None-Mother-Powder Method to Prepare Dense Li-Garnet Solid

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None-Mother-Powder Method to Prepare Dense Li-Garnet Solid Electrolytes with High Critical Current Density Xiao Huang, Yang Lu, Haojie Guo, Zhen Song, Tongping Xiu, Michael E. Badding, and Zhaoyin Wen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00976 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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ACS Applied Energy Materials

None-Mother-Powder Method to Prepare Dense Li-Garnet Solid Electrolytes with High Critical Current Density a,d

Xiao Huang , Yang Lu , Haojie Guo , Zhen Songb, Tongping Xiuc, Michael E. Baddingb, Zhaoyin Wena,d,* a

a,d

a,d

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

Abstract Cubic Li-Garnet Li7La3Zr2O12 (c-LLZO) is a promising Li+ ion conductor for applications as a ceramic solid electrolyte in next generation high safety lithium batteries. The sintering temperature of c-LLZO is usually higher than 1100°C, where Li-loss is severe, especially in conventional air ambient sintering method. Covering the green body with “mother powder” is often adopted for compensating the Li-loss. The mother powder having the same composition as the green body cannot be repeatedly use, which raises the cost of the c-LLZO ceramics. A self-compensating Li-loss method without mother powder is proposed and investigated to prepare high-quality c-LLZO ceramics. In this method, excess lithium is added to c-LLZO green pellets to self-compensate Li-loss at high temperature. The impact of different amounts of excess Li and crucible material, such as Pt, MgO, Al2O3 and ZrO2 is studied. With optimized such sintering method, Ta doped LLZO pellets with 10% excess Li can be well sintered inside low-cost MgO crucible without mother powder at 1250°C for only 40 min and laboratory scale production is demonstrated. The ceramics have relative densities of ~96%, conductivities of ~6.47×10−4 S cm−1 and critical current density of 1.15 mA cm−2 at 25 ℃, which is fundamental for further researches on solid-state batteries. Keywords: Li-Garnet; c-LLZO; solid electrolyte; Li-loss; CCD; high-quality.

Corresponding author Tel: +86-21-52411704, Fax: =86-21-52413903 *Email address: [email protected]

1.

Introduction Cubic phase Li7La3Zr2O12 (LLZO) garnets are promising polycrystalline solid electrolytes for next generation

high-safety Li ion batteries due to their high Li ion conductivity (>10-4 S/cm), chemical and electrochemical stability against Li metal (0~3.2 V),1-2 and feasibility of production in air atmosphere 3. Elements such as Al, Ga, Nb and Ta have been doped into the garnet to stabilize the cubic phase and improve ionic conductivity. Various methods have been adopted to prepare dense c-LLZO ceramics. Hot-pressing sintering technique

7-9

and spark plasma sintering (SPS)

10-12

4-6

, field assisted

have been applied to sinter garnet in a tightly sealed

chamber by compacting the powder at high pressure and in-situ heating. Such prepared LLZO ceramics are dense and have high Li-ion conductivity, however, the equipment and process are complicated and costive. Conventional air ambient sintering method is more often adopted for sintering the garnets. Usually, 1000~1100°C high temperature sintering is essential for high activity powders prepared via wet chemistry process 1100~1300°C for powders prepared via solid state reaction (SSR)

15-17

13-14

and

. The Li-loss problem is severe at such high

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temperatures. A typical solution is to cover the green bodies with the “mother powder” of the same compositions 18-25

. However, c-LLZO contains 40 wt% or more rare earth element Lanthanum, resulting in a high cost of the

material. In addition, ceramics prepared via such kind of burying method are always sintered for a long duration (>5 h) and often suffer abnormal grain growth (AGG), which will decrease the mechanical strength of LLZO ceramics 26. The relative density and conductivity at 25°C of these ceramics are usually lower than 94% and 5×10−4 S cm−1, respectively, in published works

