[bhuber]

This makes sense for lithium based batteries, but those aren't the only ones around. Iron flow batteries, for example, consist mainly of iron, salt, and water. There's no shortage of ingredients. They're too big and heavy to be practical for most transportation applications, but they have a number of desirable properties for utility-scale energy storage. https://www.technologyreview.com/2022/02/23/1046365/grid-sto... is a pretty good writeup.

[ncmncm]

Molten metal batteries have the advantage of not wearing out, and of welcoming heat produced when charging.

[scythe]

Flow batteries are pretty questionable. Moving parts suck. Zinc-bromine in particular was around for a long time as a flow technology but only started to move towards mass production when a non-flow version was developed.

But in particular, the iron-flow battery story going around recently stinks pretty badly. It's being heavily promoted almost entirely by one company, ESS, and their first real client is -- wait for it -- SoftBank [1]. The academic reports on iron-flow batteries [2] make the technology sound a lot less mature than the ESS website [3], which incorrectly refers to vanadium and lithium as "rare-earth metals".

A 2018 publication [4] from Narayan's group at USC boasts that:

>Thus, by operating at 60°C and a pH of 3 with ascorbic acid and ammonium chloride, we achieved a coulombic efficiency of 97.9%. While this value of coulombic efficiency is among the highest values reported for the iron electrode in the context of the all-iron flow battery, further improvement in efficiency is needed for supporting repeated cycling.

However, further work by Narayan's group led them to replace iron chloride by iron sulfate in 2020 [5] which was celebrated by USC in a press release [6].

It was shortly after this that ESS burst onto the scene claiming iron chloride batteries with extremely long cycle life using "carbon composite" electrodes, "porous polyethylene separator" and a "polypropene spacer" [3], which are suspiciously similar to the graphite electrodes, mesoporous hydrocarbon-polymer-not-disclosed (Tokuyama A901 [7]) anion-exchange membrane, and polypropylene housing used in the Narayan group's 2016 paper [8] proposing all-iron-flow batteries for grid storage. It's worth noting that chemically unmodified polyethylene is probably not a suitable material for an ion-selective membrane, but it wouldn't even be the second-worst mistake on the page.

Yet ESS, despite having supposedly solved major problems that are obviously of scientific interest to active researchers, does not appear to have any names on its website, and cites no publications. Frankly, it sounds like another EEStor.

1: https://cleantechnica.com/2021/10/07/first-ess-iron-flow-bat...

2: https://dornsife.usc.edu/labs/narayan/all-iron-redox-flow-ba...

3: https://essinc.com/iron-flow-chemistry/

4: https://www.sciencedirect.com/science/article/pii/S245191031...

5: https://iopscience.iop.org/article/10.1149/1945-7111/ab84f8/...

6: https://news.usc.edu/166306/flow-battery-renewable-energy-el...

7: https://watermark.silverchair.com/jeecs_18_2_024001.pdf

8: https://iopscience.iop.org/article/10.1149/2.0161601jes/pdf

[mbjorkegren]

Here’s another interesting one, storing energy in heated metal, and later converting the emitted light back to electricity using specialized PV cells: https://youtu.be/Gn7pfYKB7DA