Dendrite-Free PotassiumâOxygen Battery Based on a Liquid Alloy

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A Dendrite-Free Potassium-Oxygen Battery Based On a Liquid Alloy Anode Wei Yu, Kah Chun Lau, Yu Lei, Ruliang Liu, Lei Qin, Wei Yang, Baohua Li, Larry A Curtiss, Dengyun Zhai, and Feiyu Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08962 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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ACS Applied Materials & Interfaces

A Dendrite-Free Potassium-Oxygen Battery Based On a Liquid Alloy Anode Wei Yua,b, Kah Chun Lauc, Yu Leia, Ruliang Liud, Lei Qina,b, Wei Yanga, Baohua Lia, Larry A. Curtisse, Dengyun Zhaia,*, Feiyu Kanga,b,* a.

Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

b.

School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

c.

Department of Physics and Astronomy, California State University Northridge, 18111 Nordhoff Street, Northridge, California 91330-8268, USA

d.

Materials Science Institute, PCFM Lab and GDHPPC Lab, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China

e.

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

KEYWORDS: Na-K alloy; K-O2 battery; Dendrite growth; Liquid-liquid interface; Superoxide

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ABSTRACT The safety issue caused by the dendrite growth is not only a key research problem in lithium-ion batteries, but also a critical concern in alkali metal (i.e., Li, Na and K)-oxygen batteries where a solid metal is usually used as the anode. Herein, we demonstrate first dendrite-free K-O2 battery at ambient temperature based on a liquid Na-K alloy anode. The unique liquid-liquid connection between the liquid alloy and the electrolyte in our alloy anode based battery provides a homogeneous and robust anode-electrolyte interface. Meanwhile, we manage to show that the Na-K alloy is only compatible in K-O2 batteries but not in Na-O2 batteries, which is mainly attributed to the stronger reducibility of potassium and relatively more favorable thermodynamic formation of KO2 over NaO2 during the discharge process. It is observed that our K-O2 battery based on liquid alloy anode shows a long cycle life (over 620 h) and a low discharge-charge overpotential (about 0.05 V at initial cycles). Moreover, the mechanism investigation into the K-O2 cell degradation shows that the O2 crossover effect and ether-electrolyte instability are the critical problems for K-O2 batteries. In a word, this study provides a new route to solve the problems caused by dendrite growth in alkali metal-oxygen batteries.

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1. INTRODUCTION In recent years, rechargeable non-aqueous “alkali metal-oxygen (A-O2) batteries” (i.e. lithium-oxygen

(Li-O2)

battery,1

sodium-oxygen

(Na-O2)

battery,2

and

potassium-oxygen (K-O2) battery3 have attracted much attention because of their much higher theoretical energy density compared with conventional lithium-ion batteries. The Li-O2 battery possesses the highest energy density due to its use of the lightest metal Li as the anode (Table S1), but its discharge and charge mechanism is complicated. Currently, the predominant mechanism for Li-O2 batteries is the electrochemical reaction based on the solid discharge product Li2O2 which is generated by either disproportionation or electrochemical reaction of the intermediate LiO2.4-6 Compared with insulating Li2O2, LiO2 is found to have better electronic conductivity and is produced/decomposed through a one-electron transfer process, achieving the low charge overpotential for the OER process.7-10 For Na-O2 batteries, both NaO2 and Na2O2 can be identified as the stable discharge product, which decompose at around 2.5 V and above 3.5 V (vs. Na+/Na), corresponding to one- and two-electron transfer processes, respectively.2,11,12 However, the fundamental mechanism for the formation of main product is still not clear.13,14 Different from Li-O2 and Na-O2 batteries, the superoxide, i.e., KO2, is the sole discharge product in K-O2 batteries although K2O2 could be formed by the further reduction of KO2.3,15,16 The thermodynamically stable KO2 is produced/decomposed through one-electron transfer and the discharge/charge voltage gap is found to be as low as 0.05 V.3,17,18 It is

