The problem is not the receiver but the servos. The large majority of servos will only handle up to a nominal 6 volts. There are some new high voltage servos but the tend to be expensive. If you want to use something other than NiCad/NiMh batteries you might look into using two A123 cell battery packs or get a 6 volt regulator to step down the voltage for a LiPo pack.

Bruce