Can Lithium-Ion batteries be recharged by placing them in sunlight?

No, Lithium Iron batteries can not be recharged without a charger (which may run off solar power) but “Li-ion (Lithium Iron batteries) also performs better at high temperatures than at low ones. Heat lowers the internal resistance but this stresses the battery. Warming a dying flash-light or cellular phone battery in your jean pocket might provide additional runtime in the winter.” (Discharging at High and Low Temperatures)
Similarly if a battery is cold then warming it in the sun may provide some additional runtime, not by charging it, but by decreasing its internal resistance.