The building blocks of energy storage
The materials behind lithium ion batteries
“First off, Sasha, why should I read your 13 minute article???”
The device you are reading this on right now is likely powered by a lithium ion battery.
If you’ve ever used a phone, computer, EV, hand-held power tool, and so many other battery operated devices, there’s a huge chance you depend on lithium ion batteries to power your life.
These devices are essential now, and are likely going to become even more essential in the coming years. Might be worth it to understand them.
Plus, they’re just kinda cool :D
hii! If you want to learn how lithium ion batteries work as a preface to reading this article, check out this video :)
Why lithium-ion?
There is basically one reason we use lithium ion batteries (Li-ion) over other types of batteries: the energy density. The higher the energy density in the material, the more energy the battery can store.
If we look at this table of electrochemical equivalence, we can see commonly used anode (blue) and cathode (red) materials along with their Ampere hours per gram (4th column).
Electrochemical equivalence compares the amount of a substance that undergoes a chemical reaction to the amount of electricity produced. Although it is not the same as energy density, higher electrochemical equivalence tends to equal higher energy density.
An ampere, or an amp, is a measure of current, or how many electrons are flowing through a circuit. In our case, with an ampere hour, its how much current can flow per hour.
So with hydrogen, with 1g, you can generate 26.59 ampere hours (Ah).
As you can see, lithium (Li) is second on that list, with 3.86 Ah/g, which is pretty good. (Note: I’m looking at Ah/g, not g/Ah). It has the highest of any other material, which means it’s going to give us the highest energy density.
Additionally, Li is fairly high up on the periodic table, meaning it’s going to be quite light. (This contributes to the high energy density.) This also makes it attractive for batteries, because I don’t know about you but I wouldn’t want my phone to weigh like 8 pounds. XD
You might have realized that lithium is listed under the anode materials. It was originally used in anodes, however, they switched it to the cathode. The biggest reason for this was the safety. If you have heard anything about Li-ion battery thermal runaway, then you have seen the effects of dendrite formation.
Dendrites are metallic nanostructures that form on the negative electrode (anode) of the battery. These can end up piercing the separator (which separates the anode (-) and cathode (+) and prevents them from shorting), and cause thermal runaway.
Likely something we’d want to avoid. There is also one more relevant reason as to why we use lithium in the anode, and that is the standard reduction potential.
Basically, in a battery (Voltaic cell) something called a redox reaction occurs. One element gains electrons and is reduced, and the other loses electrons and is oxidized. (I know, its confusing. Here’s a trick: Anode = Oxidation, Reduction = Cathode. An Ox, Red Cat. This tells you where the oxidation and reduction reactions happen.)
Standard reduction potential is how likely/how easy it is for that element to undergo that reduction reaction.
Learn more about redox reactions -> Electrochemistry: Crash Course Chemistry #36 — YouTube
As you can see, Lithium again has the highest. But anyways, by using some variation of lithium as the cathode, we get a higher energy density which allows us to output more power for longer.
TL;DR of why we use Lithium:
- light weight (+ high energy density)
- high voltage (from the high standard reduction potential)
- high electrochemical equivalence
- good conductivity
- soft + ductile (easy to manipulate into thin sheets for use in a battery)
And for why we use lithium ion, that’s because Li+ ions are the transport carriers in this battery, which comes as a result of using lithium in the cathode. Lithium ion batteries have the added benefits of high energy density, long lifetime, and low weight.
The catch
Unfortunately, as with almost any seemingly perfect thing, there are a few slight drawbacks. The biggest one, in the case of Li-ion batteries being the sustainability. I’ve dove deep into that previously so I’ll spare you the details, but if you’re interested you can learn more here.
Basically, Li-ion batteries depend on things like cobalt and nickel which are toxic both while mining and after use, where 95% of all batteries end up in a landfill, leaching toxins and causing fires.
So yeah, not the best per say. :’)
And then the other relatively significant problem is that these batteries are too expensive. As I’m sure you’ve heard, we’re trying to switch away from fossil fuels and to things like solar and wind. But since the sun isn’t always shining and the wind isn’t always blowing, we need a way to store that energy for later use.
And while there are a whole bunch of other ways to store energy, it seems like that for the coming years, batteries are still going to top the list.
To be able to make the switch to renewables, the generation plus the storage needs to be less than the cost of natural gas. Tall order, but huge impact if it’s done.
Soooo. How can we fix this?
I’m pretty sure the simplest way to reduce the cost and make them more sustainable is to change the materials these batteries are made of.
The toxic nature of cobalt and nickel (+ other materials we’ll dive into later) as well as the environmentally intensive mining processes are what’s making these batteries unsustainable.
And similarly, the cost of these materials due to their relative scarcity and complicated mining processes can be quite high.
Here’s a table I made of all the elements and their cost written below them. Look at the cost of lithium. 👀I also put a square around the elements that are $4> in blue, and those that are 4–7$ in red.
