The battery pack for the Volt is shaped like a capitol T.  It starts under the hood and runs through the center of the car.  The shape of the battery pack limits the Volt to only four passengers since it runs through the middle of the rear seat.  There is no room for a center seat unless it sits much too high to allow for adequate headroom and still be safe.

GM has used this shape in their EV-1 that they made back in the mid 90s and it so far has survived today.  But if they wanted to port the e-flex technology that drives the Volt to a different more family friendly platform the battery pack would need to change shape.  Luckily it is configurable so GM can choose whatever shape they desire.  The T works well in a car because of the limited space available in the car.  A minivan or SUV might allow the shape to change.