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Lithium Tracer Diffusion in Lithium-Metal-Oxide Compounds for Battery Cathodes

Subject Area Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
Term from 2019 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 413672097
 
Final Report Year 2025

Final Report Abstract

The topic of this work was the study of lithium diffusion in lithium-metal-oxide compounds used as cathode materials in lithium-ion batteries. Understanding of diffusion is crucial for improving the performance and efficiency of these batteries. Systematic Li tracer diffusion studies were carried out on the layered structures LiNi0.33Mn0.33Co0.33O2 (NMC) and LiCoO2 (LCO), as well as on the spinel structure LiNi0.5Mn1.5O4 (LNMO) in form of sintered bulk materials and thin films. Investigations were done using 6Li tracers and Secondary Ion Mass Spectrometry (SIMS) and the results were compared to those of electrochemical methods (PITT, EIS). The influence of temperature, Li concentration and structural state was examined. The main achievements are: The Li tracer diffusivities in NMC and LCO polycrystalline sintered bulk samples are identical, and the activation enthalpy of Li diffusion was determined to ~ 0.8 eV, indicating a migration mechanism through structural vacancies. • Li-deficient NMC and LCO samples prepared by solid-state synthesis revealed no significant modification of diffusivities, indicating that vacancy formation does not directly correlate to the nominal Li content. Consequently, the use of a cathode initially made of a Lideficient material will not improve the battery performance kinetics. • In contrast, studies on electrochemically slightly (by about 10 %) delithiated NMC samples demonstrated enhanced Li tracer diffusion. • The results of Li diffusion studies using electrochemical methods (PITT, EIS) and the SIMS tracer experiments on NMC are in agreement. • Experiments on LCO single crystals prove that diffusion is anisotropic, being slower along the c-direction. Diffusion along the a/b direction is identical to that in polycrystalline samples, indicating that diffusion along grain boundaries is similar to bulk diffusion and does not play a dominant role for overall lithium-ion migration. Since grain boundaries are detrimental to mechanical stability, this work shows that there is no advantage in using polycrystalline cathode materials. • Crystalline NMC films made with ion-beam sputtering show identical Li diffusivities as bulk sintered samples. • Amorphous NMC films exhibit reduced Li diffusion, which is attributed to their unique structure. • After continuous electrochemical cycling both types of NMC films show enhanced tracer diffusivities and reduced activation enthalpies as measured by SIMS. This result can tentatively be explained with the assumption that the electrode remains still in a Li-deficient state after cycling. • In contrast to bulk materials, NMC films do not show a significant increase in chemical diffusivities after the onset of electrochemical delithiation. This explains the measured dependence of the gravimetric capacity on the current density due to low Li diffusion. • Diffusivities in LNMO bulk samples were also assessed and discussed in the framework of literature. Here, Li migration is likely to be due to thermal defects at high temperatures and structural defects at low temperatures. Combining the methodologies of SIMS-based Li tracer diffusion and electrochemical techniques provides valuable insights into optimizing material properties to enhance battery performance.

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