In-Situ Dendrite Suppression Study of Nanolayer Encapsulated Li

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Surfaces, Interfaces, and Applications

In-Situ Dendrite Suppression Study of Nanolayer Encapsulated Li Metal Enabled by Zirconia Atomic Layer Deposition Pankaj K Alaboina, Stanley Rodrigues, Michael Rottmayer, and Sung-Jin Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08585 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Applied Materials & Interfaces

1

In-Situ Dendrite Suppression Study of Nanolayer Encapsulated Li Metal Enabled by Zirconia Atomic Layer Deposition

Pankaj K Alaboina†, Stanley Rodrigues¥, Michael Rottmayer¥, and Sung-Jin Cho†,∗ †

Joint School of Nanoscience and Nanoengineering, North Carolina Agricultural and Technical State University, Greensboro, NC, 27401, USA ¥

Air Force Research Laboratory, Aerospace Systems Directorate, Wright-Patterson Air Force Base, Ohio 45433-7252, USA

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2 ABSTRACT Progressing towards the emerging era of high energy density batteries, stable and safe employment of lithium (Li) metal anodes is highly desired. The primary concern with Li metal anodes is their uncontrollable dendrite growths and extreme sensitivity to parasitic degradation reactions raising the alarms for battery safety and shelf life. Nanolayer protection encapsulation which is conformal and ionically conductive with high-κ dielectric property can suppress the degradation and empower stabilization of Li metal. In this work, engineering of zirconia (ZrO2) encapsulation layer on Li metal enabled by atomic layer deposition (ALD) was employed and investigated for surface-enhanced dendrite suppression properties using in-situ optical observations and electrochemical cycling. The ALD process involved a combination of plasma sub-cycle activation and thermal sub-cycle activation in increasing the surface functionalization and chemisorption sites for precursors to obtain highly dense and conformal deposition. The encapsulation of Li with ZrO2 ALD nanolayer further demonstrated excellent tolerance to atmospheric exposure for at least 1-5 hours due to conformal physical barrier, and excellent heat tolerance up-to 170-180 °C (close to Li melting point) and high rate capability due to thermal resistive property and high ionic transport property, respectively, of the ZrO2 ceramic. The results establish a technology transferable to other metal anode chemistries and offer a potential insight to carry out solid-state electrolyte multilayer coatings with high processing temperature flexibility and thereby providing a leap in the advancing of a range of high energy density allsolid-state batteries. KEYWORDS Lithium stabilization; zirconia atomic layer deposition; nanolayer encapsulation; high kdielectric coating; physical-thermal barrier; in-situ dendrite observation


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4 1. INTRODUCTION Lithium (Li) metal is an ideal anode material with a theoretical capacity of 3861 mAh g-1 which is extremely high, “Holy Grail” compared to the conventional anodes1,2. However, the primary concern with Li metal anodes is their chemical/electrochemical instability, uncontrollable dendrite growths, and extreme sensitivity to parasitic degradation reactions with electrolyte raising the alarms for battery safety, Coulombic efficiency decay, and shelf life3. To address these issues, one of the popular tactics is forming an ‘artificial’ surface protection layer on the Li metal as a physical barrier layer, which is conductive to Li ions transport but insulating for the electronic conduction to protect the lithium from undesirous chemical reactions and dendrite growth. In this trend, strategies in prior works included the use of functional additives in electrolytes4–6, surface functionalization coatings with polar groups7,8, or by directly employing solid-state electrolytes9–11. Passivation of Li by surface modification using functional additives, organic molecules, and polymeric materials has proven limited success due to the lack of thickness uniformity and compositional control

12–14

. On the other hand, sputtering with solid

state electrolytes has shown itself to inhibit the electrolyte decomposition on the Li surface. However, they resulted in large overpotentials due to their large thickness, variation in thickness, lower ionic conductivity, and huge interface resistances15–17. Atomic layer deposition (ALD) technique with its self-limiting, layer by layer deposition growths capability enables to control nanoscale conformity, thickness, and composition. greatly, thereby serving as an excellent nanoscale protective coating technique on Li to rectify the issues. Molecular layer deposition (MLD) where you can grow molecular layers by layer can be considered as an extension of ALD which is also popularly employed to deposit especially polymer organics or inorganic-organic hybrid coating materials18–20. In a study by Kozen et al., a 14 nm layer of Al2O3 has been


