To design the buck for battery charging, you should choose your charging current (which will be the average inductor current), then choose switching frequency, amount of ripple current in the inductor (for example, 30-40% of average current), and finally, inductor L so that you get your desired current triangle ramp, from V(t) = L * di/dt.
Control the switching to maintain the current you want - peak current mode control is fine, for example. Use an outer, slower control loop to maintain voltage regulation, by controlling the current setpoint. Or in the simplest case, just turn the control off once a certain voltage is reached, thus no voltage control loop necessary.
"Impedance of a battery" is never part of the equation, and only becomes relevant if you want to model the losses in the battery.