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K6Nb10.8O30 groove nanobelts as high performance lithium-ion battery anode towards long-life energy storage
Author’s Accepted Manuscript K6Nb10.8O30 Groove Nanobelts as High Performance Lithium-Ion Battery Anode towards Long-Life Energy Storage Haojie Zhu, Xing Cheng, Haoxiang Yu, Wuquan Ye, Na Peng, Runtian Zheng, Tingting Liu, Miao Shui, Jie Shu www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(18)30545-7 https://doi.org/10.1016/j.nanoen.2018.07.057 NANOEN2924
To appear in: Nano Energy Received date: 10 June 2018 Revised date: 21 July 2018 Accepted date: 25 July 2018 Cite this article as: Haojie Zhu, Xing Cheng, Haoxiang Yu, Wuquan Ye, Na Peng, Runtian Zheng, Tingting Liu, Miao Shui and Jie Shu, K 6Nb10.8O30 Groove Nanobelts as High Performance Lithium-Ion Battery Anode towards Long-Life Energy Storage, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.07.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
K6Nb10.8O30 Groove Nanobelts as High Performance Lithium-Ion Battery Anode towards Long-Life Energy Storage Haojie Zhu, Xing Cheng, Haoxiang Yu, Wuquan Ye, Na Peng, Runtian Zheng, Tingting Liu, Miao Shui, Jie Shu* Faculty of Materials Science and Chemical Engineering, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo 315211, Zhejiang Province, People’s Republic of China *Corresponding author: Jie Shu, Tel.: +86-574-87600787; Fax: +86-574-87609987. [email protected]
Abstract Owing to the multiple redox couples of Nb5+/Nb4+ and Nb4+/Nb3+, Nb-based compounds have attracted great attention to be promising high-capacity anode materials for rechargeable batteries. Here, K6Nb10.8O30 groove nanobelts (GNB) are synthesized through heat-treating the adjustable electrospun potassium niobate nanofibers, thereupon the structural change in the lithiation and delithiation is evidently imaged via in situ transmission electron microscopy (TEM). From in situ observations, the K6Nb10.8O30 GNB, in virtue of its stability, is ascertained to be adopted as anode material in lithium-ion batteries (LIBs). Evaluated as lithium storage host, GNB outstrip nanowires (NW) in cyclicity and in reversible capacity. Even after 1000 cycles, the retention capacity of K6Nb10.8O30 GNB is as high as 69 %. Furthermore, the lithium-storage mechanism is also investigeted via in situ X-ray
1
diffraction (XRD). It is proved that the phase transition takes place from Li-poor K6Nb10.8O30 phase to Li-rich Li22K6Nb10.8O30 phase via two steps. In addition, the results obtained from ex situ TEM and ex situ X-ray photoelectron spectroscopy (XPS) also prove that the behavior of lithiation and de-lithiation for K6Nb10.8O30 GNB is highly reversible, suggesting that it can be a possible anode material.
Graphical Abstract
Keywords K6Nb10.8O30 groove nanobelts; Electrospinning; In situ transmission electron microscopy; In situ X-ray diffraction; Lithium-ion batteries.
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1. Introduction In the past decades, the climate has been changed sharply for the greenhouse gas (GHG) emissions. Broadly, the modern-day GHG comes from the combustion of non-renewable fossil fuels, which provides us with 85% of the total ouput of energy. To reduce the dependence on fossil fuels, various possible energy conversion and storage routes are used in the modern life. Today, rechargeable batteries as an alternative energy are widely applied in the field of consumer electronics. And the great success of using rechargeable batteries in portable consumer electronics is now being extended to electric vehicles. For instance, the conventional gasoline-powered cars are replaced by electric vehicles equipped with rechargeable batteries. This trend has spawned many new energy vehicles such as hybrid vehicles, electric vehicles and plug-in hybrid vehicles, because of the advantages of high power, long cycle stability, high energy density and light weight for rechargeable batteries. These factors are essential for electric vehicles and grid storage. In order to further improve the energy and power densities of rechargeable batteries (especially for lead-acid batteries), much effort has been currently devoted to develop new types of batteries such as lithium-ion batteries,[1-6] all solid-state batteries and lithium-sulfur batteries.[7-10] The development of advanced lithium-ion batteries (LIBs) is still one of the hottest global research aspects in the new energy fields. Recently intercalation-type anode materials have been the new orientation of the development of LIBs.[11-13] However, the commercial graphite anode may suffer from possible stafety problem during rapid charge/discharge process due to its low working 3
potential (close to 0.0 V vs. Li+/Li). Among many candidate materials, Li4Ti5O12 as anode material for LIBs has been widely studied because of its excellent rate performance and cycling stability, but the low specific capacity (175 mAh g-1) limits its application in the future. Compared with ordinary graphite or Li4Ti5O12, niobium-based materials (including Nb2O5, Nb12O29, TiNb2O7, etc.) are promising anodes for LIBs due to their safe working potentials (~1.65 V) and considerable lithium storage capacity (250-300 mAh g-1).[14-18] Particularly, monoclinic Nb12O29 as anode electrode was researched by galvanostatic charge/discharge test in the potential of 1.0-2.5 V at the current density from 6.6 to 660 mA g-1.[16] It shows that Nb12O29 can reversibly incorporate a relatively large amount of lithium ions (14.3 Li per unit, 243 mAh g-1) while retaining structural integrity. It may also signal that Nb12O29 as the anode ability to maintain a long-calendar cycle is general. On the other hand, TiNb2O7 has also been widely investigated as a possible solid anode with a higher theoretical capacity (388 mAh g-1) in recent years. Unfortunately, it suffers from the issue of rapid attenuation for lithium storage capability.[19, 20] However, the long-term cycling ability and high reversible specific capacity are two essential and important factors in the performance of LIBs. Thus, the potassium-niobate family has been attentioned in recent years. Among the potassium niobates, K6Nb10.8O30 is a novel multifunctional material with opened tetragonal tungsten bronze-type structure.[21] It is not only a good photocatalyst,[22, 23] but also a probable lithium storage material.[24] In the previous report, its electrochemical behavior is briefly introduced without any structure descriptions. However, further investigations have not been undertaken till 4
now. In this paper, K6Nb10.8O30 groove nanobelts are fabricated via electrospinning method under different experimental conditions. The relationships between electrochemical performances (cycling stability and capacity) and structural characteristics (Bragg position and c-axis) are unveiled by in situ X-ray diffraction. It is found that K6Nb10.8O30 groove nanobelts possess a good ability of lithium ion storage and reversible structure. Besides, the change of microstructure in the process of insertion-extraction is also demonstrated by ex situ TEM technique. It once again proves the excellent electrochemical performance of K6Nb10.8O30.
2. Experimental Section Electrospun K6Nb10.8O30 Groove Nanobelts and Nanowires 2.2g polyvinylpyrrolidone (PVP, Mw: 1300000, Macklin) was dissolved in a mixture of 16 mL absolute ethanol (AR, Macklin) and 6mL distilled water. Meanwhile, niobium oxalate (AR, Macklin) and potassium acetate (AR, Macklin) were added to the above solution under magnetic stirring overnight. Then, the mixed solution was transferred to a 10 mL syringe with a 21G needle. Whereafter, the process of electrospinning was performed in a controlled electrospinning setup (JDF04, Changsha). A positive direct current voltage of 20 kV and a grounding voltage of 0 kV were applied to the needle tip and stainless steel roller, respectively. The distance between syringe needle and alumina boat collector was kept at 15 cm and the solution was supplied at a flow rate of 1 mL h-1 using a syringe pump. Next, it 5
was easy to observe the white as-electrospun nanofibers membrane on the alumina boat collector. This experiment maintained the ambient humidity of the whole electrospinning process at around 40%. Finally, a polymer of electrospun nanofibers attached to the alumina boat was calcined at 750 oC for 10 h in air at a ramping rate of 10 oC min-1, which resulted in the formation of the final K6Nb10.8O30 groove nanobelts. As a control, K6Nb10.8O30 nanowires were also synthesized in a similar way. First, the ingredients of the precursor solution remain the same as the above starting materials. But the parameters of electrospinning setup were changed by using different flow rate (at 2 mL h-1) and high-voltage (at 25 kV). In addition, the collection mode was changed from alumina boat to stainless steel roller. Then the precursor was put into muffle furnace and calcined for 10 h in air at 800 oC to obtain the K6Nb10.8O30 nanowires. Materials Characterization The crystal structure of the two samples was characterized by a Bruker D8 X-ray diffractometer (XRD) with Cu-Kα radiations in the 2θ range 10° to 80°. In situ XRD measurements were characterized via using the aforementioned XRD equipment. The in situ cell for XRD detection was a homemade device equipped with beryllium disc as X-ray window. The working principle and structure of the in situ XRD cell were depicted in our previous paper.[25, 26] The morphology of K6Nb10.8O30 was verified by Hitachi SU-70 scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS), JEOL JEM-2010F high-resolution transmission electron microscope (HRTEM). In situ TEM observation was performed on the 6
aforementioned TEM equipment. A nano lithium battery device consisting of K6Nb10.8O30 GNB cathode, Li2O solid electrolyte, and lithium metal anode was designed to perform the real time TEM observation of the structural changes of the K6Nb10.8O30 GNB during the lithiation process. The elemental valence states in K6Nb10.8O30 at different lithiated/delithiated conditions were characterized by X-ray photoelectron spectroscopy (XPS, Theta Probe AR-XPS System, Al Kα radiation). Inductive coupled plasma (ICP) emission spectrometer analysis was performed by utilizing a PerkinElmer OPTIMA 4300 DV model. Electrochemical Measurements of Half Cells For electrochemical tests, the anode consisted of 80 wt.% active substance, 10 wt.% conductivity agent (carbon black), and 10 wt.% polyvinylidenefluoride dispersed in N-methyl-2-pyrrolidone (NMP, Macklin) to generate a slurry. This slurry was coated on copper foil by a tablet machine, dried at 80 oC overnight to ensure removal of the NMP. Its loading mass was about 2.0 mg for each electrode. The Swagelok batteries were assembled in an argon-filled glove box using the as-prepared film as the working electrode, Li metal foil as the counter electrode, glass fiber as the separator and 1 mol L-1 LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 in volume) as the electrolyte. The charge-discharge performance was evaluated by multichannel LANHE battery tester at a constant current density of 100 mA g-1. Cyclic voltammetry (CV) was carried out at a scan rate of 0.1 mV s-1 between 0.5 and 3.0 V on a CHI1000B electrochemical workstation. Assembly of Full Cells 7
The full cell was fabricated using the as-prepared K6Nb10.8O30 anode and the synthetic LiMn1.5Ni0.5O4 cathode at a mass ratio of 1:2. The mass loadings of the K6Nb10.8O30 groove nanobelts and LiMn1.5Ni0.5O4 were in the range of 1.0-2.0 mg cm-2. The separator and electrolyte of full cell are the same as that of half cell.
