DOI: 10.1002/cssc.201500256

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Lithium Storage in Microstructures of Amorphous MixedValence Vanadium Oxide as Anode Materials Di Zhao,[a] Lirong Zheng,[b] Ying Xiao,[a] Xia Wang,[a] and Minhua Cao*[a] Constructing three-dimensional (3 D) nanostructures with excellent structural stability is an important approach for realizing high-rate capability and a high capacity of the electrode materials in lithium-ion batteries (LIBs). Herein, we report the synthesis of hydrangea-like amorphous mixed-valence VOx microspheres (a-VOx MSs) through a facile solvothermal method followed by controlled calcination. The resultant hydrangealike a-VOx MSs are composed of intercrossed nanosheets and, thus, construct a 3 D network structure. Upon evaluation as an anode material for LIBs, the a-VOx MSs show excellent lithiumstorage performance in terms of high capacity, good rate capability, and long-term stability upon extended cycling. Specifically, they exhibit very stable cycling behavior with a highly rever-

sible capacity of 1050 mA h g¢1 at a rate of 0.1 A g¢1 after 140 cycles. They also show excellent rate capability, with a capacity of 390 mA h g¢1 at a rate as high as 10 A g¢1. Detailed investigations on the morphological and structural changes of the a-VOx MSs upon cycling demonstrated that the a-VOx MSs went through modification of the local V¢O coordinations accompanied with the formation of a higher oxidation state of V, but still with an amorphous state throughout the whole discharge/charge process. Moreover, the a-VOx MSs can buffer huge volumetric changes during the insertion/extraction process, and at the same time they remain intact even after 200 cycles of the charge/discharge process. Thus, these microspheres may be a promising anode material for LIBs.

Introduction Energy is pervasive in nature and is one of the most pressing problems of the human race. Developing a sustainable and renewable energy has been one of the most important tasks of worldwide scientists to address increasing global energy consumption as well as the critical issue of climate change. Furthermore, efficient energy-storage systems are also needed for the electricity generated from intermittent, renewable sources.[1–3] Rechargeable lithium-ion batteries (LIBs), which possess superior advantages, such as high energy density, long lifespan, no memory effect, and environmental benignity, have been the best system for electric energy storage and the most popular power source for high-end consumer electronics for many years. This creates a great deal of interest in seeking high-performance electrode materials that can store and deliver more energy efficiently.[4, 5] On this basis, the major challenges in designing next-generation LIBs include the need to in[a] D. Zhao, Y. Xiao, X. Wang, Prof. M. Cao Key Laboratory of Cluster Science Ministry of Education of China Beijing Key Laboratory of Photoelectronic Electrophotonic Conversion Materials Department of Chemistry Beijing Institute of Technology Beijing 100081 (PR China) E-mail: [email protected] [b] L. Zheng Institute of High Energy Physics The Chinese Academy of Sciences Beijing 100049 (PR China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500256.

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crease their energy density, stable cycling, and charge/discharge rate capability. To deal with these problems, many researchers have focused on developing multiple approaches in recent years, including: 1) designing nanostructures with special features (a high surface area, a fine feature size, hollow channels, some void space, and stratified structure);[6–9] 2) tolerance enhancement and accommodation (decreasing dimensional size, core–shell structure, amorphization, and pore formation);[4, 10–15] 3) buffering [making composites with carbon and/or inactive components, carbon coating, alloying, thin filming, and modifying binders and the solid electrolyte interface (SEI) layer coating];[16–18] 4) new electrode materials with good electrochemical performance.[19] Among all the efforts, amorphization is a dominant approach. Wang et al.[11] reported that an amorphous structure is an important factor directly related to a stable capacity and that structural defects in amorphous materials can serve as reversible Li + storage sites for LIBs, which contribute to a high capacity. It has been demonstrated that amorphous materials can effectively accommodate stress more successfully during the lithiation/delithiation process, as they can effectively undergo reversible shape and volume changes.[11, 20] Moreover, amorphous materials bring additional benefits by reducing the stress/strain of the conversion reaction, which thus lowers the lithiation/delithiation overpotential.[21, 22] Therefore, it might be of great interest and necessary to find ideal anode materials with an amorphous structure. Among various transition-metal oxides, layered vanadium oxides, such as V2O5, V6O13, LiV3O8, and VO2, have been extensively explored as effective cathode host materials for Li + inter-

