This is logically efficient from a technical standpoint, but from a practical perspective is a terrible idea. You're only getting 2-2.5x th energy storage out of the process, but in return you're effectively requiring that the entire fluid system be isolated from the environment. Toxicity aside, you can't do anything with the fluid outside of the system. It's probably not something you want local fauna drinking, nor do you want even the slightest chance of this leaking into the local aquifers. I presume that, if it's not fully isolated, the fluid mix balance would have to be adjusted to offset evaporation of the water. And if the plant turns out not to be as great at you hoped hat do you do with the fluid?

Some numbers - a quick google says "According to Ofgem, the typical household in Britain uses approximately 2,900 kWh of electricity annually." I'm going to round that up to 8kWh/day. For a small village of, say, 1250 homes and a three day storage capacity, that's 30MWh. 1MJ (MWs) is 1000kg (one metric ton) stored at 100m - the upper end of this project. Since 3600 seconds per hour x 1MWs = 1 MWh, and we want 30, that's 1MT x 3600 x 30 = 108,000 Metric Tons of this high density liquid needed for a small project to put a 3 power day buffer in place for a town of 1250 houses. WTF are you going to do with 108,000 metric tons of high-density fluid if you decide is isn't working? Your reservoir would only need to be 25% bigger (wider, longer, and deeper/taller) to just do the whole thing with water and you wouldn't need to figure out how to get 3500 full size tanker trucks to transport it all away somewhere for a different project for for de-slurry processing.