Synthesis of High-Quality α-MnSe Nanostructures with Superior

Mar 10, 2016 - Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, ... an anode material for a lithium ion battery, which exhibited su...
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Synthesis of High-Quality α‑MnSe Nanostructures with Superior Lithium Storage Properties Na Li, Yi Zhang, Hongyang Zhao, Zhengqing Liu, Xinyu Zhang, and Yaping Du* Frontier Institute of Chemistry, Frontier Institute of Science and Technology and College of Science, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shaanxi Province 710054, China S Supporting Information *

ABSTRACT: High-quality α-MnSe nanocubes were successfully prepared for the first time by an effective hot injection synthesis strategy. This approach was simple but robust and had been applied to the controllable synthesis of different sizes and diverse morphologies of α-MnSe nanostructures. The crystal phases, compositions, and microstructures of these nanostructures had been systematically characterized with a series of techniques. As a proof-of-concept application, the as-prepared α-MnSe nanocubes were used as an anode material for a lithium ion battery, which exhibited superior rate ability and ultralong cycle stability in half-cell and full-cell tests. Importantly, the phase transition from α-MnSe to β-MnSe during the electrochemical process was proved by ex situ X-ray diffraction and selected area electron diffraction. The excellent electrochemical performance of α-MnSe endowed its potential as an anode material candidate for high performance lithium storage.

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INTRODUCTION

Manganese selenide (MnSe), as a close analogue to MnS, is a p-type semiconductor (band gap = 2.0 eV).15 MnSe can crystallize into three kinds of structure forms: rock salt (RS),16,17 zinc blende (ZB),18 and wurtzite (WZ).19,20 Among them, rock salt MnSe (α-MnSe) is a stable phase, and the majority of relevant studies always focused on its optical and magnetic properties while a few investigations concerned its electrochemical performance as an energy storage material.18,21 To date, only one study22 has reported an αMnSe thin film LIB anode fabricated by reactive pulsed laser deposition. When used as anode material for LIBs, α-MnSe electrode can give a rather low voltage plateau (about 0.6 V vs Li/Li+) with a theoretical capacity of 400 mAh g−1 and remain stable for 120 cycles. However, further investigation about the controllable synthesis of high-quality (single-crystalline structure, monodisperse size and shape, and pure phase) α-MnSe nanostructures and their electrochemical performances in full cell configuration have not been reported yet.22−24 On the other hand, tailoring the dimension of electrode material into nanoscale is a simple and effective strategy to optimize the electrochemical performance since nanostructure can enhance the contact with the electrolyte, increase the surface reactivity, and shorten both ionic and electronic transport length. What is more, for a single-crystalline particle with a size of ∼100 nm, the channel-blocking point defects could be reduced and hence the Li ion diffusion would be facilitated.25 Therefore, it would be more appealing if singlecrystalline α-MnSe could be prepared with a uniform nanoscale.

Rechargeable lithium ion batteries (LIBs) are indisputably one promising technology to fulfill not only the miniature electronic market such as cell phones, camcorders, and laptops, but also upcoming large scale applications in the field of electric vehicles.1 These aforementioned applications require high power capability, high energy density, and long cycling stability, and there are still some obstacles for our commercially available LIB systems to achieve these requirements.2 Currently, although graphites have been widely applied as the conventional LIB anode material, their inferior cycling stability caused by volume expansion during Li ion intercalation and extraction vastly limit their applications for the future electronic market.3 Zero strain Li4Ti5O12 (LTO), an insertion-type anode material, can deliver a stable charge/discharge platform and good cycling performance. However, this anode electrode shows rather higher insertion potential (about 1.55 V vs Li/Li+) and lower capacity (about 175 mAh g−1) than the commercial graphitic anodes (about 0.1 V vs Li/Li+ with theoretical capacity of 372 mAh g−1) and hence it exhibits unsatisfactory energy density.4 Therefore, searching for alternative anode materials is highly needed. Recently, transition metal monochalcogenides such as MnS,5,6 FeS,7,8 CoS,9 NiS,10 CuS,11 NiSe,12 CuSe,13 and FeSe14 have been well documented as LIB anode materials since the conversion reaction based on transition metal compounds can provide capacities several times higher than those of insertion-type anodes. Moreover, this conversion reaction has been reported to be reversible so that transition metal monochalcogenides are valuable for the application of lithium ion storage. © XXXX American Chemical Society

