Li Diffusion Properties of LiMnxFe1-xPO4 Cathode Materials for Li-ion Batteries

• Applicant: Professor Dr. Harald Schmidt
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 567205453

The present proposal deals with the phenomenon of Li diffusion in LiMnxFe1-xPO4 compounds as cathode materials for Li-ion batteries. In general, Li diffusion is important for a fundamental understanding of kinetic processes at electrodes and for basic battery performance (charging/discharging times, maximum capacity, side reactions). Some advantages of LiMnxFe1-xPO4 compared to other electrode materials are long cycle life, thermal and chemical stability, safety aspects, and environmental friendliness. However, there are also disadvantages like a low Li diffusivity together with a low electronic conductivity close to room temperature. This leads to a sub-optimal rate performance and to incomplete capacity utilization, limiting the large-scale application in the field of batteries. Consequently, a detailed investigation of Li tracer diffusion and an understanding of the diffusion mechanism is of special importance for improvement. The aim of the proposal is to carry out the first systematic Li tracer diffusion studies in LiMnxFe1-xPO4. In order to probe intrinsic properties, diffusion is measured in binder and additive free material on polycrystalline sintered bulk ceramics. The material will be produced by solid-state reactions and sintering. Tracer diffusion experiments will be done by depositing a thin 6Li isotope enriched tracer layer on a sample with natural isotope composition (6LiMnxFe1-xPO4 / LiMnxFe1-xPO4). Afterwards the samples are diffusion annealed, leading to a penetration of 6Li into the sample under investigation. Isotope depth profiles are recorded by Secondary Ion Mass Spectrometry. By comparison of the 6Li depth profile before and after annealing to adequate solutions of the diffusion equation, diffusivities can be derived. The experiments will be done as a function of temperature. The activation enthalpy of diffusion will be determined and analysed in the framework of theoretical literature work, which allows conclusions about the point defect structure. The results will be compared to those of electrochemical methods: Potentiostatic Intermittent Titration Technique (PITT) and Electrochemical impedance spectroscopy (EIS) on the same type of material at room temperature. The difference between the results will be discussed and interpreted. The exactness and validity of electrochemical methods for diffusivity determination should be elaborated. It will be identified for which Mn/Fe ratio x in LiMnxFe1-xPO4 the highest diffusivities and the lowest activation energies occur for an optimized use as cathode materials from the viewpoint of kinetics. Strategies for an improvement of Li diffusion in these materials will be derived.

• DFG Programme: Research Grants