FeSO4F-based frameworks have recently emerged as attractive candidates for alkali insertion electrodes. Mainly owing to their rich crystal chemistry, they offer a variety of new host structures with different electrochemical performances and physical properties. In this paper we report the thermodynamic stability of two such K-based “FeSO4F” host structures based on direct solution calorimetric measurements. KFeSO4F has been reported to crystallize in two different polymorphic modifications—monoclinic and orthorhombic. The obtained enthalpies of formation from binary components (KF plus FeSO4) are negative for both polymorphs, indicating that they are thermodynamically stable at room temperature, which is very promising for the future exploration of sulfate based cathode materials. Our measurements show that the low-temperature monoclinic polymorph is enthalpically more stable than the orthorhombic phase by ≈10 kJ mol−1, which is consistent with the preferential formation of monoclinic KFeSO4F at low temperature. Furthermore, observed phase transformations and difficulties in the synthesis process can be explained based on the obtained calorimetric results. The KMnSO4F orthorhombic phase is more stable than both polymorphs of KFeSO4F.
- fluorosulfate cathode
- Li-ion batteries
- phase transition
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics
- Physical and Theoretical Chemistry