Received: November 9, 2015

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DOI: 10.1021/acs.inorgchem.5b02558 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry In this paper, for the f irst time, we successfully established a simple but effective method to synthesize high-quality rock salt manganese selenide (α-MnSe) nanostructures based on a hot injection strategy. For example, high-quality cubic phase αMnSe nanocubes (referred to as MS1), with ∼100 nm cube average side length, have been synthesized. Additionally, our method can control the α-MnSe nanostructures with different sizes and morphologies, i.e. high-quality α-MnSe spherical nanoparticles with an average size of ∼25 nm (referred to as MS2) and α-MnSe nanocubes with a uniform side length of ∼50 nm (referred to as MS3). The electrochemical performance of α-MnSe as anode material for LIBs has been studied in detail. In the half-cell test system, the α-MnSe nanocubes can give a low discharge voltage plateau around 0.6 V with a reversible discharge capacity as high as 400 mAh g−1. Ultralong cycling performance and a capacity retention of 150 mAh g−1 at 2 C for 5000 cycles is achieved, and the phase transition phenomenon during the electrochemical process has been verified by selected area electron diffraction (SAED) and ex situ X-ray diffraction (XRD) analysis. Furthermore, the α-MnSe nanocube anode is coupled with Li2NaV2(PO4)3 (LNVP) cathode to fabricate a full cell configuration, which exhibits a discharge capacity of 150.1 mAh g−1 with an average output voltage of about 2.3 V at a high current density of 1 C. The outstanding electrochemical performance demonstrates the potential of α-MnSe as anode material for advanced LIBs with ultralong cycle stability. To the best of our knowledge, it is the first report about the synthesis and exploration of highquality α-MnSe with uniform nanostructure and its outstanding electrochemical performance for full cell configuration (Scheme 1). Scheme 1. Schematic Illustration for the Synthesis of αMnSe Nanocubes and Its Application for Lithium Storage

Figure 1. (a) XRD pattern of MS1 sample. (b) SEM, (c) TEM, (d) HRTEM, and (e) SAED images of MS1 sample. XPS spectra of (f) Mn 2p and (g) Se 3d for MS1 sample.

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The shape of the as-synthesized MS1 sample was observed by field emission scanning electron microscopy (SEM, Figure 1b) and transmission electron microscopy (TEM, Figure 1c,d). The SEM image in Figure 1b demonstrated the formation of monodisperse α-MnSe nanocubes with a high morphological purity yield (∼100%). The structures of as-synthesized α-MnSe nanocubes were further studied by TEM (Figure 1c). As seen from Figure 1c, the average side length of the MS1 sample was ∼100 nm. From the high resolution transmission electron microscopy (HRTEM) image of MS1 sample in Figure 1d, the spacing of lattice fringes was ∼0.27 nm, attributed to the (200) plane spacing of α-MnSe. The SAED pattern in Figure 1e further verified the single-crystalline feature of MS1 sample and could be labeled as a cubic phase. We employed X-ray photoelectron spectroscopy (XPS) to investigate the chemical

RESULTS AND DISCUSSION Characterization of the Materials. The MS1 sample was synthesized through a hot-injection method (see the Experimental Section for details). The as-synthesized MS1 sample presented an exclusively cubic phase (NaCl type, space group Fm3m, JCPDS 11-0683), as confirmed by the powder XRD analysis (Figure 1a), with the crystal lattice parameters calculated to be a = b = c = 5.462 Å. The energy dispersive X-ray analysis (EDAX) spectra of α-MnSe nanocube data (Figure S1 in the Supporting Information) illustrated the presence of Mn and Se elements with a stoichiometric composition (Mn/Se atomic ratio = 1:0.92), confirming the formation of the stoichiometric α-MnSe compounds. B

