Na-ion: A battery worth its salt?
Frank Wunderlich-Pfeiffer evangelizes us on sodium-ion batteries
Today we’re featuring a guest piece on sodium-ion batteries by Frank Wunderlich-Pfeiffer. Those of us on #BatteryTwitter will be familiar with Frank’s evangelism for Na-ion. His acerbic reporting/analysis has been prescient in the growth of Na-ion battery chemistries, particularly in Asia (watch out for the bonus response on where this evangelism stemmed from).
In fact, just this week the largest battery producer in the world announced their sodium-ion cells will be used in Chery EV cars, calling Na-ion: “a cost-effective solution” that "breaks the bottleneck of limited raw materials”.
So, here we get Frank to lay it all out: what’s the big deal with sodium?
The shape of batteries to come
When Hina presented their lineup of sodium-ion battery cells, together with a prototype of an electric car, it took a lot of people by surprise. (Including, seemingly, Farasis who presented their own EV sodium battery pack just days later.) But this had been a long time coming. In December, the company commissioned its first 1 GWh/year production line, half a year after it was finished in 2022. The facility will house five such lines; another one for 30 GWh/year is planned and more batteries will be made under license.
Plenty of competition, like CATL, SVolt, Farasis, Lifun, Transimage, and others are currently working on their own sodium-ion batteries to be mass-produced this year. Announcements of planned production capacities of battery materials in coming years already add up to several 100kt annually, without accounting for the fact that kilns built for making lithium-based layered oxide materials work just as well with sodium, and shortages of lithium will leave a lot of kilns unused. The scene is set for another upset in the battery industry in the coming years, similar to the large-scale emergence of LFP in 2020.
The Hina batteries are based on work conducted by a Beijing research group under Professor Hu Yong-sheng since 2011, who presented results for commercially viable chemistry in 2015, founding Hina Batteries (the original Chinese “Haina” means “sea salt”) two years later in 2017.
The anode is a hard carbon with 300 mAh/g, using anthracite coal as feedstock. It is a cost-effective choice, ultimately based on the work of David Stevens and Jeff Dahn two decades ago. Meanwhile, hard carbons based on synthetic materials have demonstrated capacities over 450 mAh/g, and CATL expects 350 mAh/g from theirs. The cathode is based on sodium-ferromanganese-oxide, stabilized using 22% copper and ferromanganese. The cathode delivers just 110 mAh/g (40% of the theoretical maximum) for about 350 Wh/kg, a typical value for first-generation sodium layered-oxide cathode materials.
Research has long demonstrated cathode materials of much better performance. Several manganese-oxide-based materials have demonstrated stable cycling at more than 600 Wh/kg in lab settings and many more materials around the 500 Wh/kg mark typical for LFP cathodes. But limited funding slowed industrial development to a crawl, especially for the sodium-deficient P2-layered oxides that will need an added process of pre-sodiation. That is, until the recent shortage in lithium resulted in high lithium prices and intense interest in alternatives to lithium.
The active materials in Hina’s battery deliver 210 Wh/kg, not accounting for the rest of the battery. In practice, it is 140 Wh/kg for the all-purpose cylindrical cells and 145-155 Wh/kg for the larger prismatic cells, which are intended for light and heavy EVs and stationary storage. When the research results were published in 2015, such energy densities would have been comparable to contemporary LFP batteries, just like the expectation of 2,000-3,000 charge cycles.
While LFP batteries have improved since 2015, what has improved even more is the engineering of battery packs. The Sehol E10X prototype, a low-cost EV usually sold with 20 to 31 kWh of lithium-ion batteries for the equivalent of 6,500 to 10,500 US dollars, was equipped with 25 kWh of sodium-ion batteries. The ingenious construction of the low-cost battery pack is air-cooled, using racks of cylindrical cells, retaining an energy density of 120 Wh/kg on the pack level. In terms of weight, this is 96% of the 125 Wh/kg energy density of the 54 kWh LFP pack in the standard range Tesla Model 3 first announced in 2020.
