How do lithium-ion batteries actually work? David Roberts explains

It’s still battery week, and it’s finally time to get anodes and cathodes straight.

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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.

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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.

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 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.

LIB manufacturing

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.

Image credit: Tesla

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

Image credit: Tesla via Electrek

I’m not going to get into this aspect of LIB innovation too deeply, other than to note there’s a lot going on there.

The manufacturing techniques that produce LIBs are continuously being refined, a process that is accelerated by scale. According to S&P Global, global LIB capacity is set to increase 218% between 2020 and 2025.” That’s a lot of scale.

The key takeaway from the boom in LIB manufacturing is that any competitor to LIBs will need to take advantage of existing manufacturing processes. According to Dan Steingart, a materials scientist and co-director of Columbia University’s Electrochemical Energy Center, The way these battery factories are building up now, they’re so capital-intensive that whatever chemistries come next will be produced and manufactured in such a way that they leverage existing infrastructure if at all possible.”

For Battery Week, I’m going to focus less on manufacturing (and disposal) and more on the battery chemistries themselves.

LIBs are a family of battery chemistries

LIBs are not a singular thing, but rather a family of batteries. What they have in common is that they use a lithium compound as either the cathode or anode and exchange charged lithium ions.

This leaves quite a bit of room for different chemistries. There are many types of lithium compounds, many choices of anode or cathode materials to pair with them, and many choices of electrolytes.

That yields a very large matrix of possible combinations and chemistries, each with its own unique performance characteristics (and, sigh, acronym). We’re not going to cover all of them, though — only some of the most-discussed alternatives.

The most common LIB chemistries used today are lithium nickel manganese cobalt oxide (NMC) and lithium nickel cobalt aluminum (NCA), which use compounds of those metals as the cathode. Lithium and nickel turn out to be a knockout combo: light and energy-dense. Nonetheless, there are others.

Here’s a list of the LIB chemistries we will at least touch on starting in my next post:

  • Lithium nickel manganese cobalt oxide (NMC cathode)
  • Lithium nickel cobalt aluminum (NCA cathode)
  • Lithium ferro phosphate or lithium iron phosphate (LFP cathode)
  • Lithium manganese oxide (LMO cathode) and lithium manganese nickel oxide (LMNO cathode)
  • Lithium sulfur (Li-S, sulfur cathode)
  • Lithium metal (anode) and solid state
  • Lithium titanate (LTO anode)
  • Lithium air (Li-air, lithium anode)

You can find people in the battery field who think that conventional LIBs have too big a head start for anything else to be able to catch up. And you can find more bullish analysts who believe the market will begin to diversify soon, like those at RMI, who wrote a report in 2019 called Breakthrough Batteries that surveyed possible competitors to conventional LIBs. They write:

Unlike the market development pathway for solar photovoltaic (PV) technology, battery [research and development] and manufacturing investment continue to pursue a wide range of chemistries, configurations, and battery types with performance attributes that are better suited to specific use cases.

RMI is convinced that other battery chemistries with other performance attributes will start to find niche applications and scale up by the mid-2020s. To wrap up our battery basics post, let’s take a look at the performance attributes in question.

Metrics of battery performance

RMI measures different types of batteries against eight separate categories of performance:

  • Energy cost: cost per unit of energy output, which will depend not only on the chemistry but also on the total pack price, including cooling systems and casing.
  • Energy density (Wh/​l): energy per unit of volume, or more prosaically, energy relative to size.
  • Specific energy (Wh/​kg): energy per unit of weight
  • Power cost: cost per unit of power (i.e., oomph)
  • Cycle life: how many times a battery can discharge and recharge before it falls below some threshold of capacity (usually 80 percent) due to degradation
  • Fast charge: how fast the battery can charge
  • Safety: some batteries, particularly those with cobalt, are prone to thermal runaway,” which means that if one cell goes haywire and heats up, it heats up the next one, and so on and so on in a self-reinforcing process that results in fires and battery recalls
  • Temperature range: the range of temperatures in which a battery can effectively operate

It’s possible to optimize for one or a small set of these, but as I said in my last post, doing so inevitably involves trade-offs in others.

That brings us to this graphic from RMI, which compares some LIB chemistries along all these axes. The dark green lines are current performance and the light green is highest theoretically achievable level:

Aside from all the standard business motivations you’d expect — cost, manufacturing scale, etc. — there are two overarching factors driving innovation in the LIB space.

The first is performance diversity. Some use cases call for more energy density; others call for more safety or lower cost; and so on. Most innovation to date has focused on energy density, but as use cases diversify, so do performance demands.

The second is materials. Cobalt, used in standard NMC and NCA chemistries, is highly toxic, comes almost entirely from the Democratic Republic of the Congo, and is mined in terrible working conditions that frequently spur charges of human rights abuses. Nickel and lithium are less nasty in and of themselves, but they may run into supply constraints as the market grows. (Nickel, in particular, is a source of current stress.)

Most of America’s EV material supply chain is imported. U.S. manufacturers have been pushing to develop domestic supply of both lithium and nickel — Biden’s infrastructure plan could help.

Smart manufacturers such as Tesla and others are diversifying their battery lines in anticipation of supply issues, trying to evolve away from cobalt and attempting to secure a steady supply of lithium and nickel.

So there you have the basics of LIBs. In my next post, I’ll get into the innovation going on around various competing LIB chemistries.

This article was originally published at Volts. Image article courtesy of Brookhaven National Laboratory.

David Roberts is editor-at-large at Canary Media. He writes about clean energy and politics at his newsletter, Volts. Previously, he covered the same subjects at Vox and Grist.