Characterisation of charge and discharge behaviour of lithium ion batteries with olivine based cathode active material

This paper gives insight to the physical processes taking part during the two-phase transition in lithium intercalation compounds. The behaviour of olivine based electrodes is in the special focus of this work. These electrodes exhibit phase juxtaposition within the electrode particles over a wide state of charge (SOC) range. Measurements were made to explore effects related to the formation of distinct phase sequences within the particles. Asymmetric charge characteristics, a load history dependency of the internal resistance and a voltage effect related to the disappearance of certain phase regions (the later on called vanishing phase effect) were identified. Moreover, these measurements give evidence to the existence of stable phase regions inside the electrode active material. An intuitive model is given to visualize the phase regions within spherical olivine particles. Therefore an analytical approach is developed in order to take the geometry of the particles, the ion permeability as well as the size distribution of the particles in consideration. According to the developed approach and the obtained measurement results, an enhanced cell equivalent electrical circuit is evaluated, considering phase shell development effects.