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Full Papers calation owing to their higher insertion potentials (higher than 2.5 V vs. Li/Li + ).[19, 23–25] Furthermore, vanadium oxides also possess some unique advantages, including abundant material source,[26] high energy density,[27] and wide potential window arising from their various vanadium oxidation states (V–II),[28] which render them promising candidates as anode materials for LIBs.[29, 30] For example, double-shelled nanocapsules of V2O5-based composites have a reversible capacity of 947 mA h g¢1 and they retain a high capacity of 673 mA h g¢1 after 50 cycles as high-performance anode materials for LIBs.[30] However, their rate capability is not satisfactory. Recently, the groups of Zhou and Wu reported a new kind of intercalation anode material, Li3VO4.[2, 19] Li3VO4 can intercalate Li ions at a voltage mainly between 0.5 and 1.0 V versus Li + /Li; this is lower than the voltage required for Li4Ti5O12 and higher (safer) than that of graphite. However, its theoretical capacity is only 394 mA h g¢1, which is far lower than that of other transitionmetal oxides ( … 1000 mA h g¢1).[2, 19] In addition, V2O3@carbon nanocomposites and VOx/carbon nanofibers have also been reported as novel anode materials for LIBs.[26, 31] There are currently only a few reports on vanadium oxide based anode materials, and the available anode materials mainly focus on carbon materials or other metal oxides, both of which have several evident drawbacks,[11] for example, unsatisfactory capacitive performance. So, searching for an inexpensive, highperformance material without a carbon coating has always been greatly attractive but very challenging. Herein, we report, for the first time, a simple and facile method to synthesize hydrangea-like-structured amorphous mixed-valence VOx microspheres (a-VOx MSs) and their application as an anode material for LIBs. The a-VOx MSs with a hydrangea-like structure were easily prepared by annealing the presynthesized vanadium-based glycolate (alkoxide) precursor at 350 8C. Compared to samples obtained at higher annealing temperatures (550 and 750 8C), the resultant hydrangea-like aVOx MSs exhibit highly stable cycling behavior with a highly reversible capacity of 1050 mA h g¢1 at a current density of 0.1 A g¢1 after 140 cycles, and even at a higher current density of 1 A g¢1, a high discharge capacity of 683 mA h g¢1 after 250 cycles can be maintained. Furthermore, the a-VOx MSs also show excellent rate capability, with a capacity of 390 mA h g¢1 at a current density as high as 10 A g¢1. The superior lithium storage of the a-VOx MSs can be attributed to their special microstructures, mixed valency, amorphous nature, and unique 3 D groove networks, which significantly improve the ionic and electronic transport and intercalation kinetics of the Li ions.

Results and Discussion The hydrangea-like a-VOx MSs were synthesized by a solvothermal method followed by thermal treatment at 350 8C in H2/Ar for 3 h, as shown in Scheme 1. A precursor was first synthesized by a solvothermal process, and the precursor was then subjected to thermal treatment to form the target product. For comparison, we also annealed this precursor at 550 and 750 8C, and the resultant samples are denoted VOx-550 and V2O3-750. The crystal structure of the resultant four representative samChemSusChem 2015, 8, 2212 – 2222

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Scheme 1. Schematic illustration of the fabrication of a-VOx MSs and their application in LIBs.

ples were determined by X-ray diffraction (XRD) measurements. As shown in Figure 1 a, the XRD pattern of the precursor shows two sharp peaks at 2 q = 11.1 and 18.28, which cannot be indexed to a known vanadium oxide phase. However, according to reported studies,[32] the precursor might be a vanadium-based glycolate or alkoxide, because the ethylene glycol solvent used in our experiment can serve as a ligand to form coordination complexes with transition-metal cations upon heating by alcoholysis and coordination. Surprisingly, if the precursor was annealed at 350 8C, the resultant product exhibited a different XRD pattern. As displayed in Figure 1 a (c c), the XRD profile shows only one broad peak at 2 q … 248, which indicates that the precursor changed to an amorphous structure. If the annealing temperature was increased to 550 or 750 8C, monoclinic V2O3 (JCPDS card no. 74-2037) was obtained in both cases (Figure 1 a, c and c curves). The only difference between the two cases is that the diffraction peaks for VOx-550 are wider and weaker than those for V2O3-750, which is indicative of a smaller particle size and lower crystallinity of the VOx-550 sample. These results demonstrate that with an increase in the temperature, the amorphous structure transforms into a crystalline phase. The thermogravimetry (TG) curve of the precursor and the Fourier-transform infrared (FTIR) spectra of all the samples are shown in Figure 1 b and Figure S1 (Supporting Information). The downward slope from ambient temperature to 350 8C in the TG curve can be ascribed to the loss of absorbed moisture, water molecules, and the ethylene glycol solvent trapped inside the metallic precursor, as well as to the decomposition of the residual organic molecules of the metallic precursor coming from vanadyl acetylacetonate (the strong band of the precursor at n˜ = 1362 cm¢l corresponds to a degenerate CH3 symmetric vibration adjacent to the C=C bond in acetylacetone).[33, 34] Then, as the temperature increases, the curve continues to fall and becomes stable at 750 8C. Detailed analysis of the FTIR spectra is shown in the Supporting Information. On the basis of the analysis of the TG and FTIR spectroscopy data, it can be proposed that during the annealing process, the crystal structure undergoes rearrangement with the loss of crystal water and the decomposition of organic compounds. Interestingly, the amorphous structure can be obtained only at 350 8C.

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Figure 1. (a) XRD patterns of the as-obtained precursor and VOx compounds obtained under different calcination temperatures. (b) TG curve of the precursor measured under a N2 atmosphere; the inset is the corresponding FTIR spectra. (c) The vanadium K-edge spectra of the precursor; samples annealed at 350, 550, and 750 8C (standard V2O3); and standards (VO2, V2O5). (d) The main absorption edge area of the vanadium K-edge spectra. (e) V 2p XPS spectra of the aVOx MSs. (f) The k3-weighted EXAFS curves at the vanadium K-edge of as-obtained VOx compounds and standards.

