Rational Construction of Multivoids-Assembled Hybrid Nanospheres

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Rational Construction of Multivoids-Assembled Hybrid Nanospheres Based on VPO4 Encapsulated in Porous Carbon with Superior Lithium Storage Performance Di Zhao, Tao Meng, Jinwen Qin, Wei Wang, Zhigang Yin, and Minhua Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11670 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Rational Construction of Multivoids-Assembled Hybrid Nanospheres Based on VPO4 Encapsulated in Porous Carbon with Superior Lithium Storage Performance Di Zhao, Tao Meng, Jinwen Qin, Wei Wang, Zhigang Yin, Minhua Cao* Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China.

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 34

ABSTRACT: The design of a new nanostructured anode material with high tap density while still keeping the common advantages of the hollow structure is a great challenge for future lithium-ion batteries (LIBs). Here, multivoids-assembled hierarchically meso-macroporous nanospheres based on VPO4 encapsulated in porous carbon (MVHP-VPO4@C NSs) were designed and fabricated. This unique structure can evidently decrease the excessive interior space in hollow spheres or multishelled hollow spheres to gain high volumetric energy density, and at the same time can alleviate the large mechanical strain during the cycling process. As expected, MVHP-VPO4@C NSs show good lithium storage behavior with gravimetric discharge capacity of 628 mAh g−1 after 100 cycles at a current density of 100 mA g−1. Furthermore, the full cell (LiFePO4 cathode//MVHP-VPO4@C NSs anode) also exhibits outstanding lithium storage performance. The insight obtained from this structure may provide guidance for the design of other electrode materials experiencing large volume variation during the lithiation-delithiation process.

KEYWORDS: Vanadium phosphate; multivoids; assembly; nanospheres; lithium ion batteries

2


Page 3 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION The development of environmentally-benign nanomaterials for lithium-ion batteries (LIBs) with high tap density, high power density, and outstanding electrochemical stability is still a big challenge for power-intensive energy storage applications, such as consumer devices, large-scale power-grid storage, etc..1,2 However, taking into account both the high tap density and the outstanding cycling stability usually is very difficult. For example, transition-metal oxides, sulfides and phosphates are very attractive candidates considering their abundance, environmental benignity, and high theoretical capacities.3,4 Unfortunately, a common drawback of these materials is rapid capacity fade, which is due to volume variation in these materials during the lithiation-delithiation process leading to pulverization and aggregation of electrochemically active particles, and formation of unstable solid-electrolyte interphase (SEI) film.1,4-6 To address these problems, one general strategy is to engineer the electrolyte blocking layer with conductive and elastic matrices, such as amorphous carbon and graphene, which could buffer the large volume change during cycling, and maintain a mechanically and chemically stable thin SEI film to some extent.2,4-8 Another commonly used approach is to design porous nanostructured materials, which can not only accommodate volume change to enhance the electrochemical properties of the electrode but also facilitate electronic and ionic transports between the electrolyte and electrode.9,10 Besides, engineering optimized nanostructures with void spaces such as hollow nanotubes and hollow nanospheres (NSs) has been proven to be a very powerful strategy. This unique porous structure could ensure easy access of the electrolyte

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 34

to the electrode and at the same time might accommodate the huge volume variation during the discharge-charge process compared with solid counterparts.2,6,11-13 However, these structures with excessive hollow interior space would dramatically sacrifice the packing density of the materials, thus inevitably resulting in relatively low volumetric energy and power densities.2,5,6 In order to make better use of the inner hollow cavity, complex hollow structures have been constructed, such as bowl-like hollow particles,2,4,6 double- or multi-shelled hollow spheres,4,13 single- or double-shelled yolk-shell hollow powders,11 and tube-in-tube hollow tubes.14 Although some hollow-structured materials have resolved some major problems, they cannot be applied as the electrode materials in practical applications because of their low energy density coming from low volumetric density. That is to say, the large interior space significantly lowers the volumetric density of the electrode materials. Up to date, the maximum number of the shells is four in quadruple-shelled hollow microspheres.13 However, it has been demonstrated that the quadruple-shelled hollow microspheres exhibit poor cycling performance compared with the triple-shelled counterpart due to their larger volume-occupying rate, indicating that the strategy of constructing multishelled NSs might be nonfeasible.13 Taking this into account, it is highly desirable to construct new complex hollow structures with relatively high volumetric energy density. VPO4, as one of inexpensive transition-metal phosphates, possesses great promise as an anode material for LIBs because of its relatively high theoretical specific capacity (550 mAh g−1) compared to conventional graphite.15-16,18 Besides, VPO4 has a stable structure due to the large

4


Page 5 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


volume of PO43−. Therefore, the high structural stability of VPO4 is beneficial for alleviating the volume change during the lithiation-delithiation process, thus weakening the polarization of the electrode materials.19,20 However, lithium ion-coupled charge transfer reactions in VPO4 still induce a certain degree of volume changes due to the formation of Li3PO4 and V0+ phases.19,20 This unavoidably results in some adverse processes including aggregation, pulverization and formation of unstable SEI film, thus leading to rapid capacity fading.19,20 Moreover, VPO4 has poor conductivity, which is its main drawback. To solve the problems of VPO4 mentioned above and at the same time to improve its tap density, herein, novel multivoids-assembled hierarchically meso-macroporous nanospheres based on VPO4 encapsulated in porous carbon (denoted as MVHP-VPO4@C NSs) were designed and synthesized by a freeze-drying-assisted annealing method. This distinguished material design inherits the advantages of the hollow spheres, such as the meso- and macro-scopic voids, which can accommodate volume expansion upon the lithiation-delithiation process and shorten electron/ion transport pathways. Furthermore, compared with conventional hollow spheres, the MVHP-VPO4@C NSs exclude the unnecessary internal spaces and contain a higher percentage of the active component, thus improving the energy density. Scheme 1a schematically illustrates different hollow structures including our as-synthesized multivoids-assembled NSs. Clearly, more active components (blue areas) for the multivoids-assembled spheres could be closely packed within given amount of spaces compared to hollow spheres and multi-shelled hollow spheres with the same diameter. In fact, the bubble-like macropores in the multivoids-assembled

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 34

a

b

Scheme 1. (a) Comparison of packing of hollow NSs, multishelled hollow NSs, MVHP-VPO4 NSs and solid NSs; (b) The schematic structure of hollow NSs, multishelled hollow NSs, MVHP-VPO4 NSs and solid NSs during the lithiation-delithiation process.

spheres can act as the voids, which is equivalent to build more shells in conventional hollow spheres. In view of this fact, the number of the multishells in this kind of multivoids-assembled structure is obviously larger than those of traditional multishelled structures, in which currently reported maximal number of the shells is four. Thus, the tap density of the as-fabricated electrode based on the multivoids-assembled spheres would be substantially increased. As demonstrated, the tap density of MVHP-VPO4 NSs is measured to be 0.52 g cm–3, which is lower than that of graphite (1.3 g cm−3), but higher than those of annealed carbon black (0.22 g cm−3),21 graphene (0.015 g cm−3),22 and carbon aerogels or mesoporous carbon (0.08-0.45 g cm-3).23 Furthermore, the tap density of MVHP-VPO4 NSs is also higher than those of reported hollow rutile TiO2 submicroboxes (0.44 g cm-3),24 hollow LiMn2O4 microspheres (0.27 g cm-3),25 and even higher than that of multi-shelled MgCo2O4 hollow microspheres (0.47 g cm-3).26 This value is

