Browsing by Author "Narayanan, Sumaletha"

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    Development of Novel Garnet-Type Solid Electrolytes for Potential Application in Li Ion Batteries
    (2012-09-06) Narayanan, Sumaletha; Thangadurai, Venkataraman
    The development of promising solid electrolytes having a garnet-like structure has been successfully achieved through solid state (ceramic) method. Various approaches to improve the Li ion conductivity were employed. The first approach involved creating oxide ion vacancies into the crystal structure of parent garnet-like oxide, Li5La3Nb2O12 to create a novel family of compounds with nominal composition, Li5La3Nb2-xYxO12-δ (0 ≤ x ≤ 1). The second approach was Li stuffing into the garnet-like oxides to develop a series of Li stuffed novel Li5+2xLa3Nb2-xYxO12 (0.05 ≤ x ≤ 0.75) and Li6.5La2.5Ba0.5ZrTaO12. Powder X-ray diffraction (PXRD), thermo gravimetric analysis (TGA), scanning electron microscopy (SEM), electron probe microanalysis (EPMA) coupled with a wavelength-dispersive spectrometer (WDS), 7Li nuclear magnetic resonance (Li-NMR), and AC impedance spectroscopy were employed to characterize the structure, morphology, elemental composition, Li ion sites, and Li ion conductivity. Studies have shown that Li5+2xLa3Nb2-xYxO12 have turned out to be promising solid electrolytes with high Li ion conductivity (10-4 Scm-1 at ambient temperatures). In addition, all families of garnets are found to be chemically stable with Li cathode materials (Li2MMn3O8, where M = Fe, Co) up to 400 oC in air. The developed electrolyte materials have the potential to be used in all-solid-state Li ion batteries.
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    Dielectric characteristics of fast Li ion conducting garnet-type Li5+2xLa3Nb2 xYxO12 (x = 0.25, 0.5 and 0.75)
    (Royal Society of Chemistry, 2016) Narayanan, Sumaletha; Baral, Ashok Kumar; Thangadurai, Venkataraman
    Here, we report the dielectric characteristics of Li-stuffed garnet-type Li5+2xLa3Nb2−xYxO12 (x = 0.25, 0.5 and 0.75) in the temperature range about −53 to 50 °C using AC impedance spectroscopy. All the investigated Li-stuffed garnet compounds were prepared, under the same condition, using conventional solid-state reaction at elevated temperature in air. The Nyquist plots show mainly bulk contribution to the total Li+ ion conductivity for Li5.5La3Nb1.75Y0.25O12 (Li5.5–Nb) and Li6La3Nb1.5Y0.5O12 (Li6–Nb), while both bulk and grain-boundary effects are visible in the case of Li6.5La3Nb1.25Y0.75O12 (Li6.5–Nb) phase at ∼−22 °C. Non-Debye relaxation process was observed in the modulus AC impedance plots. The dielectric loss tangent of Li5+2xLa3Nb2−xYxO12 are compared with that of the corresponding Ta analogue, Li5+2xLa3Ta2−xYxO12 and showed a decrease in peak intensity for the Nb-based garnet samples which may be attributed to a slight increase in their Li+ ion conductivity. The relative dielectric constant values were also found to be higher for the Ta member (>60 for Li5+2xLa3Ta2−xYxO12) than that of the corresponding Nb analogue (∼50 for Li5+2xLa3Nb2−xYxO12) at below room temperature. A long-range order Li+ ion migration pathway with relaxation time (τ0) 10−18–10−15 s and an activation energy of 0.59–0.40 eV was observed for the investigated Li5+2xLa3Nb2−xYxO12 garnets and is comparable to that of the corresponding Ta-based Li5+2xLa3Ta2−xYxO12 garnets.