To reveal the oxidation state of the a-VOx MSs sample, Kedge X-ray absorption near-edge structure (XANES) measurements were performed, including the energy of the pre-edge peak and the main absorption edge (Figure 1 c, d). As shown in Figure 1 c and Figure S2 a, b, XANES analyses of all the samples reveal that the vanadium oxidation states during the calcination process undergo evident change from V4 + and V5 + of the precursor to V4 + and V3 + of the a-VOx MSs and VOx-550, and finally to V3 + of V2O3-750. Figure 1 d and Figure S2 c show the corresponding XANES results of the a-VOx MSs, and its main absorption edge is between that of VO2 and that of the V2O3750 standard. Likewise, the energy position of the pre-edge peak (Figure S2 b) is close to that of VO2 and the V2O3 standard and is slightly farther away from that of the V2O5 standard. These results confirm that the vanadium oxidation states in the a-VOx MS samples can be determined as + 4 and + 3.[35, 36] The vanadium oxidation state in the a-VOx MSs were further investigated by using X-ray photoelectron spectroscopy (XPS). The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing C 1s to 284.81 eV. Figure S3 a shows the survey XPS spectrum of the a-VOx MSs, which indicates that the a-VOx MSs mainly consist of V and O elements. Figure 1 e shows a fitted high-resolution XPS spectrum of V 2p, and the core-level binding energies of 516.9 and 524.4 eV can be ascribed to V 2p3/2 and V 2p1/2, respectively. The V 2p3/2 peak was fit into three peaks at binding energies of 517.6 and 516.9 eV for V5 + and 516.2 eV for V4 + , whereas the V 2p1/2 peak was fit into two peaks at binding energies of 523.3 eV for V4 + and 524.5 eV for V5 + .[37, 38] The corresponding O 1s spectrum has three peaks, which is indicative of the existence of three different oxygen species (Figure S3 b). The peak centered at a binding energy of 530.1 eV can be assigned to the O2¢ species in V¢O, whereas the peaks at binding energies ChemSusChem 2015, 8, 2212 – 2222

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of 531.9 and 533.2 eV can be attributed to OH and H2O molecules physically absorbed onto the sample from the atmosphere, respectively.[39] It can be clearly seen that the XPS results are different from the XANES results, which is probably due to the oxidation of V during the XPS measurement process (vanadium is susceptible to oxidation). In addition, the content of carbon in the a-VOx MSs is 5.5 wt %, and the atomic ratio of O to V is approximately 2 according to inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and elemental analysis results. On the basis of the above results, we can deduce that the chemical states of the V element in the a-VOx MSs are mainly V4 + and V3 + , and a small amount of V5 + may also exist in this sample. For this reason, this sample was designated as a-VOx MSs. Moreover, further observation of Figure 1 d revealed that there are evident differences in terms of the intensity and features of the edge resonance between the a-VOx MSs and the other samples, which can be ascribed to a different local structural arrangement between them and the less distorted arrangement of the basal oxygen atoms.[35, 40] Then, the k3weighted extended X-ray absorption fine structure (EXAFS) spectra (Figure 1 f), the structural parameters of which can be obtained by converting the photon energy into the photoelectron wavevector k, show that the EXAFS oscillation of the aVOx MSs is similar to that of the VO2 and V2O3-750 standards and is partly similar to that of V2O5. The results confirm the similarities in the short-range atomic arrangement of the a-VOx MSs and VO2, V2O3-750, and V2O5.[35] Moreover, the crystalline data are considerably more detailed than those of the amorphous material, although the major point is that they coincide.[41] Therefore, the differences between the a-VOx MSs and the standards at k = 7–9 æ¢1, which is due to modification of the V¢O bonds, as mentioned above, can be used to distin-

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Full Papers guish amorphous and crystalline VO2, V2O3-750, and V2O5.[35, 40] On the basis of the EXAFS and the above XRD results, we can assign the a-VOx MSs as the amorphous structure, V2O3-750 as the crystalline phase, and VOx-550 as a material with a certain degree of crystallinity. Field-emission scanning microscopy (FE-SEM) and transmission electron microscopy (TEM) provide insight into the morphology and detailed structure of the as-synthesized samples. Figure 2 a is a representative low-magnification FE-SEM image of the as-synthesized precursor; it reveals that this sample consists entirely of uniformly dispersed microspheres with an average diameter of approximately 4 mm. The high-magnification FE-SEM image (Figure 2 b) shows that the microspheres have a perfect flowerlike morphology, a typical hierarchical structure. Detailed observation of a single microsphere (Figure 2 c) revealed that the flowerlike architecture is constructed from many petal-like nanosheets with a thickness of approximately 25 nm. These nanosheets are connected to each other to form flowerlike hierarchical microstructures through self-assembly. After annealing the flowerlike microsphere precursor at 350 8C in H2/Ar for 3 h, the resultant a-VOx sample retains the flowerlike microsphere morphology of the precursor. The micro-

spheres have an average diameter of approximately 2.97 mm (Figure 2 d, e), which is smaller than that of the precursor; this suggests that marked shrinkage occurs during the annealing process. More interestingly, unlike the smooth surface of the precursor, the surface of the annealed sample is very rough (Figure 2 f), probably because of the growth and rearrangement of smaller subunits. Figure 2 g clearly displays the subtle change in the morphology of the precursor before and after annealing. The flowerlike microsphere is very similar to the structure of a hydrangea. Though similar morphology has been reported by other groups, the hydrangea-like microspheres in this paper have much smaller subunits in the petallike nanosheets.[42, 43] More importantly, relatively regular triangular prismlike pores are formed as a result of the interconnected nanosheets (marked by red lines). The pores can accommodate the electrolyte, which thus improves the performance of the electrode materials by facilitating the insertion/extraction of Li ions.[15] Moreover, the microsized spheres assembled from nanosized particles might inherit the advantages of enhanced electrochemical reactivity from the nanosized building blocks, which would thus improve the structural stability and packing density from the secondary microsized

Figure 2. (a–c) FESEM images of the as-obtained flowerlike microsphere precursor at different magnifications; the inset in a) is the corresponding pore-diameter distribution. (d–f) FESEM images of the hydrangea-like-structured amorphous VOx microspheres after annealing the precursor at 350 8C in H2/Ar at different magnifications; the inset in (d) is the corresponding pore-diameter distribution, and the inset in (f) is a yellow hydrangea. (g) One hydrangea-like-structured microsphere before and after calcination at 350 8C. (h) Elemental mappings of one hydrangea-like-structured microsphere after calcination at 350 8C (vanadium, oxygen).

