High Anode Performance of in Situ Formed Cu2Sb Nanoparticles

Dec 27, 2016 - Webb , S. A.; Baggetto , L.; Bridges , C. A.; Veith , G. M. The Electrochemical Reactions of Pure Indium with Li and Na: Anomalous Elec...
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High Anode Performance of in Situ Formed Cu2Sb Nanoparticles Integrated on Cu Foil via Replacement Reaction for Sodium-Ion Batteries Liubin Wang, Chenchen Wang, Ning Zhang, Fujun Li,* Fangyi Cheng, and Jun Chen Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and College of Chemistry & Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, PR China S Supporting Information *

ABSTRACT: A scalable and binder-free Cu2Sb/Cu electrode has been synthesized via replacement reaction as an anode for sodiumion batteries (SIBs). The thickness of Cu2Sb formed on Cu foil can be facilely tuned by adjusting the concentration of Sb3+ and reaction time. A high capacity of 318.4 mAh g−1 is obtained at 0.08 A g−1, and a capacity retention of 98.5% is also maintained after 200 cycles at 0.8 A g−1. The reaction mechanism of the as-synthesized Cu2Sb/Cu is investigated by using ex situ X-ray diffraction and transmission electron microscopy. The porous Cu2Sb nanoparticle film and its intrinsic contact with the Cu foil result in the good electrochemical performance. This facile synthetic route for the integrity of Cu2Sb nanoparticles and Cu foil allow for simplifying the preparation processes of SIBs in large-scale applications and is applicable for advanced material design and synthesis.

S

decay. Accordingly, two methods, construction of metal/carbon nanocomposites and formation of nanosized intermetallics with other elements, are applied.22−25 The first method generally leads to low energy density because of the large amount of carbon involved. The latter one incorporates active or inactive components into Sn, Sb, or Bi nanomaterials, etc.; benefits the energy density; and can, to some extent, promote the electrochemical performance.26−30 Typically, nanosized Cu2Sb intermetallics have been demonstrated as anodes in LIBs with improved performance and attempted in SIBs with large capacity and energy density.31−34 The preparation methods for Cu2Sb range from high-energy mechanical milling to pulsed laser deposition, chemical reduction by KBH4, solvothermal synthesis, electrochemical deposition, etc.31,35−37 However, these methods are complicated and difficult to control. Moreover, many procedures are needed for electrode preparation of the resultant Cu2Sb and do not favor large-scale production. Therefore, facile processes for the preparation of Cu2Sb are desirable and of significance for large-scale production with low cost, especially in the field of large-scale electric energy storage.38,39

odium-ion batteries (SIBs), owing to the low cost, natural abundance, and wide distribution of sodium resources, have recently attracted great interest in the field of largescale electric energy storage.1−4 However, compared to the Li+ (0.76 Å) counterpart, the larger size of Na+ (1.02 Å) and the resulting sluggish kinetics make it difficult to develop appropriate host materials for fast and stable Na+ insertion and extraction.5,6 Much progress on cathode materials has been achieved, while suitable anodes remain a challenge.7−9 Graphite, widely used in commercial lithium-ion batteries (LIBs), delivers a small capacity of ∼110 mAh g−1 and hence one-third of the energy density of LIBs.10,11 The promising hard carbon exhibits a higher capacity of ∼300 mAh g−1 with low Na+ insertion potential of around 0.1 V vs Na+/Na, which favors formation of Na dendrites, especially at fast charge.12−14 This would cause severe hazard, considering the more active nature of Na compared to that of Li. Therefore, developing safe anodes with redox potential of >0.3 V,15,16 large capacity, and good electrochemical performance is urgently important for the commercialization of SIBs in the near future. Metals in groups 13−15 of the periodic table have attracted considerable attention because of their high theoretical specific capacities.7,17−21 The main drawback of these metal anodes is the dramatic volume change during Na+ insertion and extraction. It causes pulverization and loss of electrical contact between active materials and conductive carbon and thereby results in capacity © XXXX American Chemical Society