6


Page 7 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


comparable to those of previously reported Nb2O5@Nb4C3Tx composite (0.68 g cm-3),27 mesoporous Si/C nanocomposite (0.48 g cm-3)28 and carbon coated Na3V2(PO4)3 (0.68 g cm-3).29 Meanwhile, the integration of an interconnected carbon support and abundant voids not only offers shorter electron/ion transport pathways, but also might alleviate the strain associated with repeated lithiation-delithiation process, thus leading to a remarkably improved electrochemical performance (Scheme 1b).4,6,8 However, for conventional solid sphere, although it has the highest tap density among all the structures described above, its dense structure would greatly hinder the access of the electrolyte and thus the utilization of all active material is relatively low. With all these advantages of the multivoids-assembled structure, the MVHP-VPO4@C NSs electrode exhibits high specific capacities (628 mAh g-1 at the 100th cycle at a current density of 0.1 A g-1 and 385 mAh g-1 over 1000 cycles at a current density of 10 A g-1) and good cycling stability. More importantly, the assembled MVHP-VPO4@C//LiFePO4 full cell also shows outstanding cycling performance and rate capacity. Such a unique multivoids-assembled structure might be mainly responsible for the significantly improved lithium storage performance of the MVHP-VPO4@C NSs. 2. EXPERIMENTAL SECTION 2.1. Synthesis of MVHP-VPO4@C NSs: For a typical synthesis, 1 g of Pluronic F127 was dissolved into 10 mL of deionized water. Then, 1 mmol of NH4VO3 and 1 mmol of (NH4)2HPO4 were added with magnetically stirring. After 30 min, the transparent solution was put into the liquid nitrogen rapidly and then freeze-dried for 24 h at -50 °C. The freeze-drying can not only

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 34

ensure homogeneous dispersion of each component, avoiding the phase separation, but also prevent particle aggregation. Finally, the obtained precursor was annealed at 850 °C for 5 h under H2/Ar (H2, 7 vol.%) atmosphere with a heating rate of 3 °C min-1 to yield the sample (MVHP-VPO4@C NSs). In addition, pure VPO4 and VPO4@C hybrids with different morphologies and carbon contents were also prepared by adjusting the usage of F127 (0.0, 0.25, 0.5 and 1.5 g) and the amount of NH4VO3 [NH4VO3:(NH4)2HPO4 = 0.5:0.5, 2:2], while keeping other conditions constant. 2.2. Characterizations: The size and morphology were characterized by the field emission scanning electron microscopy (FESEM) on a HITACHI S-4800 microscope and the transmission electron microscopy (TEM) on a JEOL 2100F microscope operated at 200 kV coupled with an energy dispersive X-ray spectrometer (EDS). The crystalline phases were identified using powder X-ray diffraction (XRD) on a Bruker D8. In addition, after testing, the coin cells were disassembled and washed with dimethyl carbonate (DEC) and then dried in vacuum chamber. The obtained electrodes were characterized by FESEM and TEM techniques. All Raman spectra were collected by using an Invia Raman spectrometer at 633 nm laser excitation. The surface composition was characterized by the X-ray photoelectron spectra (XPS) on ESCALAB 250 spectrometer (Perkin-Elmer). The carbon and nitrogen contents were calculated by a CHN elemental analyzer (Vario EI). The specific surface areas were calculated by a standard BrunauerEmmett-Teller (BET) method on a Belsorp-max surface area detecting instrument. The tap density was acquired by a powder tap density tester (HY-100, Bettersize).

8


Page 9 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.3. Electrochemical measurements: The obtained products were incorporated into 2025 coin-type cells. For the anode preparation, the active material, conductive acetylene black and sodium carboxymethyl cellulose binder (CMC) were mixed into slurry with a weight ratio of 8:1:1. The as-resultant slurry was uniformly pasted on a Cu foil and dried at 120 °C for 24 h under vacuum oven. Coin-type cells were fabricated using lithium foil as the counter electrode, Celgard 2400 microporous polypropylene membrane as the separator and 1 M LiPF6 dissolved in an ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) mixture (1:1:1, v/v/v) as electrolyte. The loading of the active material was 0.8-1.1 mg cm-2. The coin cells were galvanostatically charged and discharged on a multichannel battery testing system (LAND CT2001A) in the voltage range of 0.01-3.0 V (vs Li+/Li). In addition, a full cell was also assembled into coin-type cells by combining a pre-lithiated MVHP-VPO4@C NSs anode, LiFePO4 cathode, and the same electrolyte. The pre-lithiation of the MVHP-VPO4@C NSs electrode was performed according to our previously reported method.30 Full cells were tested on the same battery tester in the voltage range 1.0-3.7 V. Cyclic voltammetry (CV) measurements were conducted on a CHI-760E electrochemical workstation at 0.3 mV s-1. The electrochemical impedance spectra (EIS) measurements were carried out over a frequency range of 100 kHz to 0.01 Hz. 3. RESULT AND DISCUSSION The formation process of MVHP-VPO4@C NSs involves two steps. First, an emulsion containing Pluronic F127, NH4VO3, and (NH4)2HPO4 is freeze-dried to form organic-inorganic

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hybrid

via

evaporation-induced

self-assembly

(EISA).

Afterwards,

Page 10 of 34

the

as-prepared

organic-inorganic hybrid is used as a precursor to be subjected to treating in H2/Ar atmosphere, during which the carbonization of Pluronic F127 leads to the formation of carbon matrix. Meanwhile, during this annealing process, a large amount of gases are also generated accompanying with the formation of VPO4 from the reaction between NH4VO3 and (NH4)2HPO4, thus resulting in the formation of voids. The morphology of the organic-inorganic hybrid is investigated by field-emission scanning electron microscopy (FESEM). As shown in Figure S1a, the precursor shows a sheet-like structure with a relatively rough surface. Careful observation from a high-magnification FESEM image found that the nanosheets are assembled closely by NSs with diameters of 500-800 nm (Figure S1b). After thermally treating in H2/Ar atmosphere, the rough nanosheets with nanosphere (NS) units spontaneously transform into monodispersed NSs with an average diameter of about 300 nm (Figure 1a), which may be as a result of shrinking during the annealing process. The high-magnification FESEM image further reveals that the resultant NSs show a rough surface and are composed of nanoparticle subunits (Figure 1b). In order to examine the internal structure of the NSs, high-magnification transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) are performed. It is interesting to find that the NSs are not solid and there exist lots of bubble-like voids within the NSs (Figure 1c,d), which probably result from the release of gases during the annealing process. Furthermore, the high-magnification TEM image reveals that the bubble-like voids have a wide diameter range of 10-60 nm (Figure 1e). A

10


Page 11 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

c

d

e

f

gO

i

h

V

V P

k

j C

V

Cu

O 0

V

P

2

4 Energy (Kv)

6

8

C

Figure 1. (a,b) FESEM images of MVHP-VPO4@C NSs. (c-f) TEM and HRTEM images of MVHP-VPO4@C NSs. (g) EDS spectrum and STEM image, and (h-k) corresponding element mappings of MVHP-VPO4@C NSs.