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Full Papers structure.[44] Notably, even at higher annealing temperatures (550 and 750 8C), the resultant samples still maintain the integrity of the flowerlike microspheres (Figure S4), which suggests excellent structural stability. Representative TEM images of the a-VOx MSs are presented in Figure 3 a–e. The low-magnification TEM image (Figure 3 a) clearly shows spherical particles with a uniform size, and highermagnification TEM images (Figure 3 b, c) further reveal that the microspheres are composed of nanosheets, which agrees well with the observations taken from the SEM images. Moreover, Figure 3. (a, b) TEM images of the a-VOx MSs after annealing the precursor at 350 8C in H2/Ar for 3 h. (c–e) HRTEM the high-resolution TEM (HRTEM) images of the edge of one microsphere. (f) Nitrogen adsorption/desorption isotherms and corresponding poreimage does not show that the size distribution curve of the a-VOx MSs. sample has crystalline lattice fringes, which further confirms the amorphous texture of the a-VOx MSs (Figure 3 d); this is strain/stress energy in the a-VOx MSs. Given that the integrated consistent with previous XRD and EXAFS results. Furthermore, peak area is equal to the capacity, the continuous CV cycling pores formed from self-assembly of the nanosheets are also behavior of the a-VOx MSs is consistent with the decreasing– observed in Figure 3 c, e. The porous nature of the a-VOx MSs stabilizing behavior of the capacity retention curve (Figure 4 b). was further investigated by N2 adsorption/desorption measureSimilarly, the CV plot of VOx-550 (Figure 4 c) has a new redox ments (Figure 3 f). The N2 adsorption isotherms show type IV pair as well, which may be due to the nature of a certain crysisotherms having a hysteresis loop in the pressure range of 0.4 tallinity. For comparison, we also show the CV plots of V2O3to 0.9 (P/P0), which indicates the presence of mesopores. More750 (Figure 4 e). Clearly, they exhibit flat circulars, and no signifover, the Brunauer–Emmett–Teller (BET) surface area of the aicant lithiation–delithiation process is observed between 3.0 VOx MSs was determined to be 60.37 m2 g¢1 with a total pore and 1.0 V, which might due to crystallographic constraints.[12] 3 ¢1 volume of 14.56 cm g ; this surface area is two times higher Figure 4 b, d, f display the corresponding charge/discharge profiles at a current density of 0.1 A g¢1 from the first to sixth than that of the precursor (30.28 m2 g¢1). The lithium-storage performance of the resultant a-VOx MSs cycle. It can be seen that all the CV peak potentials match well as an anode material for LIBs was evaluated, and the perwith the voltage plateaus in the charge/discharge curves and formance of VOx-550 and V2O3-750 was also investigated for remain almost unchanged in subsequent cycles, which is indicomparison. Figure 4 a–f shows the cyclic voltammetry (CV) cative of reversible insertion/extraction reactions for the first curves recorded at a scan rate of 0.5 mV s¢1 in the potential six cycles for all samples. However, by comparison, we can see window of 3 to 0.01 V. It can be clearly seen that these three from the charge/discharge profiles that the a-VOx MSs show samples exhibit different CV curves, which implies their electroa higher specific capacity and better cycle stability. chemical behavior is also different. For the VOx-550 sample, Figure 5 a and Figure S6 a give the initial discharge/charge three cathodic peaks at potentials of approximately 1.67, 1.3, voltage profiles of all the samples for the first cycle at a conand 0.4 V are observed in the first discharge cycle, which correstant current density of 0.1 A g¢1. Clearly, the electrochemical + spond to the intercalation of Li into VOx to form a LiyVOx performance of the a-VOx MSs is greatly enhanced relative to solid solution (Figure 4 c). In contrast, for the a-VOx MSs, the CV that of VOx-550 and V2O3-750, and the initial discharge and curve of the first discharge cycle reveals only one discernible charge capacities are 1136 and 1020 mA h g¢1, respectively, and cathodic peak owing to its amorphous structure (Figure 4 a), in the initial coulomb efficiency (CE) is as high as 90 %, which which the structural defects act as stable Li + storage sites.[12] suggests that the amorphous structure of the a-VOx MSs can From the second cycle, a new redox pair appears at high poform a thin and stable SEI layer; this feature is particularly bententials (2.47 V for reduction and 2.64 V for oxidation), which eficial for practical applications. Moreover, the special micromay be due to the oxidation and reduction of the V cations. structure of the a-VOx MSs can evidently increase the contact The integrated peak areas go through a decreasing–stabilizing area of the active materials with the electrolyte and serve as process, accompanied with a change in peak shape. This evoaccommodations for the electrolyte, which thus improves the lution might be associated with accumulation of defects/ lithium-storage behavior of the electrode material by facilitatChemSusChem 2015, 8, 2212 – 2222

