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Improvement of Cycling Phosphorous-Based Anode with LiFRich Solid Electrolyte Interphase for Reversible Lithium Storage Wenya Lei, Yangyang Liu, Xingxing Jiao, Chaofan Zhang, Shizhao Xiong, Bing Li, and Jiangxuan Song ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00025 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Applied Energy Materials

Improvement of Cycling Phosphorous-Based Anode with LiF-Rich Solid Electrolyte Interphase for Reversible Lithium Storage

Wenya Lei+, Yangyang Liu+, Xingxing Jiao, Chaofan Zhang, Shizhao Xiong, Bing Li, Jiangxuan Song* State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China * Corresponding author. E-mail address: [email protected] + These authors contributed equally to this work.

Abstract Red phosphorus (P) is considered as an alternative anode material for high-energy-density lithium-ion batteries (LIBs) due to its high theoretical specific capacity and cost-effectiveness. Whereas practical application of phosphorus is severely hindered by the nature of low electrical conductivity and its huge volume change during lithiation and delithiation process that leads to the failure of electron conductive network and solid electrolyte interphase (SEI). Here, we reported a highly reversible P based anode material for long cycling and high capacity LIBs. This hybrid anode (P-SP) is prepared with commercial red P and carbon black (Super-P) via ball-milling to obtain a nanoscale amorphous structure for excellent electron conductivity and minimized volume change. Furthermore, fluoroethylene carbonate (FEC) is introduced as additive to form a LiF-rich SEI which enables stable cycling of P-SP hybrid anode. As a result, the P-SP hybrid anode operating with the electrolyte of 10 vol.% FEC exhibits high capacity (2236.2 mAh g-1 at 0.3 C) and stable cycling stability (86% retention of the second-cycle capacity after 300 cycles). Considering low-cost manufacture at large scale and excellent electrochemical performances, our work may promote an effective strategy for practical application of P anode material in LIBs.

Keywords:

Lithium-ion battery, Anode, Phosphorus, Solid electrolyte interphase (SEI), Fluoroethylene

carbonate (FEC)

Introduction Commercial lithium ion batteries (LIBs) have been widely used to meet various demands of energy storage, especially in the fields of mobile electronic devices, electric vehicles and renewable power sources.1-3 However, the development of cathode and anode materials with high energy density is strongly desired to fulfill the fast increasing requirements on high gravimetric and volumetric energy densities of battery technology.4-8 Phosphorus (P) is regard as a promising substitute for LIBs due to its high theoretical capacity (2596 mAh g-1, when P fully react with Li to

ACS Paragon Plus Environment

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form Li3P) which is ~7 times that of commercial graphite (theoretical capacity is only 372 mAh g-1).9-12 Among the P allotropes, red P is chemically stable and amorphous and has been widely used in commercial applications.9, 13 The cost of red P is comparable to graphite, therefore it is quite cost-effective for the development of lithium ion batteries with high specific capacity.10 However, the practical application of red P-based anode for LIBs is severely stumbled by two major challenges: (i) large volume change (over 300% for lithiation and delithiation process), leading to unstable solid electrolyte interphase (SEI) and rapid capacity fading; (ii) low ionic conductivity (~10-14 S/cm), resulting in poor rate performance for sluggish electrode kinetics and large polarization.10, 14-16 To overcome these challenges, researchers generally combine P with carbon materials (C) to obtain P/C hybrid anodes.9, 13, 17. Yu at al. demonstrated that amorphous red P coupled with mesoporous carbon matrix (P@CMK-3) delivered outstanding capacity performance in both sodium ion batteries (SIBs) and LIBs.18 In addition, various amorphous P /C composites with nanostructure are reported as stable and high reversible anode material for high-performance LIBs.9, 13, 19-20 Although combination of P with C is demonstrated to be an effective strategy to mitigate the volume change of hybrid anode materials during repeated charge-discharge process, its forward advances are still restricted by unstable SEI that invariably leads to accelerated consumption of electrolyte, rapidly capacity fading and low Coulombic efficiency.9, 13-14, 19 For this perspective, fabricating a stable SEI on the P hybrid anodes in situ or ex situ is another important route to address these issues. As a distinguished additive in non-aqueous electrolytes of LIBs, fluoroethylene carbonate (FEC) is widely used to form highly stable SEI on anode material because of the ringopening polymerization to generate lithium fluoride (LiF) and thin polymer film on the surface of electrode.21-24 The LiF-rich SEI can effectively improve CE and capacity retention, meanwhile lower the interface impedance for Li-ion migration.25-27 Herein, we report a facile method to synthesize highly reversible P-based hybrid anode (P-SP) by high-energy ballmilling of red phosphorus and carbon black (Super-P). The P-SP anode is fine size, which can shorten migration pathways of electron/ions and mitigate the volume expansion during lithiation and delithiation process. Further of 10 vol.% FEC as film-form additive significantly enhances the stability of SEI, enabling stable cycling of P-SP anode for high performance LIBs. Because of these advantages, outstanding specific capacity of P-SP hybrid anode is obtained as 2236.2 mAh g-1 at a 0.3 C. After 300 cycles, the high retention capacity of 1564.4 mA g-1 demonstrated its stable capability in long-term cycling. This sheds the light on studying and applying P-based anode materials for high-energy-density LIBs.


