The Chevy Volt, which has recently passed a cumulative half-a-billion electric miles, is expected to be updated for its second generation next year. In the meantime, the 2015 model year (which is released in 2014; automakers are always a bit calendar-challenged), while mostly the same as the 2014 model, will get a slightly larger battery pack.
The 2011 and 2012 Volts had a 16 kWh battery. That was bumped up to 16.5 kWh for the 2013 and 2014 models, and the 2015 will now get another bump, this one to 17.1 kWh.
It seems like GM didn't go through the whole process of getting the Volt re-certified by the EPA, so the official MPG-equivalent rating will stay at 98 MPGe and official electric range will stay 38 miles. But a bigger battery can't do anything other than help, so real world performance should be a bit improved. It's only a 600Wh difference, so don't expect it to be night and day, but the 2015 Volt might get slightly better MPG and drive a bit farther in electric mode...
Chances are, though, it's the second generation Volt that will get the real goodies. I wouldn't be surprised if it had a battery pack that held a more significant amount of extra energy, maybe some more weight-saving tweaks, etc.. I'd also love to see an electric-only version of the Volt 2.0!
GM has recently invested $449 million in Detroit plant that will make next-generation Volt. Of the $449 million, $384 million will go to Detroit-Hamtramck for new Body Shop tooling, equipment, and additional plant upgrades to build the next generation Chevrolet Volt, and $65 million will go to the Brownstown Battery Assembly for "next generation of lithium-ion battery production and future battery systems." This brings GM’s total investment at Detroit-Hamtramck to more than $1 billion over the last 5 years.