Received: December 2, 2016 Accepted: December 27, 2016

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side of Cu. No cracks are observed, indicative of good flexibility of Cu2Sb/Cu. The thickness of Cu2Sb on Cu foil is estimated to ∼15 μm (mass loading, 0.6 mg cm−2) from the scanning electron microscopy (SEM) image of the cross section of Cu2Sb/Cu in Figure 1d. It can be facilely tuned from 6 to 15 μm by adjusting the solution concentration of Sb3+ and reaction time, as presented in Figure S2. From the SEM image of the top view of Cu2Sb/Cu in Figure 1e, the Cu2Sb is shown to be composed of integrated nanoparticles of 20−50 nm in size. This may be attributed to the unique reaction process, in which the precipitate CuCl was washed away after reaction (Figure S3) to leave behind void spaces between these nanoparticles. The transmission electron microscopy (TEM) image in Figure 1f displays the interconnecting nature of the resultant Cu2Sb nanoparticles. In the high-resolution TEM image of Figure 1g, well-developed lattices with d-spacing of 0.21 nm of a Cu2Sb nanoparticle can be observed, which correspond to the (112) facets. The strong diffraction spots of Cu2Sb in the selected area electron diffraction (SAED) pattern in the inset of Figure 1g is consistent with the XRD patterns in Figure 1a. The Cu2Sb is uniformly formed, as presented in Figure S4. In addition, energy-dispersive X-ray spectroscopy (EDS) mappings of Cu2Sb/Cu in Figure 1h,i indicate uniform distribution of Cu and Sb, which are consistent with the XPS and ICP data. These results imply that the crystallized Cu2Sb/Cu integrity is obtained by the replacement reaction. The as-synthesized Cu2Sb/Cu was cut into electrode pellets and assembled in coin cells with sodium foil as the counter electrode and 1.0 M NaClO4 in propylene carbonate (PC) containing 5 vol % of fluoroethylene carbonate (FEC) as the electrolyte.43−45 Figure 2a shows the cyclic voltammetry (CV) curves of the Cu2Sb/Cu for the first 8 cycles between 0.05 and 2.0 V at a scan rate of 0.1 mV s−1. In the first cathodic process, a sharp reduction peak below 0.25 V corresponds to Na+ insertion into Cu2Sb nanoparticles and formation of a solid-electrolyte interphase (SEI) layer.46 The electrode is then gradually activated in the following cycle and presents a reduction peak at a little positive potential of 0.25 V. Beyond the first two cathodic scans, three typical reduction peaks appear. They are ascribed to the stepwise Na alloying with Cu2Sb to form NaxCu(2−2x/3)Sb (x ≤ 3) and Na3Sb.43,44 The overlapped sharp oxidation peak around 0.75 V and small shoulder peak around 0.9 V in the anodic scans correspond to the desodiation reactions of Na3Sb and NaxCu(2−2x/3)Sb, respectively. Figure 2b shows the discharge−charge profiles of the Cu2Sb/Cu electrode at 0.2 A g−1, which are in good agreement with the CV results. The initial sodiation process presents a discharge capacity of 407.6 mAh g−1, leading to a Coulombic efficiency of 72.3%. The irreversible capacity mainly stems from electrolyte decomposition for the formation of SEI. After the first cycle, stable discharge−charge capacities of ∼300 mAh g−1 are obtained, suggesting good cycling stability. Rate performance of the Cu2Sb/Cu electrode is evaluated at varied current density from 0.08 to 8 A g−1. The electrode delivers a reversible capacity of 318.4, 288.2, 279.5, 271.9, 267.9, 263.5, and 256.1 mAh g−1 in the second cycle at the current density of 0.08, 0.2, 0.4, 0.8, 2, 4, and 8 A g−1, respectively, as shown in the Figure 2c. The corresponding discharge−charge profiles are provided in Figure S5. When the current density is set back to 0.08 A g−1, the electrode regains a charge capacity of 312.8 mAh g−1 with extended cycling stability. Remarkably, a high discharge capacity of ∼270 mAh g−1 at 0.8 A g−1 is sustained with a Coulombic efficiency of ∼98.5% for 200 cycles in Figure 2d. This is in sharp contrast with the quickly

