Hi, it looks far to late to speak about this for FP2, but instead of producing specific FP2 lithium batteries, it would look fairer to me to allow owners to use existing standard batteries to power their phone.
Instead of a dedicated (complicated to recycle) FP2 battery, a simple AAA battery tray (small cylinders form factor) with 2 units would provide more energy than the dedicated battery it seems as you can buy regular AA batteries of 2700mAh each in every large store.
It would be easier to replace old batteries, find spares (anywhere, anytime…).
I’m not an expert on electronics, but I would assume this requires hardware to charge the batteries properly (and I’m not even sure NiMh, which is most AA/AAA batteries are well suited for use in a smartphone*), or using cells with built-in circuits like a 10440. Total capacity wouldn’t be higher than the current battery. At the same time it would add bulk and weight that most users would find annoying. That being said, it’s conceivable that an enthusiast may be able to design and 3D print a custom back cover that could accommodate required adaptor and circuitry.
Somewhat relevant: There’s some discussion about battery frames (though not for standard batteries) in this topic here.
* Charging in phone may produce too much heat; not sure whether it can reliably discharge fast or long enough enough to keep up with demand (discharge curves); suffers too much from a memory effect (leading to more battery waste).
NiMh batteries (that you can buy in every store) have a greatly less energy density than Li-Io batteries. For that you need to look at the capacity in relation to the voltage.
The energy in a battery is the voltage multiplied by the capacity.
So a Lithium battery has around 2000 mAh at 4V, which is 8 Wh.
One single NiMh battery (AA) has about 2500 mAh at 1.2 V, which is 3 Wh.
NiMh batteries don’t support fast charging, they do have a memory effect and the life cycle is only about 500 charges. Li-Io batteries support about the double amount of charging cycles.
There is a reason why most electronic devices start having Li-Io batteries…