DOI: 10.1021/acs.inorgchem.5b02558 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry states of Mn and Se in the MS1 sample (Figure 1f,g). As shown in Figure 1f, the binding energies of Mn 2p3/2 and Mn 2p1/2 peaks were located at 642.1 and 653.9 eV,16 respectively, suggesting that the Mn ion was positive divalent in the assynthesized MS1 sample. Meanwhile, the peaks located at 55.1 eV (Figure 1g) could be assignable to the binding energy of Se 3d and thus the Se ion was negative divalent in the MS1.16 Note that we have carried out condition based experiments to synthesize the MS1 sample with high quality. In our synthesis, it was discovered that the precursor mole ratios, the surfactant composition, the reaction temperature, and time jointly affected the generation of high-quality MS1 product. For instance, under the identical condition, i.e. oleylamine (26.6 mmol), oleic acid (3.4 mmol), and l-octadecene (10 mmol), the reaction was fixed at 180 °C for 72 h, while tuning the mole ratio of Mn(CH3COO)2·4H2O/Se into 2/1 (1.0 mmol:0.5 mmol) or 1/2 (0.5 mmol:1.0 mmol), only irregular α-MnSe nanoparticles were harvested (Figure S2). Besides the mole ratios of precursors, the compositions of surfactants were necessary for the formation of high-quality MS1 product. For instance, keeping other reaction conditions unchanged, when the surfactant was pure OM (30.0 mmol), only severely aggregated and ill-shaped α-MnSe nanocubes were obtained (Figure S3a), while when the mole ratio of OM and OA was reduced to 16:1 (28.2 mmol:1.8 mmol) and 12:1 (27.7 mmol:2.3 mmol), relatively nonuniform α-MnSe nanocubes were produced (Figure S3b,c). When the mole ratio of OM and OA was further decreased to 6:1 (25.7 mmol:4.3 mmol), the nanocubes changed into aggregated nanoparticles (Figure S3d). To conclude, high-quality MS1 with high yield was only obtained at an optimized OM/OA mole ratio of 8:1 (26.6 mmol:3.4 mmol) (Figure 1c). Additionally, both the reaction temperature and time affected the quality of the products. For instance, at the same conditions, when the reaction temperature was decreased to 200 and 210 °C, respectively, we only obtained nonuniform α-MnSe nanocubes with large size distribution (Figure S4a,b); however, when the temperature was promoted to 240 °C, some of α-MnSe nanocubes turned into aggregation nanoparticles (Figure S4c). On the other side, 220 °C and fixed other conditions, for a short reaction time (t = 30 min), resulted in relatively nonuniform α-MnSe nanocubes (Figure S5a). When the reaction time was prolonged to 4 and 8 h, respectively, the as-obtained α-MnSe nanocubes had large size distribution and coexisted with some discernible impurities (Figure S5b,c). However, when the reaction time was further extended up to 18 h, aggregated α-MnSe nanocubes were produced (Figure S5d), indicating that a distinct “Ostwald ripening” process occurred.26 Importantly, our current method can be applied to manipulate the sizes and morphologies of α-MnSe nanocrystals in a controlled manner. For example, in our synthesis, we fixed the molar ratios of Mn(acac)2/Se = 1:1 (1.0 mmol:1.0 mmol), reaction time, and temperature (220 °C, 2 h), solvents were composed of OM (20 mmol) and ODE (20 mmol), the precursor was changed from Mn(CH3COO)2·4H2O to Mn(acac)2, and monodisperse α-MnSe nanoparticles (MS2) with size of ∼25 nm were obtained (Figure 2a). The SAED pattern with clear diffraction spots and lattice fringes shown in the HRTEM image proved that the MS2 sample was of high crystallization, and the lattice fringes with a spacing of ∼0.27 nm corresponded to the spacing of the (200) plane of cubic phase α-MnSe. When the temperature was elevated to 280 °C, uniform α-MnSe nanocubes (MS3) with a side length of ∼50

Figure 2. (a) TEM image and SAED pattern (inset) of MS2 sample. (b) HRTEM image of a single MS2 sample. (c) TEM image and SAED pattern (inset) of MS3 sample. (d) HRTEM image of a single MS3 sample.