Abundant materials have a tendency to be cheaper and also less toxic. So: can we make a battery out of abundant materials?
That’s what I want to figure out.
What are Lithium ion batteries even made of?
First thing I want to mention is that a battery has a ton of different parts and pieces, most of which I didn’t know about until a few weeks ago when I started diving deep. I’ve broken them down into 9 major components. As a general structure, for each component, I’ll tell you:
- the name of the component,
- show you where it is on a diagram,
- what it does in first principles,
- the status quo of it in Li-ion,
- ideal properties and
- potential improvements (assuming we are only looking for performance enhancements)
Also, keep in mind this is an interconnected system, so the properties of each depend on compatibility of each component. We’ll look at that later on.
Ready? 🚀
Anode
The anode is the negative terminal in the battery, and if you remember from before its where oxidation reactions happen (An Ox).
Purpose: is to store ions when the battery is charged.
Status Quo: in LIBS, it’s graphite: the same stuff that makes up your pencil. It’s a hexagon lattice of carbon atoms, stacked in layers. Specifically, mesophase carbon microbeads (MCMBs).
Ideal Properties: This material needs to allow for good Li+ mobility and storage capacity. It should also be able to withstand high charge and discharge rates. It should also have minimal volume changes during this, and have thermal stability.
Advancements: One company, called Sila, is working on this: having an anode made of silicon and carbon. Another alternative is silicon coated with carbon. This is because silicon has a higher energy density and also lets us charge and discharge faster. With things such as EVs, this is a massive factor. I mean, would you rather wait 4 hours or 10 mins for your car to charge?
One study also found that CoSi2, TiP, and NiSi2 have superior properties when compared to graphite. The properties they analyzed included the cell potential, volume change per lithium incorporated, and gravimetric and volumetric capacity.
cell potential: how much voltage the battery can theoretically generate
gravimetric capacity: amount of electrical charge a material can store/release per gram
volumetric capacity: amount of electrical charge a material can store/release per cm2/mm2.
Using Li metal as an anode also has higher specific capacity (charge stored per unit mass).
Cathode
The cathode is the positive terminal in the battery, and it’s where reduction reactions happen.
Purpose: accept Li+ ions during charging.
Status Quo: depends. There are a variety of different cathode materials. The one in your phone is LiCoO2, there are also LiMnO2 and LiFePO4, to name a few. These can be notated as LCO, LMO, and LFP, respectively. When it comes to grid storage, LFP is preferred because of its’ high safety.
Ideal Properties: high energy density, thermally and chemically stable, able to tolerate high rates of charge/discharge, and lightweight.
Advancements: (For LFP) The major drawback of the LFP anode is it’s low conductivity, and by coating the LiFePO4 anode with carbon (conductive layer), we can improve the electrochemical performance without sacrificing energy density. The ideal amount is >2 wt% (percent of the weight). Combining this carbon coating with FeP/FeP2 can also help enhance this effect.
Mn (manganese) can also be doped into the lattice, because it helps to improve the conductivity as well as reduces the charge transfer interface between the electrode (anode/cathode) and electrolyte. And because of Mn’s impact on the voltage potential, it can lead to higher energy density.
Separator
Purpose: separate the cathode and anode physically and electrically while allowing ion transport.
Status Quo: typical LIBs use polyolefin, which is a type of polymer. It has weak resistance to heat and poor interaction with electrolytes, so could use some improvements. Most separators have a porosity of 40–50%.
Ideal properties: high porosity and smaller pore size (good for high energy + preventing shorts), mechanical strength, thin for high energy (but not too thin so that is is not strong), thermal stability, compatibility with electrodes and electrolyte, and the ability to trigger battery shutdown (to prevent thermal runaway, aka burned down Tesla).
Advancements: nanofiber based non-woven mats, because they have enhanced turtuosity (full of twists and turns), porosity, and chemical stability.
Electrolyte
The electrolyte is typically a solution dispersed throughout the whole cell.
Purpose: allow ions to move easily back and forth between cathode and anode.
Status Quo: usually propylene carbonate based. Electrolytes have a salt and a solvent, the first of which is typically LiFP6 (lithium hexafluorophosphate) and the second of which is typically a combo of organic carbonates — eg ethylene carbonate and diethyl carbonate.
Ideal Properties: high ionic conductivity, low flammability, thermally stable.
Advancements: low concentration (diluted solution of salts + solvents) leads to better performance. One paper found that lithium bis(oxalate)borate salt with a concentration of 0.6M/L combined with 0.1M/L of LiBF4 and ethyl methyl carbonate, ethylene carbonate, and diethyl carbonate. Another study found that lithium bis(fluorosulfonyl)imide, dimethoxymethane and tetrafluoropropene with a weight ration of 1:0.5:4.92, respectively.
As you can probably tell, the electrolyte is one of the more complicated parts of the battery. And it has a very important role to play. People have even looked into batteries with a solid electrolyte, called Solid State batteries. These have some benefits such as no fire risk and also smaller volume. However, they also have some challenges such as low contact between electrolyte and electrode. (I saw a cool solution to that last issue, check it out here.)
