Li-Ion batteries, why does the capacity sag?

Question

Charger

I bought a device for measuring capacitance connected between an energy source and a consumer.
I have a three-year-old smartphone, with a passport capacity of 2000mA, I used to work freely for several days in the supine, on the table mode, now the charge flows away literally in a day. And while using a smartphone, it sits down very, very noticeably.
I measured the charging capacity, from ~ 18% to 100% (when the device stopped showing current> 30mA [its passport min limit]) - the value is 1500mA,
Unfortunately, I have no initial data, there is nothing to compare with yet.
If we take an inaccurate calculation of efficiency (converter for charging a battery with 5V) for approx. 89%,
then the new battery should have consumed 2000*89% = ~2247mA,
my battery ate 1500 + 18% of the remaining charge, which obviously will not be enough to select up to 2247mA.
Now the question, given that on a win81 smartphone, what is the subsidence of battery capacity over time, if they consume not much less than the same amount of mA for charging as "healthy" ones (there are no measurements for a healthy phone, maybe I'm wrong)?