New sodium metal battery design charges in just 4 minutes and retains its capacity for years

Researchers in China have announced a radical sodium metal battery (SMB) design that can fully charge in just four minutes and will retain its capacity for years of use.

SMBs are a form of ultrafast-charging, stable batteries that scientists say could one day be a cheap alternative to today's lithium-ion (Li-ion) batteries, which rely on geographically concentrated metals and easily catch fire. SMBs also differ from sodium-ion (Na-ion) batteries in that they use a metallic sodium anode rather than a graphite or hard carbon anode.

However, SMBs remain largely theoretical because they are prone to a type of degradation known as dendrite formation. This is when the sodium ions passing through the electrode deposit onto the highly reactive, pure-metal sodium anode in spiky, stalagmite-like structures. Over time, this forms a bridge between the cathode and the anode, short-circuiting the battery.

Dendrite formation is especially common in sodium batteries because sodium is a highly reactive metal. When charge runs through a Li-ion, Na-ion, or sodium metal battery, the anode always reacts with the electrolyte to form an oxide layer known as the SEI. This is typically 10 to 50 nanometers thick — about as wide as a small virus — but generally harmless. But with sodium, the SEI often cracks, forming bumps that attract sodium ions, which pile into dendrites.

Now, researchers say they have solved this issue by using a tough, quasi-solid gel electrolyte — dubbed Sn-FB QSE — which strengthens the battery against punctures and provides a semisolid internal structure that prevents dendrites from forming. They outlined their findings in a study published May 21 in the journal Nano-Micro Letters.

To confirm the longevity of this approach, the scientists charged and discharged the battery for over 6,000 hours without dendrites short-circuiting the battery. They also noted that when they charged the battery from zero to 100% capacity in just four minutes, it retained electrical charge, measured in milliampere-hours per gram (mAh g^–1), of 80.1. This is the equivalent of around half that retained in Li-ion batteries.

When charged at a slightly slower rate of zero to 100% in 20 minutes, the battery retained 90% of its charge capacity over 2,000 cycles — matching the theoretical limits for Li-ion batteries, the scientists said in the study. This slower speed lowered the cost and improved the safety.

This is notable because the scientists achieved this in the new battery while still charging it quicker than Li-ion batteries can be charged. This is relevant because charging speed remains a sticking point for battery deployment in electric vehicles (EVs). The fastest charging EV today is the BYD Denza, which the Chinese automaker says can go from 10-70% in just five minutes. But this requires highly specialised, 1MW proprietary chargers.

Most EVs charge much slower — Tesla representatives say its Model 3 can recharge from 10-70% in approximately 15 minutes using Tesla’s own 250kW flash chargers, but representatives from the EV routing platform Zapmap say the same vehicle will take 90 minutes to charge to 80% on 50kW chargers.

Indeed, most batteries used for modern technologies, such as smartphones and EVs, are Li-ion. However, Li-ion batteries are expensive to produce because they contain the hard-to-obtain metals lithium and cobalt, and they are prone to catching fire.

Increasingly, battery manufacturers are looking to bring Na-ion batteries to commercial scale because they are cheaper and safer. However, they are heavier and larger than Li-on batteries.

SMBs are the focus of intense research because they theoretically combine the best of both types of batteries. Because SMBs use a sodium anode, rather Na-ion batteries that use graphite or hard carbon anode, they are lighter and cheaper to produce and therefore much more comparable to Li-ion in terms of size and weight. They are also safer because they operate using sodium ions, which are bulky and cannot flow to breaches in a battery wall fast enough to cause thermal runaway. This is the self-sustaining chain reaction that causes batteries to ignite when damaged.

If the issues of dendrite formation and stability at lower temperatures can be resolved, replicated and scaled, SMBs could reshape the economics of battery deployment over the next decade, the scientists said.

SMBs could be excellent choices for EVs in public transport or within commuter cars, the scientists belive, because although they have lower ranges than Na-ion and Li-ion vehicles do, they charge faster. However, they won't be available for some time, either in vehicles or smaller devices like consumer electronics.

That's because devices like smartphones are subject to harsh temperature changes that affect the internal chemistry of batteries that rely on gel electrolytes. The research must first be replicated before manufacturers feel comfortable using pure sodium metal in place of well-understood graphite configurations.