nm were harvested for 30 min (Figure 2c). The SAED pattern (inset of Figure 2c) and the HRTEM image (Figure 2d) provided the single-crystalline nature of the MS3, and the lattice fringes with a spacing of ∼0.27 nm corresponded to the (200) plane of cubic phase α-MnSe. Moreover, the XRD patterns of MS2 and MS3 products (Figure S6) could be also readily indexed as the pure cubic α-MnSe structure phase (NaCl type, space group Fm3m, JCPDS 11-0683), and the broadening of diffraction peaks indicated the nanocrystallinity of the samples. Lithium Ion Storage Performance. The electrochemical performance of the MS1, MS2, and MS3 samples as anodes for LIBs were evaluated by assembling CR2032 coin type half-cells (metallic lithium serves as the counter electrode). Figure 3a shows the cyclic voltammograms (CV) curves for the initial three cycles of MS1 electrode at a scan rate of 0.5 mV s−1 in a voltage range from 0 to 3.0 V vs Li/Li+. In the first cathodic sweep the sharp reduction peak close to 0.1 V agreed well with the reduction of Mn2+ to LixMnSe and the formation of the solid electrolyte interface (SEI) layers.6,27 The peaks at 2.0 and 1.3 V implied the change of the structure or composition of the α-MnSe after the first cycle. This phenomenon was similar to the previous report.22 During the following anodic scan, a wide oxidation peak around 1.3 V could be observable since β-MnSe was formed22 (we will discuss it later in detail). Starting from the second cycle, the profiles of CV curves were overlapped very well, suggesting a good reversibility of the electrochemical reaction. The electrochemical behavior of pure MS2 and MS3 electrodes was similar to that of MS1 (Figure S7a,b). Figure 3b depicts the galvanostatic charge/discharge profiles of the MS1 half-cell in the first, second, third, and tenth cycles at a current density of 0.25 C. Two distinct voltage plateaus around 0.6 and 1.2 V could be found during the discharge/ charge process, matching well with the CV analysis. The first discharge and charge capacities of the MS1 sample were 790 and 401 mAh g−1, respectively. There was a large capacity loss C

DOI: 10.1021/acs.inorgchem.5b02558 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

shown in Figure S7c. This excellent rate performance of the MS1 electrode proved the good kinetics since this nanocubic morphology could not only offer large contact areas between active materials and the electrolyte, but also shorten the lithium ion pathways. In addition, electrochemical impedance spectroscopy (EIS) (Figure S8) measurement was carried out to prove that the MS1 sample has a higher lithium ion diffusion rate than the MS2 and MS3 samples. Therefore, the low voltage plateau (lower than 0.7 V), high theoretical capacity (about 400 mAh g−1), and ultralong stability (at 2 C for 5000 cycles) demonstrated its potential application for LIBs as anode material. In order to explain the reaction mechanism of the MS1 electrode, ex situ XRD, SAED, and HRTEM were carried out after the first, 100th, 300th, and 500th cycles, respectively. Figure 4a shows the XRD patterns of the MS1 electrode at

Figure 3. Electrochemical performance of MS1 sample for half-cell test. (a) CV curves at 0.5 mV s−1. (b) Charge/discharge curves at 0.25 C. (c) Cyclic properties at 2 C and rate performance (inset).

in the first cycle, and this was caused by the irreversible processes such as electrolyte decomposition and inevitable formation of the SEI layer.6,27 From the second to tenth cycles, the capacity was reduced slightly and stabilized at 398 mAh g−1, which was close to its theoretical capacity (about 400 mAh g−1).22 In addition, there was an uncommon point (marked by a black arrow) in the discharge curves and it was possible due to the converted products starting to nucleate at this stage.28 Remarkably, compared with the LTO (1.55 V with a theoretical capacity of 175 mAh g−1), the lower discharge voltage plateau and the higher capacity of MS1 could provide a higher energy density when it served as an anode material. The cycling stability test was also characterized at a high discharge−charge rate of 2 C, and the results are presented in Figure 3c. The MS1 electrode delivered a high capacity of 280 mAh g−1 for the first cycle. After the 200th cycle, the capacity started to increase gradually and reached the highest value of 320 mAh g−1 at the 260th cycle, and then reduced to about 200 mAh g−1 after 550 cycles. This cycling behavior between the 200th and 550th cycles could be accounted for by a restructuring process of the electrode29 (phase transition from α-MnSe to β-MnSe), and we will discuss it later in detail. Importantly, the MS1 electrode was cycled 5000 times at 2 C with a retention capacity about 150 mAh g−1 (Coulombic efficiency ∼ 100%, the charge capacity and discharge capacity were practically overlapped) throughout the whole process. MS1, MS2, and MS3 coin half-cells were cycled from 0.25 to 8 C in steps and then returned to 0.25 C. As shown in the inset of Figure 3c, reversible capacity of 720 mAh g−1 was achieved at the initial cycle with a discharge/charge rate of 0.25 C, and then the capacity was maintained at about 400 mAh g−1, which was almost close to the theoretical capacity.22 When the discharge/ charge rate was set at 2, 4, and 8 C, reversible capacities of 202, 143, and 90 mAh g−1 were achieved, respectively. It should be noted that this reversibility performance was much better than the other previous reports on this material.22 After 60 cycles at various current densities, the discharge capacity of the MS1 electrode could still recover to 386 mAh g−1 at 0.25 C, indicating a outstanding rate performance and resilience of the electrode. The rate performances of MS2 and MS3 are also

