That may be true but it’s extremely unlikely that you would need to support half grid for a whole week and if you did there would be a lot better options out there than lithium ion batteries.
Pumped hydro should really be the first port of call for this as the storage capacity and duration of supply can’t be matched by batteries and hydro is proven to work well.
Take the NZ power grid which is mostly run by hydropower, you’re basically doing the exact same thing except your pumping the water back upstream rather than relying on Mother Nature to do it for you.
Snowy 2.0 is going to provide 350,000MWh of storage once it’s completed. Taking your figure of 2,278,846MWh it would require a smidge over 6.5 Snowy 2.0’s to provide that amount of storage.
If Snowy 2.0 ends up costing the $12 billion it is projected to cost then that’s about $78 billion for that amount of storage. Eminently doable and not that big an expense in the great scheme of things and there are enough potential locations for pumped hydro to make that happen.
https://arena.gov.au/assets/2018/10...d-Hydro-Energy-Storage-The-Complete-Atlas.pdf