Current collectors
Purpose: collect electrons generated/consumed by reactions in the battery and transport them to the opposite electrode.
Status Quo: Aluminum for the cathode and copper for the anode.
Ideal Properties: conductive and malleable.
Advancements: If you coat the aluminum with carbon you can improve the performance. Also, 3D structing the materials can help because it can enhance conductivity and reduce weight.
Casing
Purpose: hold the materials of the battery in place, prevent the electrolyte from leaking out, and providing mechanical strength.
Status Quo: metal/hard plastic. Most common is nickel coated steel.
Ideal Properties: light, mechanically strong as well as thermally and chemically stable.
Advancements: Using aluminum allows the casing to become 63% lighter, which increases the energy density by upwards of 25%. And there are no added safety challenges when this switch is made. Additionally, boron nitride can also be used to optimize surface roughness and help with thermal properties.
Adhesive materials + binders
Dispersed throughout the battery.
These are essentially the same thing, they differ only in where they are used. Binders are used solely in the electrodes, which adhesive materials are used throughout the battery.
Purpose: hold battery materials flush with each other to allow for good conduction and no penetration of air or dust. Also, to provide flexibility when the electrodes undergo thermal expansion.
Status Quo: Polyvinylidene Fluoride (PVDF), Carboxymethyl Cellulose (CMC), Styrene-Butadiene Rubber (SBR), and Ethylene Propylene Diene Monomer (EPDM).
Ideal Properties: good adhesive, chemically and thermally stable, as well as flexible.
Advancements: polyacrylic acid as a binder can enhance performance at high temperatures.
Additive Materials
Dispersed throughout the battery.
Purpose: used to improve various aspects of the battery (eg safety, performance, longevity.)
Status Quo: really depends on what property you are trying to enhance as well as what the surrounding materials are.
Ideal properties: again, depends.
Advancements: In the first charge/discharge cycle, 5–20% of the Li can be lost, decreasing battery capacity. To improve this, adding Li2Se can be converted to Se at low voltage, supplying extra lithium. (Adding 6wt% increases initial specific capacity by 9% in Li||LFP cells and boosts energy density by 19.8% without sacrificing other battery performance). Li2NiO2 also decomposes at a low voltage during initial charging, which gives extra capacity to compensate for capacity loss in the negative electrode.
The decomposition of the carbon anode can be minimized by adding vinylene carbonate (VC).
Some specific problems with LFP cathodes are iron dissolution and capacity fading, to solve these adding additives that have the potential to react with HF and H2) are best as those compounds are what are causing the problem.
The low electronic conductivity of LFP and its rate capability at low temperature can be improved by adding low concentration BS and FEC — they form a conductive SEI film
Fluorine containing additives help to create a stable SEI layer and cathode electrolyte interface films which improve battery performance. They can also serve as a fire retardant
Succinonitrile can improve thermal stability and increase the oxidation electrochemical window of conventional electrolyte → 1wt%. Biphenyl has the potential to be an additive that prevents overcharging and thermal runaway.
Terminal Materials
Dispersed throughout the battery.
These include current collectors (but those are important so I separated them), terminal tabs, terminal caps, lead wires, and insulating materials. Seeing how most of them are similar with the exception of insulating materials, I’m going to treat insulating materials as a mini-group.
Purpose: connect to external circuitry and provide good conduction of electrons.
(For insulating materials): electrically isolate different components of the battery and help to maintain stable operating temperature.
Status Quo: For insulating materials: polyethylene and polypropylene. For most of the other terminal materials: copper and aluminum. Also interesting to note: the thickness of the terminal materials depends on the application. For example, EVs need thicker terminal materials.
Ideal Properties: Ideal terminal materials have high electrical conductivity, corrosion resistance, mechanical strength and heat resistance. Ideal insulating materials should, be obviously, insulating, and then all the other properties listed previously.
Advancements: This is more for the terminal materials, but tab design can have a huge role. Tabs are thin strips of metal that go from the electrode, connect to the current collectors, and then extend to the exterior of the battery. Opposite side tab placements allow for uniform current distribution. Widening the tabs can also greatly reduce the maximum temperature of the battery. Also, modifying the cap design of the battery to include ventilation can improve thermal stability of the overall battery.
From what I’ve read, it seems that the cathode, anode, and electrolyte are the most important and complex materials as it relates to energy storage. The separator also comes close, and plays a huge role in the safety of a battery.
Batteries are incredibly complex devices, much more complex than I had previously assumed when charging my phone/laptop. I’d say that they are something we take for granted that it just works. We don’t know how, but it does.
Learning more about the materials in lithium ion batteries was quite interesting to me, and I hope it was to you as well. :)
Thanks so much for reading!! I hope you have a wonderful rest of your day, and feel free to connect with me on LinkedIn :)