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Results and Discussion

Figure 1. Characterizations of P-SP. a-c) FESEM images of red P particles (a), Super-P carbon black (b) and asprepared P-SP hybrid (c). d-f) HRTEM images of P-SP hybrid (d) and corresponding energy filtered element mapping, including carbon (e) and phosphorus (f). g-h) Characterization of an amorphous structure by XRD (g) and Raman spectra (h). i-j) Electron density distribution of carbon materials without defect (i) and with highly concentrated defects (j).



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Commercial red P and Super P carbon black (7:3 in weight) were hand-grinded to form P-SP mixture and ballmilled to form P-SP hybrid. (see the Experimental Section for details). To investigate the morphology of as-prepared P-SP hybrid, the field emission scanning electron microscopy (FESEM) and transmission electron microscope (HRTEM) were conducted. The particle size of pristine P is more than 30 µm and that of Super-P is 20 nm approximately (Figures 1a-b). After the ball-milling procedure, the size of the P-SP hybrid is lowered to nanoscale, which is much smaller than the pristine red P and close to Super-P (Figure 1c). Meanwhile, elemental mapping images of the P-SP hybrid are shown in Figures 1d-f, revealing the chemical components of the nanoscale P-SP hybrid. Elementary C and P exhibit uniformity over the entire bulk of P-SP hybrid, indicating the even distribution and tight contact between P and Super-P. To verify the structure of the P-SP hybrid, X-ray diffraction (XRD) and Raman spectroscopy were carried out on red P, Super-P and P-SP hybrid. There exist two broad graphitic reflection peaks (24.6° and 43.4°) in pattern of Super-P and three broad peaks (15.2°, 21.2° and 34°) in pattern of red P, which confirm the amorphous structure of precursors9, 28 However, the pattern of P-SP hybrid displays only a very broad peak (centered at 21.2°), which is corresponding to red P. The absence of graphitic reflection peaks reveal that highamorphous P-SP hybrid is obtained after ball-milling procedure, by which the mechanical impact and the shearing force integrate red P and Super-P at nanoscale.29 The amorphous structure P-SP hybrid is further confirmed by Raman spectra (Figure 1h). Raman spectroscopy of P-SP hybrid shows the carbon peaks at 1322 cm-1 and 1594 cm-1 for D band and G band. The relative intensity (ID/IG) of carbon peaks represents the amorphous degree of carbon materials.9 The increased ID/IG value demonstrates that amorphous degree increases after ball-milling process (ID/IG of Super-P is 1.005 and that of P-SP hybrid is 1.188).30-31 Combing above results, we suggest that the highamorphous P-SP hybrid anode is successfully obtained via high-energy ball-milling. To obtain a better understanding of the relationship between amorphous structure and electron conductivity, first principles based on density functional theory (DFT) was conducted to reveal the electron density of amorphous carbon materials. There exist two kind of carbon atom in Super-P, sp3 carbon and sp2 carbon, which is responding to the D peak and G peak in Raman spectra, respectively. That is, higher ID/IG value of hybrid after ball-milling means higher ratio of sp3 carbon. The simulation results illustrate the local electron density of sp3 carbon at edge is higher than that of sp2 carbon in bulk (Figures 1i-j).32-36 In addition, the edge carbon atoms with high electron density in high-amorphous structure are preferred to Li-ions absorption, which contributes to better rate performance.37 Thus, we suggest that the P-SP hybrid with high degree-amorphous structure is expected to exhibit enhanced electron conductivity.


