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Energy, Environmental, and Catalysis Applications 7
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A method using water-based solvent to prepare LiLaZrO solid electrolytes Xiao Huang, Yang Lu, Jun Jin, Sui Gu, Tongping Xiu, Zhen Song, Michael E. Badding, and Zhaoyin Wen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01961 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018
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ACS Applied Materials & Interfaces
A method using water-based solvent to prepare Li7La3Zr2O12 solid electrolytes Xiao Huanga,d, Yang Lua,d,Jun Jina, Sui Gua,d, Tongping Xiuc, Zhen Songb, Michael E. Baddingb, Zhaoyin Wena,d,* a CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, P.R. China b Corning Incorporated, Corning, NY 14831, USA c Corning Research Center China, 200 Jinsu Road, Shanghai 201206, P.R. China d University of Chinese Academy of Sciences, 19 A Yuquan Rd, Shijingshan District, Beijing, 100049, P.R. China TOC graphic
Abstract Li-Garnet Li7La3Zr2O12 (LLZO) is a promising candidate for high safety solid-state Li+ ion batteries. However, due to its high reactivity to water, the preparation of LLZO powders and ceramics is not easy for large-scale amounts. Herein a method applying water-based solvent is proposed to demonstrate a possible solution. Ta doped LLZO, ie, Li6.4La3Zr1.4Ta0.6O12 (LLZTO) and its LLZTO/MgO composite ceramics are made by attrition milling followed by 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. Relative density of ~95%, 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 (PVA) 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 Corresponding author Tel: +86-21-52411704, Fax: =86-21-52413903 *Email address:
[email protected] 1.
Introduction
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Ta-doped Li7La3Zr2O12 (LLZTO) garnet solid electrolyte is promising for solid-state Li-ion batteries due to its high conductivity at room temperature 1, stability against Li metal 2 and feasibility of preparation under atmosphere 3
. The Ta5+, which substitutes the 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 (SSR)
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 sub-microns to increase their activity before forming green pellets and sintering. This powder-tailoring process is always solvent consuming. It is found that water can react with garnet and causes ion exchange of Li+/H+ and releasing LiOH
13-15
. The
PH value of water can immediately increase from 7 to above 11 when a small amount of Li-Garnet powder is put into
distilled
water.
As
for
LLZTO,
the
reaction
can
be
described
as:
Li6.4 La3 Zr1.4Ta0.6 O12+xH2 O→Li6.4-x Hx La3 Zr1.4 Ta0.6 O12 +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-propaol 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 contacting powders to suppress the ion exchange reaction between LLZO and H2O. As for large-scale powder-tailoring, 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 due to their high surface energy, which help to densify the ceramics another problem: they tend to agglomerate non-uniformly during drying
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. However, small particles bring
. Another problem is the non-uniform
devitrification of LiOH·H2O agglomerations after conventional drying process. To solve these problems, the spray drying (SD) method is used to obtain secondary granulates with uniform sizes
20-21
and distribution of LiOH·H2O.
Uniform large spherical-shape secondary granulates of 5-20 µm out of the fine sub-micron particles can be obtained by SD. This kind of secondary powders can be easily packed into dense green ceramics, which helps in obtaining uniform and dense microstructure after sintering 22. In addition, the operating temperature of SD is always higher than 100 ℃. Organic solvents, such as ethanol and 2-propanol, will bring potential safety hazard under this high temperature. The water-based solvents adopted in this work can avoid this problem.
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ACS Applied Materials & Interfaces
There are few studies applying AM and SD methods to prepare Li-Garnet ceramics. In this work, 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 electro-chemical impedance spectroscopy (EIS), X-ray 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.
2.
Experimental
2.1 The synthesis of LLZTO powder Conventional solid state reaction method is adopted to prepare cubic phase Li6.4La3Zr1.4Ta0.6O12 (LLZTO) powder and the details are published in our previous work 23. The amount of one batch of as-prepared powder in Φ10×H10 cm Al2O3 crucible is ~400 g. The X-ray diffraction pattern of the LLZTO powder was shown in Fig. S1. The obtained powder was pure cubic garnet phase without any impurity phases, indicating a complete reaction of the precursors 24.
2.2 Attrition milling and spray drying processes Different solvents’ wetting angles on the LLZTO/MgO composite ceramic surface were measured and described in the details of the supporting materials. For a certain solid loading, the pure water slurry was too viscous and during the AM process the slurry became thick and form some LLZTO agglomerates (Fig. S2A). These agglomerates could easily be de-agglomerated by adding 25 wt% ethanol (Fig. S2B) and would not appear again in the remaining AM process. Fig. S3 shows that the hybrid solvent has a smaller wetting angle on the garnet surface than pure water. Due to this trait, a preferred solvent for AM is water/ethanol mixture with 4:1 ratio. Table 1 shows a matrix experiment for testing different solid loadings and different LiOH addition concentration into the slurries, and also lists the parameters for AM and SD. The detail process for preparing the slurry is described as following: (1) Add LiOH into the solvent and use ultrasound to accelerate the dissolving. (2) Add LLZTO with 6 wt% MgO (50 nm, 99.9%, metallic basis, Aladdin. Inc) or pure LLZTO powders into the solvent. (3) Ball mill the slurry for 3
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hrs and then transferred into the AM system. (4) In AM, use 0.65 mm diameter YSZ beads (Tosoh Corporation), 1800 rpm speed for duration of 30 min. (5) Transfer the slurry to a glass container and stir vigorously. (6) Dissolve 2 g PVA (alcoholysis: 87.0~89.0% in mol/mol, 44.05 in MW, Aladdin Inc.) into small amount of hot water and then add the PVA water solution into the LLZTO slurry dropwisely. (7) Continue to vigorously stir the slurry for another 12 h to make it uniform. The slurry is ready to use for SD. In SD, the speed of peristaltic pump was set to about 5 mL/min and the entrance temperature (around the spray nozzle) was set to 220°C. SDed secondary powders was collected in the collector attached to the SD equipment. Table 1 parameters of experiments of attrition milling and spray drying.
No.
(A)
(B)
(C)
Machine
Glass chamber, diameter = 23 cm
External LiOH•H2O
2g
Solvent
260g H2O + 65g Ethanol
120g H2O + 30g Ethanol
127g H2O + 32g Ethanol
Component
100g LLZTO+6g MgO
100g LLZTO
100g LLZTO+6g MgO
1800 rpm × 30 min
Attrition milling Solid content (SC)
25%
40%
Spray nozzle
Diameter = 2.0 mm
Binder
2 g PVA (Polyvinyl alcohol)
Entrance Temperature
220˚C
Exit temperature
