Conformal, Nanoscale ZnO Surface Modification of Garnet-Based

Dec 12, 2016 - The conformal ZnO layer can be also coated onto the internal structure of 3D porous garnet SSEs used for hosting lithium metal anodes. ...
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Conformal, Nanoscale ZnO Surface Modification of Garnet-Based Solid-State Electrolyte for Lithium Metal Anodes Chengwei Wang,†,‡ Yunhui Gong,†,‡ Boyang Liu,†,‡ Kun Fu,†,‡ Yonggang Yao,† Emily Hitz,† Yiju Li,† Jiaqi Dai,† Shaomao Xu,†,‡ Wei Luo,† Eric D. Wachsman,*,†,‡ and Liangbing Hu*,†,‡ †

Department of Materials Science and Engineering and ‡University of Maryland Energy Research Center, University of Maryland College Park, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: Solid-state electrolytes are known for nonflammability, dendrite blocking, and stability over large potential windows. Garnetbased solid-state electrolytes have attracted much attention for their high ionic conductivities and stability with lithium metal anodes. However, high-interface resistance with lithium anodes hinders their application to lithium metal batteries. Here, we demonstrate an ultrathin, conformal ZnO surface coating by atomic layer deposition for improved wettability of garnet solid-state electrolytes to molten lithium that significantly decreases the interface resistance to as low as ∼20 Ω·cm2. The ZnO coating demonstrates a high reactivity with lithium metal, which is systematically characterized. As a proof-of-concept, we successfully infiltrated lithium metal into porous garnet electrolyte, which can potentially serve as a self-supported lithium metal composite anode having both high ionic and electrical conductivity for solid-state lithium metal batteries. The facile surface treatment method offers a simple strategy to solve the interface problem in solid-state lithium metal batteries with garnet solid electrolytes. KEYWORDS: Surface modification, ZnO interface, garnet solid-state electrolyte, Li metal batteries

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Li1.3Al0.3Ti1.7(PO4)3 (LATP),23,24 perovskite lithium lanthanum titanates (LLTO),25−27 and garnet-type Li-ion conductor LLMO (M = Zr, Nb, Ta).28−33 Among these SSEs, the cubic garnet phase solid-state electrolytes have attracted more interest due to their high ionic conductivities (10−4−10−3 S/ cm),28,29 stability to lithium metal up to 300 °C,34 and wide electrochemical potential ranges.35−37 A challenge for garnet solid-state batteries is their high interfacial impedance due to the poor wettability of the garnet SSEs against molten lithium, which causes a poor contact between garnet SSEs and lithium and leads to a large polarization and an uneven ion flow through the interface. Several methods have been used to modify the interface, such as applying mechanical pressure,38,39 using polymer electrolytes as a buffer layer,40 and performing preactive cycling at low current density.41 Although these methods have decreased the interfacial resistance to some extent,42 further work is needed to address the fundamental issue of wettability between garnet SSEs and lithium metal. Another challenge the lithium metal anode faces is volume change during cycling. One potentially effective strategy to combat this is the application of a 3D porous structure to serve as a host for the lithium metal anode.6−9 For this purpose, if garnet SSEs were engineered to form a porous/dense bilayer structure,43 the porous layer could serve as a perfect host layer