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Full Papers during subsequent cycles.[12, 21, 46] The charge/discharge profiles have slight fluctuation. One possible reason may be that the Li + storage at the structural defects, such as vacancies, void spaces, cluster gaps, and interstitial sites in the a-VOx MSs, is slightly different for every charge/discharge process. Interestingly, this phenomenon can be avoided if tested at a higher rate of 1 A g¢1. Figure 5 c displays the charge/ discharge profiles of the a-VOx MSs at a higher current density of 1 A g¢1. It can be observed that the charge and discharge profiles are almost overlapped from the 5th cycle to the 200th cycle, and this shows that the material has excellent electrochemical reversibility. Figure 5 d presents the cycling performance of all the electrodes at a current density of 1 A g¢1, from which we can see that all the samples show a relatively stable cycling performance. However, their final reversible capacities after 250 cycles are quite different. The a-VOx MSs deliver a high discharge capacity of ¢1 + Figure 4. CV curves recorded at a scan rate of 0.5 mV s in the range of 0.01 to 3.0 V versus Li/Li and the corre683 mA h g¢1 after 250 cycles, ¢1 sponding charge/discharge profiles at a current density of 100 mA g : (a, b) a-VOx MSs, (c, d) VOx-550, (e, f) V2O3whereas VOx-550 and V2O3-750 750. display discharge capacities of 300 and 170 mA h g¢1, respecing insertion and extraction of Li ions. Furthermore, it can also tively, both of which are far lower than the discharge capacity be seen that the intercalation of Li + occurs mainly at a voltage of the a-VOx MSs. below 1.0 V (Figure 5 a). Thus, more than two thirds of the lithTo obtain more comprehensive understanding about the exiation capacity of the a-VOx MSs is below 1.0 V, which indicates cellent cycling performance of the a-VOx MSs, the cycling perthat the a-VOx MSs have a unique active voltage window and formance was conducted under different galvanostatic condia higher energy density, and it can therefore be used as tions. As shown in Figure 6 a, the a-VOx MSs display perfect cya promising insertion anode for LIBs, consistent with the recling stability under all tested current densities and deliver results of the CV curve in Figure 4 a.[19, 45] This feature may be due versible capacities of 880, 680, 620, and 360 mA h g¢1 after 3+ to the incorporation of V into the vanadium oxides contain200 cycles at 0.2, 1.0, 2.0, and 10 A g¢1, respectively. However, 4+ 5+ ing V and V oxidation states, which could expand the pofor VOx-550 and V2O3-750, although they both have relatively tential window to the negative side.[29, 30] stable cycling performance, their reversible capacities are far Figure 5 b shows the discharge capacity as a function of lower than those of the a-VOx MSs under the same tested concycle number for the a-VOx MSs at 0.1 A g¢1 in the voltage ditions (Figure 6 b, c). Furthermore, with an increase in crystalrange of 0.01 to 3 V versus Li + /Li. During the first five cycles, linity, the V2O3-750 sample showed poorer electrochemical perthe reversible capacity decreases gradually from 1020 to formance. Notably, the charge and discharge capacities of the 970 mA h g¢1, which is probably due to the irreversible capacity a-VOx MSs for the first cycle are 880 and 955 mA h g¢1, respec+ that is caused by the formation of the SEI layer and/or Li tively, with a CE as high as 92 %, whereas the CEs of VOx-550 trapping inside the a-VOx matrix.[12] In subsequent cycles, the and V2O3-750 for the first cycle are only 66.1 and 66.2 %, rereversible capacity increases slightly and remains stable at apspectively, which indicates fewer side reactions and excellent proximately 1050 mA h g¢1 up to the 140th cycle with a CE of cycle stability for the a-VOx MSs. Clearly, the amorphous struc99.2 %, which is indicative of good stability of the SEI layer ture can effectively increase the lithium-storage capacity, ChemSusChem 2015, 8, 2212 – 2222

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Full Papers from 0.1 to 10 A g¢1 every 20 cycles within a voltage range of 0.01 to 3 V (Figure 6 d, e). Clearly, the a-VOx MSs exhibit excellent rate performance. More specifically, almost no capacity loss is observed from 0.2 to 1 A g¢1, and the capacity is mainapproximately tained at 750 mA h g¢1. Even at a high rate of 10 A g¢1, 390 mA h g¢1 can still be obtained, which is still slightly higher than the theoretical capacity of graphite (372 mA h g¢1). In contrast, VOx-550 and V2O3750 only exhibit a reversible capacity of approximately 160 mA h g¢1 upon testing at a rate of 10 A g¢1. Remarkably, if the current rate is reduced to 0.1 A g¢1 after more than 200 cycles, a stable high disFigure 5. Electrochemical performance of the samples: (a) Initial discharge/charge voltage profiles of different samples at a current density of 0.1 A g¢1; (b) cycling performance of the a-VOx MSs performed at a current density charge capacity of approximateof 0.1 A g¢1; (c) charge/discharge profiles of the a-VOx MSs at a current density of 1 A g¢1; (d) comparison of cyly 810 mA h g¢1 can be recovered ¢1 cling performance of all the samples at a current density of 1 A g . for the a-VOx MSs, whereas the discharge capacities of VOx-550 which is achieved by hosting a large amount of Li ions in the and V2O3-750 are only retained at 520 and 280 mA h g¢1, renumerous structural defects. On the other hand, for amorspectively. These results are indicative of excellent rate capabilphous materials, crystallographic confinement to the insertion/ ity and electrochemical reversibility of the a-VOx MSs for lithiextraction reactions of Li + is absent; thus, bond cleavage can um storage, which probably benefit from the unique structural be avoided, and this may play an important role in stabilizing characteristics of the microspheres. Notably, in our case, the the capacity.[12] good cyclability and rate capability of the a-VOx MSs are Furthermore, the rate capability of the a-VOx MSs was also mainly due to its unique surface, which can carry rich active evaluated to investigate the feasibility of high-power applicasites; this is in contrast to the smooth structure of the precurtions, for which the current densities were increased stepwise sor. Moreover, VOx-550 and V2O3-750 with almost similar mor-

