We’ve discussed 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 benefits and are competing for different market niches.
Before that, though, it’s worth the time to do a little review of batteries and how they work. 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 deep chemistry — believe me, no one wants that — but I thought it would help to run through some basics and some terms. It will come in handy later when we get into the competition within battery markets.
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 a battery is the cell. In the cell, two electrodes — negative (anode) and positive (cathode) — are separated by an electrolyte. The anode is a material that gives up electrons easily in chemical reactions with the electrolyte. The cathode is a material that readily absorbs 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 greater the potential.
When the anode and cathode are connected in a circuit, two things happen.
First, 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 × voltage. It’s like a river: The force exerted by the water depends on how much of it 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.
Second, 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 and returning the electrons and the ions to the anode.
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, has a long lifetime, 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 how much energy it will hold; and so on.
The battery market has seen dozens of chemistries come and go, but four have stuck and scaled to achieve mass-market penetration: lead acid, nickel-cadmium (Ni-Cd), nickel-metal hydride (NiMH) and lithium-ion (Li-ion).
Most of the developing world still uses lead-acid batteries, a $45 billion global market. But lithium-ion batteries have been gaining ground rapidly in wealthy markets.
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 have become cheap, and manufacturing capacity has converged around them.
Let’s take a closer look at LIBs.
Lithium-ion batteries 101
LIBs have been around in commercial form since the early 1990s, though obviously they’ve improved quite a bit since then.
Today’s most common and popular LIBs use graphite (carbon) as the anode, a lithium compound as the cathode and some organic goo as an electrolyte. They boast two key advantages over prior battery chemistries.
First, they need very little electrolyte. They are what’s known as “intercalation” batteries, which means the same lithium ions nestled (intercalated) in the structure of the anode transfer to be intercalated in the cathode during discharge. The electrolyte only has to serve as a conduit; it doesn’t have to store many ions. Consequently, the cell doesn’t need much of it. Saving on electrolyte saves space and weight. (Bonus: The process is almost perfectly reversible, which gives LIBs their high cycle life.)
Second, LIBs squeeze lots of energy into a small space. Lithium is the lightest metal (at the upper left corner of the periodic table) and extremely energy-dense, so LIB cells can work with electrodes only 0.1 millimeters thick. (Compare lead-acid electrodes, which are several millimeters thick.) This also makes LIBs smaller and lighter.
Because they are lightweight and have high energy density, LIBs got their initial foothold in small electronic devices, phones, laptops and the like. They scaled up quickly to run handheld power tools and lawnmowers and then completely took over electric vehicles. Recently they’ve scaled up further to create giant stationary battery arrays for grid storage.
It’s worth noting that even the biggest grid battery is just stacks upon stacks of cells, like Lego bricks. LIBs are extremely modular — they can be scaled precisely to need.
There are a number of ways to manufacture LIB cells, but the most common for portable and EV applications is the cylindrical cell. Think of it like a jelly roll. A super-thin metal anode is coated with a film (usually graphite). Then a super-thin separator is laid on top. Then a super-thin metal cathode coated with a film (usually some lithium compound) is laid on top of that. Several layers are stacked this way, and then then whole thing is rolled up and packed into a cylinder. Before the cylinder is capped, electrolyte goo is injected to infuse between the layers.
Cells are then clustered together into modules, which in turn are clustered together into packs.
There’s a whole active area of LIB innovation around cell design. Tesla recently debuted a new, bigger cylinder cell, the 4680 (46 millimeters wide, 80 mm tall), with improved… well, everything.