15-16, 27-32

. Various kinds of crucibles are adopted in these works, such as

Al2O3, ZrO2 and Pt, however, the impact of different crucibles on the sintering of the same c-LLZO samples has not yet been systematically studied. No researches on air ambient sintering without using “mother powder” are reported yet. In addition, the internal lithium dendrite occurring inside ceramics draws lots of attention in LLZO area 33-40. As for Li/LLZO/Li symmetry cell, the ceramic electrolyte will be short circuited by Li dendrites as the testing current density increases. The critical current density (CCD) just before short circuit is one of the key performances of LLZO electrolytes. The CCD values of LLZO ceramics prepared via ambient air sintering method published up to now are not satisfactory 35, 41-45. In this work, air ambient sintering method without mother powder is used to prepare dense c-LLZO ceramic electrolytes with high CCD at 25°C. The impact of crucible material, such as Pt, MgO, ZrO2 and Al2O3 are explored. Besides, c-LLZO green pellets with 0~20% excess Li from the stoichiometry are prepared and sintered in different crucibles. Properties of ceramics are studied and evaluated to find the optimum amount of excess Li and sintering condition. Then laboratory production of Ta-LLZO pellets are conducted using MgO as the sintering container and the as prepared ceramics were tested with Li-Li symmetry and Li-S full batteries to determine the CCD value.

2.

Experiment

2.1 Preparation of Ta-LLZO green pellets The introduction of Ta5+ to LLZO lattice generates enough Li vacancies in Li7−xLa3Zr2−xTaxO12 when x=0.4 or higher to stabilize the cubic phase at room temperature 46. Li6.4La3Zr1.4Ta0.6O12 (Ta-LLZO) is adopted as an example of the c-LLZO for its high Li-ion conductivity and feasibility in preparation 21. Different amounts (0, 2, 5, 10, 15 and 20%) of excess Li is added in Ta-LLZO powders, denoted as Li0, Li2, Li5, Li10, Li15 and Li20, respectively, in following text. The powders are prepared by conventional solid-state reaction method. Precursors powders of LiOH•H2O (AR), La2O3 (99.99%), ZrO2 (AR) and Ta2O5 (99.99%) with the above ratios are mixed in isopropanol, planetary ball milled at 400 rpm for 6 h, dried, and calcinated in an Al2O3 crucible at 950°C for 6 h. After the first firing, these powders were ball milled again at 250 rpm for 12 h to further decrease the particle size and to increase the reactivity of the powders. After drying at 70 °C overnight, these powders were sieved through 200 grits. Green garnet pellets were prepared by uniaxial pressing 1.5 g of the above powders into Φ18 mm × 2 mm pellets, followed with isostatic pressing under 200 MPa.

2.2 Sintering furnitures As shown in Fig. 1, five kinds of crucibles, Pt, MgO (small), MgO (large), ZrO2 and Al2O3 are selected as the containers to sinter the Ta-LLZO green pellets. Table 1 provides detail descriptions of the sintering setups. The synthesis of mother powder used in the No. 1 crucible is described in the supporting information (SI-1). All the


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green pellets were put on Pt wire supports and separated with the mother powder or the bottom of crucibles. All the samples were sintered at 1250°C for 40 min.

Fig. 1 Scheme of sintering setups Table 1. Detail information of the sintering furniture shown in Fig.1

No. 1 2 3 4 5 6 7

Abbreviation Pt+MP Pt MgO+HT.A MgO ZrO2 Al2O3 MgO-LV

Description Platinum crucible (Pt lid) with mother powder (MP) (Reference) Platinum crucible (Pt lid) without MP MgO crucible (MgO lid sealed by a high temperature adhesive) without MP MgO crucible (MgO lid without sealing) without MP ZrO2 crucible (ZrO2 lid without sealing) without MP Al2O3 crucible (Al2O3 lid without sealing) without MP Large volume MgO crucible (MgO lid without sealing) without MP

Volume 130 ml 130 ml 35 ml 35 ml 98 ml 98 ml 280 ml

2.3 Laboratory production of Ta-LLZO pellets Li6.4La3Zr1.4Ta0.6O12 with 10 wt% excess Li is adopted as the production composition. Two green pellets are put in one MgO crucible as shown by the video supporting information. Ten MgO crucibles are set in one box furnace as shown in SI-2 Fig. S1. The maximum production is ~40 pcs per day. The characterization of ceramic powders and pellets can be found in supporting information SI-3.