representative HRTEM image is shown in Figure 1f, which displays clear lattice fringes. The observed interplane spacing is 0.43 nm, which matches well to the (110) plane of orthorhombic VPO4 (PDF No. 86-1194). In addition, we also observe some disordered worm-like channels around the well-crystalline VPO4 particles, which should be attributed to amorphous carbon and it will be further confirmed later. To illustrate the element distribution in this sample, the scanning transmission electron microscopy (STEM)-energy-dispersive X-ray spectroscopy (EDS) is performed on a typical single NS. The EDS measurement confirms the co-existence of V, O, P and C elements (Figure 1g). The STEM image and the corresponding EDS elemental mapping images of V, O, P and C further reveal that all elements are uniformly distributed in the whole

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 34

NSs (Figure 1g-k, Figure S2). The X-ray diffraction (XRD) and Raman spectroscopy were further used to confirm the phase composition of the typical sample. As shown in Figure 2a, all of the main diffraction peaks in XRD pattern can be indexed to orthorhombic VPO4 phase [space group Cmcm (63), a = 5.23 Å, b = 7.767 Å, c = 6.243 Å; JCPDS No. 86-1194]. Besides the typical diffraction peaks of the VPO4 phase, several peaks at around 30o and 44o are also observed. These peaks can be indexed to different phases of VO(PO3)2 (JCPDS No.77-995 and JCPDS No. 84-48), which may be due to that the vanadium source has not been reduced completely. Furthermore, these peaks are very weak in intensity, indicating a very low content. So, these impurities can be ignored. The Raman spectroscopy (Figure 2b) shows two strong peaks at 1341 and 1590 cm−1, corresponding to the characteristic D and G bands of carbon materials, respectively,31,32 further confirming the existence of carbon in MVHP-VPO4 NSs. It should be noted that the carbon not only can serve as a rigid support to prevent VPO4 particles from agglomeration, but also is of great benefit to the fast electron transfer.32 The amount of the carbon in this hybrid is determined to be about 22.8 wt% by element analysis (Table S1). Furthermore, another two peaks at 915 and 1023 cm-1 can be assigned to the totally symmetric PO4 “ breathing” vibration and the V-O-P stretch, respectively.33,34 The surface chemical composition of MVHP-VPO4 NSs is further investigated by X-ray photoelectron spectroscopy (XPS) measurements. As expected, the survey spectrum (Figure S3) shows the existence of V, P, O and C elements, which is in agreement with above EDS elemental mapping results. Figure 2c shows high-resolution V 2p XPS spectrum, in which

12


Page 13 of 34

MVHP-VPO4@C NSs

♦

∗

∗∗

c

D band

V 2p3/2 516.7 eV

PDF No. 77-995

G band

Intensity (a.u.)

∗ Intensity (a.u.）

b

♦ PDF No. 84-48

♦ ♦

PO4

V-O-P

Intensity (a.u.)

a

V 2p1/2 523.8 eV

V 2p

VPO4 PDF No. 86-1194

20

25

30

35

40 45 2θ (degree)

50

d

55

60 500

750

e

O 1s

513

1000 1250 1500-1 1750 2000 Wavenumber (cm )

525 C 1s

Intensity (a.u.)

O=C V=O

528 530 532 534 Binding Energy (eV)

536

538

Intensity (a.u.)

C-C, C=C O-C

526

516 519 522 Binding Energy (eV)

f

P 2p

P=O

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


P-O

P-C

129

132 135 Binding Energy (eV)

138

282

C-P C-O

O-C=O

284 286 288 Binding Energy (eV)

290

Figure 2. (a) XRD pattern, (b) Raman spectrum, (c,d,e,f) high-resolution V 2p, O1s, P 2p, and C 1s XPS spectra for the resultant MVHP-VPO4@C NSs.

the binding energies at 516.7 and 523.8 eV can be assigned to V 2p3/2 and V 2p1/2 of V3+.35 The high-resolution O 1s XPS spectrum (Figure 2d) indicates the presence of V-O and P-O bonds.36-39 The high-resolution P 2p XPS spectrum (Figure 2e) can be deconvoluted into two sub-peaks located at 132 and 133.5 eV, which correspond to P-C and P-O bonds, respectively.37,40 Moreover, from the C 1s XPS spectrum, the P-C bond (285.1 eV) is further confirmed (Figure2f).37,38 The formation of the P-C bond may result from the high temperature reaction between Pluronic F127 and (NH4)2HPO4 in Ar/H2 atmosphere.41 These results suggest that VPO4 is chemically bonded with carbon framework via the P-C bond. This kind of coupling should be favorable for alleviating the volume change and maintaining the integration of the NS structure during repeated cycling.42 It should be noted that the microstructure of the final product can be tuned by changing the amount of Pluronic F127 used in our experiments (Table S1). The typical sample discussed

13



a VPO4@C-0 Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 34

b

c

VPO4@C-0

[email protected]

d

e

[email protected]

[email protected]

[email protected]

[email protected]

[email protected]

20

30

40 50 2θ (degree)

60

70

Figure 3. (a) XRD patterns of VPO4 samples obtained by using different amounts of F127 (0, 0.25, 0.5 and 1.5 g). (b-e) The corresponding FESEM and TEM images.

above is obtained with 1 g of F127. We also try different amounts of F127 (0, 0.25, 0.5 and 1.5 g) and the resultant samples are analyzed by XRD and FESEM measurements (Figure 3a-e). The XRD patterns indicate that the crystalline component in all these samples is VPO4 phase. When no F127 or a small amount of F127 (0.25 g) is used, resultant VPO4@C-0 and [email protected] samples both present large aggregates with irregular morphology. If 0.5 g of F127 is used, some bowl-like NSs in this sample ([email protected]) are observed. With increasing F127 to 1 g, the product is almost completely composed of NSs. When the amount of F127 is further increased to1.5 g, the morphology of [email protected] displayed by FESEM and TEM images is similar to that of the product obtained with 1 g of F127 (Figure 3e and Figure S4). Based on these results, we can see that F127 plays a pivotal role as a morphology directing agent for the formation of NSs. Besides, the amount of NH4VO3 is also a key factor for the formation of well-define NSs. When 0.5 mmol of NH4VO3 is used while the amount of F127 is kept at 1.0 g and the molar ratio of NH4VO3:(NH4)2HPO4 is still kept at 1:1, the product ([email protected]:0.5) displays poor crystallinity (Figure S5a) and is composed of plenty of hemispheres (Figure S5b). However, if 14