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generally believed that the enhanced round trip efficiency of A-O2 batteries based on the superoxide is related to the better conductivity and faster kinetics via one-electron transfer.3,19 But recent calculation and experiments studies found that superoxides (NaO2 and KO2) were also poor electrical conductors at the ambient temperature and could unlikely contribute to the low overpotential.20-22 In order to be more practical in application, there are two critical problems needed to be solved for A-O2 batteries. The first challenge is to exploit a more stable electrolyte system. Generally, the irreversible side reactions involving electrolyte degradation occur during cells cycling and the unwanted byproducts are accumulated continuously within cells, eventually leading to the capacity fading.2,18,23-25 Although the electrolyte degradation is also inevitable, the ether is still one of the most stable electrolytes widely applied in A-O2 batteries.2,3,7,14,18,26-28 Recently, the K+-ion selective membrane was introduced into the solid K-O2 battery as the separator. This membrane can not only alleviate the parasitic reactions of ether electrolyte caused by O2 crossover but also improve the cycleability, although the larger overpotential was formed compared with the cell without K+-ion selective membrane.3,18 Another critical problem is the safety issue caused by the dendrite growth at solid metal anode. In today’s lithium-ion batteries,29,30 the inhibition of lithium anode dendrite remains a very important research area. Similarly, the dendrite formation is also a critical safety issue in A-O2 batteries where the solid alkali metals (i.e., Li, Na and K) are usually employed as the anode, and replacement of a solid anode by a liquid one seems a possible solution. Actually, some liquid metal based batteries have

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been reported, such as Li-Sb-Pb battery, Na-S battery and Mg-Sb battery.31-35 Although those liquid electrodes effectively eliminate the dendrite formation, safety and durability concerns are aroused by their high operating temperature. Hence, it is of great importance to develop a new battery based on a room-temperature operating dendrite-free liquid anode. The Na-K alloy remains liquid state at room temperature when the mass percent of K is in the range of 40 to 90% (Figure S1).36 Very recently, Goodenough et al.37 reported a potassium ion battery using the Na-K alloy as the anode, but the preparation of Na-K alloy anode by absorbing it into carbon paper requires a quite high temperature 450 oC. Compared with ion or sulfur batteries, few fundamental investigations have been done on alloy anodes for A-O2 batteries.38-40 Only Wu et al.41 employed the solid K3Sb alloy as the anode for K-O2 batteries to address the issues from dendrite growth and oxygen crossover. In this work, an ambient-temperature K-O2 battery based on a dendrite-free Na-K liquid alloy anode is designed. The electrochemical properties and underlying mechanism are deeply investigated. By replacing a conventional solid-liquid interface between the anode and the electrolyte by a liquid-liquid one, the problem of potassium dendrite formation has been completely solved in our new type of K-O2 battery. Furthermore, the new liquid anode-electrolyte interface contributes to the long-time cycle performance and the extremely low overpotential. 2. EXPERIMENTAL SECTION 2.1. K+ (Na+) selective membranes K+ selective membranes were prepared by exchanging the origin cation H+ of nafion

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to K+, which was reported in previous work in detail.42 In this work, Nafion 211 membranes (purchased from DuPont) were immersed in 0.5 M KOH solution of dimethyl sulfoxide (DMSO, >99.9 %, Sigma-Aldrich) and deionized water (volume ratio = 1:1) at 60 oC for 2 h. The as-treated membranes were carefully washed with deionized water several times to remove residual KOH and solvents, and then treated using water bath at 80 oC for 4h. The Nafion-K+ membranes were finally dried at 110 o

