1 Anchoring an Artificial Protective Layer to Stabilize Potassium Metal

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Anchoring an Artificial Protective Layer to Stabilize Potassium Metal Anode in Rechargeable K–O2 Batteries Neng Xiao, Jingfeng Zheng, Gerald Gourdin, Luke Schkeryantz, and Yiying Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02116 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Materials & Interfaces

Anchoring an Artificial Protective Layer to Stabilize Potassium Metal Anode in Rechargeable K–O2 Batteries Neng Xiao, Jingfeng Zheng, Gerald Gourdin, Luke Schkeryantz, and Yiying Wu* Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Ohio 43210, United States E-mail: [email protected] Keywords: potassium secondary batteries; artificial protective layer; potassium metal anode; potassium–oxygen batteries; alkali metal anodes; surface treatment; anode stability.

Abstract: Rechargeable potassium batteries, including the potassium–oxygen (K–O2) battery, are deemed as promising low-cost energy storage solutions. Nevertheless, the chemical stability of the K metal anode remains problematic and hinders their development. In the K– O2 battery, electrolyte and dissolved oxygen tend to be reduced on the K metal anode, which consumes the active material continuously. Herein, an artificial protective layer is engineered on the K metal anode via a one-step method to mitigate side reactions induced by the solvent and reactive oxygen species. The chemical reaction between K and SbF3 leads to an inorganic composite layer that consists of KF, Sb, and KSbxFy on the surface. This in situ synthesized layer effectively prevents K anode corrosion while maintaining good K+ ionic conductivity in K–O2 batteries. Protection from O2 and moisture also ensures battery safety. Improved anode lifespan and cycling performance (>30 days) are further demonstrated. This work introduces a novel strategy to stabilize the K anode for rechargeable potassium metal batteries.

Introduction Rechargeable potassium batteries (K–ion, K–O2, and K–S batteries etc.) have attracted broad interest in recent years as promising substitutes for current Li-ion technology owing to the high abundance, low cost, and substantial potential (-2.92 V vs SHE) offered by potassium.1–7 Based on the one-electron O2/KO2 redox couple, catalyst-free rechargeable K– 1 ACS Paragon Plus Environment

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O2 batteries have provided an elegant solution to the low cathode efficiency and chemical instability challenges in the oxygen reduction/evolution reaction.1,8,9 The highly reversible cathode reaction achieved by the O2/KO2 redox chemistry also outperforms traditional Li–O2 chemistry.8,10 Since our group introduced the concept of K–O2 batteries in 2013, our efforts have been focused on understanding the fundamentals and developing electrolyte and electrode materials to realize the high-performance K–O2 battery chemistry.11–17 In the past few years, more research groups have also contributed significantly to this field. Kang et al. demonstrated a Na-K alloy anode in K–O2 battery.18 Lu et al. and Ramani et al. used DMSO to replace ether-based solvents on the cathode and increased electrode kinetics for better rate performance.19,20 Bruce et al. discovered that the discharge capacity of the K–O2 battery is not limited by the growth of KO2 via the surface route.21 It has been concluded that the O2/KO2 redox couple resolves the problems of the oxygen cathode, thus improving the reversibility of the K anode becomes the major obstacle and the focus of our research for commercializing rechargeable K–O2 batteries.1 K metal is the ideal anode material for rechargeable potassium batteries owing to its high specific capacity (687 mAh/g), low cost, and flat voltage plateau compared with alloybased and carbonaceous materials.3,13,22 However, major challenges arise with the direct use of K metal and impede the progress of research. Unlike commercialized Li foils, there is no standard potassium foil available due to the difficulty in fabricating and preserving a foil disc of uniform shape and size. Unfortunately, the lack of a standard K metal anode also compromises the reproducibility of experiments. Furthermore, potassium metal suffers from poor electrochemical reversibility upon plating and stripping due to the decomposition caused by its great reducing power. Thus, it is extremely crucial to form a stable interface and prevent the continuous reductive decomposition of salt and solvent on the K surface. Although the KFSI-DME electrolyte was found to enable reversible K plating and stripping by forming a KF-abundant solid electrolyte interphase (SEI), it is decomposed by the cathode 2 ACS Paragon Plus Environment

