K0.5FeF3 as a New Zero-Strain Cathode Material for Lithium-Ion Batteries

Abstract

The stability of the cathode is of crucial importance for the service life of lithium-ion batteries (LIB). In the cathode, the lattice parameter usually changes during (de-)insertion of lithium, which causes various signs of aging that have a negative effect on the capacity of the cell. This problem can be tackled using a specific type of materials called zero-strain (ZS) materials. These materials do not show any (de-)intercalation-induced structural changes and stresses. Such a zero-strain effect was already demonstrated with fluorine-containing materials [1] and even for the similar material K0.6FeF3 [2] for sodium-ion batteries (SIB). In this case, potassium ions are removed in the first cycle and the vacant lattice sites can be used for the incorporation of sodium ions, while keeping the structural integrity of the host material.Complementary to experiments, theoretical atomistic simulations are powerful to develop design criteria for further ZS-materials. Combining experiments and simulations in this work, the structural behavior of the material K0.5FeF3 was investigated when used as a cathode material in a LIB. The synthesis of K0.5FeF3 was achieved by a Rapid Microwave-Enhanced Solvothermal Process, which allowed to obtain the material in a TTB-type (Tetragonal Tungsten Bronze) phase with a high degree of purity. The material was then processed into electrodes, which were galvanostatically cycled against lithium metal. It was shown that the material is suitable for LIB as a cathode material. X-ray diffraction experiments showed that the lattice parameter changed by less than 1% after the incorporation of Li-Ions, in good agreement with the theoretical calculations conducted in parallel. Lattice Parameter Charged Discharged a[Å] 12.74±0.024 12.73±0.006 c[Å] 3.99±0.008 4.00±0.002 Volume [Å3] 647.57±2.67 647.75±0.76 This work was supported by the German Research Foundation (DFG) BI 1636/7-1 and EL 155/29-1.