Battery alone will never work. Just tonight I went outside to take my son to work, and my garden lights were not working. It has been overcast all day long and raining so they did not get a charge. Now I know that the solar panels on them are small and not high quality but extrapolate that out and you see that they need almost daily charging to be totally reliable.
Garden lights have cheap, usually poorly made, solar cells. If you bought them more than a year or two ago, they have NiCad batteries--cheap ones that are less than 500 milliamps, and low quality. The NiMH batteries in the new ones are cheaper and low quality. If you change the batteries in your garden lights to Duracell/Eveready/Rayovac 2000+ mA your lights will charge better and last longer, even on cloudy days [but the batteries might cost more than the lights.
Lithium batteries for heavy use are entirely different from NiCad and NiMH anyway--no comparison. Aside from cell phones and other electronics, they're used in cars and for storing power generated by solar PV shingled roofs.
We've been following the development of PV power and storage for over 20 years, but only recently has it become more affordable, thanks in part to the investment by the German government that encourages homeowners to install the solar shingles. Similar projects are in progress in Japan. In the US? For now, open your wallet and pay for it, unless you live in a state that can afford the tax credit.
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