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nucleophiles (O2-) in the K–O2 battery.13,14 The TFSI- anion is more chemically stable but results in poor reversibility and low conductivity on the anode interface.11,14 So far, the KPF6DME electrolyte still represents the best compromise for balancing anode reversibility and cathode stability. However, its cycling performance and shelf-life are hindered by severe K anode corrosion in the presence of the solvent and reactive oxygen species.12 While a K+Nafion membrane was demonstrated to decrease the oxygen permeation rate and increase anode lifetime to over 400 hours, it added significantly to the total cost and internal resistance.12 In order to unlock the full potential of the K–O2 battery, solvent and oxygen decomposition on the K anode must be mitigated by a facile, effective, and low-cost method. Although several emerging strategies have been proven to protect Li and Na metal anodes lately, there has yet to be a desired approach for K metal batteries.23–32 Herein, we demonstrate a novel method that was developed to address the aforementioned challenges based on a one-step surface reaction. Inspired by the earlier work from the Nazar group,23 a procedure to fabricate standard K metal foil is established and then followed by an in situ chemical reaction to anchor an artificial protective layer onto the surface of the K metal. The protective layer is formed by the direct reduction of SbF3 on the K metal and can protect the K metal anode from solvent, oxygen, and moisture-induced decomposition. Owing to the restrained anode corrosion, K–O2 batteries with a SbF3-treated K metal anode achieved superior cycle-life and shelf-life (over a month) without sacrificing round-trip energy efficiency. This strategy presents a simple and effective way to stabilize the K metal anode in rechargeable K–O2 batteries. It also opens up a promising avenue for anode protection in rechargeable potassium metal batteries.

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Experimental Section Materials: Potassium hexafluorophosphate (KPF6) (≥99%, Sigma-Aldrich) and Antimony (III) fluoride (SbF3) (99.8%, Sigma-Aldrich) powder were both dried under high vacuum at 100 °C for 72 h prior to use. 1,2-dimethoxyethane (DME, Gotion Inc.) was stored over 3 Å molecular sieves (Sigma-Aldrich). The water content of the electrolyte was below 10 ppm as determined by Karl-Fischer titration. Celgard (PP-PE-PP, 25 μm thickness), Cu foil (99.99%) were purchased from the MTI corporation. All materials were stored and handled in an argon-filled glovebox (< 0.5 ppm H2O and < 1.5 ppm O2). Characterization: The pristine K anode, SbF3-treated K metal anode, and aged electrodes were characterized using an X-ray diffractometer (Bruker D8 Advance, Cu- Kα source, 40 kV, 40 mA). A polymer thermoplastic sealant (3M Company) was applied to seal the XRD holder against the ambient air. Scanning electron microscopy (SEM) was performed using an FEI Quanta 200 SEM to image the morphological characteristics with an accelerating voltage of 5 kV. Electrode samples were prepared in a glovebox and transferred to the SEM chamber using an air-free SEM holder to prevent sample exposure to the ambient air. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos Axis Ultra XPS spectrometer using monochromatic Al Kα radiation. All spectra were calibrated by referencing the C 1s peak position of the C−C peak at a binding energy of 284.8 eV. The spectra curve was fitted using a combined Gaussian−Lorentzian profile using the CasaXPS program. 1H NMR and 19F NMR were carried out using a 400 MHz NMR spectrometer (Bruker, Avance III) after immersing the samples in D2O (99.9 atom % D, Sigma-Aldrich). Fabrication of the SbF3 treated K metal anode: Preparation of the potassium metal electrode was carried out in an argon filled glovebox. First, raw potassium metal (99.5%, Sigma-Aldrich) was cut and polished to remove the oxide layer on the surface. The K cube was then sandwiched in between two pieces of Celgard membranes and placed into the pasta machine. The K metal was rolled through the rollers of the pasta machine to obtain a uniform 4 ACS Paragon Plus Environment

