It's an interesting question, and one I've been pondering. It was part of the discussion I had with Bent Flyvbjerg, author of the much recommended book "How Big Things Get Done". He and his co-author included a bit of my China-oriented analysis in book, and so we had the opportunity to talk.

The modularity and manufacturing scale of cell-based batteries does make them cheap, but the gas tank and the engine are tightly coupled, metaphorically speaking. I'm still leaning toward pumped hydro and redox flow for that reason, but as a contraindication cell-based batteries won an 8 hour storage request for proposal recently.

I'm watching this space. I might be wrong. ;)