Sorry, as far as I am aware, there is no other battery with higher capacity.
In fact, it seems the update to Marshmallow (Android 6.0) even made the battery draining worse.
You will find lots of discussions on this topic in the forum, even concerning different OS. Maybe these threads give you some insight
And you will find even more links there.
Maybe Fairphone is working on it and we will be surprised when the new Generation (FP2.1) is delivered. I read somewhere in this forum they promised some surprise or the like. Guess was a new and better camera module, but a bigger battery would be another option. (Just guesswork of course ;))
Up till then you could always opt for a power bank to accompany your phone or for a second battery. There is even a cover-design that allows for carrying a second battery (although not connected; unfortunately):