Are you sure NiMH has better capacity? You have to look at mAh * voltage (or just mWh) for a fair comparison.
NiMH is safer, and cheaper for now, but those factors are irrelevant when 1.2V is insufficient for a particular device. If a device accepts 3.6V with overdischarge protection, then regular Li-ion makes sense, but there are a lot of 1.5V-3.0V AA/AAA devices out there.
> Are you sure NiMH has better capacity? You have to look at mAh * voltage (or just mWh) for a fair comparison.
Great question. Yes. At least I was, until I just found some newish 1000mAh 14500 cells. Last I knew, 800mAh was the highest capacity around. Ironically, I think in a lot of cases the 1.5V Li-ion cells use Wh rather than Ah to obfuscate the fact that the cell has less considerably less capacity than a 30¢ alkaline. So there is now about the same energy in an AA 1.2V NiMH cell as in a top capacity 14500 3.6V Li-ion cell. But this doesn't stay true for the lifetime of the cell as opposed to a single charge, for the most part because NiMH cells can tolerate 4X the recharges before resistance gets too high.
> NiMH is safer, and cheaper for now, but those factors are irrelevant when 1.2V is insufficient for a particular device. If a device accepts 3.6V with overdischarge protection, then regular Li-ion makes sense, but there are a lot of 1.5V-3.0V AA/AAA devices out there.
In hindsight, the observation I made does not apply much beyond flashlights. You're right, a 3.6V cell is not going to help someone whose device takes 2xAA cells without Dr. Frankenstein's assistance. However, if 1.2V is insufficient to power a device that was designed around alkaline chemistry, then it's a problem with the design, not the chemistry (granted, I have heard stories that NiMH doesn't work in some devices, but this wasn't most devices nor devastating to consumers). Alkaline cells aren't 1.5V for very long, it's a pretty steady discharge curve from ~1.65V down to 1V. So a device with such strict power parameters designed around drawing power from alkaline cells, without taking into account the discharge curve of alkaline chemistry, is thus poorly designed and wasting upwards of a third of your batteries' capacity (if 1.2V no longer works, and an alkaline cell is fully charged at 1.65V).
NiMH is safer, and cheaper for now, but those factors are irrelevant when 1.2V is insufficient for a particular device. If a device accepts 3.6V with overdischarge protection, then regular Li-ion makes sense, but there are a lot of 1.5V-3.0V AA/AAA devices out there.