C under vacuum for 24 h and stored in the glove box filled with high-purity Ar

(99.999 %). Na+ selective membranes were prepared in terms of the above procedure and the membranes first were immersed in 0.5 M NaOH solution of DMSO and deionized water. The thickness of selective membranes was 0.031 mm. The detailed measurements and results of membrane resistance and ionic conductivity were described in supplementary information (Table S2). 2.2. Electrode preparation The carbon nanotube (CNT, 80 wt%, purchased from Cnano Technology Ltd.) was employed as cathode material, which was mixed with polyvinylidene fluoride binder (PVDF, 20 wt%) in N-methyl-2-pyrrolidone (NMP) and coated on Torray carbon paper. The loading weight of CNT was about 0.5 mg cm-2. Then the cathode was dried under vacuum at 110 oC for 24h. In addition, all the components of cell, including steel base, cathode connector, porous collector and glass fiber separators (GF/A, Whatman) were also dried under vacuum at 110 oC for 24 h before transferred into the glove box. 2.3. Cell assembly

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The K-O2 cell using Na-K liquid alloy as the anode was assembled in high purify Ar filled glove box. The electrolyte was 0.5 M KPF6 dissolved in diethylene glycol diethyl ether (DEGDME). DEGDME solvent was refluxed under reduced pressure and stored over 4 Å molecular sieves in glove box. The water content of DEGDME was around 30 ppm by Karl Fischer titration (Metrohm 831). The Na-K liquid alloy was prepared in glove box. The solid Na and solid K (mass ratio of Na:K = 3:7) were placed in the beaker and stirred slowly using glass rod to form the homogenous liquid alloy with a metallic sheen. The adequate Na-K liquid alloy was added drop by drop into the hollow cylinder on the bottom of steel base to keep the convex surface and the top of the hollow cylinder at the same height. Note that the mass of added Na-K alloy is about 0.3 g, which is far more than the necessary amount in discharge or cycling cell tests in order to guarantee the stable composition of this liquid anode. The sandwich-type separators which contained two glass fiber separators (GF/A, Whatman) and one Nafion-K+ membrane were covered on the stage. About 70 µl of electrolyte (0.5 M KPF6 in DEGDME) is infused between the anode and the cathode. Then the CNT cathode, the porous collector and cathode connector was stacked in turn. The assembled cell was sealed in glass chamber and then filled with 1 atm of high-purity O2 (99.995 %). The cell stood for 2 h before test. All the galvanostatic discharge/charge tests were performed using a Land 2001A battery testing system at 298 K with the cutoff voltages set between 2 and 3 V (vs. Na-K alloy). The current density was 50 µA cm-2. 2.4. Characterization

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The disassembly of the K-O2 batteries was carried out in the glove box filling high purity Ar atmosphere. Crystalline characterization of discharge product on cathode and residues on both cathode and anode side after cycle were measured by the X-ray diffraction (XRD, Rigaku D/max 2500/PC) in which Cu-Kα (λ=0.154 nm) was used as the radiation source. The samples were sealed in the glass holder under the Kapton® film to isolate air/humidity in Ar glove box. Similarly, the electrode samples for Raman spectra were loaded inside of a sealing test device with glass or quartz window under the condition of air isolation. The Raman spectrum was detected by a LabRAM HR 800 which was set up in a 180 o reflective mode. A semiconductor laser at 532 nm exciting wavelength was employed and 25 % of maximum laser intensity 50 mW was applied, approximately. 1H nuclear magnetic resonance (NMR) spectrums of DEGDME before and after cycle tests were collected by a Bruker AVANCE III 400 instrument. The residual electrolyte component from the separator was dissolved in CDCl3. A field-emission scanning electron microscope (SEM, Hitachi SU-8010) was used to show the morphology of glass fiber separators and depositions on the anode side. 2.5. Computational methods To model the Na-K liquid alloy that used in experiment, the Ab Initio Molecular Dynamics (AIMD) and density functional theory (DFT) calculations are used. To construct a Na-K alloy, the bulk Na4K5 alloy with the Na:K mass ratio = 32 % : 68 % that is close to ~ 3:7 found in experiment is modeled with a (18 × 18 × 12 Å3) simulation cell. The simulation cell consists of 28 Na and 35 K atoms which

