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1. Observation of Elemental Inhomogeneity and Its Impact on Ionic Conductivity in Li‐Conducting Garnets Prepared with Different Synthesis Methods

   https://doi.org/10.1002/aesr.202000109  

   Weller, J. Mark ; Dopilka, Andrew ; Chan, Candace K. ( March 2021 , Advanced Energy and Sustainability Research)

   Tantalum‐doped lithium lanthanum zirconate garnet (Li_7−xLa₃Zr_2−xTa_xO₁₂[LLZTO]) has received interest as a solid electrolyte for solid‐state lithium batteries due to its good electrochemical properties and ionic conductivity. However, the source of discrepancies for reported values of ionic conductivity in nominally or nearly equivalent compositions of LLZTO is not completely clear. Herein, synthesis‐related factors that may contribute to the differences in performance of garnet electrolytes are systematically characterized. The conductivity of samples with composition Li_6.4La₃Zr_1.4Ta_0.6O₁₂prepared by various methods including solid‐state reaction (SSR), combustion, and molten salt synthesis is compared. Varying levels of elemental inhomogeneity, comprising a variation in Ta and Zr content on the level of individual LLZTO particles, are identified. The elemental inhomogeneity is found to be largely preserved even after high‐temperature sintering and correlated with reduced ionic conductivity. It is shown that the various synthesis and processing‐related variables in each of the preparation methods play a role in these compositional variations, and that even LLZTO synthesized via conventional, high‐temperature SSR can exhibit substantial variability in local composition. However, by improving reagent mixing and using LLZTO powder with low agglomeration and small particle size distribution, the compositional uniformity, and hence, ionic conductivity, of sintered garnet electrolytes can be improved.

    

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2. Time‐Temperature‐Transformation (TTT) Diagram of Battery‐Grade Li‐Garnet Electrolytes for Low‐Temperature Sustainable Synthesis

   https://doi.org/10.1002/ange.202304581  

   Zhu, Yuntong ; Chon, Michael ; Thompson, Carl V. ; Rupp, Jennifer L. M. ( October 2023 , Angewandte Chemie)

   Efficient and affordable synthesis of Li⁺functional ceramics is crucial for the scalable production of solid electrolytes for batteries. Li‐garnet Li₇La₃Zr₂O_12−d(LLZO), especially its cubic phase (cLLZO), attracts attention due to its high Li⁺conductivity and wide electrochemical stability window. However, high sintering temperatures raise concerns about the cathode interface stability, production costs, and energy consumption for scalable manufacture. We show an alternative “sinter‐free” route to stabilize cLLZO as films at half of its sinter temperature. Specifically, we establish a time‐temperature‐transformation (TTT) diagram which captures the amorphous‐to‐crystalline LLZO transformation based on crystallization enthalpy analysis and confirm stabilization of thin‐film cLLZO at record low temperatures of 500 °C. Our findings pave the way for low‐temperature processing via TTT diagrams, which can be used for battery cell design targeting reduced carbon footprints in manufacturing.

    

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3. Time‐Temperature‐Transformation (TTT) Diagram of Battery‐Grade Li‐Garnet Electrolytes for Low‐Temperature Sustainable Synthesis

   https://doi.org/10.1002/anie.202304581  

   Zhu, Yuntong ; Chon, Michael ; Thompson, Carl V. ; Rupp, Jennifer L. M. ( October 2023 , Angewandte Chemie International Edition)

   Efficient and affordable synthesis of Li⁺functional ceramics is crucial for the scalable production of solid electrolytes for batteries. Li‐garnet Li₇La₃Zr₂O_12−d(LLZO), especially its cubic phase (cLLZO), attracts attention due to its high Li⁺conductivity and wide electrochemical stability window. However, high sintering temperatures raise concerns about the cathode interface stability, production costs, and energy consumption for scalable manufacture. We show an alternative “sinter‐free” route to stabilize cLLZO as films at half of its sinter temperature. Specifically, we establish a time‐temperature‐transformation (TTT) diagram which captures the amorphous‐to‐crystalline LLZO transformation based on crystallization enthalpy analysis and confirm stabilization of thin‐film cLLZO at record low temperatures of 500 °C. Our findings pave the way for low‐temperature processing via TTT diagrams, which can be used for battery cell design targeting reduced carbon footprints in manufacturing.

