- Email: [email protected]
Nanosheets assembling hierarchical starfish-like Cu2−xSe as advanced cathode for rechargeable Mg batteries
Chemical Engineering Journal xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Nanosheets assembling hierarchical starfish-like Cu2−xSe as advanced cathode for rechargeable Mg batteries ⁎
Dong Chena, Yujie Zhanga, Xue Lia, Jingwei Shena, Zhongxue Chena, Shun-an Caoa, Ting Lib, , ⁎ Fei Xua, a
Key Laboratory of Hydraulic Machinery Transients, Ministry of Education, School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission, Ministry of Education, College of Chemistry and Materials Science, SouthCentral University for Nationalities, Wuhan 430074, China b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Hierarchical starfish-like Cu Se of • nanosheets is used for Mg batteries. cathode delivers a high capacity • The at 100 mA g . of 210 mAh g long-term cycleability over • A300superior cycles is obtained. thin nanosheets enhance solid• The diffusion kinetics. state Mg 2−x
−1
−1
2+
A R T I C LE I N FO
A B S T R A C T
Keywords: Rechargeable Mg batteries Electrochemical conversion reaction Cu2−xSe Hierarchical structure Nanosheet
Rechargeable Mg battery is a low-cost and highly safe energy storage technique suitable for large-scale application; however, searching for high-performance cathodes remains a large challenge blocking their development. Non-stoichiometric Cu2−xSe has extraordinarily high conductivity and its unique crystal structure could better accommodate Mg2+ cations. In this work, Cu2−xSe with nanosheets assembling starfish-like hierarchical structure (S-Cu2−xSe) and nanocrystal (C-Cu2−xSe) are synthesized by facile one-step hydrothermal methods and used as cathode for rechargeable Mg batteries. It is observed that S-Cu2−xSe exhibits a high specific capacity of 210 mAh g−1 at 100 mA g−1, an excellent rate capability of 100 mA h g−1 at 1000 mA g−1, and a long-term cycling stability over 300 cycles. Mechanism investigation demonstrates that the Cu2−xSe cathodes experience a two-step magnesiation/demagnesiation process during cycling, in which phase transformation is involved. The high-performance of S-Cu2−xSe is owing to the fast solid-state Mg2+ diffusion in the thin nanosheets and the material integrity of the hierarchical structure. This work not only delivers facile approaches for the construction of high-performance Cu2−xSe Mg cathodes, but also highlights scientific insights for the exploration of conversion-type Mg-storage materials.
1. Introduction Rechargeable Mg batteries have drawn an increasing research
⁎
interest as an efficient and sustainable energy storage technique because of the low redox potential (−2.36 V vs SHE), high natural abundance and high capacity (2205 mAh g−1 and 3833 mAh cm−3) of
Corresponding authors. E-mail addresses: [email protected] (T. Li), [email protected] (F. Xu).
https://doi.org/10.1016/j.cej.2019.123235 Received 3 September 2019; Received in revised form 14 October 2019; Accepted 17 October 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Dong Chen, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123235
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
stainless-steel autoclave (100 mL) and kept at 180 ℃ for 24 h. The final product was separated by high-speed centrifugation and washed with deionized water and ethanol for three times to obtain S-Cu2−xSe sample. For comparison, C-Cu2−xSe sample was synthesized via the same procedure by adding 10 mmol of NaBH4 into the precursor solution before heating.
metallic Mg anode [1–5]. More importantly, Mg anode does not generate dendrite during electrochemical deposition process in certain electrolytes, which ensures high safety for Mg batteries [6]. However, due to the bivalent Mg2+ cation and its strong interaction with the host lattice, Mg-intercalation cathode usually suffers from sluggish solidstate diffusion kinetics and poor performance [7–10]. As an alternative strategy, electrochemical conversion reaction provides more Mg-storage cathode options beyond intercalation mechanism since it is not constrained by crystal lattice [11–13]. For an effective development of conversion-type materials, it is thus of great significance to analyze the factors determining Mg-storage performance. The hard and soft acids and bases theory (HSAB) could well interpret the Mg-storage reaction reversibility [14,15]. When a hard base anion such as O2− or a soft base anion such as Se2− is used, discharge product of MgO or MgSe would form. Since Mg2+ is a hard acid, the cation–anion interaction involved in MgO would be much stronger than MgSe, and hence Se2− seems to be favorable for construction of the cathode host framework in light of the reaction reversibility. On the other hand, a soft acid cation such like Cu+ would have a relatively stronger interaction with the soft anion of Se2− according to HSAB, and thus could assist the dissociation of discharge product. As a result, promising Mg-storage materials would be combinations of soft acid cation and soft base anion [14–20]. Besides the material selection criteria, the morphology also plays a significant role in the electrochemical performance of conversion-type materials owing to the bivalent Mg2+ cation. For example, 1D nanorod or 2D nanosheet structures would be preferred as they construct short-range solid-state diffusion path for Mg2+ cations [21,22], and thus high capacity and good rate capability could be achieved. Moreover, for a long-term cycling stability, hierarchical structure is usually advantageous over simple lowdimensional nanorods or nanosheets, as it could maintain the material integrity and prevent particle aggregation during cycling. According to the discussion above, non-stoichiometric Cu2−xSe is considered as a promising Mg-storage material. As a chalcogenide, nonstoichiometric Cu2−xSe exhibits a unique crystal structure and many excellent physicochemical properties beyond stoichiometric Cu2Se [23–25]. In the cubic unit cell (Fig. 1a), the highly mobile Cu+ cations are disorderly distributed among Se atoms, making Cu2−xSe a p-type conductor with electronic conductivity 3000 times higher than stoichiometric Cu2Se [26]. Furthermore, the large space among the atoms in Cu2−xSe could better accommodate Mg2+ cations, resulting in better Mg-storage reaction reversibility and better kinetic performance. Herein, a starfish-like Cu2−xSe with hierarchical structure assembled by nanosheets (S-Cu2−xSe) is reported as a high-performance cathode for rechargeable Mg batteries. A comparison is conducted with Cu2−xSe nanocrystal (C-Cu2−xSe) to elucidate the structure–property relationship. It is observed that S-Cu2−xSe deliveries a high reversible capacity of 210 mAh g−1 at 100 mA g−1, an excellent rate capability of 100 mA h g−1 at 1000 mA g−1, and a superior cycling stability over 300 cycles. The nanosheet structure of S-Cu2−xSe enhances the solid-state Mg2+ diffusion kinetics and thus improves the reversible capacity and rate capability. On the other hand, its hierarchical structure guarantees the cycling stability. This work not only delivers facile approaches for the construction of high-performance rechargeable Mg battery cathode of Cu2−xSe, but also highlights scientific insights for the exploration of conversion-type Mg-storage materials.
2.2. Characterization The crystalline compositions of the samples were characterized by X-ray diffraction (XRD, Bruker, D8 advance). X-ray photoelectron spectroscopy (XPS) was performed using VG MultiLab 2000 spectrometer, and the aliphatic C 1 s signal of carbon black at 284.6 eV was selected to calibrate the binding energies. The morphologies were observed using scanning electron microscopy (SEM, FEI Quanta-200) and transmission electron microscopy (TEM, JEOL, JEM-2100FEF). As the active material tends to remain on the separator, ex-situ characterizations were conducted using the cycled separators, which were taken out from the disassembled cells at different discharge/charge states. Prior to the characterizations, the separators were washed with dehydrated dimethoxethane solvent in an Ar-filled glove box (Mikrouna, Super, H2O and O2 < 0.1 ppm). 2.3. Mg Electrolyte preparation Mg electrolyte was prepared as follows in the Ar-filled glove box [16]: First of all, magnesium bis(hexamethyldisilazide) (Mg(HMDS)2, Sigma-Aldrich, 97%, 2 mmol) was dissolved in dehydrated diglyme (Alfa asear, extra dry, 99%, 10 mL) to form a homogeneous solution. Then, AlCl3 (Alfa asear, ultra dry, 99.999%, 4 mmol) were slowly added and the solution was stirred for 96 h for a thorough reaction. At last, magnesium chloride (MgCl2, Alfa asear, extra dry, 99%, 2 mmol) was added to the above solution, followed by a 24 h stirring. 2.4. Electrochemical measurements The Cu2−xSe electrodes were fabricated as follows: First, a homogeneous mixture was prepared using the active materials, conductive carbon (Ketjen Black) and polytetrafluoroethylene (PTFE) with a weight ratio of 7: 2: 1 in isopropyl alcohol solvent. Then the mixture was coated onto current collector (carbon cloth), dried under vacuum at 70 ℃ for 24 h, and cut into disks (12 mm diameter, active material loading of 1.5 mg cm−2). Electrochemical measurements were conducted using the as-prepared electrodes as cathode, Mg foil as anode, a glass microfiber film as separator, and the as-prepared Mg electrolyte. A lab-made cell was used to prevent corrosion (Fig. S1) [27–29]. The cell is made of PTFE body and carbon rod electrode. The crack is sealed with epoxy resin. PTFE tape is used for the sealing during the fabrication. Galvanostatic cycling measurements were performed on a LAND cycler (Wuhan Kingnuo Electronic Co., China). Cyclic voltammogram (CV) measurements were performed using a CHI 660e electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was performed on an impedance measuring unit (IM 6e, Zahner) with an AC voltage of 5 mV amplitude in the frequency range from 100 kHz to 0.01 Hz. 3. Results and discussion
2. Experimental Two Cu2−xSe samples are prepared herein by facile hydrothermal reactions, namely, starfish-like Cu2−xSe constructed by nanosheets (SCu2−xSe) and Cu2−xSe nanocrystals (C-Cu2−xSe). Fig. 1b shows the XRD patterns of S-Cu2−xSe and C-Cu2−xSe. All the diffraction peaks are in accordance with cubic-phase Cu2−xSe (JCPDS No. 06–0680) with a space group of F-43 m, where a cubic sub-lattice is constructed with Se atoms located at the face-centered sites and Cu atoms statistically dispersed at the interstitial sites in the sub-lattice (Fig. 1a). SEAD patterns
2.1. Materials and synthesis Cu2−xSe materials were fabricated by a facile one-step hydrothermal route. Typically, 4 mmol of copper acetate monohydrate (Cu (ac)2·H2O), 2 mmol of Na2SeO3 and 3 mL of ethylenediamine (EDA) were dissolved in 22 mL of deionized water via vigorous stirring for 30 min. Then the precursor solution was transferred to a Teflon-lined 2
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 1. (a) Crystal structure of Cu2-xSe with cubic phase. (b) XRD patterns of S-Cu2-xSe and C-Cu2-xSe. High-resolution XPS spectra of (c) Cu 2p and (d) Se 3d for SCu2−xSe and C-Cu2−xSe.
