Why does smartphone battery capacity decreases over time?

Battery used in smartphone are lithium-ion battery due to higher electrode potential but there is some limitations.

Battery used in smartphone, during charging, lithium gravitates to the graphite anode (negative electrode). Removing the  lithium again during discharge does not reset the battery fully. A layer of Solid Electrolyte Interface ( SEI ) grown in the surface of anode. Composed of  lithium oxide and lithium carbonate. As, battery cycle increase, the SEI layer grows and get thicker and thicker which is irreversible and eventually forms a barrier that obstructs interaction with graphite.

On the other hand, cathode (positive electrode) develops a similar restrictive layer known as electrolyte oxidation. It is observed that above 4V it is occurred, longer the battery in high voltage higher is the  development of restrictive layer hence, the faster the degradation of battery occurs. A battery loses 20% of its capacity after 400 complete charging cycle.

This phenomena can quantitatively measured by coulombic efficiency or ( CE ) . More the CE ( 0 < CE < 1 ), more the battery life or battery capacity reduce slower. CE alo depends on temperature and charging/discharging rate ( c-rate ). As coulombic efficiency increase, battery life also increase, hence the longevity increase or capacity reduce slower.

Five experimental batteries were tested for coulombic efficiency. A higher CE provides a longer life.
Courtesy of the Dalhousie University.

Relationship of coulombic efficiency and cycle life. High CE values live the longest; low values die first.

So, it is observed that, if you higher the voltage above 4.0V the electrolyte oxidation take place at cathode and if you lower the voltage then SEI layer will grown up at anode.

The loss of battery capacity also can be explained by C-rate or charging/discharging rate. Higher the C-rate lower the CE , hence, lower the battery life or battery capacity reduce faster. The elevated capacity loss at higher C-rates is lithium plating at the anode caused by rapid charging.

Cycle performance of Li-ion with 1C, 2C and 3C charge and discharge

Moderate charge and discharge currents reduce structural degradation of battery capacity. So, it is better to change your phone by normal charger and reduce the use of firs charger.

By this way temperature also effects your phone. At high temperatures the SEI layer at anode ( negative electrode ) and electrolyte oxidation layer at positive electrode take place faster, which farther reduce the battery capacity. So, keep your phone and laptop cool, as much as possible. The life expectancy of a smartphone battery is on an average 2.5 to 3 year and for a laptop 3.5 to 5 year. Battery dies after 1000 charging cycle.

Here is the some basic tricks to keep your smartphone and laptop battery cool.

• Don't switch on wifi, bluetooth and internet while not using.

• Don't use your phone or laptop on direct sunlight unnecessary or keep your phone at room temperature.

• Don't keep your phone or laptop in fridg!

• Don't keep brightness at 100%.

• Use power saving mode when possible.

To know more about reduction of battery capacity in detail visit: What courses Li-ion to die? 

And watch this:

To know more about battery mechanism watch this:

The Credit of this article goes to https://batteryuniversity.com/learn/article/bu_808b_what_causes_li_ion_to_die

and learn engineering YouTube channel, Quora, Wikipedia.