Electric dreams

With sales of EVs surging, improvements in electric batteries are likewise racing ahead, Anthony King reports

Sales of electric cars tipped 6.6m in 2021 – more than tripling their market share from two years earlier, according to the International Energy Agency (IEA). Europe and China led the way, with electric car sales soaring to 3.4m in China and up 70% to 2.3m in Europe in 2021. Worldwide, there are now around 16m electric cars on the road.

A range of cathode chemistries are at work in today’s electric vehicles (EVs), and one prevailing anode material. State-of-the-art for EVs currently is nickel manganese cobalt (NMC) or nickel cobalt aluminium oxide (NCA) cathodes, paired with graphite anodes. In recent years, however, there has been a push to increase the nickel content, while reducing expensive cobalt. ‘As you increase nickel, you get increasing energy density benefits,’ says Billy Wu at Imperial College London, UK. Meaning the same sized battery can deliver further range for the EV. The ratio has gone from 1-1-1 to 6-2-2 to 8-1-1 now for NMC, so eight times more nickel.

‘High nickel cathodes are increasingly being used in electric vehicles,’ says Wu, ‘but one of the challenges is materials availability, where people essentially see a shortfall in the next few years in both lithium and nickel.’ The price of nickel has been rising precipitously, and NMC still includes expensive cobalt as an ingredient, which must be sourced mostly from the politically unstable Democratic Republic of Congo. There is unlikely to be enough nickel or cobalt to build hundreds of millions of EVs. Moreover, as a cathode becomes more nickel rich, it becomes more energy dense and more unstable. ‘You need more safety systems to make sure there’s no catastrophic failure,’ Wu explains, adding both weight and cost.

An alternative is manganese-rich chemistries. For example, the original Nissan Leaf ran on lithium manganese oxide (LMO) batteries – cheap, but with lifetime downsides. The newer Leaf runs on an NMC battery. Still, companies such as Volkswagen and Tesla remain interested in manganese-rich battery chemistry, since manganese is far more abundant and cheaper than nickel and cobalt.

Iron-rich cathodes

Supply issues around cobalt and nickel have led to the emergence of LFP (lithium iron phosphate) batteries as an option to power vehicles. LFP batteries use LiFePO4 as the cathode material, and usually graphite as anode. LFP is cheaper than NMC but needs more volume.

‘NMC was dominant in the last couple of years, but companies are moving back to LFP,’ says battery chemist Huge Geaney at the University of Limerick in Ireland. ‘Cost and sustainability are in play.’ Iron and phosphorus are abundant elements and fewer carbon dioxide emissions are released in producing LFP than NMC batteries.

Meanwhile, ‘LFP is a fundamentally safer chemistry,’ notes Wu, and requires fewer packing materials and safety systems. Until recently, however, there remained a significant gap in terms of energy density, with the best NMC batteries pushing on 270Wh/kg and LFP historically around 150Wh/kg, adds Wu.

What shifted was how battery packs are put together to power a car. ‘Current battery packs for cars take dozens or hundreds of smaller units, a little bigger than chocolate bars, and maybe pack 20 of them into a shoe-sized box,’ explains Maximilian Fichtner, energy storage researcher at Karlsruhe Institute of Technology in Baden-Württemberg, Germany. Ten to 20 such modules may be stacked to make a container, which means the actual storage material gets more diluted with packaging materials. ‘Eventually there’s only 25% or so of active material in the battery pack,’ says Fichtner.

High nickel cathodes are increasingly being used in electric vehicles right now, but one of the challenges is obviously materials availability, where people essentially see a shortfall in the next few years in both lithium and nickel.
Billy Wu, Faculty of Engineering, Imperial College London, UK

Some major developments in the last two years underpinned LFP’s comeback. So-called cell-to-pack technology was made possible by progress in manufacturing enabling large pieces of electrodes to be produced. Instead of hundreds of cells stacked together, there is one cell from left to right. ‘Instead of 16 modules, there are just three or four,’ says Fichtner. This culminated in the launch by BYD of the Blade Battery, ca 10-15cm high and 1cm thick, but almost a 1m long. The design allowed for 50% less parts, compared with a conventional LFP block battery, and therefore 50% more energy, according to BYD.

Macrophotograph of a sample of high purity 99.8% electrolytic cobalt metal.