nanosheets of S-Cu2−xSe. In a HRTEM image (Fig. 2i), lattice fringes with spacing of 0.20 nm is also observed, corresponding to the (2 2 0) plane of Cu2−xSe. It is speculated that S-Cu2−xSe should have a better Mg-storage performance than C-Cu2−xSe, as the small thickness of the nanosheets is favorable for the solid-state Mg2+ diffusion kinetics. Fig. 2j schematically illustrates the overall reaction procedure and formation mechanism of S-Cu2−xSe and C-Cu2−xSe. The morphology difference of the two samples is owing to the different formation rates of Se2−. The Se2− anion is originated from reduction of Na2SeO3, and the formation rate is related to the reductive circumstance. For SCu2−xSe, a small amount of EDA (3 mL) was added and thus Se2− was formed at a slow rate, providing enough time for the growth of Cu2−xSe nanosheets and assemble into regular starfish-like structure. Another important factor is the Cu resource (Cu(ac)2·H2O) [35]. The relatively low dissociation degree of Cu(ac)2·H2O makes Cu2+ cation released with a low rate, facilitating the formation of S-Cu2−xSe. Whereas for CCu2−xSe, large amount of NaBH4 built a strong reductive environment, and the fast crystal growth rate resulted in C-Cu2−xSe nanoscale crystals with smaller size but irregular shapes. Interestingly, it is found that Cu2−xSe could form only upon addition of EDA, while other reduction agents would lead to formation of Cu2Se. Such a phenomenon is possibly owing to the coordination of EDA with Cu2+ cation. Till now, the high-performance of Cu2−xSe electrode has been demonstrated in Li, Na and Al batteries [23–26], but not for Mg batteries yet. The typical Mg cell discharge/charge profiles for S-Cu2−xSe and CCu2−xSe at 100 mA g−1 are shown in Fig. 3a and b, respectively. An electrochemical activation process is observed for both of the cathodes, during which a decrease in polarization occurs with discharge voltage increase and charge voltage decrease. Obviously, the activation degree of S-Cu2−xSe is much higher than that of C-Cu2−xSe, due to the relatively larger size of nanosheets. At the 2nd cycle for S-Cu2−xSe, a discharge plateau at 0.8 V and a tiny charge plateau at 2.0 V are observed. After 60 cycles of activation, the discharge and charge plateaus become
shown in Fig. S2 confirm the XRD results. XPS measurement was performed to confirm the surface composition and valance states of Cu2−xSe. As exhibited in Fig. S3 for overall XPS spectra, the signals of Cu 2p and Se 3d can be easily recognized. In high-resolution Cu 2p spectra shown in Fig. 1c, two peaks at 932.5 and 952.6 eV are originated from Cu 2p3/2 and Cu 2p1/2 of Cu+, respectively [30,31]. The two peaks at 935.4 and 955.0 eV are attributed to Cu 2p3/2 and Cu 2p1/2 of Cu2+, respectively, which are origin from surface oxide layer [30,31]. Satellite peaks of Cu2+ are also observed at 943.1 and 963.2 eV [32]. It is noteworthy that the ratio of Cu2+ to Cu+ for C-Cu2−xSe is higher than that of S-Cu2−xSe, which indicates a relatively higher degree of surface oxidation, while it does not affect the structure of the bulk phase (Fig. 1b). As shown in Fig. 1d, high-resolution Se 3d spectra contain two peaks at 53.6 and 54.5 eV, corresponding to Se 3d5/2 and 3d3/2, respectively [33,34]. A broad peak at 58.7 eV is observed, which is originated from the surface oxidation of Se species [34]. Fig. 2 shows the morphologies of S-Cu2−xSe and C-Cu2−xSe captured by SEM and TEM, in which a sharp difference can be observed between the two materials. As shown in Fig. 2a, b and S4, S-Cu2−xSe displays a starfish-like microstructure aggregated by several leafy components. It can be further illustrated from the magnified image (Fig. 2c) that the components are composed of laminated nanosheets with a diameter of ~300 nm. It is interesting that such a special hierarchical structure is obtained by a facile synthesis without adopting templates or using other assistant methods. TEM image (Fig. 2d) of SCu2−xSe confirms the size of the laminated nanosheets is about 200–400 nm. In a HRTEM image (Fig. 2e), the interlayer spacing of 0.33 nm is well observed and assigned to the crystal plane of (1 1 1) for cubic-phase Cu2-xSe. On the other hand for C-Cu2−xSe, a number of well-dispersed nanocrystals are observed from Fig. 2f. Magnified image (Fig. 2g) suggests the nanocrystals show smaller particle size, but the thickness is larger than that of S-Cu2−xSe. TEM image shown in Fig. 2h confirms the smaller particle size of C-Cu2−xSe (50 ~ 200 nm) than the 3
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 2. (a–c) SEM, (d) TEM and (e) HRTEM images of S-Cu2−xSe. (f, g) SEM, (h) TEM and (i) HRTEM images of C-Cu2−xSe. (j) The diagram of formation mechanism for S-Cu2−xSe and C-Cu2−xSe.
first five cycles (Fig. 3e and f, activation process), reproducible CV curves are obtained for activated electrodes (Fig. 3g and h). The activated electrode of S-Cu2−xSe (Fig. 3g) shows current densities much higher than C-Cu2−xSe (Fig. 3h), in good accordance with the capacity of the activate cells (Fig. 3c and d). The capacity decrease of C-Cu2−xSe during initial cycles is possibly due to irreversible conversion of Cu2+ to Cu+ [20]. Fig. 4 shows the rate capability and long-term cycling stability of SCu2−xSe and C-Cu2−xSe cathodes. As exhibited in Fig. 4a and b, the two discharge plateaus become narrower as the current density increases. In Fig. 4c, S-Cu2−xSe delivers high capacities of 209, 185, 139 and 100 mAh g−1 at the current densities of 100, 200, 500 and 1000 mA g−1, respectively. A reversible capacity of 200 mAh g−1 could be obtained upon a recover of current density to 100 mA g−1, retaining 96% of its original value (see Fig. S5 for detail). In contrast, C-Cu2−xSe only delivers 48 mA h g−1 at 1000 mA g−1. Fig. 4d compares the rate capability of S-Cu2−xSe with several reported copper chalcogenides for rechargeable Mg batteries [14,15,17–20], and such an excellent rate capability of S-Cu2−xSe is almost of the highest level. The high rate capability of S-Cu2−xSe is owing to both of intrinsic nature of Cu2−xSe and thin nanosheet morphology, which facilitates the solid-state Mg2+
wider with voltage shifting to 0.85 V and 1.75 V, respectively. Meanwhile, another discharge plateau emerges at 1.25 V, as well as another charge plateau at 1.35 V. As the reduction of Cu+ to Cu0 only corresponds to a discharge plateau at about 0.95 V [17,18], the 1.25 V discharge plateau would possibly indicate phase transformation of Cu2−xSe during the electrochemical reaction. The activation process is also evidenced by the cycling performance. As shown in Fig. 3c, the reversible capacity of S-Cu2−xSe gradually increases during the initial 60 cycles from 70 to 210 mA h g−1. In contrast, C-Cu2−xSe shows relatively inferior performance, providing a capacity of 138 mA h g−1 after activation (Fig. 3d). The difference in capacities is owing to the different morphologies of S-Cu2−xSe and C-Cu2−xSe. The thin-layered nanosheets have relatively short-range solid-state Mg2+ diffusion path, which enable a higher capacity utilization. CV curves of the two Cu2−xSe cathodes were measured for initial (Fig. 3e and f) and activated cycles (Fig. 3g and h). As seen in Fig. 3e for S-Cu2−xSe, two pairs of redox peaks can be clearly recognized at 0.85/ 1.35 and 1.25/1.75 V, suggesting a two-step magnesiation/demagnesiation reaction process. Similar curves can be observed for C-Cu2−xSe with higher current densities (Fig. 3f), which agrees with the different initial capacities (Fig. 3c and d). After the change of CV curves in the 4
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 3. Discharge/charge profiles of (a) S-Cu2−xSe and (b) C-Cu2−xSe cathodes in the potential range of 0.3–2.2 V. Cycling performance of (c) S-Cu2−xSe and (d) CCu2−xSe at 100 mA g−1. CV curves in the first five cycles of (e) S-Cu2−xSe and (f) C-Cu2−xSe at a scan rate of 0.1 mV s−1. CV curves of the activated (g) S-Cu2−xSe and (h) C-Cu2−xSe.