2.4 Construction and characterization of Li/Ta-LLZO/Li and Li/Ta-LLZO/S batteries Li/LLZO/Li cell: All electrolyte pellets (thickness: 1.7-1.8 mm) were dry polished with 400 followed by 1200 grit SiC sandpaper in ambient air. Two parallel sides of the pellet were sputtered with Au under 7~8 mA at 2.5~3.2 kV for 10 min. The Au layer is shining gold in appearance. Then these pellets were transferred to glove box filled with argon. A piece of Φ12 mm lithium metal film was put in the center of one face of Au/LLZO, and then heated to 250~300 °C upon a heater. After ~1 min the melted lithium would spread on the surface of the ceramic pellet. The well melted Li-Au-LLZO was transferred to a steel plate and cooled. The other face was attached with melting Li via the same method. Then the Li/LLZO/Li symmetry batteries were sealed in CR2032 cells. These cells were



firstly tested by EIS (Autolab Model PGSTAT302N) and then tested by rate cycling under the initial current density of 0.1 mA cm−2 with an increasing step of 0.05 mA cm−2 to determine the critical current density (CCD) of LLZO ceramic pellets. The charge and discharge duration are set to be30 min. The cells are also tested by galvanostatic cycling under the current density of 0.1 and 0.3 mA cm−2. The discharge and charge duration are set to be 10 min. All battery tests are conducted at 25 °C. Cathode: Ketjen black (KB) and Sulfur were mixed according to the weight ratio of 1:4 by ball milling process in ethanol. The slurry was dried and sieved, and then heated to 155 °C under vacuum to obtain sulfur composite material. This 80 wt% composite was mixed with 10 wt% Super P conductive carbon black, 5 wt% Carboxy Methylated Cellulose and 5 wt% Styrene Butadiene Rubber binder in distilled water. This slurry was uniformly spread onto aluminum foil. The dried cathode on the foil was cut into sheets with 12 mm in diameter and further dried under vacuum. The sulfur loading is 0.8 mg cm−2. Li-S battery configuration: The ceramic pellets were ground to 1.1-1.2 mm. The high interfacial resistance of Li metal and electrolyte was reduced by a thin Au interlayer sputtered onto the surface of LLZO ceramic as described above. 10 µL liquid electrolyte was applied to wet the interface between the cathode and ceramic. The batteries were tested by galvanostatic cycling in the voltage range of 1.5-2.8 V vs Li/Li+ at 0.2 C (the theoretical capacity of sulfur was set to 1675 mAh g−1, typical current density=0.267 mA cm−2) at 25°C on a LAND CT2001A battery test system (Wuhan, China). Specific capacities were calculated based on sulfur mass. After 30 cycles, the rate was raised to 0.5 C. After 100 cycles, the rate was further raised to 1 C. The current densities of 0.5 and 1 C are 0.667 and 1.334 mA cm−2.

3.

Results and discussion

3.1 Properties of powders

Fig. 2 (A): Particle sizes distributions of Li0-20 powders after ball milling; (B): SEM image of Li0 powder in (A).


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All the Li0-20 powders calcinated in Al2O3 crucibles are pure cubic LLZO phase as shown in SI-4 Fig. S2. Fig. 2A shows the particle size distributions of Li0-20 powders after ball milling at 250 rpm for 12 h. All curves have three peaks. The D10, D50 and D90 values for all samples varied within 0.15 to 0.18 µm, 0.25 to 0.50 µm and 2.0 to 3.0 µm, respectively, (see details in SI-5 Table S1), indicating a similar particle size distribution for these six samples with different excess Li amounts. The particle morphologies are shown by SEM images in Fig. 2B and SI-6 Fig. S3. Combining the particle size distribution curves with the SEM results, the peaks near 0.2 µm, 0.6 µm, and the flat peak ranging from 1 to 10 µm correspond to the primary small particles shown in the SEM image in area 1, the agglomerations of small primary particles in area 2 and the inadequately ground large primary particles or their agglomerations in area 3, respectively. The distribution of the ceramic particle size is one of the critical parameters that impact its sintering and its final microstructure of the ceramics 47-48. The cross-sectional microstructures of green pellets pressed from Li0-20 powders are shown in SI-7 Fig. S4. The particles are uniformly and tightly compacted together. and the typical density of a green pellet is 3.1 g cm−3. In addition, the six samples with different amounts of excess Li show similar morphologies in cross-sections.

