There’s little direct info on the Fairphone battery but generally the battery would typically last 2 or three years.
This means than on average if your battery is well over a year old it will discharge, in terms of percentage charge at a faster rate.
There are different make-ups of Li -ion which have longer life LiFePo are renowned.
One problem with Li-ion is that if they are discharged to zero and charged to 100% the life is shortened.
The Fairphone battery Charges up to 4.4V, on my phone, which adds stress to the battery although it does not get hot.
Here are a couple of links to help understand the issues.
The Lifespan Of Lithium-Ion Battery
The typical lifespan of lithium-ion battery is around 2-3 years or 300-500 charge cycles – whichever happens first. One charge cycle is calculated as the period of use from fully charged to discharged and fully recharged once again.
If the battery doesn’t run the complete charge cycle, you should rely on the 2-3 year lifespan of the battery.
The theoretical life of a Li(NiCoMn)O2 is approximately 800 cycles, which is medium in the commercial rechargeable lithium batteries. Currently, the cell manufacturers promise more than 500 times in the Li(NiCoMn)O2 specifications produced by them (charge and discharge under standard conditions). Nevertheless, following the assembly of the batteries into the battery packs, owing to the consistency issues, voltage and internal resistance primarily cannot be exactly same, and its cycle life is approximately 400 times. The battery pack manufacturer recommends the SOC as 10%~90%. It is not recommended to carry out deep charge and discharge. Or else, it is expected to cause irreversible damage to the positive and negative structures of the battery."