Crystal radios are only good in urban areas where there's a very strong xmit signal that's not too close to other strong xmit signals. In the boonies (no strong signals) or in denser cities (too many strong signals) they don't have the sensitivity and/or selectivity needed.

There are all kinds of xtal radio kits that let you do the whole thing with the crystal and cat whisker, but if you just want a simple no-battery radio, you could use a germanium diode detector (1n34 maybe) instead of messing with a crystal. Scrounge a loopstick antenna and tuning cap out of an old AM transistor radio and use a high impedance earphone.

Really though, an earphone radio doesn't take much battery power, so a conventional radio makes a lot more sense.