Lithium-iron phosphate in LiFePO4 batteries consists of a large number of small, micrometer-sized particles which are charged and discharged individually one after the other. Charging proceeds particle by particle, and involves the release of lithium ions. A fully charged particle is therefore lithium-free and comprises only iron phosphate. Discharge in turn involves the re-incorporation of lithium atoms into the electrode particles, so that iron phosphate becomes lithium-iron phosphate once more.
The changes in the amount of lithium associated with charging and discharging induce a change in the chemical potential of the individual particles, which in turn changes the voltage of the battery. However, charging and discharging are not linear processes. During charging, chemical potential initially increases, with the progressive release of lithium ions. But then, the particles reach a critical lithium-content value and chemical potential. At this point, there is an abrupt transition – the particles give up their remaining lithium ions very rapidly, but are not allowed to change their chemical potential. This is the transition that explains why battery voltage remains practically unchanged over a wide region.
The time that elapses between charging and discharging a battery plays an important role. Charging and discharging are processes that alter the thermodynamic equilibrium of the 18650 battery, and this equilibrium can be achieved after some time. Scientists have found that idling a sufficiently long period of time can be used to erase the memory effect. However, in accordance with the many particles model, this only happens under certain conditions.
The memory effect only vanished if one waited a sufficiently long time after a cycle of partial charging followed by full discharge. In such cases, the two particle groups were still separated after the full discharge, but were found on the same side of the potential barrier. Thus, the separation disappeared, because particles attained an equilibrium state, in which they all had the same Polymer Lithium ion Battery-content. The memory effect remained however providing you waited after the partial charging and before the incomplete discharge. Here, the particles were on opposite sides of the potential barrier, and this prevented a reverse of their division into lithium-rich and lithium-poor.