Here is a thread from awhile back discussing the 20700, and 21700 cells.
I believe they would be fine for our purposes, the only problem is that they will need a 'custom' size host rather than the repurposed flashlight hosts we use that only accept the traditional size li-ions.
There is nothing special about the 18650 cell size, it was just something that was very widely used in laptop batteries in the past. With laptops getting thinner there often is no place for these cells anymore, and many modern laptops used some kind of envelope-filling solution with oddly shaped cells nowadays.
Things like flashlights and lasers taking 18650 batteries are only there because this form factor is/was widely available. If something like 21700 becomes more readily available producers will adapt to it.
Perhaps things will change again and we could see common battery sizes like AAA, AA, C and D again in lithium technology. The AAA and AA sizes have been on the market for a while, but there is nothing inherently problematic in producing C or D cell sized ones.
The only issue is that they would fit into the wrong application, giving 3.7 volts per cell where the device expects 1.5 maximum. This is also a problem for AA/14500 sized cells as it is though.
Is about double the energy denisty of a 18650 according to reports 21700 is about 5750-6000 mAh vs 1500 - 3400 mAh for 18650. Not bad if anyone wants to make a laser host that fits it.
"The most important piece of technology that Tesla is working on isn't an electric car. It isn't a solar roof. It's a battery. Not the Powerwall or the new Powerwall 2, but a chunky, squat cylinder — like an oversized shotgun shell — called the 2170.
The commercial and industrial battery cell of choice for the last few years has been the venerable 18650, measuring a maximum of 65mm long with an 18-19mm diameter. Cells of the 18650 size have internal energy capacities of between 1500 and 3000mAh and energy density of around 250Wh/kg in applications like the Model S' skateboard chassis, but engineers have struggled to increase its energy density beyond current levels.
The thicker, longer 2170 — named for its approximate 21mm diameter and 70mm length — increases on the 18650's energy density significantly. Where the 66cm3 volume of a 18650 translates into a maximum electric charge of around 3000mAh, the 97cm3 volume of a 2170 has been tested within the region of 5750-6000mAh, doubling the level of charge for a 50 per cent increase in volume. That's a huge jump in energy density, and it's what allows Tesla's new Powerwall 2 battery pack to boast twice the energy despite being more compact than the original.
The 2170 could massively boost the battery life of everything from laptops to electric cars. In a recent earnings call with Tesla company investors and analysts, Musk said that the 2170 had the best energy density of any currently produced battery cell, beating out the 18650
The 2170, co-developed with Tesla's close working partner Panasonic — on which it has collaborated on the Gigafactory in Nevada — is also the cheapest battery cell currently produced according to Musk, due to the economies of scale of the Gigafactory and the batteries' ability to fit more raw materials inside to store energy."
I anticipated this battery would work. With the higher output lasers on the market a beefier power source would keep any laser working for a longer total time at max to near max output. Now some hobbyist has to utilize said battery in a custom design.