Charge 14500 LiFePO4 lithium batteries from an Arduino Nano I've recently started tinkering with Arduino, and I'm attempting to build a little 2WD robot based on an Arduino Nano.

I was thinking of powering the robot using two 14500 LiFePO4 batteries, which I understand will fit in a standard 2 x AA battery holder, and in series will give me about 6.4V, which should be ideal.

If I were instead going to use AA batteries, I think I'd need 4 of them, which will make my robot a fair bit bulkier.

I would like to be able to charge the batteries simply by plugging a standard USB cable into the Nano's USB socket.

Is there some easy way to safely do this? Do I need a separate charging module board, or is there some way to do it directly from the Nano?
No, you cannot charge a Lithium-Ion battery (of any variant) directly from an Arduino. You require a Lithium-Ion battery charge controller. 

The best source for these kinds of things at a small size is retailers of model vehicle parts.