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1. Pyrochlore nanocrystals as versatile quasi-single-source precursors to lithium conducting garnets

   https://doi.org/10.1039/d0ta05842d  

   Weller, J. Mark ; Chan, Candace K. ( September 2020 , Journal of Materials Chemistry A)

   null (Ed.)

   Lithium conducting garnets are attractive solid electrolytes for solid-state lithium batteries but are difficult to process, generally requiring high reaction and sintering temperatures with long durations. In this work, we demonstrate a synthetic route to obtain Ta-doped garnet (Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 ) utilizing La- and Ta-doped lanthanum zirconate (La 2.4 Zr 1.12 Ta 0.48 O 7.04 ) pyrochlore nanocrystals as quasi-single-source precursors. Via molten salt synthesis (MSS) in a highly basic flux, the pyrochlore nanocrystals transform to Li-garnet at reaction temperatures as low as 400 °C. We also show that the pyrochlore-to-garnet conversion can take place in one step using reactive sintering, resulting in densified garnet ceramics with high ionic conductivity (0.53 mS cm −1 at 21 °C) and relative density (up to 94.7%). This approach opens new avenues for lower temperature synthesis of lithium garnets using a quasi-single-source precursor and provides an alternative route to highly dense garnet solid electrolytes without requiring advanced sintering processes. 

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2. Atomic-scale origin of the low grain-boundary resistance in perovskite solid electrolyte Li0.375Sr0.4375Ta0.75Zr0.25O3

   https://doi.org/10.1038/s41467-023-37115-6  

   Lee, Tom ; Qi, Ji ; Gadre, Chaitanya A. ; Huyan, Huaixun ; Ko, Shu-Ting ; Zuo, Yunxing ; Du, Chaojie ; Li, Jie ; Aoki, Toshihiro ; Wu, Ruqian ; et al ( April 2023 , Nature Communications)

   Oxide solid electrolytes (OSEs) have the potential to achieve improved safety and energy density for lithium-ion batteries, but their high grain-boundary (GB) resistance generally is a bottleneck. In the well-studied perovskite oxide solid electrolyte, Li_3xLa_2/3-xTiO₃(LLTO), the ionic conductivity of grain boundaries is about three orders of magnitude lower than that of the bulk. In contrast, the related Li_0.375Sr_0.4375Ta_0.75Zr_0.25O₃(LSTZ0.75) perovskite exhibits low grain boundary resistance for reasons yet unknown. Here, we use aberration-corrected scanning transmission electron microscopy and spectroscopy, along with an active learning moment tensor potential, to reveal the atomic scale structure and composition of LSTZ0.75 grain boundaries. Vibrational electron energy loss spectroscopy is applied for the first time to reveal atomically resolved vibrations at grain boundaries of LSTZ0.75 and to characterize the otherwise unmeasurable Li distribution therein. We find that Li depletion, which is a major reason for the low grain boundary ionic conductivity of LLTO, is absent for the grain boundaries of LSTZ0.75. Instead, the low grain boundary resistivity of LSTZ0.75 is attributed to the formation of a nanoscale defective cubic perovskite interfacial structure that contained abundant vacancies. Our study provides new insights into the atomic scale mechanisms of low grain boundary resistivity.

    

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3. Lithium Ion Pathway within Li 7 La 3 Zr 2 O 12 ‐Polyethylene Oxide Composite Electrolytes

   https://doi.org/10.1002/ange.201607539  

   Zheng, Jin ; Tang, Mingxue ; Hu, Yan‐Yan ( September 2016 , Angewandte Chemie)