ithium-ion batteries have been widely used in various applications for the last two decades.1−4 With the rapid development of portable electronic devices and electric vehicles, the demand for safe, high energy density batteries has grown. Using lithium metal as the anode is an attractive way to increase the energy density of batteries due to the high theoretical specific capacity (3.86 Ah/g) and low reduction potential (−3.05 V) of lithium metal. However, the growth of lithium dendrites can lead to battery performance decay and cause safety concerns, especially when flammable organic liquid electrolytes are used. Recently, many strategies have been developed to address the dendrite challenge in the lithium metal batteries, such as using 3D structured current collectors to lower the current density,5−10 mixing the electrolyte with additives to form a protective layer,11−13 and engineering modified separators to block or detect dendrites.14−16 Although these strategies have addressed some of the challenges associated with lithium dendrite, the dendritic lithium growth remains an inevitable issue, and flammable liquid electrolytes still present a safety concern. Solid-state electrolytes (SSEs) are a fundamental strategy to achieve practical Li metal batteries free of the safety and performance issues resulting from other electrolytes. They are of interest due to their ability to mechanically block lithium dendrite growth17,18 and their nonflammability compared to organic liquid counterparts. There have been many types of SSEs under study, including Li2.88PO3.73N0.14 (LiPON),19,20 Li10GeP2S12, Li9.54Si1.74P1.44S11.7Cl0.3, and Li9.6P3S12 (LGPS),21,22 © XXXX American Chemical Society

Received: November 9, 2016 Revised: December 8, 2016 Published: December 12, 2016 A

DOI: 10.1021/acs.nanolett.6b04695 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Schematic of surface-treated garnet wetted with molten lithium. (a) Lithium shows a large contact angle on the pristine garnet. (b) Wetting process of the molten lithium on the ZnO coated surface of garnet SSE, where the molten lithium diffuses into the ZnO layer to form Li−Zn alloy and wets the surface of the garnet.

Figure 2. Characterization of the ZnO coating to improve the surface wettability of garnet electrolyte with molten lithium. Cross-section (a) SEM images and (b) elemental mapping of the garnet electrolyte coated with a 50 nm ALD ZnO layer. The inset of (a) is a cross-section SEM image of the garnet/ZnO interface at higher magnification. (c) Schematic of the lithium diffusion process along the ZnO coating layer on the garnet surface. Digital images of (d) the top side and (e) the back side of the lithium-wetted ZnO-modified garnet after the lithium diffusion process. The marked region in (d) is of the area being polished afterward, indicating that the dark materials are from the surface layer. (f) Cross-section SEM images of the Li wetted garnet.

due to its high hardness and ionic conductivity, while the dense layer could serve as the electrolyte and the separator. Moreover, the 3D porous structures of the SSEs can also increase the contact area with the electrode materials, further lowering the interface resistance and the specific current density. However, due to the high tortuosity of the porous structure, molten lithium metal needs to overcome surface tension to infiltrate into the pores. Therefore, it is strongly desired to develop a method to improve the surface wettability of garnet SSEs with lithium metal.

In this work, we address the interfacial resistance challenge by forming a nanoscale zinc oxide (ZnO) surface modification layer to improve the surface wettability of garnet electrolytes, significantly enhancing the contact between lithium metal and garnet electrolytes and decreasing the interface resistance. With the ultrathin, conformal ZnO surface coating, the molten lithium can react with ZnO so as to have better contact with the surface of the garnet electrolyte, resulting in a significantly decreased interfacial resistance. The conformal ZnO layer can be also coated onto the internal structure of 3D porous garnet SSEs used for hosting lithium metal anodes. Because of the B

DOI: 10.1021/acs.nanolett.6b04695 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 3. Schematics and the cross-section SEM images of the lithium infiltration into porous garnet with a ALD-coated ZnO surface modification layer. (a) An illustration of the lithium infiltration into porous garnet with or without surface modification. The cross-section SEM images of (b) the pristine and (c) the lithium infiltrated porous garnet with a porosity of 60%−70%. (d) The cross-section SEM image of the porous garnet coated with conformal ZnO surface layer using ALD process. (e) The cross-section SEM images of lithium infiltrated porous garnet with ZnO surface treatment, where almost all pores have been filled with lithium metal. The lithium metal area has been marked with a cyan dashed line.

