The long-term stability of Li-ion batteries can be negatively affected by mechanical stresses in the microstructure of the cathode material during charge/discharge cycles. Therefore, materials are of great interest which show a zero-strain (ZS) behavior, i.e. a negligible volume change during insertion or extraction of Li ions.
Within a joint project funded by the German Research Foundation (DFG), we combine experimental measurements with theoretical calculations to explore the suitability of iron-based flourides with tungsten-bronze structures as ZS cathode materials for Li-ion batteries with the goal to identify design criteria for ZS behavior in this material class.
We carried out atomistic simulations to systematically investigate intercalation of alkali metal ions (A = Li, Na, K) into TTB and two other tungsten-bronze crystal structures: the perovskite tungsten bronze (PTB) and the hexagonal tungsten bronze (HTB). Using a method of density functional theory with a Hubbard U correction (DFT+U) we systematically calculated formation energies and volume changes of both unit cells and local coordination polyhedra in these structure for different A-ion concentrations.
During the intercalation of the A ions, the formal oxidation state of the Fe ions changes and their ionic radii increase, which leads to an expansion of the surrounding octahedra due to the increased bond lengths. This effect is partly compensated by the volume reduction of the polyhedra surrounding the A ions due to reduced repulsion between the negatively charged fluorine anions. Our results indicate the needed structures of the insertion cavities and of the host lattice to achieve a ZS insertion mechanism.