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5 deposited on Li metal using ALD which proved to be successful in passivating the Li metal against different kinds of corrosive environments including air and solvent exposure21. The ALD coating formed a passivation layer that helped the Li to withstand in the air without reacting. The ALD layer thus can act as a passivation layer inside the battery and can also prevent the side reactions. Chaun-Fu et al. presented a quantification study proving the ALD protection layer on Li metal was suppressing the corrosion reactions in an organic solvent22. Similarly, Sun et al. demonstrated the protection of sodium metal anodes from dendrites and mossy formations by ultrathin Al2O3 layer23. The prior works related to metal anode ALD protection focused popularly on employing Al2O3, or emerging Al2O3 derived alucone using MLD20–23. However, Li with Al2O3 ALD coating reported an increase in impedance with increasing thickness due to insulation from the Al2O3 layer. In another recent work, Dasgupta et al. have treated Li metal electrodes with ultrathin (∼2 nm) Al2O3 layers using ALD to leverage the impedance increase 21,24

. They have tested the ALD treated electrodes at a current density of 1 mA cm-2 using Li-Li

symmetric cells and found them to be stable for more than 1000 cycles before the failure occurs. Clearly, ALD treatment of the Li metal electrodes has suppressed the dendrite formation and has improved the electrode/electrolyte interface. In this work, for the first-time surface engineering of ionically conductive zirconia (ZrO2) as a protective encapsulation layer on Li metal was employed by using ALD and investigated for surface-enhanced dendrite suppression properties using in-situ optical observations and electrochemical cycling. This work considers the high Kdielectric (25 dielectric constant, 2-3 times higher than Al2O3

25,26

) and high thermally resistive

(~1.7 W m-1 K-1, 10 times thermally insulating than Al2O3 27,28) properties of ZrO2 which served as a great encapsulation layer exhibiting appealing benefits surpassing the use of Al2O3. The encapsulation process optimization by using ALD involved a combination of initial plasma



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6 activation in increasing the surface functionalization and chemisorption sites for precursors followed by thermal sub-cycle deposition loops to obtain highly dense, smooth and conformal deposition. ZrO2 ALD encapsulation layer with its smooth morphology, high dielectric, and high carrier density properties were believed to promote high ionic interface transfers and induce uniform spread of Li kinetics to suppress dendrite growths and stabilize Li as an artificial protective layer, as shown in Figure 1 schematic. Optical microscopy has been used to observe the surface changes at the Li/electrolyte interface which gives great visuals of the dendrite growths 25–28. The encapsulation of Li with ZrO2 ALD nanolayer offered physical protection, and an excellent heat insulating barrier and high rate capability credited to thermal resistive property and high ionic transport property of the ZrO2 ceramic, which is discussed in detail in the next sections showcasing its great potential for advancing the Li metal anodes for high energy density batteries.

Figure 1. Schematic of the dendrite suppression and enhanced stability of Li encapsulated with ZrO2 ALD. 2. EXPERIMENTAL SECTION


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7 Atomic Layer Deposition of ZrO2 on Li Metal. Gemstar XT-DP Dual Manifold Thermal/Plasma-enhanced atomic layer deposition benchtop system integrated to argon filled glovebox was used for the experiment. Li-metal discs of size 15.6 mm diameter x 0.25 mm thickness (from MTI Corp.) was used and placed on steel bed substrate during the deposition process. The chamber temperature was maintained at 140-145 °C during deposition which is much below the melting point of Li metal (170-180 °C). Tetrakis(dimethylamino)zirconium (TDMAZ), C8H24N4Zr (Strem Chemicals Inc.), was used as the precursor and was maintained at 75 °C at the precursor cylinder. To optimize the atomic layer deposition (ALD) process, deposition concentrations were compared for thermal ALD at 145 °C, plasma ALD with Argon plasma source, and a plasma-thermal activated ALD process, each run for 100 cycles. The processing cycle for thermal ALD was: H2O oxidant pulse of 0.06 sec followed by TDMAZ injection pulse of 1.5 sec looped for 100 cycles at 145 °C. Similarly, the processing cycle for plasma ALD was: Ar plasma at 300 W for 10 sec followed by TDMAZ injection pulse of 1.5 sec looped for 100 cycles. In the plasma-thermal activated ALD process the sequence included a combination of single cycle of Ar plasma activation at 300 W for 10 sec and followed by a loop of 100 cycles of H2O oxidant sub-cycle pulse of 0.06 sec and TDMAZ injection sub-cycle pulse of 1.5 sec at 145 °C with argon carrier gas, as shown in Figure 2(a). The deposition process in each case was performed on the two sides of Li-metal discs for full complete protection coverage and encapsulation. Scanning Electron Microscopy (SEM) and Elemental Characterization. SEM imaging was performed by using Carl Zeiss Auriga which was integrated with Bruker Nano X-Flash Detector 5030 for energy dispersive X-ray spectrometer (EDS) elemental composition mapping.