With this surface modification layer, the garnet electrolyte demonstrates a much improved wettability to molten lithium. Note that the lithiation process of the ZnO thin layer is possible even at room temperature. After a small piece of lithium was melted onto the ZnO-coated garnet surface at about 230 °C for 30 min, it attached to the garnet surface firmly while a dark ring of lithiated ZnO formed at the edge of the lithium. Even after the pellets cooled to room temperature, the dark lithiated area continued to propagate along the surface and ultimately reached the back side in about 8 h via the edge of the garnet pellet (Figure 2c). The photographs in Figure 2d,e show the lithium-wetted garnet with a dark-lithiated ZnO area on the front and back sides, respectively. To confirm that the reacted layer is limited to the surface coating, part of the black garnet surface was polished with sandpaper to expose the white garnet color beneath (Figure 2d). The center area of the backside remained white, while the area near the edge turned black, demonstrating that the lithium diffused to the backside along the edge instead of through the volume of the garnet pellet. Figure 2f depicts the cross-section SEM image of lithiumcoated garnet, where the garnet displays an excellent wetted interface with lithium metal. This indicates that the ZnO coating layer can effectively improve the surface wettability of the garnet electrolyte with the molten lithium. ALD not only has great thickness control of the ZnO coating layer but also is an effective method for 3D conformal coating of porous structure, which then makes it possible to infiltrate lithium metal. Considering the volume change of the lithium anode during the charge−discharge process of the lithium battery, a supporting material is necessary to maintain the structure of the battery and good contact between the lithium anode and the electrolyte. For this reason, a porous, ionically conductive solid-state electrolyte would be an ideal supporting material for lithium metal anode, because the porous structure can offer more contact surface for lithium to further decrease the interfacial resistance while maintaining the volume of the anode. However, due to the poor wettability between molten lithium and the solid-state electrolyte, it is difficult to directly infiltrate lithium into porous garnet without surface mod-

high tortuosity of the 3D porous structure, most common coating techniques such as sputtering44 and chemical vapor deposition45 cannot achieve a uniform coating. To ensure a conformal coating on the surface of the porous structure, atomic layer deposition (ALD) is successfully applied to improve the wettability of the 3D porous garnet SSE, which results in excellent infiltration of lithium metal into the porous garnet. Figure 1 exhibits the effect of the surface treatment on the improvement of the wettability of garnet SSEs with lithium metal. Because of the large difference in the specific surface energies of molten lithium and garnet, the molten lithium has a large contact angle on the pristine garnet (Figure 1a). After application of a thin layer of ZnO as a surface treatment, the garnet SSE shows much greater wettability to molten lithium (Figure 1b). This change is brought about when the molten lithium contacts the ZnO coating, because ZnO can be reduced by lithium to form a LiZn alloy. During the alloying process, lithium diffuses along the ZnO coating layer and therefore wets the surface of the garnet (Figure 1b). Because of its excellent reactivity and wettability with molten lithium, ZnO is selected as the coating material for the surface of garnet SSE to improve the electrolyte−anode interface characteristics. Among the many coating techniques, ALD has superior control over layer thickness and uniformity. Here, ALD was employed to deposit a 30−50 nm ZnO layer on the surface of garnet solid electrolytes and study their interface properties. Figure 2a,b exhibits the scanning electron microscopy (SEM) images and the energy dispersive spectroscopy (EDS) elemental mapping of the cross-section of an ALD ZnO-coated garnet pellet. For better observation, a thicker, ∼50 nm ZnO layer was made. Because of the large surface roughness of the garnet pellet, the ZnO layer does not appear smooth, though it maintains tight and continuous contact with the garnet (Figure 2a). The elemental mapping in Figure 2b further confirms the continuous layer of ZnO, where the deposited layer is shown to reach and tightly coat the curved surface of the garnet electrolyte. C

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Figure 4. Characterization of the reaction between ZnO and molten lithium at ∼250 °C. Digital photographs of the reaction processes between ZnO pellets and (a) sufficient and (b) limited molten lithium at different time intervals. (c) The phase diagram of Li−Zn.46 (d) X-ray diffraction (XRD) patterns of the reaction products between ZnO and molten lithium. The bright and dark patterns are from the reaction product of ZnO with sufficient and limited lithium, as marked in panels a and b, respectively.

