sodium_battery

Editor's note: This article is part of the specialty theme, E-mobility meets the road, by Boyd theme, hosted by GlobalSpec News & Analysis.

To meet the net zero carbon emissions agreed to by the U.S. in the 2015 Paris Accord, it is projected that the number of electric vehicles (EVs) will need to increase from 2.5 million today to 44 million by 2030. This illustrates the extreme demand on the EV battery supply chain, and whether that goal is attainable or not, drives research into new battery technologies that harmonize costs, safety, performance, and sustainability.

Although lithium (Li)-ion batteries currently dominate the EV traction battery market, manufacturers are under pressure to come up with new chemistries, technologies and manufacturing processes to support an even more rapid uptake of battery electric vehicles.

So even as the industry rolls out Gen 2 and Gen 3 Li-ion batteries, manufacturers and researchers are intensifying their investments and collaborations to transition to more cost-effective and higher-performance storage devices, such as Gen 4 batteries with a specific energy density equal to, or greater than, 400 Wh/kg.

Nascent technologies driving next-generation EV batteries

The energy density of Li-ion batteries has increased from 80 Wh/kg to around 300 Wh/kg since 1991, yet EVs are still have a limited range, which is further hampered by an overwieght battery chemistry.

Modern EV traction batteries require even more energy density along with faster charging.

The problem is that today’s Li-ion batteries mainly contain intercalation-type cathodes (LiFePO₄, LiCoO_2, or LiNiMnCoO₂) and graphite-based anodes. The energy density of these electrodes is approaching its upper limit. As a result, researchers are exploring new materials, including 100% silicon and Li-metal anode technologies, to increase performance.

While the addition of small amounts of silicon – typically well under 10% - to the graphite anode has delivered impressive results, the amount of silicon is limited by the material’s charge-induced volume expansion. This expansion, of up to 400% during cycling, results in material fractures that degrade performance and reduce the lifespan of the battery.

To overcome the consequences of this expansion, U.S.-based battery technology company GDI has developed a 100% silicon anode using an environmentally friendly priority “roll-to-roll” process. This novel process also reduces manufacturing greenhouse gas emissions by 80% compared to graphite anodes.

These silicon anodes, with an energy density of 3200 mAh/g, exhibit a 30% increase in energy density compared to traditional graphite anodes, and allow Li-ion batteries to charge up to 80% in just 15 minutes, without sacrificing cyclability.

Having completed initial testing, production is planned to ramp up to 10 GWh by 2028, paving the way for its widespread use in high-performance EVs in the next decade.

Another anode material currently attracting great interest is Li-metal, with extremely high theoretical specific capacities of 3860 mAh/g and 2061 mAh/ cm³, and a low negative electrochemical potential of −3.04 V.

However, despite these potential performance gains, Li-metal batteries face several challenges, including susceptibility to dendrite growth, ‘dead lithium,’ corrosion and volume expansion of the anode. By switching to a solid-state electrolyte, companies such as Factorial Energy have overcome many of these challenges, en route to achieving specific energy densities exceeding 450 Wh/kg in solid-state batteries (SSBs) by 2030.

With SSBs being touted as a game-changer for EV battery technology, American SSB tech company QuantumScape has adopted a novel approach to battery design by doing away with the anode. Instead of an anode, QuantumScape plates Li-metal directly onto the collector during the first charge.

Even though this contributes to an impressive specific energy density of over 450 Wh/kg, during the charge-discharge cycle, the depletion and regeneration of the Li-metal causes the cell to swell and contract. To counter this QuantumScape proposes a new cell format – something between a pouch and prismatic form. QuantumScape expects to start commercializing its batteries in 2024 and ramp up production through 2028.

Adopting an entirely different approach to battery technology, researchers from the Institute of Physics, Chinese Academy of Sciences in Beijing, have managed to produce pouch-type rechargeable Li batteries with a record-breaking energy density of 711.3 Wh/kg and a volumetric energy density of 1,653.65 Wh/l. The design is underpinned by a high-capacity Li-rich manganese-based cathode with an areal capacity exceeding 10 mAh/cm² and a thin Li-metal anode with high specific energy.

To simplify manufacturing and optimize performance, the design has been tailored to minimize the use of auxiliary materials while increasing the percentage of active materials used in the battery cell, thereby delivering the ultrahigh energy density.

At this stage, many of these technologies are still in the laboratory and development phases, so cost is not yet defined. Frankly, it is unlikely that any of these batteries can overcome the mature state of Li-ion technologies.

For that, and to realize the cost reductions needed to accelerate BEV adoption in mass market segments, the industry is turning to sodium-ion batteries.

Sodium-ion chemistry: A cost-effective alternative to Li-ion EV batteries

While sodium-ion batteries (SIBs) have always held the potential to significantly reduce the cost of EV batteries - approximately 20 to 30% cheaper than that of LFP batteries - the marginal energy density and poor cyclability have restricted their use to two-wheeled vehicles.

That is until automaker JAC released the Yiwei, an EV with a range of 157 miles extracted from a 25 kWh battery, boasting 120 Wh/kg of energy density, a 3 to 4C charge-rate, from a NaCR32140 cell produced by Chinese manufacturer HiNa.

This limited range is partly due to sodium metal being about three times heavier than Li, which adds considerably to the battery weight, reduces energy density and translates into a shorter driving range.

Following their research into energy-boosting nickel-manganese-cobalt (NMC) cathodes for use in Li-ion batteries, researchers at the U.S. Department of Energy’s Argonne National Laboratory developed a novel variation of the NMC cathode for SIBs. The new cathode, comprising nickel-manganese-iron (NMF), features a layered structure for efficient insertion and extraction of sodium ions. The absence of cobalt in the cathode mitigates cost, scarcity and social concerns associated with the metal.

The increase in energy density stemming from the NMF cathode is expected to enable a projected driving range of about 180-200 miles on a single charge. Moreover, the cathode also overcomes the SIB’s limited cyclability allowing the battery cells to be charged and discharged over the same number of cycles as their lithium-ion counterparts. Having transitioned from the laboratory phase, the cathode is now set for testing in battery cells similar to those used in an EV battery. This testing will be done in Argonne’s Cell Analysis, Modeling and Prototyping Facility.

Whilst the energy density and range of SIBs are not yet quite up to that of Li-ion, or even LFP batteries, the team continues working to develop different materials for the two other main components of the battery — the electrolyte and anode — to boost energy density even further.

So, even though the SIB might not appeal to those seeking long driving ranges, it could attract budget-conscious consumers, particularly urban dwellers whose daily driving falls within the range per charge offered by the SIB.

Conclusion

At the rate that battery technologies are advancing, it is difficult to quantify the important metrics, or even the exact chemistries, of the nascent EV traction batteries moving into the 2030s. And it might be less important, if engineers and OEMs stop thinking in absolute terms about which single battery technology serves the future.

To meet the ambitious climate goals of 2030s, EV battery supplies need to diversity and adapt to multiple technologies. It is easy to image SIBs as ideally fit for small vehicles and short commutes, whereas advanced li-ion technologies serve a general purpose role. And extreme density technologies supply the power needed for long distance or power intensive applications - like urban air mobility.

Even though there is not likely to be an outright winner of the battery technology race, the technologies, and chemistries described in this article will undoubtedly form part of the mix as the industry continues its quest to deliver cheaper, fast-charging, safe EVs with abundant range.