30.0°, 36.8° and 41.0° are ascribed to the separator. At the fully discharge state (0.3 V, point a) after activation process, weak peaks for Cu2−xSe are still observed, indicating a small part of Cu2−xSe did not participate in the reaction. Clear observation of MgSe peaks at 32.8° and 47.0° confirms the occurrence of conversion reaction [14]. However, diffraction peaks of metallic Cu0 cannot be well recognized in the discharge state, possibly owing to the nanoscale particles and poor crystallinity. When charged to 1.75 V through the first charge plateau (point b), Cu2−xSe peaks enhance slightly and MgSe peaks weaken, indicating formation of Cu2−xSe from MgSe and metallic Cu0. At fully charged state of 2.2 V (point c), Cu2−xSe and MgSe peaks disappear entirely, and peaks for Cu5Se4 emerge at 27.5° and 45.4° instead. These results indicate a phase transformation during the activation process, which is common for selenides to be used as conversation-type cathodes for Mg batteries [14,15]. When discharge to 0.85 V through the first discharge plateau (point d), Cu2−xSe phase formed again as well as
diffusion. The long-term cycling stability of S-Cu2−xSe was examined at 200 mA g−1 after three cycles of activation at 50 mA g−1. As exhibited in Fig. 4e, the activation process lasted till 100th cycle, at which the capacity reached 170 mAh g−1, and then gradually faded. Such a long activation process is possibly due to the bivalent Mg2+ cation. A capacity of 109 mAh g−1 was maintained after 300 cycles for S-Cu2−xSe, corresponding to a retention rate of 65%, which is much higher than that for C-Cu2−xSe (Fig. 4f). The hierarchical starfish-like structure enhances the material integrity and prevents particle aggregation, thus resulting in a good cycleability. It is worthy to note that the coulombic efficiency is almost 100% all through the cycling, which is of special importance for the application of cathode materials. Ex-situ XRD measurements were used to investigate the electrochemical reaction mechanism of Cu2−xSe upon cycling. Cycled separator is used for the characterization since the active material tends to remain on the separator. As seen in Fig. 5a, the peaks at 25.9°, 28.1°, 5
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 4. (a) Discharge/charge profiles of (a) S-Cu2−xSe and (b) C-Cu2−xSe cathodes at different current densities. (c) Rate capability of S-Cu2−xSe and C-Cu2−xSe cathodes at different current densities varies from 100 mA g−1 to 1 A g−1. (d) Comparison for rate capability of S-Cu2−xSe with reported copper chalcogenides [14,15,17–20]. Long-term cycling stability of (e) S-Cu2−xSe and (f) C-Cu2−xSe at 200 mA g−1.
electrode materials at fully discharge (0.3 V) state. It is revealed that uniformly distributed nanoparticles appeared, which suggests the activation process occurs with downsize of Cu2−xSe, establishing large interface favorable for solid-state Mg2+ diffusion and improving the reversible capacity. Furthermore, in the HRTEM image (Fig. 5e), lattice fringes of MgSe (2 0 0) plane (0.27 nm) and metallic Cu0 (2 0 0) plane (0.18 nm) are clearly observed, which confirm the occurrence of conversion reaction. Hence, the overall conversion reaction of Cu2−xSe cathode occurs between Cu5Se4 and metallic Cu0 with intermediate of Cu2−xSe. Specifically, conversion reaction of Cu5Se4 to Cu2−xSe is
MgSe. Subsequently, Cu2−xSe peaks weakened and MgSe peaks enhanced at the second discharge state of 0.3 V (point e). Ex-situ XPS measurements were also conducted to confirm the reaction mechanism. Fig. 5b shows the high-resolution XPS spectra of Cu 2p for Cu2−xSe cathode at the fully charge (2.2 V) and discharge (0.3 V) states. Compared with charge state of Cu5Se4, the satellite peaks for Cu2+ almost vanish at the discharge state, suggesting conversion of Cu2+ to Cu+ during the reaction [26]. Hence, the two peaks at 932.5 and 952.6 eV are originated from Cu+ and metallic Cu0, respectively [19]. Fig. 5c and d show TEM images of the activated S-Cu2−xSe
6
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 5. (a) Ex-situ XRD patterns for Cu2−xSe cathodes at different discharge/charge states. (b) Ex-situ XPS spectra of Cu 2p for Cu2−xSe cathodes at the fully charged (2.2 V) and discharged (0.3 V) states. (c, d) TEM and (e) HRTEM images of activated S-Cu2−xSe at fully discharge (0.3 V) state.
thorough but that of Cu2−xSe to Cu0 is not, resulting in slightly lower capacity than theoretical value (220 mAh g−1) for the second discharge plateau. To further understand the high performance of S-Cu2-xSe cathode, the electrochemical behavior was evaluated by kinetic investigation. Fig. 6a–c depicts the CV curves at various scan rates range from 0.2 to 1.0 mV s−1. Fig. 6a–c show fresh S-Cu2-xSe, activated S-Cu2-xSe and activated C-Cu2−xSe electrodes, respectively. All the CV curves exhibit coherent redox peaks, and the currents increase and potentials shift with the increasing scan rates. The relationship between the peak current (i) and scan rate (v) could be described as i = avb, where a and b are adjustable constant parameters. A b value of 1.0 indicates a surfacecontrolled pseudo-capacitive behavior, whereas a b value of 0.5 suggests a diffusion-controlled feature [36–38]. As shown in Fig. 6d, the b values of peaks C1, C2, A1 and A2 for fresh S-Cu2-xSe cathode approach 1.0, suggesting the high pseudo-capacitive behavior. While after the activation process (Fig. 6e), the b values largely decrease, illustrating the diffusion contribution increases significantly. Such results reflect the morphology feature of S-Cu2-xSe. Due to the thin nanosheet structure providing large exposed interfaces and active sites, pseudo-capacitance (near-surface Faradic process) plays a dominant role in initials
cycles. During the activation process, the electrode experiences internal structure modification to establish framework favorable for fast solidstate Mg2+ diffusion kinetics. On the other hand, C-Cu2−xSe gives b values much lower than S-Cu2−xSe (Fig. 6f), which is owing to its thicker particles and thus lower pseudo-capacitance contribution (Fig. 6g). Furthermore, the CV curves are also used to evaluate the solid-state Mg2+ diffusion kinetics of the Cu2−xSe cathodes in Mg cells. The diffusion coefficient of Mg2+ is calculated by the following equation [39–41]:
Ip = 2.69 × 105n3/2AD1/2ν1/2C0
(1)
where Ip is the peak current (A), n is the number of electrons per molecule during the reaction, A is the contact area between the electrode and electrolyte (cm2), D is the diffusion coefficient of Mg2+ (cm2 s−1), C0 is the concentration of Mg2+ ion in the electrode material (mol cm−3), and v is the scan rate (V s−1). DMg2+ of peaks C1, C2, A1, A2 for S-Cu2-xSe are determined to be 2.5 × 10−11, 2.4 × 10−10, 2.5 × 10−10 and 1.6 × 10−11, respectively, which are much higher than those for CCu2-xSe (Fig. 6h, see Table S1 for the calculation details). Benefited from the thin nanosheet morphology and modified solid-state diffusion 7
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 6. CV curves of (a) fresh S-Cu2−xSe, (b) activated S-Cu2−xSe and (c) C-Cu2−xSe cathodes at various scan rates. The corresponding log(peak current)-log(scan rate) plots of the redox peaks for (d) fresh S-Cu2−xSe, (e) activated S-Cu2−xSe and (f) C-Cu2−xSe cathodes. (g) Contribution ratio of diffusion capacities at different scan rates. (h) Corresponding peak current-(scan rate)1/2 plots of the redox peaks for activated S-Cu2−xSe and C-Cu2−xSe cathodes. (i) Nyquist plots of the activated S-Cu2−xSe and C-Cu2−xSe cathodes.
paths, faster solid-state Mg2+ diffusion is achieved in the hierarchical SCu2−xSe. Further Nyquist plots obtained by EIS measurements (Fig. 6i) also reveal that S-Cu2−xSe exhibits much lower resistance than CCu2−xSe, indicating a faster charge transfer rate [42,43].
Declaration of Competing Interest
4. Conclusions
Acknowledgements
In summary, we introduce facile one-step hydrothermal methods to fabricate nanosheets assembling hierarchical starfish-like Cu2−xSe (SCu2−xSe) and nanocrystals (C-Cu2−xSe), and their applications as Mgstorage cathodes are systematically demonstrated. It is observed that SCu2−xSe exhibits a high reversible capacity of 210 mAh g−1 at 100 mA g−1, an excellent rate capability of 100 mA h g−1 at 1 A g−1, and a long-term cycling stability over 300 cycles. The high capacity and outstanding rate capability of S-Cu2−xSe is owing to the thin nanosheet structures, which facilitate solid-state Mg2+ diffusion. The long-term cycling stability of S-Cu2−xSe is attributed to the hierarchical morphology, which maintain the material integrity during cycling. Mechanism investigation reveals that the electrochemical reaction is a typical conversion-type reaction occurring between Cu5Se4 and metallic Cu0 with intermediate of Cu2−xSe. This work delivers facile approaches for the construction of high-performance Cu2−xSe Mg cathodes with delicate morphology tailoring, and more significantly, highlights scientific insights for the exploration of conversion-type Mg-storage materials.
This work was financially supported by the Hubei Provincial Natural Science Foundation (2019CFB452 and 2019CFB620), the National Natural Science Foundation of China (Nos. 51402113 and 21403305), the Fundamental Research Funds for the Central Universities and the Open Fund of Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123235. References [1] M. Mao, T. Gao, S. Hou, C. Wang, A critical review of cathodes for rechargeable Mg batteries, Chem. Soc. Rev. 47 (2018) 8804–8841. [2] E. Levi, Y. Gofer, D. Aurbach, On the way to rechargeable Mg batteries: the challenge of new cathode materials, Chem. Mater. 22 (2010) 860–868.