   Polymer–ceramic composite electrolytes are emerging as a promising solution to deliver high ionic conductivity, optimal mechanical properties, and good safety for developing high‐performance all‐solid‐state rechargeable batteries. Composite electrolytes have been prepared with cubic‐phase Li₇La₃Zr₂O₁₂(LLZO) garnet and polyethylene oxide (PEO) and employed in symmetric lithium battery cells. By combining selective isotope labeling and high‐resolution solid‐state Li NMR, we are able to track Li ion pathways within LLZO‐PEO composite electrolytes by monitoring the replacement of⁷Li in the composite electrolyte by⁶Li from the⁶Li metal electrodes during battery cycling. We have provided the first experimental evidence to show that Li ions favor the pathway through the LLZO ceramic phase instead of the PEO‐LLZO interface or PEO. This approach can be widely applied to study ion pathways in ionic conductors and to provide useful insights for developing composite materials for energy storage and harvesting.

    

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4. Elucidating Phase Transformation and Surface Amorphization of Li 7 La 3 Zr 2 O 12 by In Situ Heating TEM

   https://doi.org/10.1002/smll.202304799  

   Zheng, Hongkui ; Xu, Mingjie ; He, Kai ( October 2023 , Small)

   Garnet‐type Li₇La₃Zr₂O₁₂(LLZO) solid‐state electrolytes hold great promise for the next‐generation all‐solid‐state batteries. An in‐depth understanding of the phase transformation during synthetic processes is required for better control of the crystallinity and improvement of the ionic conductivity of LLZO. Herein, the phase transformation pathways and the associated surface amorphization are comparatively investigated during the sol–gel and solid‐state syntheses of LLZO using in situ heating transmission electron microscopy (TEM). The combined ex situ X‐ray diffraction and in situ TEM techniques are used to reveal two distinct phase transformation pathways (precursors → La₂Zr₂O₇ → LLZO and precursors → LLZO) and the subsequent layer‐by‐layer crystal growth of LLZO on the atomic scale. It is also demonstrated that the surface amorphization surrounding the LLZO crystals is sensitive to the postsynthesis cooling rate and significantly affects the ionic conductivity of pelletized LLZO. This work brings up a critical but often overlooked issue that may greatly exacerbate the Li‐ion conductivity by undesired synthetic conditions, which can be leveraged to ameliorate the overall crystallinity to improve the electrochemical performance of LLZO. These findings also shed light on the significance of optimizing surface structure to ensure superior performance of Li‐ion conductors.

    

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5. Microstructural impacts on ionic conductivity of oxide solid electrolytes from a combined atomistic-mesoscale approach

   https://doi.org/10.1038/s41524-021-00681-8  

   Heo, Tae Wook ; Grieder, Andrew ; Wang, Bo ; Wood, Marissa ; Hsu, Tim ; Akhade, Sneha A. ; Wan, Liwen F. ; Chen, Long-Qing ; Adelstein, Nicole ; Wood, Brandon C. ( December 2021 , npj Computational Materials)

   Although multiple oxide-based solid electrolyte materials with intrinsically high ionic conductivities have emerged, practical processing and synthesis routes introduce grain boundaries and other interfaces that can perturb primary conduction channels. To directly probe these effects, we demonstrate an efficient and general mesoscopic computational method capable of predicting effective ionic conductivity through a complex polycrystalline oxide-based solid electrolyte microstructure without relying on simplified equivalent circuit description. We parameterize the framework for Li_7-xLa₃Zr₂O₁₂(LLZO) garnet solid electrolyte by combining synthetic microstructures from phase-field simulations with diffusivities from molecular dynamics simulations of ordered and disordered systems. Systematically designed simulations reveal an interdependence between atomistic and mesoscopic microstructural impacts on the effective ionic conductivity of polycrystalline LLZO, quantified by newly defined metrics that characterize the complex ionic transport mechanism. Our results provide fundamental understanding of the physical origins of the reported variability in ionic conductivities based on an extensive analysis of literature data, while simultaneously outlining practical design guidance for achieving desired ionic transport properties based on conditions for which sensitivity to microstructural features is highest. Additional implications of our results are discussed, including a possible connection between ion conduction behavior and dendrite formation.

    

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