3.2 The impact of crucibles on the sintering results of Li15 samples without mother powder

Fig. 3 The phases of Li15 pellets sintered in No. 1-7 setups as shown in Fig. 1

Fig. 3 is the XRD patterns of the Li15 green pellets sintered in the different setups shown in Fig. 1 at 1250°C for 40 min. The pellets sintered in No. 1-Pt+MP setup, i.e., the reference condition that used mother powder to compensate the Li loss, are in pure cubic phase. The pellets sintered in No. 2-Pt, No. 3-MgO+HT.A and No. 4-MgO setups without mother powder are also in pure cubic phase. However, the pellets sintered in No. 5-ZrO2, No. 6-Al2O3 and No. 7-MgO-LV setups contain the impurity phase of La2Zr2O7, whose characteristic peak is denoted by red inverted triangles. La2Zr2O7 formed at high temperature (>1000 ℃) is usually from Li loss and insufficient Li source in the sintering system to compensate the Li loss. It was also observed in Al-LLZO ceramics sintered at



1250°C for 35 h inside the covering powder 49 and in the Li-Garnet thin films prepared by deposition methods 50-52. Ikeda, Y. et. al. studied the vapor pressure and vapor components that accompanied with the Li loss using mass-spectrometric Knudsen diffusion method

53-56

, and concluded that the main component is Li2O gas. Pt and

MgO are relatively chemical-stable with Li2O at high temperature, while ZrO2 and Al2O3 are reactive to Li2O with the following reactions57-58:

2 − 3 + → 2 − 2 + → These reactions lower down the Li2O vapor pressure during garnet sintering, and as a result enhance more Li2O to escape from the pellets. Therefore, firing in Al2O3 and ZrO2 crucibles need much more Li inside the systems. The above result indicates that when firing in a Pt or a small MgO crucible, the excess Li in the Li15 pellets is enough to compensate the Li loss. The excess Li forms certain vapor pressure of Li2O, which suppresses further vaporization of Li2O from the pellets. For a larger volume MgO crucible, more Li loss can occur and therefore more Li is needed to achieve a similar vapor pressure of Li2O; besides, the MgO lid on the crucible is a relatively poor seal to contain the Li2O gas; therefore, a small amount of La2Zr2O7 phase formed on samples fired in the large MgO crucible. According to these results and analysis, an appropriate crucible is important to ensure pure cubic phase for c-LLZO ceramics. Fig. 4A-G shows the cross-sectional microstructures of Li15 pellets sintered in the above seven setups. The reference pellets that sintered in No. 1-Pt+MP (Fig. 4A) with mother powder have dense structure and well grown grains with smooth facets and clear grain boundaries. Pellets sintered in No. 2-Pt (Fig. 4B), No. 4-MgO (Fig. 4D) and No. 7-MgO-LV (Fig. 4G) without mother powder show similar morphologies to the reference sample. Pellets sintered in No. 3-MgO+HT.A (Fig. 4C) and No. 6-Al2O3 (Fig. 4F) have grains covered with “glass-like materials” on the surfaces and obvious porosity, indicating a deficient sintering degree of these samples. Pellets sintered in No. 5-ZrO2 (Fig. 4E) have polyhedron grains with rough surfaces and obvious porosity. It was expected that the samples in No. 3 setup with sealed lid to have better sintering than those in the No. 4 setup without sealing. However, the experiment shows a result opposite to our expectation. Further analysis shows the problem is from the sealant used for hermetically sealing the lid to the crucible body. The major component of the HT.A sealant is Al2O3, which can absorb the Li2O gas evaporated from the green pellet, and accelerate the Li loss, therefore resulting a lower degree of sintering for the ceramics. As shown in SI-8 Fig. S5, the pellets sintered in the small MgO crucible (No. 4) were in tan color while the pellets sintered in the large MgO crucible (No. 7) were in white color with cracked edges. The white color is usually from not enough sintering, and the cracks at edge result from non-uniform shrinkage. The SEM image in Fig.4G was taken at the center part of the pellet, which has denser morphology than the rest area, indicating the sintering of pellets in large volume MgO crucible is not uniform. According to the above results and analysis, an appropriate crucible is important to ensure good sintering of the c-LLZO ceramics. The EIS curves of Li15 pellets sintered in No. 2-Pt, No. 4-MgO and No. 6-Al2O3 conditions are shown in Fig. 4H&I. Huge grain boundary impedance of 2 MΩ cm is observed for No. 6 sample, while the No. 2 and No. 4 samples show much lower grain boundary impedances (1.5 kΩ cm). The Z′ value of the reflection point in EIS curves is considered as the total impedance and used to calculate the total conductivity. Fig. 4J shows the relative density and conductivity of the above Li15 pellets. The reference sample, i.e. the pellets sintered in No. 1-Pt+MP, has a relative density of 94% and a conductivity of 6.35×10−4 S cm−1. In comparison, the samples sintered in No.


