I think you misunderstood the article - they're talking about the capacity of the battery in terms of kilowatt-hours, not its cost relative to round-trip effici

I think you misunderstood the article - they're talking about the capacity of the battery in terms of kilowatt-hours, not its cost relative to round-trip efficiency. If the cost were $6/kwh, you could build a 100kwh battery for $600, and charge/discharge that many times. I think this is supposed to be interesting both for its relatively cheap materials (and useful level of energy density), and its ability to be 'frozen' in a room-temperature charged state for long periods of time without much loss of charge.