Lion rechargeable batteries will produce a higher output voltage for a longer time than Alkaline batteries, energy density, but initially their output voltage could be lower than Alkaline batteries. Lion batteries keep a charge longer during storage, low internal resistance. When the Lion batteries run down the voltage drop is faster than Alkaline. The Lion batteries do not last as long a the Alkaline batteries and they require a special charger.
If you can afford it, I would look at the external power pack for the SB-800. With this pack you keep 4 batteries in the Speedlight to power the control circuits and the power pack will recharge the strobe light circuitry and provide a larger cache of power.