ification. Even with surface treatment, most surface modification techniques cannot easily coat the surface of a porous structure having high tortuosity.44,45 Using ALD technique, the inner surface of the porous structure can be uniformly coated. Figure 3a shows the schematic of lithium infiltration into the porous garnet with or without surface treatment, while Figure 3b−e are the corresponding cross-section SEM images. The porous garnet consists of many interconnected microsized pores (Figure 3b), which cannot be wetted and infiltrated by the molten lithium due to the high tortuosity. In Figure 3c, most of the lithium remains on the surface of the porous garnet without filling the inner pores. After being coated with the ZnO surface modification layer, the porous structure of the garnet electrolyte still remains (Figure 3d). The porous garnet was then successfully infiltrated with lithium metal at ∼300 °C for 30 min (Figure 3e). From the SEM images in Figure 3e, we can clearly see that almost all the pores have been filled with lithium metal, indicating that the ZnO coating layer can significantly improve the wettability of the garnet electrolyte for lithium metal. To better understand the mechanism of the wetting process, pure ZnO pellets pressed from ZnO nanoparticles were used to study the reaction with molten lithium. According to the images shown in Figure 4a, after contacting the ZnO pellet at ∼250 °C the molten lithium quickly wetted and corroded the ZnO. After about 10 min, almost all of the molten lithium was absorbed by the ZnO pellet, while most of ZnO was reduced to zinc metal and alloyed with lithium. The amount of lithium

present was more than sufficient to reduce ZnO to zinc metal and form a Li−Zn alloy. Therefore, even though ZnO partially oxidized the lithium, the final product still exhibited a shiny metallic color. In another case, a limited amount of lithium was used to react with the pressed ZnO pellet, where the lithium quickly wetted the surface of ZnO pellet and then was fully absorbed by ZnO pellet in about 1 min (Figure 4b). The final product in this situation was much darker than the one in the former situation. The dark product also agrees with the black lithiated ZnO coating on garnet surface at room temperature (Figure 2d, e). According to the Li−Zn phase diagram in Figure 4c,46 there are many alloy phases between lithium and zinc existing at different atomic ratios. To identify the reaction products, X-ray diffraction was conducted on the two final products aforementioned. The patterns labeled with “Bright” and “Dark” in Figure 4d are from the shiny product marked in Figure 4a and the dark product marked in Figure 4b, respectively. Although the relative intensities of the peaks are different, the patterns are identified to LiZn alloy phase (JCPDF# 03-0954), which is the most lithium-rich alloy phase on the Li−Zn phase diagram (Figure 4c). Therefore, the difference of the color is believed to come from the various sizes and shapes of the LiZn grains instead of the variation of the composition, because the amount of lithium and reaction time differ significantly to affect the growth of LiZn grains. The results demonstrate a continuous and firm contact between lithium metal and garnet electrolyte due to the excellent reactivity and wettability of the ultrathin ZnO coating D

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Figure 5. Electrochemical characterization of Li/Garnet/Li symmetric cells. Nyquist plots of (a) the symmetric cells without ZnO surface treatment and with ZnO treatment heated at different temperatures and (b) the ZnO-treated symmetric cells heated at 300 °C before and after the stripping− plating test at a current density of 0.1 mA/cm2. (c) The bulk and interfacial ASR of a 200 μm thick garnet pellet with or without ZnO surface treatment. (d) The voltage profile of the symmetric cell during the stripping−plating test at a current density of 0.1 mA/cm2.

