High-Performance Aluminum-Ion Battery with CuS@C Microsphere

Dec 15, 2016 - (24) In earlier research, aluminum was used as an anode for Al-air batteries on account of its high theoretical capacity and overall sp...
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High-Performance Aluminum-Ion Battery with CuS@C Microsphere Composite Cathode Shuai Wang,† Shuqiang Jiao,*,† Junxiang Wang,† Hao-Sen Chen,*,‡ Donghua Tian,† Haiping Lei,† and Dai-Ning Fang‡ †

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, People’s Republic of China ‡ Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, People’s Republic of China S Supporting Information *

ABSTRACT: On the basis of low-cost, rich resources, and safety performance, aluminum-ion batteries have been regarded as a promising candidate for next-generation energy storage batteries in large-scale energy applications. A rechargeable aluminum-ion battery has been fabricated based on a 3D hierarchical copper sulfide (CuS) microsphere composed of nanoflakes as cathode material and room-temperature ionic liquid containing AlCl3 and 1ethyl-3-methylimidazolium chloride ([EMIm]Cl) as electrolyte. The aluminum-ion battery with a microsphere electrode exhibits a high average discharge voltage of ∼1.0 V vs Al/AlCl4−, reversible specific capacity of about 90 mA h g−1 at 20 mA g−1, and good cyclability of nearly 100% Coulombic efficiency after 100 cycles. Such remarkable electrochemical performance is attributed to the well-defined nanostructure of the cathode material facilitating the electron and ion transfer, especially for chloroaluminate ions with large size, which is desirable for aluminum-ion battery applications. KEYWORDS: copper sulfide, microsphere, cathode material, aluminum-ion battery

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involves three-electron transfer during the electrochemical reaction, which offers higher energy density and capacities.24 In earlier research, aluminum was used as an anode for Al-air batteries on account of its high theoretical capacity and overall specific energy.28−30 It is worth noting that the role of electrolyte is critical in the battery system.31,32 Unfortunately, the large overpotentials on the aluminum and the air electrodes and the decomposition of the aqueous electrolyte during the discharge reaction lead to poorer performance of the Al-air batteries than theoretical expectation,33 and an aqueous electrolyte results in high corrosion in combination with hydrogen evolution on the aluminum electrode, which also leads to a serious decline in the efficiency.34 Therefore, a nonaqueous electrolyte has been widely considered for Albased batteries. It is known that the cathode material is the key component in building battery systems with high power and energy densities for large-scale applications.35 Over the past years, some cathode materials have been proposed for Al-based batteries in

owadays, due to the increasing concerns about fossil fuel depletion, renewable energy sources such as solar and wind power have been widely developed for grid applications.1−4 However, these renewable energy sources are intermittent, requiring integration with large-scale energy storage systems.5 Therefore, developing new types of energy storage with high energy density and long lifetime has become a significant research topic. Electrochemical batteries can efficiently store and release electricity through chemical reactions and show the most promise in portable electronic devices. Lithium-ion batteries (LIBs) are now the most widely used type of electrochemical battery for portable electronic devices.6−13 However, there are some critical drawbacks limiting their application, such as stability and safety problems, but more seriously, the rapidly increasing demand for LIBs faces the challenge of shortages in lithium resources.14,15 Therefore, Na-ion,16−18 Mg-ion,19−21 Ca-ion,22,23 and Alion24−27 batteries are being considered as alternative candidates due to their low cost and abundance. Aluminum-ion batteries (AIBs) have recently attracted significant interest because of aluminum being the most abundant metal and the third most abundant element in the earth’s crust. In addition, an Al-based redox couple usually © 2016 American Chemical Society

Received: September 24, 2016 Accepted: December 15, 2016 Published: December 15, 2016 469

DOI: 10.1021/acsnano.6b06446 ACS Nano 2017, 11, 469−477

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Figure 1. (a) XRD pattern of the as-prepared CuS. (b) Raman spectrum of the as-prepared CuS. (c) Schematic representation of CuS crystal structures, shown perpendicular to different axes. Brownish-red and yellow spheres represent copper and sulfur atoms, respectively. (d) Schematic illustration of the solvothermal process for the formation of CuS.