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Figure 2. Electrochemical performance of P-SP hybrid anode. Cycling stability of P-SP hybrid anode (a), initial charge-discharge voltage profiles (b) and Electrochemical impedance spectra (EIS) spectra after first cycle (c). All the cells are tested at 0.1 C for activation and then at 0.3 C in the subsequent cycles, using 1M LiPF6 in EC/DEC as electrolyte.

As previously reported, red P has high theoretical capacity (2596 mAh g-1) for the three-electron reaction to form Li3P.12, 38 To evaluate the electrochemical performance of P-SP hybrid as anode material for LIBs, half-cells were assembled using 1 M LiPF6in ethylene carbonate and diethyl carbonate (EC: DEC=1:1) as the electrolyte and tested at 0.3 C. As shown in Figure 2a, the half-cell with P-SP hybrid via ball-milling delivers high capacity (1646 mAh g-1) among 100 cycles lifespan. By contrast, hand-grinding P-SP composite anode presents knockdown capacity after initial cycle. Figure 2b displays initial change-discharge curves, showing that P-SP hybrid via ball-milling exhibits higher initial Coulombic efficiency (ICE) (78.34% for ball milling and 7.05% for hand grinding), better reversibility and lower polarization. Meanwhile, the electrochemical impedance spectra (EIS) analysis was conducted to investigate the integrity of electrode structure.10, 31, 39-40 P-SP hybrid anode delivers lower interfacial impedance than P-SP mixture after first cycle (Figure 2c). Predictably, it is demonstrated that the nanoscale and amorphous P-SP hybrid anode enables less volume change of anode during cycling, which is beneficial to enhance the reversibility of high capacity and electron conductivity.18, 41



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Figure 3. Electrochemical performance of P-SP hybrid anode that using 1M LiPF6 in EC/DEC with FEC. a) Cyclic performance of P-SP hybrid in the electrolytes with different FEC contents at 0.3 C. b-d) Charge-discharge voltage profiles of P-SP hybrid with 2 vol.% FEC (b), 5 vol.% FEC (c) and 10 vol.% FEC (d). e) Capacity retention of P-SP in different electrolytes after 300 cycles. f) Rate capability of the P-SP hybrid anode. g) Initial cyclic voltammetry curves of the P-SP hybrid in the electrolyte without and with 10 vol.% FEC at scanning rate of 0.05 mV s-1.

To obtain a robust SEI for adopting the volumetric expansion of P-based anode, FEC was added to the conventional electrolyte (1M LiPF6 in EC/DEC), ranging from 2 vol.% to 10 vol.%. Figure 3a shows cyclic performance of P-SP half-cells with FEC-added electrolyte. The capacity of the cell with 2 vol.% remains 24.2 mAh g-1 after 300 cycles, whereas the corresponding capacity of the cell with 10 vol.% FEC increases to 1564.4 mAh g-1 over 300 chargedischarge steps. The remarkable increase suggests that FEC is effective to deliver high capacity retention over longterm operation. The voltage profiles displayed in Figures 3b-d further confirm the function of FEC to stabilize the cyclic performance and lowering the fading rate of capacity. As summarized in Figure 3e, addition of FEC can significantly extends the cyclic life of cell with P-SP hybrid. We demonstrate that the optimum cyclic performance is achieved with addition of 10 vol.% FEC. Furthermore, the rate capability of P-SP half-cells with FEC addictive was tested at various C-rate of 0.2, 0.5, 0.75, 1, 1.5 and back to 0.2 C, which deliver 1849.8, 1758.0, 1656.0, 1492.7,


