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Functional Inorganic Materials and Devices

Neat Design for the Structure of Electrode to Optimize the Lithium Ion Battery Performance Yongjie Zhao, Caihua Ding, Yanan Hao, Ximei Zhai, Chengzhi Wang, Yutao Li, Jingbo Li, and Haibo Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00873 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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ACS Applied Materials & Interfaces

Neat Design for the Structure of Electrode to Optimize the Lithium Ion Battery Performance Yongjie Zhao1*, Caihua Ding1, Yanan Hao2*, Ximei Zhai1, Chengzhi Wang1, Yutao Li3, Jingbo Li1 and Haibo Jin1* 1

Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green

Applications, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing, 100081, China 2

State Key Laboratory of Information Photonics and Optical Communications, School of Science,

Beijing University of Posts and Telecommunications, Beijing 100876, PR China 3

Materials Science and Engineering Program and Texas Materials Institute, University of Texas at

Austin, Austin, TX 78712 (USA) *

Corresponding Author: Yongjie Zhao, Email: [email protected]; Yanan Hao, Email: [email protected]

KEYWORDS:

Intergranular

crack,

Nb2O5/graphene

composite,

Soft-agglomeration, Li-ion batteries

1

ACS Paragon Plus Environment

graphene

nanoscroll,

ACS Applied Materials & Interfaces 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

ABSTRACT The appearance of mechanical cracks originated from anisotropic expansion and shrinkage of electrode particles during Li+ de/intercalation is a major cause of the capacity fading in Li-ion batteries. Well-designed and controlled nanostructures of electrodes have shown prominent prospect for solving this obstacle. Here we report a novel and convenient strategy for preparation of graphene nanoscroll wrapping Nb2O5 nanoparticles (denoted as T-Nb2O5/G). Firstly a high energy ball-milling is conducted to acquire softly-agglomerated T-Nb2O5 nanoparticles owing to its spontaneous reduction of surface energy among these single particles. Then freeze-drying leads to the formation of graphene nanoscroll, which easily realizes the in-situ wrapping over softly-agglomerated T-Nb2O5 nanoparticles. Extended cycling tests demonstrate that such T-Nb2O5/G yields a high reversible specific capacity of 222 mA h g-1 over 700 cycles at 1C. The dominated surface capacitive insertion processes possessing favorable kinetics enable the T-Nb2O5/G to exhibit excellent rate performance, which achieve capacity of 110 mA h g-1 at 10 C. A combined ex-situ XRD, SEM and TEM investigation reveal that the long-term cycling stability of T-Nb2O5/G is attributed to the excellent structural stability of electrode, in which the synergistic effect between the softly-agglomerated T-Nb2O5 nanoparticles and graphene nanoscroll prevents the formation of mechanical cracks.

2


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INTRODUCTION For lithium-ion batteries (LIBs), the performance degeneration may occur progressively over hundreds of cycles and eventually lead to failure. Many processes can contribute to these failure, and the intrinsic factors of the electrode, such as side reactions between the electrode and electrolyte, and mechanical cracks, often hold dominant action.1-5 Recently, the occurence of in/ex-situ observation offers a feasible and convenient way to monitor the processes of Li+ intercalation/deintercalation process in real-time, which plays an significant role in exploring the degradation mechanisms of electrodes.6-10 Wang et al. report the observations on the nucleation and growth of intragranular cracks in a commercial LiNi1/3Mn1/3Co1/3O2 cathode, finding that the formation of the intragranular cracks is directly associated with cycling stability and diffusion-controlled process.11 Chupas et al. find that the capacity loss is linked to intergranular cracking at the boundary between primary particles due to anisotropic changes in lattice dimension during Li extraction/insertion.12 In fact, intergranular crack formation is one of the most well-known material degradation mechanisms.13-19 In virtue of increasing the volumetric energy density of the electrode for lithium-ion batteries (LIBs), the packing density of the active components are required to increased accordingly, and one useful strategy to reach this is to ultilize primary nanoparticles to form densely packed secondary nano/micro particles. However, the anisotropic expansion and shrinkage in lattice dimension of the primary particles leads to occurrence of cracks between these particles during Li+ intercalation/extraction. With the prolonging of cycling, the cracks expand between different primary particles, which hinder the transfer of ions and electrons in the secondary particles. Moreover, the enlarging of cracks could result in the formation of passive film derived from the contact reactions between boundaries and electrolyte, which intensely impedes the transfer of ions and electrons in the boundaries, and 3



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eventually causes the deactivation of active materials, poor electrical conductivity and other mechanical failure in electrode.11, 20-22 The above analysis shows that in order to prolong the cycle life of electrode materials, one effective strategy is to depress the anisotropic expansion and shrinkage in lattice dimension of the primary particles. Generally speaking, the approaches include: (1) synthesizing of small-sized particles to reduce the stress at boundaries between particles; (2) Coating secondary particles with carbonous materials, which is a mechanical method to cope with the change of stress. Conclusions as a result, alleviation of intragranular cracking requires a stable structural framework of the electrode material. In the synthesis of densely packed secondary nano/micro particles, there are two kinds of agglomeration

behaviors

of

primary

nanoparticles,

known

as

hard-agglomeration

and

soft-agglomeration. The former refers to the agglomerated primary particles via strong interaction such as the chemical bond force or hydrogen bond force. The hard-agglomerated nanoparticles are combined intensely and revealed inferior activity, which should be avoided both in the preparation of nanostructured material and in the applications for energy storage fields. The latter represents the agglomerated primary particles induced by weak interaction forces like the Van der Waals' force and Coulomb forces between particles, which can be eliminated by some chemical action or mechanical energy. This feature makes the softly-agglomerated nanoparticles benifite for alleviating the stress induced by intergranular cracks during cycling, due to the relativly expansile and retractable rooms between each primary nanoparticles. In this paper, we conceived a convenient approach to scalable synthesis of softlyagglomerated T-Nb2O5 nanoparticles wrapped with graphene nanoscroll (donated as T-Nb2O5/G). The typical synthesis process involved using a ball milling to produce softly-agolometrated T-Nb2O5 nanoparticles, and a freeze-drying treatment to fabricate graphene nanoscroll via a cold 4


