Does anyone know a general answer to these questions?
In my opinion when a battery is charged electric energy (potential difference) is converted to potential chemical energy.
In an ideal battery only reversible chemical reactions occur, so that it can deliver many discharge/recharge cycles. However, it’s not possible to design a battery in such a way that no unwanted reactions occur at all. Over time products of unwanted reactions foul up the mix. Battery design is about trade-offs. The lead-acid batteries that are used in cars are good for many cycles (indicating that very little capacity is lost to irriversible chemical reactions), but the lead-acid chemistry is not suitable for portable devices.
But there are also forms of storage of electric energy that do not convert it. A capacitor stores electric energy directly.
In a capacitor some regions of its interior get a surplus of electrons, and other regions (separated by an insulation with special properties) become proportionally electron depleted.
The electric force is mind-bogglingly strong, and it’s a long-range force. That long range is the big problem. To avoid concentration of electric force the electron-enriched and electron-depleted regions must have a very large surface area, and all of that surface must be very close-by to each other. (A common way to fabricate a capacitor is to stack ribbons of foil and then roll up that ribbon to a cylinder.)
When a capacitor fails internally there is a runaway effect. A capacitor must have very strong safeguards against failing, because if it does it’s catastrophic failure.
The reason that fuel such as gasoline is so efficient as a means of high-density stored energy is that the chemical force between the atoms of a molecule is very short-range.
Once fuel is burning a lot of heat becomes available. That heat is generated by the attraction between the oxygen atoms from the air and the carbon and hydrogen atoms in the fuel. But even a mixture of air and fuel vapor is still very stable; the oxygen is bound in oxygen molecules, the carbon and hydrogen are bound in the fuel molecules. It takes a pretty strong trigger to start combusting. Fuel is so stable (comparitively) because the force of chemical attraction between atoms is very short-range.