Figure 4. (a) Ex situ XRD patterns of the electrodes after different cycles.Observable Cu signals were from the current collector. (b) SAED pattern of the as-formed electrodes after 500 cycles. (c) Phase transition illustration of α-MnSe to β-MnSe.

different cycles when the electrode was discharged to 0 V. There were also current collector (Cu) peaks in every pattern of the tested samples. After the first cycle, the peaks of β-MnSe were found at 26.5 and 29.8° but the peaks of α-MnSe were dominant. After the 100th and 300th cycles, the peaks of βMnSe at 26.5 and 29.8° became gradually sharp, and after the 500th cycle, there were only peaks of β-MnSe and the peaks of α-MnSe totally disappeared, indicating the phase transition from α-MnSe to β-MnSe during the electrochemical process. The SAED patterns could also be used to further confirm the formation of β-MnSe species. After 500 cycles, the rings of the (111), (220), and (311) planes of β-MnSe were detected (Figure 4b). Based on the above experimental results, the phase transition from α-MnSe to β-MnSe was confirmed to exist during the electrochemical process (Figure 4c). Importantly, in our work, NASICON-type Li2NaV2(PO4)3 (LNVP)30 was employed as a cathode material to couple with MS1 anode for full cell assembly. Scheme 2 schematically illustrates the construction of the LNVP//MS1 full cell configuration. CV studies were conducted from 0 to 4.0 V at D

DOI: 10.1021/acs.inorgchem.5b02558 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Schematic Illustration of the Lithium Ion Full Cell with the LNVP//MS1 Couple

even after 120 cycles, indicating the excellent cyclability of the LNVP//MS1 full cell.

a scan rate of 0.25 mV s−1, and the results are shown in Figure 5a. In the first cycle, three predominant oxidation peaks

CONCLUSION In summary, we have established a simple but effective approach for hot injection synthesis of high-quality α-MnSe nanocubes. By varying the experimental conditions, the sizes and morphologies of the α-MnSe nanostructures were readily manipulated. The as-obtained α-MnSe nanocubes exhibited outstanding electrochemical performance as anode material for LIBs. In the half-cell test, the as-obtained α-MnSe electrode can give a stable discharge capacity of about 150 mAh g−1 at 2 C even after 5000 cycles, and the phase transition from α-MnSe to β-MnSe during electrochemical process has been proved in detail. When coupled with Li2NaV2(PO4)3 cathode for the full cell test, it can still deliver a stable discharge capacity of 150.1 mAh g−1 at 1 C. We expected this synthetic methodology could be broadly applicable for the facile and high-yield production of a wide variety of high-quality transition metal selenide nanostructures with various potential applications, and the current proof-of-concept study may pave new avenues for the future developments of ultralong cycle rechargeable LIBs.