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quenching of heated mixed suspension of GO sheets and Nb2O5 nanoparticles. Such 3D pomegranate-like architecture maintained structural stability for the volume swelling/shrink during Li+ insertion/extraction, in which the softly-agglomerated T-Nb2O5 effectively prevented the formation of mechanical cracks between particles, and simultaneously, the graphene nanoscroll offered excellent conductivity, flexible accommodation to volume change, and strong electron interaction with the T-Nb2O5 nanoparticles. Consequently, the resultant T-Nb2O5/G delivered a high reversible specific capacity of 222 mA h g-1 over 700 cycles at the rate of 1 C, and high rate capability of 1600th reversible capacity of 163 mA h g-1 at 10 C, presenting a great promising candidate for the traditional Li4Ti5O12 anode material. EXPERIMENTAL SECTION Preparation of Graphene oxide and Nb2O5 Nanoparticles First, Graphene oxide (GO) sheets were synthesized using a modified Hummer’s method. T-Nb2O5 nanoparticles were prepared via a convenient nanocrystallization process of modified ball milling. Typically, commercial niobium pentoxide (Nb2O5) was first crushed into millimeter range and then a high energy mechanical milling at a speed of 1000 r/min was employed to produce refined T-Nb2O5 nanoparticles. Preparation of Nb2O5 Nanoparticles/Graphene Composite (T-Nb2O5/G) The typical synthesis process of T-Nb2O5/G composites was done as follows. (1) The T-Nb2O5 nanoparticles and the GO nanosheets were respectively dispersed in tertiary-butanol solution (the mass ratio of Nb2O5 and GO were 4:1) and then the Nb2O5 dispersion solution and GO suspension were mixed together with aid of magnetic stirring (80 °C) and sonication for 30 min. (2) A steel salver sprawled with the resulting homegenous mixed-solution was firstly put into liquid nitrogen 5



and then vacuum drying freezer (-50 °C) for 30 min until it was fully transformed into an ice-cake. After that, the completely frozen solid was suffered the vacuum freeze-drying at -50 °C for 24 h to remove vapor and tertiary-butanol. (3) The obtained dried solid precursor was then reduced by thermal treatment at 400 °C in argon atmosphere with a heating rate of 3 °C /min. Finally, the T-Nb2O5/G nanocomposite was obtained, in which the T-Nb2O5 nanoparticles were regularly wrapped by ultrathin graphene layers. Material Characterization The microstructure and morphology of the as-prepared materials were characterized by powder X-ray diffraction (XRD, Rigaku D8 advance X-ray diffraction-meter with Cu Kɑ radiation ( λ = 1.5418 Å ), scanning electron microscopy (SEM, LEO-1530, Oberkochen, Germany) and transmission electron microscope (TEM, JEM-2100F). Raman scattering was performed using the 633 nm radiation from He-Ne laser and was collected by a micro-Raman spectrometer in the 100~2000 cm−1 range at room temperature. X-ray photoelectron spectroscopy (XPS) was carried out on a PHI quantera scanning X-ray microprobe instrument employing monochromatic Al-Kɑ radiation as the excitation source, to evaluate the chemical composition of the synthesized samples. Fourier transform infrared spectra (FTIR, Nicolet iS50, Thermo SCIENTIFIC) were recorded over the wavenumber from 4000 to 400 cm-1. The specimens were pressed into a spectroscopically pure KBr matrix. The specific surface area and pore sizes distribution were investigated using Brunauer-Emmett-Teller (3H-2000PS2), and Barrett-Joyner-Halenda (BJH) desorption analyses, respectively. Electrochemical Characterization The electrochemical performances were assessed using LIR2032 coin-type cells. The working 6


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electrode was fabricated by mixing the active material, acetylene black, and polyvinylidene difluoride (PVDF) in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidinone (NMP) solvent. After fully grinding, they as-prepared working electrode was coated onto a copper foil substrate and dried in a vacuum drying oven at 80 °C, 24 h. Lithium metal was served as both the counter electrode and the reference electrode for lithium-ion battery, respectively. The electrolyte for LIBs was LiPF6 (1.0 M) in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) with a volume ratio of 1:1:1. A micro-porous polyethylene was used as a separator. The discharge-charge cycles were conducted on a LAND multi-channel battery test system (Wuhan, China) under diverse rates within a voltage window of 0-2.5 V. The Cyclic voltammetry (0-2.5 V) was performed at different scan rates within a voltage window of 0-2.5 V. The electrochemical impedance measurements were investigated on an electrochemical workstation (CHI 660B, Shanghai Chenhua instrument Co., LTD) with a AC voltage of 5 mV amplitude in the frequency range of 10 kHz to 0.01Hz. RESULTS AND DISCUSSION Figure 1 schematically illustrated the typical synthetic procedure of T-Nb2O5/G composites. First,

softly-agglomerated

T-Nb2O5

nanoparticles

were

prepared

via

a

convenient

nanocrystallization process of high-energy ball milling. Subsequently, Graphene oxide (GO) sheets were synthesized using a modified Hummer’s method.23 For fabrication of graphene wrapping T-Nb2O5 nanoparticle composite, we conceived a convenient strategy derived from previous reports about synthesis of graphene nanoscroll via change of tensile stress caused by sharply reducing of temperature.24-26 In a typical route: (1) The as-prepared T-Nb2O5 nanoparticles and the GO nanosheets were respectively dispersed in tertiary-butanol solution (the mass ratio of Nb2O5 and GO

7



were 4:1) and then the Nb2O5 dispersion solution and GO suspension were mixed together with aid of magnetic stirring (80 °C) and sonication for 30 min. (2) A steel salver sprawled with the resulting homogenous mixed-solution was firstly put into liquid nitrogen and then vacuum drying freezer at -50 °C for 24 h to remove vapor and tertiary-butanol. (3) The obtained dried solid precursor was then reduced by thermal treatment at 400 °C in argon atmosphere with a heating rate of 3 °C /min. Finally, the graphene nanoscroll wrapping T-Nb2O5 nanoparticles were prepared. As shown in the low-magnification SEM image in Figure 2a, the T-Nb2O5 consisted of large amounts of uniform nanoparticles, which softly agglomerated together due to high surface energy of single particle after ball-milling. The inset of Figure 2a was the statistical analysis of diameter for the T-Nb2O5 nanoparticles, which confirmed that the average sizes of the T-Nb2O5 nanoparticles were in the region of 80 to 120 nm in diameter. The TEM image in Figure 2b exhibited the disperse T-Nb2O5 nanoparticles, indicating that the softly-agglomerated T-Nb2O5 nanoparticles could be easily separated after ultrasonic dispersion for TEM characterization. The high-resolution TEM image (Figure 2c) from the selected area revealed high crystallinity with a lattice fringe distance of 0.39 nm, corresponding to the (001) d-spacing of the orthorhombic (T) Nb2O5 nanoparticles. Figure 2d presented the SEM image of the T-Nb2O5/G. It can be found that the softly-agglomerated T-Nb2O5 nanoparticles were closely encapsulated by wrinkled graphene nanoscroll, forming a pomegranate-like nanostructure. The TEM images in Figure 2e depicted that the T-Nb2O5 nanoparticles firmly attached to the graphene layers even after ultrasonic dispersion for TEM characterization, which demonstrated the existence of graphene nanoscroll wrapping T-Nb2O5 nanoparticles architecture. Referring to pomegranate structures, the graphene nanoscroll and T-Nb2O5 nanoparticles were corresponded to “films” and “granules”, respectively, showing as the inset of Figure 2d. This well-designed nanostructure endowed the T-Nb2O5/G with enough space to 8