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2-Pt and No. 4-MgO without any mother powder have similar relative densities and conductivities. As for the No. 3-MgO+HT.A, No. 5-ZrO2 and No. 6-Al2O3 samples sintered inside crucibles that contain materials reactive to Li2O (g), the relative densities are much lower than the reference sample. No. 6 sample was selected to conduct EIS testament due to the universal application of Al2O3 crucible in published papers. The conductivity is as low as 5.22×10−7 S cm−1, which is three orders of magnitude lower than that of the reference sample. The relative density of No. 7 sample sintered in the large MgO crucible (280 ml) is 90%, which is lower than No. 4 sample sintered in small volume MgO crucible (35 ml). These results demonstrate the import impacts of crucible conditions on the final relative density and conductivity of the Ta-LLZO ceramics.

Fig. 4 (A-G): SEM images of the cross-sections of Li15 pellets sintered in No. 1-7 setups. (H&I): Normalized EIS curves of Li15 samples sintered in No. 2, 4 & 6 setups and the inserted graph is the fitting circuit; (J): Relative density (black) and total conductivity (red) at 25°C of Li15 pellets sintered in No. 1-7 setups.

Table 2 summaries the properties of Li15 pellets sintered in No. 1-7 setups. It is obvious the Al2O3, and ZrO2 crucibles are not desired for the sintering of Ta-LLZO ceramics while the MgO crucible seems to be a good



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substitution to the Pt crucible. In addition, mother powder is not a must when using MgO crucible for sintering dense LLZO.

Table 2 Summary of the phase, cross-sectional morphology, relative density and total conductivity at 25°C of Li15 pellets sintered in seven sintering sets

Sintering conditions No. 1 Pt+MP No. 2 Pt No. 3 MgO+HT.A No. 4 MgO No. 5 ZrO2 No. 6 Al2O3 No. 7 MgO-LV

Phase

Cross-sectional morphology

C C C C C+La2Zr2O7 C+La2Zr2O7 C+La2Zr2O7

Smooth polyhedron grains Smooth polyhedron grains Rough worm-like grains Smooth polyhedron grains Rough polyhedron grains Rough worm-like grains Smooth polyhedron grains

Relative density 0.94 0.95 0.82 0.95 0.81 0.79 0.90

Conductivity ×10−4 S cm−1 6.35 6.65 N/A 6.32 N/A 0.00522 N/A

3.3 The impact of excess Li amounts on garnet pellets sintering in No. 1-Pt+MP, No. 2-Pt and No. 4-MgO sintering setups

Fig. 5 SEM images of cross-sections of (A~F) Li0~20 pellets sintered in No. 4-MgO set-up. All images use same scale bar.