high as 1900 Ω·cm2 after annealing at 300 °C for 30 min (Figure 5c). With a 30 nm ZnO surface modification layer, the interfacial ASR drops dramatically to about 450 Ω·cm2, even with heating temperatures as low as 230 °C. In this case, the ZnO coating is believed to be just prelithiated to form the dark phase, as mentioned in Figures 2d and 4b, which was found to have lower electrical conductivity (∼kΩ/□ for dark lithiated ZnO coating in Figure 2d) than the shiny lithium-rich alloy phase in Figure 4a. The oxygen from the conformal ZnO coating may also cause some oxidation in the lithium to form Li2O and slow down lithium diffusion into the interface layer. Additionally, after further heating at 300 °C for 30 min, the interface became fully lithiated to the lithium-rich alloy phase, and the interfacial ASR decreased to as low as ∼100 Ω·cm2 (Figure 5c), which is almost 20 times lower than the interfacial ASR for samples without surface treatment. Galvanostatic cycling was performed on the cells, where lithium was plated back and forth between the two electrodes at a constant current density of 0.1 mA/cm2. After about 50 h of stripping−plating cycles, the interfacial ASR further decreased to about 20 Ω·cm2 (Figure 5b,c). This decrease could be attributed to further lithiation and activation of the interface layer during the stripping−plating process, which agrees with previous liter-

with molten lithium. To further study the interface properties, a Li/Garnet/Li symmetric cell was studied via electrochemical measurement. The conductivity of the dense garnet electrolyte pellet used in this work was measured to be about 2.2 × 10−4 S/cm. The morphology of the garnet electrolyte can be seen in the cross-section SEM image in Figure S1. Its crystallographic structure was confirmed to be cubic garnet phase, according to XRD patterns (Figure S2). Two ∼0.2 cm2 area circular pieces of lithium were punched from lithium metal sheet pressed from clean lithium pellet and melted onto a ∼0.5 cm2 garnet surface that had been coated with ∼30 nm ZnO by ALD. Electrochemical impedance spectroscopy (EIS) was used to measure the interfacial resistance between lithium and the garnet electrolyte. Figure 5a shows the Nyquist plots of the Li/ Garnet/Li symmetric cells treated with different conditions. The interfacial area specific resistance (ASR) is calculated from the value on the real axis of the semicircle of the Nyquist plot at low frequency. After subtracting the electrolyte ASR (∼90 Ω· cm2) of the garnet pellet (∼200 μm thick), we consider the two Li/Garnet interfaces, one at either side of the cell, and determine that the interfacial ASR is half of the remaining resistance multiplied by the area of the electrode materials. For the sample without surface treatment, the interfacial ASR is as E

