Co3+/La3+ Cross-Diffusion at the Li7La3Zr2O12 | LiCoO2 Interface
Authors: Ladenstein Lukas , Ring Joseph , Smetazcek Stefan , Kubicek Markus , Ganschow Steffen , Redhammer Günther , Knez Daniel , Kothleitner Gerald , Dugulan Achim Iulian , Smith Jeffrey , Siegel Donald , Limbeck Andreas , Wilkening H. Martin R. , Fleig Juergen , Rettenwander Daniel 
Published: July 2021

Cubic Li7La3Zr2O12 (LLZO) garnets have attracted lots of attention in previous years as they show promising properties as a solid electrolyte for Solid-state Li batteries (SSLB), such as high ionic conductivity in the range of 1 mS cm-1 and high electrochemical stability up to 6 V.1 Despite of these promising prerequisites, its implementation in SSLB leads to high interfacial resistance that limits its current use in such devices. This high interface resistance relates to the required high temperature treatment to form a good contact between LLZO and, e.g., LiCoO2.2

It is known that this is related to the formation of interdiffusion layers that imped the ionic transport across the interface. Herein, we present that Co does not only lead to the formation of resistive interlayers, such as La2CoO4, but also leads to the incorporation of Co into the garnet lattice. Since, Co is a transition metal its incorporation could change, e.g., (i) the electronic conductivity, being suggested to play a critical role for dendrite formation within the garnet, and (ii) the band gap, hence, the electrochemical stability. Moreover, we also observed, that La diffuses into LiCoO2, which potentially changes the electrochemical properties of the cathode material as well.3
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We mirrored the properties of the interface by incorporating Co from LiCoO2 powder into Czochralski-grown Li6.4Ga0.2La3Zr2O12 single crystals over the gas phase at high temperature. Interestingly, the color changed from yellow, orange to dark blue depending on the heating history. Thereafter we applied a wide spectrum of techniques, such as SC XRD, UV-VIS, TOF-SIMS, 57Emission Mößbauer spectroscopy, DFT/AIMD simulations, impedance spectroscopy and cyclic voltammetry to understand the role of Co in LLZO.
For example, we found that the band gap decreases up to 2 eV by the incorporation of 0.1 Co per formula unit (pfu) and a La deficit of 0.12 pfu. This change in the band gap manifests itself in a decreased electrochemical stability of LLZO indicated by an oxidative peak below 4 V as observed in the CV measurement. Moreover, we found that the incorporation of Co leads to a significant change in the Li ion transport. The impedance analysis revealed a low activated (E
a = 0.43 eV) process at temperatures between -100 °C and 110 °C and another highly activated (E
a = 0.7 eV) diffusion process predominantly present at temperatures > 160 °C. By extrapolating the highly activated process to room temperature, an ionic conductivity σ = 2.91 x 10-6 S cm-1 could be calculated which is about two orders of magnitude lower than values found for pristine LLZO.

References:

Hofstetter, K. et al. Electrochemical Stability of Garnet-Type Li7La2.75Ca0.25Zr1.75Nb0.25O12 with and without Atomic Layer Deposited-Al2O3 under CO2 and Humidity . J. Electrochem. Soc.
166, A1844–A1852 (2019).

Park, K. et al. Electrochemical Nature of the Cathode Interface for a Solid-State Lithium-Ion Battery: Interface between LiCoO2 and Garnet-Li7La3Zr2O12. Chem. Mater.
28, 8051–8059 (2016).

Kim, K. H. et al. Characterization of the interface between LiCoO2 and Li7La3Zr2O12 in an all-solid-state rechargeable lithium battery. J. Power Sources
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