What is the voltage, capacity, and discharge rate? And are they available in sizes that fit existing flashlight hosts?
Some specs of a 18650 NaIon cell were shared up here: 【Technology】Will the Sodium-ion batteries be the game-changer in the battery industry? - #8 by INeedMoreLumens
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Huge cells is definitely NOT a niche market. Remember the good “old” Tesla type “S”? The top of the line needed more than 8,000 pieces of 18650. Tesla model “3” was more or less the introduction of the 21700 cell. And now the Tesla cybertruck, and future BMW models, are equipped with 46800 (and taller) cells. Though cells and batteries for flashlights will always represent a minute portion of the total LiIon market.
Nevertheless: it might be a good idea to anticipate a charger for 46120+ cells. The diameter of it means a huge increase of the existing chargers. OTOH: given the enormous capacity of these cells I don’t think we will need a charger for 4,6, or 8 cells 
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I’d like to see a charger made looking a bit like a jar lid covering the top of the 46xx series battery. Screen for charge/discharge capacity. 3 amp charging to not overheat older charging ports. And please bring back those little carry bags that came with your chargers.
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I would welcome all kinds of batteries which are saver than LiIon, even if it had less capacity.
A half year ago a paper recycling plant had a major fire because of a Lithium battery from a greeting card. 80% of fire in recycling plants are caused by Lithium or Lithium-Ion batteries.
And if the raw materials are less critical and easier to recycle, that would be worth another technology
Yeah, maybe not for things you carry everyday but for other use for sure. Would be nice if my lamps with motion sensor would be shine longer than actual 
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After the 26800 debacle, I will stick to batteries that are produced in sufficient quantities to benefit from economies of scale, R&D focus, and multiple vetted suppliers.
Perhaps a larger battery would be useful in SaR (Search and Rescue) type lights, but its not something I would EDC, use for walks, hiking, running, or anything I would be carrying often.
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50A is pretty hot for 18650 but of course that would drain the cell in a minute. If the capacity could double up to 2500 mAh, even at half discharge rate, then these become a lot more interesting for their trade offs.
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It’s told that a bunch of EV companies that used the 26800 batteries went bankrupt. And QB is stopping 26800 batteries’ production. Many users think the 26800 is great and it’s disappointing to see it vanish. While, the bigger size 46800 and 46950 Li-ion batteries appear in the market. Not sure these bigger batteries will be the trend? It seems there is just not enough market support for them…
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For flashlights… Mh. maybe for the huge soda can-like lights, but most lights would be too chonky and heavy with such huge cells.
26800 made for great palm-grip-sized powerful lights, like big throwers etc. The formfactor is perfect for those. 50mm diameter is already hard to firmly grasp in your palm for a long time.
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Actually I doubt the big size cells will be a trend.
I see the benefit in delivering much power for long time but TBH, for flashlights you use e.g. for a longer walk you will prefer something with ligther weight.
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Supply on the open market for 46800 (and taller) Li-ion batteries is very little. BMW expects to use a lot of those and new factories are built as we speak. Samsung SDI will upscale/start the production of 46800, 46950, and 46120 batteries in Goed (Hungary).
Other new battery tech on the horizon https://www.yahoo.com/news/electric-vehicle-battery-could-recharge-013000849.html
when you storing energy, you can not avoid fires, or explosions upon its catastrophic failures, the more energy you store the more violently it will be released, it is very different from storing fuel.
How easily extinguished is the difference I imagine.
pretty much yea, fuel needs oxygen and temp, so remove either, it will extinguish, energy doesn’t need anything, it will keep going until all is released, you can’t really put it out,
From the Fire Protection Ass.:
“The flammable gases generated often ignite immediately, but may also spread out unignited, until an external ignition source is encountered, resulting in an explosion. Lithium-ion battery fires generate their own oxygen and can be very difficult to extinguish.”
Lithium ION may, although that can be considered many different compositions.
Lithium IRON however itself will not create enough heat to release oxygen and cause thermal runaway.
That only applies to some batteries, not everything coined “lithium”
The chemistry does not matter, if it stores energy it will release energy during catastrophic failure, even lifepo4. refer to post#6
You looking at it as a combustion, look at it as energy release,
Nice @$$! 