(If you don’t want to read, you can listen! Just click play above.)Hello! Welcome back to Battery Week here at Volts … where we use the term “week” somewhat loosely.Up to now, we’ve been focusing on lithium-on batteries (LIBs) — why they are so important, how they work, and the varieties of LIBs that are battling it out for the biggest battery market, electric vehicles (EVs). It’s fairly clear from that discussion that LIBs, in some incarnation, are going to dominate EVs for a long while to come. There is no other commercial battery that can pack as much power into as small a space and lightweight a package. Plus, LIBs have built up a large manufacturing base, driving down prices with scale and learning. Their lock on the EV market is likely unbreakable, at least for the foreseeable future. But there’s another battery market where some competitors hope to get a foothold: grid storage. They think there’s space in that market waiting to be claimed.Currently, there’s a robust and growing short-duration grid storage market, offering storage of anywhere from seconds (to provide grid services like voltage and frequency regulation) to four hours. LIBs have about 99 percent of that market locked up; in some areas, projects with solar power coupled with four hours of storage are bidding in competitively with natural gas. Most energy wonks believe that, to fully shift the grid to zero-carbon energy, we will eventually need long-duration storage as well, to the tune of weeks, months, or even seasons. LIBs are almost certainly not going to cut it for that purpose, so it will be some combination of other technologies. (I’ll write about long-duration storage some other time.)In between short and long, there’s something that might be called mid-duration storage, covering the range between four and 24 hours. What technologies will cover that range? LIBs can do it, of course — theoretically they can cover any duration; you just stack more and more batteries — but the economics get extremely difficult. Mid-duration projects will require lots of capacity but might run comparatively rarely. As duration gets to four hours and above, the cost of LIBs, at least today’s LIBs, starts to get prohibitive.This is where other batteries come in, challengers to LIBs that hope to beat them at longer durations — though they aren’t quite there yet. “There really aren't competitive technologies in the battery electric vehicle space aside from all these different lithium ion batteries,” says Chloe Holzinger, an energy storage analyst at IHS Markit, but “there's a ton of different battery technologies for grid storage. They just tend to be significantly more expensive than lithium ion batteries.”These challengers believe they are better suited to the needs of the mid-duration grid storage market, where energy density matters less than capacity, calendar and cycle life, and safety. They think they can bring costs down to competitive levels at those durations. (Some of them think they can find other niches as well, but it’s grid storage that offers the most realistic shot.)Flow batteriesFlow batteries operate on a fundamentally different principle than the batteries we’ve looked at so far. Rather than storing energy in metals on the electrodes, energy is stored as a dissolved metal in an aqueous electrolyte. The anolyte is stored in one tank; the catholyte is stored in another; pumps circulate the fluids past electrodes (sometimes in a fuel cell), where they don’t quite mix, thanks to a thin separator, but they exchange ions and electrons, generating electricity.The key conceptual difference is that flow batteries separate energy (the amount stored) from power (the rate at which it can be released). If you want more power, you make the electrodes bigger. If you want to store more energy, you make the tanks of electrolytes bigger. And electrolytes are fairly cheap, so it’s cheap to increase capacity. This is in contrast to LIBs, which double in cost with each doubli