3. Result and Discussion The schematic diagram of electrospinning in Figure 1 shows the morphology formation process of the groove nanobelts and nanowires on different stages. Obviously, the solution of precursor mixed with polyvinylpyrrolidone (PVP) in ethanol is delivered through a syringe at a flow rate of 1 mL h-1 which is installed in the electrospinning machine. When the machine is applied with high voltage, it sprays nanofibers on the alumina boat collector (or stainless steel roller). K6Nb10.8O30 groove nanobelts (K6Nb10.8O30 GNB) are finally obtained via calcinations at high temperature. The formation process of K6Nb10.8O30 GNB includes three steps, forming nanowires, twining and merging, which is probably contributed to the evolution of the neighbouring nanowires under continuous calcinations. Briefly, in the initial heat-up region (<500 oC), the PVP and ethanol in the precursor gradually volatilized and decomposed, and then the coarse nanowire with a thick diameter was formed. In the subsequent heat-up circumstance at normal pressure (< 750 oC), adjacent nanowires were prone to collide and intertwined with each other due to the acceleration of thermal motion. After calcinations treatment at 750 oC for a period of time, the entangled nanowires were finally fused together to form nanobelts. Meanwhile, the 8
groove is formed at the joint of the neighbouring nanowires naturally. For comparison, the nanowires are formed due to the dense accumulation of precursor fibers on the stainless steel roller. As shown in Figure 2a, a twisted ribbon structure can be clearly identified, of which the width is about 600-900 nm and the thickness is approximately 80-100 nm. And its length can generally reach more than 10 μm. Figure 2b shows a representative panoramic TEM image of K6Nb10.8O30 GNB. The bending form shows the most distinctive features of groove nanobelts, which agrees well with the SEM image. Observed from Figure 2b and c, the K6Nb10.8O30 GNB is constructed by many small nanoparticles with the diameter of 15-25 nm. Figure 2d demonstrates that the K6Nb10.8O30 GNB is composed of the crystalline nanoparticles which are defined with the (110) plane. The polycrystalline structure of K6Nb10.8O30 GNB is confirmed by selected area electron diffraction (SAED) characterization in Figure 2e, which indicates a typical polycrystalline ring pattern. Additionally, the EDS elemental mapping shows a homogeneous dispersion of K, Nb and O elements within the entire zonal framework (Figure 2g-i), and the upper half of initial image has obvious groove (Figure 2f), separately confirms the key structural characteristics of the K6Nb10.8O30 GNB again. Moreover, to confirm the practical stoichiometric ratio of as prepared materials, EDS and ICP results are also given in Table S1 and S2. The potassium/niobium stoichiometric values of the K6Nb10.8O30 GNB and the K6Nb10.8O30 NW from the ICP data are 5.97/10.8 and 5.94/10.8, respectively, which is close to the theoretical stoichiometric value of K6Nb10.8O30. It suggests the successful 9
preparation of K6Nb10.8O30 GNB and K6Nb10.8O30 NW in this work, which is demonstrated by the Rietveld XRD patterns as shown in Figure S1. To gain more insight, the morphologic features of as-prepared K6Nb10.8O30 GNB and K6Nb10.8O30 NW are characterized by SEM, as shown in Figure 3. As can be seen from a low magnification SEM image of Figure 3a, it illustrates the one dimensional K6Nb10.8O30 GNB with a width of 500-700 nm and length of several micrometers. The longitudinal thickness of K6Nb10.8O30 GNB is about 90 nm. It can be also obviously observed that as-fabricated polycrystalline K6Nb10.8O30 GNB is composed of numerous nanograins with uneven surface, which can be further confirmed by a high magnification SEM picture in Figure 3b. Figure 3c provides a schematic diagram to classify the formation process of K6Nb10.8O30 GNB into three types, including straight-type, folded-type and screw-type, all of which appear in the SEM images. Additionally, the detailed groove structure of K6Nb10.8O30 GNB can also be observed from Figure S2. In contrast, the K6Nb10.8O30 NW in Figure 3d and e is straight-type nanowire with the diameter of 750 nm, and the length of 20 μm. Furthermore, Figure 3f-i proves the homogeneous distribution of K, Nb and O elements in the polycrystalline nanowires. It is noticeable that the superficial area of the K6Nb10.8O30 NW is much smaller than that of the K6Nb10.8O30 GNB, which suggests that K6Nb10.8O30 GNB can provide larger reaction places for lithium ions. To
reveal
the
structural
stability
of
K6Nb10.8O30
GNB
during
lithiation/delithiation process, the ex situ TEM technique has been used. Figure 4a-f show three stages at different charge/discharge cycles (1st, 100th and 400th). It is 10
apparent that K6Nb10.8O30 GNB with well-defined two-dimeonsinal band-like structure are surrounded by a lot of carbon black particles after the first cycle. Concurrently, a solid electrolyte interphase (SEI) film is formed on the surface of the K6Nb10.8O30 GNB, which leads to significantly irreversible capacity in the first cycle. After 400 cycles, the structure of K6Nb10.8O30 GNB is maintained well (Figure 4c). Figure 4d-f presents the HRTEM images of K6Nb10.8O30 GNB at the different cycles (1st, 100th and 400th). The polycrystalline structure is well-defined after repeated cycles. Moreover, the lattice distance of the K6Nb10.8O30 GNB displays no changes (d=0.268 nm for (321) at different states (1 cycle, 100 cycles and 400 cycles). Thus, it demonstrates the excellent cycle stability of K6Nb10.8O30 GNB during long-life cycles. To verify whether K+ ions can move out/into the structure of K6Nb10.8O30 GNB during repeated charging/discharging or not, two experiments are designed. Firstly, an ex situ XRD experiment is performed for K6Nb10.8O30 GNB to probe the possible Li+/K+-exchange behaviors in a lithium-containing LiPF6-based electrolyte, as shown in Figure S3. It is hard to observe the shifting of the featured (001) and (410) peaks for K6Nb10.8O30 GNB after being soaked in electrolyte for 96 h, which indicates that K6Nb10.8O30 GNB has good chemical stability in organic electrolyte. Secondly, to further check the possible dissolution of potassium ions during the electrochemical reaction, the ex situ ICP results for K6Nb10.8O30 GNB before and after different cycles are given in Table S3 and S4. The detected results show that the K/Nb ratio in the cycled electrode decreases from 0.232 to 0.225 after 400 cycles. Furthermore, only 31 mg Kg-1 of potassium element is found in electrolyte (1M LiPF6) after 400 cycles 11
(Table S4). All these evidences indicate that only very few potassium ions can move out/into the structure during charging/discharging. Therefore, K6Nb10.8O30 GNB has a stable framework for repeated lithium storage. Figure 5a gives a schematic illustration of LIBs based on as-prepared product. The K6Nb10.8O30 GNB arranged to each other is served as the working electrode and lithium plate attached to the stainless steel collector column is served as the counter electrode. In this cell, K6Nb10.8O30 GNB is cycled at different conditions. Figure 5b shows the HRTEM images of K6Nb10.8O30 GNB at different lithiated/delithiated states (initial, lithiated and delithiated). Before cycles, the pristine sample exhibits a polycrystalline structure with featured (311) and (410) lattice planes (d=0.281 nm for (311) plane and d=0.304 nm for (410) plane). The lattice distances of K6Nb10.8O30 (d=0.291 nm for (311) plane and d=0.313 nm for (410) plane) expand slightly in the lithiation state. As expected, the lattice distances of the nanocrystallines shrink to the original values after the first de-lithiation. This means that the reaction of K6Nb10.8O30 GNB with lithium is reversible. Figure 5c-h illustrates the nanoscale electrochemical device for in situ TEM and its patterns. The K6Nb10.8O30 GNB is attached to a platinum needle and used as the working electrode. The Li metal clung to the tungsten needle is served as the counter electrode and lithium source, and the naturally grown Li2O layer on the surface of the Li metal is used as a solid electrolyte to allow the transportion of Li+ ions. Figure 5d-h presents the TEM images of the K6Nb10.8O30 GNB during the lithiation process. It can be observed that the whole nanostructure of the K6Nb10.8O30 GNB changes 12