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alleviate stress caused by expansion and contraction in lithiation/delithiation process. Figure 2f exhibited the HRTEM image of the T-Nb2O5/G, in which both crystalline and amorphous regions coexisted. The crystal regions showed clear lattices with spacing of 0.39 and 0.25 nm, corresponding to (001) and (181) lattice plane of the orthorhombic Nb2O5. The amorphous region represented the existence of graphene. The inset of Figure 2f presented the selected area electron diffraction (SAED) of the T-Nb2O5/G, exhibiting excellent crystallinity of orthorhombic Nb2O5 in nanocomposite. The XRD patterns of the T-Nb2O5 and T-Nb2O5/G were showed in Figure 2g. The T-Nb2O5 and T-Nb2O5/G samples exhibited high crystallinity with sharp diffraction peaks, which could be indexed to the orthorhombic (T) Nb2O5 phase (PDF# 30-0873). As shown in Figure S1 (seeing Supporting Information), a single peak at 11.6° corresponding to the (002) plane of GO with d-spacing of 0.82 nm could be observed, suggesting the wrecked original sp2-bond of graphite during oxidization. For the XRD patterns of the T-Nb2O5/G, no characteristic peaks at 26.6° derived from the stacking of graphene layers were detected, confirming that the long-range order or aggregation of graphene was destroyed owing to the freeze-drying process. 27 Figure 2h displayed the Raman spectra of the GO, T-Nb2O5 and T-Nb2O5/G. As for the pristine T-Nb2O5, the peaks located at 128, 226, 311, and 685 cm-1 arised from T-Nb2O5.28 Besides the characteristic peaks of the T-Nb2O5, the spectra of the T-Nb2O5/G showed two characteristic peaks at 1332.3 cm-1 and 1594.2 cm-1, which can be assigned to the disorder induced D-band and the graphitic G-band caused by E2g vibrational mode within aromatic carbon rings, respectively.29,30 The D and G band are the characteristic peaks of carbonaceous materials. The D/G intensity ratios (ID/IG) for the T-Nb2O5/G (1.25) was much higher than that of GO (1.01), which indicated a largely

9



disordered structure of graphene.31 The detailed surface elemental composition of the T-Nb2O5 and T-Nb2O5/G was characterized by X-ray photoelectron spectroscopy (XPS). The survey XPS spectrum of the T-Nb2O5/G (Figure S2a) exhibited the presence of the Nb 3d, C 1s and O 1s, without evidence of any impurities. The fitting results of the C 1s spectrum in the GO and T-Nb2O5/G were showed in Figure 2i. The C 1s spectrum of the T-Nb2O5/G revealed three different peaks located at 281.9, 283.0, and 286.7 eV, corresponding to C-C, C˗O, and C=O, respectively.32 Compared with that of in the GO (the inset of Figure 2i), the high intensity of C-C and low intensity of C=O (almost vanished) evidently suggested the thermally reduced of GO after calcination.33 The O 1s spectrum of the T-Nb2O5/G and the Nb 3d spectrum of the T-Nb2O5 and T-Nb2O5/G were showed in supporting information Figure S2b and Figure S2c-d. The FTIR spectra of the GO, T-Nb2O5 and T-Nb2O5/G composites were showed in Figure S3. The Nitrogen sorption isotherms were measured to investigate the Brunauer-Emmet-Teller (BET) surface area and the pore size distribution of the T-Nb2O5/G. As shown in Figure S4a, the nitrogen adsorption/desorption curve depicted a typical type IV hysteresis loop, suggesting the T-Nb2O5/G possessed a mesoporous structure.34 The BET surface area of the T-Nb2O5/G was calculated to be 54.01 m2 g-1. The Barrett-Joyner-Halenda (BJH) pore-size distribution curve (the inset in Figure S4b) exhibited that the pore size mainly centered in the range of 2~20 nm. It can be found that the obtained BET surface area of the T-Nb2O5/G is somewhat lower, which was primarily attributed to the soft-agglomeration of Nb2O5 nanoparticles. Simultaneously, the thermal treatment during reduction of GO may also result in the lowering of the surface area value. Moreover, active material possessing a very high surface area always renders redundant side reactions with the electrolyte and lower initial Coulombic efficiency, which should be avoid for being high

10


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performance LIBs electrodes. Cyclic voltammetry (CV) is critical to characterize the electrochemical properties of Li+ intercalation/deintercalation into/from the T-Nb2O5/G. The typical CVs at a scan rate of 2 mV s-1 in a potential window of 0 to 2.5 V (vs. Li+/Li) were showed in Figure 3a. In the initial cathodic scanning, a sharp and irreversible cathodic peak located at 0.54 V could be observed, which was mainly due to the reactions of electrolyte with the carbon additive in the electrode and the formation of irreversible solid electrolyte interphase (SEI) layer.35,36 The cathodic peaks at 1.42 and 1.58 V were attributed to the lithium intercalation process and the corresponding broad anodic peak at 2.3 V was assigned to the delithiation process. The lithium storage mechanism of T-Nb2O5 can be expressed as: xLi+ + xe- + Nb2O5 → LixNb2O5, and the lithiation of LixNb2O5 can be reached up to x = 2, corresponding to a maximum theoretical capacity of 200 mA h g-1.28, 37 In the subsequent cycles, the CVs revealed symmetric cathodic and anodic peaks, suggesting a highly reversible redox reactions process. Figure 3b exhibited the representative galvanostatic discharge/charge profiles of the T-Nb2O5/G electrode at a current density of 1C in a potential window of 0 to 2.5 V (1 C= 200 mA g-1). It can be noted that the first discharge profile with one distinct plateau during the lithiation process was significantly different from the subsequent cycles. The first discharge and charge capacities were 754 and 458 mA h g-1, respectively, rendering an initial Coulombic efficiency of 60.7 %. The large initial capacity may benefit from the full utilization of Nb2O5 nanoparticles and the irreversible formation of a solid electrolyte interphase (SEI) layer on the surface of the composite. The severe capacity loss in the first cycle could be ascribed to the irreversible formation of SEI layers and the incomplete reduction of Li2O.38 The discharge-charge curves of T-Nb2O5

