It’s a question often asked by people who have little to no experience with 18650 batteries and look at them from an appearance aspect only, so lets start there;
Cylindrical lithium batteries are named by their size. A bare “unprotected” and “flat top” 18650 battery will always be 18mm in diameter (width) and are 65mm long (give or take a little manufacturing tolerance). AA batteries on the other-hand are 13.5-14.5mm in diameter, and 49-51mm long. This tolerance is much wider, largely because AA batteries are normally fitted into spring loaded compartments and a millimetre or two difference is of no consequence.
The most common nominal voltage for almost every NiMh or NiCD AA battery is 1.2V-1.5v. 18650 batteries have a nominal voltage of 3.6V, so this means you would need 3 AA batteries in series configuration (attached end to end) to get the same voltage as just one 18650.
Capacity, current and “Energy density”
The highest capacity AA batteries on the market right now are made by Ansmann with 2850mAh but this is the very top end of rechargeable AA form factor cells. For the most part, 2500mAh is more readily available and considerably cheaper and we have a fantastic range of Eneloop products in this top end category. The highest current that can be drawn from any AA battery however, is only a few amps. Some 18650 batteries can be discharged at rates more than 10 times the ability of an AA option. The Sony VTC5A 18650 battery for example can be discharged at 25A continuously.
The biggest reason to use 18650 over AA is Energy Density. We won’t get too far into it here, but far enough to demonstrate some easy figures. Energy density is the measure of how much energy is contained within a battery or cell measured in Watt hours. Calculating the Watt hours in a battery is very simple. Take the mAh rating, multiply it by the nominal voltage, then divide by 1000.
For example, a normal Duracell Alkaline battery has around 1500mAh. Its nominal voltage is 1.5V. This means that (1500*1.5)/1000 = 2.25Wh. Not a lot at all
An LG MJ1 has 3500mAh and a nominal voltage of 3.6V. (3500*3.6)/1000 = 12.6Wh
That’s a big difference and is what (appearance and small increase in size aside) sets 18650 apart from AA batteries.
Ultimately, the best battery for the job is decided on by the manufacturer of the product and its rare that they get this wrong. To use 18650 batteries in a TV remote is overkill and would make them much larger. To use AA batteries for vaping doesn’t work because they don’t have the required current output ability or energy density. What does remain however is that they are two very different products, each with their own set purpose, characteristics and applications. If you’re ever in doubt about what to use in your device, you should always contact the manufacturer of the product or a reputable battery retailer for advice.