18650 Na-ion Prototype may Challenge Li-ions

November 29, 2015 by Jeff Shepard

French researchers from the RS2E network today revealed the first prototype of the sodium-ion (Na-ion) "18650" battery, a standard format used notably in portable computers. The information may not sound exciting to non-specialists... Yet scientists across the globe, including the US, Japan, the UK, and Israel, are working on this technology – which today is considered the most serious alternative to the lithium-ion batteries that equip practically all portable electronic devices (portable computers, tablets, smartphones...) – and are beginning to take a serious look at electric vehicles. The battery used for Tesla cars, for example, is nothing more than the combination of several thousand "18650" lithium-ion batteries.

"The sodium-ion battery unveiled today is directly inspired by lithium-ion technology," explains Jean-Marie Tarascon, the French battery "guru" who is a solid-state chemist at the CNRS, and professor at the Collège de France.2 Like lithium ions, sodium ions "travel" from one electrode to another, during the course of charging and discharging cycles, and they do so without in any way modifying the "host materials" located at each electrode, as the latter take the form of crystalline structures the ions can smoothly enter into... Its format, called "18650," indicates that it is presented in the form of a cylinder, with a diameter of 1.8 centimeters and a height of 6.5 centimeters.

For the moment, its creators have not disclosed the composition of the materials wrapped around the two electrodes of their sodium-ion battery—a trade secret. However, the performance of the prototype presented today is better known. With 90 Watt-hours/kilogram, "its energy density (the quantity of energy that can be stored per kilo of battery) is comparable to certain lithium-ion batteries, such as the Li-ion iron/phosphate battery," points out Loïc Simonin, a researcher at LITEN, a CEA laboratory associated with the development of the prototype. And its life span (maximum number of charge-discharge cycles) exceeds 2000 cycles. These initial results are thus highly encouraging, all the more so as they can be improved.

Considered desirable objects today, sodium batteries have nevertheless come a long way. In the late 1980s, this technology had in fact been set aside in favor of lithium, whose superiority seemed obvious to all: thanks to a voltage of 3.5V, lithium in theory provides the most energy. Being three times lighter than sodium ions, lithium ions also make it possible to produce very lightweight batteries, an undeniable asset when it comes to portable electronics. Lithium's only disadvantage is its (relative) rareness, and the fact that it is only found in a few specific locations (Columbia, Chile, China, etc.).

"When the electric vehicle market began to develop, we feared a surge in lithium prices, and sodium thus entered the race again,” Tarascon recalls. True enough, it has the significant advantage of being abundant (2.6% sodium can be found in the Earth's crust, compared with barely 0.06% lithium), and widely accessible, notably in seawater in the form of sodium chloride (NaCl).

"The 18650 format enables us to provide proof of concept, and compare the performance of our batteries with those of similar format that are already available on the market. However, other formats will need to be designed to meet new requirements," explains Simonin. Time is of the essence, as Toyota are working relentlessly on a prototype of a sodium-ion car battery, while the British startup Faradion, in association with Oxford University, made a first demonstration this year of an electric bicycle powered by a sodium-ion battery.