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K film. A punch with the desired diameter was then used to cut the film into a disc of K foil, which was placed onto a Cu substrate. The K foil used in our work weighs ca. 84 mg (ca. 44 mg/cm2). After that, the pristine K foil was immersed into 1 mL of 0.2 M SbF3 in DME (maximum solubility) solution for 3 min. Upon removal from the solution, the K foil was rinsed with 500 L DME for 3 times and dried in the glovebox antechamber before use. Electrochemical measurement: The K−O2 batteries were assembled by stacking a pristine potassium metal anode (99.5% from Sigma-Aldrich) or pretreated K Metal anode, trilayer Celgard separator (25 μm thickness), glass fiber separator (GF/D, Whatman) soaked with 300 μL DME-based electrolyte, and carbon paper electrode (d = 12 mm, P50, Fuel Cell Store) in a stainless-steel battery cell of our own design. All battery assembly was performed in an Argon-filled glovebox. The carbon electrodes were dried at 120 °C under vacuum for 2 days prior to battery assembly. Additionally, 250 μL DME solvent was added to the oxygen chamber to saturate the oxygen atmosphere and prevent solvent vaporization. After purging the oxygen chamber with high purity oxygen (99.993% UHP, ca. 1 atm), the batteries were allowed to rest for 2 hours prior to discharge to ensure sufficient oxygen dissolution and diffusion. Galvanostatic cycling tests were carried out using an MTI battery analyzer (BST8WA) with the cutoff voltages set at 1.8 and 3.0 V (vs. K/K+). Aging test in Moisture, solvent, and oxygen: The aging test for K metal anode were performed under three different conditions: (1) 2 mL of pure tetraglyme solvent, (2) 1000 ppm H2O in DME solution, and (3) DME saturated with pure O2. Both the pristine K metal anode and the SbF3-treated K metal anode were immersed into each solution and kept air-tight during the aging test.



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Results and Discussion In order to obtain a standard K metal foil, a procedure was developed in Figure 1 to control the thickness and size of the pristine K anode. Briefly, the oxide layer on a raw K cube was first removed. Then, a pasta machine and punch were utilized to produce the K metal foil anode at the desired thickness and diameter. The standard K foil was then placed onto a Cu substrate. Lastly, the as-obtained K foil was immersed into a saturated SbF3-DME solution (0.2 M) to anchor an artificial protective layer onto the fresh surface. The rapid chemical reaction led to the formation of a thin black inorganic layer on the surface of the K metal foil, which reduces the contact between the electrolyte and pristine K underneath. It is worth noting that reacting SbCl3 and K to form a protective layer has also been examined (Figure S1). However, the SbCl3 solution resulted in a violent reaction and consumed all of the bulk K. Therefore, the SbF3 solution was chosen for building an artificial protective layer on the surface of K. The rinsed sample was applied as the anode in the battery assembly.

Figure 1. Fabrication of standard K metal foil followed by surface treatment with SbF3 solution to anchor an artificial protective layer on the anode surface. The expected chemical reactions upon treating the surface are as follows: 3K + SbF3 = Sb + 3KF

(1)

KF + SbF3 = KSbF4

(2)

KSbF4 + SbF3 = KSb2F7

(3)

KSbF4 + KF = K2SbF5

(4)


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For the main reaction in Equation 1, fresh K metal reacts rapidly with the SbF3 solution and forms Sb metal and KF particles on the surface. However, depending on the stoichiometry in the subsequent reactions (Equation 2, 3, and 4), KF could further react with SbF3 and eventually introduce small amounts of KSbF4, KSb2F7, and K2SbF5.33

Figure 2. SEM analysis on the in situ formed artificial protective layer (a-b) surface imaging (inset in 1a: photo of SbF3-surface treated K metal), and (c-d) cross-section imaging (sample was cut by razor blade). Surface and cross-section morphology of the SbF3-treated K metal anode was then characterized by scanning electron microscopy (SEM), as shown in Figure 2. Compared to the smooth surface of a pristine K metal anode (Figure S2), the as-formed artificial composite 7 ACS Paragon Plus Environment


layer consisted of numerous nanoparticles that densely covered the surface of the K metal. Energy dispersive X-ray spectroscopy (EDS) also verified a uniform presence of Sb and F elements on the surface (Figure S3). The inset picture in Figure 2a shows that this artificial layer is black in color and maintains a rather flat morphology on the surface. Shown in Figure 2c-d, cross-sectional imaging further indicates that the as-formed layer has a thickness of ca. 10 m, while the bulk K metal underneath is about 650 m thick. The in situ formed composite layer laid directly on top of the K metal, which is expected to reduce direct contact between the highly reducing K and the solvent or oxygen species in electrochemical cells.