11



without GO was shown in Fig. S-5 in the supporting information. Figure 3c depicted the dQ/dV curves of the T-Nb2O5/G electrode derived from the corresponding discharge/charge curves in Figure 3b. The intensities of the anodic and cathodic peaks in all profiles remained highly reversible with little decay from the second cycle, indicating enhanced reaction kinetics during long cycling. The extraordinary rate capability of the T-Nb2O5/G electrode was further demonstrated by cycling the batteries at various current densities (Figure 3d). The T-Nb2O5/G electrode delivered specific capacities of 281, 261, 238 and 224 mA h g-1 as the current density increased stepwise from 1 to 1.5, 2.5 and 5 C, respectively. It can be seen that the specific capacity under each stage showed good reversibility with high Coulombic efficiency over 98%. Moreover, even under an ultrahigh current density of 10 C, the T-Nb2O5/G electrode still achieved specific capacity of as high as 210 mA h g-1. When the current density gradually decreased from 10 C to 5, 2.5, 1.5 and 1 C, the specific capacities restored from 210 mA h g-1 to 239, 267, 284 and 295 mA h g-1, respectively, which were almost equal to the corresponding initial values. The vibration of the capacities showed highly symmetric with the change of current density, confirming the excellent electronic/ionic transport properties and improved reaction kinetic. The cycling performances of the T-Nb2O5 and the T-Nb2O5/G electrodes at a current density of 1C were shown in Figure 3e. The T-Nb2O5 electrode showed slight decay and obtained a reversible capacity of about 117 mA h g-1 after 300 cycles. In contrast, the T-Nb2O5/G electrode delivered a reversible capacity of about 222 mA h g-1 after 700 cycles with superior cycling stability. The Coulombic efficiency of the T-Nb2O5/G electrode could preserve nearly at 100% from the second cycle. The resulting performances indicated that the T-Nb2O5/G electrode exhibited enhanced lithium storage in terms of high specific capacity and excellent cycling performance compared with pristine T-Nb2O5. The electrochemical stability of the T-Nb2O5/G electrode at a very high rate of 10 12


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C was showed in Figure 3f. The T-Nb2O5/G electrode exhibited 1600th reversible capacity of 163 mA h g-1 with the average Coulombic efficiency of 99.8 %, indicating remarkable cycle stability. For the sake of exploring why the T-Nb2O5/G electrode exhibited enhanced electrochemical performances compared with the pristine T-Nb2O5, the electrochemical kinetics of Li+ de/intercalation processes within electrodes were investigated. Cyclic voltammetry of the T-Nb2O5/G electrode at scan rates ranging from 0.2 to 1 mV s-1 at a voltage range of 0~2.5 V was firstly measured. As shown in Figure 4a, the CV curves at various scanning rates exhibited similar shapes with broad cathodic and anodic peaks, suggesting obvious electrochemical reactions involved during Li+ insertion/extraction. The fitting results of b-value were obtained which illustrated in Figure 4b using the slope of the log (v)–log (i) plots, obeying the power law: i = avb, where a and b are appropriate values. In particular, the b-value of 0.5 represents a total diffusion-controlled behavior, whereas b-value of 1.0 suggests a capacitive process.39,40 The b values of anodic and cathodic peaks for the T-Nb2O5/G electrode were quantified as 0.94 and 0.87, respectively, corresponding to the dominated surface capacitive insertion processes. This result verified the significant effect of the favorable kinetics contributed by the appropriate structure on the outstanding rate performance of the T-Nb2O5/G electrode. Investigation of the sweep-rate dependence of the response current in CV curves can determine whether the process involved was a surface mechanism or a diffusion controlled mechanism. Thus, the capacitive and diffusion-limited contributions of the T-Nb2O5/G electrode was quantitatively separated, to further elucidate the effect of softly-agglomerated T-Nb2O5 nanoparticles and graphene layers in the improved rate capability and capacity. According to the relationship between measured current (i) and scan rate (ν) from the CV curves in Figure 4a, at a fixed potential, the measured current (i) can be divided into two terms: 13



i (V) = k1 ν + k2 ν1/2

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(1)

The surface capacitive effect represents fast kinetics which can be expressed as k1 ν, whereas the diffusion-controlled insertion process can be expressed as k2 ν1/2.41-43 By dividing both sides of Equation (1) by ν1/2, a rearranged equation (2) was obtained as follows:

i (V) / ν1/2 = k1 ν1/2 + k2

(2)

So k1 and k2 can be determined by plotting ν1/2 vs i (V) / ν1/2, which was depicted in Figure 4c, where the values of k1 were the gradients and values of k2 were the y-intercept, respectively. According to this analysis, by determining k1 and k2, it can be quantified the fraction of the current due to each of these contributions at specific potentials. Seeing the grey area in Figure 4d, the quantitative result verified that about 48% of the total lithium charge was attributed to surface capacitive mechanisms (at 1.6 V). Moreover, the data of surface capacitive contributions obtained here were for a relatively slow sweep rate (1 mV/s), which was beneficial for lithium intercalation processes. However, at higher sweep rates the capacitive processes associated with rapid discharge/charge characteristics would be more pronounced. The graphene contributed to the good conductivity of the T-Nb2O5/G electrode, which directly related to the superior electrochemical performance. The electrochemical impedance spectroscopy (EIS) measurement was further performed to confirm the enhanced conductivity of the T-Nb2O5/G electrode over a frequency range of 100 kHz to 0.01 Hz. The Nyquist plots of the pristine T-Nb2O5 and the T-Nb2O5/G electrodes before cycling were shown in Figure 4e, which were constituted by a depressed semicircle in high frequency region and a sloping line in low-frequency region, representing the charge-transfer impedance (Rct) in the electrode/electrolyte and lithium-diffusion process of the electrodes (Zw), respectively.44,45 The inset of Figure 4e is the equivalent circuit model simulated through Z-view software. The Rct values for the pristine T-Nb2O5 and the 14


