Battery storage is mature, yes.

Unfortunately, there is not enough (economically extractable) Lithium to go around. Experts estimate a Lithium "hole" of up to 300.000 tons of missing yearly production by 2030.

There is nothing we can do about it, only prospecting for more Lithium, reducing red tape for Lithium mining (with all the negative environmental impacts), or further increasing efficiency of Lithium use in batteries. As you said, Lithium battery technology is mature, so we have hit diminishing returns here.

This is why the feasibility of sodium batteries that was announced a year or so ago was such a big deal. Getting Sodium (e.g. from table salt, mined as excess in potassium mines, with a price of around zero) is easy and cheap.