Page 15 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NH4VO3 is increased to 2 mmol, the resultant NSs (VPO4@C-2:2) become larger (with an average diameter of 500 nm) and irregular (Figure S5c). Moreover, NH4+ derived from NH4VO3 and (NH4)2HPO4 also has an important effect on the formation of multivoids-assembled porous NSs. To confirm this fact, NH4VO3 and (NH4)2HPO4 are replaced by vanadium (IV) oxy acetylacetonate and H3PO4. It is interesting to find that the resultant sample does no contain pores and voids under the same annealing conditions, although its component still is phosphate phase (Figures S6,S7). This result indicates that NH4+ may be responsible for the formation of pores/voids because of the release of internally generated ammonia (NH3) during the annealing process. The CHN element analysis further confirms the complete removal of NH4+ since all the as-synthesized VPO4 samples almost do not contain nitrogen element (Table S1). To reveal the porosity properties of all the resultant samples, N2 absorption-desorption isotherms are measured. As shown in Figure 4a, they all display type-IV isotherms with a relatively unusual combination of type-H2 and -H4 hysteresis loop in the relative pressure range of 0.5-1.0 P/P0. The pore size distributions of all the samples calculated by the desorption branch corroborate the co-existence of mesopores and macropores (Figure 4b). Typically, for the MVHP-VPO4@C NSs, there is a wide peak with the maximum located at about 66 nm, suggesting a unique characteristic of macropores (the macro-voids) embedded in a carbon matrix with small-size pores.6 However, for the [email protected] sample, the peaks for mesopores and macropores shift towards small pore size, and meanwhile the micropores appear obviously (Figure 4a,b). Furthermore, the MVHP-VPO4@C NSs display the specific surface area of 81.5

15



m2 g-1, which is larger than those of VPO4@C-0 (6 m2 g-1), [email protected] (28.2 m2 g-1), [email protected] (36.9 m2 g-1), [email protected]:0.5 (31.2 m2 g-1), and VPO4@C-2:2 (32.4 m2 g-1), but smaller than that of [email protected] (117.7 m2 g-1) (Figure 4c, Table S1 and Figure S8). These results show that with the increase of the amount of F127, the specific surface area is getting

a 200

[email protected] MVHP-VPO4@C NSs

100

[email protected] [email protected]

0

VPO4@C-0

0.0

c

b

0.004

300

dV/dD (cm3 g-1 nm-1)

Volume adsorbed (cm3 g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

0.2 0.4 0.6 0.8 Relative pressure (p/p0)

Samples VPO4@C-0 [email protected] [email protected] MVHP-VPO4@C NSs [email protected]

1.0

[email protected] MVHP-VPO4@C NSs [email protected] [email protected] VPO4@C-0

0.003 0.002 0.001 0.000 0

30

60 90 120 Pore diameter (nm)

Carbon content (wt%) 0.4 8.8 11.30 22.8 28.2

150

SBET ( m2 g-1) 6.1 28. 2 36.9 81.5 117.7

Figure 4. (a) N2 adsorption-desorption isotherms and (b) the corresponding pore size distribution curves of MVHP-VPO4@C NSs and other control samples (the isotherms of [email protected], [email protected], MVHP-VPO4@C NSs, and [email protected] were offset vertically by 30, 90, 120 and 170 cm3 g-1 STP, respectively); (c) the carbon contents and specific surface areas of all samples.

larger, indicating that F127 acts as not only a morphology directing agent but also a nanopore-forming agent. However, the porous materials fabricated with the assistance of F127 usually possess ordered mesopores because the spherical F127 micelles have a narrow size distribution around 10 nm.43 Thus it can be deduced that the large pores observed in our case could be ascribed to the usage of the ammonium sources, which may induce the aggregation of small micelles into large clusters with diameters over 10 nm via hydrogen bond or chemical bond

16


Page 17 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


interactions. Based on above experimental results, a possible mechanism for the formation of MVHP-VPO4@C NSs is proposed in Scheme 2. First, amphiphilic F127 is dissolved in water, and then self-assemble into micelles with hydrophilic PEO chains surrounding the hydrophobic

Scheme 2. Illustration of the formation of the MVHP-VPO4@C NSs.

micelle core. Then upon incorporation of NH4VO3 and (NH4)2HPO4, the small micelles aggregate to large spherical clusters by hydrogen- or chemical-bonding interactions between PEO chains and NH4+, which is electrically neutralized by VO3- and HPO42-. Finally, when the precursor is annealed at 850 °C under H2/Ar atmosphere, VO3- and HPO42- react to form VPO4 and at the same time the soft template F127 is carbonized accompanied with large NH3 release resulting from NH4+, thus generating mesopores and macropores in MVHP-VPO4@C NSs. The electrochemical performance of MVHP-VPO4@C NSs is examined as an anode material for LIBs and other VPO4 samples are also tested for comparison. Figure 5a shows the cyclic voltammograms (CVs) of the MVHP-VPO4@C NSs electrode for the first five cycles at a scan rate of 0.3 mV s-1 in the range of 0.01-3.0 V. A pronounced cathodic peak at around 0.1 V is observed in the first cycle, which can be ascribed to the reduction of VPO4 as well as some

17



3

a

c

b

Specific capacity (mAh g-1)

0.3

900

-0.3 1st ----- 5th

-0.6 -0.9

1st 2nd 10th 30th 100th

2

1

1.2 1.8 Potential (V)

2.4

0

3.0

d

1000

90

800

0.1 A g-1

600

60

1 A g-1

400 30 200

5 A g-1

8 A g-1

0 0

20

40 60 Cycles

80

0 100

Coulombic efficiency (%) Specific capacity (mAh g-1)

0.6

200 400 600 800 Specific capacity (mAh g-1) 1

2

4

0.1

[email protected] [email protected] MVHP-VPO4@C NSs

600

VPO4/C-0

15

350

5

VPO4/C-1.5

0 0

1000

e 900 0.1 0.2 0.4 0.8 A g-1

VPO4/C-0.5 VPO4/C-0.25

300

0

0.0

MVHP-VPO4/C NSs

600

300

30

45 60 Cycles

75

90

f MVHP-VPO4@C NSs VPO4@C-0 [email protected] [email protected] [email protected]

250 200

-Z'' ohm

Voltage (V)

Current (mA)

0.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

150 100

300 [email protected]

50 VPO4@C-0

0

0

20

0

40 Cycles

60

80

0

50

100 150 200 250 300 350 Z' ohm

Figure 5. Electrochemical performance: (a) CVs at a scan rate of 0.3 mV s-1 and (b) discharge-charge curves at 0.1 A g-1 for MVHP-VPO4@C NSs electrode; (c) cycling performance of all the VPO4 electrodes at 0.1 A g-1; (d) cycling performance at different current densities of MVHP-VPO4@C NSs electrode; (e) rate performances; (f) Nyquist plots of all the VPO4 electrodes after fitting using the equivalent electrical circuit shown in Figure S11.

irreversible reactions that are related to the decomposition of the electrolyte and the formation of SEI layer.19,20 From the 2nd cycle, this peak disappears, suggesting that the SEI layer is relatively stable after the 1st cycle. So, the second discharge profile is different from the first one, implying dissimilar electrochemical reactions. From the 2nd cycle, a new cathodic peak appears at about 0.88 V and then finally fixes at 0.95 V at the 5th cycle, and meanwhile there are several anodic peaks appearing. The involved reversible conversion reaction is as follows: VPO4 + 3Li+ + 3e- ↔ V + Li3PO4.19,20 Besides, after the first cycle, the CV profiles do not change significantly with subsequent sweeps, suggesting a good stability and reversibility of the electrochemical reactions in

MVHP-VPO4@C

NSs.