8
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
[25] Y. Li, X. Sun, Z. Cheng, X. Xu, J. Pan, X. Yang, F. Tian, Y. Li, J. Yang, Y. Qian, Mesoporous Cu2-xSe nanocrystals as an ultrahigh-rate and long-lifespan anode material for sodium-ion batteries, Energy Storage Mater. (2019). [26] H. Li, J. Jiang, J. Huang, Y. Wang, Y. Peng, Y. Zhang, B.J. Hwang, J. Zhao, Investigation of the Na storage property of one-dimensional Cu2−xSe nanorods, ACS Appl. Mater. Interfaces 10 (2018) 13491–13498. [27] Y. Cheng, T. Liu, Y. Shao, M. Engelhard, J. Liu, G. Li, Electrochemically stable cathode current collectors for rechargeable magnesium batteries, J. Mater. Chem. A 2 (2014) 2473–2477. [28] J. Muldoon, C.B. Bucur, A.G. Oliver, J. Zajicek, G.D. Allred, W.C. Boggess, Corrosion of magnesium electrolytes: chlorides – the culprit, Energy Environ. Sci. 6 (2013) 482–487. [29] C. Wall, Z. Zhao-Karger, M. Fichtner, Corrosion resistance of current collector materials in bisamide based electrolyte for magnesium batteries, Electrochem. Lett. 4 (2015) C8–C10. [30] R. Jin, X. Liu, L. Yang, G. Li, S. Gao, Sandwich-like Cu2-xSe@C@MoSe2 nanosheets as an improved-performance anode for lithium-ion battery, Electrochim. Acta 259 (2018) 841–849. [31] Y. Liu, S. Shen, J. Zhang, W. Zhong, X. Huang, Cu2-xSe/CdS composite photocatalyst with enhanced visible light photocatalysis activity, Appl. Surf. Sci. 478 (2019) 762–769. [32] L. Zhu, H. Xie, Y. Liu, D. Chen, M. Bian, W. Zheng, Novel ultralong hollow hyperbranched Cu2-xSe with nanosheets hierarchical structure: preparation, formation mechanism and properties, J. Alloy. Compd. 802 (2019) 430–436. [33] Y. Tang, Z. Zhao, X. Hao, Y. Wang, Y. Liu, Y. Hou, Q. Yang, X. Wang, J. Qiu, Engineering hollow polyhedrons structured from carbon-coated CoSe2 nanospheres bridged by CNTs with boosted sodium storage performance, J. Mater. Chem. A 26 (2017) 13591–13600. [34] B. Yu, Y. Hu, F. Qi, X. Wang, B. Zheng, K. Liu, W. Zhang, Y. Li, Y. Chen, Nanocrystalline Ni0.85Se as efficient non-noble-metal electrocatalyst for hydrogen evolution reaction, Electrochim. Acta 242 (2017) 25–30. [35] C. Wang, T. Wang, J. Liu, Y. Zhou, D. Yu, J. Kuei, F. Han, Q. Li, J. Chen, Y. Huang, Facile synthesis of silk-cocoon S-rich cobalt polysulfide as an efficient catalyst for hydrogen evolution reaction, Energy Environ. Sci. 11 (2018) 2467–2475. [36] D. Chao, P. Liang, Z. Chen, L. Bai, H. Shen, X. Liu, X. Xia, Y. Zhao, S.V. Savilov, J. Lin, Z.X. Shen, Pseudocapacitive Na-ion storage boosts high-rate and areal capacity of self-branched 2D layered metal chalcogenide nanoarrays, ACS Nano 10 (2016) 10211–10219. [37] D. Chao, C. Zhu, P. Yang, X. Xia, J. Liu, J. Wang, X. Fan, S.V. Savilov, J. Lin, H.J. Fan, Z.X. Shen, Array of nanosheets render ultrafast and high-capacity Na-ion storage by tunable pseudocapacitance, Nat. Commun. 7 (2016) 12122. [38] S.H. Yang, S.K. Park, Y.C. Kang, Mesoporous CoSe2 nanoclusters threaded with nitrogen-doped carbon nanotubes for high-performance sodium-ion battery anodes, Chem. Eng. J. 370 (2019) 1008–1018. [39] Z. Wei, D. Wang, M. Li, Y. Gao, C. Wang, G. Chen, F. Du, Fabrication of hierarchical potassium titanium phosphate spheroids: a host material for sodium-ion and potassium-ion storage, Adv. Energy Mater. 8 (2018) 1801102. [40] Y. Yang, Y. Tang, G. Fang, L. Shan, J. Guo, W. Zhang, J. Zhou, C. Wang, L. Wang, S. Liang, Li+ intercalated V2O5·nH2O with enlarged layer spacing and fast ion diffusion as an aqueous zinc-ion battery cathode, Energy Environ. Sci. 11 (2018) 3157–3162. [41] D. Yu, M. Li, T. Yu, C. Wang, Y. Zeng, X. Hu, G. Chen, G. Yang, F. Du, Nanotubeassembled pine-needle-like CuS as an effective energy booster for sodium-ion storage, J. Mater. Chem. A 7 (2019) 10619–10628. [42] Y. Tang, Z. Zhao, X. Hao, Y. Wei, H. Zhang, Y. Dong, Y. Wang, X. Pan, Y. Hou, X. Wang, J. Qiu, Cellular carbon-wrapped FeSe2 nanocavities with ultrathin walls and multiple rooms for ion diffusion-confined ultrafast sodium storage, J. Mater. Chem. A 7 (2019) 4469–4479. [43] F. Liu, Z. Chen, G. Fang, Z. Wang, Y. Cai, B. Tang, J. Zhou, S. Liang, V2O5 nanospheres with mixed vanadium valences as high electrochemically active aqueous zinc-ion battery cathode, Nano-Micro Lett. 11 (2019) 1–19.
[3] F. Murgia, P. Antitomaso, L. Stievano, L. Monconduit, R. Berthelot, Express and low-cost microwave synthesis of the ternary chevrel phase Cu2Mo6S8 for application in rechargeable magnesium batteries, J. Solid State Chem. 242 (2016) 151–154. [4] L. Wang, K. Asheim, P.E. Vullum, A.M. Svensson, F. Vullum-Bruer, Sponge-like porous manganese (II, III) oxide as a highly efficient cathode material for rechargeable magnesium ion batteries, Chem. Mater. 28 (2016) 6459–6470. [5] Y. Liang, H.D. Yoo, Y. Li, J. Shuai, H.A. Calderon, F.C.R. Hernandez, L.C. Grabow, Y. Yao, Interlayer-expanded molybdenum disulfide nanocomposites for electrochemical magnesium storage, Nano Lett. 15 (2015) 2194–2202. [6] J. Song, E. Sahadeo, M. Noked, S.B. Lee, Mapping the challenges of magnesium battery, J. Phys. Chem. Lett. 7 (2016) 1736–1749. [7] P. Canepa, G.S. Gautam, D.C. Hannah, R. Malik, M. Liu, K.G. Gallagher, K.A. Persson, G. Ceder, Odyssey of multivalent cathode materials: open questions and future challenges, Chem. Rev. 117 (2017) 4287–4341. [8] J. Muldoon, C.B. Bucur, T. Gregory, Quest for nonaqueous multivalent secondary batteries: magnesium and beyond, Chem. Rev. 114 (2014) 11683–11720. [9] X. Sun, P. Bonnick, L.F. Nazar, Layered TiS2 positive electrode for Mg batteries, ACS Energy Lett. 1 (2016) 297–301. [10] X. Sun, P. Bonnick, V. Duffort, M. Liu, Z. Rong, K.A. Persson, C. Gerbrand, L.F. Nazar, A high capacity thiospinel cathode for Mg batteries, Energy Environ. Sci. 9 (2016) 2273–2277. [11] Z. Zhang, S. Dong, Z. Cui, A. Du, G. Li, G. Cui, Rechargeable magnesium batteries using conversion-type cathodes: a perspective and minireview, Small Methods 2 (2018) 1800020. [12] R. Zhang, C. Ling, F. Mizuno, A conceptual magnesium battery with ultrahigh rate capability, Chem. Commun. 51 (2015) 1487–1490. [13] Z. Chen, Z. Zhang, A. Du, Y. Zhang, M. Men, G. Li, G. Cui, Fast magnesiation kinetics in α-Ag2S nanostructures enabled by an in situ generated silver matrix, Chem. Commun. 55 (2019) 4431–4434. [14] Y. Tashiro, K. Taniguchi, H. Miyasaka, Copper selenide as a new cathode material based on displacement reaction for rechargeable magnesium batteries, Electrochim. Acta 210 (2016) 655–661. [15] Y. Tashiro, K. Taniguchi, H. Miyasaka, The effect of anion-sublattice structure on the displacement reaction in copper sulfide cathodes of rechargeable magnesium batteries, Chem. Lett. 46 (2017) 1240–1242. [16] V. Duffort, X. Sun, L.F. Nazar, Screening for positive electrodes for magnesium batteries: a protocol for studies at elevated temperatures, Chem. Commun. 52 (2016) 12458–12461. [17] M. Wu, Y. Zhang, T. Li, Z. Chen, S. Cao, F. Xu, Copper sulfide nanoparticles as highperformance cathode materials for magnesium secondary batteries, Nanoscale 10 (2018) 12526–12534. [18] F. Xiong, Y. Fan, S. Tan, L. Zhou, Y. Xu, C. Pei, Q. An, L. Mai, Magnesium storage performance and mechanism of CuS cathode, Nano Energy 47 (2018) 210–216. [19] Z. Wang, S. Rafai, C. Qiao, J. Jia, Y. Zhu, X. Ma, C. Cao, Microwave-assisted synthesis of CuS hierarchical nanosheets as the cathode material for high-capacity rechargeable magnesium batteries, ACS Appl. Mater. Interfaces 11 (2019) 7046–7054. [20] M. Wu, Y. Zhang, H. Wu, A. Qin, X. Li, J. Shen, Z. Chen, S. Cao, T. Li, F. Xu, Cu9S5 nanoflower cathode for Mg secondary batteries: high performance and reaction mechanism, Energy Technol. 7 (2019) 1800777. [21] B. Liu, T. Luo, G. Mu, X. Wang, D. Chen, G. Shen, Rechargeable Mg-ion batteries based on WSe2 nanowire cathodes, ACS Nano 7 (2013) 8051–8058. [22] D. He, D. Wu, J. Gao, X. Wu, X. Zeng, W. Ding, Flower-like CoS with nanostructures as a new cathode-active material for rechargeable magnesium batteries, J. Power Sources 294 (2015) 643–649. [23] H. Li, J. Jiang, F. Wang, J. Huang, Y. Wang, Y. Zhang, J. Zhao, Facile synthesis of rod-like Cu2-xSe and insight into its improved lithium-storage property, ChemSusChem 10 (2017) 2235–2241. [24] J. Jiang, H. Li, T. Fu, B.J. Hwang, X. Li, J. Zhao, One-dimensional Cu2−xSe nanorods as the cathode material for high-performance aluminum-ion battery, ACS Appl. Mater. Interfaces 10 (2018) 17942–17949.