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8 Atmospheric Exposure. To examine the extent of atmospheric exposure passivation, bare Li metal,15.6 mm diameter x 0.25 mm thickness (from MTI Corp.), were coated with 20, 50, and 100 cycles of ALD process.

Samples were exposed to atmosphere environment at room

temperature (~24 °C) and relative humidity around 50%. The samples were monitored by capturing photograph images using 8-megapixel iPhone 6 camera for up to 10 hours. Thermal Exposure. To examine the thermal tolerance, bare Li metal, 15.6 mm diameter x 0.25 mm thickness (from MTI Corp.), was coated with 100 cycles of ALD for a complete encapsulated protection and tested for its thermal tolerance for up to 170-180 °C, i.e., close to the melting temperature of Li. The thermal tolerance test was conducted for over 5-hours on a hotplate inside an argon-filled glove box and was monitored for any changes in weight, area, appearance and other physical properties. The carbon-based paper was used as a substrate on the hot plate to carry the Li samples under test, and the skin temperature on the carbon paper was regulated to around 170-180 °C. Chronopotentiometry Cycling and In-Situ Dendrite Optical Microscopy. Dendrite growth behaviors were investigated by building symmetric cells with optically transparent window facility. The optical cells were constructed with acrylic or quartz glass material. Bare Li metal strip of 0.75 mm thickness (from Alfa Aesar) was cut into 10 mm x 10 mm chips for the optical cell symmetric electrodes. The sample of interest included the bare Li metal chips coated with 100 cycles of ZrO2 ALD encapsulation. Inside the optical cell assembly, the electrodes were placed separated at 5 mm distance, and nickel tabs were used as current collectors at both the electrodes. The space between the electrodes is filled with 250 µl of 1M LiPF6 in ethylene carbonate (EC)/ diethyl carbonate (DEC) – 1/1 (v/v) (from Panax Etec Co., Ltd.-Starlyte) as the electrolyte bridge. The assembly was tightly sealed by using silicone gasket and screw fittings.


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9 Chronopotentiometry cycling and electrochemical impedance spectroscopy (EIS) measurements were performed by using Biologic SP-300 potentiostat. The chronopotentiometry voltage behavior was investigated by subjected the symmetric optical cells to very high current densities of 4 mA cm-2 with Li stripping/platting for 10 minute each for up to 100 cycles. EIS measurements were performed after the 1st cycle and after the 100th cycle using the scan range of 200 KHz-10 mHz and voltage amplitude of 10 mV. Optical microscopy for the dendrite growth observation at different stages of cycling was performed at the electrode/electrolyte cross-section interface by using Zeiss Axio imager-Z2M. Electrochemical Characterization. To investigate the effect of ZrO2 protection barrier on Li metal electrochemical properties CR2032 coin cells were built using LTO (Li4Ti5O12) electrodes. The LTO electrodes were prepared by mixing LTO (Panax Etec Co.), carbon black (Super C65, Timcal), and polyvinylidene fluoride (PVDF-5130, Solvay) binder in the ratio 8:1:1, dissolved in 1-methyl-2-pyrrolidinone (NMP, ≥99.5%, Aldrich) as the solvent. The homogenous electrode slurry obtained after ball mill mixing using planetary ball mill (MSE-PMV1-0.4L) was cast onto aluminum-foil current collector using a doctor blade and CV-400 Rotech Lab Coater. The electrodes were dried in a vacuum oven at 110 °C for at least 10 hours and punched into 14 mm discs which measured an active material loading of ~ 1.65 mg cm-2. The CR2032 coin cells were assembled inside a high purity argon-filled glovebox with Celgard C480 as the separator, and 60 µl of 1M LiPF6 in ethylene carbonate (EC)/ diethyl carbonate (DEC) – 1/1 (v/v) was used as the electrolyte. Electrochemical characterization was performed using a Toyo TOSCAT 3100 battery cycler. The cells were cycled at 0.1C rate (0.01 mA cm-2) charge/discharge rate for the formation cycle at a temperature of 23 °C in the potential range between 1.2 V - 2.8 V. To further investigate the electrochemical properties, the coin cells after the formation cycle were



