Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 17147−17155
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Method Using Water-Based Solvent to Prepare Li7La3Zr2O12 Solid Electrolytes Xiao Huang,†,∥ Yang Lu,†,∥ Jun Jin,† Sui Gu,†,∥ Tongping Xiu,§ Zhen Song,‡ Michael E. Badding,‡ and Zhaoyin Wen*,†,∥ †
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, P. R. China ‡ Corning Incorporated, Corning, New York 14831, United States § Corning Research Center China, 200 Jinsu Road, Shanghai 201206, P. R. China ∥ University of Chinese Academy of Sciences, 19 A Yuquan Rd, Shijingshan District, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Li-garnet Li7La3Zr2O12 (LLZO) is a promising candidate of solid electrolytes for high-safety solid-state Li+ ion batteries. However, because of its high reactivity to water, the preparation of LLZO powders and ceramics is not easy for largescale amounts. Herein, a method applying water-based solvent is proposed to demonstrate a possible solution. Ta-doped LLZO, that is, Li6.4La3Zr1.4Ta0.6O12 (LLZTO), and its LLZTO/MgO composite ceramics are made by attrition milling, followed by a spray-drying process using water-based slurries. The impacts of parameters of the method on the structure and properties of green and sintered pellets are studied. A relative density of ∼95%, a Li-ion conductivity of ∼3.5 × 10−4 S/cm, and uniform grain size LLZTO/MgO garnet composite ceramics are obtained with an attrition-milled LLZTO/MgO slurry that contains 40 wt % solids and 2 wt % polyvinyl alcohol binder. Li−sulfur batteries based on these ceramics are fabricated and work under 25 °C for 20 cycles with a Coulombic efficiency of 100%. This research demonstrates a promising mass production method for the preparation of Li-garnet ceramics. KEYWORDS: attrition mill, spray dry, Li-garnet, solid electrolyte, LLZO, Li−sulfur battery
1. INTRODUCTION Ta-doped Li7La3Zr2O12 (LLZTO) garnet solid electrolyte is promising for solid-state Li-ion batteries because of its high conductivity at room temperature,1 stability against Li metal,2 and feasibility of preparation under atmosphere.3 Ta5+, which substitutes Zr4+, stabilizes the cubic phase and increases the conductivity by creating Li vacancies.4 Among many methods for the preparation of LLZTO powders, such as sol−gel,5 citrate-nitrate methods,6,7 solid-state reactions (SSRs),8−10 and nebulized spray pyrolysis,11,12 the SSR method is the most promising for mass production of coarse powders. These powders are further milled to submicrons to increase their activity before forming green pellets and sintered ceramics. This powder-tailoring process is always solvent-consuming. It is found that water can react with garnet and causes ion exchange of Li+/H+, releasing LiOH.13−15 The pH value of water can immediately increase from 7 to above 11 when a © 2018 American Chemical Society
small amount of Li-garnet powder is put into distilled water. As for LLZTO, the reaction can be described as: Li6.4La3Zr1.4Ta0.6O12 + xH2O → Li6.4−xHxLa3Zr1.4Ta0.6O12 + xLiOH. The LLZTO powder after ion exchange is thermally unstable and will decompose during the sintering process. Therefore, organic solvents such as ethanol and 2-propanol are adopted in the above-mentioned powder-tailoring process for small-scale production.5−12 When handling large-scale powders, these organic solvents bring in safety, cost, and recycling problems. Herein, a method using water-based solvents is proposed to demonstrate the possibility to solve this dilemma. As for solvents, additional LiOH is added into the water before Received: February 2, 2018 Accepted: April 27, 2018 Published: April 27, 2018 17147
DOI: 10.1021/acsami.8b01961 ACS Appl. Mater. Interfaces 2018, 10, 17147−17155
Research Article
ACS Applied Materials & Interfaces contacting powders to suppress the ion-exchange reaction between LLZO and H2O. As for large-scale powder tailoring, an attrition milling (AM) method is adopted for mass production of fine ceramic powder with uniform particle size.16,17 This method is also broadly used in food processing industry, such as chocolate powder production. Fine and uniform LLZTO and LLZTO/MgO powder at particle sizes of around 300 nm can be easily prepared by AM. Such small particles provide high activity for sintering because of their high surface energy, which helps to densify the ceramics.18 However, small particles bring another problem: they tend to agglomerate nonuniformly during drying. 19 Another problem is the nonuniform devitrification of LiOH·H2O agglomerations after the conventional drying process. To solve these problems, the spray-drying (SD) method is used to obtain secondary granulates with uniform sizes20,21 and distribution of LiOH·H2O. Uniform large spherical-shape secondary granulates of 5−20 μm out of the fine submicron particles can be obtained by SD. This kind of secondary powders can be easily packed into dense green ceramics, which helps in obtaining a uniform and dense microstructure after sintering.22 In addition, the operating temperature of SD is always higher than 100 °C. Organic solvents, such as ethanol and 2-propanol, will bring a potential safety hazard under this high temperature. The water-based solvents adopted in this work can avoid this problem. There are few studies applying AM and SD methods to prepare Li-garnet ceramics. In this work, a systematic research was conducted to demonstrate the application of these methods to prepare dense LLZTO and LLZTO/MgO ceramics with high Li-ion conductivity. The wetting angle of different solvents on LLZTO/MgO ceramics was measured to optimize the component. The parameters of AM and SD methods were carefully adjusted to obtain fine primary and secondary particles. The morphology of these particles was detected by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical properties, phase component, and microstructure of the ceramic pellets were studied by electrochemical impedance spectroscopy (EIS), Xray diffraction (XRD), and SEM, respectively. A Li−sulfur battery based on the LLZTO/MgO ceramics was fabricated and tested at room temperature to demonstrate the feasibility of applying this ceramic in Li-metal batteries.
Table 1. Parameters of Experiments of AM and SD no. machine external LiOH·H2O solvent
component
AM solid content (SC) spray nozzle binder entrance temperature exit temperature
(A)
(B) (C) glass chamber, diameter = 23 cm 2g 260 g 120 g H2O + 30 g 127 g ethanol H2O + 32 g H2O + 65 g ethanol ethanol 100 g 100 g LLZTO 100 g LLZTO + 6g LLZTO + 6g MgO MgO 1800 rpm × 30 min 25% 40% diameter = 2.0 mm 2 g PVA 220 °C
