By Tina Casey
Earlier this year, we thought we caught hints that sodium-ion energy storage technology is ready for its closeup, and this makes it official. The University of Texas at Austin has just let out word that a research team guided by professor John Goodenough has come up with a new cathode material leading to the development of a marketable sodium-ion battery. That’s right, the man who invented lithium-ion batteries is ready to move on.
E Is For Energy Storage (And Eldfellite)
The new energy storage breakthrough involves a small translucent yellowy-green mineral with a Lord of the Ringsy name, eldfellite. If you never heard of eldfellite before, join the club. Eldfellite is one of 27 new mineral species discovered at volcano eruption sites in Iceland in the last part of the 20th century, and one of only two in that group to earn the title “new world mineral” (for the record, heklaite is the other one).
Eldfellite — aka NaFe(SO4)2 — is so named because it was found on the northeast rim of Eldfell, a volcano on Iceland’s Heimaey island that nearly caused the entire population to permanently relocate when it unexpectedly erupted in 1973.
Energy Storage With Sodium
Where were we? Oh, right, the new sodium-ion battery. The findings were reported last month in the journal Energy & Environmental Science under the title “Eldfellite, NaFe(SO4)2: an intercalation cathode host for low-cost Na-ion batteries.”
The abstract doesn’t give away too many clues, unless you can decipher this:
The mineral eldfellite, NaFe(SO4)2, is characterized as a potential cathode for a Na-ion battery that is fabricated in charged state; its 3 V discharge versus sodium for reversible Na+ intercalation is shown to have a better capacity, but lower insertion rate than Li+ intercalation. The theoretical specific capacity for Na+ insertion is 99 mA h g−1. After 80 cycles at 0.1C versus a Na anode, the specific capacity was 78 mA h g−1 with a coulomb efficiency approaching 100%.
Intercalation refers to the ability of a material to host “guest” molecules or ions that come and go, which is basically what happens in a battery. Here’s a nice plain-language explanation from Chemistry World:
Na-ion batteries work in the same way as Li-ion batteries. During discharge, Na+ ions migrate from the anode to the cathode, while the balancing electrons travel to the cathode via an external circuit, where they can be used to perform electrical work. At the cathode, the arriving electrons are accommodated by utilising a redox couple while the sodium ions intercalate into the cathode structure – this process is reversible on charging.
With that in mind, let’s focus on the middle where it says “better capacity, but lower insertion rate.”
Professor Goodenough and his team were looking to solve one of the issues blocking commercialization of sodium-ion batteries, which is the instability of the cathode material. The new cathode consists of fixed layers of sodium and iron, which remain that way as sodium is inserted and removed, resulting in a battery with better capacity than lithium-ion batteries.
With the capacity problem in hand, the team is now tackling the lower energy density issue, which is a significant one. Lower energy density translates into a heavier battery, which is one of the other issues with sodium-ion technology.
More And Better Sodium-Ion Energy Storage, Eventually
As for why the Goodenough team is so excited over winning just half the battle, that’s because sodium-ion chemistry provides a low-cost, abundant material for battery energy storage, which would result in a huge drop in the cost of electric vehicles, and in the cost of stationary battery systems for storing wind or solar energy.
That could be a long way off. Back in 2013 our sister site Planetsave caught wind of a microscale sodium-ion battery based on wood and tin, but that doesn’t seem to have gotten anywhere yet (if you’ve heard anything, drop us a note in the comment thread).
More promisingly, last year, a research team at Kansas State University came up with a sodium-ion variant based on graphene, and a team from the University of Maryland announced progress on an “expanded” graphite anode for sodium-ion chemistry.
Earlier this week, Purdue University also weighed in with a new anode based on “tailored” carbon and tin.
At least one company isn’t waiting around for the research. The UK company Faradion has come up with a sodium-ion energy storage solution, which it has already tested out on an electric bicycle as proof of concept. Next steps include more R&D and an energy storage project co-funded by the agency Innovate UK.
Image credit: Crystal structure of the eldfellite cathode for a sodium-ion battery via Cockrell School of Engineering.