But in terms of volume, this air-cooled pack would not be competitive. Nor do the prismatic cells with their volumetric energy density of 263-283 Wh/l seem to be a useful replacement for LFP cells with 380-450 Wh/l at first glance. However, the engineering of cell-to-pack (CTP) battery packs has improved immensely even in the few years since they were first talked about in 2019.
CATL introduced the first-generation CTP with a volumetric “integration efficiency” of 55%, an immense improvement over the more typical 40% at the time. Meanwhile, CATL introduced the Qilin battery pack for high-nickel batteries with 72% efficiency. Our Next Energy and SVolt offer 76% in their LFP packs and CATL announced their sodium-ion batteries in 2021 alongside a prospected integration efficiency of over 80%. Such packs make even first-generation sodium-ion batteries viable as a direct replacement for older generations of cars with LFP battery packs of the early 2020s.
The numbers are straightforward. To illustrate, we will use the 80 Ah Hina cell with 145 Wh/kg and 263 Wh/l and the dimensions of battery packs of Tesla’s Model 3/Y and VW’s MEB battery pack as an example. They are very similar in having a volume of 320 liters and a maximum total weight of 500 kg. Using 76% integration efficiency, the result is a 64 kWh battery pack. Assuming a mass efficiency of 80% (a typical value, exact numbers will depend on the optimization of the pack) the weight is about 550 kg.
The result is a typical standard-range battery pack for premium EVs, even if the capacity of the pack has to be reduced to stay within the 500 kg limit. Future improvements in cell chemistry will see a further increase in capacity.
This should end all claims that sodium-ion batteries are unsuitable for regular EVs. Even based on performance, the sodium-based battery would be preferable to cars with older LFP packs. They can fast charge from 10% to 80% within 15 to 20 minutes and only lose 10% of capacity at -20 degrees Celsius. Sodium-ion batteries can also be fully discharged without the risk of catastrophic failure of the battery upon recharge, as would be the case in lithium-ion batteries due to copper current collectors in the anode. This allows repair and maintenance work near high-voltage components that would normally require a specialist.
And, sodium-ion batteries are cheaper. Hina has a target cost in large-scale production of $50/kWh. Even with a discounted lithium price of $30/kg lithium carbonate, as offered by CATL in limited contracts to some customers, LFP is expected to be 50% more expensive. The price of lithium carbonate topped $80/kg in November 2022 but fell to $40/kg following not only CATL’s announcement but also signs of economic problems in China and the end of EV subsidies there, as well as an unprecedented price war among manufacturers of combustion cars, all depressing lithium demand and thus prices.
At least some of these factors will be temporary. But even at the low lithium costs before 2021, the cost advantage of using sodium is substantial. Anodes don’t use copper current collectors or the more expensive graphite and electrolyte salts can be made directly, without having to convert sodium precursors to lithium compounds. CATL expects to sell their batteries with Prussian blue cathodes for as little as $30/kWh to $45/kWh in mass production, due to the lower material cost of iron-based cathode and synthesis without the need for high-temperature calcination.
Cells with more advanced layered oxide cathodes are widely expected to reach 200-220 Wh/kg and between 400-500 Wh/l in the next few years, before the end of the decade. This will enable EVs over 500km range without the use of lithium. This will fundamentally change the battery market. However, the energy density of high nickel NMC cathodes is 20-40% higher than any known sodium-based cathodes that might be commercialized. Graphite/silicon anodes are lighter and more compact than any commercial hard carbon. Although some research has recently been picked up that should result in feasible alloy-type anodes, which could close this gap in the near term. Still, even with hard carbon, the difference between sodium and lithium is surprisingly small in light of the many dismissive comments relegating sodium-ion batteries to stationary storage only.
The gap will widen only once the light lithium atoms are no longer paired with heavy transition metal oxides or phosphates. One day (many days away), practical lithium-sulfur batteries will be made. This is where all the arguments of sodium being uncompetitive with lithium due to the atoms being too heavy, too large, and delivering lower voltage comes to bear. Greater expansion of sodium-sulfur cathodes will make them harder to manufacture than their lithium counterparts. Sodium sulfide is also 70% heavier than lithium sulfide and delivers less voltage, resulting in less than half the energy density, in the best case.