slightly during the lithiation process. And the K6Nb10.8O30 GNB expands slightly in the horizontal and vertical directions. The fact can be confirmed by the variation of the marked red rectangle and straight line. After 120 seconds of lithiation process, the volume expansion is invisible and the morphological framework remains stable. It strongly illustrates the structural stability of the K6Nb10.8O30 GNB. And intuitive in situ TEM video of the K6Nb10.8O30 GNB is also shown in Video S1. As shown in Figure 6a, the tetragonal bronze structure of K6Nb10.8O30 is built upon the connection of NbO6 octahedra with corner-sharing configurations, which consists of some triangles, quadrilateral and pentagonal tunnels. Although the pentagonal and quadrilateral tunnels are occupied by K cations, these two tunnels can still provide spaces to accommodate inserted lithium ions. In contrast, the triangle tunnels are fully blocked by Nb cations, which prohibit the insertion of lithium ions. Here, the powder X-ray diffraction (XRD) pattern of the typical K6Nb10.8O30 is plotted in Figure 6a. Correspondingly, the XRD Rietveld refinements of the K6Nb10.8O30 GNB and K6Nb10.8O30 NW by materials studio software also are demonstrated in Figure S1. It can be found that the Rietveld parameters of the two samples are close to the standard card. No peaks related to impurity phases such as Nb2O5 are observed. All peaks can be indexed with a tetragonal bronze structure (space group: P4/mbm), indicating the formation of pure K6Nb10.8O30 phase. As previous ex situ TEM discussed, the synergistic effects of K6Nb10.8O30 nanocrystals and nanobelts may contribute to the extraordinary electrochemical performances of the K6Nb10.8O30 GNB. To clearly reveal this synergistic effects 13
inherent to multi-crystal-nanobelt structure, we describe the structural evolution of the K6Nb10.8O30 GNB by in situ XRD during lithium-ion intercalation and deintercalation process (Figure 6b). Two galvanostatic charge/discharge cycles are also presented along with the in situ XRD patterns. An appreciable variation of the positive region within a 2θ range of 19.8-38.2° is observed, indicating the appearance of phase transitions during the charge/discharge process. As can be seen in Figure 6b, an obvious two-phase reaction occurs during the first discharge process, followed with a solid solution reaction upon further discharge. At the beginning of the constant-current reduction process from 0 to 37 min, the Li-poor K6Nb10.8O30 phase fades gradually along with the disappearance of featured reflections and a Li-rich Li3K6Nb10.8O30 phase is observed with the appearance of new reflections. As the discharge time increases from 37 to 265 min, the continuous peak shifting towards lower angle indicates a solid solution transformation in this region. During the charging, a quasi-reversible process occurs, except a slight irreversibility at the end of delithiation. In above process, the phase transition in the initial cycle may be briefly summarized in the following equations: 3.0-1.6 V: K6 Nb10.8 O30 + 3Li+ + 3e- = Li3 K6 Nb10.8 O30
(1)
1.6-0.5 V: Li3 K6 Nb10.8 O30 + 19Li+ + 19e- = Li22 K6 Nb10.8 O30
(2)
0.5-1.8 V: Li22 K6 Nb10.8 O30 = Li8.5 K6 Nb10.8 O30 + 13.5Li+ + 13.5e-
(3)
1.8-3.0 V: Li8.5 K6 Nb10.8 O30 = Li7 K6 Nb10.8 O30 + 1.5Li+ + 1.5e-
(4)
The in situ XRD patterns exhibit three main characteristics (Figure 6c-e). First, the (001) reflection of K6Nb10.8O30 GNB shows the evolutions of relative intensity and 14
Bragg position. It shifts and recovers repeatedly during the initial two charge/discharge processes (Figure 6c). The corresponding ex situ HRTEM images are also provided to confirm the recovery of the structure in the above discussion (Figure 5b). When lithium ions insert into the structure of K6Nb10.8O30 GNB along c-axis direction, the corresponding peaks shift toward lower angles. When lithium ions extract, the peaks turn to their initial positions, corresponding to the traditional process of storing electricity. In this observation, the XRD patterns of K6Nb10.8O30 GNB exhibit four clear slopes corresponding to adjacent charge/discharge curves. Second, when the charge/discharge is performed in the range of 0.5-3.0 V, Bragg positions of the (211), (400), and (221) reflections gradually shift toward lower 2θ values upon lithiation, and then these Bragg positions reverts to the original angles upon delithiation (Figure 6d), which is validated by the reversible changes of the lattice fringe of (410) plane in the TEM images (Figure 5b). Third, the angle variation of the (311) peak is similar to the (410) peak in the local colormap image in 31-35.5° (Figure 6e), which gradually moves to low-angle region, and then goes back to high angles. All these above discussed reflections have the same trend as the charge/discharge curves of K6Nb10.8O30 GNB. Hence, K6Nb10.8O30 GNB as an intercalation-type anode material can achieve highly efficient lithium ion storage in terms of specific capacity and cycling stability. In addition, to confirm the reversibility of K6Nb10.8O30 GNB in the charge/discharge process, the changes of lattice parameters (a, c and V) are calculated as the scatter diagram in Figure S4. The initial lattice parameters a, c and unit-cell 15
volume V at the open circuit potential state for K6Nb10.8O30 GNB are 12.476 Å, 3.961 Å and 616.53 Å3, respectively. When the in situ XRD cell is discharged to 0.5 V, the lattice parameters a and c are 12.484 and 4.059 Å, respectively. The lattice volume can be calculated, which is 632.59 Å3, suggesting the whole volume expansion of 2.60% after a full lithiation. In addition, it is obvious that all the lattice parameters (a, c and V) present good mirror symmetry during two charge/discharge cycles. After these processes, all the parameters can almost recover to their initial values, which further confirm the structural reversibility of K6Nb10.8O30 GNB anode material. Figure 7a shows the crystal structure of K6Nb10.8O30 viewed along the c-axis and [001] direction. This structure of K6Nb10.8O30 has a basic framework based on that of the tetragonal bronze structure in which cooperative lattice distortions take place for corner-shared NbO6 octahedra along c-axis, providing the triangular, square and pentagonal tunnels in cross-section.[16, 24] Figure 7b exhibits the specific shapes of these three channels. As the chart shows, the pentagonal-type tunnel is described as channel I, the square-type tunnel is described as channel II and the triangular-type tunnel is described as channel III. As described in the above discussion, only pentagonal and square tunnels parallel to the c-axis can store lithium ions, while triangular tunnels are blocked for stereo-hindrance effect induced by Nb cations. Remarkably, combined with the aforementioned conclusion, two tunnels along c-axis present open spaces for lithium ions insertion into 4g and 2a sites. A schematic diagram of transport path for lithium ions in the discharge process is shown in the Figure 7c. In the discharge process, lithium ions initially occupy the empty positions 16
in the pentagonal tunnel, and then take the available positions in the square tunnel.[25] In addition, it is known that about five lithium ions per unit can be embedded at the positions in pentagonal tunnel and about two lithium ions are inserted at the positions in the square tunnel by calculation (Figure S5). Upon a reverse delithiation, the lithium ion battery shows a phase-transition range between 0.5 and 1.75 V with a reversible capacity of about 216 mAh g-1. As a result, the reversibility of lithium removal mechanism in the [001] direction can be obtained according to the variation of the Bragg position of the (001) peak in the in situ XRD (Figure 6c). To evaluate the obtained K6Nb10.8O30 GNB and K6Nb10.8O30 NW materials as electrodes for lithium ion battery, the electrochemical properties are systematically studied as presented in Figure 8. Figure 8a shows the cyclic voltammetry test of the K6Nb10.8O30 GNB at the scanning rates of 0.1, 0.2, 0.3, 0.5, 1.0 and 2.0 mV s-1. It can be noticed that there are two pairs of broad peaks appeared in the cyclic voltammograms (CVs), which changes as the scanning rate increases. During the negative scan, a clear cathodic peak is located at about 1.59 V. And during the positive scan, a highlighted anodic peak is seated at 1.75 V. The positions of this redox couple coincide with the previous report.[24] The couple of intensive cathodic/anodic peaks at 1.59/1.72 V can be attributed to the valence variation of Nb4+/Nb5+ redox couple.[26] For comparison, the broad shoulders ranging from 1.2 to 0.8 V can be assigned to the Nb3+/Nb4+ redox couple. It suggests that the lithiation/delithiation process is reversible for K6Nb10.8O30 GNB, which can be further demonstrated by the reverisble transformation of Nb element between Nb4+/Nb5+ and Nb3+/Nb4+ redox couples in the 17