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randomly packed in the simulation cell initially with a predefined atomic density, i.e. 0.87 g cm-3 as determined by experiment. For both AIMD and DFT simulation, the calculations were carried out with plane wave basis sets as implemented in the Vienna Ab-initio Simulation Package (VASP) code.43,44 All the calculations were spin-polarized and carried out using the gradient corrected exchange-correlation functional of PBE (Perdew, Burke, and Ernzerhof) under the projector augmented wave method, with plane wave basis set up to a kinetic energy cutoff of 500 eV within Γ-point.45,46 The van der Waals method of Grimme, DFT-D2 was used throughout AIMD calculations with the convergence criterion of the total energy set to be within 1 × 10−5 Ev.47 To obtain a thermodynamic stable of Na-K alloy at room temperature, the system was thermally equilibrated at T = 300 K based on Nose–Hoover thermostat within NVT thermodynamic ensemble with a time step of 1 fs, and the production run (~ 2 ps) was obtained after thermal equilibration of ≈ 4 ps. For the DEGDME solvents adsorption study on both Na4K5 alloy and potassium (100) surface, the adsorption energy or interaction energy is obtained from the lowest energy structure after geometry optimization of several trial configurations. 3. RESULTS AND DISCUSSION 3.1. Set-up of a K-O2 cell using the Na-K liquid alloy anode Considering the fluidity of the Na-K liquid alloy, we designed a special configuration for the K-O2 cell, as shown in Scheme 1. At the bottom of the steel base, there is a hollow cylinder with a diameter of 12 millimeters, which is filled with Na-K alloy. A piece of Nafion-K+ membrane is used here as the separator, which is sandwiched by

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two glass fiber containing certain amount of electrolyte. It is worth noting that Nafion membrane is used mainly to inhibit O2 crossover from the cathode to the anode. The carbon paper cathodes coated with the CNT are stacked sequentially. In the end, a porous collector is placed on the CNT cathode and a cathode connector is fixed tightly in the base by screw threads. The cell assembly in detail is described in the experimental section. In addition, the Na-K alloy is found to be metallic according to the DFT calculations (Figure S2), which is consistent with the experimental and theoretical results in previous studies.48-50 Therefore, the important electrical contact of metallic anode can be established.

Scheme 1 The K-O2 cell configuration and assembly based on Na-K liquid alloy as the anode. 3.2. Discharge performance of the K-O2 cell A K-O2 cell is assembled and manipulated the discharge test. After discharge to the limited capacity of 1.0 mAh, the discharge product on cathode is detected by XRD and Raman spectroscopy, as shown in Figure 1. The XRD result shows that the main discharge product is KO2, although a very small amount of KOH is also produced. The Raman peak at 1142 cm-1 in Figure 1b confirms the generation of KO2 on the cathode.3,51,52 In addition, the XRD and Raman tests of the cathodes after full

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discharge to a cut-off voltage 2.0 V exhibit the similar results that the KO2 dominates the discharge products (Figure S3). For comparison, the discharge cathode in K-O2 cell without the Nafion-K+ membrane is also characterized by XRD (Figure S4). Compared with the peak in Figure 1a, the intensity of KOH peak is much stronger in Figure S4, which reveals that a larger amount of KOH is produced along with KO2. This is probably attributed to the effect of Nafion membrane which can inhibit O2 crossover effectively and further reduce the side reactions (more detailed discussion in following section). According to the results above, in a K-O2 cell employing Na-K alloy anode, the expected discharge product KO2 is produced, which indicates that Na-K alloy can work effectively as the anode in K-O2 batteries.