Furthermore,

compared

to

other

VPO4

electrodes,

the

MVHP-VPO4@C NSs electrode has the largest area bounded by the CV curves of discharge-charge processes (Figure S9a), which ascribes to the energy of the electrode.19 The 18


Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


first-cycle galvanostatic discharge-charge profile of MVHP-VPO4@C NSs electrode at 0.1 A g-1 is shown in Figure 5b. It can be clearly seen that MVHP-VPO4@C NSs electrode has a long voltage plateau located at ca. 0.75 V in the initial discharge process, which belongs to a typical two-phase reaction involving the crystal structure fracture and the formation of nanosize V dispersed in amorphous Li2O. The long voltage plateau is followed by a sloping region extended to 0.01 V, implying a single-phase Li reaction.44 However, it should be noted that except the initial discharge process, no obvious phase transition plateaus can be observed in the following discharge-charge processes, although there are some obvious cathodic and anodic peaks in the CV curves. This is probably due to that there is a single-phase-like reaction with a continuous transformation of crystal structure occurring during the lithiation-delithiation processes.3,43 Furthermore, Figure S9b presents the first-cycle galvanostatic discharge-charge profiles of all the VPO4 electrodes at 0.1 A g-1. Obviously, MVHP-VPO4@C NSs and [email protected] electrodes have the distinct longest voltage plateau, indicating the highest power output of these two samples among all the tested samples.13,45 Moreover, the initial discharge capacities of MVHP-VPO4@C NSs and [email protected] electrodes are 943 and 855 mAh g-1, respectively, which are much higher than those of other VPO4 materials. The outstanding lithium storage capacities are associated with the unique structure of the multivoids-assembled NSs, consisting of

highly

meso-macroporous

carbon

support.

Compared

with

[email protected],

the

MVHP-VPO4@C NSs electrode exhibits slightly higher discharge and charge specific capacities, which may be due to its lower carbon content (note that the specific capacity is calculated based

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

on the total mass of the VPO4 and the carbon support) (Table S1). In addition, for the MVHPVPO4@C NSs electrode, a high initial charge capacity of 678 mAh g-1 is obtained with an initial irreversible loss of about 28%, which is likely attribute to some irreversible reactions.2 However, for MVHP-VPO4@C-2, the initial charge capacity (601 mAh g-1) and the initial Coulombic efficiency (CE, about 70 %) are slightly lower, which may be due to the deep-seated micropores and smaller size mesopores in its structure (Figure 4).45 Moreover, compared with other VPO4 samples (66% for [email protected], 64% for [email protected], and 62% for VPO4@C-0) (Figure S9b), the CE (72%) of the MVHP-VPO4@C NSs electrode is also the highest, further demonstrating its excellent electrochemical performance. Moreover, from Figure 5b, we can also see that the capacities of the MVHP-VPO4@C NSs electrode range from 670 to 630 mAh g-1 from the 2nd to 100th cycle, and no obvious decay is observed. All of these capacities are higher than the theoretical capacity (550 mAh g-1) of VPO4, which might be attributed to the structure characteristics of the MVHP-VPO4@C NSs, including the high surface area, the comparatively appropriate pore size and the suitable carbon content, providing extra active sites for Li+ storage.47,48 In addition, the significantly improved electrochemical performance may benefit from the C-P interaction, because it can induce a lot of topological defects on the carbon surface, thus leading to a disordered carbon structure that further improves Li+ storage property.49 Moreover, the C-P bond could maintain good electric connection between VPO4 and the conductive carbon network during the lithiation process, thus enhancing the structural robustness of the electrode.41 As further shown in Figure 5b, the discharge-charge voltage profiles of the

20


Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


subsequent cycles almost overlap, indicating a good reversibility of the electrochemical reactions. To our knowledge, the performance of the VPO4-based anode materials for LIBs presented here is by far the best.19,20 Figure 5c shows the cycling performance of all the VPO4@C electrodes as well as bulk VPO4. Clearly, the MVHP-VPO4@C NSs possess the highest specific capacity and the best stability. After 100 cycles at 100 mA g-1, the specific capacities of MVHP-VPO4@C NSs, [email protected], [email protected] and [email protected] remain 628, 560, 350 and 265 mAh g-1, respectively, while that of the bulk VPO4 is only 185 mAh g-1. These results suggest that the optimized carbon content and porous multivoid structure can not only increase the electronic conductivity, but also alleviate the volume change.2,50 Moreover, Figure 5d indicates that the MVHP-VPO4@C NSs exhibit very high cycle stability at various current rates. Specifically, the corresponding CE at the current density of 0.1 A g-1 is close to 100% and high capacities of 426, 380 and 280 mAh g-1 are maintained after 100 cycles at high current densities of 1, 5 and 8 A g-1, respectively. Furthermore, the rate capability of the MVHP-VPO4@C NSs, which is critical for practical applications, is also satisfactory. As shown in Figure 5e, the specific capacity decreases slowly after the continuous cycling process with increasing current rates. Even at a high current density of 5 A g-1, the MVHP-VPO4@C NSs can deliver the specific capacity as high as 420 mAh g-1, which is higher than the theoretical capacity of graphite (372 mAh g-1). Remarkably, a stable capacity of 620 mAh g-1 can still be resumed when the current density goes back to 0.1 A g-1. The result indicates that the elastic multivoid structure indeed might be very “breathable”.13 To reveal

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

the excellent electrochemical behavior of such a electrode, the electrochemical impedance spectra (EIS) is carried out in the range of 100 kHz to 0.01 Hz (Figure 5f). The Nyquist plots are analyzed and fitted using an equivalent circuit model (Figure S10). The high frequency semicircle includes the resistance of the surface layer formed on the electrode (Rf) and the SEI film capacitance (Q1), while the intercept of the high-frequency semicircle on the Z’ axis can be assigned to the resistance of the electrolyte (Re). The medium frequency semicircle can be explained as the charge-transfer impedance (Rct) and the double-layer capacitance (Q2). In addition, the low-frequency sloping straight line corresponds to the Warburg impedance (Zw), which is due to the solid-state diffusion of lithium ions in the bulk electrode.16 And Q3 is related to the insertion capacitance reflecting the occupation of lithium into the inserted active sites.17 After fitting, the MVHP-VPO4@C NSs electrode shows the smallest charge transfer resistance (74.9 Ω) compared to other VPO4 samples and [email protected] with the highest carbon content (Table S2). Meanwhile, the MVHP-VPO4@C NSs electrode possesses the lowest electronic conductivity (σ = 8000 S m-1). One possible reason may be that the porous multivoid structure of the MVHP-VPO4@C NSs with the optimized surface area, the appropriate carbon content and the P-C bond increases the electrical conductivity, and more importantly shortens the transform paths for Li+ and electron.6,13 To further confirm the cycling performance of MVHP-VPO4@C NSs hybrid at high current density, the MVHP-VPO4@C NS electrode is tested at a higher current density of 10 A g-1 for 1000 cycles. As shown in Figure 6a, the electrode delivers a capacity as high as 385 mAh g-1

22


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b


Page 23 of 34

1000 800 -1

0.05 A g

600

-1

10 A g 400 200 0 0

200

400

600

800

1000

Cycle number (n)

200th

50th Lithiation delithiation

Figure 6. (a) Cycling performance of MVHP-VPO4@C NSs electrode at 10 A g-1 for 1000 cycles; (b) TEM images of the MVHP-VPO4@C NSs electrode after being tested for 50 (the left) and 200 (the right) cycles.

after 1000 cycles, which is still higher than the theoretical capacity of graphite (372 mAh g-1). It is worth noting that, from the 10th cycle to the 500th cycle, the capacity increases upon extended cycling, which may be related to the gradual access of the electrolyte into the porous multivoids of the MVHP-VPO4@C NSs and the gradual activation process of the electrochemical conversion reactions between VPO4 and V metal particles.51 Meanwhile the small-size effect of V nanocrystallines embedded in the amorphous Li2O matrix, greatly increases the electrochemical activity of reaction and will promote extra Li2O reversibly convert to Li+.44.51 These results confirm the high dependence of the cycling performance of the MVHP-VPO4@C NSs hybrid on the multivoids-assembled nanostructure. The structural stability of MVHP-VPO4@C NSs is further characterized by FESEM (Figure S11) and TEM images (Figure 6b) of the electrode cycled after 50 and 200 cycles, which maintain their morphology very well.