But this will not make sodium-ion batteries redundant. Cost matters more than energy density. It is important to remember that to this day, there is a 400 GWh/year industry for lead-acid batteries, despite their low energy density of just 35 Wh/kg.
Sodium-ion batteries, based only on abundant materials like sodium, iron, manganese, carbon, aluminum, and the like, are sufficient for EVs with a practical range of over 300 km — even in their current primitive form. With no constraints on critical resources, this should help alleviate many geopolitical tensions currently building up around access to battery materials like lithium, nickel, cobalt, and graphite. And while the news from the London Metal Exchange would become less entertaining, miners and metal traders should not despair. There will always be a premium for lithium and nickel products, especially in a richer world that has more access to batteries and renewable resources instead of less.
The next step will be supporting the development of sodium-ion batteries outside of China. Some small developments can be seen. Altris, for example, is developing a pilot plant for Prussian Blue cathodes in Sweden and Stora Enso is using waste lignin from paper mills to make hard carbon anode material in cooperation with Northvolt, with plans for a 100,000t production capacity. But if the development of LFP is any guide, it will take three years until action is taken to provide the necessary direct support for large-scale production.
So Frank, tell us how this acerbic prescience originated.
“Between 2015 and 2018 several companies like Tiamat, Novasis, Faradion, or Hina had already demonstrated the production of sodium-ion batteries. Performance data for energy density, power, and cycle life was available and cells had been produced in anything from batches of 18650 to pouch cells, e-bike packs, and 100-1000 kWh stationary storage demonstrators. Yet, in discussions of 2020 and early 2021, sodium-ion batteries were almost universally dismissed as merely experimental and some people even cast doubt cast on whether they could work at all. Despite the demonstrations and a large depth and breadth of research, producing several thousand papers on the topic each year.
The attitude hardly changed after CATL, as the world’s largest battery manufacturer with its reputation at stake, announced its sodium-ion batteries in 2021. They were presented as batteries for electric cars and had enough performance and energy density for that. Material data for the cathode (160 mAh/g) and anode (350 mAh/g) was typical for prussian blue and synthetic hard carbons. They were well below the highest demonstrated values and in line with achieving the claimed energy density of 160 Wh/kg - either through bottom-up calculation or extrapolating from the 130 Wh/kg already demonstrated in 2015 by Sharp Labs/Novasis using similar but less developed chemistry.
There were no surprises from a technical point of view. Low-cost materials and synthesis for the cathode, anode, and current collectors ensured economic viability, even at low lithium prices. It was all a matter of investment and execution, not technology. The investment came as lithium prices rose due to limitations in lithium mining (eventually staying above the equivalent of $50 per kWh for a year!). By 2021 the investment was there (with CATL lending credibility to the technology at least in China, where most battery supply chains are), and with the experience of the rapid growth of LFP and NMC811 in recent years, prescience was not required to see sodium ion batteries commercialized and mass-produced within two years.
But industry analysts and even a lot of lithium-focused scientists dismissed sodium ion batteries outright or accepted them as useful only in stationary storage and two or three-wheelers, in short: Purely a replacement for lead-acid batteries, despite having 4-5 times more energy density already. None of these arguments were made using numbers, despite everyone involved being fluent in basic arithmetic. Those industry analysts who said sodium ion batteries were viable for stationary storage were clearly not doing so in good faith, as even their long-term projections for battery materials needed by 2030 or 2040 didn't include them.
It was not the response you would expect after decades of research and a company that produced one third of all lithium ion batteries worldwide announced plans for mass production of sodium ion batteries. They were not seen as an actually exsting technology, despite abundant evidence to the contrary. The situation lend itself to develop a (not always healthy) amount of cynicism, especially in light of an abundance of supportive comments for billion dollar SPAC companies like Quantumscape at the same time. Their unrealistic plans to start mass productions by 2024 were defended quite vigorously. Despite showing no more than lab-scale single or few-layer prototypes, that were clearly hand-picked and betrayed no evidence of the state of development needed for mass production by 2024. With Quantumscape withholding all data on energy density to this day, even from independent lab reports.”
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