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Figure 1 (a) XRD patterns and (b) Raman spectrum of the discharged cathodes in the K-O2 cell with the Nafion-K+ membrane. The electrolyte is 0.5 M KPF6 in diethylene glycol diethyl ether (DEGDME) and limited discharged capacity is about 1 mAh. 3.3. Principle analysis of Na-K alloy anode in Na- or K-O2 system Since the metal K in the alloy works as the anode in K-O2 batteries according to the above results, the question whether the metal Na in Na-K alloy also can be used as the anode for Na-O2 batteries is worth to be studied. A direct comparison in the redox reactions for the cathode and the anode of Na- and K-O2 batteries under standard hydrogen electrode (SHE, in aqueous solution) is shown in Figure S5. For the anode

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of Na- and K-O2 batteries, the theoretical redox potential of Na/Na+ and K/K+ is -2.71 and -2.92 V vs. SHE, respectively.53 Although the potentials in DEGDME might be different, the change in potential would be the same for the same cation. For the redox reaction at the cathode, the theoretical reaction potentials (E0) based on NaO2 and KO2 as discharge product are 2.27 V (vs. Na/Na+) and 2.48 V (vs. K/K+) (referred to Table S1), respectively, which reveals the reaction potential difference between the cathode and the anode. Actually, the formation potentials of NaO2 and KO2 at the cathode are the same under the uniform reference potential (i.e. -0.44 V vs. SHE), while the oxidation potentials of Na and K at the anode are different (-2.71 and -2.92 V vs. SHE). Thus, in a mixed Na-K system where the redox reactions of Na- or K-O2 battery could possibly occur, we can deduce the reaction at the anode according to the discharge potential. For the oxidation reaction at Na-K alloy anode, the reaction (K →K+ + e-) would tend to occur especially in Na-O2 system (the electrolyte containing Na+) because of the stronger reducibility of the metal K and larger oxidation driving force owing to ion concentration difference. Although the formation of NaO2 and KO2 at the cathode during discharge process is both found to be thermodynamic feasible (∆rG < 0) and share the same theoretical redox potential (i.e. -0.44 V vs. SHE), the formation of KO2 would be theoretically more favorable because it is thermodynamically more exergonic in terms of Gibbs free energy of formation (Table S1). Besides, another potential factor from the cation concentration difference should be taken into consideration. For example, during the initial stage of discharge process for a Na-O2

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cell based on the Na-K alloy anode, the concentration of Na+ is much higher than the K+ concentration in the electrolyte, and the initial high concentration gradient would promote the formation of NaO2 kinetically at the cathode. Thus both the thermodynamic and kinetic factors may play a critical role in dictating the fundamental electrochemical reaction at the cathode. To investigate whether the Na-K alloy can be used as the anode for Na-O2 batteries, we carried out the following experiments. In terms of assembling procedure of K-O2 cell in Figure 1, two Na-O2 cells are assembled in glove box, consisting of a Na-K alloy as the anode, CNT cathode, 0.5 M NaPF6 in DEGDME as the electrolyte and two glass fiber separators, with and without Nafion-Na+ membrane, respectively. Figure 2a shows the discharge curves of two cells. The cell without Nafion-Na+ membrane is discharged to about 2.0 mAh, and the voltage at the beginning stage is about 2.45 V which is close to 2.48 V. Based on this observation, the reaction at the anode should be K → K+ + e-. Subsequently the K+ diffuses into the electrolyte containing Na salt and the concentration of K+ increases. The reaction at the cathode can be inferred by identifying the discharge product on the cathode. The XRD result in Figure 2b shows that the discharge product is composed of KO2, KOH and NaO2. The presence of KO2 and KOH in Na-O2 cell with a liquid Na-K alloy anode may be attributed to the following electrochemical process. At the initial stage of discharge process, the concentration of Na+ is much higher than K+ in electrolyte. The large concentration difference of Na+ in electrolytes during initial discharge process drives the formation of NaO2 by consuming Na+ in electrolyte, although the formation of