X-ray photoelectron spectroscopy (XPS, Figure 9).[27] Figure 8b illustrates the charge/discharge profiles of K6Nb10.8O30 GNB cycled at a current density of 100 mA g-1. The K6Nb10.8O30 GNB delivers the discharge/charge specific capacity of 343.4/216.4 mAh g-1, corresponding to a Coulombic efficiency of 63 % at the first cycle. The irreversible capacity of 127 mAh g-1 in the first discharge-charge step results from the decomposition of the electrolyte, which causes the formation of solid electrolyte interphase (SEI) layer on the electrode surface and the irreversible consumption of lithium ions.[28-31] After 400 cycles, the K6Nb10.8O30 GNB remains a specific charge capacity of 172.1 mAh g-1 (Figure 8c), having the capacity retention of 81.1%, which is much better than the electrochemical performance of K6Nb10.8O30 NW (Figure S6). According to the transverse comparison, the lithium storage ability of K6Nb10.8O30 GNB is superior to many other niobium based anode materials (Figure 8d, Table S5). The dark blue dots represent reversible specific capacity and the red dots represent the cycle number of capacity retention at 80 %. Regardless of reversible specific capacity or capacity retention, it is clear that the electrochemical property of K6Nb10.8O30 GNB is far beyond other Nb- or Ti-based anode materials reported recently.[32-37] According to the longitudinal comparison, Table S5 also shows the comparison of K6Nb10.8O30 GNB with representative anode materials (such as graphite, alloys, Li4Ti5O12 and niobium compounds) in terms of their reversible capacity and cycle performance.[38-43] The results exhibit that the lithium storage capability of the K6Nb10.8O30 GNB is acceptable among the representative anode materials reported in recent years. 18
Based on the comparison of the obtained literature, the results show that the morphology of K6Nb10.8O30 GNB has a great influence on its electrochemical properties (Figure 8d). In order to further check the performance of K6Nb10.8O30 anode, electrochemical experiments with rate capability and cyclability are designed to test. Figure 10a and c show the electrochemical performances of K6Nb10.8O30 NW. As charge/discharge current density increases from 100 to 800 mA g-1, its lithium storage capacity gradually decreases from 150 to 74 mAh g-1 (Figure 10a). Upon a long-life cycles at 100 mA g-1, as seen in the Figure 10c, the charge capacity of K6Nb10.8O30 NW stabilizes at 85.19 mAh g-1 after 1000 cycles with about 50 % capacity decay. Remarkably, the electrochemical performance of K6Nb10.8O30 GNB is much better than that of the K6Nb10.8O30 NW. At the same current density (100-800 mA g-1), K6Nb10.8O30 GNB delivers the reversible capacity of 235.2 mAh g-1 at 100 mA g-1 and 137.1 mAh g-1 at 800 mA g-1 (Figure 10b). Furthermore, its long-life capability at 100 mA g-1 is also superior to K6Nb10.8O30 NW. As exhibited in Figure 10d, the charge capacity remains at 161.5 mAh g-1 after 1000 cycles with about 69 % of capacity retention. Cyclability is an important electrochemical performance for LIBs. Thus the cycle performance of the K6Nb10.8O30 GNB and K6Nb10.8O30 NW at different current densities is demonstrated in Figure S7-S10. The reversible capacity of K6Nb10.8O30 GNB fades slowly from 200.8 mAh g-1 at 300 mA g-1 to 174.6 mAh g-1 at 500 mA g-1 and 150.8 mAh g-1 at 700 mA g-1. More importantly, K6Nb10.8O30 GNB shows a capacity of 127.4 mAh g-1 at a high current density of 1000 mA g-1 and maintains 19
73.9 % of capacity after 550 cycles. In contrast, K6Nb10.8O30 NW only delivers capacities of 126.1, 103.6, 74.6 and 51 mAh g-1 at the current densities of 300, 500, 700 and 1000 mA g-1, respectively. It suggests that electrochemical properties of K6Nb10.8O30 GNB are better than that of K6Nb10.8O30 NW. Besides, the initial Coulombic efficiency is also increased from 53 % (K6Nb10.8O30 NW) to 63 % (K6Nb10.8O30 GNB) owing to the special nanobelt structure. Based on above various analyses, it can be concluded that K6Nb10.8O30 GNB is undoubtedly a superior anode for high performance lithium storage, which can be further proved by the light-emitting diodes (LEDs) powered with a LiMn1.5Ni0.5O4/K6Nb10.8O30 cell (Figure 10e).
4. Conclusions In this work, K6Nb10.8O30 GNB with a thickness of 80-100 nm is successfully synthesized by using a facile electrospinning method. The phase transition and lithium storage behaviors of K6Nb10.8O30 GNB from Li-poor K6Nb10.8O30 phase to Li-rich Li22K6Nb10.8O30 phase during intercalation of lithium ions are analyzed by in situ XRD and ex situ XPS. It reveals that lithium storage at 4g and 2a sites in quadrilateral and pentagonal tunnels is based on the multiple redox couples of Nb5+/Nb4+ and Nb4+/Nb3+. Moreover, in situ TEM, ex situ TEM and in situ XRD tests demonstrate the structural stability and reversiblity of K6Nb10.8O30 GNB as an anode material for LIBs. Electrochemical tests show that nanobelt structure brings ultra-long cycling stability and super rate performance. Cycled at 100 mA g-1, it has the charge 20
capacities of 192.6, 182.1, 176.9 and 172.1 mAh g-1 in the 100th, 200th, 300th and 400th cycles, respectively. It also exhibits a capacity retention as high as 69 % after 1000 cycles.
Owing
to
the
outstanding
electrochemical
properties,
a
LiMn1.5Ni0.5O4/K6Nb10.8O30 full cell is fabricated to power the LEDs at the first time.
Acknowledgement This work is sponsored by National Natural Science Foundation of China (U1632114), Ningbo Key Innovation Team (2014B81005), and K.C. Wong Magna Fund in Ningbo University.
Supporting Information Supplementary data associated with this article can be found in this submission, including Rietveld refinement profiles, SEM images, ex situ XRD patterns, in situ XRD evolution, changes of lattice parameters, Li content x versus voltage, in situ TEM video, Nb 3d XPS spectra, charge/discharge curves, cycling performances, comparative capacities, electrochemical kinetic parameters and ICP-AES data of K6Nb10.8O30 GNB and K6Nb10.8O30 NW.
Video S1. In situ TEM of K6Nb10.8O30 GNB upon discharge.
21
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Figure 1. The formation process of K6Nb10.8O30 GNB and K6Nb10.8O30 NW.
28
Figure 2. (a) SEM, (b, c) TEM, (d) HRTEM, (e) SAED and (f-i) EDS elemental mappings characterizations of K6Nb10.8O30 GNB.
29
Figure 3. (a, b) SEM images of K6Nb10.8O3 GNB, and (c) corresponding sketch illustration of three-type groove nanobelts; (d-f) SEM images of K6Nb10.8O3 NW, and (g-i) corresponding EDS elemental mapping images of K, Nb and O elements.
30
Figure 4. Schematic diagrams and ex situ TEM observations of electrochemical process of K6Nb10.8O3 GNB. (a, d) 1 cycle, (b, e) after 100 cycles, and (c,f) after 400 cycles.
31
Figure 5. (a) Schematic illustration of lithium storage in K6Nb10.8O3; (b) HRTEM images of K6Nb10.8O3 GNB at pristine, lithiated and delithiated states; (c) Schematic illustration of in situ experimental setup inside a TEM; (d) In situ TEM observation of electrochemical process of K6Nb10.8O3 GNB in the initial stage, (e, f, g, h) in the lithiated stages.
32
Figure 6. (a) Crystal structure and XRD pattern of K6Nb10.8O3 GNB; (b) Overall in situ XRD pattern with its charge/discharge curve; (c) Local colormap image in 20-24.5°; (d) Local colormap image in 26.5-31°; (e) Local colormap image in 31-35.5°.
33
Figure 7. (a) Crystal structure of K6Nb10.8O30 viewed along the c-axis; (b) Three types of channels for lithium ions transportation in K6Nb10.8O30; (c) Possible sites for lithium ions storage in K6Nb10.8O30.
