Sumaletha Narayanan

Fast Solid-State Li Ion Conducting Garnet-Type Structure Metal Oxides for Energy Storage

Lithium ion batteries are the most promising energy storage system on the market today; however, safety issues associated with the use of flammable organic polymer-based electrolytes with poor electrochemical and chemical stabilities prevent this technology from reaching maturity. Solid lithium ion electrolytes (SLIEs) are being considered as potential replacements for the organic electrolytes to develop all-solid-state Li ion batteries. Out of the recently discovered SLIEs, the garnet-related structured Li-stuffed metal oxides are the most promising electrolytes due to their high total (bulk + grain boundary) Li ion conductivity, high electrochemical stability window (∼6 V versus Li(+)/Li at room temperature), and chemical stability against reaction with an elemental Li anode and high-voltage metal oxide Li cathodes. This Perspective discusses the structural-chemical composition-ionic conductivity relationship of Li-stuffed garnets, followed by a discussion on the Li ion conduction mechanism, as well as the electrochemical and chemical stability of these materials. The performance of a number of all-solid-state batteries employing garnet-type Li ion electrolytes is also discussed.

Macroscopic and microscopic Li+ transport parameters in cubic garnet-type “Li6.5La2.5Ba0.5ZrTaO12” as probed by impedance spectroscopy and NMR

The garnet-type “Li6.5La2.5Ba0.5ZrTaO12”, crystallizing with cubic symmetry was prepared according to a conventional solid state synthesis method using metal oxides and salt precursors of high purity. The formation of the “single-phase” garnet-type structure was studied by powder X-ray diffraction (PXRD). Electron microprobe analysis (EMPA) coupled with a wavelength-dispersive spectrometer (WDS) showed a rather homogeneous distribution of Ta ions and Zr ions compared to that of Ba ions and La ions in “Li6.5La2.5Ba0.5ZrTaO12”. Li ion dynamics were complementarily studied using variable-temperature AC-impedance spectroscopy and 7Li NMR measurements. The bulk (ion) conductivities probed are in very good agreement with results reported earlier, illustrating the excellent reproducibility of the Li transport properties of “Li6.5La2.5Ba0.5ZrTaO12”. In particular, AC impedance and NMR results indicate that the Li transport process studied is of long-range nature. Finally, the chemical compatibility of the electrolyte “Li6.5La2.5Ba0.5ZrTaO12” was tested with Li2FeMn3O8, being a high-voltage cathode material. As shown by variable-temperature PXRD measurements, the garnet-type structure (bulk) was found to be stable up to 673 K.

Dopant Concentration–Porosity–Li-Ion Conductivity Relationship in Garnet-Type Li5+2xLa3Ta2–xYxO12 (0.05 ≤ x ≤ 0.75) and Their Stability in Water and 1 M LiCl

Highly Li-ion conductive Y-doped garnet-type Li5+2xLa3Ta2-xYxO12 (0.05 ≤ x ≤ 0.75) were studied to understand the effects of yttrium- and lithium-doping on crystal structure, porosity, and Li-ion conductivity using (7)Li MAS NMR, electrochemical ac impedance spectroscopy, and scanning electron microscopy (SEM), as well as ex situ and in situ powder X-ray diffraction (PXRD) to further explore the potential application of garnets in all-solid-state Li-ion batteries. Solid-state (7)Li MAS NMR studies showed an increase in the Li-ion mobility as a function of Y- and Li-doping in Li5+2xLa3Ta2-xYxO12, which is consistent with the results from ac impedance spectroscopy. The SEM studies on sintered pellets indicated a systematic decrease in porosity and an increase in sinterability as the Y- and Li-doping levels increase in Li5+2xLa3Ta2-xYxO12. These results are consistent with the calculated porosity and densities using the Archimedes method. Using the variable-temperature in situ PXRD in the temperature range of 30-700 °C, a thermal expansion coefficient of 7.25 × 10(-6) K(-1) was observed for Li6La3Ta1.5Y0.5O12. To further explore the possibility of a new application for the Li-stuffed garnets, the stability of these materials in aqueous LiCl solution was also studied. A high degree of structural stability was observed in these materials upon 1 M LiCl treatment, making them suitable candidates for further studies as protective layers for lithium electrodes in aqueous lithium batteries.

The synthesis and electrical properties of hybrid gel electrolytes derived from Keggin-type heteropoly acids and 3-(pyridin-1-ium-1-yl)propane-1-sulfonate (PyPs)

Herein, we report the effect of the proton concentration in polyoxometalates (POMs) upon hybrid formation with ionic liquids (ILs), and their ionic conductivity relationship to optimize their ionic conductivity. The hybrid gels were derived from Keggin-type heteropoly acids containing different proton concentrations, such as H3PW11MoO40, H4PMo11VO40 and H5PMo10V2O40, and 3-(pyridin-1-ium-1-yl)propane-1-sulfonate (PyPs) IL. Elemental C, H, and N analysis was found to be consistent with the theoretical composition within 4% for C and N, whereas H content was found to be slightly higher than the anticipated value, which may be due to potential uptake of water during the sample preparation. 1H and 13C nuclear magnetic resonance and Fourier transform infrared spectroscopy (FTIR) confirmed the presence of functional groups of PyPs in the hybrids. In situ variable temperature powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), electrochemical AC impedance spectroscopy and cyclic voltammetry studies showed excellent thermal (up to ∼300 °C) and electrochemical (3 V at room temperature) stability of [PyPs]3PW11MoO40. The structural characterizations confirmed the interaction between the organic cation and Keggin-type inorganic heteropoly anion in the hybrid material. The bulk ionic conductivity of 0.1, 0.01 and 0.0003 S cm−1 at ∼90 °C was obtained for [PyPs]3PW11MoO40, [PyPs]4PMo11VO40 and [PyPs]5PMo10V2O40, respectively.

Correction to "Fast Solid-State Li Ion Conducting Garnet-Type Structure Metal Oxides for Energy Storage".

Structure Metal Oxides for Energy Storage” Venkataraman Thangadurai,* Dana Pinzaru, Sumaletha Narayanan, and Ashok Kumar Baral J. Phys. Chem. Lett. 2010, 1, 3530−3537. DOI: 10.1021/jz501828v T publication information (year, volume, and pagination) were incorrect when this Perspective was originally published ASAP on January 6, 2015. The information was corrected to J. Phys. Chem. Lett. 2015, 6, 292−299, and the Perspective was republished.