In a previous job I once evaluated some returned product that wasn't having the best battery life or wasn't working right. It was specialized expensive equipment that used regular alkaline batteries.

TL;DR: ordinary batteries can have negative voltage if you don't give a fuck and just mix your old and new ones in a drawer before you put them into your high-cost life-saving equipment.

Lithium-* batteries don't actually have any specific useful numbers. It's something like this (the actual numbers are pulled out of my ass and depends on battery time and test parameters and even then I'm simplifying):

• At 0 volts, the battery is dead.

• At 1 volts, the battery is practically dead.

• Discharging to 2 volts kills it after around 100 times.

• Discharging to 3 volts kills it after around 10 000 times

• Discharging to 3.5 volts kills it after 100 000 times

• Charging to 4 volts kills it after 100 000 times

• Charging to 4.2 volts kills it after 10 000 times

• Charging to 4.3 volt kills it after 1000 times

• Charging to 4.4 volts kills it after 100 times

• Charging to 4.5 has s significant chance of it catching fire

Now choose how many charge cycles you want it to survive, and you know which voltage to consider 0% and which to consider 100%. The bigger difference, the bigger capacity with the same battery.

This is why a phone with 0% battery can tell you that it's out of battery.

You can also adjust what "killed" means. Is it when battery capacity is reduced to 80%? 50%?

I have to repeat - the numbers are not accurate, and this is strongly simplified.

It's just an illustration of what 0% and 100% means it's just where you are on the useful range, according to the manufacturers definition of useful.

The battery has a charge curve. What does the most wear or damage to the battery is the ends of the curve - either deep discharging the battery or charging it up fully to the point where it cannot take any more charge. It's up to the manufacturer where they want to put 0% and 100% on the curve - to protect and extend the life of the battery most manufacturers don't put 0% and 100% at the extreme ends of the curve.

A cell's voltage will change with how much energy it stores, but if you keep applying current to force more charge to move you can cause voltages to be quite far outside of the proper range. However you don't want to do this as at minimum you are damaging the materials in the cell, or worse, cause a significant safety hazard where the cell could catch on fire.

You can look at the Discharge Curve of a cell which compares voltage vs capacity, as a rule of thumb, essentially the steeper the curve changes, the more damage you are doing to the cell by operating in the range.

Best source I can find: https://eugen-barilyuk.medium.com/how-i-realized-android-battery-percentage-wasnt-serious-enough-0310e484d874

3.7v is 0%, 4.2v is 100%. But a lithium battery can go higher and lower, it's just that doing that can harm the battery, perhaps spectacularly. OEMs just narrow the voltage range to extend life. When you set a charge limit, it narrows that range further.