How to Adequately Describe Full Range Intercalation-A Two-Sided Approach

One of the key challenges in battery research is to quantitatively describe the intercalation storage capacity as a function of the reversible cell voltage. The reason that such endeavors are not yet very successful, lies in the lack of an adequate charge carrier treatment. Using the most challenging example of nanocrystalline lithium iron phosphate, where the full range from FePO₄ to LiFePO₄ is accessible without miscibility gap, this study shows how a quantitative description of literature results can be achieved even for such a huge window. For this purpose, point-defect thermodynamics is applied and the problem is tackled from the two end-member sides including saturation effects. A first, rather heuristic treatment interpolates in-between using the safe thermodynamic criterion of local phase stability. Already this straightforward approach works very satisfactorily. In order to also gain mechanistic insight, interactions among and between ions and electrons have to be taken account of. This study shows how to implement them into the analysis.