In spite of fire concerns here on the ground, lithium-ion batteries could be best solution for backup power on the International Space Station.
Just like we have concerns with renewable energy being intermittent here on the ground, the International Space Station [ISS] is entirely dependent on renewable energy. Solar panels provide energy when the sun shines on them, about 66% of each day. [≈35 minutes of each 90-minute orbit is in complete darkness], so when the ISS is in the dark, backup battery power keeps everything running. Backup batteries are critical on the ISS, because ongoing experiments and computers need to keep running [don’t forget life support!]
Current ISS nickel-hydrogen [NiHy] backup batteries have a lifespan of 6.5yrs, which means new ones have to be brought up from time to time. Weight, when it comes to space travel, especially getting anything into orbit, is especially critical. If batteries can carry more energy for their weight, then it’ll be easier to get these into orbit. Lithium-ion [Li-ion] batteries are more energy-dense than the current NiHy batteries, so these might be the next set of batteries to get sent up to replace the old ones.
Some have raised concerns over Li-ion batteries and possible fires, citing recent problems with Boeing 787 Dreamliners, which are still grounded following fires in their lithium-ion battery packs. Don’t forget, fire need an oxidizer which, in the atmosphere is plentiful, in the near-vacuum of the ISS orbit, is nearly nonexistent if the battery is placed outside on one of the struts.