Lithium- und Sauerstoffdiffusion in undotiertem und Al-dotiertem LiMn$_{2}$O$_{4}$

Schwab, Christian; Martin, Manfred (Thesis advisor); Eichel, Rüdiger-Albert (Thesis advisor)

Aachen (2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

LiMn$_{2}$O$_{4}$ (LMO) is a candidate for the cathode material in the next generation of lithium ion batteries. To specifically tailor and optimize the material, knowledge of the defect chemistry, microstructure, diffusion and the effect of doping is required. In this study oxygen and lithium diffusion of dense, undoped and Al-doped LMO-pellets is investigated using isotope tracer diffusion experiments. The analysis is carried out by secondary ion mass spectrometry (SIMS). The pellet samples are prepared by spray drying and subsequent field assisted sintering technique (FAST). To investigate the oxygen diffusion in undoped and Al-doped LMO pellets oxygen exchange experiments in 18O2-enriched atmosphere are performed and analyzed by means of SIMS. Due to an occurring phase transition at the surface during the experiments in the temperature range of 400 °C to 700 °C and a pO2 of 200 mbar no results are obtained without the knowledge of the transition kinetics. The lithium diffusion in the undoped and Al-doped LMO pellets is investigated using an amorphous 6Li enriched thin-film, deposited on the pellets by pulsed laser deposition (PLD). Diffusion anneals in air in a temperature range of 20 °C to 210 °C were carried out to introduce tracer diffusion profiles between thin-film and pellet. The resulting diffusion profiles show several features assigned to film, bulk and grain boundary diffusion. To obtain tracer diffusion coefficients from the experimental profiles two two-dimensional numeric models are used and compared. The first consisting of infinite parallel grain boundaries and grains and the second being the brick-layer model. The modelling yields lithium tracer diffusion coefficients for the amorphous film, the bulk material, and the grain boundaries. The results show a fast grain boundary diffusion being orders of magnitude faster than bulk diffusion. Results obtained from the numerical models are compared to Harrison’s type A and B regimes of classic diffusion kinetics in polycrystalline samples. Activation energies of the corresponding diffusion processes are obtained by temperature dependent experiments. In this study, bulk and grain boundary diffusion in LMO are d for the first time using one experiment only.

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