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KO2 is favored thermodynamically. However, along with the increasing concentration of K+ and consumption of Na+ over time during discharge process, the concentration gradients of Na+ and K+ cations in electrolyte will gradually change, and subsequently, both kinetic and thermodynamic favorable reactions for the growth of KO2 will dominate the discharge process after a small amount of NaO2 formed at the cathode. Thus, with the discharge processing, a Na-O2 cell based on the Na-K alloy anode will eventually turn into a K-O2 cell. It can be concluded that the Na-O2 battery based on the liquid Na-K alloy as anode is not sustainable due to the stronger reducibility of K and competing thermodynamic driven growth of KO2 over NaO2 during the discharge process. In the Na-O2 cell with Nafion-Na+ membrane, the ion selectivity of Nafion-Na+ membrane hinders the diffusion of K+ ions from the anode to cathode, which leads to the very limited discharge capacity. Large amount of KOH are detected in the after discharged cathode (Figure 2b), which is consistent with the results in Figure S4 (SI).

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Figure 2 (a) The galvanostatic discharge curve of Na-O2 batteries based on the Na-K alloy as the anode at 0.05 mA cm-2, with and without Nafion-Na+ membrane; (b) XRD pattern of the discharged cathode in a Na-O2 cell without Nafion-Na+ membrane. 3.4. Liquid-liquid interface between the Na-K alloy anode and the electrolyte To elucidate the differences between the liquid alloy anode-electrolyte and K solid anode-electrolyte interface, we investigated the contact interface of anode-electrolyte shown in Figure 3a. During cell cycling of the K-O2 battery, the fundamental reaction (K ↔ K+ + e-) is assumed to be taking place at the anode-electrolyte interface. For a conventional solid metal K anode, the surface of this solid metal is usually rough (Figure 3a) and this subsequently leads to a far from ideal contact interface between anode and electrolyte. For the solid metal-electrolyte interface, the non-uniform 16


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connection and unprotected reaction sites are predictable, which will eventually lead to unavoidable dendrite formation on the anode surface after repeated plating and stripping cell cycling. Owing to the immiscibility with DEGDME solvent (Figure S6), the high surface tension of Na-K liquid alloy prevents it from wetting and achieves a uniform robust liquid-liquid interface compared to solid potassium metal. Obviously, the strong surface tension of the liquid phase Na-K alloy remains a stable and uniform convex surface during cycles, but even the fresh solid K metal cannot guarantee a smooth surface. Furthermore, considering the volume change caused by dendrite growth in solid potassium metal surface during repeated cycles, we believe the dendrite-free liquid alloy offers a predictable better interface between Na-K alloy anode and electrolyte. From DFT calculations (Figure 3b-e), the interaction of DEGDME solvents on Na-K alloy surface is found to be less favorable (i.e. with adsorption energy ∆E ~ 0.18 to -0.34 eV, which is found to be slightly exothermic) and substantially more inert than the metal potassium surface (i.e. with adsorption energy ∆E ~ -1.67 to -2.80 eV, which is found to be highly exothermic). Therefore, this further demonstrates that the chemical or electrochemical interaction between electrolyte and liquid Na-K alloy anode will be less active than that on a solid-liquid anode-electrolyte interface. Thus, we believe that the employment of a liquid Na-K alloy anode will eliminate the partial non-uniform plating or stripping process of anode and subsequently exclude the safety issue caused by dendrite formation.

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Figure 3 (a) Graphic illustration of liquid-liquid and solid-liquid interfaces in K-O2 batteries based on a Na-K liquid alloy and solid metal K as the anode, respectively. (b) The lowest energy configuration of a single DEGDME solvent adsorbed on Na4K5 surface with adsorption energy, ∆E = 0.18 eV. (c) The lowest energy configuration of a fully cover Na4K5 surface with DEGDME solvents which the adsorption energy is found to be slightly exothermic, ∆E = -0.34 eV. (d) The lowest energy configuration of a single DEGDME solvent adsorbed on potassium (100) surface with adsorption energy, ∆E = -1.67 eV. (e) The lowest energy configuration of a fully cover potassium (100) surface with DEGDME solvents which the adsorption energy is found to be highly exothermic, ∆E = -2.80 eV.

