Intrinsic Improvement of LLZO Solid-State Electrolyte to Suppress Li Dendrite Growth
Authors: Arinicheva Yulia , Zheng Hao , Tsai Chih Long , Nonemacher Juliane , Malzbender Jürgen , Fattakhova-Rohlfing Dina , Guillon Olivier , Finsterbusch Martin 
Published: February 2020

After the unexpected discovery of similar metal dendrite issues in dense ceramic electrolytes as in conventional liquid ones, the key factors governing the lithium dendrite growth e.g. in Li7La3Zr2O12 are still not fully understood. Possible factors include lithium ion diffusion kinetics at grain boundaries, influenced by microstructure [1, 2] and density [3], as well as inhomogeneous contact between LLZ 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 [4], second, the effective contact area can be increased [5], third, surface defects can be reduced [6], 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 LLZ:Ta precursor powders with larger (≈5 μm) and nano-sized particles were synthesized via solid-state synthesis and solution-assisted solid-state synthesis, respectively. LLZ: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’s modulus), ionic conductivity, cycling stability, stability in contact with humid air was investigated. The conductivity was separated into grain and grain boundary contributions. Activation energies of conductivity for the samples with larger and smaller grains were determined. Lower interfacial resistances and better cycling behaviour was found for the specimens with smaller grains and attributed to surface quality and mechanical properties of the material.
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