The stability and kinetics of the Li-Li 7 La 3 Zr 2 O 12 (LLZO) interface were characterized as a function of temperature and current density. Polycrystalline LLZO was densified using a rapid hot-pressing technique achieving 97±1% relative density, and < 10% grain boundary resistance; effectively consisting of an ensemble of single LLZO crystals. It was determined that by heating to 175 • C, the room temperature Li-LLZO interface resistance decreases dramatically from 5822 (as-assembled) to 514 Ω.cm 2 ; a > 10-fold decrease. In characterizing the maximum sustainable current density (or critical current density-CCD) of the Li-LLZO interface, several signs of degradation were observed. In DC cycling tests, significant deviation from Ohmic behavior was observed. In post-cycling tests, regions of metallic Li were observed; propagating parallel to the ionic current. For the cells cycled at 30, 70, 100, 130 and 160 • C, the CCD was determined to be 50, 200, 800, 3500, and 20000 μA.cm-2 , respectively. The relationships and phenomena observed
The oxygen transport properties of several organic electrolytes were characterized through measurements of oxygen solubility and electrolyte viscosity. Oxygen diffusion coefficients were calculated from electrolyte viscosities using the Stokes-Einstein relation. Oxygen solubility, electrolyte viscosity, and oxygen partial pressure were all directly correlated to discharge capacity and rate capability. Substantial improvement in cell performance was achieved through electrolyte optimization and increased oxygen partial pressure. The concentration of oxygen in the electrode under discharge was calculated using a semi-infinite medium model with simultaneous diffusion and reaction. The model was used to explain the dependence of cell performance on oxygen transport in organic electrolyte.
The oxide known as LLZO, with nominal
composition Li7La3Zr2O12, is a promising solid
electrolyte for Li-based batteries due to its high Li-ion conductivity
and chemical stability with respect to lithium. Solid electrolytes
may also enable the use of metallic Li anodes by serving as a physical
barrier that suppresses dendrite initiation and propagation during
cycling. Prior linear elasticity models of the Li electrode/solid
electrolyte interface suggest that the stability of this interface
is highly dependent on the elastic properties of the solid separator.
For example, dendritic suppression is predicted to be enhanced as
the electrolyte’s shear modulus increases. In the present study
a combination of first-principles calculations, acoustic impulse excitation
measurements, and nanoindentation experiments are used to determine
the elastic constants and moduli for high-conductivity LLZO compositions
based on Al and Ta doping. The calculated and measured isotropic shear
moduli are in good agreement and fall within the range of 56–61
GPa. These values are an order of magnitude larger than that for Li
metal and far exceed the minimum value (∼8.5 GPa) believed
to be necessary to suppress dendrite initiation. These data suggest
that LLZO exhibits sufficient stiffness to warrant additional development
as a solid electrolyte for Li batteries.
The strong correlation between LLZO grain size and the Li–LLZO stability as a function of Li plating rate is demonstrated. The increase in grain size reduces the grain boundary area and hence the number of possible failure points leading to an increased maximum tolerable current density.
X-ray and neutron diffraction, Raman spectroscopy, complex impedance spectroscopy and electron microscopy were used to characterize the tetragonal vs. cubic phase stability in superionic conducting garnet-oxide electrolyte.
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