The E-M1 uses the same BLN-1 battery that is used by the E-M5 and E-P5. It's a 9.2Wh unit, that is good for around 350 shots (according the CIPA standard testing).
For such a high-end camera, this number is a little low - we're a bit surprised that Olympus hasn't used the additional space created by the larger grip to provide a larger battery.