Figure 6. (a–c) Cycling performance of the a-VOx MSs, VOx-550, and V2O3-750 at current densities of 0.2, 1, 2, and 10 A g¢1. (d, e) Comparison of the rate performance of all samples. (f) Comparison of the electrochemical impedance spectra of all samples in the frequency range of 0.01 to 100 kHz.

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Full Papers phologies and surface structures exhibit excellent long-term after lithium insertion and extraction, the main-edge shifts tostability for lithium storage as well. However, their irreversible wards higher energies and the intensity of the pre-edge inspecific capacities are much lower than that of the a-VOx MSs creases relative to that of the electrode before discharge upon testing under the same conditions, which indicates that [open-circuit voltage (OCV)]. Wong et al.[36] demonstrated that the amorphous structure of the a-VOx MSs also plays a signifithe main-edge energy position of vanadium compounds concant role in the excellent electrochemical properties. In this tains information about the valence of the V cations, and the regard, the unique microstructure and the amorphous texture pre-edge intensity contains information about the coordination of V. On this basis, the continuous change in the pre-edge inof the a-VOx MSs may be the main contributors to their excellent lithium-storage performance. To further confirm the entensity in the spectra during the charge/discharge process may hanced lithium-storage performance, electrochemical impebe attributed to modification of the local structure of the V cations by Li-ion insertion and extraction. Symmetrical vanadidance spectroscopy (EIS) was performed (Figure 6 f). As can be seen, the semicircle diameter observed for the a-VOx MSs indium ligand coordinations with inversion of symmetry, such as regular “VO6” octahedra, have very low pre-edge intensities, cates that the SEI resistance (Rf) and charge-transfer impedance (Rct) are close to those of VOx-550, but much lower than those whereas coordinations without inversion of symmetry, such as of V2O3-750. Moreover, the inclined line in the low-frequency distorted “VO6” units or “VO4” tetrahedra, have significant high region may be attributed to the Warburg impedance (Zw), pre-edge intensities.[48] As shown in Figure 7 b, c, the pre-edge + which is related to Li diffusion in the solid state. The greater intensity observed for V at the OCV is 0.34. Upon discharging slope of the Zw of the a-VOx MSs electrode indicates more to 1.2 V, the observed pre-edge intensity increases to 0.69, and facile diffusion of Li + in the electroactive and defect-rich vanaif it continues to be discharged to 0.01 V and charged to 2 V, dium oxide.[47] In addition, the Nyquist plots of the a-VOx MSs the pre-edge intensity decreases to 0.58. Moreover, upon for the 1st, 100th, and 200th cycles are similar (Figure S6 b). charging to 2.4 and 2.7 V, the pre-edge intensity becomes 0.69 However, the value of Rct decreases with an increase in the again. From these results and according to literature precenumber of cycles, which indicates gradual activation and imdent,[48, 49] coordinations of the V¢O type most probably proved kinetics of the reaction upon cycling. Therefore, the EIS change from the slightly distorted “VO6” octahedra at the OCV measurements corroborate the effective electronic and ionic to “VO4” tetrahedra after discharging and charging. Moreover, mobility within the amorphous structure of the VOx matrix for the vanadium K-edge XANES spectra of the a-VOx MSs electhe insertion and extraction reactions of lithium ions. trode for the sixth cycle at different discharge/charge voltages On the basis of the above experimental results, we can see are shown in Figure S8 a. From these spectra, it can be seen that the a-VOx MSs exhibit high capacity and superior capacity that the pre-edge intensity is between 0.67 and 0.78 over the retention, which should be attributed to the amorphous nature of the microspheres and their unique hydrangea-like microstructure. To disclose the mechanism for the excellent electrochemical performance and structure change of the aVOx MSs, the coin cells were taken apart after the electrochemical performance test at different charge/discharge voltages and at different cycles at 200 mA g¢1. The morphology and structure of the a-VOx MSs were investigated by using XANES, SEM, XRD, and XPS. The vanadium K-edge X-ray absorption spectroscopy (XAS) spectra were collected in situ at different charge/discharge voltages according to their CV plots (Figure S7) to examine variations in the oxidation states and structure. The corresponding XANES Figure 7. (a, b) Vanadium K-edge XANES spectra of the a-VOx MSs tested at different charge/discharge voltages for results for the first and sixth the first and sixth cycles according to their CV plots (Figure S7); OCV represents the electrode before discharge. cycles are shown in Figure 7 a– (c) Pre-edge region of the a-VOx MSs tested at different charge/discharge voltages for the first cycle. (d) V 2p XPS c and Figure S8 a. Unexpectedly, spectra of the a-VOx MSs upon charging to 3 V for the sixth cycle. ChemSusChem 2015, 8, 2212 – 2222

