Asma Sharafi

Impact of air exposure and surface chemistry on Li–Li7La3Zr2O12 interfacial resistance

Li7La3Zr2O12 (LLZO) is a promising solid-state electrolyte that could enable solid-state-batteries (SSB) employing metallic Li anodes. For a SSB to be viable, the stability and charge transfer kinetics at the Li–LLZO interface should foster facile plating and stripping of Li. Contrary to these goals, recent studies have reported high Li–LLZO interfacial resistance which was attributed to a contamination layer that forms upon exposure of LLZO to air. This study clarifies the mechanisms and consequences associated with air exposure of LLZO; additionally, strategies to minimize these effects are described. First-principles calculations reveal that LLZO readily reacts with humid air; the most favorable reaction pathway involves protonation of LLZO and formation of Li2CO3. X-ray photoelectron spectroscopy, scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy were used to characterize the surface and subsurface chemistry of LLZO as a function of relative humidity and exposure time. Additionally, electrochemical impedance spectroscopy was used to measure the Li–LLZO interfacial resistance as a function of surface contamination. These data indicate that air exposure-induced contamination impacts the interfacial resistance significantly, when exposure time exceeds 24 h. The results of this study provide valuable insight into the sensitivity of LLZO to air and how the effects of air contamination can be reversed.

Electrochemical Stability of Li6.5La3Zr1.5M0.5O12 (M = Nb or Ta) against Metallic Lithium

The electrochemical stability of Li6.5La3Zr1.5Nb0.5O12 (LLZNO) and Li6.5La3Zr1.5Ta0.5O12 (LLZTO) against metallic Li was studied using direct current (DC) and electrochemical impedance spectroscopy (EIS). Dense polycrystalline LLZNO (ρ=97 %) and LLZTO (ρ=92 %) were made using sol-gel synthesis and rapid induction hot-pressing at 1100 °C and 15.8 MPa. During DC cycling tests at room temperature (±0.01 mA/cm2 for 36 cycles), LLZNO exhibited an increase in Li-LLZNO interface resistance and eventually short-circuiting while the LLZTO was stable. After DC cycling, LLZNO appeared severely discolored while the LLZTO did not change in appearance. We believe the increase in Li-LLZNO interfacial resistance and discoloration are due to reduction of Nb5+ to Nb4+. The negligible change in interfacial resistance and no color change in LLZTO suggest that Ta5+ may be more stable against reduction than Nb5+ in cubic garnet versus Li during cycling.

Controlling and correlating the effect of grain size with the mechanical and electrochemical properties of Li7La3Zr2O12 solid-state electrolyte

Li7La3Zr2O12 (LLZO) solid-state electrolyte is garnering interest due to its potential to enable solid-state batteries (SSBs) using metallic Li anodes. However, Li metal propagates along LLZO grain boundaries at high Li plating current densities (above the critical current density, CCD). In the present study, we examined whether microstructural aspects, such as grain size, could influence mechanical and electrochemical properties thereby affecting the CCD. A unique densification technique (heating between 1100 and 1300 °C) was used to control grain size. Electron backscatter diffraction determined that the grain size and the misorientation angle varied from 5 to 600 μm and 20 to 40°, respectively. Vickers indentation was used to characterize the mechanical properties and revealed that hardness decreased (9.9–6.8 GPa) with increasing grain size, but the fracture toughness was invariant (0.6 MPa m−1/2) at grain sizes ≥40 μm. DC and AC techniques were used to measure and correlate the CCD with grain size and showed that the CCD increased with increasing grain size achieving a maximum of 0.6 mA cm−2. We believe the implications of this work could be far-reaching in that they represent a significant step towards understanding the mechanism(s) that control the stability of the Li–LLZO interface and a rational approach to increase the CCD in SSBs.

Crystal Orientation-Dependent Reactivity of Oxide Surfaces in Contact with Lithium Metal

Understanding ionic transport across interfaces between dissimilar materials and the intrinsic chemical stability of such interfaces is a fundamental challenge spanning many disciplines and is of particular importance for designing conductive and stable solid electrolytes for solid-state Li-ion batteries. In this work, we establish a surface science-based approach for assessing the intrinsic stability of oxide materials in contact with Li metal. Through a combination of experimental and computational insights, using Nb-doped SrTiO3 (Nb/STO) single crystals as a model system, we were able to understand the impact of crystallographic orientation and surface morphology on the extent of the chemical reactions that take place between surface Nb, Ti, and Sr upon reaction with Li. By expanding our approach to investigate the intrinsic stability of the technologically relevant, polycrystalline Nb-doped lithium lanthanum zirconium oxide (Li6.5La3Zr1.5Nb0.5O12) system, we found that this material reacts with Li metal through the reduction of Nb, similar to that observed for Nb/STO. These results clearly demonstrate the feasibility of our approach to assess the intrinsic (in)stability of oxide materials for solid-state batteries and point to new strategies for understanding the performance of such systems.