As discussed in section 3.2, Li15 pellets can be densified in No. 2-Pt and No. 4-MgO sintering setups without any mother powder. The excess Li amount is also a critical parameter for the sintering of Ta-LLZO pellets. Green pellets of Li0-20 are sintered in three sintering setups, No. 1, No. 2 and No. 4, to explore the relationship between Li amount and sintering results for different crucibles. Table 3 summarizes the properties of these samples. When there is external mother powder to compensate the Li loss, all Li0-20 pellets sintered in No. 1-Pt+MP show similar cross-sectional morphologies with smooth polyhedron grains and clear grain boundaries as shown in SI-9 Fig. S6. When removing the mother powder and sintering in the No. 2-Pt setup, except the Li0 pellet which is not dense, the rest Li2-20 pellets are all well sintered as shown in SI-9 Fig. S7. When further substituting the Pt crucible with MgO, which comes to the No. 4 setup, higher amount of Li is needed for sintering dense garnet. As shown in Fig. 5, Li0-5 pellets are not densified, and dense morphology starts to show after Li10 sample. The cross-section of Li0 sample contains worm-like grains with rough surfaces (Fig. 5A). Li2 pellets show a better cross-section morphology than Li0 with smooth polyhedron grains and loose grain boundaries. Trans-granular fracture and pores


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can be observed in Li5 pellets. When the excessive amount of Li is higher than 10%, dense cross-sections can be obtained. It can be inferred that 2% excess Li in Ta-LLZO green pellet is enough to compensate the Li loss when using a covered Pt crucible without mother powder, and this excess Li amount increases to 10% when using a lid covered MgO crucible. The difference originates from the tightness of lids for the Pt and the MgO crucibles. Pt is softened at 1250°C and the lid and crucible can be sealed together. Force is always needed to remove the Pt lid after sintering. The seal quality of MgO lid does not change with heating. More Li2O gas is lost in MgO crucible compared to the Pt crucible, and therefore more excess Li is needed to compensate the greater Li loss when firing in MgO crucible.

Fig. 6 (A-C): EIS curves of Li0-20 pellets sintered in No. 4-MgO set, where (B) is the enlarged view of green square in (A) and (C) is the enlarged view of blue square in (B). Inserted graph is the fitting circuit.

Fig. 6A-C show the EIS curves of Li0-20 samples sintered in No. 4-MgO without mother powder. In the full-frequency-range view (Fig. 6A), Li0 pellet shows a huge grain boundary impedance semi-circle in Fig. 6A, while curves of the other samples all curl inside the green square. The semi-circle corresponds to the grain boundary impedance with the terminal frequency (TF) of 70 Hz. In a middle-frequency-range view (Fig. 6B), dual semi-circles are observed for Li2 sample. The one with the TF of 17 kHz corresponds to the bulk impedance, and the one with the TF of 700 Hz corresponds to the grain boundaries. Only a flattened semi-circle with the TF of 70 kHz is observed for Li5 sample, which corresponds to the grain boundaries. In a high-frequency-view (Fig. 6C), Li10-20 samples show similar curves with a semi-circle corresponding to the grain boundaries with the TF of 1.1 MHz plus a diffusion tail in low frequencies. Similar phenomenon is also observed for Li0-20 pellets sintered in the No. 2-Pt setup as shown in SI-10 Fig. S8. The grain boundary impedance decreases with the increase of the excessive amount Li in the pellets, indicating a better sintering in the higher Li containing pellets. This is consistent with the SEM morphology study shown in Fig. 6. The conductivities and relative densities of Li0-20 samples sintered in the No. 1, 2 and 4 setups are shown in Table 3 and Fig. 7. In the Pt crucible with mother powder, all sintered samples have high conductivities, ~6×10-4 S/cm. When removing the mother powder, the Li0 sample’s conductivity drops to 4.36×10−7 S cm−1, while the rest samples’ conductivities are still at high level. When substituting the Pt crucible with MgO, Li0, Li2 and Li5 samples’ conductivity significantly decreased, while the rest samples still keep the same high conductivity as those reference samples. The trend of the relative densities of these samples is similar. In the No. 1 setup, all samples have similar high relative densities (~94%) and a slow decrease with the excess Li amount can be observed. In No.