    

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4. Perovskite Na-ion conductors developed from analogous Li 3x La 2/3−x TiO 3 (LLTO): chemo-mechanical and defect engineering

   https://doi.org/10.1039/d1ta04252a  

   Lin, Yu-Ying ; Gustafson, William J. ; Murray, Shannon E. ; Shoemaker, Daniel P. ; Ertekin, Elif ; Krogstad, Jessica A. ; Perry, Nicola H. ( September 2021 , Journal of Materials Chemistry A)

   Na-ion conducting solid electrolytes can enable both the enhanced safety profile of all-solid-state-batteries and the transition to an earth-abundant charge-carrier for large-scale stationary storage. In this work, we developed new perovskite-structured Na-ion conductors from the analogous fast Li-ion conducting Li 3 x La 2/3− x TiO 3 (LLTO), testing strategies of chemo-mechanical and defect engineering. Na x La 2/3−1/3 x ZrO 3 (NLZ) and Na x La 1/3−1/3 x Ba 0.5 ZrO 3 (NLBZ) were prepared using a modified Pechini method with varying initial stoichiometries and sintering temperatures. With the substitution of larger framework cations Zr 4+ and Ba 2+ on B- and A-sites respectively, NLZ and NLBZ both had larger lattice parameters compared to LLTO, in order to accommodate and potentially enhance the transport of larger Na ions. Additionally, we sought to introduce Na vacancies through (a) sub-stoichiometric Na : La ratios, (b) Na loss during sintering, and (c) donor doping with Nb. AC impedance spectroscopy and DC polarization experiments were performed on both Na 0.5 La 0.5 ZrO 3 and Na 0.25 La 0.25 Ba 0.5 ZrO 3 in controlled gas environments (variable oxygen partial pressure, humidity) at elevated temperatures to quantify the contributions of various possible charge carriers (sodium ions, holes, electrons, oxygen ions, protons). Our results showed that the lattice-enlarged NLZ and NLBZ exhibited ∼19× (conventional sintering)/49× (spark plasma sintering) and ∼7× higher Na-ion conductivities, respectively, compared to unexpanded Na 0.42 La 0.525 TiO 3 . Moreover, the Na-ion conductivity of Na 0.5 La 0.5 ZrO 3 is comparable with that of NaNbO 3 , despite having half the carrier concentration. Additionally, more than 96% of the total conductivity in dry conditions was contributed by sodium ions for both compositions, with negligible electronic conductivity and little oxygen ion conductivity. We also identified factors that limited Na-ion transport: NLZ and NLBZ were both challenging to densify using conventional sintering without the loss of Na because of its volatility. With spark plasma sintering, higher density can be achieved. In addition, the NLZ perovskite phase appeared unable to accommodate significant Na deficiency, whereas NLBZ allowed some. Density functional theory calculations supported a thermodynamic limitation to creation of Na-deficient NLZ in favor of a pyrochlore-type phase. Humid environments generated different behavior: in Na 0.25 La 0.25 Ba 0.5 ZrO 3 , incorporated protons raised total conductivity, whereas in Na 0.5 La 0.5 ZrO 3 , they lowered total conductivity. Ultimately, this systematic approach revealed both effective approaches and limitations to achieving super-ionic Na-ion conductivity, which may eventually be overcome through alternative processing routes. 

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5. Decomposition of Trace Li 2 CO 3 During Charging Leads to Cathode Interface Degradation with the Solid Electrolyte LLZO

   https://doi.org/10.1002/adfm.202103716  

   Delluva, Alexander A. ; Kulberg‐Savercool, Jonas ; Holewinski, Adam ( July 2021 , Advanced Functional Materials)

   A major challenge for lithium‐containing electrochemical systems is the formation of lithium carbonates. Solid‐state electrolytes circumvent the use of organic liquids that can generate these species, but they are still susceptible to Li₂CO₃formation from exposure to water vapor and carbon dioxide. It is reported here that trace quantities of Li₂CO₃, which are re‐formed following standard mitigation and handling procedures, can decompose at high charging potentials and degrade the electrolyte–cathode interface. Operando electrochemical mass spectrometry (EC–MS) is employed to monitor the outgassing of solid‐state batteries containing the garnet electrolyte Li₇La₃Zr₂O₁₂(LLZO) and using appropriate controls CO₂and O₂are identified to emanate from the electrolyte–cathode interface at charging potentials > 3.8 V (vs Li/Li⁺). The gas evolution is correlated with a large increase in cathode interfacial resistance observed by potential‐resolved impedance spectroscopy. This is the first evidence of electrochemical decomposition of interfacial Li₂CO₃in garnet cells and suggests a need to report “time‐to‐assembly” for cell preparation methods.

    

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