Figure 2. Morphologies and compositions of the as-prepared 3D hierarchical nanostructured CuS microspheres. (a−c) FE-SEM images at different magnification, (d, e) elemental mapping images of Cu and S, (f, g) TEM images, (h−j) HRTEM images and corresponding SAED pattern, and (k) representative EDS spectrum of the as-prepared CuS.

nonaqueous eletrolytes, such as V2O5,24 fluorinated natural graphite,36 and conductive polymers.37 However, these materials exhibited a very low discharge voltage of about 0.55−0.65 V vs Al/Al3+ and poor cyclability. Very recently, our group at USTB and Dai’s group at Stanford University reported that chloroaluminate ions (AlCl4−) can be deintercalated from the graphite cathodes in a nonflammable ionic liquid electrolyte at a higher voltage plateau around 2.0 V vs Al3+/Al.26,27 Our recent aluminum-ion battery used Ni3S2 microflakes as cathode and exhibited a relative low capacity, which is due to the different crystal structures, and Ni exists in Ni3S2 in different valence states.38

In this study, we successfully fabricated 3D hierarchical nanostructured single-crystalline copper sulfide (CuS) microspheres with diameters of 3−5 μm comprising nanoflakes via a facile method. The obtained CuS microspheres exhibit high storage capacity and a high cyclic stability as cathode material for AIBs in an ionic liquid electrolyte. Benefiting from both the special crystalline structure and uniform nanoflakes, the asprepared CuS facilitates electron and ion transfer during the discharge and charge reactions. Furthermore, acetylene black nanoparticles uniformly dispersed on the surface of CuS nanoflakes, which enhances the charge transfer. These properties significantly enhance the performance of the aluminum-ion battery. The battery delivers a specific capacity 470

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ACS Nano of about 90 mA h g−1 with nearly 100% Coulombic efficiency after 100 cycles at a current density of 20 mA g−1.

which clearly indicates the presence of Cu and S elements again. To further explore the chemical binding states and elemental composition of the as-prepared CuS microspheres, the X-ray photoelectron spectroscopy (XPS) spectra were recorded, and the results manipulated keeping the C 1s peak as reference (Supporting Information, Figure S1). Figure S1a shows the wide survey XPS spectrum of the as-prepared CuS microspheres. No obvious peaks for other elements or impurities were observed in the survey spectrum. The C 1s peak is given in Figure S1b, and two Gaussian peaks centered at 282.3 and 286.1 eV could be observed. The C−C bond of the sp2 carbon atom could be assigned to 282.3 eV, and the other peak (286.1 eV) is due to C−O.43,44 Figure S1c shows the XPS spectrum of Cu 2p for CuS microspheres. The two main peaks located at 932.5 and 952.4 eV were assigned to Cu 2p3/2 and Cu 2p1/2, respectively. Moreover, there was a shakeup satellite peak at around 943.0 eV (rectangle marked in Supporting Information Figure S1c), suggesting the presence of the paramagnetic chemical state of Cu2+.45 In Figure S1d, a doublet peak at around 163.0 eV can be indexed to S 2p3/2 and S 2p1/2 binding energies, respectively, and an additional small peak at around 161.2 eV is attributed to CuS.46 To demonstrate the advantages of CuS microspheres, we evaluate their performance as cathode material in AIBs. Figure S2 shows the morphologies and compositions of CuS@C microsphere composites. The specific element contents are shown in Table S1. It can be seen that the mass ratio of C can reach about 30%, which facilitates the conductivity of the CuS@C microsphere composite electrode. More interestingly, due to the excellent nature of the well-dispersed superstructures, such integrated CuS@C-based electrode can be directly used as a cathode for aluminum-ion batteries, which exhibit excellent conductivity and stable Al-ion storage performance. Figure S3 shows a schematic diagram of the soft package aluminum-ion battery and the voltage measurements of an aluminum-ion battery. Clearly, the fully charged batteries in series can light the light emitting diode (LED) lamps, and the voltage of the batteries in series can reach about 2.45 V. To evaluate the electrochemical properties of CuS microspheres, further galvanostatic discharge/charge measurements were performed. Figure 3a shows the initial discharge/charge curves at a current of 20 mA g−1. Obviously, there is a long and glossy voltage plateau (∼1.9 V vs Al/AlCl4−) in the initial charge process, which could be related to the oxidation of S2−. In our previous work,38 a similar phenomenon also occurred at the first charge step. The peak of S6+ around 168.9 eV occurred or increased compared with the original materials, which is due to the oxidation of S2− during the first cycle of charging under a high potential. In addition, the initial cycle has two discharge voltage plateaus (∼1.0 and 0.4 V vs Al/AlCl4−) with the capacity of about 240 mA h g−1. The theoretical capacity of the battery can be calculated by Faraday’s law, according to the following equation:

RESULTS AND DISCUSSION The phase compositions and crystal structures of the asprepared CuS were studied with X-ray diffraction (XRD), and the resulting pattern is given in Figure 1a. The well-defined peaks in the XRD pattern indicate the formation of pure hexagonal phase CuS with high crystallinity.39 All the diffraction peaks can be perfectly indexed as hexagonal CuS (space group: P63/mmc) with lattice parameters a = 3.802 Å and c = 16.43 Å, which are well matched with the standard values (JCPDS No. 03-1090). The absence of any other peaks indicates the high purity of products. A Raman spectrum of the as-prepared CuS is shown in Figure 1b. A strong and sharp band at 473.6 cm−1 probably originates from the lattice vibration. Figure 1c shows the crystal structure of CuS. In this structure, both copper and sulfur atoms adopt two distinct coordination environments:40 two-thirds of the copper atoms are at the center of a triangle of sulfur atoms, and the remaining one-third of copper atoms is at the center of the layer. The designed route for the preparation of CuS is illustrated in Figure 1d. Generally, the growth process of crystals includes two stages: an initial stage and a subsequent growth stage.41 At the initial stage, CuS nuclei are formed in ethylene glycol (EG) solution, and Tu and Cu2+ can form a coordination compound [Cu(Tu)m(EG)n]2+ while stirring and heating. In the subsequent step, the coordination compound decomposes into CuS through solvothermal treatment, and these nuclei preferentially are grown in the same direction. Finally, the as-prepared CuS was obtained. Under the present solvothermal conditions, the following reactions may occur:42 Cu 2 + + mTu + n EG → [Cu(Tu)m (EG)n ]2 +

(1)

[Cu(Tu)m (EG)n ]2 + → CuS↓

(2)

Field emission scanning electron microscope (FE-SEM) was used to study the morphologies of the as-prepared CuS. The SEM images shown in Figure 2a−c indicate that the asprepared CuS is composed of well-dispersed superstructures with a well-defined uniform 3D hierarchical nanostructured morphology. Interestingly, these superstructures are in fact built from nanoplates with a mean edge length of about 1 μm and an average thickness of about 13 nm. The elemental mapping images shown in Figure 2d,e indicate the uniform distribution of Cu and S all over CuS microspheres. In order to reveal the fine microstructure, the as-prepared CuS microspheres were characterized using a transmission electron microscope (TEM). Figure 2f,g show the TEM images, and it can be clearly seen that CuS microspheres are highly dispersed, which is in accordance with the result of SEM images. Further microstructure information is obtained from the high-resolution transmission electron microscope (HRTEM) images and the selected area electron diffraction (SAED) pattern (Figure 2h− j). The HRTEM images show an interplanar spacing of 3.04 Å, which corresponds to the d spacing for the (102) planes of the CuS crystal. The corresponding SAED pattern reveals the single-crystalline nature of CuS, and the SAED ring patterns can be readily matched with the (102) and (110) planes, which agrees well with the XRD data. Energy-dispersive X-ray spectroscopy (EDS) spectra of CuS microspheres were collected to obtain information on the local chemical composition and uniformity of the as-prepared materials,

C0 = nF /3.6M mA h g −1

(3)