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10 transferred to a 45 °C high-temperature chamber and tested for high charge-discharge current rates. The electrochemical cycling included three phases as follows -Phase-I = Cycling at 1C, 2C, 4C, 8C and back to 1C in same steps for 3 cycles each; Phase–II = 1C rate for 100 cycles aging; and Phase–III = Cycling at 1C, 2C, 4C, 8C and back to 1C in same steps for 3 cycles each where 1C rate is equivalent to 0.16 mA cm-2 current density. 3. RESULTS AND DISCUSSION Process Optimization of ZrO2 Atomic Layer Deposition on Li Metal. Protective zirconium dioxide (ZrO2) coating on Li metal was tuned and optimized by using a plasma-thermal activated ALD process which demonstrated high deposition concentrations of elemental zirconia (Zr) compared to that obtained using plasma ALD and thermal ALD processes. SEM-EDS results comparison mapping the Zr- concentration on the surface and cross-section of the Li samples obtained from 100 ALD cycles of ZrO2 indicated the highest concentration of Zr per scan area for plasma-thermal activated ALD process recipe compared to that of plasma ALD and thermal ALD (Figure S1). The high deposition density of ZrO2 indicated by the Zr-concentration was achieved due to the employment of both plasma sub-cycle activation and thermal sub-cycle activation which increased the surface functionalization and chemisorption sites for precursors reactions. The initial high-power plasma sub-cycle functionalization was believed to help etch away the native oxide layers22 present on bare Li-metal and effectively functionalize the surface ready for the next precursor sub-cycles. Li metal before and after 100 cycles of ALD coating with ZrO2 is shown in Figure 2(a) photograph images. The ZrO2 ALD coating appears as a white-silver layer on the surface as shown in Figure 2(b). High magnification SEM-EDS in Figure 2(c) show the confirmation of ZrO2 deposition on Li metal indicated by Zr and oxygen (O) mapping on the internal cross-section of Li samples at two different scales, 100 µm and 400


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11 nm scale. Very conformal, smooth, and pinhole free ZrO2 coating on Li metal can be observed in Figure 2(c) images with a thickness of approximately 190 nm protection achieved after 100 cycles of ALD process on both sides of Li metal. The mapping images and distribution confirm the elemental composition of the ZrO2 deposition. The O-mapping is slightly spread and detected in bulk of the cross-section exposed Li-metal zone as observed in Figure 2(c) which is due to the oxidation of Li-metal exposed to the atmosphere for a short interval during the cross-section sample preparation and transfer into SEM-EDS machine. Plasma-thermal activated ALD was able to deposit a conformal protection layer of ZrO2 encapsulating Li metal at a lower processing temperature without causing any thermal damage.

Figure 2. (a) Process flow chart for plasma-thermal activated ALD of ZrO2 on Li metal. (b) Photograph images of the bare Li metal (diameter 19mm) and after coating with ZrO2 ALD. (c) SEM-EDS confirmation of ZrO2 deposition on Li metal cross-section indicated by Zr and O mapping at different magnification scales of 100 µm and 400 nm.



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12 Atmospheric and Thermal Passivation. The atmosphere exposure study of Li samples for around 10-hours is displayed in Figure 3(a)- showing the spectra of passivation for different Li samples. The results in Figure 3(a) show that the bare Li in the exposure test turns black in just 5 min due to its rapid reaction with oxygen and moisture from the air. However, under the same air exposure conditions, all the ZrO2 ALD coated sample irrespective of 20 - 100 ALD cycles did not show any surface color changes for at least 1 hour indicating the uniform protection and passivation as expected due to the physical barrier layer

21,24

. It can be considered that with

increasing number of ALD cycles the physical barrier layer protection can be increased, and as such Li metal double side coated with 100 ALD cycles showed the relatively highest tolerance to air exposure and passivation up to at least 5-10 hours.

Figure 3. (a) Air exposure comparison of bare Li metal, and after ZrO2 coating with 20, 50, and 100 ALD cycles. (b) Thermal exposure comparison of bare-Li metal and Li after ZrO2 ALD (100 cycles) conducted on a hot-plate at 170-180 °C skin temperature for over 5 hours inside an argon-filled glove-box.