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Full Papers whole cycle of insertion and extraction, which indicates that the local structure of the V element tends to stabilize. This estimate is in good agreement with the above excellent cycling stability. Notably, the main-edge shifts towards higher energies for the first and sixth cycles relative to that of the OCV, which confirms that the V cations in the a-VOx MSs could be re-oxidized to a higher oxidation state. This phenomenon is also reflected in the charge/discharge performance and the CV profiles. As shown in Figure 5 d, the specific capacity goes through a decreasing–stabilizing process at a current density of 1 A g¢1 for 250 cycles. Furthermore, the new redox pair, 2.47 V for lithiation and 2.64 V for delithiation in the CV plot in Figure 4 a, can be attributed to the transformation between the low-oxidation state V cations (V3 + or V4 + ) and the high-oxidation state cations (V4 + or V5 + ), which may change the local V¢O coordinations in the amorphous structure of the a-VOx MSs. Thus, the excellent performance may be due to accessible redox couples and the structural versatility of the a-VOx MSs, which make them attractive for reversible Li + -ion intercalation.[29, 30, 50] XPS measurements were used to further examine the valence states of the V cations in the a-VOx MSs electrode after charging to 3 V in the sixth cycle at 200 mA g¢1 (Figure 7 d and Figure S8 b). As clearly displayed in Figure S8 b, two weak peaks in the XPS spectrum of V 2p are detected, and their binding energies are shifted to higher values relative to those of the OCV, which is indicative of a change in the oxidation state of V during the electrochemical process. Figure 7 d shows the high-resolution V 2p XPS spectra, in which the peaks centered at binding energies of 517.1, 517.8, and 525.0 eV correspond to V5 + , whereas the weak peak at a binding energy of 516.3 eV can be assigned to V4 + . Clearly, according to the V 2p XPS spectra at the OCV in Figure 1 e and their integral area, the electrode charged to 3 V after the sixth cycle involves a smaller amount of V4 + . This phenomenon can also be observed in other anode materials.[21, 51, 52] For instance, Guo et al. reported that Mn2 + could be reoxidized to a higher oxidation state in amorphous MnOx/carbon electrodes owing to synergistic effects of carbon and the amorphous structure.[21] It has been widely accepted that Mn2 + in well-designed manganese oxide electrodes can be further oxidized to a higher oxidation state, which can improve the conversion-reaction kinetics of the electrode and the specific capacity.[21, 51–53] Therefore, it can be speculated that the a-VOx MSs electrode exhibits high reversible capacity and excellent cycling stability mainly or partly as a result of the modification of the local V¢O coordinations accompanied with the oxidation of the V ions to a higher oxidation state, which may originate from defects in the amorphous structure. Figure 8 a, b shows the SEM images of the aVOx MSs after 100 and 200 cycles at a current density of 0.2 A g¢1. As is well known, the insertion/deinsertion of Li + generates a certain amount of structure stress on the host material, especially for the insertion of a large amount of Li + ions. However, for the a-VOx MSs electrode (Figure 8 a), the structures of the microspheres remain intact; the nanoparticles and the nanosheets can still be seen, and almost no SEI layer is observed. Even tested after 200 cycles, the microspheres are still ChemSusChem 2015, 8, 2212 – 2222

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Figure 8. (a, b) SEM images of the a-VOx MSs after 100 and 200 cycles at a current density of 0.2 A g¢1. (c, d) SEM images of the VOx-550 and V2O3-750 electrodes after 200 cycles at a current density of 0.2 A g¢1.

well maintained and the 3 D networks are also clearly visible (Figure 8 b). In contrast, for the VOx-550 electrode (Figure 8 c), although the microsphere morphology can be observed, the nanosheets become thicker, whereas for the V2O3-750 electrode (Figure 8 d), the hydrangea-like microspheres change into very smooth microspheres with the formation of a thick SEI layer on the electrode surface. This result also explains why the V2O3-750 sample has lower CE and lower specific capacity. In short, the aforementioned experimental results strongly suggest that only the a-VOx MSs remain intact after 200 cycles of the charge/discharge process; this could provide numerous structural defects to host more Li + ions without bond cleavage, which efficiently buffers the strong volumetric expansion. To investigate the structural reversibility of the a-VOx MSs upon Li insertion/deinsertion, ex situ XRD was performed to characterize the phase changes of the electrodes at different charge/discharge states. The strong peaks in all the XRD patterns at 2 q = 43.5, 50.5, and 73.68 can be indexed to Cu current collector. As shown in Figure 9 a, no diffraction peaks can be observed in the XRD pattern of the electrode at the OCV, which is consistent with the amorphous structure of the a-VOx MSs. Upon discharging to 0.01 V, and then charging again to 3.0 V, only the diffraction peaks of Cu can be observed. Moreover, upon conducting the ex situ XRD measurements for the sixth cycle at different charge/discharge states (Figure 9 c), no new phases emerge in either of the XRD patterns at complete discharging to 0.01 V and subsequent charging to 3.0 V. Namely, despite modification of the coordination environment of the local vanadium ligands during the charge/discharge process, the a-VOx MSs still stay in an amorphous state throughout the whole discharge/charge process in subsequent cycles. This structural consistency could effectively buffer the huge volumetric change during the insertion/extraction of the Li ions so as to sustain the high cycling stability of the electrode. The above electrochemical characterizations demonstrate that the a-VOx MSs exhibit excellent lithium-storage per-

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Full Papers works, which could significantly buffer the huge volumetric change, improve the ionic and electronic transport, and improve the intercalation kinetics of the Li ions. Furthermore, this strategy for constructing this unique structure and composition would be very helpful in boosting the electrochemical performance of other nanomaterials.

