li-On charger Reverse polarity protection
I have not seen electronic polarity protection on lithium batteries. Normally a polarized connector is used so that it is not possible to connect the battery with reverse polarity. Barring that, I would think that red/black color coding would be enough to prevent reverse polarity connections.

But if you REALLY want a circuit solution, here it is.



Q1 should be a PMOS FET with low Rds(on) at 2.7V. One possibility is alpha and omega p/n AO3401A. Q2 can be a 2n3904 or any typical logic BJT.

Note that this circuit will prevent a totally dead battery from charging. The battery must be at least 0.6V or so to turn on Q2.