What kind of capacitor should I use in Buck topology if the load is a lead-acid battery?
Firstly, I'd use this guide to the BQ2031 - it has a fuller description and includes a circuit diagram. I'm saying this because the diagram in the question is probably not going to work.

As for the output capacitor, \$C_O\$, this should be a decent low ESR type and placed close to where the inductor and \$R_S\$ is on the circuit board: -



I've circled the diode that is needed (and missing from the diagram in the question). Also circled is \$R_S\$ which is in the milli-ohm range and is required when the battery leads are very short.

It should also be noted that the diode (preventing battery discharge when the charger is not powered) should not be used where the question diagram proposes. See DB1 on diagram above.