Herein, we report a new one-step method for scalable fabrication of a binder-free and flexible Cu2Sb/Cu electrode for high-performance SIBs. The Cu2Sb nanoparticles are in situ formed via the replacement reaction between Cu foil and Sb3+ in ethanol at room temperature, the thickness of which can be facilely tuned by either concentration of Sb3+ or reaction time. The resultant Cu2Sb/Cu integrity can be directly used as an anode for a SIB without addition of conductive agent and polymer binder. It shows remarkable cycling stability and rate capability. This facile synthetic route and good electrochemical performance of the integrity of Cu2Sb nanoparticles and Cu foil allow for simplifying the preparation processes of SIBs and merit it a promising anode in large-scale application. In principle, a reactant possessing lower redox potential can replace another reactant with higher redox potential in solutions. The redox potential of Cu+/Cu versus Sb3+/Sb (0.137 vs 0.241 V against normal hydrogen electrode, NHE) ensures the replacement of Sb3+ by Cu. The fabrication process of the Cu2Sb/Cu anode is schematically illustrated in Scheme 1. Briefly, Scheme 1. Schematic Illustration of the Formation Mechanism of Cu2Sb Nanoparticles Integrated on Cu Foil

a Cu foil is immersed into an ethanol solution of SbCl3, where Sb3+ is reduced to Sb, and it immediately forms intermetallics Cu2Sb with Cu.40,41 At the same time, Cu is oxidized to Cu+, which reacts with Cl− to generate washable CuCl precipitate. The formation mechanism is depicted in Scheme 1. This process can facilely produce Cu2Sb nanoparticles integrated onto both sides of the Cu substrate in large scale, which is important for industrial application. The in situ formed Cu2Sb nanoparticles integrate with the substrate of Cu and can be directly used as anodes of SIBs without further treatment. The as-obtained Cu2Sb/Cu was characterized by X-ray diffraction (XRD). As shown in Figure 1a, all the diffraction peaks are consistent with JCPDS No. 65-2815 of Cu2Sb intermetallics, besides two characteristic peaks of the pristine Cu. It shows that the resultant Cu2Sb is well-crystallized and belongs to the tetragonal P4/nnm space group. Raman spectra of Cu2Sb as well as commercial Sb and Cu are shown in Figure 1b. Two peaks at 115.0 and 177.7 cm−1 are observed, which correspond to the rocking and stretching vibrations of Cu−Sb and Sb−Sb bonds or their combination.34,42 In addition, the chemical composition of Cu2Sb was examined by X-ray photoelectron spectroscopy (XPS), as shown in Figure S1. The atomic ratio of Cu and Sb is estimated to be around 1.8 from the spectra of Cu 2p3 and Sb 3d5, which is in agreement with ∼2.0 by the inductively coupled plasma analysis (ICP), as listed in Table S1. These confirm the successful synthesis of Cu 2 Sb intermetallics. The Cu2Sb/Cu is twisted in the photograph in Figure 1c, in which the black side with Cu2Sb is in contrast with the yellow 257

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Figure 1. (a) XRD patterns of the as-synthesized Cu2Sb/Cu and pristine Cu foil; (b) Raman spectra of Cu2Sb/Cu, Cu, and Sb; (c) photograph of the twisted Cu2Sb/Cu; (d) SEM image of the cross section of Cu2Sb/Cu; (e) SEM image of the top view of Cu2Sb/Cu; (f) TEM image of Cu2Sb; (g) high-resolution TEM image of Cu2Sb nanoparticle and selected-area electron diffraction pattern (inset); and EDS mappings of selected area, (h) Cu and (i) Sb.

Figure 2. (a) CV curves at 0.1 mV s−1 between 0.05 and 2.0 V; (b) discharge−charge profiles at 0.2 A g−1; (c) rate capability at different current densities from 0.08 to 8 A g−1; and (d) cycling stability at 0.8 A g−1 of the Cu2Sb/Cu electrode.

decaying capacities of the ball-milled Sb even at a small current density of 0.2 A g−1 in Figure S6.45 These indicate the advantage of the unique structure of the Cu2Sb/Cu electrode and the resultant excellent rate capability and cycling performance. Figure 3a exhibits the CV responses of the Cu2Sb/Cu electrode at varied scan rates from 0.1 to 1.0 mV s−1 between

0.05 and 2.0 V. The peak currents increase with scan rates, which is consistent with the discharge−charge profiles at different current densities in Figure S5. However, the peak current (i) is not proportional to the square root of scan rate (υ). This implies that the discharge−charge process is dominated by nonfaradaic and faradaic behaviors.47,48 Alternatively, the relationship 258

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Figure 3. (a) CV curves at different scan rates; (b) pseudocapacitive contributions of peaks 1−3 at different scan rates; (c) EIS spectra of Cu2Sb/ Cu electrode at open-circuit voltage before test and after first, second, 51st, and 101st charge; and (d) magnified spectra in high-frequency region of panel c.

