I use LifePO4 (Life) Battery's in my 1/8th RallyX Buggy they offer a 6.6v rating so they can be used with standard servo but they do liven them up slightly, they are in the same price point of Nimh receiver battery's but they can come more compact allowing higher capacity with the same size battery, They also weigh a load less than a Nimh pack, they do require a propriety charger but most multi-chemistry chargers will have the option to charge them, Hope this Helps
Mark