In this equation, n is the number of transferred electrons, F is the Faraday constant, and M is the molecular weight of the material. After the discharge process to 0.1 V vs Al/AlCl4−, the discharge theoretical capacity is about 280.3 mA h g−1. However, the practical capacity is about 240 mA h g−1 in the initial cycle (Figure 3a), which is due to the incomplete 471

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impedance changes little.48 The cycling performance at various current densities over 100 cycles has also been demonstrated (Figure S5d). Although it delivered an excellent initial discharge capacity of about 150 mA h g−1, the electrode showed poor discharge capacity at a high current density (150 mA g−1) after 100 cycles. More importantly, after nearly 10 cycles, the smaller the current density, the better the cycle stability. Cyclic voltammogram (CV) curves recorded in a voltage range of 0.1−2.0 V (vs Al/AlCl4−) at a scan rate of 5 mV s−1 are shown in Figure S6a. It is found that the first scan has a high oxidation peak at about 1.9 V vs Al/AlCl4−, which is consistent with the initial charge curve in Figure 3a. In the subsequent scans, two oxidation peaks are observed: a significant peak at about 1.50 V vs Al/AlCl4− (O) and a tiny peak at 0.85 V vs Al/ AlCl4− (O′). The intensity of peak (O′) gradually decreases. In contrast, there are also two peaks present around 0.90 V vs Al/ AlCl4− (R) and 0.35 V vs Al/AlCl4− (R′) in the reduction process. The peak (R′) also gradually disappears in the subsequent scans, which can be attributed to the formation of an SEI layer on the active material surface in the first few cycles.49 The result agrees well with the galvanostatic discharge/charge curves described above. Figure S6b shows CVs of the CuS electrode, C electrode, and Ta electrode, respectively. It can be seen that there are no oxidation or reduction peaks in the voltage range of 0.1−2.0 V (vs Al/ AlCl4−), and the second discharge/charge curves of the C electrode and Ta electrode were measured also in the same voltage range (Figure S6c). Obviously, the C and Ta provide only very small amounts of capacity (3−6 mA h g−1) during the discharge and charge process. Raman spectra have been measured in order to extract information on the structure of the ionic liquid electrolyte, their composition, and decomposition reactions during the discharge and charge process (Figure 4). They reveal that both AlCl4−

Figure 3. Discharge−charge curves and cycling performance of a CuS@C microsphere cathode. (a) Initial discharge/charge curves at a current density of 20 mA g−1. (b) Typical discharge/charge curves in the voltage range of 0.1−2.0 V vs Al/AlCl4− from the second to the fourth cycle. (c) Cycling performance with Coulombic efficiency at a current density of 20 mA g−1.

reactions and the impact of internal resistance. The subsequent cycles from the second to the fourth cycle are shown in Figure 3b. Interestingly, it can be found that two charge plateaus exist at about 0.8 and 1.5 V vs Al/AlCl4−, and in the second to fourth cycle, the discharge capacities of the electrode decreased to 214.6, 197.8, and 169.1 mA h g−1, respectively. The irreversible capacity loss for the subsequent cycles can be attributed to the formation of a solid electrolyte interface (SEI) layer and possible incomplete reactions.47 Figure 3c shows the cycling performance and corresponding Coulombic efficiency at a current of 20 mA g−1. The capacities decreased gradually with an increment in cycle number, and the discharge and charge capacities at the 100th cycle are both about 90 mA h g−1, which corresponds to a Coulombic efficiency of nearly 100%. Especially, the rapid capacity decay in the first 10 cycles could be attributed to the formation of an SEI layer and the decomposition of the liquid electrolyte or possible incomplete reactions. The detailed reasons will be discussed in the following. Even after 300 cycles, the discharge and charge capacities are both about 80 mA h g−1 (Figure S4). The issue of capacity decay of CuS microspheres should be addressed and solved in future work for a long stability. Figure S5a,b also show the electrochemical performances of CuS microspheres. Typical galvanostatic curves of the electrode at a high current density (100 mA g−1) exhibit the same voltage plateaus as Figure 3b. Moreover, the second discharge and charge capacities are 161.3 and 174.5 mA h g−1, respectively. The corresponding cycling performance is displayed in Figure S5b. After initial equilibrium cycles, the electrode still possesses a discharge capacity of about 70 mA h g−1 after 100 cycles. Figure S5c displays the galvanostatic discharge/charge curves of the electrode evaluated at various current densities (from 20 to 150 mA g−1). With an increase in the current density, the specific capacity declines. In addition, the discharge/charge voltage plateaus are essentially the same at various current densities, from which it can be deduced that the charge-transfer