9
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Nanosheets assembling hierarchical starfish-like Cu2−xSe as advanced cathode for rechargeable Mg batteries ⁎
Dong Chena, Yujie Zhanga, Xue Lia, Jingwei Shena, Zhongxue Chena, Shun-an Caoa, Ting Lib, , ⁎ Fei Xua, a
Key Laboratory of Hydraulic Machinery Transients, Ministry of Education, School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission, Ministry of Education, College of Chemistry and Materials Science, SouthCentral University for Nationalities, Wuhan 430074, China b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Hierarchical starfish-like Cu Se of • nanosheets is used for Mg batteries. cathode delivers a high capacity • The at 100 mA g . of 210 mAh g long-term cycleability over • A300superior cycles is obtained. thin nanosheets enhance solid• The diffusion kinetics. state Mg 2−x
−1
−1
2+
A R T I C LE I N FO
A B S T R A C T
Keywords: Rechargeable Mg batteries Electrochemical conversion reaction Cu2−xSe Hierarchical structure Nanosheet
Rechargeable Mg battery is a low-cost and highly safe energy storage technique suitable for large-scale application; however, searching for high-performance cathodes remains a large challenge blocking their development. Non-stoichiometric Cu2−xSe has extraordinarily high conductivity and its unique crystal structure could better accommodate Mg2+ cations. In this work, Cu2−xSe with nanosheets assembling starfish-like hierarchical structure (S-Cu2−xSe) and nanocrystal (C-Cu2−xSe) are synthesized by facile one-step hydrothermal methods and used as cathode for rechargeable Mg batteries. It is observed that S-Cu2−xSe exhibits a high specific capacity of 210 mAh g−1 at 100 mA g−1, an excellent rate capability of 100 mA h g−1 at 1000 mA g−1, and a long-term cycling stability over 300 cycles. Mechanism investigation demonstrates that the Cu2−xSe cathodes experience a two-step magnesiation/demagnesiation process during cycling, in which phase transformation is involved. The high-performance of S-Cu2−xSe is owing to the fast solid-state Mg2+ diffusion in the thin nanosheets and the material integrity of the hierarchical structure. This work not only delivers facile approaches for the construction of high-performance Cu2−xSe Mg cathodes, but also highlights scientific insights for the exploration of conversion-type Mg-storage materials.
1. Introduction Rechargeable Mg batteries have drawn an increasing research
⁎
interest as an efficient and sustainable energy storage technique because of the low redox potential (−2.36 V vs SHE), high natural abundance and high capacity (2205 mAh g−1 and 3833 mAh cm−3) of
Corresponding authors. E-mail addresses: [email protected] (T. Li), [email protected] (F. Xu).
https://doi.org/10.1016/j.cej.2019.123235 Received 3 September 2019; Received in revised form 14 October 2019; Accepted 17 October 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Dong Chen, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123235
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
stainless-steel autoclave (100 mL) and kept at 180 ℃ for 24 h. The final product was separated by high-speed centrifugation and washed with deionized water and ethanol for three times to obtain S-Cu2−xSe sample. For comparison, C-Cu2−xSe sample was synthesized via the same procedure by adding 10 mmol of NaBH4 into the precursor solution before heating.
metallic Mg anode [1–5]. More importantly, Mg anode does not generate dendrite during electrochemical deposition process in certain electrolytes, which ensures high safety for Mg batteries [6]. However, due to the bivalent Mg2+ cation and its strong interaction with the host lattice, Mg-intercalation cathode usually suffers from sluggish solidstate diffusion kinetics and poor performance [7–10]. As an alternative strategy, electrochemical conversion reaction provides more Mg-storage cathode options beyond intercalation mechanism since it is not constrained by crystal lattice [11–13]. For an effective development of conversion-type materials, it is thus of great significance to analyze the factors determining Mg-storage performance. The hard and soft acids and bases theory (HSAB) could well interpret the Mg-storage reaction reversibility [14,15]. When a hard base anion such as O2− or a soft base anion such as Se2− is used, discharge product of MgO or MgSe would form. Since Mg2+ is a hard acid, the cation–anion interaction involved in MgO would be much stronger than MgSe, and hence Se2− seems to be favorable for construction of the cathode host framework in light of the reaction reversibility. On the other hand, a soft acid cation such like Cu+ would have a relatively stronger interaction with the soft anion of Se2− according to HSAB, and thus could assist the dissociation of discharge product. As a result, promising Mg-storage materials would be combinations of soft acid cation and soft base anion [14–20]. Besides the material selection criteria, the morphology also plays a significant role in the electrochemical performance of conversion-type materials owing to the bivalent Mg2+ cation. For example, 1D nanorod or 2D nanosheet structures would be preferred as they construct short-range solid-state diffusion path for Mg2+ cations [21,22], and thus high capacity and good rate capability could be achieved. Moreover, for a long-term cycling stability, hierarchical structure is usually advantageous over simple lowdimensional nanorods or nanosheets, as it could maintain the material integrity and prevent particle aggregation during cycling. According to the discussion above, non-stoichiometric Cu2−xSe is considered as a promising Mg-storage material. As a chalcogenide, nonstoichiometric Cu2−xSe exhibits a unique crystal structure and many excellent physicochemical properties beyond stoichiometric Cu2Se [23–25]. In the cubic unit cell (Fig. 1a), the highly mobile Cu+ cations are disorderly distributed among Se atoms, making Cu2−xSe a p-type conductor with electronic conductivity 3000 times higher than stoichiometric Cu2Se [26]. Furthermore, the large space among the atoms in Cu2−xSe could better accommodate Mg2+ cations, resulting in better Mg-storage reaction reversibility and better kinetic performance. Herein, a starfish-like Cu2−xSe with hierarchical structure assembled by nanosheets (S-Cu2−xSe) is reported as a high-performance cathode for rechargeable Mg batteries. A comparison is conducted with Cu2−xSe nanocrystal (C-Cu2−xSe) to elucidate the structure–property relationship. It is observed that S-Cu2−xSe deliveries a high reversible capacity of 210 mAh g−1 at 100 mA g−1, an excellent rate capability of 100 mA h g−1 at 1000 mA g−1, and a superior cycling stability over 300 cycles. The nanosheet structure of S-Cu2−xSe enhances the solid-state Mg2+ diffusion kinetics and thus improves the reversible capacity and rate capability. On the other hand, its hierarchical structure guarantees the cycling stability. This work not only delivers facile approaches for the construction of high-performance rechargeable Mg battery cathode of Cu2−xSe, but also highlights scientific insights for the exploration of conversion-type Mg-storage materials.
2.2. Characterization The crystalline compositions of the samples were characterized by X-ray diffraction (XRD, Bruker, D8 advance). X-ray photoelectron spectroscopy (XPS) was performed using VG MultiLab 2000 spectrometer, and the aliphatic C 1 s signal of carbon black at 284.6 eV was selected to calibrate the binding energies. The morphologies were observed using scanning electron microscopy (SEM, FEI Quanta-200) and transmission electron microscopy (TEM, JEOL, JEM-2100FEF). As the active material tends to remain on the separator, ex-situ characterizations were conducted using the cycled separators, which were taken out from the disassembled cells at different discharge/charge states. Prior to the characterizations, the separators were washed with dehydrated dimethoxethane solvent in an Ar-filled glove box (Mikrouna, Super, H2O and O2 < 0.1 ppm). 2.3. Mg Electrolyte preparation Mg electrolyte was prepared as follows in the Ar-filled glove box [16]: First of all, magnesium bis(hexamethyldisilazide) (Mg(HMDS)2, Sigma-Aldrich, 97%, 2 mmol) was dissolved in dehydrated diglyme (Alfa asear, extra dry, 99%, 10 mL) to form a homogeneous solution. Then, AlCl3 (Alfa asear, ultra dry, 99.999%, 4 mmol) were slowly added and the solution was stirred for 96 h for a thorough reaction. At last, magnesium chloride (MgCl2, Alfa asear, extra dry, 99%, 2 mmol) was added to the above solution, followed by a 24 h stirring. 2.4. Electrochemical measurements The Cu2−xSe electrodes were fabricated as follows: First, a homogeneous mixture was prepared using the active materials, conductive carbon (Ketjen Black) and polytetrafluoroethylene (PTFE) with a weight ratio of 7: 2: 1 in isopropyl alcohol solvent. Then the mixture was coated onto current collector (carbon cloth), dried under vacuum at 70 ℃ for 24 h, and cut into disks (12 mm diameter, active material loading of 1.5 mg cm−2). Electrochemical measurements were conducted using the as-prepared electrodes as cathode, Mg foil as anode, a glass microfiber film as separator, and the as-prepared Mg electrolyte. A lab-made cell was used to prevent corrosion (Fig. S1) [27–29]. The cell is made of PTFE body and carbon rod electrode. The crack is sealed with epoxy resin. PTFE tape is used for the sealing during the fabrication. Galvanostatic cycling measurements were performed on a LAND cycler (Wuhan Kingnuo Electronic Co., China). Cyclic voltammogram (CV) measurements were performed using a CHI 660e electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was performed on an impedance measuring unit (IM 6e, Zahner) with an AC voltage of 5 mV amplitude in the frequency range from 100 kHz to 0.01 Hz. 3. Results and discussion
2. Experimental Two Cu2−xSe samples are prepared herein by facile hydrothermal reactions, namely, starfish-like Cu2−xSe constructed by nanosheets (SCu2−xSe) and Cu2−xSe nanocrystals (C-Cu2−xSe). Fig. 1b shows the XRD patterns of S-Cu2−xSe and C-Cu2−xSe. All the diffraction peaks are in accordance with cubic-phase Cu2−xSe (JCPDS No. 06–0680) with a space group of F-43 m, where a cubic sub-lattice is constructed with Se atoms located at the face-centered sites and Cu atoms statistically dispersed at the interstitial sites in the sub-lattice (Fig. 1a). SEAD patterns
2.1. Materials and synthesis Cu2−xSe materials were fabricated by a facile one-step hydrothermal route. Typically, 4 mmol of copper acetate monohydrate (Cu (ac)2·H2O), 2 mmol of Na2SeO3 and 3 mL of ethylenediamine (EDA) were dissolved in 22 mL of deionized water via vigorous stirring for 30 min. Then the precursor solution was transferred to a Teflon-lined 2
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 1. (a) Crystal structure of Cu2-xSe with cubic phase. (b) XRD patterns of S-Cu2-xSe and C-Cu2-xSe. High-resolution XPS spectra of (c) Cu 2p and (d) Se 3d for SCu2−xSe and C-Cu2−xSe.
