Energy efficiency of Li-Ion Battery
Generally, the Coulomb efficiency of a Li-Ion cell is quite high (> 99 % when not overcharged), but the voltage efficiency isn't as good. This is basically due to internal resistance of the cell (and effective resistance because of the diffusion of ions inside the cell). So, charging and discharging at high currents basically creates higher I*R drops across these resistances, and this wastes energy, but most of the Coulombs you put in (or out) get stored.