Figure 3. (a) XRD and (b-d) XPS analysis on the chemical components of as-formed artificial protective layer on K metal anode. The chemical composition of this artificial protective layer was further analyzed by Xray diffraction (XRD), as shown in Figure 3a. Compared with the pristine K metal, multiple 8 ACS Paragon Plus Environment

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new peaks appeared in the SbF3-treated K metal anode due to the as-proposed chemical reactions. As expected, the two peaks at 34 and 49 are assigned to KF,13 while other major peaks at 40, 42, 52 and 59 can be attributed to the formation of Sb metal particles on the K metal surface. Furthermore, X-ray photoelectron spectroscopy (XPS) was also applied to probe the composition of the inorganic composite layer. In Figure 3b-d, the main features showed up in the F 1s and Sb 3d regions. In the F 1s spectrum, the characteristic signal at 684.4 eV confirms a significant amount of KF in the artificial protective layer. An adjacent small peak at 683.2 eV was assigned to the species of KSbxFy produced by the subsequent reaction between KF and SbF3. In terms of the Sb 3d5/2 region, three different species were identified: Sb metal (527.9 eV), Sb2O3 (529.8 eV), and KSbxFy compounds possibly with the formula of K2SbF5 and/or K2SbF7 (531.2 eV).34 Furthermore, only adventitious carbon was detected in the C 1s region, which was supported by the fact that no formate or acetate was detected in the 1H NMR spectra (Figure S4). Therefore, ether solvent decomposition was not involved in the rapid formation of the artificial protective layer. The compounds identified by XRD and XPS also confirmed that the chemical reactions occurred upon treating the surface with the SbF3 solution.



Figure 4. Stability test of pristine K metal anode (left) and SbF3-treated K metal anode (right) upon (a) immersing into (a) TEGDME solvent and (b) 1000 ppm water in DME solution (yellow circles indicated the region that H2 gas bubbles generated). (c) Aging in DME with saturated O2 for 2 weeks. (d) XRD analysis on the aged samples with and without surface treatment. To further evaluate the advantages of the artificial protective layer, both the pristine K anode and SbF3-treated K metal anode were exposed to tetraglyme (TEGDME), watercontaining DME, and DME saturated with pure O2 for aging tests. As shown in Figure 4a, when pristine K metal was immersed into the TEGDME, the solution turned into blue immediately due to the formation of solvated electrons (long-chain ether coordinated with 10 ACS Paragon Plus Environment

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K+).12 In stark contrast, a SbF3-treated K metal resulted in no obvious color change, indicating that contact between the K metal and TEGDME molecules was blocked by the protective layer. Similarly, anode stability in water-containing electrolyte was examined in Figure 4b, where the K metal anodes with and without treatment were immersed into a DME solution containing 1000 ppm H2O. Due to the high reactivity of pristine K metal, a large amount of H2 gas bubbles was generated continuously (2K + 2H2O = 2KOH +H2). As for the SbF3treated K metal anode, there was little H2 produced owing to the decreased interfacial contact. This significant difference also ensures better safety for rechargeable K metal batteries. Furthermore, aging in the DME/pure O2 environment was carried out to mimic the K–O2 cell condition (Figure 4c). After two weeks, the metallic luster of the fresh K metal foil gradually gained a yellowish-white passivation layer on the surface, while the SbF3-treated K metal anode remained unchanged. XRD of the aged samples revealed the consequences of solvent and O2 decomposition on the K metal anodes (Figure 4d and Figure S5). Indeed, the strongest peaks at 31, 34, 39, 45 and 46 are attributed to the potassium hydroxide hydrate (KOH•H2O), which was formed by the decomposition of DME molecules in the presence of O2 on the K anode according to our former mechanism study.12 The pristine K metal anode resulted in more pronounced byproduct peaks compared with the SbF3-treated anode, which confirmed our observation of the thick yellowish-white (color of KO2 and KOH) layer formed after aging. This also agrees with the fact that very little formate, acetate, and methoxyacetate accumulated in the SbF3-treated anode (Figure S6), which indicated that the solvent and O2induced parasitic reactions were effectively suppressed by the artificial protective layer.