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2 setup, except the Li0 sample, whose relative density reduces to 77%, the rest samples all have high relative density (~95%). In No. 4 setup, i.e., the MgO crucible, Li0, Li2, and Li5 samples have significantly lower relative density, while the rest samples have similar density as the reference samples. It is interesting that all curves when reached to the highest point, the relative density starts to decrease with the excess Li amount. It is considered that when the excess Li is more than enough, some of the Li will stay at the green boundary region, and may evaporate in form of Li2O gas, and leave gaps in the sample, causing slightly decreasing in density. These results demonstrate the importance of amount of excess Li, whereas in a specific condition, there is a best excessive value for obtaining high-quality LLZO ceramics.

Fig. 7 (A) Total conductivity and (B) Relative density of pellets at 25°C sintered in No. 1-Pt+MP, No. 2-Pt, No. 4-MgO and No. 6-Al2O3 sintering sets versus the excessive amounts of Li. Table 3 Summary of total conductivity at 25°C and relative density of Li0-20 pellets sintered in No. 1, 2, 4 and 6 sets.

Sintering sets

0%

No. 1-Pt+MP No. 2-Pt No. 4-MgO No. 6-Al2O3

6.33 0.00436 0.00308

No. 1-Pt+MP No. 2-Pt No. 4-MgO No. 6-Al2O3

0.94 0.77 0.75

Excessive amount of Li 2% 5% 10% Total Conductivity at 25°C (×10−4 S cm−1) 6.36 5.51 6.33 6.37 6.60 6.52 0.088 0.537 6.47

0.94 0.96 0.89

Relative density 0.93 0.94 0.96 0.95 0.90 0.96

15%

20%

6.35 6.65 6.32 0.00522

6.30 6.28 6.37

0.94 0.95 0.95 0.79

0.93 0.94 0.94

3.4 The electro-chemical performance of Li-Li and Li-S batteries constructed by laboratory production pellets


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The sintering method and results for production of Ta-LLZO small pellets are shown in Fig. 8A. As-prepared pellets have uniform colors and dense appearances. The relative density, conductivity (25°C) and activation energy of these pellets are >96%, >6.4×10−4 S cm−1 and 0.43 eV (SI-11), respectively. Table. S2 (SI-12) compares the properties of c-LLZO ceramics prepared via different methods. Ta-LLZO pellets sintered at 1250°C for only 40 min without mother powder show close performances comparing to samples sintered via hot-pressing method.

Fig. 8 (A) Digital images showing the sintering method for laboratory production of Ta-LLZO pellets. (B) Scheme and AC impedance curves of Li/Garnet/Li and Li/Garnet/S batteries. (C) Rate performance of Li-Li symmetry cell. (D&E)



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Discharge-charge profiles and cycling performance of Li-S full cell. (F) Cycling performance of Li-Li symmetry cell.

The AC impedance curves of Li-Li and Li-S batteries constructed of these pellets are shown by black squares and red spheres in Fig. 8B. Au is used to improve the wetting of Li on LLZO surface and thus to decrease the interfacial resistance 59-61. Li-Li cell has two semi-circles: the small one from 200 kHz to 2 kHz corresponds to the interface between Li-Au alloy and LLZO; the big one from 2 kHz to 0.1 Hz may correspond to the electro-chemical reaction of Li-Au alloying

61-62

. The Li/LLZO interfacial resistance (excluding the Li-Au alloying contribution) is

~3 Ω cm2, which is considered to be low enough to exclude the impact of non-uniform contacting between Li and LLZO on the CCD results. Li-S cell has one semi-circle, one flat line and an upward tail: the large semi-circle from 200 kHz to 50 Hz corresponds to the two interfaces of Li-Au/LLZO/liquid electrolyte (LE), the diffusion of Li+ from LE to sulfur cathode; the flat line from 50 Hz to 0.7 Hz corresponds to the charge transfer at the cathode side. The LLZO electrolyte with a thickness of 1.2 mm contributes ~120 Ω cm2 and the total resistance of Li-S full cell is ~220 Ω cm2, which are higher than pure liquid electrolyte based Li-S battery but lower than LLZO based batteries published recently