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3.5. Cycle performance of the K-O2 batteries based on the Na-K alloy anode The cycle performance of the K-O2 battery using Na-K alloy anode is shown in Figure 4 and Figure S7. In order to alleviate the O2 crossover effect, a piece of Nafion-K+ membrane was added in our cycling battery. The discharge and charge plateaus of the first few cycles are 2.45 and 2.50 V, respectively, and this optimal performance can be sustained as long as 70 cycles (over 620 h, Figure S8). As reported in previous work, although the Nafion membrane could reduce the O2 crossover and subsequently improved the cell cycle life up to 40 cycles in the solid K anode based K-O2 batteries, the membrane also leads to inevitable increase of the discharge-charge overpotential from less than 0.05 to 0.3 V.18 Compared to recent reported solid anode based K-O2 batteries (Table S3), our current result shows that Na-K liquid alloy employed as the anode for K-O2 batteries improves the cell cyclability significantly and remains the discharge-charge overpotential within 0.05 V, even when the Nafion membrane is introduced.3,17,18,41,54

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Figure 4 (a) Galvanostatic discharge-charge curves of the first five discharge-charge cycles and (b) the first seventy cycles at 0.05 mA cm-2 with the controlled capacity of 500 mAh g-1 and voltage range from 2.0 V to 3.0 V. 3.6. The mechanism of cycling capacity fading The cycling performance of K-O2 cell (Figure 4b) shows that the discharge plateau reduces gradually from 2.45 to 2.25 V and the charging voltage increases from 2.5 to 2.6 V with cycle numbers, which is probably attributed to the accumulation of byproducts. These similar observations were reported in previous studies of Na- and K-O2 batteries.17,18,38,55 In order to investigate the composition of residual products after cycles, the K-O2 cell after 70 cycles (Figure 4) is disassembled in glove box, and

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the measurements are carried out as shown in Figure 5. As we can see in Figure 5c, after the long time cycles the Na-K alloy anode still kees liquid phase, just like the fresh Na-K alloy and the anode after discharge tests in Figure S9 (SI), which confirms the liquid existence of alloy anode in all circumstance of cell tests. In addition, the cathode and the separators are relatively dry. After the sandwiched separators are removed, a piece of white powder is found to be deposited on the top of Na-K alloy. The cathode and the white powder are characterized using XRD and the possible decomposition byproducts attributed to electrolyte degradation are analyzed based on 1

H NMR measurements. From the XRD results in Figure 5d, the residual product

deposited on the cathode contains K-salt (i.e. KPF6), undecomposed KO2 and the byproduct KOH. The white powder deposited on the anode is mainly consisted of alkali hydroxide (KOH and NaOH) and possible superoxide (KO2), which possibly refers to the byproduct of DEGDME degradation owing to the O2 crossover into the Na-K alloy anode.18 The microstructure of the white powder and glass fiber separator are shown in Figure S10 (SI). Based on the 1H NMR results in Figure 5e, only H2O (δ = 1.56 ppm, singlet) and some small molecules containing methyl (CH3-; δ = 1.26 ppm and 0.86 ppm) are identified in the CDCl3 solution and scarcely any peaks concerning DEGDME can be detected, which indicates that the DEGDME solvent is completely decomposed after 70 cycles. As shown in Figure S11, the 1H NMR characterization of electrolyte after two week’s aging test under O2 atmosphere in our Na-K alloy based K-O2 batteries are consistent with the results after 70 cycles, which means the degradation of DEGDME caused by crossover O2 on alloy anode side

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seems a spontaneous reaction and the previous study based on solid potassium showed the similar results.18 Besides, the 1H NMR results in Figure S11 also proves that the Nafion-K+ membrane can alleviate the degradation to some extent, although the crossover O2 problem can not be completely solved.