Figure 5. (a) The principle of how artificial protective layer works in K–O2 batteries. (b) Voltage profiles of K–O2 batteries with pristine and SbF3-treated K metal anodes. (c) Longterm cycling of K–O2 battery with SbF3-treated K anode. (d) Comparison of cycle life with and without SbF3 treatment. (e) Enhanced shelf-life with SbF3-treated K metal anode.

With the beneficial features presented by the SbF3 surface treatment, anchoring an artificial barrier is expected to help stabilize K metal anode in the K–O2 battery by mitigating solvent and O2-induced decomposition reactions, as depicted in Figure 5a. To further prove this assertion, the SbF3-treated K metal anode was employed in the K–O2 batteries. Although the SEI formed in KPF6-DME electrolyte suffered most from oxygen-induced anode corrosion,12 it demonstrated decent plating and stripping efficiency in the absence of oxygen (Figure S7). Therefore, the artificial protective layer should increase the lifetime of K anode 12 ACS Paragon Plus Environment

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in a K–O2 battery by alleviating the corrosion rate. Figure 5b compares the voltage profiles of the pristine K metal anode and SbF3-treated K metal anode. They exhibited overlapping curves with similar overpotentials in K–O2 batteries, indicating the artificial protective layer maintained high K+ ionic conductivity. This was further verified by electrochemical impedance spectra analysis using symmetric cells (Figure S8) and a comparable voltage profile at a current up to 1 mA (Figure S9). Cycling performance of the K–O2 batteries are further compared in Figure 5c-d. For a K–O2 cell with a pristine K anode, sudden death occurred around the 20th cycle due to serious anode corrosion (Figure S10). The metallic K metal eventually turned into a yellow disc after cell failure (Figure S11). In sharp contrast, the SbF3-treated K metal anode demonstrated an excellent cycle life of over 70 cycles (>30 days). The lifespan of the SbF3-treated K metal anode even reached 1000 hours when the K– O2 battery was cycled with a lower depth of discharge (Figure S12). This greatly enhanced cycling performance can be attributed to the function of the artificial protective layer, which serves as a solvent and O2 barrier. Furthermore, an improvement in shelf-life was also observed with the SbF3-treated K metal anode. In our prior research, the pristine K metal anode suffered from O2 corrosion upon aging and the battery died within 10 days.8 Na–O2 battery was known to suffer even more from poor stability of NaO2 and oxygen species cross over.35,36 In Figure 5e, with an artificial protective layer on the K anode, the battery maintained a stable open-circuit potential and was successfully cycled with a high coulombic efficiency of 97% even after aging for 20 days. This strategy can also be used in combination with methods such as an oxygen-blocking membrane (Figure S13). Put simply, the artificial protective layer introduced by the simple SbF3 treatment significantly extends the lifetime of the K metal anode in K–O2 batteries.

Conclusion



In summary, we for the first time developed an effective strategy to fabricate standard K metal anodes with an artificial protective layer via a one-step rapid chemical reaction between K and SbF3. The inorganic layer mainly consists of KF, Sb, and KSbxFy species. With the established protective layer, solvent decomposition and oxygen crossover issues have been effectively suppressed in K–O2 batteries. The lifespan of SbF3-treated K anode was significantly improved, which ensured an increased cycle-life (>750 hours) and shelf-life (>20 days). Owing to the high K+ ion conductivity of this inorganic layer, the K–O2 battery maintains a high round-trip energy efficiency. This work paves the way to develop a stable K metal anode for rechargeable K metal batteries, and especially for high-performance K–O2 batteries.

Supporting Information Supporting Information is available. Supplementary figures are included. Acknowledgements This work was financially supported by the National Science Foundation (Grant Number: CBET-1512405). We also thank Sichen Gu, Songwei Zhang, Lei Qin, and Wanwan Wang for many fruitful discussions.

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1 Anchoring an Artificial Protective Layer to Stabilize Potassium Metal

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