LFP Cells - Case to internal thermal resistance
The concept of "thermal resistance" with regard to "power output" assumes Joule heat dissipation in one area (junction or resistive element), and heat transfer across the surface of a device. In battery cells the major process is electro-chemical transformation, not resistive heat dissipation. 

Consider a Li-ion cell under charge. Say, the charger produces 1 A of current, and the battery is under 4 V level. You have 4 W of electricity flowing in, but the cell remains COLD, because the entire energy transforms into electro-chemical potential. The only heat production comes from the parasitic internal resistance, which is about 100 mOhms (on large signals), so the heat dissipation is just 100 mW, which is negligible for the typical cell geometry/size. That's why the Li-ion batteries stay cool, and start overheating only when there is a problem with chemical process. 

The same parasitic dissipation happens on the way of battery discharge - the heat is generated mostly on parasitic resistance. And this parasitic resistance changes over time as the battery ages.

Therefore, the cell temperature can't be calculated solely on "power output", and mostly depends on variable over time parameter - internal resistance (with some chemistry likely involved). I think this is the reason why manufacturers can't and won't specify the thermal resistance of battery cells.