Figure 5 (a), (b) and (c) Disassembling illustration of the K-O2 cell after 70 cycles in glove box; (d) XRD pattern of the cathode; (e) 1H NMR spectra of electrolyte before and after cycle test; (f) XRD pattern of white powder on the surface of Na-K alloy. In our opinion, the decomposition of solvent is the primary reason that leads to the significant capacity fading, and therefore, the remaining KPF6 is precipitated at the electrode after cycles. According to the studies from Wu et al.,18 the reduced ether molecules (i.e. negatively charged ether radical) at anode might interact with O2 crossover from the cathode to the anode leads to the decomposition of solvent, and subsequently leads to the formation of an insulating layer (i.e., KOH) at the Na-K anode side. Thus during the cycle tests, the crossover O2 binds to the DEGDME radical and is reduced to superoxide radical (O2-) and will initiate the decomposition

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of solvents as shown in Figure S12. The attack of the induced superoxide radical that leads to the DEGDME decomposition from the C-O bond cleavage, and the subsequent formation of the methoxy radical and methoxyacetate are the critical pathways for DEGDME decomposition. In addition, hydrogen abstraction and dissociation of methoxyacetate could potentially lead to the generation of hydroxide radical (OH−) and hydroxide species (i.e. KOH and NaOH). Moreover, the generation of H2O in the electrolyte possibly accelerates the formation of insulated hydroxide, which is consisted with the XRD results in Figure 5d and 5f. As to the interaction between discharge products KO2 and DEGDME, the 1H NMR results in Figure S13 verify the relative stability of KO2 in DEGDME, which is consistent with previous reports.56 The small amount of electrolyte decomposition is also caused by the crossover O2-induced parasitic reaction. Thus based on the observations reported in this work, we can conclude that the problems of the O2 crossover and the instability of glyme-based solvent remain the key challenges that need to be addressed for a practical K-O2 battery. Ironically, these problems generally persist in all A-O2 batteries.18,38,57 Although the liquid Na-K alloy anode can be used to thoroughly eliminate the metal dendrite growth at anode as we have shown in this work, a robust separator or membrane that can efficiently suppress the O2 crossover is critically needed. 4. CONCLUSION In this work, a new type of K-O2 battery based on the liquid Na-K alloy anode instead of solid metal K at ambient temperature is demonstrated. Thanks to this unique

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liquid-liquid interface between the liquid Na-K alloy anode and the electrolyte, the battery shows a gratifying cycling performance as long as 70 cycles (beyond 620 h) at low discharge-charging overpotential about 0.05 V in initial cycles. On the basis of our DFT calculation, the dendrite-free liquid Na-K alloy anode provides a homogeneous and robust liquid-liquid anode-electrolyte interface which is relatively more stable than the traditional solid-liquid interface contributes to the remarkable cycle stability. By systematically comparing the discharge processes of K-O2 and Na-O2 batteries which are both based on the Na-K liquid alloy, we found that the Na-K alloy is only suitable for K-O2 batteries but not for Na-O2 batteries due to the competing electrochemical reactions between K+ and Na+ ions. This phenomenon is attributed to the stronger reducibility of K and the thermodynamically more favorable of KO2 formation during discharge process. Moreover, the problems caused by the O2 crossover and the instability of electrolyte remain the critical challenges that restrict the cyclic stability of K-O2 batteries. In general, the application of the liquid Na-K alloy in a K-O2 battery provides an effective approach for solving the dendrite growth problem of the anode. However, exploiting a more robust electrolyte system and inhibiting the reaction between the crossover O2 and the anode will remain the critical subjects of investigation for the improved design of a practical K-O2 battery.

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ASSOCIATED CONTENT Supporting Information. DFT calculation, XRD, Raman, Digital photo, SEM and additional 1H NMR analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mails: [email protected]; [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Key Basic Research Program of China (Grant No. 2014CB932400), the National Natural Science Foundation of China (Grant

No.

51232005)

and

Shenzhen

Basic

Research

Project

(No.

JCYJ20170412171311288). K.C.L. acknowledge the grants of computer time through the LCRC Blues Clusters at Argonne National Laboratory.

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