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    Dopant Concentration - Porosity - Li-ion Conductivity Relationship in Garnet-Type Li5+2xLa3Ta2-xYxO12 (0.05 ≤ x ≤ 0.75) and Their Stability in Water and 1M LiCl
    (American Chemical Society, 2015) Narayanan, Sumaletha; Ramezanipour, Farshid; Thangadurai, Venkataraman
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    Effect of Excess Li on the Structural and Electrical Properties of Garnet-Type Li6La3Ta1.5Y0.5O12
    (Electrochemical Society, 2015) Narayanan, Sumaletha; Hitz, Gregory T.; Wachsman, Eric D.; Thangadurai, Venkataraman
    Volatility of lithium during preparation of lithium-stuffed garnet-type metal oxide solid Li ion electrolytes is a common problem, which affects phase formation, ionic conductivity, mechanical strength and density. Synthesis of Li-stuffed garnets has been performed generally using the conventional solid-state reactions at elevated temperature in air. The present study describes the effect of excess LiNO3 (2.5 to 15 wt.%) addition during the ceramic synthesis on the structural and electrical properties of garnet-type Li6La3Ta1.5Y0.5O12. Powder X-ray diffraction (PXRD) confirmed that cubic phase was formed in all tested cases, and there is no significant variation in lattice parameter with amount of excess LiNO3 used. However, increasing amounts of excess lithium decreased inter-particle contact and increased grain growth during sintering, producing sharply varied microstructures. PXRD showed no secondary phase and scanning electron microscopy (SEM) analysis showed rather uniform morphology and absence of "glassy" materials at the grain-boundaries. The bulk Li ion conductivity was found to increase with amount of excess lithium, reaching a maximum room temperature conductivity of 1.62 × 10−4 Scm−1 for the sample prepared using 10 wt.% excess LiNO3. Raman microscopy study indicated the presence of Li2CO3 in all aged Li6La3Ta1.5Y0.5O12 samples prepared using excess LiNO3.
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    Fast Solid State Li ion Conducting Garnet-Type Structure Metal Oxides for Energy Storage
    (American Chemical Society, 2015) Thangadurai, Venkataraman; Pinzaru, Dana; Narayanan, Sumaletha; Baral, Ashok K.
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    Solid State Electrolytes for Energy Storage and Conversion Devices
    (2016) Narayanan, Sumaletha; Thangadurai, Venkataraman; Nagarajan, Rajamani; Sundararaj, Uttandaraman; Sutherland, Todd; Shi, Yujun
    Electrochemical energy conversion and storage devices hold significant importance in the successful implementation of renewable energy systems. Solid-state electrolytes, with garnet-type crystal structure for lithium ion batteries, have been synthesized using ceramic method for energy storage application. A systematic study on lithium-stuffed garnet-type Li5+2xLa3Ta2-xYxO12 (0.05 ≤ x ≤ 0.75) have been carried out for the understanding of the effect of Y- and Li- doping on the structural, electrical, chemical and electrochemical properties. Powder X-ray diffraction (PXRD) studies have revealed the cubic garnet-type structure of the materials. The AC electrochemical impedance spectroscopy (EIS) has shown that the sample with highest Li and Y content show the best Li+ ion conductivity of 10-4 Scm-1 at room temperature. Optimization of lithium salt added during the solid-state preparation of garnets to compensate lithium loss at higher sintering temperatures was also carried out on Li6La3Ta1.5Y0.5O12. Crystal structure was unaltered with the change in lithium salt addition, whereas the porosity and conductivity were affected. Evaluation of fundamental transport properties of Li5+2xLa3M2-xYxO12 (M = Nb, Ta) (x = 0.25, 0.50 and 0.75) has been carried out by employing AC EIS method. The dielectric analysis performed below room temperature suggested that the Li+ ion conduction in garnet-type materials takes place through a hopping mechanism following Path A (low energy route) or Path B (high energy route). Hybrid proton conducting materials derived from ionic liquid and polyoxometalates have been developed for the potential application in energy conversion devices. The effect of different heteropoly acids such as H3PW11MoO40, H4PMo11VO40 and H5PMo11V2O40 used along with (3-(pyridin-1-ium-1-yl) propane-1-sulfonate (PyPs)) ionic liquid on the properties of hybrid materials were studied. A high thermal stability up to ~ 300 °C, electrochemical stability of ~ 3V, and ionic conductivity of 0.01 Scm-1 at 90 °C were observed for the hybrid proton conductors. A soft-chemistry approach was proposed for the synthesis of another class of proton conductors, from layered perovskites such as KLaNb2O7 and K2La2Ti3O10, and an imidazolium based ionic liquid. PXRD, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) were used to understand the ion exchange chemistry.
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    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)
    (Royal Society of Chemistry (RSC), 2016-10-12) Narayanan, Sumaletha; Tong, Xia; Thangadurai, Venkataraman
    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.