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T-Nb2O5/G electrode before cycling was 110.3 and 32.5 Ω, respectively. As shown in Figure 4f, the Rct values of the T-Nb2O5/G electrode after 700 cycles had increased to 240.2 Ω compared with that before cycling. The T-Nb2O5/G electrode with lower Rct value exhibited improved electrical conductivity compared with the pristine T-Nb2O5. More importantly, the notable decrease in Rct value after introduction of graphene indicated that the electrochemical reactions within the T-Nb2O5/G electrode became much easier than that within the pristine T-Nb2O5 electrode, which was favorable for achieving high rate capability. Li+ intercalating into the T-Nb2O5/G electrode can be divided into two steps during the discharge process, and the intercalation quantities of Li+ during each step was calculated according to the discharge curve cycled at a current density of 1 C (Figure 4g). The electrochemical reactions in the T-Nb2O5/G electrode can be elaborated as below: xLi+ + xe- + T-Nb2O5/G → Lix (T-Nb2O5/G) (x = 0.85)

(3)

yLi+ + ye- + Lix(T-Nb2O5/G) → Lix+y (T-Nb2O5/G) (y= 2.83)

(4)

Firstly, in the voltage range of 2.0~1.5 V, this step represented the phase transition from T-Nb2O5/G to Li0.85 (T-Nb2O5/G). A following phase transition from Li0.85 (T-Nb2O5/G) to Li3.68 (T-Nb2O5/G) occurred in the voltage range of 1.5~0.2 V, which offered a significant contribution to capacities. In the case of charge processes, Li+ stepwise deintercalated from Li3.68 (T-Nb2O5/G) to the T-Nb2O5/G phase. It should be mentioned that the gross intercalation quantity of Li+ simultaneously included the quantity of Li+ into graphene was utilized for calculating x and y. To further verify the Li+ de/intercalation mechanism in the T-Nb2O5/G electrode, the ex-situ XRD characterization of was performed within a selected 2θ range operated at different states of discharge and charge against the voltage during the initial cycle (Figure 4h). The two distinct peaks at the range of 36° to 38° were derived from T-Nb2O5 at the as-prepared state. It can be found the 15



XRD peaks of T-Nb2O5 slightly shifted to the lower 2θ and the intensity gradually became weakened upon discharge from 2.5 to 0 V (vs. Li), indicating the intercalation of Li+ into T-Nb2O5 crystal. When the T-Nb2O5/G composites electrode was recharged back to 2.5 V, the peaks of T-Nb2O5 gradually recovered to the higher 2θ and the intensity slowly became strengthened, suggesting the deintercalation of Li+ from T-Nb2O5 crystal. The ex-situ SEM and TEM analyses were conducted to illustrate the structural stability of the T-Nb2O5/G electrode during and after cycling (Figure 5 and Figure S6). Figure 5a and b displayed the SEM images of the T-Nb2O5/G electrode after 5000 cycles under an ultrahigh current density of 10 C. The softly-agglomerated T-Nb2O5 nanoparticles still remained its pristine nanostructure (Figure 5a), suggesting that the pulverization and fracture was effectively avoided in this nanocomposite. From Figure 5b it can be found that cracks were clearly absent between pristine particles, indicating the well-designed nanostructure successfully prevented the emerging of cracks. The inset of Figure 5b exhibited the copper foil coated with active materials after 5000 discharge-charge cycles, in which no exfoliation of active material from the copper foil was observed, indicating that intergranular crack formation was effectively prohibited. Figure 5c showed the corresponding EDS spectrum of the T-Nb2O5/G electrode after 5000 cycles, which disclosed the existence and homogeneous distribution of Nb, O, and C elements. Figure 5d-f exhibited the ex-situ TEM images of the T-Nb2O5/G electrode after 1000, 2000 and 5000 cycles, where the integration and soft-agglomeration nanostructures of T-Nb2O5 are clearly observed. The insets of Figure 5d-f are the corresponding HRTEM images of the T-Nb2O5/G electrode after different cycles. The size of single nanoparticle with interplanar spacing of 3.9 Å (the inset of Figure 5f) was still about 80~120 nm, coinciding with the sizes of the pristine T-Nb2O5. To illustrate the structural stability of the T-Nb2O5/G electrode, a model representing the evolution of nanostructures of the pristine T-Nb2O5 16


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and T-Nb2O5/G electrode during lithiation/delithiation process was proposed as shown in Figure 5g and h. For the pristine T-Nb2O5 nanoparticles (Figure 5g), the nanostructure could preserve integration when Li+ intercalated into crystal structure at the beginning, but suffered from huge crumbling and collapsing of particles after long-term cycling, which was ascribed to the mechanical cracks between particles caused by stress. By contrast, the T-Nb2O5/G electrode revealed that the electrodes all preserved structural integration under long cycling (Figure 5f). Although the T-Nb2O5 nanoparticles showed volume expansion during Li+ intercalation, however, the external wrapping graphene effectively limited the volume change. Therefore, it could be inferred that this synergistic effect between softly-agglomerated nanostructures and graphene not only helped to accommodate the volume change of the active materials, but also effectively alleviated intergranular cracks, which maintained the integration of macroscopic structure in electrode. This accounts for why the T-Nb2O5/G electrode possessed long-term cycling stability. The excellent cycling stability and high rate capability of the T-Nb2O5/G electrode as anode material for LIBs were achieved due to the structural merits derived from the distinctive design route. (i) The well-tailored nanoscaled T-Nb2O5 nanoparticles shortened the diffusion lengths, benefitting for both electronic transport and Li+ transport (excellent rate capability). (ii) The softly-agglomerated T-Nb2O5 nanoparticles could promptly alleviate the stress induced by expansion/shrink of particles during cycling, which can effectively avoid the emergence of intergranular cracks. (iii) The wrapping graphene nanoscroll not only enhanced the conductivity of composites, but also preserved the structural-integration of composites (long-time cycling stability). CONCLUSIONS In conclusion, a well-conceived T-Nb2O5/G electrode consisting of softly-agglomerated Nb2O5 nanoparticles wrapped by graphene nanoscroll has been synthesized via a convenient and scalable 17



