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Microstructural and Electrochemical Properties of Alkaline Earth Metal-doped Li Garnet-Type Solid Electrolytes Prepared by Solid-State Sintering and Spark Plasma Sintering Methods Sanoop Palakkathodi Kammampata, Rajendra Hongahally Basappa, Tomoko Ito, Hirotoshi Yamada, and Venkataraman Thangadurai ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01899 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019
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Microstructural and Electrochemical Properties of Alkaline Earth Metaldoped Li Garnet-Type Solid Electrolytes Prepared by Solid-State Sintering and Spark Plasma Sintering Methods Sanoop Palakkathodi Kammampata†, Rajendra Hongahally Basappa‡, Tomoko Ito‡, Hirotoshi Yamada‡, Venkataraman Thangadurai †* †Department
of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4. ‡Graduate School of Engineering, Nagasaki University, 1-14, Bunkyo-machi, Nagasaki 852-8521, Japan Keywords: Li-stuffed garnets, sintering, Li ion charge transfer, ionic conductivity, doping
ABSTRACT: Li-stuffed garnet-type solid Li ion electrolytes have been considered as a promising candidate for all-solid-state Li batteries. In this work, alkaline earth metal-doped Li garnet-type solid Li ion electrolytes, Li6.5La2.5A0.5TaZrO12 (A = Ca, Sr, Ba) were prepared by conventional solid-state sintering (CSS) and spark plasma sintering (SPS) methods. The effect of sintering methods on the structural and electrochemical properties of the solid electrolytes was investigated by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). Among the investigated compounds, Sr-doped garnet-type Li6.5La2.5Sr0.5TaZrO12 prepared by SPS method showed the highest Li ion conductivity of 3.08 × 10-4 S cm-1 at 20 °C and the lowest activation energy of 0.35 eV. Both Sr- and Ba-doped samples exhibited a critical current density of 0.15 mA cm-2, and the Sr-doped Li6.5La2.5Sr0.5TaZrO12 sample showed the lowest Li ion charge transfer resistance of 139 cm2 at room temperature.
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INTRODUCTION Rechargeable lithium-ion batteries (LIBs) have received great attention among the electrochemical energy storage systems due to their high volumetric and gravimetric energy densities and their wide range of applications.1–4 Even though organic electrolytes are widely used in LIBs due to their higher ionic conductivities (in the range of 10-2 S cm-1), they have several disadvantages, such as flammability, toxicity, and potential for leakage.5,6 Therefore, recent research on electrolytes for LIBs has been focused on developing a non-flammable inorganic solid Li-ion electrolyte with high ionic conductivity (10-3 S cm-1). Among the various known solid-state inorganic electrolytes, Li-ion conducting garnet-type electrolytes are considered as the prospective electrolytes for all-solid-state LIBs due to their stability toward Li metal, wide electrochemical stability window (> 6 V vs. Li+/Li), and ionic conductivity of 10-4 S cm-1.7,8 Ideal garnets are group of orthosilicates having the general formula of AII3BIII2(SiO4)3 (A = Ca, Mg; B = Al, Cr, Fe) where A, B, and Si cations occupy the eight, six, and four oxygen coordination sites, respectively.9 Garnet-type Li5La3M3O12 (M = Nb, Ta), Li6La2AMO12 (A = Ca, Sr, Ba) and Li7La3Zr2O12 Li-ion conductors have been intensively studied as solid Li ion electrolytes for LIBs7-16 following the first report by Thangadurai et al in 2003.10 The garnet-type electrolytes are being prepared by conventional solid-state sintering (CSS) in which the precursor materials are homogeneously mixed and heat-treated at different stages. Garnet-type solid Li ion electrolytes prepared by CSS generally require long sintering time (>10 h) and high temperatures (>1000 °C), which could lead to lithium loss. The Li evaporation at high temperature sintering can be avoided to a certain extent by covering the pellets with powder having the same composition and also using adding slightly excess (~ 10 to 15 wt. %) of Li precursor.
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Spark plasma sintering (SPS) is considered as a rapid sintering method where the heating power is dissipated exactly at the contact points of the powder particles.17 Garnet-type electrolytes have been prepared by SPS process that enables the densification of the electrolytes at lower temperature (
