What is the efficiency of a lithium polymer battery?
Efficiency is a complicated idea to apply to a battery of any chemistry.

The losses in a cell are going to be the impedance of the cell during charging and discharging.

If you have cells of a known resistance, an known steady currents for charge and discharge, you can calculate the losses by using P=I^2R.

The problem is that these losses are dependant on the current. Current varies significantly between applications. As an example, I have designed things which charge at 0.5A or 2.2A. And they output anything from 1A to 10A (or more). Charging at half an amp is nice an efficient, but very slow. Higher charging currents lead to more thermal losses.

You've then got the complexity of how high currents and how they effect the cell chemistry itself. In a very top level and basic view, the higher current used to charge a pack, the lower the usable capacity tends to be.

Then there are lots of questions about how to rate the capacity and how it's valued etc. I'd recommend you do some serious research into battery chemistry if you are going to use cell capacity as your yard-stick.

TL:DR - losses from cells are pretty much only P=I^2R losses, don't get confused about capacity when talking about efficiency.