A primer on lithium-ion batteries: how they work and how they are changing

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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.
According to RMI, “lithium-ion battery suppliers are poised to
reach at least 1,330 GWh of combined annual manufacturing
capacity by 2023.” According to S&P Global, “global LIB
capacity is set to increase 218% between 2020 and 2025.” That’s a
lot of scale.


The main thing to take from the boom in LIB manufacturing is that
any competitor to LIBs will need to take advantage of existing
manufacturing processes. “The way these battery factories are
building up now,” says Dan Steingart, a materials scientist and
co-director of Columbia University’s Electrochemical Energy
Center, “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.”


This will be important later; some LIB competitors can slipstream
into existing manufacturing and some can’t.


For Battery Week, I’m going to focus less on manufacturing (and
disposal) and more on the battery chemistries themselves — which
ones are dominating and which have a chance of catching on.


Li-ion is a family of battery chemistries


LIBs are not a singular thing, but a family. They have in common
that they use lithium in 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 different performance characteristics
(and, sigh, acronym). We’re not going to cover all of them,
though — even I have my limits. We’ll just hit 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 — incredibly
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 (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)


Why bother with any of these alternatives? Why not just stick to
NMC and NCA?


There are two sources of pressure on the industry to diversify.


LIBs face pressure to diversify performance


The first is performance. Most LIB innovation to date has focused
on energy density, for passenger EVs. In some applications,
though, like home energy storage or fleet vehicles, energy
density matters less than safety and cost. As use cases
diversify, so do performance demands.


With that in mind, let’s take a quick look at the various metrics
used to judge battery performance. RMI uses eight:


* energy density (Wh/L): energy per unit of
volume, or more prosaically, energy relative to space occupied,
sometimes called “volumetric energy density”;


* specific energy (Wh/kg): energy per unit of
weight, sometimes called “gravimetric energy density”;


* power cost ($/kW): cost per unit of power
output (to return to our river analogy: cost per unit of force
the river is capable of exerting at its peak);


* energy cost ($/kWh): cost per unit of energy
output (the amount of force exerted by the river over an hour);


* cycle life: the number of times a battery can
discharge and recharge before it falls below some threshold of
capacity (usually set at 80 percent) due to degradation;


* fast charge: how fast the battery can charge,
i.e., how fast it can accept power;


* safety: some batteries, particularly those
with cobalt, suffer from “thermal runaway,” which means if one
cell goes haywire and heats up, it heats up the next one, and so
on in a self-reinforcing cycle that results in fires and battery
recalls;


* temperature range: the range of temperatures
in which a battery can effectively operate.


As I said, it’s possible to optimize for one or a small set of
these, but doing so inevitably involves trade-offs in others.


This graphic from RMI compares some LIB chemistries along all
these axes. The dark green lines are current performance and the
light green is highest theoretically achievable level:


As you can see, different chemistries excel on different metrics
and will target different applications.


LIBs face pressure to diversify 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 amidst terrible human rights abuses. Lithium
and nickel are fairly nasty too, and may run into supply
constraints as the market grows (nickel, in particular, is a
source of current stress).


There’s lots of innovation underway to reduce the social and
environmental impacts of materials mining, and increase supply,
but, as we will see next week, there are also competing battery
chemistries that eschew these problematic materials entirely.


Smart manufacturers like Tesla are diversifying their battery
lines in anticipation of supply issues, trying to evolve away
from cobalt and secure a steady domestic supply of lithium and
nickel. (Biden’s infrastructure plan, which aims to kickstart a
domestic EV supply chain, could help.)


Some battery diversity will happen, the question is how much


You can find people in the battery field who stress that
conventional LIBs have too great a head start for anything else
to catch up. In its white paper on the future of LIBs, SILA
Nanotechnologies writes:


Technologies that claim they will replace Li-ion often grab
headlines, but scale limitations make that impractical within a
generation. It is for this reason that by 2050, while Li-ion will
not constitute all of energy storage, it will be the most
dominant chemistry by far, with most everything else relegated to
niche applications.


Lou Schick, director of investments at Clean Energy Ventures, a
venture capital firm that invests in clean technology projects,
stressed to me the importance of scale and familiarity:


The only selection criteria for any project is, is it bankable?
Can I get insurance for it? Is it consumer product? Any insurgent
that wants to get to that state and is in a lab right now is 10
years away from being bankable, if they are very successful. So
you're never catching up. And it has nothing to do with chemistry
or physics.


You can find others who believe diversity is inevitable. “It's
not like the Lord of the Rings, one ring to rule them all,” says
Michael Burz, an engineer who founded and runs battery company
EnZinc. “There will be different chemistries for different
applications.”


Among the analysts more bullish on diversity are 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 R&D 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 begin to find markets and scale up by
the mid-2020s.


Chloe Holzinger, an energy storage analyst at the research firm
IHS Markit, told me that diversity will be a market asset:


What we're going to see in the future is increasing diversity in
all three of those areas [anode, cathode, and electrolyte].
Automakers will be able to take advantage of this diversity to
make their portfolios robust against commodity price spikes and
distinguish themselves from other automakers — “we're the only
ones that provide this kind of battery.”


It’s possible that this disagreement amounts to less than it
appears. Even skeptics agree that some competitors might find
niches; the main disagreement seems to be over how fast that
might happen and how big the niches will be.


After all, says Schick, in trillion-dollar markets, “if the
market fragments by use case, the individual use cases can be
quite enormous.”


Conventional LIBs have a huge head start, but the pressure to
diversify may offer some hope to innovators both within the LIB
family and outside it.


In my next post, I’ll get into some of that intra-family
competition within LIBs, a space rife with ongoing innovation.


Mabel wishes everyone a Happy Spring.


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