LiPo charging circuit
Try searching for "power management IC" or battery management IC"

From TI: http://www.ti.com/lsds/ti/power-management/battery-management-products-overview.page this looks like what you want.

A word of caution, though: Li-ion batteries (especially big ones, like that 6s you mentioned) have the potential for fires and explosions. If you're going to sell this to a consumer, you really want to be sure that it's safe. Judging by your question, you don't seem to have a lot of experience, so might be good to consult with somebody who does.

Is there a reason why you have to have a Li-ion battery? You might also want to take a good look at other battery chemistries, especially the lithium-iron-phosphate batteries: https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery