The 2015 Volt which is now in production Edit: is now confirmed (see post #13) to have an increased capacity 17.1KWh battery. This would be an increase from the 16.5KWh battery in the 2013 and 2014 Volts and 16 KWh in the 2011 and 2012 Volts. Though it has been suspected that many 2014's may have been manufactured with this increased capacity battery.
The 2015 Volt is listed on the Chevrolet website in several locations as having a 17.1KWh battery. At this time I do not have information as to what impact this has on usable state of charge window and overall EV range.
(might need to select 2015 Volt near top left)