approach. Primary T-Nb2O5 nanoparticles in the region of 80 to 120 nm softly agglomerated together due to high-surface energy of single nanoparticle after milling. The abruptly changing of thermal stress originated from a cold quenching of heated mixed suspension of GO sheets and Nb2O5 nanoparticles caused the formation of graphene nanoscroll and simultaneously realized the wrapping of graphene over T-Nb2O5 nanoparticles. The T-Nb2O5/G electrode exhibited high reversible specific capacity of 222 mA h g-1 over 700 cycles at 1C, and high rate capability of 1600th reversible capacity of 163 mA h g-1 at 10 C. A combined ex-situ XRD, ex-situ SEM and TEM investigations revealed that the long-term cycling stability of T-Nb2O5/G was derived from the excellent stability of structures. Such 3D framework of T-Nb2O5/G effectively limited the volume expansion of Nb2O5 nanoparticles upon Li+ intercalation, consequently avoided the formation of intergranular crack, which maintained the integration of nanostructure of electrode. Overall, the T-Nb2O5/G composite has been demonstrated as a promising anode for LIBs. ASSOCIATED CONTENT Supporting Information The Supporting Information is avaialable free of charge on the ACS Publication website at DOI:. Additional Figures S1-S6. The XRD pattern of the GO (Figure S1). XPS spectrum of the T-Nb2O5/G (Figure S2). FT-IR spectra of the GO, T-Nb2O5 and T-Nb2O5/G (Figure S3). The BET surface area and the pore size distribution of T-Nb2O5/G (Figure S4). Discharge/charge profiles of T-Nb2O5 electrode (Figure S5). The SEM images of the T-Nb2O5/G electrode after different cycles and EDX spectrum of electrode after 50 cycles (Figure S6). AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (Y. Z.). 18


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E-mail: [email protected] (Y.H.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51602023) and this project is supported by State Key Laboratory of New Ceramic and Fine Processing Tsinghua University (No. KF201816).

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REFERENCES (1) Zheng, J. M.; Gu, M.; Xiao, J.; Zuo, P. J.; Wang, C. M.; Zhang, J. G. Corrosion/Fragmentation of Layered Composite Cathode and Related Capacity/Voltage Fading During Cycling Process. Nano Lett. 2013, 13, 3824-3830. (2) Gu, L.; Xiao, D.; Hu, Y. S.; Li, H.; Ikuhara, Y. Atomic-Scale Structure Evolution in a Quasi-Equilibrated Electrochemical Process of Electrode Materials for Rechargeable Batteries. Adv. Mater. 2015, 27, 2134-2149. (3) Robert, R.; Novák, P. Structural Changes and Microstrain Generated on LiNi0.80Co0.15Al0.05O2 During Cycling: Effects on the Electrochemical Performance, J. Electrochem. Soc. 2015, 162, A1823-A1828. (4) Sathiya, M.; Abakumov, A. M.;

Foix, D.;

Rousse, G.; Ramesha, K.;

Doublet, M. L.; Vezin, H.; Laisa, C. P.; Prakash, A. S.;

Gonbeau, D.;

Saubanère,

VanTendeloo,

M.; G.;

Tarascon, J. M. Origin of Voltage Decay in High-Capacity Layered Oxide Electrodes. Nat. Mater. 2015, 14, 230-238. (5) Rana, J.; Stan, M.; Kloepsch, R. K.; Li, J.; Schumacher, G.; Welter, E.; Zizak, I.; Banhart, J.; Winter, M. Structural Changes in Li2MnO3 Cathode Material for Li-Ion Batteries. Adv. Energy Mater. 2014, 4, 1300998. (6) Ou, X.; Yang, C. H.; Xiong, X. H.; Zheng, F. H.; Pan, Q. C.; Jin, C.; Liu, M. L.; Huang, K. A New rGO-Overcoated Sb2Se3 Nanorods Anode for Na+ Battery: In Situ X-Ray Diffraction Study on a Live Sodiation/Desodiation Process. Adv. Funct. Mater. 2017, 27, 1606242-1606252. (7) Jiang, Y. Z.; Li, Y.; Sun, W. P.; Huang, W.; Liu, J. B.; Xu, B.; Jin, C. H.; Ma, T. Y.; Wu, C. Z.; Yan, M.; Spatially-Confined Lithiation-Delithiation in Highly Dense Nanocomposite Anodes Towards Advanced Lithium-Ion Batteries. Energy Environ. Sci. 2015, 8, 1471-1479. 20


Page 21 of 32 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


(8) Xing, C. C.; Zhang, D.; Cao, K.; Zhao, S. M.; Wang, X.; Qin, H. Y.; Liu, J. B.; Jiang, Y. Z.; Meng, L.; In Situ Growth of FeS Microsheet Networks with Enhanced Electrochemical Performance for Lithium-Ion Batteries. J. Mater. Chem. A, 2015, 3, 8742 -8749. (9) Li, H. S.; Zhu, Y.; Dong S. Y.; Shen, L. F.; Chen, Z. J.; Zhang, X. G.; Yu. G. H. Self-Assembled Nb2O5 Nanosheets for High Energy-High Power Sodium Ion Capacitors. Chem. Mater. 2016, 28, 5753−5760 (10) Zhang, K.; Hu, Z.; Liu, X.; Tao, Z. L.; Chen, J. FeSe2 Microspheres as A High-Performance Anode Material for Na-ion Batteries. Adv. Mater. 2015, 27, 3305-3309. (11) Yan, P. F.; Zheng, J. M.; Gu, M.; Xiao, J.; Zhang, J. G.; Wang, C. M. Intragranular Cracking as A Critical Barrier for High-Voltage Usage of Layer-Structured Cathode for Lithium-Ion Batteries. Nat. Commun. 2017, 8,14101-141010. (12) Liu, H.; Wolf, M.; Karki, K.; Yu, Y. S.; Stach, E. A.; Cabana, J.; Chapman, K. W.; Chupas, P. J.; Inter-Granular Cracking as A Major Cause of Long-Term Capacity Fading of Layered Cathodes. Nano Lett. 2017, 17, 3452-3457. (13) Makimura, Y.; Zheng, S. J.; Ikuhara, Y. C.; Ukyo, Y. J. Microstructural Observation of LiNi0.8Co0.15Al0.05O2 after Charge and Discharge by Scanning Transmission Electron Microscopy. J. Electrochem. Soc. 2012, 159, A1070-A1073. (14) Chen, G.; Song, X. Y.; Richardson, T. J. Electron Microscopy Study of the LiFePO4 to FePO4 Phase Transition. Electrochem. Solid-State Lett. 2006, 9, A295-A298. (15) Wang, H. F.; Jang, Y.; Huang, B. Y.; Sadoway, D. R.; Chiang, Y. M. TEM Study of Electrochemical Cycling-Induced Damage and Disorder in LiCoO2 Cathodes for Rechargeable Lithium Batteries. J. Electrochem. Soc. 1999, 146, 473-480. (16) Kiziltas-Yavuz, N.; Herklotz, M.; Hashem, A. M.; Abuzeid, H. M.; Schwarz, B.; Ehrenberg, H.; 21