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EXPERIMENTAL SECTION

Synthesis of α-MnSe Nanocubes (MS1). For the synthesis of MS1 sample, at room temperature, 0.5 mmol of manganese(II) acetate tetrahydrate (Mn(CH3COO)3·4H2O) was added to a mixture of 26.6 mmol of oleylamine (OM), 3.4 mmol of oleic acid (OA), and 10 mmol 1-octadecene (ODE) in a three-neck flask (50 mL). The slurry was heated to 120 °C in a vacuum for 60 min, and a transparent solution was formed. Then, 0.5 mmol of selenium (Se) was dispersed in a mixture of 0.1 mL of 1-dodecanethiol (1-DDT) and 3.0 mL of OM to form a colloid solution, and this solution was swiftly injected into the aforementioned solution at 220 °C. The mixture was maintained at 220 °C for 2 h. Finally, the α-MnSe nanocubes were precipitated by adding excess absolute ethanol and collected by centrifugation at 8500 rpm for 10 min. The as-harvested α-MnSe nanocubes could be readily dispersed in cyclohexane. The yield of MS1 sample was ca. ∼ 80%. Synthesis of α-MnSe Nanoparticles (MS2). The synthetic process was the same as that of α-MnSe nanocubes, with the exception of 1.0 mmol of Mn(acac)2 was added to a three-neck flask (50 mL) containing 20 mmol of OM and 20 mmol of ODE. Then, the mixture was heated to 120 °C in a vacuum about 60 min. Then, 1.0 mmol of Se powder was dispersed in the mixture of 0.1 mL of 1-DDT and 3 mL of OM to form a colloid solution, and this solution was swiftly injected into the aforementioned solution at 220 °C. The mixture was then maintained at 220 °C for 2 h under pure N2 atmosphere. Synthesis of α-MnSe Small Nanocubes (MS3). The synthetic process was the same as that of α-MnSe nanocubes, with the exception of 1.0 mmol of Mn(acac)2 was added to a three-neck flask (50 mL) containing 20 mmol of OM and 20 mmol of ODE. Then, the mixture was heated to 120 °C in a vacuum about 60 min. After that, 1.0 mmol of Se powder was dispersed in the mixture of 0.1 mL of 1-DDT and 3 mL of OM to form a colloid solution, and this solution was swiftly injected into the aforementioned solution at 280 °C. The mixture was maintained at 280 °C for 30 min under pure N2 atmosphere. Electrochemical Measurements for LIB. The electrochemical tests of the α-MnSe half-cell were performed by using CR2032 cointype cells, comprised of an α-MnSe cathode and lithium metal anode parted by polypropylene (PP) film. The α-MnSe electrodes were fabricated by milling 80 wt % active materials, 10 wt % acetylene black, and 10 wt % poly(vinyl difluoride) (PVDF) in N-methylpyrrolidinone (NMP) to form a homogeneous slurry. The slurry was pasted uniformly on a Cu foil current collector, and then the electrode was dried under vacuum at 110 °C for 12 h. The cells were assembled in a glovebox with dried argon gas. The electrolyte was 1 M LiPF6 in a

Figure 5. Electrochemical performance of LNVP//MS1 for full cell test. (a) CV curves at a scan rates of 0.25 mV s−1. (b) Rate performance. (c) Cyclic properties at 1 C.

appeared at 1.7, 2.25, and 3.2 V and the corresponding reduction peak could be observable at 2.3 V. From the second cycle, the original oxidation peaks at 1.7 and 2.25 V disappeared and no obvious changes were detected for the following process. Figure 5b shows the rate ability of the LNVP//MS1 full cell. Specifically, this full cell was cycled from 0.25 to 1.0 C in steps and then returned to 0.25 C. As shown in Figure 5b, the MS1 electrode exhibited final discharge capacities of 440.4, 287.0, and 140.7 mAh g−1 at different current densities of 0.25, 0.5, and 1.0 C, respectively, and these results clearly demonstrated the high rate capability of the MS1 electrode in the lithium ion full cell system. The cycling performance of the LNVP//MS1 full cell was also tested by galvanostatic measurement at 1 C. As shown in Figure 5c, this full cell provided a discharge capacity of 423.8 mAh g−1 based on the mass of MS1 nanocubes at the first cycle and the discharge capacity could still remain 150.1 mAh g−1 E

DOI: 10.1021/acs.inorgchem.5b02558 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v). For full cell assembly, the Li2NaV2(PO4)3 electrode was prepared under the same conditions except that Al foil was employed as current collector. Then the Li2NaV2(PO4)3 electrode was used as cathode and α-MnSe was used as anode separated by polypropylene (PP) film; the electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v). To investigate electrochemical performance, cyclic voltammetry (CV) and charge/discharge measurements were carried out on a CHI660D electrochemistry workstation and Land Battery Measurement System at room temperature. For the half-cell test, the electrochemical performance was conducted at various current densities in the voltage range of 0−3 V. Cyclic voltammetry (CV) studies were carried out between 0 and 3 V at a scan rate of 0.5 mV s−1. For the full cell test, the electrochemical performance was conducted at various current densities in the voltage range of 0−4.0 V. Cyclic voltammetry (CV) studies were carried out between 0 and 4.0 V at a scan rate of 0.25 mV s−1.

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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/acs.inorgchem.5b02558. EDAX spectra, TEM images, XRD pattern, and other characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

Na Li and Yi Zhang contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial aid from the start-up funding from Xi’an Jiaotong University, the Fundamental Research Funds for the Central Universities (2015qngz12), the China National Funds for Excellent Young Scientists (Grant 21522106), and NSFC (Grant 21371140). We also appreciate Dr. Xing-Hua Li at Northwest University for his kind help in obtaining HRTEM images.

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DOI: 10.1021/acs.inorgchem.5b02558 Inorg. Chem. XXXX, XXX, XXX−XXX