Efficiency of battery???
Li-ion cells have a HIGHER internal resistance then a lead-acid cell.
ALL rechargeable cells have a common problem: electrode distortion.
Recharging a battery is essentially an electroplating process. How the material is deposited on that electrode is going to be different then the way the electrode was originally machined. Ions are kinda dumb, ya know. They don't know or care what the electrode originally looked like.
Enough distortion and the cell won't charge anymore.
NiCad batteries had a big problem with distortion, which is why they tended to die after awhile and wouldn't hold a charge.
Lead acid cells have an additional problem if allowed to discharge through their internal resistance and to sit for a while...sulfating. Lead sulfate builds up in the bottom of the battery box over time. When it touches plates, they won't hold a charge anymore. You can actually rejuvenate a lead-acid battery by dumping out the electrolyte, washing out the lead sulfate, and filling with fresh electrolyte, then putting the initial charge on it again. The distortion in the plates as still occurred so it won't last as long, but it does extend the life of said battery.
Ni-Fe cells are heavy, just like NiCad, but they don't suffer electrode distortion quite as fast, and they have a lower internal resistance.
Chemically, all batteries are the same. All are based on dissimilar metals and an electrolyte, even the lead-acid battery (the dissimilar metal is a lead oxide coated plate, and a just a regular lead plate, producing a potential of 2.6v). These lead-acid batteries are normally charged at a voltage of 3.1v.
You should be aware that voltage is not joules. Battery capacity is measured in joules. You might be more familiar with watts. One joule is one watt per second. Watt-hours are really just joules. One watt-hour is 3600 joules.
Batteries often have an amp-hour rating. This is kinda fake. Amps mean nothing without voltage to drive the current. What is really being described here is watt-hours, or joules.
Voltage is just like pressure in a pipe. Current is just that...the flow in the pipe. The power such a flow can do requires both the pressure and the flow. Yes...Ohm's law works for pipes. Even resistance is there (the friction in the pipe).
Voltage (pressure) and no flow is no work. If the battery had infinite internal resistance, it would never discharge. Of course, all electrical and electronic components are combinations of resistance, inductance, and capacitance. Even a wire, which is why you have to meet certain wire sizes depending on length of run and the current flowing on it, and why you have to use a conduit larger than the wires inside it to allow them to cool.
A battery is no different.