2, Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research: Microstructure and Properties of Materials (IEK-2), Juelich, , Germany
3, Central Facility for Electron Microscopy, RWTH Aachen University, Aachen, , Germany
After the unexpected discovery of similar metal dendrite issues in dense ceramic electrolytes as in conventional liquid ones, the key factors governing the Li dendrite formation e.g. in LLZO are still not fully understood. Possible factors include lithium ion diffusion kinetics at grain boundaries, influenced by microstructure [1, 2] and density , as well as inhomogeneous contact between LLZO solid electrolyte and Li electrodes, leading to high contact resistance. Multiple strategies can be employed to reduce the contact resistance: first, the surface can be treated in order to remove LiOH/Li2CO3-contamination , second, the effective contact area can be increased  and third, surface defects can be reduced , and finally, the surface can be coated to increase the wettability [7-9].
To elucidate the interdependence of the various possibilities, the present work focuses on the effect of doping, microstructure, surface properties and density of the Li6.6La3Zr1.6Ta0.4O12 solid state electrolyte on its electrochemical performance, especially the resistance to dendrite penetration. Al-doped and Al-free LLZO:Ta precursor powders with larger (≈5 μm) and nano-sized particles were synthesized via solid-state synthesis and solution-assisted solid-state synthesis, respectively. LLZO:Ta pellets with high density (>99% of the theoretical density), high conductivity (8*10-4 S/cm ) and various grain sizes were obtained for both precursor powders by hot pressing. The grain size dependence of mechanical properties (fracture toughness, micro hardness, Young modulus), ionic conductivity, cycling stability, stability in contact with humid air on microstructure was investigated. The conductivity was separated into grain and grain boundary contributions. Activation energies of polycrystalline conductivity for the samples with larger and smaller grains were determined. Lower interfacial resistances and better cycling behaviour was found and attributed to surface quality and mechanical properties of the material.
1. Sakamoto, J. et al. Nanotechnology 2013
2. Cheng, L. et al.ACS Appl Mater Interfaces, 2015
3. Ren, Y. et al. Electrochemistry Communications, 2015
4. Sharafi, A. et al. Chemistry of Materials, 2017
5. Basappa, R.H. et al. Journal of The Electrochemical Society, 2017
6. Porz, L. et al. Advanced Energy Materials, 2017
7. Tsai, C.L. et al. ACS Appl Mater Interfaces 2016
8. Wang, C. et al. Nano Lett, 2017
9. Han, X. et al. Nat Mater, 2017