nanosheets of S-Cu2−xSe. In a HRTEM image (Fig. 2i), lattice fringes with spacing of 0.20 nm is also observed, corresponding to the (2 2 0) plane of Cu2−xSe. It is speculated that S-Cu2−xSe should have a better Mg-storage performance than C-Cu2−xSe, as the small thickness of the nanosheets is favorable for the solid-state Mg2+ diffusion kinetics. Fig. 2j schematically illustrates the overall reaction procedure and formation mechanism of S-Cu2−xSe and C-Cu2−xSe. The morphology difference of the two samples is owing to the different formation rates of Se2−. The Se2− anion is originated from reduction of Na2SeO3, and the formation rate is related to the reductive circumstance. For SCu2−xSe, a small amount of EDA (3 mL) was added and thus Se2− was formed at a slow rate, providing enough time for the growth of Cu2−xSe nanosheets and assemble into regular starfish-like structure. Another important factor is the Cu resource (Cu(ac)2·H2O) [35]. The relatively low dissociation degree of Cu(ac)2·H2O makes Cu2+ cation released with a low rate, facilitating the formation of S-Cu2−xSe. Whereas for CCu2−xSe, large amount of NaBH4 built a strong reductive environment, and the fast crystal growth rate resulted in C-Cu2−xSe nanoscale crystals with smaller size but irregular shapes. Interestingly, it is found that Cu2−xSe could form only upon addition of EDA, while other reduction agents would lead to formation of Cu2Se. Such a phenomenon is possibly owing to the coordination of EDA with Cu2+ cation. Till now, the high-performance of Cu2−xSe electrode has been demonstrated in Li, Na and Al batteries [23–26], but not for Mg batteries yet. The typical Mg cell discharge/charge profiles for S-Cu2−xSe and CCu2−xSe at 100 mA g−1 are shown in Fig. 3a and b, respectively. An electrochemical activation process is observed for both of the cathodes, during which a decrease in polarization occurs with discharge voltage increase and charge voltage decrease. Obviously, the activation degree of S-Cu2−xSe is much higher than that of C-Cu2−xSe, due to the relatively larger size of nanosheets. At the 2nd cycle for S-Cu2−xSe, a discharge plateau at 0.8 V and a tiny charge plateau at 2.0 V are observed. After 60 cycles of activation, the discharge and charge plateaus become
shown in Fig. S2 confirm the XRD results. XPS measurement was performed to confirm the surface composition and valance states of Cu2−xSe. As exhibited in Fig. S3 for overall XPS spectra, the signals of Cu 2p and Se 3d can be easily recognized. In high-resolution Cu 2p spectra shown in Fig. 1c, two peaks at 932.5 and 952.6 eV are originated from Cu 2p3/2 and Cu 2p1/2 of Cu+, respectively [30,31]. The two peaks at 935.4 and 955.0 eV are attributed to Cu 2p3/2 and Cu 2p1/2 of Cu2+, respectively, which are origin from surface oxide layer [30,31]. Satellite peaks of Cu2+ are also observed at 943.1 and 963.2 eV [32]. It is noteworthy that the ratio of Cu2+ to Cu+ for C-Cu2−xSe is higher than that of S-Cu2−xSe, which indicates a relatively higher degree of surface oxidation, while it does not affect the structure of the bulk phase (Fig. 1b). As shown in Fig. 1d, high-resolution Se 3d spectra contain two peaks at 53.6 and 54.5 eV, corresponding to Se 3d5/2 and 3d3/2, respectively [33,34]. A broad peak at 58.7 eV is observed, which is originated from the surface oxidation of Se species [34]. Fig. 2 shows the morphologies of S-Cu2−xSe and C-Cu2−xSe captured by SEM and TEM, in which a sharp difference can be observed between the two materials. As shown in Fig. 2a, b and S4, S-Cu2−xSe displays a starfish-like microstructure aggregated by several leafy components. It can be further illustrated from the magnified image (Fig. 2c) that the components are composed of laminated nanosheets with a diameter of ~300 nm. It is interesting that such a special hierarchical structure is obtained by a facile synthesis without adopting templates or using other assistant methods. TEM image (Fig. 2d) of SCu2−xSe confirms the size of the laminated nanosheets is about 200–400 nm. In a HRTEM image (Fig. 2e), the interlayer spacing of 0.33 nm is well observed and assigned to the crystal plane of (1 1 1) for cubic-phase Cu2-xSe. On the other hand for C-Cu2−xSe, a number of well-dispersed nanocrystals are observed from Fig. 2f. Magnified image (Fig. 2g) suggests the nanocrystals show smaller particle size, but the thickness is larger than that of S-Cu2−xSe. TEM image shown in Fig. 2h confirms the smaller particle size of C-Cu2−xSe (50 ~ 200 nm) than the 3
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 2. (a–c) SEM, (d) TEM and (e) HRTEM images of S-Cu2−xSe. (f, g) SEM, (h) TEM and (i) HRTEM images of C-Cu2−xSe. (j) The diagram of formation mechanism for S-Cu2−xSe and C-Cu2−xSe.
first five cycles (Fig. 3e and f, activation process), reproducible CV curves are obtained for activated electrodes (Fig. 3g and h). The activated electrode of S-Cu2−xSe (Fig. 3g) shows current densities much higher than C-Cu2−xSe (Fig. 3h), in good accordance with the capacity of the activate cells (Fig. 3c and d). The capacity decrease of C-Cu2−xSe during initial cycles is possibly due to irreversible conversion of Cu2+ to Cu+ [20]. Fig. 4 shows the rate capability and long-term cycling stability of SCu2−xSe and C-Cu2−xSe cathodes. As exhibited in Fig. 4a and b, the two discharge plateaus become narrower as the current density increases. In Fig. 4c, S-Cu2−xSe delivers high capacities of 209, 185, 139 and 100 mAh g−1 at the current densities of 100, 200, 500 and 1000 mA g−1, respectively. A reversible capacity of 200 mAh g−1 could be obtained upon a recover of current density to 100 mA g−1, retaining 96% of its original value (see Fig. S5 for detail). In contrast, C-Cu2−xSe only delivers 48 mA h g−1 at 1000 mA g−1. Fig. 4d compares the rate capability of S-Cu2−xSe with several reported copper chalcogenides for rechargeable Mg batteries [14,15,17–20], and such an excellent rate capability of S-Cu2−xSe is almost of the highest level. The high rate capability of S-Cu2−xSe is owing to both of intrinsic nature of Cu2−xSe and thin nanosheet morphology, which facilitates the solid-state Mg2+
wider with voltage shifting to 0.85 V and 1.75 V, respectively. Meanwhile, another discharge plateau emerges at 1.25 V, as well as another charge plateau at 1.35 V. As the reduction of Cu+ to Cu0 only corresponds to a discharge plateau at about 0.95 V [17,18], the 1.25 V discharge plateau would possibly indicate phase transformation of Cu2−xSe during the electrochemical reaction. The activation process is also evidenced by the cycling performance. As shown in Fig. 3c, the reversible capacity of S-Cu2−xSe gradually increases during the initial 60 cycles from 70 to 210 mA h g−1. In contrast, C-Cu2−xSe shows relatively inferior performance, providing a capacity of 138 mA h g−1 after activation (Fig. 3d). The difference in capacities is owing to the different morphologies of S-Cu2−xSe and C-Cu2−xSe. The thin-layered nanosheets have relatively short-range solid-state Mg2+ diffusion path, which enable a higher capacity utilization. CV curves of the two Cu2−xSe cathodes were measured for initial (Fig. 3e and f) and activated cycles (Fig. 3g and h). As seen in Fig. 3e for S-Cu2−xSe, two pairs of redox peaks can be clearly recognized at 0.85/ 1.35 and 1.25/1.75 V, suggesting a two-step magnesiation/demagnesiation reaction process. Similar curves can be observed for C-Cu2−xSe with higher current densities (Fig. 3f), which agrees with the different initial capacities (Fig. 3c and d). After the change of CV curves in the 4
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 3. Discharge/charge profiles of (a) S-Cu2−xSe and (b) C-Cu2−xSe cathodes in the potential range of 0.3–2.2 V. Cycling performance of (c) S-Cu2−xSe and (d) CCu2−xSe at 100 mA g−1. CV curves in the first five cycles of (e) S-Cu2−xSe and (f) C-Cu2−xSe at a scan rate of 0.1 mV s−1. CV curves of the activated (g) S-Cu2−xSe and (h) C-Cu2−xSe.