Mauger, A.; Julien, C. M.; Synthesis, Structural, Magnetic and Electrochemical Properties of LiNi1/3Mn1/3Co1/3O2 Prepared by A Sol-Gel Method Using Table Sugar as Chelating Agent. Electrochim. Acta 2013, 113, 313-321. (17) Kim, H.; Kim, M. G.; Jeong, H. Y.; Nam, H.; Cho, J. A New Coating Method for Alleviating Surface Degradation of LiNi0.6Co0.2Mn0.2O2 Cathode Material: Nanoscale Surface Treatment of Primary Particles. Nano Lett. 2015, 15, 2111-2119. (18) Mukhopadhyay, A.; Sheldon, B. W. Deformation and Stress in Electrode Materials for Li-Ion Batteries. Prog. Mater. Sci. 2014, 63, 58-116. (19) Nadimpalli, S. P. V.; Sethuraman, V. A.; Abraham, D. P.; Bower, A. F.; Guduru, P. R. Stress Evolution in Lithium-Ion Composite Electrodes during Electrochemical Cycling and Resulting Internal Pressures on the Cell Casing. J. Electrochem. Soc. 2015, 162, A2656-A2663. (20) Itou, Y.; Ukyo, Y. Performance of LiNiCoO2 Materials for Advanced Lithium-Ion Batteries. J. Power Sources 2005, 146, 39-44. (21) Lee, E. J.; Chen, Z. H.; Noh, H. J.; Nam, S. C.; Kim, D. H.; Amine, K.; Sun, Y. K. Development of Microstrain in Aged Lithium Transition Metal Oxides. Nano Lett. 2014, 14, 4873-4880. (22) Miller, D. J.; Proff, C.; Wen, J. G.; Abraham, D. P.; Bareno, J.; Observation of Microstructural Evolution in Li Battery Cathode Oxide Particles by In Situ Electron Microscopy. Adv. Energy Mater. 2013, 3, 1098-1103. (23) Choi, S. H.; Kang, Y. C. Crumpled Graphene-Molybdenum Oxide Composite Powders: Preparation and Application in LithiumIon Batteries. ChemSusChem 2014, 7, 523-528 (24) Zhao, J. P.; Yang, B. J.; Zheng, Z. M.; Yang, J.; Yang, Z.; Zhang, P.; Ren, W. C.; Yan, X. B.; Facile Preparation of One-Dimensional Wrapping Structure: Graphene Nanoscroll-Wrapped of 22


Page 22 of 32

Page 23 of 32 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


Fe3O4 Nanoparticles and Its Application for Lithium-Ion Battery. ACS Appl. Mater. Interfaces 2014, 6, 9890-9896 (25) Chabot, V.; Feng, K.; Park, H. W.; Hassan, F. M.; Elsayed, A. R.; Yu, A. P.; Xiao, X. C.; Chen, Z. W. Graphene Wrapped Silicon Nanocomposites for Enhanced Electrochemical Performance in Lithium Ion Batteries. Electrochim. Acta 2014, 130, 127-134 (26) Zhang, X. Q.; Huang, X. X.; Xia, L. X.; Zhong, B.; Zhang, X. D.; Zhang, T.; Wen, G. W. Flexible Freestanding Cotton-Graphene Composites for Lithium-Ion Batteries. J. Appl. Polym. Sci. 2017, 134, 44727-44734. (27) Zhao, Y. J.; Yan, D.; Ding, C. H.; Su, D. Z.; Ge, Y. Y.; Zhao, Y. Z.; Zhou, H. P.; Li, J. B.; Jin, H. B. Fe2O3 Nanocubes Exposed (012) Active Facets Combination with Graphene Rendering Enhanced Lithium Storage Capability. J. Power Sources 2016, 327, 658-665. (28) Lübke, M.; Sumboja, A.; Johnson, I. D.; Brett, D. J. L.; Shearing, P. R.; Liu, Z. L.; Darr, J. A.; High Power Nano-Nb2O5 Negative Electrodes for Lithium-Ion Batteries. Electrochim. Acta 2016, 192, 363-369. (29) Cao, X. H.; Zheng, B.; Shi, W. H.; Yang, J.; Fan, Z. X.; Luo, Z. M.; Rui, X. H.; Chen, B.; Yan, Q. Y.; Zhang, H. Reduced Graphene Oxide-Wrapped MoO3 Composites Prepared by Using Metal-Organic Frameworks as Precursor for All-Solid-State Flexible Supercapacitors. Adv. Mater. 2015, 27, 4695-4701. (30) Lu, Y.; Wu, J.; Liu, J.; Lei, M.; Tang, S. S.; Lu, P. J.; Yang, H. R.; Yang, Q. Facile Synthesis of Na0.33V2O5 Nanosheet-Graphene Hybrids as Ultrahigh Performance Cathode Materials for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 17433-17440. (31) Jin, S. X.; Wang, C. X. Synthesis and First Investigation of Excellent Lithium Storage Performances of Fe2GeO4/Reduced Graphene Oxide Nanocomposite. Nano Energy 2014, 7, 63-71. 23