23



a

b

4


150

Potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

120

3 Half-cell: LiFePO4 Full-cell: MVHP-VPO4@C//LiFePO4

2

1 0

30 60 90 120 150 Specific capacity (mAh g-1)

90 60

Unit: C

0.1 0.2 0.4 0.6 0.8 30

0

10

20

1

30

5

10 0.1

40

Cycles

Figure 7. (a) Typical charge/discharge profiles of Li-LiFePO4 half-cell and MVHP-VPO4@C//LiFePO4 full cell; (b) rate performance of MVHP-VPO4@C//LiFePO4 full cell.

Thus it can be seen that the MVHP- VPO4@C NSs can act as an ideal candidate for an anode materials in LIBS. Ultimately, to further estimate the practical application of the as-prepared MVHP-VPO4@C NSs in LIBs, a full cell is constructed using the MVHP-VPO4@C NSs as the anode and commercially available LiFePO4 as the cathode. Before the full cell is assembled, to eliminate the large capacity loss in the first cycle and realize the optimal capacity balance, the MVHP-VPO4@C anode and the LiFePO4 cathode are electrochemically activated for three cycles in a half cell between 0.01 and 3.0 V and 2.8-4.0 V (Figure S12,S13), respectively. In order to approach the cell capacity balance to the 1:1 ratio with a slight excess of the anode capacity, the mass ratio of MVHP-VPO4@C in respect to LiFePO4 is determined to be about 1:3.4 to ensure the efficient utilization of the cathode material (the loadings of MVHP-VPO4@C NSs and LiFePO4 in our experiment are 0.8–1.1 and 2.72–3.74 mg cm-2, respectively).30 Figure 7a shows typical charge/discharge profiles of LiFePO4 half-cell and MVHP-VPO4@C//LiFePO4 full-cell. Clearly, LiFePO4 half-cell exhibits a high voltage plateau and a high capacity of 153 24


Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mAh g−1 at 0.1 C (1 C = 170 mA g−1 vs. LiFePO4). For the full-cell, a discharge capacity of 147 mAh g−1 (based on the cathode mass) is obtained operating at 0.1 C. Furthermore, the full cell has two discharge plateaus at 3.34 and 2.67 V, and exhibits an average discharge plateau of 3.0 V.52 Moreover, the full cell displays relatively stable cycling behavior at 0.1 C (Figure S14). Even cycled at high current densities, it exhibits high capacity and good stability. As we can see from Figure 7b, the cell can deliver 100 mAh g-1 with 68% capacity retention at 1 C, and 65 mAh g−1 at 10 C. The better performance in terms of specific capacity and cyclability confirms the potentiality of the MVHP-VPO4@C NSs as an innovative electrode material for the progress of lithium-based energy storage systems. 4. CONCLUSION In

summary,

novel

multivoids-assembled

VPO4@C

hierarchically

porous

NSs

(MVHP-VPO4@C NSs) were successfully synthesized. The rich bubble-like voids can provide enough space to accommodate the large volume change upon the lithiation process. In addition, the unique architecture of MVHP-VPO4@C NSs not only maintains the advantages of the conventional hollow structure or multishelled hollow structure but also brings additional benefits such as the increased packing density and extra active sites for Li+ storage. When tested as an anode material for LIBs, the MVHP-VPO4@C NSs exhibit excellent rate capacity, good cycling performance,

and

a

high

specific

capacity.

More

importantly,

the

assembled

MVHP-VPO4@C//LiFePO4 full cell also shows outstanding cycling performance and rate capacity. The concept demonstrated here may be generalized to design and synthesize other

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

materials with large volume expansion in next-generation LIBs. ASSOCIATED CONTENT

Supporting Information. Table S1 for Experimental details. FESEM images of the precursor for MVHP-VPO4@C NSs. STEM-EDS mapping images and XPS survey spectrum of the typical MVHP-VPO4@C NSs. TEM, FESEM images and XRD patterns of control samples. N2 adsorption-desorption isotherms of [email protected]:0.5 and VPO4@C-2:2. The first-cycle CVs and the first-cycle discharge-charge curves for all electrodes. FESEM images of the MVHP-VPO4@C NS electrode after different cycles. The equivalent circuit model and kinetic parameters of VPO4-based electrodes. Additional electrochemical test results of MVHP-VPO4@C//LiFePO4 full cell. The Supporting Informationis available free of charge via the Internet at http://pubs.acs.org/ AUTHOR INFORMATION Corresponding Author *Fax: +86-10-81381360. Tel: +86-10-81381360. E-mail: [email protected]

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21601014, 21471016 and 21271023) and the 111 Project (B07012). The authors would like to thank the Analysis & Testing Center of Beijing Insititute of Technology for performing FESEM and TEM measurements. REFERENCES

26


Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Chen, Y.; Li, X.; Park, K.; Song, J.; Hong, J.; Zhou, L.; Mai, Y. W.; Huang, H.; Goodenough, J. B. Hollow Carbon-Nanotube/Carbon-Nanofiber Hybrid Anodes for Li-Ion Batteries. J. Am. Chem. Soc. 2013, 135, 16280-16283. 2.

Liang, J.; Hu, H.; Park, H.; Xiao, C.; Ding, S.; Paik, U.; Lou, X. W. Construction of Hybrid Bowl-Like Structures by Anchoring NiO Nanosheets on Flat Carbon Hollow Particles with Enhanced Lithium Storage Properties. Energy Environ. Sci. 2015, 8, 1707-1711.

3. Nan, X.; Liu, C.; Zhang, C.; Ma, W.; Wang, K.; Li, Z.; Cao, G. A New Anode Material for High Performance Lithium-Ion Batteries: V2(PO4)O/C. J. Mater. Chem. A 2016, 4, 9789-9796. 4.

Xu, S.; Hessel, C. M.; Ren, H.; Yu, R.; Jin, Q.; Yang, M.; Zhao, H.; Wang, D. α-Fe2O3 Multi-Shelled Hollow Microspheres for Lithium Ion Battery Anodes with Superior Capacity and Charge Retention. Energy Environ. Sci. 2014, 7, 632-637.

5.

Lin, D.; Lu, Z.; Hsu, P. C.; Lee, H. R.; Liu, N.; Zhao, J.; Wang, H.; Liu, C.; Cui, Y. A High Tap Density Secondary Silicon Particle Anode Fabricated by Scalable Mechanical Pressing for Lithium-Ion Batteries. Energy Environ. Sci. 2015, 8, 2371-2376.