Experimental Section Synthesis of hydrangea-like a-VOx MSs

Figure 9. (a) The ex situ XRD patterns of the a-VOx MSs electrode at different insertion/extraction depths during the first discharge/charge process. (b) CV curve of the first charge/discharge process at a scan rate of 0.5 mV s¢1 in the range of 0.01 to 3.0 V versus Li/Li + . (c) The ex situ XRD patterns during the sixth discharge/charge process for the a-VOx MSs electrode at different insertion/extraction depths.

formance in terms of reversible capacity, cyclability, and rate capability. The a-VOx MSs with inverted quasipyramid grooves can increase the contact area with the electrolyte and serve as accommodations for the electrolyte, which hence improves the electrochemical performance of the electrode materials by facilitating insertion and extraction of Li ions. In addition, the above electrochemical performance also demonstrates that the amorphous nature can avoid lattice stress and provide open vacancies and voids for faster Li + diffusion so as to greatly improve the reversible capacity and the structural stability. Moreover, the amorphous nature favors modification of the coordination environment of the local vanadium ligands during the charge/discharge process along with the oxidation of the lowoxidation state V cations to higher states, which can improve the electrochemical kinetics of the electrode and increase the specific capacity of the anode materials.

Conclusions We demonstrated the successful fabrication of amorphous mixed-valence VOx microspheres (a-VOx MSs) by first designing an appropriate precursor and then finely controlling the calcination process. The resultant a-VOx MSs exhibit a hydrangealike structure assembled from nanosheets. Upon evaluating these a-VOx MSs as an anode material for lithium-ion batteries, the a-VOx MSs showed excellent lithium-storage performance in terms of high capacity, good rate capability, and long-term stability upon extended cycling. The excellent electrochemical performance may originate from its amorphous nature, the mixed valency, the particular rough surface, and the 3 D netChemSusChem 2015, 8, 2212 – 2222

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In a typical synthesis, vanadyl acetylacetonate [VO(acac)2] (0.397 g, 1.5 mmol) was dissolved in ethylene glycol (50 mL) at room temperature. After magnetically stirring the solution for 1 h, the formed grass-green transparent solution was transferred to an 80 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed tightly and maintained at 200 8C for 48 h. After cooling to room temperature naturally, the resultant green precipitate was washed and separated by centrifugation–redispersion cycles with deionized water several times, and it was then dried under ambient atmosphere. Finally, the precursor was calcined at different temperatures (350, 550, and 750 8C) for 3 h under a H2/Ar flow (7 % H2 by volume) of 200 mL min¢1, and the thus-obtained samples are abbreviated as a-VOx MSs, VOx-550, and V2O3-750, respectively.

Characterization The crystal structure of the hydrangea-like a-VOx MSs was characterized by X-ray power diffraction (XRD, Shimadzu XRD-6000, CuKa, l = 1.54178 æ) at a scanning rate of 108 min¢1 with 40 kV voltage and 50 mA current. Thermogravimetry (TG) analysis was performed with a DTG-60AH instrument with a heating rate of 20 8C min¢1 from 25 to 700 8C in air. The microstructure and morphology were investigated by using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800 SEM unit) with an energy-dispersive X-ray (EDS) spectrometer, transmission electron microscopy (TEM, JEOL, JEM-2010, 200 kV), and high-resolution transmission electron microscopy (HRTEM). X-ray photoelectron spectra (XPS) were recorded with an ESCALAB 250 spectrometer (PerkinElmer) to characterize the surface composition. The element distribution was detected by combustion by using an elemental analyzer (Vario EI) and an inductively coupled plasma (ICP) spectrometer. The surface area and pore-size distribution were measured by using a Belsorp-max surface area detecting instrument by N2 physisorption at 77 K. The oxidation state and the surrounding areas of the vanadium atoms were measured by X-ray absorption spectroscopy (XAS), including X-ray absorption near-edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS), which were conducted at Beamlines 1W1B at the Beijing Synchrotron Radiation Facility (BSRF) by using transmission X-ray absorption fine structure (XAFS) technology. For the V2O3-750 sample, we used V2O3 as a standard, and for the product annealed at 350 8C for 2 h in air, we used V2O5 as a standard. In addition, the VO2 standard was synthesized by the solvothermal method outlined in Ref. [54].

Electrochemical measurements The electrochemical behavior of the a-VOx MSs was examined by using a two-electrode CR2025 coin cell with a lithium sheet as both the counter and reference electrodes. To prepare the working electrode, the as-prepared active material (70 %), carbon black (20 %), and polyvinylidene fluoride binder (10 %) were mixed with

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Full Papers N-methylpyrrolidone as the solvent, and the resultant slurry was then uniformly pasted on a Cu foil current collector. After coating, the electrode was pressed at 10 MPa and dried at 120 8C under vacuum for 24 h. The cell was assembled in an argon-filled glove box with a solution of 1 m LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1 in volume) as non-aqueous electrolyte. The galvanostatic charge/discharge tests were recorded by using a multichannel battery testing system (LAND CT2001A) with a cutoff voltage of 0.01–3.00 V versus Li/Li + at room temperature. Cyclic voltammetry (CV) was performed by using a CHI-660D potentiostat in the voltage range of 3 to 0.01 V at a scanning rate of 0.5 mV s¢1. Moreover, the electrochemical impedance spectroscopy (EIS) of the cells was measured with an electrochemical workstation (CHI 604 B) in the frequency range of 0.01 to 100 kHz.

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