63-65

. The rate performance of Li/Ta-LLZO/Li cell is shown in Fig. 8C. As the current

density increases, the voltage increases linearly until 1.2 mA cm−2. Short circuit occurs at this high current density as the voltage suddenly drops to a low level. The CCD is determined to be 1.15 mA cm−2 in this experiment. Li-S full batteries are constructed to demonstrate the application of LLZO. As shown in Fig. 8D&E, the capacity retention is acceptable from 1446 mAh g−1 (1st) to 835 mAh g−1 (100th) and the Coulombic efficiency is higher than 99% before the rate increases to 1C. It should be addressed that the CCD of laboratory production Ta-LLZO is 1.15 mA cm−2 and 10 µL liquid electrolyte is applied at the cathode side to decrease the interfacial resistance between LLZO and S. When the charge current density is 1C=1.334 mA cm−2, the lithium dendrites will penetrate through LLZO pellet and react with soluble poly-sulfides at the cathode side. Thence the Coulombic efficiency becomes less than 99% as shown by orange dashed box in Fig. 8E, the capacity degradation becomes serious in 100~150 cycles and the charge profile of 113th cycle is twisty as shown by red line in Fig. 8D. These Ta-LLZO pellets can be stable at 0.5C=0.667 mA cm−2 in Li-S cell, however, unstable at 1C=1.334 mA cm−2. Although the CCD value obtained in this work is higher than most of the published works up to now

39, 66-70

, it is

still not high enough for the operation of Li/Ta-LLZO/S at 1C. In addition, after long-duration (>550 h) cycling test at 0.1 and 0.3 mA cm−2, the polarization of Li/Ta-LLZO/Li symmetry cell becomes higher as shown in Fig. 8F. This may be due to the pulverization of Lithium electrodes 59, which needs further effort to overcome.

4.

Conclusion The impact of the crucible material on garnet sintering was studied by sintering Ta-LLZO pellets with 15%

excess Li in seven different sintering set-ups. Without mother powder, green garnet pellets cannot be densified in setups that contain materials reactive to Li2O (g), such as ZrO2, Al2O3 crucibles and MgO crucible sealed by Al2O3 based high temperature adhesive. The impact of the excess Li on sintering in Pt and MgO crucibles without mother powder is studied with excess Li in the range of 0-20%. 2% excess Li included in the green pellet can ensure good sintering in a Pt crucible while 10% is needed for the MgO crucible. Finally, Ta-LLZO pellets (laboratory scale production) with 10% excess Li can be well sintered in a low-cost MgO crucible with relative densities higher than 96% and total conductivities higher than 6.4×10−4 S cm−1 at 25°C. These pellets are also tested with Li-Li symmetry and Li-S full batteries to determine the critical current density and to demonstrate the application in solid batteries. The CDD value is 1.15 mA cm−2 at 25°C, which is higher than most of the published works up to now.


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Comparing with other ambient air sintering method, our method shows advantages in feasibility, material cost saving, short sintering duration and simultaneously preparing high-quality ceramics. This work is fundamental for further research works of solid-state batteries.

Acknowledgements This work was ﬁnancially supported by the National Key R&D Program of China under Grant No. 2018YFB0905400, Corning Incorporated, the National Natural Science Foundation of China under Grant No. 51772315, No. 51432010 and Key Fundamental Research Project from Science and Technology Commission of Shanghai Municipality (14JC1493000).

Supporting Information Available: The synthesis of mother powder, the images demonstrating laboratory production of Ta-LLZO pellets, the characterization of ceramic powders and pellets, the XRD patterns of Li0~20 powders, the particle sizes distribution and morphology of Li0~20 powders after ball milling, the cross-sectional microstructure of Li0~20 green pellets, the digital images of all pellets prepared in this work, the cross-sectional microstructure and EIS curves of Li0~20 pellets sintered in Pt crucible with/without mother powder are available in supporting information.

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None-Mother-Powder Method to Prepare Dense Li-Garnet Solid

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