Dr Euan McTurk, Plug Life Consulting Ltd
Electric vehicles saw a subtle, but significant evolution in batteries during the 2010s. For example, the BMW i3’s battery capacity doubled between 2013 and 2019, using the same physical size of cells. However, nothing particularly exciting happened regarding the chemistry of those batteries. They were almost always NMC – lithium nickel manganese cobalt oxide – with gradually decreasing cobalt content and improved energy density (in other words, how much range you can pack into a given weight or size of battery).
Now, we’re starting to see some seriously exciting battery developments on the horizon, all of which have important implications for EVs, and will result in a diversification of the chemistries offered in different makes and models of electric vehicles. Here, we’ll look at my personal top three favourite battery chemistries that are due to hit EVs soon – and despite the hype, you may be surprised to learn that none of them are solid-state.
Today’s EV batteries
First, let’s look at what’s in an EV battery today. Lithium-ion cells consist of a negative electrode (anode) that is usually made of graphite – into which lithium ions are safely intercalated (pigeon-holed) – and a positive electrode (cathode) made of some form of lithium metal oxide, usually NMC or the similar NCA (lithium nickel cobalt aluminium oxide). These are coated on copper and aluminium “current collector” foils, respectively. The anode and cathode are separated by a polymer separator, wetted in an organic electrolyte containing a lithium salt, which allows lithium ions to travel between the two electrodes, but not electrons – otherwise you’d have an internal short circuit and the cell would discharge itself very quickly. Graphite is bulky, and cobalt is expensive. All of these are important points upon which to improve.
Lithium iron phosphate and LMFP
The first of my top three battery chemistries is already offered in some of the best selling EVs today, including the Standard Range Tesla Model 3 and Y, MG4 and BYD’s electric cars. Lithium iron phosphate (LFP) replaces NMC in the cathode. It contains no cobalt or nickel, so is cheaper and more ethical, and on top of this, it has a longer cycle life (can be charged and discharged more times before its capacity degrades to the point that it’s no longer suitable for your needs) and doesn’t catch fire if severely damaged. BYD’s Blade LFP cell doesn’t even get hot if drilled straight through when fully charged!
However, this article is meant to be about upcoming chemistries. It would be cheating to merely point out that established automotive giants like Ford, Mercedes and Volkswagen all plan to offer LFP in some of their electric vehicles in the near future. However, LFP has one small drawback: it isn’t as energy dense as NMC, so electric vehicles equipped with LFP batteries can’t travel as far on a single charge. This is because the voltage of LFP cells is less than NMC, and since the energy contained in a cell is equal to cell voltage multiplied by its capacity in ampere hours, the range per charge consequently takes a hit.
LFP’s range issue is on the cusp of being solved by LMFP, whereby manganese (or another metal) is added alongside the iron and phosphorus in the cathode. This boosts the cell’s voltage without adding much cost, and brings the energy density of this cobalt-free, cheaper, safer and more ethical chemistry close to that of NMC. Tesla was rumoured to be introducing LMFP in the refreshed Model 3, which would give the Standard Range model a range in excess of 300 miles per charge, but this has yet to happen. However, watch this space: in addition to the Tesla rumours, Gotion, a cell manufacturer backed by Volkswagen, plans to start manufacturing its LMFP cells this year.
Silicon anodes
As mentioned previously, graphite is bulky: to safely store just one lithium atom in the anode, six carbon atoms are required. Ideally, we want to minimise the amount of material that stores the material that actually stores energy, and one way to do that is with silicon anodes. One silicon atom can hold four lithium atoms, and allows them to move in and out of the anode much quicker than in graphite, which gives the win-win of improved energy density and faster charging.
However, silicon has a problem: it expands a lot during charging – up to 300% of its original size. As the silicon anode swells and contracts during cycling, it mechanically breaks itself apart, resulting in a short lifespan for the cell. The good news is that a number of startups have overcome this problem. One example is Amprius, which grows silicon nanowires onto the current collector that connects the anode with the outside world (and the electric vehicle that the battery is powering). This forest of silicon trees can expand and contract without breaking each other, resulting in a much-improved lifespan. Amprius’ approach results in double the gravimetric and volumetric energy density vs today’s leading NMC batteries – in other words, the range per charge of today’s electric vehicles could be doubled by using this silicon anode, or could stay the same but with half the weight and space required, which is ideal for smaller, lighter, more efficient cars.
Several electric vehicle manufacturers are working with silicon anodes for upcoming models. Mercedes claims that their all-electric EQG shall offer a silicon anode battery pack when launched this year, which makes use of Sila’s anode material and vastly reduces not just charging times, but the carbon footprint of the battery too. Meanwhile, Polestar is working with StoreDot to test the latter’s silicon-dominant, ultra-fast charging batteries in the Polestar 5 prototype.
Sodium-ion
While there is a lot of focus on how make batteries that provide the most range for an electric vehicle, some of the most exciting developments will result in slightly less range vs today, but with huge advantages in other areas. Sodium-ion is a prime example of this: it eliminates cobalt, nickel, copper and lithium, replacing them with cheaper and far more abundant materials with well-established supply chains. After all, sodium is found in sea salt.
Sodium-ion cells don’t pose a fire hazard if severely damaged; perform better than lithium-ion in freezing temperatures; don’t require as much thermal management (battery cooling/heating) as lithium-ion cells; and are much easier and safer to ship. So, not only are they cheaper to make and manage, but they’re cheaper to transport too, which delivers a double cost saving to consumers. Their downside is that their energy density is less than LFP, so the range per charge of EVs equipped with the first generation of sodium-ion cells will arguably best suited to urban drivers, but that’s still a valid part of the market, and one that needs affordable cars. On that note, the first two sodium-ion equipped city cars have gone on sale in China, both with price tags of under £8,500. With Chinese, European and US manufacturers working on sodium-ion cells as we speak – including BYD building a new gigafactory dedicated to sodium-ion cell production – it shouldn’t be too long before we see the first sodium-ion EVs being offered for sale in the UK.
What about solid-state?
The term “solid-state” encompasses a broad church of different electrolytes and chemistries, rather than being a single type of cell. “Solid-state” refers to the replacement of the liquid electrolyte and polymer separator with a solid electrolyte that does both jobs, ideally while offering faster charge and discharge times and improved safety. This means that, for example, a solid-state electrolyte could be used with a silicon anode and an LMFP cathode. The electrolyte may be so safe that the silicon anode would no longer be required, allowing for a pure lithium anode to be used instead and further reducing the bulkiness – and increasing the energy density – of the battery pack. This means that some of our cheapest and most ethical chemistries today could be made so compact that they exceed the energy density of today’s more expensive market-leading NMC chemistries, while solid-state electrolytes used in conjunction with NMC cathodes could result in seriously impressive electric vehicle ranges, including “city cars” that are more than capable of taking on cross-country journeys with ease.
On top of this, some startups are now turning their attention to how to make solid-state electrolytes for sodium-ion cells. This would further reduce the cost of batteries by allowing more abundant and ethical sodium to be used instead of lithium, in batteries that rival the range per charge of today’s EVs. While early versions of solid-state batteries are already in use in niche applications today, it will be a few years before we see them offered in mass produced electric cars and vans.
One thing is for certain, though: electric vehicle battery chemistries are diversifying, and shall continue to do so, bringing many cost, range and ethics advantages which are great news for electric vehicles and their buyers.
