This CO2 is acting as a reusable fluid in a closed loop. The initial capture of the CO2 costs energy, but the battery keeps using the same CO2 over and over again. So the question of efficiency should be more about land usage and maintenance of the rest of the parts and the labor needed for each megawatt stored vs what other grid scale energy storage costs in materials and labor.
The rough reality is that batteries aren't going to be up to the task of grid scale energy storage unless they have a couple huge breakthroughs. Something like this is a far less materially expensive way to store energy for later use.
Currently most grid scale energy storage is just pumping water up a hill and letting it back down through a generator. It is extremely limited in where it can be used and requires tremendous space to be effective.
There's not enough minable copper or lithium to make all of the batteries we will need. So e alternatives will have to emerge if we are to reduce the need for generating power on demand with fuels.