I’ve seen a 12V computer fan work in a canal boat bathroom, but have to agree with SG that the energy required to move useful amounts of air is going to kill even a bank of 18650’s pretty quickly.
EDIT: Data point - my bathroom fan is rated 25W @ 230V, which were the power linear (it isn’t, but…), 25W/ 12V = over 2A! Even the best Noctua PC fan is going to pull quite a current from batteries.