6.

Liang, J.; Yu, X. Y.; Zhou, H.; Wu, H. B.; Ding, S.; Lou, X. W. Bowl-Like SnO2@Carbon Hollow Particles as an Advanced Anode Material for Lithium-Ion Batteries. Angew. Chem. Int. Ed. 2014, 53, 12803-12807.

7.

Jia, X. L.; Cheng, Y. H.; Lu, Y. F.; Wei, F. Building Robust Carbon Nanotube Interweaved-Nanocrystal Architecture. ACS Nano 2014, 8, 9265-9273.

8.

Han, F.; Li, W. C.; Lei, C.; He, B.; Oshida, K.; Lu, A. H. Selective Formation of Carbon-Coated,

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Metastable Amorphous ZnSnO3 Nanocubes Containing Mesopores

Page 28 of 34

for Use as High-Capacity

Lithium-Ion Battery. Small 2014, 10, 2637-2644. 9.

Fei, L.; Xu, Y.; Wu, X.; Chen, G.; Li, Y.; Li, B.; Deng, S.; Smirnov, S.; Fan, H.; Luo, H. Instant Gelation Synthesis of 3D Porous MoS2@C Nanocomposites for Lithium Ion Batteries. Nanoscale 2014, 6, 3664-3669.

10. Wang, B.; Chen, J. S.; Wu, H. B.; Wang, Z.; Lou, X. W. Quasiemulsion-Templated Formation of α-Fe2O3 Hollow Spheres with Enhanced Lithium Storage Properties. J. Am. Chem. Soc. 2011, 133, 17146-17148. 11.

Hong, Y. J.; Son, M. Y.; Kang, Y. C. One-Pot Facile Synthesis of Double-Shelled SnO2 Yolk-Shell-Structured Powders by Continuous Process as Anode Materials for Li-Ion Batteries. Adv. Mater. 2013, 25, 2279-2283.

12.

Liu, J.; Xia, H.; Xue, D.; Lu, L. Double-Shelled Nanocapsules of V2O5-Based Composites as High-Performance Anode and Cathode Materials for Li Ion Batteries. J. Am. Chem. Soc. 2009, 131, 12086-12087.

13. Wang, J.; Yang, N.; Tang, H.; Dong, Z.; Jin, Q.; Yang, M.; Kisailus, D.; Zhao, H.; Tang, Z.; Wang, D. Accurate Control of Multishelled Co3O4 Hollow Microspheres as High-Performance Anode Materials in Lithium-Ion Batteries. Angew. Chem. Int. Ed. 2013, 52, 6417-6420. 14.

Zhang, G. Q.; Xia, B. Y.; Xiao, C.; Yu, L.; Wang, X.; Xie, Y.; Lou, X. W. General Formation of Complex Tubular Nanostructures of Metal Oxides for the Oxygen Reduction Reaction and Lithium-Ion Batteries. Angew. Chem. Int. Ed. 2013, 52, 8643-8647.

28


Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15. Hall, J. W.; Membreno, N.; Wu, J.; Celio, H.; Jones, R. A.; Stevenson, K. J. Low-Temperature Synthesis of Amorphous FeP2 and Its Use as Anodes for Li Ion Batteries. J. Am. Chem. Soc. 2012, 134, 5532-5535. 16. Wang, J. X.; Li X. H.; Wang Z. X.; Huang B.; Wang Z. G.; Guo, H. J. Nanosized LiVPO4F/Graphene Composite: A Promising Anode Material for Lithium Ion Batteries. J. Power Sources 2014, 251, 325-330. 17.

Ou, J. K.; Zhang, Y. Z; Chen, L.; Yuan, H. Y.; Xiao, D. Heteroatom Doped Porous Carbon Derived from Hair as An Anode with High Performance for Lithium Ion Batteries. RSC Adv. 2014, 4, 63784-63791.

18.

Liu, Z. M.; Peng, W. J.; Shih, K.; Wang, J. X.; Wang, Z. X.; Guo, H. J.; Yan, G. C.; Li, X. H.; Song, L. B. A MoS2 Coating Strategy to Improve the Comprehensive Electrochemical Performance of LiVPO4F. J. Power Sources 2016, 315, 294-301.

19. Zheng, J. C.; Han, Y. D.; Zhang, B.; Shen, C.; Ming, L.; Ou, X.; Zhang, J. F. Electrochemical Properties of VPO4/C Nanosheets and Microspheres as Anode Materials for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6, 6223-6226. 20. Zhang, Y.; Zhang, X. J.; Tang, Q.; Wu, D. H.; Zhou, Z. Core–Shell VPO4/C Anode Materials for Li Ion Batteries: Computational Investigation and Sol–Gel Synthesis. J. Alloys Compd. 2012, 522, 167-171. 21. Magasinski, A.; Dixon, P.; Hertzberg, B.; Kvit, A.; Ayala, J.; Yushin, G. High-Performance Lithium-Ion Anodes Using a Hierarchical Bottom-up Approach. Nature Mater. 2010, 9, 353-358.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

22. Wu, Z. S.; Ren, W. C.; Xu, L.; Feng L.; Cheng, H .M. Doped Graphene Sheets As Anode Materials with Superhigh Rate and Large Capacity for Lithium Ion Batteries. ACS Nano. 2011, 5, 5463-5471. 23. Guo, D. C.; Mi, J.; Hao, G. P.; Dong, W.; Xiong, G.; Li, W. C.; Lu, A. H. Ionic Liquid C16mimBF4 Assisted Synthesis of Poly(Benzoxazine-co-Resol)-Based Hierarchically Porous Carbons with Superior Performance in Supercapacitors. Energy Environ. Sci. 2013, 6, 652-659. 24. Yu, X. Y.; Wu, H. B.; Yu, L.; Ma, F. X.; Lou, X. W. Rutile TiO2 Submicroboxes with Superior Lithium Storage Properties. Angew. Chem. Int. Ed. 2015, 54, 4001-4004. 25. Xiao, X.; Lu, J.; Li, Y. LiMn2O4 Microspheres: Synthesis, Characterization and Use as a Cathode in Lithium Ion Batteries. Nano Res. 2010, 3, 733-737. 26. Shin, H.; Lee, W. J. Multi-Shelled MgCo2O4 Hollow Microspheres as Anodes for Lithium Ion Batteries. J. Mater. Chem. A 2016, 4, 12263-12272. 27. Zhang, C. J.; Kim, S. J.; Ghidiu, M.; Zhao, M. Q.; Barsoum, M. W.; Nicolosi, V.; Gogotsi, Y. Layered Orthorhombic Nb2O5@Nb4C3Tx and TiO2@Ti3C2Tx Hierarchical Composites for High Performance Li-Ion Batteries. Adv. Funct. Mater. 2016, 26, 4143-4151. 28. Zhang, R.; Du, Y.; Li, D.; Shen, D.; Yang, J.; Guo, Z.; Liu, H. K.; Elzatahry, A. A.; Zhao, D. Highly Reversible and Large Lithium Storage in Mesoporous Si/C Nanocomposite Anodes with Silicon Nanoparticles Embedded in a Carbon Framework. Adv. Mater. 2014, 26, 6749-6755. 29. Rui, X.; Sun, W.; Wu, C.; Yu, Y.; Yan, Q. An Advanced Sodium-Ion Battery Composed of Carbon Coated Na3V2(PO)4 in a Porous Graphene Network. Adv. Mater. 2015, 27, 6670-6676. 30. Wang, W.; Sun, Y.; Liu, B.; Wang, S.; Cao, M. Porous Carbon Nanofiber Webs Derived from Bacterial