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13 The thermal exposure test results in Figure 3(b) show that the bare Li metal at 170-180 °C heat exposure quickly turns into a molten form losing its physical shape integrity which was expected considering its melting temperature in the nearby heat testing range. Carbon paper was selected as a substrate due to its lithiophobic nature to allow easy handling of Li samples during the thermal test and prevent any sticking of the molten Li when heating close to Li melting temperature. The use of lithiophobic carbon substrate evidently shows the melting of bare Li as it causes the molten Li to shrink to a smaller surface area. It was observed that the bare Li metal had an initial area of ~188.69 cm2 which shrink to around ~141.82 cm2 with a generation of a large number of wrinkle belts on its surface. In addition, the bare Li appearance changed from a fresh golden color to a dark reddish golden color due to heating. However, in the same thermal conditions at 170-180 °C for over 5 hours, the ZrO2 ALD coated Li metal as observed in Figure 3(b) images showed excellent thermal tolerance and maintained its overall shape integrity. There was almost no change in surface area and overall physical shape which can be attributed to the thermal insulation property of the ZrO2 ceramic material used as the protection encapsulation 24,29,30

. Slight color change of the ZrO2 ALD Li samples was observed after 3 hours thermal

exposure with relatively fewer wrinkles generated indicating its thermal rigidness and capability to maintain shape integrity. However, no loss or gain of weight was noticed throughout the thermal test for either of the samples indicating inert atmospheric conditions with almost no occurrence of evaporation or oxidation-based reactions, respectively. In summary, Li metal with ZrO2 ALD thermal barrier showed increased thermal tolerance indicating its excellent potential to carry out multilayer coating at relatively high processing temperature if needed. In-Situ Dendrite Characterization. Photograph image of an optical cell constructed is shown in Figure 4(a). The optical cell has an optically transparent window to closely analyze the



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14 dendrite growths at the Li electrode/electrolyte cross-section interface. The cell was sealed using the silicone gasket in white and screw fittings. The Ni collector tabs were extending at the sides of the optical cell through which it was connected to the electrochemical workstation. The optical cells were subjected to a very high current density of 4 mA cm-2 with 10 min each Li stripping/plating sub-cycles to induce the dendritic growth conditions. The variation of voltage and overpotential for over 100 cycles for the bare Li symmetry cell and Li with ZrO2 ALD symmetry cell is shown in Figure 4(b). Both the cells start with similar voltage profile of around 0.5 V in the first cycle. However, the bare Li symmetry cells show a rapid increasing overpotential over the course of 100 cycles indicating the increasing charge transfer resistance due to the buildup of poorly conductive surface and unstable dendrite growths. Li with ZrO2 ALD symmetry cell showed a stable voltage cycling indicating its effective passivation to side reactions with electrolyte preventing huge impedance rise and suppressing the dendrite growth. The variations in impedances with cycling are confirmed by the EIS measurements. Figure 4(c) shows the EIS measurements of the optical symmetry cells after the 1st cycle and after 100th cycle along with their equivalent circuit. The equivalent circuit represents Rb as the bulk resistance from the electrolyte and electrode components; Cs capacitance and Rs resistance as the surface layer interface at the high frequency; Cct and Rct as the charge transfer components at medium frequencies; and W (Warburg element) at low frequencies corresponding to diffusion information. The EIS spectra after the 1st cycle, also shown in the inset of Figure 4(c) illustrates similar impedance profiles for both the samples, however, the symmetry cell with ALD samples showing slightly higher semicircle depression width indicated higher surface impedance due to the ZrO2 ceramic coating layer. It was significant to notice that even after having a ZrO2 encapsulation of around 190 nm thick (as measured from Figure 2(c)), the increase in impedance