34
Figure 8. (a) Cyclic voltammograms of K6Nb10.8O30 GNB recorded at different scan rates (0.1, 0.2, 0.3, 0.5, 1.0 and 2.0 mV s-1); (b) The 1st, 2nd, 100th, 200th and 400th charge/discharge curves of K6Nb10.8O30 GNB; (c) Tong-term cycling and capacity retention stability of K6Nb10.8O30 GNB at a current density of 100 mA g-1; (d) The comparison of reversible capacity and cycling stability between K6Nb10.8O30 and previous reported anode materials. 35
Figure 9. The Nb 3d XPS spectra of K6Nb10.8O30 GNB at different discharge/charge states. (a) Pristine state; (b) Lithiation to 0.5 V; (c) Delithiation to 3.0 V.
36
Figure 10. (a) Rate capability of K6Nb10.8O30 NW; (b) Rate capability of K6Nb10.8O30 GNB; (c) Long-term cycle performance of K6Nb10.8O30 NW at 100 mA g-1; (d) Long-term cycle performance of K6Nb10.8O30 GNB at 100 mA g-1; (d) An LEDs array powered by a LiMn1.5Ni0.5O4/K6Nb10.8O30 full cell. 37
Haojie Zhu received his B.E. degree in Chemical Engineering and Technology from Jiaxing University in 2016. Now he is pursuing his M.S. degree at Ningbo University. His research interest is focused on nanomaterials for rechargeable lithium/sodium ion batteries.
Xing Cheng received her B.S. degree in Chemistry from Xinyang Normal University in 2016 and she is currently a Master candidate at Ningbo University. Her current research focuses on electrospun nanomaterials for rechargeable lithium-ion batteries.
Haoxiang Yu received his B.E. degree in Applied Chemistry from College of Science & Technology Ningbo University in 2012 and he is now pursuing his M.S. degree at Ningbo University. His current research focuses on advanced materials for rechargeable batteries. 38
Wuquan Ye received his B.S. degree in Chemistry from Quzhou University in 2016. He is currently a Master candidate at Ningbo University. He is interested in synthesized niobium oxide nanoparticles for energy conversion and storage.
Na Peng received her B.S. degree in Chemistry from Ningbo University in 2017 and she is now pursuing her M.S. degree at Ningbo University. Her current research focuses on nanomaterials for rechargeable lithium-ion batteries.
Runtian Zheng received his B.S. degree in Chemistry from Xinyang Normal University in 2017 and he is now pursuing her M.S. degree at Ningbo university. His current research focuses on the design and synthesis of nanomaterials, and investigation of their fundamental properties and applications in energy storage devices. 39
Tingting Liu received her B.S. degree from Mudanjiang Normal University in 2017 and she is now pursuing her M.S. degree at the Ningbo University. Her current research focuses on metal sulfides for rechargeable alkali-ion batteries.
Shui Miao received his Ph.D degree in Chemistry from Zhejiang University in 2002. He was a visiting scholar at University of Ottawa in 2010-2011. Now he is an associate professor at Ningbo University. His research interest is focused on energy storage materials.
Jie Shu is currently an associate professor at Ningbo University. He received his Ph.D degree from Institute of Physics, Chinese Academy of Sciences in 2007. He ever worked as a postdoctoral researcher at National Institute 40
of Advanced Industrial Science and Technology (Japan), Université de Picardie Jules Verne and Centre national de la recherché scientifique (France). His research interest is focused on energy storage materials. He has published more than 100 papers, 2 book chapters, and 30 patents.
Highlights
K6Nb10.8O30 groove nanobelts are prepared by electrospinning.
K6Nb10.8O30 groove nanobelts show superior performance to nanowires.
A charge capacity of 161.5 mAh g-1 is remained after 1000 cycles.
Structural evolution of K6Nb10.8O30 is analyzed by in situ XRD and TEM.
Cycling stability of K6Nb10.8O30 is demonstrated by ex situ TEM and XPS.
41
PII: DOI: Reference:
S2211-2855(18)30545-7 https://doi.org/10.1016/j.nanoen.2018.07.057 NANOEN2924
To appear in: Nano Energy Received date: 10 June 2018 Revised date: 21 July 2018 Accepted date: 25 July 2018 Cite this article as: Haojie Zhu, Xing Cheng, Haoxiang Yu, Wuquan Ye, Na Peng, Runtian Zheng, Tingting Liu, Miao Shui and Jie Shu, K 6Nb10.8O30 Groove Nanobelts as High Performance Lithium-Ion Battery Anode towards Long-Life Energy Storage, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.07.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
K6Nb10.8O30 Groove Nanobelts as High Performance Lithium-Ion Battery Anode towards Long-Life Energy Storage Haojie Zhu, Xing Cheng, Haoxiang Yu, Wuquan Ye, Na Peng, Runtian Zheng, Tingting Liu, Miao Shui, Jie Shu* Faculty of Materials Science and Chemical Engineering, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo 315211, Zhejiang Province, People’s Republic of China *Corresponding author: Jie Shu, Tel.: +86-574-87600787; Fax: +86-574-87609987. [email protected]
Abstract Owing to the multiple redox couples of Nb5+/Nb4+ and Nb4+/Nb3+, Nb-based compounds have attracted great attention to be promising high-capacity anode materials for rechargeable batteries. Here, K6Nb10.8O30 groove nanobelts (GNB) are synthesized through heat-treating the adjustable electrospun potassium niobate nanofibers, thereupon the structural change in the lithiation and delithiation is evidently imaged via in situ transmission electron microscopy (TEM). From in situ observations, the K6Nb10.8O30 GNB, in virtue of its stability, is ascertained to be adopted as anode material in lithium-ion batteries (LIBs). Evaluated as lithium storage host, GNB outstrip nanowires (NW) in cyclicity and in reversible capacity. Even after 1000 cycles, the retention capacity of K6Nb10.8O30 GNB is as high as 69 %. Furthermore, the lithium-storage mechanism is also investigeted via in situ X-ray
1
diffraction (XRD). It is proved that the phase transition takes place from Li-poor K6Nb10.8O30 phase to Li-rich Li22K6Nb10.8O30 phase via two steps. In addition, the results obtained from ex situ TEM and ex situ X-ray photoelectron spectroscopy (XPS) also prove that the behavior of lithiation and de-lithiation for K6Nb10.8O30 GNB is highly reversible, suggesting that it can be a possible anode material.
Graphical Abstract
Keywords K6Nb10.8O30 groove nanobelts; Electrospinning; In situ transmission electron microscopy; In situ X-ray diffraction; Lithium-ion batteries.
2
1. Introduction In the past decades, the climate has been changed sharply for the greenhouse gas (GHG) emissions. Broadly, the modern-day GHG comes from the combustion of non-renewable fossil fuels, which provides us with 85% of the total ouput of energy. To reduce the dependence on fossil fuels, various possible energy conversion and storage routes are used in the modern life. Today, rechargeable batteries as an alternative energy are widely applied in the field of consumer electronics. And the great success of using rechargeable batteries in portable consumer electronics is now being extended to electric vehicles. For instance, the conventional gasoline-powered cars are replaced by electric vehicles equipped with rechargeable batteries. This trend has spawned many new energy vehicles such as hybrid vehicles, electric vehicles and plug-in hybrid vehicles, because of the advantages of high power, long cycle stability, high energy density and light weight for rechargeable batteries. These factors are essential for electric vehicles and grid storage. In order to further improve the energy and power densities of rechargeable batteries (especially for lead-acid batteries), much effort has been currently devoted to develop new types of batteries such as lithium-ion batteries,[1-6] all solid-state batteries and lithium-sulfur batteries.[7-10] The development of advanced lithium-ion batteries (LIBs) is still one of the hottest global research aspects in the new energy fields. Recently intercalation-type anode materials have been the new orientation of the development of LIBs.[11-13] However, the commercial graphite anode may suffer from possible stafety problem during rapid charge/discharge process due to its low working 3
potential (close to 0.0 V vs. Li+/Li). Among many candidate materials, Li4Ti5O12 as anode material for LIBs has been widely studied because of its excellent rate performance and cycling stability, but the low specific capacity (175 mAh g-1) limits its application in the future. Compared with ordinary graphite or Li4Ti5O12, niobium-based materials (including Nb2O5, Nb12O29, TiNb2O7, etc.) are promising anodes for LIBs due to their safe working potentials (~1.65 V) and considerable lithium storage capacity (250-300 mAh g-1).[14-18] Particularly, monoclinic Nb12O29 as anode electrode was researched by galvanostatic charge/discharge test in the potential of 1.0-2.5 V at the current density from 6.6 to 660 mA g-1.[16] It shows that Nb12O29 can reversibly incorporate a relatively large amount of lithium ions (14.3 Li per unit, 243 mAh g-1) while retaining structural integrity. It may also signal that Nb12O29 as the anode ability to maintain a long-calendar cycle is general. On the other hand, TiNb2O7 has also been widely investigated as a possible solid anode with a higher theoretical capacity (388 mAh g-1) in recent years. Unfortunately, it suffers from the issue of rapid attenuation for lithium storage capability.[19, 20] However, the long-term cycling ability and high reversible specific capacity are two essential and important factors in the performance of LIBs. Thus, the potassium-niobate family has been attentioned in recent years. Among the potassium niobates, K6Nb10.8O30 is a novel multifunctional material with opened tetragonal tungsten bronze-type structure.[21] It is not only a good photocatalyst,[22, 23] but also a probable lithium storage material.[24] In the previous report, its electrochemical behavior is briefly introduced without any structure descriptions. However, further investigations have not been undertaken till 4
now. In this paper, K6Nb10.8O30 groove nanobelts are fabricated via electrospinning method under different experimental conditions. The relationships between electrochemical performances (cycling stability and capacity) and structural characteristics (Bragg position and c-axis) are unveiled by in situ X-ray diffraction. It is found that K6Nb10.8O30 groove nanobelts possess a good ability of lithium ion storage and reversible structure. Besides, the change of microstructure in the process of insertion-extraction is also demonstrated by ex situ TEM technique. It once again proves the excellent electrochemical performance of K6Nb10.8O30.
