I think the issue is that li-ion isn't that much better than NiMH as to be worth it. It's also worth noting that there are already standard sized Li-ion batteri

I think the issue is that li-ion isn't that much better than NiMH as to be worth it. It's also worth noting that there are already standard sized Li-ion batteries. 18650 batteries (so called because they are 18mm diameter by 650mm long) are the most common, but there are a bunch of other standard sizes. These are commonly used in things torches (flashlights) and headlamps.