Li Diffusion in Rare-Earth Metal Doped Battery Materials

One of the key elements determining the performance in Li-ion batteries is the diffusion. All solid-state batteries require fast ionic diffusion in the electrolyte but equally important in the anode and cathode. The electrolyte is required to be electronically non-conducting, the opposite is true for anode and cathode. Various compounds are currently investigated.

It is intuitively clear that by widening the Li-ion diffusion channels the activation barrier will be reduced leading to faster diffusion. The lattice parameter can be increased by partially replacing host atoms by impurities with a larger atomic radius.  With this strategy in mind, I) the influence of the diffusivity on the lattice parameter is studied employing density functional theory. II) The thermodynamic stability of rare-earth metal atoms as impurities in existing battery materials is calculated in conjunction with the Li diffusion.

I) The influence on the lattice parameter is evaluated in two ways. Changing the lattice parameter by introducing impurities or alloying similar materials not only changes the lattice parameter but simultaneously introduces disorder along the Li-ion diffusion pathways. Therefore, changing only the lattice parameter artificially by applying an external strain allows disentangling the contributions from the lattice expansion vs. the disorder effect. At the example of Li diffusion in rare-earth oxides it is shown that the activation energy can be reduced by increasing the lattice parameter in agreement with experiments [1].

II) The thermodynamic stability of rare-earth metal atoms in materials suggested as cathode materials like spinel LiMn2O4 and LiCoO2 is calculated. The influence on the Li diffusion due to the introduced lattice distortion and reduced symmetry is discussed. A coarse-grained diffusion model on the basis of the DFT data is introduced containing the relevant changes in the local activation energies due to the presence of the rare-earth metal impurities. The numerical evaluation of the diffusion is carried out by kinetic Monte Carlo simulations.

Acknowledgements: This work was supported by the Leibniz-Gemeinschaft under the project No. SAW-2011-PDI-230.

[1] S. Tamura, A. Mori and N. Imanaka (2006). Solid State Ionics, 177(26-32), 2727–2730.