There are number of options like i-Blason, Alpatronix, Lenmars Maven and Mophies Juice Pack. However, Mophie is one of the best iPhone 6 battery cases. It comes out with a better solution, although at a higher price most likely. It doubles battery life of my iPhone 6. And if you go for Juice Pack Plus, it gives 120% extra battery to your phone. Only drawback of the Juice Pack is that it adds a significant amount of bulk to your phone
iBattz also makes an interesting battery case. It has interchangeable batteries so you can carry around a few spare batteries and put it into the case to charge your phone is you're gonna be away from an outlet for a very long time
I think the best is PhoneSuit Elite, I had one in my iPhone 4S and it worked perfectly. Check the mophies reviews in amazon, there are many bad reports about the mute switch with this case.