https://www.pveducation.org/pvcdrom/battery-characteristics/...
https://www.pveducation.org/pvcdrom/battery-basics/electroch...
Update: I see it also depends on their concentration, so the voltage changes as the cell discharges, unless all the components are solids:
https://www.pveducation.org/pvcdrom/battery-basics/nernst-eq...