The sodiation and desodiation in the discharging and charging processes of Cu2Sb are monitored by ex situ XRD and TEM. Figure 4a shows the XRD patterns of the Cu2Sb/Cu electrode at both discharge and charge states. After the electrode is discharged to 0.05 V, the diffraction peaks of Cu2Sb disappear

between peak current (i) and scan rate (υ) can be described in the following equations:

i = aυ b

(1)

log(i) = b log(υ) + log(a)

(2)

where a and b are two variables and b determines the sodiation and desodiation behaviors. If b equals 1, the electrochemical system is controlled by pseudocapacitance; as b is 0.5, the discharge−charge process is dominated by Na+ ion diffusion. The linear relationships of log(i) and log(υ) for peaks 1, 2, and 3 are shown in Figure S7a. The b values of peaks 1, 2, and 3 are estimated to be 0.92, 0.96, and 0.69, respectively, suggesting that the electrochemical reactions of Cu2Sb are partially controlled by pseudocapacitive behavior. This can lead to high-rate performance and good cyclic stability, as displayed in Figure 2. The contributions of pseudocapacitance and intercalation−alloying can be quantitatively analyzed by eq 3 i = k1υ + k 2υ0.5

(3)

where k1υ and k2υ0.5 represent the pseudocapacitive and intercalation−alloying contribution, respectively.47,48 The constants k1 and k2 can be obtained by plotting iυ−0.5 against υ0.5, and the results for peaks 1−3 are shown in Figure S7b. The pseudocapacitive contributions for peaks 1−3 at different scan rates can be quantified and are shown in Figure 3b. The pseudocapacitive domination may be partially responsible for the excellent electrochemical performance of the Cu2Sb/Cu electrode. Furthermore, the electrochemical impedance spectroscopy (EIS) of the charged Cu2Sb/Cu electrode during cycles was obtained at open-circuit voltage after a 5 h rest, which is presented in Figure 3c,d. It can be found that the resistance of the electrode becomes stable after initial activation, which is consistent with the discharge−charge profiles in Figure 2b.

Figure 4. (a) Ex situ XRD patterns at different states; HRTEM images and SAED patterns of the Cu2Sb/Cu electrode discharged to 0.05 V (b and d) and charged to 2.0 V (c and e), respectively. 259

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Figure 5. TEM images of Cu2Sb/Cu electrode at (a) discharge and (b) charge state; (c) schematic diagram of Cu2Sb/Cu sodiation/desodiation.

Figure 6. (a) Schematic illustration of the sodium-ion battery with the Na3V2(PO4)3/Cu2Sb couple; (b) typical galvanostatic discharge−charge profiles of the Na3V2(PO4)3/Cu2Sb full cell at 0.25 A g−1 (based on the anode) and a “NKU” logo powered by the full cell (inset).

Cu 2Sb + x Na + + x e− ↔ NaxCu(2 − 2x /3)Sb + (2x /3)Cu

with the presence of Na3Sb as evidenced at 18.6°, 21.3°, 34.2°, and 38.5°, and Cu at 43.3°. The other two diffraction peaks at 21.4° and 23.7° are derived from parafilm, which is used to protect the electrode from air. When the electrode is charged to 2.0 V, the diffraction peaks of Cu2Sb appear again with the absence of the characteristic diffraction peaks of Na3Sb. The direct observation of Cu and Na3Sb can be realized by the welldeveloped facets with d-spacings of 0.20 and 0.27 nm in Figure 4b, respectively. When the electrode is charged to 2.0 V, the dspacings of 0.21 and 0.28 nm, which belong to Cu2Sb, can be observed.49 This reversible behavior of sodiation and desodiation of Cu2Sb is also demonstrated by the SAED patterns in Figure 4d,e, in which the characteristic diffraction spots are in good accordance with the XRD patterns in Figure 4a. Based on the above analyses, the reaction mechanism can be concluded as follows:

(4)

NaxCu(2 − 2x /3)Sb + (3 − x)Na + + (3 − x)e− ↔ Na3Sb + (2 − 2x /3)Cu

(5)

The discharged and charged Cu2Sb/Cu electrodes are shown in the TEM images of panels a and b of Figure 5, respectively. The Cu2Sb 20−50 nm in size in Figure 1g changes to Na3Sb nanoparticles of several nanometers upon discharging, and they are connected by the in situ formed Cu nanoparticles. When charged to 2.0 V, the Cu2Sb nanoparticles can be reversibly generated, as evidenced by the SAED patterns in Figure 4e. Theoretically, the volume per unit formula of Cu (Fm3̅m), Cu2Sb (P4/nmm) and Na3Sb (P63/mmc) are 11.7, 48.8, and 118.6 Å3, respectively. The volume change of Cu2Sb during discharging 260