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1214.5 and 1702.9 mAh g-1 respectively (Figure 3f). It is worth noting that there still retain 1214.5 mAh g-1 even at a high C-rate of 1.5 C. As the testing C-rate increasing, the rate capability of P-SP anode without FEC reveals poor rate capability and rapidly fading capacity. Figure 3g shows the initial cyclic voltammetry (CV) curves of the P-SP hybrid in the electrolyte without and with 10 vol.% FEC. It is observed that the potential of the irreversible reductive peak slightly shifts to higher position when add 10 vol.% FEC, (0.54V for FEC-added electrolyte and 0.35 V for FEC-free electrolyte). This suggests that the decomposition of FEC is prior to that of main solvents (EC and DEC), forming a SEI layer of the reduction products.

21

Overall, P-SP hybrid anode with 10 vol.% FEC delivers high

reversible capacity and stable cyclic performance.

Figure 4. Characterization of solid electrolyte interphase on P-SP anode. a-d) HRTEM image of P-SP hybrid without additive (a, c) and with 10 vol.% FEC (b, d) after 100 cycles. e-f) Electrochemical impedance spectra (EIS) spectra of P-SP hybrid without additive (e) and with 10 vol.% FEC in the discharged state (f). g) The corresponding interfacial resistance of P-SP hybrid without additive and with 10 vol.% FEC. Inset: equivalent circuit model.

To elucidate the underlying mechanism of FEC additive, HRTEM and EIS spectrum were carried out. Figure 4b shows that the thickness of the SEI formed on P-SP anode without FEC is about 30-50 nm. By contrast, the generated SEI film with 10 vol.% FEC as additive is only about 3-5 nm, which is marked in yellow dashed in Figure 4d.



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Electrochemical impedance spectra of P-SP half-cells were recorded after 1st, 2nd, 5th, 10th, 60th, 100th and 150th cycle. As mentioned in prior section, the SEI film is expected to form on P-SP hybrid after the first cycle. The SEI resistance of the P-SP cell using FEC after the first cycle is 51.7 ohms, much lower than that of the cell without FEC (143.5 ohms). With the increasing cycling, the SEI formed with FEC exhibits a decreased resistance from 51.7 ohms to 17 ohms and finally a relatively stable value around 13.6 ohms. However, the corresponding resistance of the cell with no additive increased by 143.5 ohms at first times from 109.1 ohms at 2nd cycle to 473.5 ohms after 150 cycles. The above electrochemical evidence confirmed that the introduction of FEC is beneficial to obtain a thin, highly conductive and stable SEI on P-SP surface, which enables suppressed consumption of electrolyte, excellent reversibility and high capacity retention for the hybrid anode. To obtain a better understanding of properties of the SEI films, X-ray photoelectron spectroscopy (XPS) was used to acquire chemical information of the components on P-SP hybrid anode. The P 2p spectra of SEI films with and without FEC can be attributed to P-F (137.1 eV), P-O (133.0 eV) and P-P (129.5 eV) bond (Figure 5a). Meanwhile, the peaks of F 1s spectra can be verified as organic fluorides (686.9 eV) and LiF (684.7 eV).21-23, 42 The concentrations of elements in the SEI films are present in Figure 5c. The atom concentration of F in the SEI formed with 10 vol.% FEC is 15.9%, much higher than that in the SEI without FEC (5.46%). In addition, the main fluorides in the SEI film with additive is LiF and the fraction of LiF is up to 91.68%, whereas the value for SEI without additive is only 17.59%. On account of above XPS results, it is evident that the distinct character of the chemical components for the SEI formed with FEC as additive is the high abundance of LiF. As previously reported, FEC is highly preferred to be triggered for ring-opening polymerization, generating thin polymer layer and fluorides.22, 26, 42-43 To understand the decomposition of solvents on anode, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the components in the two electrolytes were calculated on first principle (Figure 5e). The LUMO value of the single FEC is the lowest one among FEC, EC and DEC. The simulating results demonstrates the decomposition of FEC on P-SP hybrid anode in a thermodynamic perspective. The XPS and computational modelling evidences suggest that the preferred decomposition of FEC on P-SP anode results a SEI film which is rich of LiF. The abundance of LiF in SEI is significant to the improved performance of PSP anode: (i) The insulativity of LiF can block electron transfer between anode and electrolyte solution, which will suppress the decomposition of electrolyte components. (ii) LiF exhibit high interfacial energy and low migrating barrier energy of LiF can accelerate the diffusion of Li-ion through the SEI film, resulting in a low interface impedance. (iii) Comparing to organic fluorides, LiF is a thermodynamically stable product under reduction, which results in enhanced stability of the SEI.27, 43-44


