I used Duracell rechargeable batteries for my lights. They're supposed to behave more like alkaline batteries, and so far I've had pretty good luck with them. I use them in a Radbot 1000 though, so I can't speak to the Planet Bike light. I do have the Cygolite Hotshot as a backup on my winter bike, and the only complaint I have about it is the power switch.

Although, if you're looking to reduce your environmental footprint, I would think recycling some rechargeable batteries in a few years is probably a lower impact than replacing a whole light that probably won't be able to be recycled.