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.

1. 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.

2. 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 (though lead-acid batteries remain a $45 billion global market). They have gotten very cheap and a large-scale manufacturing capacity has grown up 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. LIBs 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 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 high energy density, LIBs got their initial foothold in small electronic devices, phones and 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 home storage batteries and giant stationary battery arrays for grid storage.

It’s worth noting that even the biggest LIB installation is just stacks upon stacks of cells, like Legos. LIBs are extremely modular — they can be scaled precisely to need.

LIB manufacturing

There are a number of ways of manufacturing LIB cells — button cells, pouch cells, prismatic 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 the whole thing is rolled up and packed into a cylinder. Before the cylinder is capped, electrolyte goop is injected to infuse between the layers.

Cells are then clustered together into modules, which are in turn clustered together into packs.

There’s a whole active area of LIB innovation around cell design. Tesla recently debuted a new, bigger cylindrical cell, the 4680 (46 millimeters wide, 80 mm tall), with improved … everything — energy, range, and power.

Tesla is also putting these cells together into packs that form part of the structure of their vehicles, which will reduce overall weight and complexity.

I’m not going to get into LIB manufacturing innovation too much, other than to note there’s a lot going on there.

The manufacturing techniques that produce LIBs are being continuously refined, a process that is accelerated by scale.