Figure 4. Raman spectra of the AlCl3/[EMIm]Cl ionic liquid electrolyte with a mole ratio of AlCl3/[EMIm]Cl = 1.3. (a) Fully charged through the 10th cycle. (b) Fully discharged through the 10th cycle. (c) Original ionic liquid electrolyte.

and Al2Cl7− anions were present in the ionic liquid with a molar ratio of 1.3:1 before the tests (Figure 4c). Most studies of the AlCl4− anion in an ionic liquid have been interpreted in terms of the ion possessing tetrahedral (Td) symmetry, while a bent Al−Cl−Al bridge has been proposed for Al2Cl7−.50 Figure 4a shows the Raman spectrum of a fully charged ionic liquid electrolyte through the 10th cycle. (The battery system could 472

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ACS Nano reach equilibrium after ∼10 cycles according to Figure 3c.) Clearly, two bands of the Al2Cl7− anion (311.6 and 437.1 cm−1) show a definite decrease and eventually disappear, which is due to the dissociation of the complex anion (Al2Cl7−) during the charge process with the increase in voltage. It also provided a large number of simple ions to the process of redox reactions. During the discharge process, the amount of Al2Cl7− is not completely recovered compared to the original ionic liquid electrolyte when the battery reaches an equilibrium state through the ∼10th cycle (Figure 4b). However, there were still a large number of AlCl4− and Al2Cl7− anions in the electrolyte system to balance the reactions accompanied by the rapid capacity decay in the first 10 cycles, which is consistent with cycling tests in Figure 3c and Figure S5d. We further explored the chemical binding states of the electrodes during the discharge and charge process. The XPS spectrum in the Cu 2p region after the discharge process shows two strong peaks (Figure 5a). Interestingly, there is no shakeup

the discharge and charge process. It can be seen that the concentration of Al3+ after the discharge process was significantly higher than the concentration of Al3+ after the charge process, which suggests the formation of large amounts of Al2S3 after the discharge process. Additionally, to further explore the reaction mechanisms, XRD spectra of the cathode are observed after discharge and charge. When the battery is discharged to 0.1 V, mainly Cu2S and Al2S3 are found to exist in the cathode (Figure 5e), and when the battery is charged to 2.0 V, the vast majority of the cathode is transformed into CuS again (Figure 5f). In order to probe the rapid capacity decay phenomenon in the first few cycles, Figure S8a shows the 20th discharge/charge curves in the voltage range of 0.1−2.0 V vs Al/AlCl4− at different current densities when the battery system achieved a balanced state. It can be seen that the discharge capacities are all about 100 mA h g−1 in the 20th cycle, and the discharge curves are substantially coincident with each other. Figure S8b shows Raman spectra of acetylene black before and through the 20th cycle, which exhibit two peaks at ∼1360 cm−1 (D-band) and 1580 cm−1 (G-band), and the two peaks are attributed to the presence of in-plane vibration of sp2-bonded carbon atoms and the vibrational modes from sp3-bonded carbon atoms.51,52 Clearly, the value of ID/IG changed due to the occurrence of a defect position during the discharge and charge process. Figure S8c shows Raman spectra of active materials (CuS) before and through the 20th cycle. As known about CuS, the peak of the Raman spectrum shows up at 473.56 cm−1. It presents the S−S stretching of the surface, and the shifting to 470.57 cm−1 after the discharge process was reported to be related with the transformation process from CuS to Cu2S (Figure S8d).53 After the charge process, the peak of CuS cannot shift to its original position completely, which means the Cu−S or S−S bond length changes. Additionally, it can be seen from Figure 4 that the two bands of the Al2Cl7− anion show a definite decrease and eventually disappear, which is due to the dissociation of the complex anion during the charge process. However, there are still sufficient anions in the electrolyte system to balance the reaction, so the discharge capacity can reach a steady state after a few cycles. The discharge and charge processes are accompanied by some side reactions (Figure 5e,f). Therefore, the rapid capacity decay could be attributed to the formation of an SEI layer and the decomposition of the liquid electrolyte or some incomplete side reactions. Figure S9 shows the morphologies and compositions of the electrode after the discharge process. The low-resolution TEM images, as shown in Figure S9a,b, clearly indicate the many multiple microspheres united together due to the presence of a glass fiber (GF/D) binder. Further microstructure information on the products after the discharge process is obtained from the HRTEM images (Figure S9c). Obviously, the microstructure is rumpled compared with the microstructure of CuS (Figure 2h,i). The SAED pattern of acetylene black is shown in Figure S9d, and the SAED ring patterns can be well-matched with the (110), (100), and (002) planes.54 The representative EDS spectrum of the electrode after the discharge process is shown in Figure S9e, which clearly indicates the presence of C, Cu, S, Al, and Cl elements. In addition, there is a small amount of O element due to the oxidation of the electrode in air. As Figure S10 shows, the self-discharge behavior of the aluminum-ion battery exhibits a noticeable similarity at different current densities. The batteries were rested for 12 h after fully charging to 2.0 V vs Al/AlCl4− (second cycle), and the voltage will