(32) Zhou, W. W.; Cheng, C. W.; Liu, J. P.; Tay, Y. Y.; Jiang, J.; Jia, X. T.; Zhang, J. X.; Gong, H.; Hng, H. H.; Yu, T.; Fan, H. J. Epitaxial Growth of Branched α-Fe2O3/SnO2 Nano-Heterostructures with Improved Lithium-Ion Battery Performance. Adv. Funct. Mater. 2011, 21, 2439. (33) Han, S. C.; Hu, L. F.; Liang, Z.; Wageh, S.; Al-Ghamdi, A. A.; Chen, Y. S.; Fang, X. S. One-Step Hydrothermal Synthesis of 2D Hexagonal Nanoplates of α-Fe2O3/Graphene Composites with Enhanced Photocatalytic Activity, Adv. Funct. Mater. 2014, 24, 5719-5727. (34) Guo, H.; Liu, L. X.; Li, T. T.; Chen, W. W.; Liu, J. J.; Guo, Y. Y.; Guo, Y. C. Accurate Hierarchical Control of Hollow Crossed NiCo2O4 Nanocubes for Superior Lithium Storage. Nanoscale 2014, 6, 5491-5497. (35) Lu, H.; Xiang, K. X.; Bai, N. B.; Zhou, W.; Wang, S. L.; Chen, H. Urchin-Shaped Nb2O5 Microspheres Synthesized by the Facile Hydrothermal Method and Their Lithium Storage Performance. Mater. Lett. 2016, 167, 106. (36) Rudola, A.; Saravanan, K.; Mason, C. W.; Balaya, P. Na2Ti3O7: An Intercalation Based Anode for Sodium-Ion Battery Applications. J. Mater. Chem. A 2013, 1, 2653-2662. (37) Li, S.; Xu, Q.; Uchaker, E.; Cao, X.; Cao, G. Z. Comparison of Amorphous, Pseudohexagonal and Orthorhombic Nb2O5 for High-Rate Lithium Ion Insertion. CrystEngComm 2016, 18, 2532-2540. (38) Kim, G.; Jo, C. S.; Kim, W. Y.; Chun, J. Y.; Yoon, S. H.; Lee, J. W.; Choi, W. Y. TiO2 Nanodisks Designed for Li-Ion Batteries: A Novel Strategy for Obtaining an Ultrathin and High Surface Area Anode Material at the Ice Interface. Energy Environ. Sci. 2013, 6, 2932-2938. (39) Lim, E.; Jo, C.; Kim, H.; Kim, M. H.; Mun, Y.; Chun, J.; Ye, Y.; Hwang, J.; Ha, K. S.; Roh, K. C.; Kang, K.; Yoon, S.; Lee, J. W. Facile Synthesis of Nb2O5@Carbon Core-Shell Nanocrystals with Controlled Crystalline Structure for High-Power Anodes in Hybrid Supercapacitors. ACS nano 24


Page 24 of 32

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2015, 9, 7497-7505. (40) Wang, Y. G.; Hong, Z. S.; Wei, M. D.; Xia, Y. Y. Layered H2Ti6O13-Nanowires: A New Promising Pseudocapacitive Material in Non-Aqueous Electrolyte, Adv. Funct. Mater. 2012, 22, 5185-5193. (41) Khoo, E.; Wang, J. M.; Ma, J.; Lee, P. S. Electrochemical Energy Storage in a β-Na0.33V2O5 Nanobelt Network and Its Application for Supercapacitors. J. Mater. Chem. 2010, 20, 8368-8374. (42) Wang, J.; Polleux, J.; Lim, J.; Dunn, B. Pseudocapacitive Contributions to Electrochemical Energy Storage in TiO2 (Anatase) Nanoparticles. J. Phys. Chem. C 2007, 111, 14925-14931. (43) Kim, H.; Hong, J.; Park, Y. U.; Kim, J.; Hwang, I.; Kang, K. Energy Storage: Sodium Storage Behavior in Natural Graphite Using Ether-based Electrolyte Systems. Adv. Funct. Mater. 2015, 25, 652-652. Adv. Funct. Mate. 2015, 25,534-541. (44) Sun, R. M.; Wei, Q. L.; Sheng, J. Z.; Shi, C. W.; An, Q. Y.; Liu, S. J.; Mai, L. Q. Novel Layer-By-Layer Stacked VS2 Nanosheets with Intercalation Pseudocapacitance for High-Rate Sodium Ion Charge Storage. Nano Energy 2017, 35, 396-404. (45) Fu, F.; Yao, Y. Z.; Wang, H. Y.; Xu, G. L.; Amine, K.; Sun, S. G.; Shao, M. H. Structure Dependent Electrochemical Performance of Li-Rich Layered Oxides in Lithium-Ion Batteries. Nano Energy, 2017, 35, 370-378.

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Figure 1. Schematic illustration of the fabrication of T-Nb2O5/G, involving three steps: (1) Ball milling of commercial Nb2O5 to obtain softly-agglomerated Nb2O5 nanoparticles; (2) Quick-freezing and vacuum-drying process to fabricate T-Nb2O5/GO composite; (3) Calcination of the as-prepared composite to obtain T-Nb2O5/G.

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Figure 2. (a) SEM image, (b) TEM image and (c) HRTEM image of the T-Nb2O5 nanoparticles; The inset of (a) is statistical analysis of diameter for the T-Nb2O5 nanoparticles; (d) SEM images, (e) TEM images and (f) HRTEM image of the T-Nb2O5/G; the inset of (f) is the corresponding SAED pattern of the T-Nb2O5/G; (g) The XRD patterns of the T-Nb2O5 and T-Nb2O5/G; (h) The Raman spectra of the GO, T-Nb2O5 and T-Nb2O5/G; (i) The C1s XPS spectrum of the GO and T-Nb2O5/G.

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Figure 3. Electrochemical properties of the T-Nb2O5/G composites electrode for lithium-ion batteries: (a) CV curves; (b) Discharge/charge profiles at a current density of 1 C; (c) Corresponding dQ/dV curves derived from (b); (d) Rate performance at various current density; (e) Cycling performance and Coulombic efficiency of the pristine T-Nb2O5 and the T-Nb2O5/G composites electrodes at 1 C; (f) Prolonged cycle behavior of the T-Nb2O5/G composites electrode at a high current density of 10 C.

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Figure 4. (a) CV curves of the T-Nb2O5/G electrode at various scan rates from 0.1 to 1 mV s-1; (b) Plot of log (i) versus log (v) of anodic and cathodic peaks and b-value determination according to the Power law: i = avb; (c) Using equation (2) to analyze the current arising from surface effects at different potentials for the lithiation process; (d) CV curve of the T-Nb2O5/G electrode with separation between total current (solid line) and surface capacitive current (shaded regions) at 1 mV s-1; (e) Nyquist plots of the pristine T-Nb2O5 and T-Nb2O5/G electrodes before cycling, and the inset is the equivalent circuit model; (f) Nyquist plots of the T-Nb2O5/G electrode at as prepared and after 700th cycling; (g) The 5th discharge/charge profiles at a current density of 1 C; (h) The ex-situ XRD patterns within a selected 2θ range of the T-Nb2O5/G electrode operated at different states of discharge and charge against the voltage during the initial cycle. 29



Figure 5. The morphologies of the T-Nb2O5/G electrode after cycling: (a, b) SEM images and (c) corresponding EDX spectrum of electrode after 5000 cycles; The inset of Fig. 5b is the copper foil coated with active materials; The ex-situ TEM/HRTEM images after (d) 1000, (e) 2000 and (f) 5000 cycles; Schematic illustration of the lithiation/delithiation process in (g) the T-Nb2O5 and (h) the T-Nb2O5/G electrode.

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