How many large format LiFePO4 cells can be connected in series?
You will need:


A few million dollars for an engineering team to develop a good enclosure with all the spacing/creepage clearances/insulators.
A very sophisticated central BMS that can work across the full kV range.  This will likely require fiber optic insulation.
Good testing facility with very expensive battery testing equipment that works at 1 kV.
Full 40 cal PPE whenever you are close to the battery.




Mind you that Li cells come with built-in oxidant so firefighting is not an option.