There are four ways to solar charge a lithium ion battery that I know of; please comment if you know of others or better ways. Each has their advantages and disadvantages. A difficulty with solar charging lithium ion batteries for electric bicycles is that the best voltage to use for motor powers from 200W to 500W is 36V. This in nearly exclusively a voltage used for electric bicycles and it's near impossible to find commercially available systems for charging this voltage with care needed to make sure it's suitable for lithium ion chemistry (36V regulators for lead acid batteries can be found, many of these regulators can be set at 12, 24, 36 or 48V).
Here are the methods I can think of with their approximate prices and pros and cons -
1) Get a standard grid connected solar system and then use your charger from the mains power. Approximately $3000 for a 1.5kW system in Australia with current government rebate scheme. Advantages - can charge at any time you like, excess energy fed to grid and you get paid for it. Disadvantages - you need to own a home and have a spare 3K, nothing elegant about it, losses through the system through inverter, AC to DC conversions.
2) Get a 12V 80-100W panel, a 5-10A solar regulator, a 80-120Ah deep cycle lead acid (AGM) battery, an ~300W inverter (modified or true sine wave, both seem to work).Connect all of these items together then connect your regular charger to the inverter. Approximately $1300. Advantages - can charge anytime you like, system can be used for other applications, relatively small, can possibly be put in a trailer for cross county touring. Disadvantages - need an extra storage battery, losses through AC/DC conversions.
3) Get a 36-48V panel. Have an electrician construct a regulator to drop the voltage to float at approximately 42.5V. This is the voltage the regular charger puts out, I've found it to charge batteries fully and not over charge them. I've been using such a system daily for about 4 months. I don't know what the long term damage may be but nothing's exploded yet and range is still fine.If using a 100W panel (48V), then it'll take about 6-26 hours of sun to fully charge a 36V 10Ah battery from dead flat. Approximately $650. Advantages - Quite elegant direct system, can be mounted at work or home and you'll never have to charge from mains if you're in a sunny zone within bombing range of your charging capacity. Disadvantages - When the bike isn't connected the energy created is lost, can't re-charge at night, need to find an electrician to create a step down regulator to match your panel.
4) Get 10W panels (about the largest size you can comfortably attach to a bike) cut specifically to give a closed circuit voltage of ~ 42-43V open circuit voltage of about 50V) - connect this straight to the bike battery. Some Chinese factories will do this if you promise it's just a test sample and you'll by hundreds (of containers full!). Approximately $100. Advantages - small and can be put on a bicycle (see picture), most efficient use of panel energy. Disadvantages - will only allow about 1km travel per hour in the sun, un-regulated system is probably not ideal and could cause battery damage over time (though so far so good).
I've put these methods in a video (it's near the end of the conversion kit video at about 14min into this youtube clip posted here):
Matt - Solar Bike - www.solarbike.com.au