Agree with the "compete with lithium-ion" being weird. The biggest advantage of lithium-based batteries in general is the low weight of lithium providing a good power-weight ratio. For static installations like grid storage that should be mostly irrelevant. As you say, they are used currently because that's the only battery technology that is widely produced for decades.

While sodium battery tech is taking off, I am surprised that other alternatives like Nickel-Iron ("Edison") batteries and supercapacitors aren't used more for grid storage. Supercapacitors don't hold charge for long, but long enough to charge during daylight and discharge in the early evening when the demand-supply gap is lowest. Their size and weight shouldn't matter much for a static installation. Nickel-Iron batteries are heavy and need occasional topping up of electrolyte, but again those should be minor issues for a static installation.

Reminder that control of the patents for large-scale many-cycled NiMh batteries were bought by GM in 1994 then Texaco / Chevron in 2000; Chevron have refused to sell or licence the technology, preventing it from being used for anything other than AA and AAA batteries.