’s worth the time to do a little review of battery basics. If you’re like me-a-month-ago, you probably have a hazy understanding at best of the structure of batteries and the processes involved in running them.
I’m not going to get into any complicated chemistry — believe me, no one wants that — but I thought it would be helpful later, when we get into the competition within battery markets, to have some rudimentary terms and concepts clear in our heads.
Batteries 101
F’ing batteries, how do they work?
As the name suggests, electrochemical batteries store energy via chemical reaction. Discharging the battery involves a chemical reaction that produces electrons; recharging the battery involves a chemical reaction that stores electrons.
The basic unit of the electrochemical battery is the cell. In the cell, two electrodes — negative (anode) and positive (cathode) — are separated by an electrolyte.
When the anode and cathode are connected in a circuit, two things happen.
- Negatively charged electrons flow from the former to the latter, generating power. The amount of power is determined by two factors:
* current, the number of electrons traveling in a given circuit, and
* voltage, the force with which the electrons are traveling.
Power = current X voltage. It’s like a river: the force exerted by the water will depend on how much there is and how fast it’s moving. You can get the same force with less water if it moves faster, or with slower water if there’s more of it. Similarly, you can get the same power with less current if you have more voltage, and vice versa.
- The anode releases positively charged ions into the electrolyte, to balance the reaction, and the cathode absorbs a commensurate amount. (Some batteries have a thin semi-permeable barrier within the electrolyte to regulate the flow of ions.)
Recharging a battery basically involves reversing the reaction, returning the electrons and the ions to the anode.
The anode will be a material that gives up electrons easily in chemical reaction with the electrolyte. The cathode will be a material eager to absorb them. The propensity to shed/absorb electrons is known as standard potential, and the difference in standard potential between the anode and cathode will determine the battery’s total electrical potential. The bigger the difference, the more potential.
The whole game of battery design and development is to find a combination of anode, cathode, and electrolyte that performs well along a broad set of criteria — holds a lot of energy, releases energy quickly, operates safely, lasts a long time, is cheap, etc.
The tragedy of battery development is that there are always trade-offs. High performance on one criterion generally means lower performance on another. Optimize for holding more energy and you limit how quickly energy can be released; optimize for safety and you limit energy density; and so on.
Battery development has seen dozens of chemistries come and go, but four have stuck and scaled to mass-market size: lead acid, nickel cadmium (Ni-Cd), nickel metal hydride (NiMH), and lithium-ion (Li-ion).
LIBs have hit on a combination of anode, cathode, and electrolyte that performs well enough along several criteria (especially cost) to work for most short-duration applications today. They dominate consumer electronics, electric passenger vehicles, and short-duration grid-scale storage, and are expanding in other markets as well