30.0°, 36.8° and 41.0° are ascribed to the separator. At the fully discharge state (0.3 V, point a) after activation process, weak peaks for Cu2−xSe are still observed, indicating a small part of Cu2−xSe did not participate in the reaction. Clear observation of MgSe peaks at 32.8° and 47.0° confirms the occurrence of conversion reaction [14]. However, diffraction peaks of metallic Cu0 cannot be well recognized in the discharge state, possibly owing to the nanoscale particles and poor crystallinity. When charged to 1.75 V through the first charge plateau (point b), Cu2−xSe peaks enhance slightly and MgSe peaks weaken, indicating formation of Cu2−xSe from MgSe and metallic Cu0. At fully charged state of 2.2 V (point c), Cu2−xSe and MgSe peaks disappear entirely, and peaks for Cu5Se4 emerge at 27.5° and 45.4° instead. These results indicate a phase transformation during the activation process, which is common for selenides to be used as conversation-type cathodes for Mg batteries [14,15]. When discharge to 0.85 V through the first discharge plateau (point d), Cu2−xSe phase formed again as well as
diffusion. The long-term cycling stability of S-Cu2−xSe was examined at 200 mA g−1 after three cycles of activation at 50 mA g−1. As exhibited in Fig. 4e, the activation process lasted till 100th cycle, at which the capacity reached 170 mAh g−1, and then gradually faded. Such a long activation process is possibly due to the bivalent Mg2+ cation. A capacity of 109 mAh g−1 was maintained after 300 cycles for S-Cu2−xSe, corresponding to a retention rate of 65%, which is much higher than that for C-Cu2−xSe (Fig. 4f). The hierarchical starfish-like structure enhances the material integrity and prevents particle aggregation, thus resulting in a good cycleability. It is worthy to note that the coulombic efficiency is almost 100% all through the cycling, which is of special importance for the application of cathode materials. Ex-situ XRD measurements were used to investigate the electrochemical reaction mechanism of Cu2−xSe upon cycling. Cycled separator is used for the characterization since the active material tends to remain on the separator. As seen in Fig. 5a, the peaks at 25.9°, 28.1°, 5
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 4. (a) Discharge/charge profiles of (a) S-Cu2−xSe and (b) C-Cu2−xSe cathodes at different current densities. (c) Rate capability of S-Cu2−xSe and C-Cu2−xSe cathodes at different current densities varies from 100 mA g−1 to 1 A g−1. (d) Comparison for rate capability of S-Cu2−xSe with reported copper chalcogenides [14,15,17–20]. Long-term cycling stability of (e) S-Cu2−xSe and (f) C-Cu2−xSe at 200 mA g−1.
electrode materials at fully discharge (0.3 V) state. It is revealed that uniformly distributed nanoparticles appeared, which suggests the activation process occurs with downsize of Cu2−xSe, establishing large interface favorable for solid-state Mg2+ diffusion and improving the reversible capacity. Furthermore, in the HRTEM image (Fig. 5e), lattice fringes of MgSe (2 0 0) plane (0.27 nm) and metallic Cu0 (2 0 0) plane (0.18 nm) are clearly observed, which confirm the occurrence of conversion reaction. Hence, the overall conversion reaction of Cu2−xSe cathode occurs between Cu5Se4 and metallic Cu0 with intermediate of Cu2−xSe. Specifically, conversion reaction of Cu5Se4 to Cu2−xSe is
MgSe. Subsequently, Cu2−xSe peaks weakened and MgSe peaks enhanced at the second discharge state of 0.3 V (point e). Ex-situ XPS measurements were also conducted to confirm the reaction mechanism. Fig. 5b shows the high-resolution XPS spectra of Cu 2p for Cu2−xSe cathode at the fully charge (2.2 V) and discharge (0.3 V) states. Compared with charge state of Cu5Se4, the satellite peaks for Cu2+ almost vanish at the discharge state, suggesting conversion of Cu2+ to Cu+ during the reaction [26]. Hence, the two peaks at 932.5 and 952.6 eV are originated from Cu+ and metallic Cu0, respectively [19]. Fig. 5c and d show TEM images of the activated S-Cu2−xSe
6
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 5. (a) Ex-situ XRD patterns for Cu2−xSe cathodes at different discharge/charge states. (b) Ex-situ XPS spectra of Cu 2p for Cu2−xSe cathodes at the fully charged (2.2 V) and discharged (0.3 V) states. (c, d) TEM and (e) HRTEM images of activated S-Cu2−xSe at fully discharge (0.3 V) state.
thorough but that of Cu2−xSe to Cu0 is not, resulting in slightly lower capacity than theoretical value (220 mAh g−1) for the second discharge plateau. To further understand the high performance of S-Cu2-xSe cathode, the electrochemical behavior was evaluated by kinetic investigation. Fig. 6a–c depicts the CV curves at various scan rates range from 0.2 to 1.0 mV s−1. Fig. 6a–c show fresh S-Cu2-xSe, activated S-Cu2-xSe and activated C-Cu2−xSe electrodes, respectively. All the CV curves exhibit coherent redox peaks, and the currents increase and potentials shift with the increasing scan rates. The relationship between the peak current (i) and scan rate (v) could be described as i = avb, where a and b are adjustable constant parameters. A b value of 1.0 indicates a surfacecontrolled pseudo-capacitive behavior, whereas a b value of 0.5 suggests a diffusion-controlled feature [36–38]. As shown in Fig. 6d, the b values of peaks C1, C2, A1 and A2 for fresh S-Cu2-xSe cathode approach 1.0, suggesting the high pseudo-capacitive behavior. While after the activation process (Fig. 6e), the b values largely decrease, illustrating the diffusion contribution increases significantly. Such results reflect the morphology feature of S-Cu2-xSe. Due to the thin nanosheet structure providing large exposed interfaces and active sites, pseudo-capacitance (near-surface Faradic process) plays a dominant role in initials
cycles. During the activation process, the electrode experiences internal structure modification to establish framework favorable for fast solidstate Mg2+ diffusion kinetics. On the other hand, C-Cu2−xSe gives b values much lower than S-Cu2−xSe (Fig. 6f), which is owing to its thicker particles and thus lower pseudo-capacitance contribution (Fig. 6g). Furthermore, the CV curves are also used to evaluate the solid-state Mg2+ diffusion kinetics of the Cu2−xSe cathodes in Mg cells. The diffusion coefficient of Mg2+ is calculated by the following equation [39–41]:
Ip = 2.69 × 105n3/2AD1/2ν1/2C0
(1)
where Ip is the peak current (A), n is the number of electrons per molecule during the reaction, A is the contact area between the electrode and electrolyte (cm2), D is the diffusion coefficient of Mg2+ (cm2 s−1), C0 is the concentration of Mg2+ ion in the electrode material (mol cm−3), and v is the scan rate (V s−1). DMg2+ of peaks C1, C2, A1, A2 for S-Cu2-xSe are determined to be 2.5 × 10−11, 2.4 × 10−10, 2.5 × 10−10 and 1.6 × 10−11, respectively, which are much higher than those for CCu2-xSe (Fig. 6h, see Table S1 for the calculation details). Benefited from the thin nanosheet morphology and modified solid-state diffusion 7
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
Fig. 6. CV curves of (a) fresh S-Cu2−xSe, (b) activated S-Cu2−xSe and (c) C-Cu2−xSe cathodes at various scan rates. The corresponding log(peak current)-log(scan rate) plots of the redox peaks for (d) fresh S-Cu2−xSe, (e) activated S-Cu2−xSe and (f) C-Cu2−xSe cathodes. (g) Contribution ratio of diffusion capacities at different scan rates. (h) Corresponding peak current-(scan rate)1/2 plots of the redox peaks for activated S-Cu2−xSe and C-Cu2−xSe cathodes. (i) Nyquist plots of the activated S-Cu2−xSe and C-Cu2−xSe cathodes.
paths, faster solid-state Mg2+ diffusion is achieved in the hierarchical SCu2−xSe. Further Nyquist plots obtained by EIS measurements (Fig. 6i) also reveal that S-Cu2−xSe exhibits much lower resistance than CCu2−xSe, indicating a faster charge transfer rate [42,43].
Declaration of Competing Interest
4. Conclusions
Acknowledgements
In summary, we introduce facile one-step hydrothermal methods to fabricate nanosheets assembling hierarchical starfish-like Cu2−xSe (SCu2−xSe) and nanocrystals (C-Cu2−xSe), and their applications as Mgstorage cathodes are systematically demonstrated. It is observed that SCu2−xSe exhibits a high reversible capacity of 210 mAh g−1 at 100 mA g−1, an excellent rate capability of 100 mA h g−1 at 1 A g−1, and a long-term cycling stability over 300 cycles. The high capacity and outstanding rate capability of S-Cu2−xSe is owing to the thin nanosheet structures, which facilitate solid-state Mg2+ diffusion. The long-term cycling stability of S-Cu2−xSe is attributed to the hierarchical morphology, which maintain the material integrity during cycling. Mechanism investigation reveals that the electrochemical reaction is a typical conversion-type reaction occurring between Cu5Se4 and metallic Cu0 with intermediate of Cu2−xSe. This work delivers facile approaches for the construction of high-performance Cu2−xSe Mg cathodes with delicate morphology tailoring, and more significantly, highlights scientific insights for the exploration of conversion-type Mg-storage materials.
This work was financially supported by the Hubei Provincial Natural Science Foundation (2019CFB452 and 2019CFB620), the National Natural Science Foundation of China (Nos. 51402113 and 21403305), the Fundamental Research Funds for the Central Universities and the Open Fund of Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123235. References [1] M. Mao, T. Gao, S. Hou, C. Wang, A critical review of cathodes for rechargeable Mg batteries, Chem. Soc. Rev. 47 (2018) 8804–8841. [2] E. Levi, Y. Gofer, D. Aurbach, On the way to rechargeable Mg batteries: the challenge of new cathode materials, Chem. Mater. 22 (2010) 860–868.