Figure 5. XPS and XRD tests of the cathode materials through the 10th cycle. (a, b) XPS spectra of Cu 2p and S 2p, discharged: 0.1 V vs Al/AlCl4−. (c, d) XPS spectra of Cu 2p and S 2p, charged: 2.0 V vs Al/AlCl4−. (e) XRD of the cathode discharged to 0.1 V. (f) XRD of the cathode charged to 2.0 V.

satellite peak at around 943.0 eV compared with the XPS spectrum of Cu 2p for the as-prepared CuS, suggesting the disappearance of the paramagnetic chemical state of Cu2+. Figure 5b shows the XPS spectrum of S 2p, where a strong peak at 168 eV appears compared with Figure S1d, which is due to the formation of Cu2S. After the charge process, in addition to the two main peaks of Cu 2p, the shakeup satellite peak located at 943.0 eV appeared once again, suggesting the existence of Cu2+ (also CuS), as shown in Figure 5c. The XPS spectrum of S 2p after the charge process was similar to the XPS spectrum (Figure S1d). Figure S7 shows the XPS spectra of Al 2p after 473

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ACS Nano remain at about 1.2 V, which is in good agreement with the voltage measurements in Figure S3. Compared to the second discharge/charge curves at different current densities (Figure S5c), the discharge capacity decreased slightly after standing for 12 h, because there was also a small capacity loss during the rest process. As described above, the discharge/charge reaction mechanism of the battery can be formulated as the following equations: In the discharge process: Cathode: 6CuS + 2Al3 + + 6e− → 3Cu 2S + Al 2S3

(4)

Anode: Al + 7AlCl4 − − 3e− → 4Al 2Cl 7−

(5)

Figure 6. Impedance test of the CuS@C microsphere cathode. (a) Nyquist plots of the CuS microsphere electrode measured at an amplitude of 5 mV over a frequency range from 100 kHz to 0.1 Hz. (b) Linear fits of the Z′ vs ω−1/2 curve of the aluminum-ion battery in the low-frequency region. (c) Equivalent circuit model of the studied system. CPE represents the constant phase element.

