The neutrons and the antineutrons are made of multiple electrically charged particles whose electric charges sum to zero, exactly like the electric charges of a

The neutrons and the antineutrons are made of multiple electrically charged particles whose electric charges sum to zero, exactly like the electric charges of all particles that compose a neutral atom or neutral anti-atom sum to zero. Therefore there is no path to generate antineutrons in which you do not have pairs of electrically charged particles with opposite charges that are generated or annihilated. The annihilation and pair generation reactions are electromagnetic interactions between electrically charged particles and antiparticles with opposite electric charges, which are otherwise identical, in order to satisfy all conservation laws. Only the neutrinos do not have electric charge and about them it has not been proved beyond reasonable doubt that the antineutrinos are different from the neutrinos (in other ways except opposite spin). Neutrinos do not participate in annihilation and pair generation reactions. Neutrinos may appear and disappear in similar reactions that are mediated by weak interactions (i.e. by the heavy bosons), like the inter-conversions between protons and neutrons (actually between u and d quarks), but in these weak-force based reactions the energy that is produced is much less than in annihilation reactions and frequently much of it is lost by being carried away by neutrinos. It is possible to make electric generators with beta-radioactive isotopes, where this kind of reactions happen, but the only advantage of those is the extremely long lifetime, because otherwise the power density and energy density are low. So when talking about using antimatter for energy storage purposes, that refers exclusively to generating electrically-charged particle-antiparticle pairs, separating and storing the antiparticles, then annihilating the antiparticles with their corresponding particles, resulting in extremely intense gamma radiation, which carries the reaction energy. When the annihilation is done inside matter, which includes annihilation between nucleons (where there may be multiple annihilation events between the component quarks) the gamma photons interact with the surrounding matter or sub-nucleon components, producing a cascade of various accelerated particles, including many new particle-antiparticle pairs, which will cause later other annihilations. So from a single initial annihilation a great number of accelerated particles and gamma photons may result, but the first stage is always the generation of a pair of gamma photons.