2. Experimental Section Electrospun K6Nb10.8O30 Groove Nanobelts and Nanowires 2.2g polyvinylpyrrolidone (PVP, Mw: 1300000, Macklin) was dissolved in a mixture of 16 mL absolute ethanol (AR, Macklin) and 6mL distilled water. Meanwhile, niobium oxalate (AR, Macklin) and potassium acetate (AR, Macklin) were added to the above solution under magnetic stirring overnight. Then, the mixed solution was transferred to a 10 mL syringe with a 21G needle. Whereafter, the process of electrospinning was performed in a controlled electrospinning setup (JDF04, Changsha). A positive direct current voltage of 20 kV and a grounding voltage of 0 kV were applied to the needle tip and stainless steel roller, respectively. The distance between syringe needle and alumina boat collector was kept at 15 cm and the solution was supplied at a flow rate of 1 mL h-1 using a syringe pump. Next, it 5
was easy to observe the white as-electrospun nanofibers membrane on the alumina boat collector. This experiment maintained the ambient humidity of the whole electrospinning process at around 40%. Finally, a polymer of electrospun nanofibers attached to the alumina boat was calcined at 750 oC for 10 h in air at a ramping rate of 10 oC min-1, which resulted in the formation of the final K6Nb10.8O30 groove nanobelts. As a control, K6Nb10.8O30 nanowires were also synthesized in a similar way. First, the ingredients of the precursor solution remain the same as the above starting materials. But the parameters of electrospinning setup were changed by using different flow rate (at 2 mL h-1) and high-voltage (at 25 kV). In addition, the collection mode was changed from alumina boat to stainless steel roller. Then the precursor was put into muffle furnace and calcined for 10 h in air at 800 oC to obtain the K6Nb10.8O30 nanowires. Materials Characterization The crystal structure of the two samples was characterized by a Bruker D8 X-ray diffractometer (XRD) with Cu-Kα radiations in the 2θ range 10° to 80°. In situ XRD measurements were characterized via using the aforementioned XRD equipment. The in situ cell for XRD detection was a homemade device equipped with beryllium disc as X-ray window. The working principle and structure of the in situ XRD cell were depicted in our previous paper.[25, 26] The morphology of K6Nb10.8O30 was verified by Hitachi SU-70 scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS), JEOL JEM-2010F high-resolution transmission electron microscope (HRTEM). In situ TEM observation was performed on the 6
aforementioned TEM equipment. A nano lithium battery device consisting of K6Nb10.8O30 GNB cathode, Li2O solid electrolyte, and lithium metal anode was designed to perform the real time TEM observation of the structural changes of the K6Nb10.8O30 GNB during the lithiation process. The elemental valence states in K6Nb10.8O30 at different lithiated/delithiated conditions were characterized by X-ray photoelectron spectroscopy (XPS, Theta Probe AR-XPS System, Al Kα radiation). Inductive coupled plasma (ICP) emission spectrometer analysis was performed by utilizing a PerkinElmer OPTIMA 4300 DV model. Electrochemical Measurements of Half Cells For electrochemical tests, the anode consisted of 80 wt.% active substance, 10 wt.% conductivity agent (carbon black), and 10 wt.% polyvinylidenefluoride dispersed in N-methyl-2-pyrrolidone (NMP, Macklin) to generate a slurry. This slurry was coated on copper foil by a tablet machine, dried at 80 oC overnight to ensure removal of the NMP. Its loading mass was about 2.0 mg for each electrode. The Swagelok batteries were assembled in an argon-filled glove box using the as-prepared film as the working electrode, Li metal foil as the counter electrode, glass fiber as the separator and 1 mol L-1 LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 in volume) as the electrolyte. The charge-discharge performance was evaluated by multichannel LANHE battery tester at a constant current density of 100 mA g-1. Cyclic voltammetry (CV) was carried out at a scan rate of 0.1 mV s-1 between 0.5 and 3.0 V on a CHI1000B electrochemical workstation. Assembly of Full Cells 7
The full cell was fabricated using the as-prepared K6Nb10.8O30 anode and the synthetic LiMn1.5Ni0.5O4 cathode at a mass ratio of 1:2. The mass loadings of the K6Nb10.8O30 groove nanobelts and LiMn1.5Ni0.5O4 were in the range of 1.0-2.0 mg cm-2. The separator and electrolyte of full cell are the same as that of half cell.
3. Result and Discussion The schematic diagram of electrospinning in Figure 1 shows the morphology formation process of the groove nanobelts and nanowires on different stages. Obviously, the solution of precursor mixed with polyvinylpyrrolidone (PVP) in ethanol is delivered through a syringe at a flow rate of 1 mL h-1 which is installed in the electrospinning machine. When the machine is applied with high voltage, it sprays nanofibers on the alumina boat collector (or stainless steel roller). K6Nb10.8O30 groove nanobelts (K6Nb10.8O30 GNB) are finally obtained via calcinations at high temperature. The formation process of K6Nb10.8O30 GNB includes three steps, forming nanowires, twining and merging, which is probably contributed to the evolution of the neighbouring nanowires under continuous calcinations. Briefly, in the initial heat-up region (<500 oC), the PVP and ethanol in the precursor gradually volatilized and decomposed, and then the coarse nanowire with a thick diameter was formed. In the subsequent heat-up circumstance at normal pressure (< 750 oC), adjacent nanowires were prone to collide and intertwined with each other due to the acceleration of thermal motion. After calcinations treatment at 750 oC for a period of time, the entangled nanowires were finally fused together to form nanobelts. Meanwhile, the 8
groove is formed at the joint of the neighbouring nanowires naturally. For comparison, the nanowires are formed due to the dense accumulation of precursor fibers on the stainless steel roller. As shown in Figure 2a, a twisted ribbon structure can be clearly identified, of which the width is about 600-900 nm and the thickness is approximately 80-100 nm. And its length can generally reach more than 10 μm. Figure 2b shows a representative panoramic TEM image of K6Nb10.8O30 GNB. The bending form shows the most distinctive features of groove nanobelts, which agrees well with the SEM image. Observed from Figure 2b and c, the K6Nb10.8O30 GNB is constructed by many small nanoparticles with the diameter of 15-25 nm. Figure 2d demonstrates that the K6Nb10.8O30 GNB is composed of the crystalline nanoparticles which are defined with the (110) plane. The polycrystalline structure of K6Nb10.8O30 GNB is confirmed by selected area electron diffraction (SAED) characterization in Figure 2e, which indicates a typical polycrystalline ring pattern. Additionally, the EDS elemental mapping shows a homogeneous dispersion of K, Nb and O elements within the entire zonal framework (Figure 2g-i), and the upper half of initial image has obvious groove (Figure 2f), separately confirms the key structural characteristics of the K6Nb10.8O30 GNB again. Moreover, to confirm the practical stoichiometric ratio of as prepared materials, EDS and ICP results are also given in Table S1 and S2. The potassium/niobium stoichiometric values of the K6Nb10.8O30 GNB and the K6Nb10.8O30 NW from the ICP data are 5.97/10.8 and 5.94/10.8, respectively, which is close to the theoretical stoichiometric value of K6Nb10.8O30. It suggests the successful 9
preparation of K6Nb10.8O30 GNB and K6Nb10.8O30 NW in this work, which is demonstrated by the Rietveld XRD patterns as shown in Figure S1. To gain more insight, the morphologic features of as-prepared K6Nb10.8O30 GNB and K6Nb10.8O30 NW are characterized by SEM, as shown in Figure 3. As can be seen from a low magnification SEM image of Figure 3a, it illustrates the one dimensional K6Nb10.8O30 GNB with a width of 500-700 nm and length of several micrometers. The longitudinal thickness of K6Nb10.8O30 GNB is about 90 nm. It can be also obviously observed that as-fabricated polycrystalline K6Nb10.8O30 GNB is composed of numerous nanograins with uneven surface, which can be further confirmed by a high magnification SEM picture in Figure 3b. Figure 3c provides a schematic diagram to classify the formation process of K6Nb10.8O30 GNB into three types, including straight-type, folded-type and screw-type, all of which appear in the SEM images. Additionally, the detailed groove structure of K6Nb10.8O30 GNB can also be observed from Figure S2. In contrast, the K6Nb10.8O30 NW in Figure 3d and e is straight-type nanowire with the diameter of 750 nm, and the length of 20 μm. Furthermore, Figure 3f-i proves the homogeneous distribution of K, Nb and O elements in the polycrystalline nanowires. It is noticeable that the superficial area of the K6Nb10.8O30 NW is much smaller than that of the K6Nb10.8O30 GNB, which suggests that K6Nb10.8O30 GNB can provide larger reaction places for lithium ions. To
