If you use water, it has 5 times the specific heat (but less density), so you only need 1 cubic meter. Probably easier to heat/cool/use, too. Water can also be heated more than 20 degrees above ambient, too.

My old house had an electric boiler that would automatically heat up at night electricity was cheap. They have fallen out of fashion in the past two decades or so around here, but I can see them making a come back.

Of course that was direct resistive heating. Stick a heat pump in there and you got something.