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15 compared to the bare Li samples was not huge considering the prior reports limiting the ALD protection thickness with widely used Al2O3 due to impedance increase21,24. Furthermore, after 100th cycle, the bare Li symmetry cell showed a vast increase in the surface impedance as seen from the huge semicircle depression indicating the surface resistance which was almost 2.5 times higher than the ALD samples symmetry cell. The huge impedance increase with cycling can be attributed to the poorly conductive surface and accumulation of dead Li due to severe dendrite growth which is also interpreted from the increasing overpotential profiles. The in-situ optical observations capturing the dendrite growths visuals are displayed in Figure 4(d). The dotted line indicated the electrode and electrolyte interface. Optical images of the samples right after the assembly of the coin cell, and after 24 hours rest in an atmospheric environment do not show any noticeable color changes indicating no degradation reactions and good sealing of the cells. With the beginning of the Li stripping/plating cycles at 4 mA cm-2, the optical symmetry cell with bare Li electrodes shows bush shaped dendrite growths at the interface which rapidly increased in shape and size with cycling indicating its serious vulnerability to safety concerns. The optical images for the bare Li symmetry also show the formation of poorly connected and porous dead Li which gives a visual confirmation for the increasing overpotential and high impedance as observed in the chronopotentiometry cycling (Figure 4(b)) and EIS spectra (Figure 4(c)), respectively. The optical images of the symmetry cell with ZrO2 ALD encapsulated Li electrodes do not show any dendrite growth at the interface showing the excellent suppression offered even at a severe current density of 4 mA cm-2. However, around the 100th cycle aging, dendrites were also observed in ZrO2 ALD coated Li which were nevertheless flat shaped and appeared less severe. The dendrite growth even in encapsulated Li samples at the delay of 100th cycles can be considered as the tolerance limit or



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16 generation of the protected layer wear off with cycling, the confirmation of which needs further research and for now was not in the scope of the presented work. The optical images showed great evidence for the high dendrite suppression capability and ability to uniformly spread the Li kinetics which is credited to the smooth ZrO2 encapsulation layer with its dielectric and high carrier density properties promoting high ionic transfers through its structure31,32.

Figure 4. (a) Photograph image of the optical cell. (b) Chronopotentiometry cycling of the symmetric optical cell at a current density of 4 mA cm-2 with 10 min each of Li stripping/plating sub-cycles. (c) EIS measurement of the symmetric optical cells after 1st cycle and after 100th


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17 cycle of chronopotentiometry cycling. (d) Optical microscopy images at electrode/electrolyte cross-section interface during different stages from assembly to after 100 cycles of chronopotentiometry. High-Temperature Electrochemical Performance. Figure 5(a) shows the formation chargedischarge capacities of the LTO coin cells with bare Li metal, and Li protected with ZrO2 protected ALD at 0.1C rate (0.01 mA cm-2 current density) and 23 °C temperature. LTO/bare-Li cell demonstrated an initial charge capacity of 176.9 mAh g-1 and a discharge capacity of 177.0 mAh g-1 with a coulombic efficiency of ~99.9%. LTO/Li with ZrO2 ALD showed similar performance with a charge capacity of 177.6 mAh g-1, the discharge capacity of 178.4 mAh g-1, and coulombic efficiency of ~99.6%. Formation cycle results in Figure 5(a) do not show much difference in the performance between the bare Li metal and ZrO2 protected Li indication no degradation in electrochemical performance due to the ALD process and the protection encapsulation layer. To further investigate the electrochemical properties, the coin cells after the formation cycle are transferred to a 45 °C high-temperature chamber and tested for high chargedischarge current rates. A consolidation of the three-electrochemical cycling-phases (phase-I, II, and III) as described in the experimental section is shown in Figure 5(b). The electrochemical cycling results of the phase-I section demonstrated that the bare Li and the ZrO2 coated Li coin cells have similar charge-discharge cycling properties at 1C, 2C, and 4C current rates. However, at the very high current rate of 8C (1.25 mA cm-2 current density), the bare Li cell showed a relatively lower capacity of around 153 mAh g-1. On the other hand, the ZrO2 ALD coated Li sample cell exhibited a comparatively high capacity of 157 mAh g-1. The relatively improved performance of the ALD coated Li sample at the high current rate can be attributed to the ZrO2 layer with its high dielectric properties and high carrier density which promotes high ionic



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18 diffusions through its crystal structure

31,32

. In the phase-II electrochemical cycling at 1C rate

(0.16 mA cm-2 current density) for the next 100 cycles after the phase-I, the bare Li and the ZrO2 ALD coated Li showed similar charge-discharge cycling properties with capacity retention of around 99.3% in both the cases. In the next cycling conditions, phase-III cycling showed cycling behaviors in agreement with the phase-I cycling. In phase-III cycling, the bare Li and the ZrO2 ALD coated Li showed similar charge-discharge cycling properties at current rates of 1C, 2C, and 4C current rates. However, at the very high current rate of 8C (1.25 mA cm-2 current density), the ZrO2 ALD coated Li sample cell showed a relatively higher capacity of around 152 mAh g-1 when compared to the bare Li cell which delivered around 142 mAh g-1 capacity. The relatively improved performance of the ALD coated Li sample at the high current rate as observed again in phase-III is attributed to the ZrO2 layer dielectric and high carrier density properties promoting high ionic migrations through its crystal structure 31,32.


