How many electrons of charge can be stored on average inside a lithium mobile battery?

Some batteries will be labeled with their capacity in amp-hours or milliamp-hours. An amp is one coulomb/second. One coulomb of charge will account for 6.2x10^18 electrons. If you multiply the number of amp-hours by 3600 seconds/hour by 6.2x10^18 electrons/coulomb by 1 coulomb/second, you'll get the total number of electrons put out by a battery through a full discharge cycle.

However, it is not correct to say that the battery "stored" this number of electrons. You should think of the battery as a water pump that provides energy to water that is passing through it. The battery sends electrons out on a cycle through circuit and as they came back to the battery through the circuit, the battery gives them energy again and sends them out for another lap. So the battery doesn't really "store" that number of electrons any more than a water pump stores all the water that it will have pumped out in its lifetime.

Another way to estimate the number of electrons stored in a battery is see at what voltage it is rated and estimate the physical dimensions of the battery (namely the distance between the positive and negative terminals). This will allow you to estimate the electric field maintained which will give you an estimate of the charge which is generating that field. Charge is measured in coulombs, one coulomb of which is worth 6.2x10^18 electrons. The problem with this method, however, is that there is a lot of error in estimating the distance between positive and negative ends. You're not really interested in the terminals as seen by someone outside the battery, because those can be connected by wires to anodes and cathodes inside where the voltage is really generated. In short, you need to know something about the internal design of the battery to estimate the distance between the anodes and cathodes and therefore use this method to guess the number of electrons contributing to the voltage.

And finally, it is important to realize that anything made of atoms stores electrons. Every atom in the battery has a bunch of electrons. An extremely small minority of those electrons are in any position to contribute to the voltage generated by the battery.

Batteries don't store charge; a battery is overall neutral.

Batteries have chemical energy which is used to 'punp' electrons round a circuit. A bit like a water pump pumps water around a heating system - the pump is not a store of water.

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