The pack also survived a nail penetration test. Normally, nail penetration can cause an internal short circuit, heating the interior of the battery to 400-500°C, igniting the flammable electrolyte. ‘The Blade battery is not only cheap, but it is safer. It doesn’t burn when you do a nail penetration test,’ says Fichtner. In summer 2021, news broke that the Tesla Model 3 would transition to LFP batteries, which it had begun using in its Chinese cars, buying from CATL. The company confirmed in October 2021 that its standard Model 3 and Model Y models would drive on LFP. Previously, US Tesla cars in had relied on NCA chemistries, produced by Panasonic. In December 2021, it was reported that Ford in China would also start using the BYD Blade Battery.

China had a long-time interest in LFP batteries, says Wu, because it recognised restraints on nickel and cobalt, and because the country had a royalty-free license to manufacture them. That agreement is soon to expire, allowing them to sell such batteries into Europe. ‘They’re now going to push this chemistry globally,’ says Wu. Another factor in the rise of LFP is that it has gained support from high-volume battery manufacturers such as CATL, which along with Panasonic and LG Chem can produce batteries at lower cost due to economies of scale. ‘People laughed about this material three years ago,’ Fichtner says, whereas recent announcements from Tesla suggest that three-quarters of its fleet will run on iron-based cathode chemistries.

Anode antics

Li-ion batteries are usually described by their cathode chemistries. The anode is usually made from layered carbon: graphite. ‘For a long while, we had an obsession with the cathode, and we kind of forgot about the anode,’ says Wu. But doubling the energy density of the cathode will require a similarly improved anode or a doubling of the anode’s volume. Typically, the anode was a blend of 60% synthetic graphite and 40% natural graphite. There is now even greater potential to boost anode performance. ‘The developments that are going on in anodes will eventually pay out much more than what we can ever achieve with the cathode,’ says Fichtner.

Now, companies are blending small amounts of silicon into their graphite anodes. Tesla has about 5% of the metal as part of its anode recipe. It takes six carbon atoms in graphite to take up one lithium ion, whereas theoretically each silicon atom can take up four lithium ions. Normally, however, silicon cannot take four lithium ions due to volume expansion, notes Shirley Meng, a battery chemist at the University of Chicago, US. This causes anode material high in silicon to pulverise itself. ‘Silicon has about ten times the capacity of graphite,’ says Wu, ‘but the problem is it expands and contracts by about 300% when you charge and discharge it.’ While lithium ions sit between the layers of graphite, they create electrochemical alloys with silicon, meaning that they are stronger and store more energy.

A second challenge with silicon in the anode is that it made existing electrolytes incompatible. ‘Each battery developer had to modify their electrolyte formulations in order to accommodate silicon, and this slowed down silicon integration quite significantly,’ says Meng. There are signs of progress. California-based Sila Nano says its silicon-based composite material can accommodate swelling due to an engineered particle structure and stands as a drop-in replacement for graphite anodes. It describes graphite as ‘the weak link in a battery’ for taking up more space than any other component. In a white paper, Sila Nano states it expects Si-dominant anodes to increase the energy density of NCA or NCM cathodes by up to 50%, thereby dropping kWh costs by 30-40% in less than a decade.

Fichtner says silicon oxide is another material of interest to EV companies, because it doesn’t suffer from such huge expansion. Also, if you ‘integrate nanoparticles of porous carbon, then this gives some further room for expansion,’ he says. Meng is optimistic on the anode front. ‘Things are going to change very quickly in the next couple of years, because lots of investments are being made into silicon anodes, both for the electrolyte formulation, as well as how to handle the volume changes,’ she says.

But silicon is not the only potential game changer. One way to greatly improve the energy density of an anode would be to make it not out of material that accepts lithium ions, but out of lithium metal. There are even efforts to make an anode-less design, where the anode is created from lithium once you charge the battery. Meng is doubtful about the practicality, but notes lithium-rich anodes are moving towards commercialisation.

Singapore-headquartered SES announced in November 2021 that it will build the largest Li-metal battery facility in the world in Shanghai, China, by 2023. Its Li-metal anode reduces battery weight and production costs, with a projected 400Wh/kg and 80% charge in less than 15 minutes. A big problem with the first-generation Li-metal batteries was fires, but Meng says she is ‘cautiously optimistic’ about commercialisation of this technology. ‘Lithium metal anodes will eventually increase volumetric energy density by about 50%,’ she predicts, ‘so it is great news for the EV industry, because with a reservoir of lithium, you can move lithium back from anode to cathode really quickly.’ Especially interesting for race cars, for example.