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19

Figure 5. (a) Formation cycle for the bare-Li and Li with ZrO2 ALD coin cells at a 0.1C rate (0.01 mA cm-2 current density) in the potential window 1.2 V- 2.8 V performed at 23 °C temperature. (b) Electrochemical cycling of the bare-Li and Li with ZrO2 ALD coin cells at different charge-discharge current rates in the potential window 1.2 V- 2.8 V performed at 45 °C high temperature. (c) Electrochemical rate performance comparison of the bare-Li and Li with ZrO2 ALD coin cells. (d) Bar graph comparing the 8C (1.25 mA cm-2) rate capacity and recovery of the samples during the phase-I and phase-III cycling performed in the potential window 1.2 V2.8 V at 45 °C high temperature.



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20 Figure 5(c) shows a comparison of the electrochemical performance during the phase-I and phase-III cycling. As discussed earlier, the Li with ZrO2 ALD samples demonstrated relatively high rate performance at 8C (1.25 mA cm-2 current density). The bar graph in Figure 5(d) gives a closer comparison look at the delivered capacity’s at 8C in phase-I and after aging, in phase-III. It was observed that the bare Li cell with its relatively lower 8C capacity delivered 153 mAh g-1 in phase-I which significantly reduced to 142 mAh g-1 with aging in phase-III cycling. In contrast, the Li with ZrO2 ALD coating exhibited relatively low capacity degradation from 157 mAh g-1 in phase-I to 152 mAh g-1 in phase-III at 8C, which can be attributed to the corrosion protection of Li metal with the ZrO2 ALD coating allowing for improved cycle lifetime 24. The electrochemical results indicate that the Li metal electrochemical properties are not degraded due to the ALD process and ZrO2 coating but in addition, the high physical barrier protection and high rate capability was achieved. 4. CONCLUSIONS In conclusion, zirconium dioxide (ZrO2) ceramic layer was deposited on Li metal using plasmathermal activated atomic layer deposition process for a protective and stabilizing encapsulation. The physical barrier encapsulation exhibited excellent tolerance to air exposure condition for at least 1-5 hours when compared to the bare Li which quickly oxidizes to air exposure in around 5 min. Moreover, the thermal insulation property of having the ZrO2 ceramic demonstrated enhanced thermal tolerance to heat exposure of 170-180 °C temperature for over 1-5 hours, while the bare Li metal is quickly melting and losing its mechanical shape integrity at the similar thermal conditions, indicating its potential to carry out multilayer coating at relatively high processing temperature flexibility if needed. The optical images showed a great evidence for the high dendrite suppression capability, and ability to uniformly spread the Li kinetics which was


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21 arising from the smooth ZrO2 ALD encapsulation layer with its high dielectric and high carrier density properties promoting uniform and high ionic interface transfers. Regarding the electrochemical properties, no degradation in properties of Li metal was observed due to the ALD process or due to the presence of ZrO2 surface coating. In addition, the high rate capability of ZrO2 coated Li metal was observed due to the high ionic transport property of the ZrO2 ceramic. The results evidently establish a technology transferable to other metal anode chemistries with the enormous potential in advancing of a range of high energy density all-solidstate batteries.

ASSOCIATED CONTENT Supporting Information Process Optimization of ZrO2 Atomic Layer Deposition on Li Metal (Figure S1) AUTHOR INFORMATION Corresponding Author ∗

E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge funding support from the Air Force Research Laboratory Educational Partnership Agreement (AFRL-EPA) grant #: FA8650-17-2-2228 and Joint School of Nanoscience and Nanoengineering, a member of Southeastern Nanotechnology Infrastructure



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22 Corridor (SENIC) and National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-1542174).

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Abstract Graphic: For Table of Contents Only


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122x61mm (300 x 300 DPI)



187x99mm (300 x 300 DPI)


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172x88mm (300 x 300 DPI)



Graphic Abstract 78x35mm (300 x 300 DPI)


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In-Situ Dendrite Suppression Study of Nanolayer Encapsulated Li

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