How do I charge a commercial lithium battery with solar cells?
You'd probably be better off taking a bigger spare battery. 

But if you really want to charge using solar energy, you have to understand that there isn't much. If you have a 3cm^2 solar panel and go on the basis of 15% efficiency for the solar cell (which is pretty good), then plug that into the solar energy per square meter on Earth, which is from 1,413 to 1,321 W/m^2, which gives you about 59mW. Your battery is 3.4Wh, so it will take over 2 days to recharge it, nevermind that the sun is only around for a fraction of that or conversion efficiency.

Working backwards with those numbers, if one wants to charge a 3.4Wh cell within 4 hours, they would need a (3.4Wh)/(4h)/((1321W/m^2)*0.15)=43cm^2 solar cell.