In the charge process: Cathode: 3Cu 2S + Al 2S3 − 6e− → 6CuS + 2Al3 +

(6)

Anode: 4Al 2Cl 7− + 3e− → Al + 7AlCl4 −

(7)

system is provided in Figure 6c. According to the works reported by others,55 Rs represents the internal resistance of the battery, Rct1 and CPE1 are associated with the resistance and constant phase element of the SEI film, Rct2 and CPE2 are associated with the charge-transfer resistance and constant phase element of the electrode/electrolyte interface, and Zw is associated with the Warburg impedance. As shown in Figure 6a, the medium frequency semicircle is attributed to the chargetransfer resistance (Rct) at the electrode/electrolyte interface, and the low-frequency region corresponds to the aluminum ion diffusion or Warburg diffusion process into the bulk of the electrodes.56 It can be seen that the charge-transfer resistance of the CuS microsphere electrode was about 5 Ω before cycling. As cycling continues, the intermediate frequency semicircle becomes slightly larger, and a tiny impedance increase was detected (∼10 Ω), indicating the growth of SEI during cycling.57 In the case of the diffusion impedance, the sloping line in the low-frequency region deviates from the Warburg impedance with θ = 45°, which is attributed to the blocking electrode-type behavior. Figure 6b shows the relationship between Z′ and the square root of the frequency (ω−1/2) in the low-frequency region, and the low slope (K = 18.88) at low frequency indicates good kinetics in the electrode.

−

Meanwhile the complex anions (such as Al2Cl7 ) in the ionic liquid electrolyte were constantly resolved into simple ions (such as Al3+ and AlCl4−) during the charge process, which facilitates the redox reactions during the discharge/charge process. In the case of AlCl3:[EMIm]Cl, Al2Cl7− is present when there are more moles of AlCl3 than 1-ethyl-3methylimidazolium chloride ([EMIm]Cl). Thus, the molar ratio of AlCl3 to [EMIm]Cl must be greater than 1.37 Hence, a schematic illustration of the aluminum-ion battery based on CuS microspheres with an ionic liquid electrolyte during the discharging process is shown in Scheme 1. In the battery, the Scheme 1. Schematic illustration of an aluminum-ion battery during the discharging process. (−) Al foil/ionic liquid electrolyte ([EMIm]AlxCly)/GF/D sepatator/ionic liquid electrolyte ([EMIm]AlxCly)/CuS@C composites/Ta foil (+) from bottom to top.

CONCLUSION In summary, 3D hierarchical nanostructured CuS microspheres have been designed and synthesized by a facile method. The CuS microsphere electrode exhibited a high storage capacity and cyclic stability as the cathode material for AIBs in the ionic liquid of [EMIm]Cl. Interestingly, acetylene black (AB) nanoparticles uniformly dispersed on the surface of the CuS nanoflakes, resulting in significantly improved electrochemical performance. The CV and EIS results also indicated that the redox reaction kinetics can be optimized with appropriate conducting materials, which can be mainly ascribed to the charge-transfer enhancement. Particularly, the battery had a high average voltage of ∼1.0 V, and the battery delivers a specific capacity of about 90 mA h g−1 with nearly 100% Coulombic efficiency after 100 cycles at a current density of 20 mA g−1. This 3D CuS microsphere is a promising cathode material for rechargeable AIB applications.

CuS electrode was used as cathode, and the pure metallic Al foil (∼0.1 mm) was used as anode. In addition, the ionic liquid electrolyte was made from AlCl3/[EMIm]Cl, and the glass fiber (GF/D) was used as the separator. On the cathode side, CuS was transformed into the low-price Cu (Cu2S) along with the migration of Al3+ in the ionic liquid electrolyte during discharging. At the same time, on the anode side, the dissolution of Al occurred on the surface of the Al foil electrode. There is an inverse relationship between the charge process and the discharge process. In order to better investigate the electrochemical performance of the aluminum-ion batteries, electrochemical impedance spectroscopy (EIS) measurements were carried out before cycling and after 100 cycles at a current density of 20 mA g−1, as shown in Figure 6. The equivalent circuit model of the

EXPERIMENTAL SECTION Preparation of 3D Nanostructured CuS Microspheres. All chemicals were of analytical grade and used without further purification. The hierarchical nanostructured copper sulfide (CuS) 474