8
Chemical Engineering Journal xxx (xxxx) xxxx
D. Chen, et al.
[25] Y. Li, X. Sun, Z. Cheng, X. Xu, J. Pan, X. Yang, F. Tian, Y. Li, J. Yang, Y. Qian, Mesoporous Cu2-xSe nanocrystals as an ultrahigh-rate and long-lifespan anode material for sodium-ion batteries, Energy Storage Mater. (2019). [26] H. Li, J. Jiang, J. Huang, Y. Wang, Y. Peng, Y. Zhang, B.J. Hwang, J. Zhao, Investigation of the Na storage property of one-dimensional Cu2−xSe nanorods, ACS Appl. Mater. Interfaces 10 (2018) 13491–13498. [27] Y. Cheng, T. Liu, Y. Shao, M. Engelhard, J. Liu, G. Li, Electrochemically stable cathode current collectors for rechargeable magnesium batteries, J. Mater. Chem. A 2 (2014) 2473–2477. [28] J. Muldoon, C.B. Bucur, A.G. Oliver, J. Zajicek, G.D. Allred, W.C. Boggess, Corrosion of magnesium electrolytes: chlorides – the culprit, Energy Environ. Sci. 6 (2013) 482–487. [29] C. Wall, Z. Zhao-Karger, M. Fichtner, Corrosion resistance of current collector materials in bisamide based electrolyte for magnesium batteries, Electrochem. Lett. 4 (2015) C8–C10. [30] R. Jin, X. Liu, L. Yang, G. Li, S. Gao, Sandwich-like Cu2-xSe@C@MoSe2 nanosheets as an improved-performance anode for lithium-ion battery, Electrochim. Acta 259 (2018) 841–849. [31] Y. Liu, S. Shen, J. Zhang, W. Zhong, X. Huang, Cu2-xSe/CdS composite photocatalyst with enhanced visible light photocatalysis activity, Appl. Surf. Sci. 478 (2019) 762–769. [32] L. Zhu, H. Xie, Y. Liu, D. Chen, M. Bian, W. Zheng, Novel ultralong hollow hyperbranched Cu2-xSe with nanosheets hierarchical structure: preparation, formation mechanism and properties, J. Alloy. Compd. 802 (2019) 430–436. [33] Y. Tang, Z. Zhao, X. Hao, Y. Wang, Y. Liu, Y. Hou, Q. Yang, X. Wang, J. Qiu, Engineering hollow polyhedrons structured from carbon-coated CoSe2 nanospheres bridged by CNTs with boosted sodium storage performance, J. Mater. Chem. A 26 (2017) 13591–13600. [34] B. Yu, Y. Hu, F. Qi, X. Wang, B. Zheng, K. Liu, W. Zhang, Y. Li, Y. Chen, Nanocrystalline Ni0.85Se as efficient non-noble-metal electrocatalyst for hydrogen evolution reaction, Electrochim. Acta 242 (2017) 25–30. [35] C. Wang, T. Wang, J. Liu, Y. Zhou, D. Yu, J. Kuei, F. Han, Q. Li, J. Chen, Y. Huang, Facile synthesis of silk-cocoon S-rich cobalt polysulfide as an efficient catalyst for hydrogen evolution reaction, Energy Environ. Sci. 11 (2018) 2467–2475. [36] D. Chao, P. Liang, Z. Chen, L. Bai, H. Shen, X. Liu, X. Xia, Y. Zhao, S.V. Savilov, J. Lin, Z.X. Shen, Pseudocapacitive Na-ion storage boosts high-rate and areal capacity of self-branched 2D layered metal chalcogenide nanoarrays, ACS Nano 10 (2016) 10211–10219. [37] D. Chao, C. Zhu, P. Yang, X. Xia, J. Liu, J. Wang, X. Fan, S.V. Savilov, J. Lin, H.J. Fan, Z.X. Shen, Array of nanosheets render ultrafast and high-capacity Na-ion storage by tunable pseudocapacitance, Nat. Commun. 7 (2016) 12122. [38] S.H. Yang, S.K. Park, Y.C. Kang, Mesoporous CoSe2 nanoclusters threaded with nitrogen-doped carbon nanotubes for high-performance sodium-ion battery anodes, Chem. Eng. J. 370 (2019) 1008–1018. [39] Z. Wei, D. Wang, M. Li, Y. Gao, C. Wang, G. Chen, F. Du, Fabrication of hierarchical potassium titanium phosphate spheroids: a host material for sodium-ion and potassium-ion storage, Adv. Energy Mater. 8 (2018) 1801102. [40] Y. Yang, Y. Tang, G. Fang, L. Shan, J. Guo, W. Zhang, J. Zhou, C. Wang, L. Wang, S. Liang, Li+ intercalated V2O5·nH2O with enlarged layer spacing and fast ion diffusion as an aqueous zinc-ion battery cathode, Energy Environ. Sci. 11 (2018) 3157–3162. [41] D. Yu, M. Li, T. Yu, C. Wang, Y. Zeng, X. Hu, G. Chen, G. Yang, F. Du, Nanotubeassembled pine-needle-like CuS as an effective energy booster for sodium-ion storage, J. Mater. Chem. A 7 (2019) 10619–10628. [42] Y. Tang, Z. Zhao, X. Hao, Y. Wei, H. Zhang, Y. Dong, Y. Wang, X. Pan, Y. Hou, X. Wang, J. Qiu, Cellular carbon-wrapped FeSe2 nanocavities with ultrathin walls and multiple rooms for ion diffusion-confined ultrafast sodium storage, J. Mater. Chem. A 7 (2019) 4469–4479. [43] F. Liu, Z. Chen, G. Fang, Z. Wang, Y. Cai, B. Tang, J. Zhou, S. Liang, V2O5 nanospheres with mixed vanadium valences as high electrochemically active aqueous zinc-ion battery cathode, Nano-Micro Lett. 11 (2019) 1–19.
[3] F. Murgia, P. Antitomaso, L. Stievano, L. Monconduit, R. Berthelot, Express and low-cost microwave synthesis of the ternary chevrel phase Cu2Mo6S8 for application in rechargeable magnesium batteries, J. Solid State Chem. 242 (2016) 151–154. [4] L. Wang, K. Asheim, P.E. Vullum, A.M. Svensson, F. Vullum-Bruer, Sponge-like porous manganese (II, III) oxide as a highly efficient cathode material for rechargeable magnesium ion batteries, Chem. Mater. 28 (2016) 6459–6470. [5] Y. Liang, H.D. Yoo, Y. Li, J. Shuai, H.A. Calderon, F.C.R. Hernandez, L.C. Grabow, Y. Yao, Interlayer-expanded molybdenum disulfide nanocomposites for electrochemical magnesium storage, Nano Lett. 15 (2015) 2194–2202. [6] J. Song, E. Sahadeo, M. Noked, S.B. Lee, Mapping the challenges of magnesium battery, J. Phys. Chem. Lett. 7 (2016) 1736–1749. [7] P. Canepa, G.S. Gautam, D.C. Hannah, R. Malik, M. Liu, K.G. Gallagher, K.A. Persson, G. Ceder, Odyssey of multivalent cathode materials: open questions and future challenges, Chem. Rev. 117 (2017) 4287–4341. [8] J. Muldoon, C.B. Bucur, T. Gregory, Quest for nonaqueous multivalent secondary batteries: magnesium and beyond, Chem. Rev. 114 (2014) 11683–11720. [9] X. Sun, P. Bonnick, L.F. Nazar, Layered TiS2 positive electrode for Mg batteries, ACS Energy Lett. 1 (2016) 297–301. [10] X. Sun, P. Bonnick, V. Duffort, M. Liu, Z. Rong, K.A. Persson, C. Gerbrand, L.F. Nazar, A high capacity thiospinel cathode for Mg batteries, Energy Environ. Sci. 9 (2016) 2273–2277. [11] Z. Zhang, S. Dong, Z. Cui, A. Du, G. Li, G. Cui, Rechargeable magnesium batteries using conversion-type cathodes: a perspective and minireview, Small Methods 2 (2018) 1800020. [12] R. Zhang, C. Ling, F. Mizuno, A conceptual magnesium battery with ultrahigh rate capability, Chem. Commun. 51 (2015) 1487–1490. [13] Z. Chen, Z. Zhang, A. Du, Y. Zhang, M. Men, G. Li, G. Cui, Fast magnesiation kinetics in α-Ag2S nanostructures enabled by an in situ generated silver matrix, Chem. Commun. 55 (2019) 4431–4434. [14] Y. Tashiro, K. Taniguchi, H. Miyasaka, Copper selenide as a new cathode material based on displacement reaction for rechargeable magnesium batteries, Electrochim. Acta 210 (2016) 655–661. [15] Y. Tashiro, K. Taniguchi, H. Miyasaka, The effect of anion-sublattice structure on the displacement reaction in copper sulfide cathodes of rechargeable magnesium batteries, Chem. Lett. 46 (2017) 1240–1242. [16] V. Duffort, X. Sun, L.F. Nazar, Screening for positive electrodes for magnesium batteries: a protocol for studies at elevated temperatures, Chem. Commun. 52 (2016) 12458–12461. [17] M. Wu, Y. Zhang, T. Li, Z. Chen, S. Cao, F. Xu, Copper sulfide nanoparticles as highperformance cathode materials for magnesium secondary batteries, Nanoscale 10 (2018) 12526–12534. [18] F. Xiong, Y. Fan, S. Tan, L. Zhou, Y. Xu, C. Pei, Q. An, L. Mai, Magnesium storage performance and mechanism of CuS cathode, Nano Energy 47 (2018) 210–216. [19] Z. Wang, S. Rafai, C. Qiao, J. Jia, Y. Zhu, X. Ma, C. Cao, Microwave-assisted synthesis of CuS hierarchical nanosheets as the cathode material for high-capacity rechargeable magnesium batteries, ACS Appl. Mater. Interfaces 11 (2019) 7046–7054. [20] M. Wu, Y. Zhang, H. Wu, A. Qin, X. Li, J. Shen, Z. Chen, S. Cao, T. Li, F. Xu, Cu9S5 nanoflower cathode for Mg secondary batteries: high performance and reaction mechanism, Energy Technol. 7 (2019) 1800777. [21] B. Liu, T. Luo, G. Mu, X. Wang, D. Chen, G. Shen, Rechargeable Mg-ion batteries based on WSe2 nanowire cathodes, ACS Nano 7 (2013) 8051–8058. [22] D. He, D. Wu, J. Gao, X. Wu, X. Zeng, W. Ding, Flower-like CoS with nanostructures as a new cathode-active material for rechargeable magnesium batteries, J. Power Sources 294 (2015) 643–649. [23] H. Li, J. Jiang, F. Wang, J. Huang, Y. Wang, Y. Zhang, J. Zhao, Facile synthesis of rod-like Cu2-xSe and insight into its improved lithium-storage property, ChemSusChem 10 (2017) 2235–2241. [24] J. Jiang, H. Li, T. Fu, B.J. Hwang, X. Li, J. Zhao, One-dimensional Cu2−xSe nanorods as the cathode material for high-performance aluminum-ion battery, ACS Appl. Mater. Interfaces 10 (2018) 17942–17949.
9