During the Q&A session, related to the Q4 2021 financial report, Tesla CEO Elon Musk said that he expects a transition of all stationary energy storage (ESS) products to Lithium Iron Phosphate (LFP) battery chemistry.
The switch of the entry-level Model 3 and Model Y cars to iron-based batteries was confirmed several months ago and happened to be quite successful so far.
In the case of ESS, it should be even easier because the lower energy density (vs NCA or NCM types) is not a big issue. The LFP batteries will be heavier, but it matters only in the initial transport phase, while once installed, the ESS is stationary anyway.
Strong points of the LFP are affordability (low cost per kWh), great longevity, power output and safety. On top of that, those are some of the most environmentally-friendly batteries (no cobalt).
As far as we know, since the beginning, Tesla used higher-energy dense battery cells, supplied by Panasonic and also other manufacturers (smaller scale).
According to Elon Musk, besides the LFP battery chemistry, there is also an option for a manganese-based version. If specs are good enough, we assume.
Tesla is building a new, 40 GWh Megapack battery plant in California, but the company has not specified who will supply the battery cells (most likely, the LFP).
Some link this project to the Gotion High-Tech's announcement about a 200 GWh LFP battery order over a six-year period (2023-2028), but it's just speculation (a probable one).