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Nano Letters ature.41 The interface activation process can be seen from the voltage profile of the stripping−plating (Figure 5d), where the voltage slowly decreases from about 40 mV to ∼10 mV in the first 10 h and then becomes stable. Because it is difficult to obtain the surface area of the porous garnet, the interfacial ASR between Li metal and porous garnet is not studied. We expect a reduction in the interfacial resistance similar to that of Li metal and ZnO-coated dense garnet electrolyte pellet. Conclusion. In summary, we successfully demonstrated an ultrathin, conformal surface modification layer that significantly improved the wettability of the garnet electrolyte for molten lithium. The continuous, tight contact between lithium and garnet made the interfacial resistance decrease to as low as ∼20 Ω·cm2, much lower than untreated sample (∼2000 Ω·cm2) and typical values reported in literature (∼500 Ω·cm2), and maintain stability during the plating−stripping process. We also extend the conformal, nanoscale ZnO coating by ALD to a 3D porous garnet. Lithium metal was perfectly infiltrated into the surface coated porous garnet electrolyte, which can potentially be used as a self-supported composite anode for the solid-state battery applications. Therefore, we can envision that our facile surface treatment technique will inspire more effective strategies to overcome the interface problem of the solid-state electrolytes and facilitate the application of the solidstate batteries. Experiments. Synthesis of Garnet Solid-State Electrolytes. Cubic garnet electrolyte of Li6.75La2.75Ca0.25Zr1.75Nb0.25O12 composition was synthesized by conventional solid-state reaction. Stoichiometric amounts of LiOH·H2O (Alfa Aesar, 98.0%), La2O3 (Alfa Aesar, 99.9%), CaCO3 (Alfa Aesar, 99.0%), ZrO2 (Inframat Advanced Materials, 99.9%), and Nb2O5 (Alfa Aesar, 99.9%) were thoroughly ball milled in isopropanol for 24 h. Ten weight percent excess LiOH·H2O was added to compensate for vitalization of lithium during the calcination and sintering processes. The well-mixed precursors were dried, pressed, and calcined at 900 °C for 10 h. The as-calcined pellets were broken down and ball-milled in isopropanol for 48 h. The dried powders were pressed into 12.54 mm diameter pellets at 500 MPa. The pellets were fully covered by the mother powder and sintered at 1050 °C for 12 h. All the thermal processes were carried out in alumina crucibles. Before subsequent lithium metal assembling, the garnet electrolyte was mechanically polished on both sides to produce clean and flat surfaces. Atomic Layer Deposition (ALD). ALD of the ZnO coating layer was performed on the Beneq TFS 500. Pure nitrogen was used as carrier gas and preheated to 150 °C for the whole process. Typically, 5 ALD cycles were performed for 1 nm of ZnO deposition. Each cycle included alternating flows of diethyl zinc (DEZ, 1.5 s Zn precursor) and water (1.5 s, oxidant) separated by flows of pure nitrogen gas (4 and 10 s, as carrier and cleaning gas, respectively). Symmetric Cell Assembly. To make Li/Garnet/Li symmetric cells, the ZnO-coated garnet pellet was sandwiched between two thin lithium disks (∼0.5 cm in diameter and 150 μm thick) and then heated at 230 °C or additionally 300 °C for 30 min in argon filled glovebox. During heating, three pieces of stainless steel coin cell spacers were used to press the molten lithium onto the garnet surface to ensure a good contact between the molten lithium and the garnet surface. For the control sample, lithium metal was applied with the same process to the surface-polished pristine garnet. Materials characterization. Observation of the morphologies of the garnet solid-state electrolytes and elemental

mapping of the samples were conducted using a Hitachi SU70 FEG-SEM at 10 kV. Phase analysis was performed by X-ray diffraction (XRD) on a C2 Discover diffractometer (Bruker AXS, WI, USA) using a Cu Kα radiation source operated at 40 kV and 40 mA. Electrochemical Measurement. Electrochemical tests were conducted on a BioLogic VMP3 potentiostat. The electrochemical impedance spectra (EIS) were measured in the frequency range of 100 mHz to 1 MHz with a 30 mV AC amplitude. Galvanostatic stripping−plating cycling of the symmetric cells was recorded at a current density of 0.1 mA/ cm2. All measurements were conducted in an argon-filled glovebox.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b04695. The cross-section SEM images of the as-made garnet solid-state electrolyte, the XRD patterns of the asprepared garnet solid-state electrolyte and the standard Li5La3Nb2O12 shase, and the digital images for estimating contact angles of LLZO and ZnO-coated LLZO with molten lithium (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Liangbing Hu: 0000-0002-9456-9315 Author Contributions

C.W. and Y.G. contributed equally to this work. C.W. and L.H. conceived the idea of ZnO coating layer for surface modification of garnet solid-state electrolyte. Y.G. conducted synthesis of the garnet electrolyte. B. L. performed ALD deposition and XRD. Y.Y. and Y.L. carried out SEM images and EDS mapping. J.D. drew the illustration images. L.H. and E.W. supervised the project. All authors contributed to the manuscript writing and revising. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the NASA Advanced Energy Storage System Project within the Game Changing Development Program of the Space Technology Mission Directorate. We acknowledge the support of the Maryland NanoCenter and its AIMLab.

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REFERENCES

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DOI: 10.1021/acs.nanolett.6b04695 Nano Lett. XXXX, XXX, XXX−XXX