Fuel cells convert chemical -> electrical directly, batteries go both ways.
For nuclear fusion, the only practical reaction releases 80% of the energy as uncharged particles (neutrons), for which the only way to harness their energy is to convert it to heat. The 20% of the energy that is emitted as charged particles could in principle be converted with high efficiency, but the other 80% dominates.
For fission, more of the energy is emitted as charged particles, but it's not clear how one would harvest it directly in bulk. (It can be done on small scales, but not efficiently.)
For nuclear fusion, the only practical reaction releases 80% of the energy as uncharged particles (neutrons), for which the only way to harness their energy is to convert it to heat. The 20% of the energy that is emitted as charged particles could in principle be converted with high efficiency, but the other 80% dominates.
For fission, more of the energy is emitted as charged particles, but it's not clear how one would harvest it directly in bulk. (It can be done on small scales, but not efficiently.)