reveal
the
structural
stability
of
K6Nb10.8O30
GNB
during
lithiation/delithiation process, the ex situ TEM technique has been used. Figure 4a-f show three stages at different charge/discharge cycles (1st, 100th and 400th). It is 10
apparent that K6Nb10.8O30 GNB with well-defined two-dimeonsinal band-like structure are surrounded by a lot of carbon black particles after the first cycle. Concurrently, a solid electrolyte interphase (SEI) film is formed on the surface of the K6Nb10.8O30 GNB, which leads to significantly irreversible capacity in the first cycle. After 400 cycles, the structure of K6Nb10.8O30 GNB is maintained well (Figure 4c). Figure 4d-f presents the HRTEM images of K6Nb10.8O30 GNB at the different cycles (1st, 100th and 400th). The polycrystalline structure is well-defined after repeated cycles. Moreover, the lattice distance of the K6Nb10.8O30 GNB displays no changes (d=0.268 nm for (321) at different states (1 cycle, 100 cycles and 400 cycles). Thus, it demonstrates the excellent cycle stability of K6Nb10.8O30 GNB during long-life cycles. To verify whether K+ ions can move out/into the structure of K6Nb10.8O30 GNB during repeated charging/discharging or not, two experiments are designed. Firstly, an ex situ XRD experiment is performed for K6Nb10.8O30 GNB to probe the possible Li+/K+-exchange behaviors in a lithium-containing LiPF6-based electrolyte, as shown in Figure S3. It is hard to observe the shifting of the featured (001) and (410) peaks for K6Nb10.8O30 GNB after being soaked in electrolyte for 96 h, which indicates that K6Nb10.8O30 GNB has good chemical stability in organic electrolyte. Secondly, to further check the possible dissolution of potassium ions during the electrochemical reaction, the ex situ ICP results for K6Nb10.8O30 GNB before and after different cycles are given in Table S3 and S4. The detected results show that the K/Nb ratio in the cycled electrode decreases from 0.232 to 0.225 after 400 cycles. Furthermore, only 31 mg Kg-1 of potassium element is found in electrolyte (1M LiPF6) after 400 cycles 11
(Table S4). All these evidences indicate that only very few potassium ions can move out/into the structure during charging/discharging. Therefore, K6Nb10.8O30 GNB has a stable framework for repeated lithium storage. Figure 5a gives a schematic illustration of LIBs based on as-prepared product. The K6Nb10.8O30 GNB arranged to each other is served as the working electrode and lithium plate attached to the stainless steel collector column is served as the counter electrode. In this cell, K6Nb10.8O30 GNB is cycled at different conditions. Figure 5b shows the HRTEM images of K6Nb10.8O30 GNB at different lithiated/delithiated states (initial, lithiated and delithiated). Before cycles, the pristine sample exhibits a polycrystalline structure with featured (311) and (410) lattice planes (d=0.281 nm for (311) plane and d=0.304 nm for (410) plane). The lattice distances of K6Nb10.8O30 (d=0.291 nm for (311) plane and d=0.313 nm for (410) plane) expand slightly in the lithiation state. As expected, the lattice distances of the nanocrystallines shrink to the original values after the first de-lithiation. This means that the reaction of K6Nb10.8O30 GNB with lithium is reversible. Figure 5c-h illustrates the nanoscale electrochemical device for in situ TEM and its patterns. The K6Nb10.8O30 GNB is attached to a platinum needle and used as the working electrode. The Li metal clung to the tungsten needle is served as the counter electrode and lithium source, and the naturally grown Li2O layer on the surface of the Li metal is used as a solid electrolyte to allow the transportion of Li+ ions. Figure 5d-h presents the TEM images of the K6Nb10.8O30 GNB during the lithiation process. It can be observed that the whole nanostructure of the K6Nb10.8O30 GNB changes 12
slightly during the lithiation process. And the K6Nb10.8O30 GNB expands slightly in the horizontal and vertical directions. The fact can be confirmed by the variation of the marked red rectangle and straight line. After 120 seconds of lithiation process, the volume expansion is invisible and the morphological framework remains stable. It strongly illustrates the structural stability of the K6Nb10.8O30 GNB. And intuitive in situ TEM video of the K6Nb10.8O30 GNB is also shown in Video S1. As shown in Figure 6a, the tetragonal bronze structure of K6Nb10.8O30 is built upon the connection of NbO6 octahedra with corner-sharing configurations, which consists of some triangles, quadrilateral and pentagonal tunnels. Although the pentagonal and quadrilateral tunnels are occupied by K cations, these two tunnels can still provide spaces to accommodate inserted lithium ions. In contrast, the triangle tunnels are fully blocked by Nb cations, which prohibit the insertion of lithium ions. Here, the powder X-ray diffraction (XRD) pattern of the typical K6Nb10.8O30 is plotted in Figure 6a. Correspondingly, the XRD Rietveld refinements of the K6Nb10.8O30 GNB and K6Nb10.8O30 NW by materials studio software also are demonstrated in Figure S1. It can be found that the Rietveld parameters of the two samples are close to the standard card. No peaks related to impurity phases such as Nb2O5 are observed. All peaks can be indexed with a tetragonal bronze structure (space group: P4/mbm), indicating the formation of pure K6Nb10.8O30 phase. As previous ex situ TEM discussed, the synergistic effects of K6Nb10.8O30 nanocrystals and nanobelts may contribute to the extraordinary electrochemical performances of the K6Nb10.8O30 GNB. To clearly reveal this synergistic effects 13
inherent to multi-crystal-nanobelt structure, we describe the structural evolution of the K6Nb10.8O30 GNB by in situ XRD during lithium-ion intercalation and deintercalation process (Figure 6b). Two galvanostatic charge/discharge cycles are also presented along with the in situ XRD patterns. An appreciable variation of the positive region within a 2θ range of 19.8-38.2° is observed, indicating the appearance of phase transitions during the charge/discharge process. As can be seen in Figure 6b, an obvious two-phase reaction occurs during the first discharge process, followed with a solid solution reaction upon further discharge. At the beginning of the constant-current reduction process from 0 to 37 min, the Li-poor K6Nb10.8O30 phase fades gradually along with the disappearance of featured reflections and a Li-rich Li3K6Nb10.8O30 phase is observed with the appearance of new reflections. As the discharge time increases from 37 to 265 min, the continuous peak shifting towards lower angle indicates a solid solution transformation in this region. During the charging, a quasi-reversible process occurs, except a slight irreversibility at the end of delithiation. In above process, the phase transition in the initial cycle may be briefly summarized in the following equations: 3.0-1.6 V: K6 Nb10.8 O30 + 3Li+ + 3e- = Li3 K6 Nb10.8 O30
(1)
1.6-0.5 V: Li3 K6 Nb10.8 O30 + 19Li+ + 19e- = Li22 K6 Nb10.8 O30
(2)
0.5-1.8 V: Li22 K6 Nb10.8 O30 = Li8.5 K6 Nb10.8 O30 + 13.5Li+ + 13.5e-
(3)
1.8-3.0 V: Li8.5 K6 Nb10.8 O30 = Li7 K6 Nb10.8 O30 + 1.5Li+ + 1.5e-
(4)
The in situ XRD patterns exhibit three main characteristics (Figure 6c-e). First, the (001) reflection of K6Nb10.8O30 GNB shows the evolutions of relative intensity and 14
Bragg position. It shifts and recovers repeatedly during the initial two charge/discharge processes (Figure 6c). The corresponding ex situ HRTEM images are also provided to confirm the recovery of the structure in the above discussion (Figure 5b). When lithium ions insert into the structure of K6Nb10.8O30 GNB along c-axis direction, the corresponding peaks shift toward lower angles. When lithium ions extract, the peaks turn to their initial positions, corresponding to the traditional process of storing electricity. In this observation, the XRD patterns of K6Nb10.8O30 GNB exhibit four clear slopes corresponding to adjacent charge/discharge curves. Second, when the charge/discharge is performed in the range of 0.5-3.0 V, Bragg positions of the (211), (400), and (221) reflections gradually shift toward lower 2θ values upon lithiation, and then these Bragg positions reverts to the original angles upon delithiation (Figure 6d), which is validated by the reversible changes of the lattice fringe of (410) plane in the TEM images (Figure 5b). Third, the angle variation of the (311) peak is similar to the (410) peak in the local colormap image in 31-35.5° (Figure 6e), which gradually moves to low-angle region, and then goes back to high angles. All these above discussed reflections have the same trend as the charge/discharge curves of K6Nb10.8O30 GNB. Hence, K6Nb10.8O30 GNB as an intercalation-type anode material can achieve highly efficient lithium ion storage in terms of specific capacity and cycling stability. In addition, to confirm the reversibility of K6Nb10.8O30 GNB in the charge/discharge process, the changes of lattice parameters (a, c and V) are calculated as the scatter diagram in Figure S4. The initial lattice parameters a, c and unit-cell 15