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and charging is 191%, which is smaller than that of the pure Sb (293%).26 This unique structure is effective to buffer the dramatic volume change for Sb-based material during sodiation− desodiation. Furthermore, the morphologies of the Cu2Sb/Cu electrode during cycles are captured by SEM in Figure S8. It can be found that the porous nature of the pristine Cu2Sb is always sustained, which can efficiently accommodate volume changes upon Na+ insertion and extraction, and the integrity of Cu2Sb is preserved during 100 cycles of discharge and charge without showing any cracks. This is attributed to the applied in situ replacement reaction for Cu2Sb/Cu and the electron conduction matrix provided by Cu. The microstructure evolution of the Cu2Sb/Cu electrode during sodiation and desodiation is schematically shown in Figure 5c. The nanoporous integrity structure not only benefits penetration of electrolyte into the whole electrode but also favors diffusion of Na+ ions into and away from the active material. The in situ formed Cu matrix with intrinsic contact with the Sb-based nanoparticles enables fast electron transport and results in good rate capability and high reversible capacity.39,50 Besides, the void spaces between Cu2Sb nanoparticles as observed in Figure 1e,f can effectively accommodate volume change during discharge and charge. The structural advantage of the Cu2Sb/Cu integrity electrode obtained by the replacement reaction ensures high reversible electrochemical performance. The outstanding performance of the Na/Cu2Sb half cell warrants further evaluation of its potential application in a full cell. As shown in Figure 6a, the full cell is constructed with Cu2Sb/Cu as anode and Na3V2(PO4)3 as cathode. The synthesized Na3V2(PO4)3 cathode delivers a stable discharge− charge capacity of ∼96 mAh g−1 for 25 cycles at 20 mA g−1 in Figure S9. The discharge and charge profiles of the full cell are exhibited in Figure 6b with excessive Na3V2(PO4)3 between 2.2 and 3.3 V, offering an average discharge voltage of 2.7 V. The discharge capacity remains 211.3 mAh g−1 after 14 cycles, corresponding to capacity retention of 85.7% (Figure S10). As a demonstration, an “NKU” logo is successfully powered by the flexible full cell in the inset of Figure 6b. These results indicate the potential promising application of the Cu2Sb/Cu anode via the facile replacement reaction for SIBs. The binder-free, flexible Cu2Sb/Cu integrated electrode was successfully prepared via a facile replacement reaction by immersing a Cu foil into an ethanol solution of SbCl3. The resultant electrode is featured as a porous integrity of Cu2Sb nanoparticles on Cu foil, the thickness of which can be tuned by the concentration of Sb3+ and reaction time. It presents excellent rate capability and cycling performance with high capacities of 318.4 and 256 mAh g−1 at 0.08 and 8 A g−1, respectively, and capacity retention of 98.5% after 200 cycles at 0.8 A g−1. A full cell is constructed to present a capacity of ∼250 mAh g−1 based on the anode. The good electrochemical performance is enabled by the structural uniqueness of the nanoporous Cu2Sb/Cu integrity, which can efficiently accommodate its volume change during discharge−charge and preserve fast electron conduction and ion transport. These results demonstrate the promise of the potential application of the integrated electrodes in SIBs. The facile scalable synthetic method will be applicable in advanced material design and synthesis.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00649. XPS spectra of Cu2Sb/Cu, SEM images of Cu foil, cross section and morphology evolution during 100 cycles of Cu2Sb/Cu and ball-milled Sb, XRD patterns of the product during synthesis, charge−discharge curves of the Cu2Sb/Cu anode at different current densities, linear relationship of log(i) and log(υ), plots of iυ−0.5 against υ0.5, electrchemical performance of Na3V2(PO4)3, cycling performance of the Na+ full battery, and the ICP results (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fujun Li: 0000-0002-1298-0267 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Financial support from Ministry of Science and Technology of China (2016YFB0901500) and National Natural Science Foundation of China with Grants 21603108 and 51671107 is acknowledged.



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DOI: 10.1021/acsenergylett.6b00649 ACS Energy Lett. 2017, 2, 256−262