There are probably more lithium ion batteries in use than lead acid. Every cell phone, iPad, tablet, notebook and netbook computer has a lithium ion battery in it not to mention all the power tools, electric vehicles and military applications. BUT if you want something with a bit better safety record and much longer life get Lithium Iron Phosphate batteries. The common Lithium Cobalt batteries in consumer electronics have a life cycle of around 500 charges where the Lithium Iron Phosphate will get around 2000 cycles. They are still WAY more expensive than lead acid but if weight is your primary concern then it may be worth it. There have been reports of lithium batteries catching fire but it is extremely rare and often due to substandard workmanship or incorrect charging.