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Figure 5. Characterization of chemical components of SEI film. a-b) XPS spectra of P 2p (a) and F 1s (b) for P-SP hybrid electrodes that cycled in 10 vol.% FEC electrolyte and blank electrolyte. c) Concentrations of element (C, O, F, P, Li) in the SEI formed in 10 vol.% FEC electrolyte and blank electrolyte after 100 cycles. d) The ratio between LiF and organic fluorides in SEI. e) LUMO and HOMO values of solvent molecules. f) Schematic diagram for the working mechanism of preparation process and FEC as additive on high-performance P-SP hybrid anode.

Figure 5f summarizes the functions of preparation process and FEC as additive on high-performance P-SP hybrid anode. Incompact structure of P-SP hybrid prepared by simple hand milling results in poor electron conductivity and low reversibility of active material. By contrast, the high energy ball milling process compresses red P and super-P to a compact composite with ultrafine size and improved amorphous degree. There advantages of P-SP hybrid anode fabricated by ball milling not only mitigate the volume expansion during charge-discharge process, but also enhance the electron conductivity and the reversibility of the anode. On account of former results, addition of FEC promotes formation of a stable, thin and highly conductive SEI film on P-SP hybrid anode, inhibiting the continuous



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decomposition of electrolyte and maintaining the integrity of electrode.

Figure 6. Electrochemical performance of P-SP hybrid anode with 10 vol.% FEC electrolyte at 0.5 C. a) Cyclic performance of P-SP hybrid anode using 10 vol.% FEC electrolyte. b-d) Charge-discharge voltage profiles of handgrinding P-SP hybrid anode with blank electrolyte (b), ball-milling P-SP hybrid anode with blank electrolyte (c) and ball-milling P-SP hybrid anode with 10 vol.% FEC (d).

Moreover, LIBs was examined at high C-rate of 0.5 C to meet practical application because P-based anode would tolerate larger volume change and aggravated side reaction when operating at higher rate Remarkably, the P-SP hybrid with 10 vol.% FEC exhibits excellent cyclicality, maintaining about 1087.4 mAh g-1 (61% of the second-cycle capacity) over 300 cycles (Figure 6a). Figures 6b-d show the voltage profiles of the P-SP hybrids prepared by different procedures and evaluated with various electrolytes. It indicates that three kinds of P-SP anodes all deliver an initial discharge capacity about 2221.3 mAh g-1. However, the P-SP anode prepared by hand-grinding reveals a rapid decreased capacity to 128.6 mAh g-1. Owing to the multiple strategies of ball-milling and addition of 10 vol.% FEC, the as prepared P-SP hybrid anode exhibit negligible capacity fading among 300 cycles (only 0.1% per cycle). In summary, we report an associated strategy of ball-milling process and FEC addition to realize practical application of red P as anode for high performance LIBs. P-SP hybrid anode that prepared by ball-milling has nanoscale amorphous structure, which dramatically improves electron conductivity of the whole electrode and suppresses volume expansion of the hybrid anode. In addition, the introduction of FEC facilitates the generation of a robust LIF-rich SEI, accelerating the Li-ion migration and preventing decomposition of electrolyte. Impressively, PSP hybrid anode with 10 vol.% FEC additive delivers remarkable cycling stability and maintains an average


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reversible capacity of 1717.2 mAh g-1 at 0.3 C over 300 cycles. Therefore, we provide a novel perspective to achieve practical P-based anode material through the synergistic effect of preparation procedure and optimization of electrolyte.

