3.5v solar panel Li-Ion battery charger
The solar garden lights I have charge one AAA 300mAh Ni-MH cell in 10 hours from a 2V Solar panel in series with a 0.3V Schottky diode to prevent the battery from discharging into the solar panel in darkness.
Your 18650 Lithium battery needs much more voltage and current to charge. If your lithium cell has a capacity of 3000mAh then a few solar panels like I have in series will charge it in 10 days of sunlight. Sixty of these little solar panels in series/parallel will charge the Lithium battery cell in a couple of hours before it explodes and catches on fire from overcharging.
A Lithium cell needs a battery charger IC for safety.