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ACS Nano microspheres were synthesized by a facile solvothermal method. CuCl2·2H2O (1.0 g, purity 99%, Alfa Aesar) was first added to a 40 mL glycol (C2H6O2, EG, Sinopharm Chemical Reagent) solution while stirring and heating to 120 °C in a beaker. Then 1.8 g of (NH2)2CS (Tu, purity 98%, Sinopharm Chemical Reagent) was added slowly to another 40 mL of EG solution while stirring. The solutions that formed in the beakers were mixed and stirred for about 30 min and then transferred to a pressure vessel with a 100 mL size Teflon vessel for solvothermal treatment. The mixture solution was heated at 140 °C for 90 min in an oven and was subsequently cooled to room temperature naturally. The black preliminary precipitate was washed several times with deionized water and absolute alcohol. After cleaning, the precipitate was collected by a centrifuge, and then the mixture was dried in an oven at 60 °C for over 6 h. Finally, the as-prepared CuS microspheres were obtained. Preparation of Ionic Liquid Electrolyte. A room-temperature ionic liquid electrolyte was made by mixing [EMIm]Cl and anhydrous aluminum chloride (AlCl3) in an argon-atmosphere glovebox ([O2] < 0.1 ppm, [H2O] < 0.1 ppm). AlCl3 was slowly dissolved in [EMIm]Cl with a molar ratio of 1.3:1, and the resulting light yellow and transparent ionic liquid was stirred in the glovebox for 30 min and then allowed to stand for at least 12 h. Characterization. XRD measurements were performed by using a Rigaku D/max-RB instrument using Cu Kα radiation (λ = 1.5418 Å) at a range of 10−90°. The detailed morphology and microstructure of the as-prepared materials were determined by FE-SEM (JEOL, JSM6701F) with an energy-dispersive X-ray spectrometer and transmission electron microscopy (JEOL, JEM-2010). XPS (Kratos AXIS Ultra DLD) was also applied to study the change of valence state. Raman spectra of the ionic liquid electrolyte were obtained using a He−Cd laser (325 nm) with 1.6 cm−1 resolution. Raman spectra of the materials or electrode plates were obtained using a 532 nm laser with 0.65 cm−1 resolution. Electrochemical Measurements. The capacities and cycling properties of the CuS microsphere electrode were evaluated in a soft package aluminum-ion battery containing metallic Al as the counter electrode. The working electrode was prepared by making a slurry of 60 wt % of the active material (CuS, ∼100 mg), 30 wt % acetylene black, and 10 wt % polyvinylidene difluoride binder in N-methyl-2pyrrolidinone. The slurry was casted onto a tantalum (Ta) foil current collector. Finally, the electrode plates were dried at 60 °C for 12 h in an oven to remove residual alcohol. The electrolyte used for aluminum-ion batteries was a mixture of AlCl3 and [EMIm]Cl with a 1.3:1 molar ratio. The glass fiber (GF/D) was used as the separator. The discharge/charge characteristics were determined by cycling in the voltage range 0.1−2.0 V vs Al/AlCl4− at different current densities with a multichannel battery testing system (Neware BTS-53). Cyclic voltammetry measurements were conducted at 5 mV s−1 over the range 0.1−2.0 V vs Al/AlCl4− on a CHI 1140C (Shanghai, China) electrochemical workstation. The batteries were also analyzed by EIS (CHI 1140C) in the frequency range 100 kHz to 0.1 Hz.

AUTHOR INFORMATION Corresponding Authors

*E-mail (S. Jiao): [email protected]. *E-mail (H.-S. Chen): [email protected]. ORCID

Shuqiang Jiao: 0000-0001-9600-752X Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06446. XPS spectrum of the as-prepared CuS microspheres; SEM images and EDS spectrum of CuS@C microsphere composites; voltage measurements; cycling performance of CuS microsphere cathode at a current density of 20 mA g−1 over 300 cycles; discharge−charge curves and cycling performance of CuS microsphere cathode at different current densities; CV test of different electrodes; XPS spectra of Al 2p after cycling tests; SEM and TEM images of the electrode after discharge; and selfdischarge behavior (PDF) 475

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DOI: 10.1021/acsnano.6b06446 ACS Nano 2017, 11, 469−477