Yes, lithium batteries can be charged by solar panels.
Like batteries, solar cells have a voltage that is determined by the materials and the number of cells connected together. This voltage can be increased by connecting solar cells in series and parallel to get higher voltages and currents. For example, if you connect two 1V/cell cells in series, you’ll end up with 2V total (1V/cell x 2). If you connect them in parallel, though, you’ll get 4V total (1V/cell x 4).
Similarly, if you connect four 1V/cell cells in series and then combine them with two 1/4-volt/cell solar panels in parallel, you’ll end up with 5V total (3V/cell x 2 + 0.25V/cell x 4).
Note that 2.40V/cell for lead acid and 4.20V/cell for lithium-ion are the full-charge voltage thresholds. Charge controllers are also available for lithium-ion to charge 10.8V packs (3 cells in series).