Lithium-based batteries are great, the different electrode chemistries allowing them to be slotted into a variety of uses. The problem with them has nothing to do with their performance. The challenge before us is that we want to make a very of batteries; If they all use lithium, we will undoubtedly face a shortage of supplies.
One possible solution to this is simply to replace the lithium with a different ion. Alternative batteries may not be as good as the lithium variants in all the different places we currently use them. They should be good enough at just one job to remove some of the need for sticking lithium everywhere.
This is the reason behind some of the interest in sodium-based batteries. Sodium is very abundant and correspondingly cheap and can be made to behave a bit like lithium when used in batteries. But sodium batteries always carry risks associated with sodium’s tendency to react explosively. But a recently developed solid electrolyte suggests that at least some of the challenges associated with sodium can be overcome.
an accidental electrolyte
There are several battery technologies that are based on sodium, such as sodium-sulfur batteries, which have little in common with lithium batteries. But sodium-ion batteries operate on more or less the same principles as lithium-ion and may also use somewhat similar materials, such as carbon-based electrodes. Sodium is heavy, so sodium-ion batteries can’t really reach the same energy-per-weight level that lithium can. But again, sodium is plentiful and cheap, so sodium batteries may make sense in cases where weight isn’t important, such as home- and grid-level storage.
The big hang-up here is sodium itself. Many lithium-based batteries use an aqueous electrolyte to get the ions between the two electrodes. And sodium is not noted to mix well with water. In fact, it reacts energetically to liberate hydrogen, which then explodes. Fire hazards are problematic with non-aqueous electrolytes in lithium batteries; Add sodium’s reactivity with the environment, and the dangers are serious.
So, the electrolyte appears to be a reasonable target for research. This is somewhat surprising because the research team seems to have stumbled across the electrolyte by accident. The researchers refer their work to the synthesis of the electrolyte and, if you follow that reference, you will find that it is talking about an MRI contrast agent. It’s not clear exactly how anyone got the idea to try it out in batteries, but here we are.
The electrolyte is what is called a block copolymer. These are molecules that are composed of two different classes of subunits. The polymerization process is controlled in such a way that you end up with a stretch of polymer made up of repeats of one subunit, alternating with stretches made of another. (Those parts are called blocks, giving the material its name.)
In this case, one of the two blocks was based on a carbon/sulfur compound; This polymer alone serves as a control material. For the block copolymer, the second block was a hydrocarbon in which most of the hydrogens were swapped for fluorine atoms. The idea behind fluorine was to avoid the situation that occurs with associated electrolytes, where sodium was interacting with oxygens in the polymer and therefore trapped instead of moving.
While the block copolymer is solid, it undergoes a transition from glass to plastic at temperatures that are likely to occur during battery operation. In either state, it tends to form distinct domains depending on the two different blocks, with the fluorinated material forming internal channels that can accommodate sodium and the other block providing structural integrity.
How does this work?
Researchers spend a lot of paper cycling sodium in and out of polymers and see what happens. This tends to form a layer of sodium on the surface of the material – a bit like electroplating it. It is important to note that sodium created a smooth surface on the polymer. On the control polymer, on the contrary, sodium dendrites with sharp edges are formed. This is important because dendrite formation is a major point of failure for lithium-ion batteries.
The main thing is that this process was reversible; The plating of sodium on the polymer can be reversed and then re-plated. Performance was good for over 200 cycles in and out of sodium.
So, they went ahead and made two different batteries. For both batteries, one electrode was simply sodium metal (an approach that is being developed for lithium, as it would greatly increase the charge per weight). Other electrodes stored sodium in a sodium-vanadium phosphate material or sodium iron phosphate. Both batteries worked. Increasing the charge/discharge current resulted in a slight degradation in performance, but did not result in permanent damage to the polymer; Leaving the current one restores the previous display.
But the main thing was consistency. After more than 900 cycles, it still had over 97 percent of the battery’s initial capacity.
None of this is to say that sodium batteries are guaranteed to be the next big thing. Any battery that includes a sodium metal electrode is going to involve some significant engineering costs to maintain safety—which can offset some of the cost savings of using sodium and the weight savings of having a metal electrode. But the important thing is now less about being a mature technology, as much as about the development of different types of battery chemistries over the time of current battery production to the point where lithium becomes a limiting factor.
nature material2022. DOI: 10.1038/s41563-022-01296-0 (About DOI).