With outdoor temperature ranges of 0 to 100F, –18 to 38C, I keep multiple flashlights with 18650 battery’s in all vehicles. Charge typically to 4.1V, never in the vehicle. I also keep battery operated tools in the same conditions. If I am slightly shortening the battery life, then so be it.
In all of my cars you’ll find 9v PALights. Their continuous glow makes them easy to find and is a reminder that the battery is still functional. I’ve never had a problem with storing 9 volt batteries in the car, even in Florida. And as others have mentioned, in an emergency 9 volt batteries are plentiful when everything else has been exhausted.