The graph you link to says magnesium and diesel already have greater energy density than fat.
So, I think you have to specify how portable, how common or cheap, and maybe whether you are talking about rechargable or not - or the prediction is probably going to be vague - and subject to the criticism that it has already happened.
I meant commonly used for powering portable electronics. I don't assign a high probability to this. It is the upper bound of what I think worth discussing.
I would like to propose this as a thread for people to write in their predictions for the next year and the next decade, when practical with probabilities attached. I'll probably make some in the comments.