We often use batteries in standard sizes like AA. These are annoying to use, for multiple reasons. How can we improve them?
First, switch to lithium-ion. They have an energy density of 110–160 watt-hour/kg, while the next best option, NiMH, has only 60–120. This means that lithium-ion batteries will last longer. Nobody likes buying new batteries or recharging often, which are a hassle and cause environmental damage.
Lithium-ion has a voltage of 3.6 volts, compared to 1.5 volts for today’s AA batteries. This means that devices won’t be backward-compatible. That’s the price we’ll have to pay for further progress .
Second, with today’s AA batteries often need to be paired, which is a hassle. But a single lithium-ion battery has a higher voltage (3.6V ) than two AA batteries (3V), so lithium-ion batteries won’t need to be paired.
It turns out there’s already a battery that does exactly this, called 14500:
More devices should use them.
 Lithium-ion batteries should be slightly bigger than AA to avoid the risk of damaging incompatible devices by putting Lithium-ion batteries in them.