EXPERIMENTAL SECTION Preparation of P-SP Hybrid. The hybrid of commercial phosphorus (Aladdin, 98.5%) and Super-P carbon black with a mass ratio of 7:3 was mixed by ball-milling for 16h at a speed of 450 rpm. Samples are sealed in a stainless steel jar under argon atmosphere through ball-milling. Characterization. The structures of the samples were obtained by Rigaku Ultima IV X-ray diffractometer (XRD) with Cu Kα radiation. Raman spectroscopy was performed on a LabRAM HR evolution laser Raman spectrometer with 532 nm laser excitation. The microstructure investigation of the P-SP hybrid was performed via field scanning electron microscopy (Zeiss Gemini SEM 500). TEM images was obtained on JEM-2100F field emission electron microscope. X-ray photoelectrons spectroscopy (XPS) measurements were carried out with a Thermo Fisher ESCALABXi+ X-ray photoelectron spectroscopy using Al Ka radiation (hν =1486.6 eV) under ultrahigh vacuum conditions, the XPS data were collected at multiple locations on (P-SP) hybrid anodes. The calibration of C1s, O1s, F1s and P2p spectra was based on the peak binding energy of C1s, which is 284.5ev. Electrochemical Measurements. The electrochemical performances of the P–SP hybrid and P-SP mixture (hand-grinding) were realized by using 2016 coin-type half-cells assembled with lithium metal as the counter electrode in an argon-filled glovebox. The working electrode was prepared by coating a slurry containing 70 wt.% active materials, 15 wt.% NaCMC binder, and 15 wt.% Super P carbon black on a copper foil substrate, followed by drying in a vacuum oven for 2 h at 100 °C. The active material loading of the electrodes was 0.4-0.6 mg cm-2. The electrolyte used in this work was 1 mol L-1 LiPF6 in an ethylene carbonate (EC)-diethyl carbonate (DEC) solution (1:1 by volume) (DoDoChem, water content is 7.5 ppm), with or without the addition of fluoroethylene carbonate (FEC) (DoDoChem, 99.95% purity, water content is 9.8 ppm). All the cells were assembled in a glove box filled with argon and tested at room temperature. The galvanostatic charge/discharge test was conducted on a LAND battery tester between 0.01 and 2.0 V versus Li/Li+. Cyclic voltammetry measurements were conducted at a scan rate of 0.05 mV s-1 within the range of 0-2 V using a CHI 760E electrochemical workstation. Electrochemical impedance spectra were recorded by Solartron Energy Lab with an amplitude of 5 mV at the frequency range of 10 mHz to 1000 kHz. Electron density calculation37: The DFTB+ module of Materials Studio 8.0 with was employed to calculate the



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electron density of carbon materials with bulk atom and edge atom, applying the non-equilibrium Green's function (NEGF) formalism. We chose CH as Slater-Koster library and set SCC tolerance as 10-5 within 50 cycles. Calculation of LUMO and HOMO energy:21, 27, 45-46 The DMOl3 module of Materials Studio 8.0 with was employed to optimizing the geometry of EC, FEC, and DEC as well as the solvent structure. And electron exchangecorrelation was carried out with Perdew-Bruke-Ernzerhof (PBE) functional of generalized-gradient approximation (GGA). Within the calculation, the basis file was set as 3.5 and the core treatment was set as all-electron numerical basis with DNP set.

Acknowledgements We thank the National Natural Science Foundation of China (No. 51602250, No. 51802256 and No. 21875181) for supporting this work. We also would like to thank Mr. Ren and Miss Liu at Instrument Analysis Center of Xi’an Jiaotong University for their assistance with SEM and XPS analysis.

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