30


Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cellulose as an Anode for High Performance Lithium Ion Batteries. Carbon 2015, 91, 56-65. 31. Yang, W.; Liu, X.; Yue, X.; Jia, J.; Guo, S. Bamboo-Like Carbon Nanotube/Fe3C Nanoparticle Hybrids and Their Highly Efficient Catalysis for Oxygen Reduction. J. Am. Chem. Soc. 2015, 137, 1436-1439. 32. Wu, R.; Zhang, J.; Shi, Y.; Liu, D.; Zhang, B. Metallic WO2-Carbon Mesoporous Nanowires as Highly Efficient Electrocatalysts for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 6983-6986. 33. Kaminow, I. P.; Leite, R. C. C.; Porto, S. P. S. Laser-Excited Raman Spectra of Deuterated KH2PO4. J. Phys. Chem. Solids 1965, 26, 2085-2088. 34. Dong, W. S.; Bartley, J. K.; Song, N. X.; Hutchings, G. J. Synthesis and Characterization of Vanadyl Hydrogen Phosphite Hydrate. Chem. Mater. 2005, 17, 2757-2764. 35. J. Mendialdua a, R. C. a., Y. Barbaux b. XPS Studies of V2O5, V6O13, VO2 and V2O3. J. Electron Spectrosc. Relat. Phenom. 1995, 71, 249-261. 36. Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. Long-Term Cycling Studies on 4 V-Cathode, Lithium Vanadium Fluorophosphate. J. Power Sources 2010, 195, 5768-5774. 37. Yan, H.; Tian, C.; Wang, L.; Wu, A.; Meng, M.; Zhao, L.; Fu, H. Phosphorus-Modified Tungsten Nitride/Reduced Graphene Oxide as a High-Performance, Non-noble-Metal Electrocatalyst for the Hydrogen Evolution Reaction. Angew. Chem. Int. Ed. 2015, 54, 6325-6329. 38. Puziy, A. M.; Poddubnaya, O. I.; Socha, R. P.; Gurgul, J.; Wisniewski, M. XPS and NMR Studies of Phosphoric Acid Activated Carbons. Carbon 2008, 46, 2113-2123.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

39. Jeon, I. Y.; Ju, M. J.; Xu, J.; Choi, H. J.; Seo, J. M.; Kim, M. J.; Choi, I. T.; Kim, H. M.; Kim, J. C.; Lee, J. J.; Liu, H. K.; Kim, H. K.; Dou, S.; Dai, L.; Baek, J. B. Edge-Fluorinated Graphene Nanoplatelets as High Performance Electrodes for Dye-Sensitized Solar Cells and Lithium Ion Batteries. Adv. Funct. Mater. 2015, 25, 1170-1179. 40. Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nature Nanotech. 2015, 10, 444-452. 41. Sun, J.; Zheng, G.; Lee, H. W.; Liu, N.; Wang, H.; Yao, H.; Yang, W.; Cui, Y. Formation of Stable Phosphorus-Carbon Bond for Enhanced Performance in Black Phosphorus Nanoparticle-Graphite Composite Battery Anodes. Nano Lett. 2014, 14, 4573-4580. 42. Zhou, X.; Dai, Z.; Liu, S.; Bao, J.; Guo, Y. G. Ultra-Uniform SnOx /Carbon Nanohybrids toward Advanced Lithium-Ion Battery Anodes. Adv. Mater. 2014, 26, 3943-3949. 43. Pramanik, M.; Tsujimoto, Y.; Malgras, V.; Dou, S. X.; Kim, J. H.; Yamauchi, Y. Mesoporous Iron Phosphonate Electrodes with Crystalline Frameworks for Lithium-Ion Batteries. Chem. Mater. 2015, 27, 1082-1089. 44. Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries. Chem. Rev. 2013, 113, 5364-457. 45. Cabana, J.; Monconduit, L.; Larcher, D.; Palacin, M. R. Beyond Intercalation-Based Li-Ion Batteries: the State of the Art and Challenges of Electrode Materials Reacting through Conversion Reactions. Adv. Mater. 2010, 22, E170-E192. 46. Wang, S. X.; Chen, S.; Wei, Q.; Zhang, X.; Wong, S. Y.; Sun, S.; Li, X. Bioinspired Synthesis of

32


Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Hierarchical Porous Graphitic Carbon Spheres with Outstanding High-Rate Performance in Lithium-Ion Batteries. Chem. Mater. 2015, 27, 336-342. 47. Tian, L.; Zou, H.; Fu, J.; Yang, X.; Wang, Y.; Guo, H.; Fu, X.; Liang, C.; Wu, M.; Shen, P. K.; Gao, Q. Topotactic Conversion Route to Mesoporous Quasi-Single-Crystalline Co3O4 Nanobelts with Optimizable Electrochemical Performance. Adv. Funct. Mater. 2010, 20, 617-623. 48. Linares, N.; Silvestre-Albero, A. M.; Serrano, E.; Silvestre-Albero, J.; Garcia-Martinez, J. Mesoporous Materials for Clean Energy Technologies. Chem. Soc. Rev. 2014, 43, 7681-717. 49. Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y. Synthesis of Phosphorus-Doped Graphene and Its Multifunctional Applications for Oxygen Reduction Reaction and Lithium Ion Batteries. Adv. Mater. 2013, 25, 4932-4937. 50. Wang, H. G.; Yuan, S.; Ma, D. L.; Zhang, X. B.; Yan, J. M. Electrospun Materials for Lithium and Sodium Rechargeable Batteries: from Structure Evolution to Electrochemical Performance. Energy Environ. Sci. 2015, 8, 1660-1681. 51. Zhao, D.; Qin, J.; Zheng, L.; Cao, M. H. Amorphous Vanadium Oxide/Molybdenum Oxide Hybrid with Three-Dimensional Ordered Hierarchically Porous Structure as a High-Performance Li-Ion Battery Anode. Chem Mater. 2016, 28, 4180-4190. 52. Lv, D.; Gordin, M. L.; Yi, R.; Xu, T.; Song, J.; Jiang, Y. B.; Choi, D.; Wang, D. GeOx/Reduced Graphene Oxide Composite as an Anode for Li-Ion Batteries: Enhanced Capacity via Reversible Utilization of Li2O along with Improved Rate Performance. Adv. Funct. Mater. 2014, 24, 1059-1066.

33



The table of contents entry

Rational Construction of Multivoids-Assembled Hybrid Nanospheres Based on VPO4 Encapsulated in Porous Carbon with Superior Lithium Storage Performance Di Zhao, Tao Meng, Jinwen Qin, Wei Wang, Zhigang Yin, Minhua Cao*

90

800

0.1 A g-1

600

60

1 A g-1

400 30 200

5 A g-1

8 A g-1

0 0

20

40 60 Cycles

80

0 100

Coulombic efficiency (%)

1000 Specific capacity (mAh g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

34


Rational Construction of Multivoids-Assembled Hybrid Nanospheres

Recommend Documents