volume V at the open circuit potential state for K6Nb10.8O30 GNB are 12.476 Å, 3.961 Å and 616.53 Å3, respectively. When the in situ XRD cell is discharged to 0.5 V, the lattice parameters a and c are 12.484 and 4.059 Å, respectively. The lattice volume can be calculated, which is 632.59 Å3, suggesting the whole volume expansion of 2.60% after a full lithiation. In addition, it is obvious that all the lattice parameters (a, c and V) present good mirror symmetry during two charge/discharge cycles. After these processes, all the parameters can almost recover to their initial values, which further confirm the structural reversibility of K6Nb10.8O30 GNB anode material. Figure 7a shows the crystal structure of K6Nb10.8O30 viewed along the c-axis and [001] direction. This structure of K6Nb10.8O30 has a basic framework based on that of the tetragonal bronze structure in which cooperative lattice distortions take place for corner-shared NbO6 octahedra along c-axis, providing the triangular, square and pentagonal tunnels in cross-section.[16, 24] Figure 7b exhibits the specific shapes of these three channels. As the chart shows, the pentagonal-type tunnel is described as channel I, the square-type tunnel is described as channel II and the triangular-type tunnel is described as channel III. As described in the above discussion, only pentagonal and square tunnels parallel to the c-axis can store lithium ions, while triangular tunnels are blocked for stereo-hindrance effect induced by Nb cations. Remarkably, combined with the aforementioned conclusion, two tunnels along c-axis present open spaces for lithium ions insertion into 4g and 2a sites. A schematic diagram of transport path for lithium ions in the discharge process is shown in the Figure 7c. In the discharge process, lithium ions initially occupy the empty positions 16
in the pentagonal tunnel, and then take the available positions in the square tunnel.[25] In addition, it is known that about five lithium ions per unit can be embedded at the positions in pentagonal tunnel and about two lithium ions are inserted at the positions in the square tunnel by calculation (Figure S5). Upon a reverse delithiation, the lithium ion battery shows a phase-transition range between 0.5 and 1.75 V with a reversible capacity of about 216 mAh g-1. As a result, the reversibility of lithium removal mechanism in the [001] direction can be obtained according to the variation of the Bragg position of the (001) peak in the in situ XRD (Figure 6c). To evaluate the obtained K6Nb10.8O30 GNB and K6Nb10.8O30 NW materials as electrodes for lithium ion battery, the electrochemical properties are systematically studied as presented in Figure 8. Figure 8a shows the cyclic voltammetry test of the K6Nb10.8O30 GNB at the scanning rates of 0.1, 0.2, 0.3, 0.5, 1.0 and 2.0 mV s-1. It can be noticed that there are two pairs of broad peaks appeared in the cyclic voltammograms (CVs), which changes as the scanning rate increases. During the negative scan, a clear cathodic peak is located at about 1.59 V. And during the positive scan, a highlighted anodic peak is seated at 1.75 V. The positions of this redox couple coincide with the previous report.[24] The couple of intensive cathodic/anodic peaks at 1.59/1.72 V can be attributed to the valence variation of Nb4+/Nb5+ redox couple.[26] For comparison, the broad shoulders ranging from 1.2 to 0.8 V can be assigned to the Nb3+/Nb4+ redox couple. It suggests that the lithiation/delithiation process is reversible for K6Nb10.8O30 GNB, which can be further demonstrated by the reverisble transformation of Nb element between Nb4+/Nb5+ and Nb3+/Nb4+ redox couples in the 17
X-ray photoelectron spectroscopy (XPS, Figure 9).[27] Figure 8b illustrates the charge/discharge profiles of K6Nb10.8O30 GNB cycled at a current density of 100 mA g-1. The K6Nb10.8O30 GNB delivers the discharge/charge specific capacity of 343.4/216.4 mAh g-1, corresponding to a Coulombic efficiency of 63 % at the first cycle. The irreversible capacity of 127 mAh g-1 in the first discharge-charge step results from the decomposition of the electrolyte, which causes the formation of solid electrolyte interphase (SEI) layer on the electrode surface and the irreversible consumption of lithium ions.[28-31] After 400 cycles, the K6Nb10.8O30 GNB remains a specific charge capacity of 172.1 mAh g-1 (Figure 8c), having the capacity retention of 81.1%, which is much better than the electrochemical performance of K6Nb10.8O30 NW (Figure S6). According to the transverse comparison, the lithium storage ability of K6Nb10.8O30 GNB is superior to many other niobium based anode materials (Figure 8d, Table S5). The dark blue dots represent reversible specific capacity and the red dots represent the cycle number of capacity retention at 80 %. Regardless of reversible specific capacity or capacity retention, it is clear that the electrochemical property of K6Nb10.8O30 GNB is far beyond other Nb- or Ti-based anode materials reported recently.[32-37] According to the longitudinal comparison, Table S5 also shows the comparison of K6Nb10.8O30 GNB with representative anode materials (such as graphite, alloys, Li4Ti5O12 and niobium compounds) in terms of their reversible capacity and cycle performance.[38-43] The results exhibit that the lithium storage capability of the K6Nb10.8O30 GNB is acceptable among the representative anode materials reported in recent years. 18
Based on the comparison of the obtained literature, the results show that the morphology of K6Nb10.8O30 GNB has a great influence on its electrochemical properties (Figure 8d). In order to further check the performance of K6Nb10.8O30 anode, electrochemical experiments with rate capability and cyclability are designed to test. Figure 10a and c show the electrochemical performances of K6Nb10.8O30 NW. As charge/discharge current density increases from 100 to 800 mA g-1, its lithium storage capacity gradually decreases from 150 to 74 mAh g-1 (Figure 10a). Upon a long-life cycles at 100 mA g-1, as seen in the Figure 10c, the charge capacity of K6Nb10.8O30 NW stabilizes at 85.19 mAh g-1 after 1000 cycles with about 50 % capacity decay. Remarkably, the electrochemical performance of K6Nb10.8O30 GNB is much better than that of the K6Nb10.8O30 NW. At the same current density (100-800 mA g-1), K6Nb10.8O30 GNB delivers the reversible capacity of 235.2 mAh g-1 at 100 mA g-1 and 137.1 mAh g-1 at 800 mA g-1 (Figure 10b). Furthermore, its long-life capability at 100 mA g-1 is also superior to K6Nb10.8O30 NW. As exhibited in Figure 10d, the charge capacity remains at 161.5 mAh g-1 after 1000 cycles with about 69 % of capacity retention. Cyclability is an important electrochemical performance for LIBs. Thus the cycle performance of the K6Nb10.8O30 GNB and K6Nb10.8O30 NW at different current densities is demonstrated in Figure S7-S10. The reversible capacity of K6Nb10.8O30 GNB fades slowly from 200.8 mAh g-1 at 300 mA g-1 to 174.6 mAh g-1 at 500 mA g-1 and 150.8 mAh g-1 at 700 mA g-1. More importantly, K6Nb10.8O30 GNB shows a capacity of 127.4 mAh g-1 at a high current density of 1000 mA g-1 and maintains 19
73.9 % of capacity after 550 cycles. In contrast, K6Nb10.8O30 NW only delivers capacities of 126.1, 103.6, 74.6 and 51 mAh g-1 at the current densities of 300, 500, 700 and 1000 mA g-1, respectively. It suggests that electrochemical properties of K6Nb10.8O30 GNB are better than that of K6Nb10.8O30 NW. Besides, the initial Coulombic efficiency is also increased from 53 % (K6Nb10.8O30 NW) to 63 % (K6Nb10.8O30 GNB) owing to the special nanobelt structure. Based on above various analyses, it can be concluded that K6Nb10.8O30 GNB is undoubtedly a superior anode for high performance lithium storage, which can be further proved by the light-emitting diodes (LEDs) powered with a LiMn1.5Ni0.5O4/K6Nb10.8O30 cell (Figure 10e).
4. Conclusions In this work, K6Nb10.8O30 GNB with a thickness of 80-100 nm is successfully synthesized by using a facile electrospinning method. The phase transition and lithium storage behaviors of K6Nb10.8O30 GNB from Li-poor K6Nb10.8O30 phase to Li-rich Li22K6Nb10.8O30 phase during intercalation of lithium ions are analyzed by in situ XRD and ex situ XPS. It reveals that lithium storage at 4g and 2a sites in quadrilateral and pentagonal tunnels is based on the multiple redox couples of Nb5+/Nb4+ and Nb4+/Nb3+. Moreover, in situ TEM, ex situ TEM and in situ XRD tests demonstrate the structural stability and reversiblity of K6Nb10.8O30 GNB as an anode material for LIBs. Electrochemical tests show that nanobelt structure brings ultra-long cycling stability and super rate performance. Cycled at 100 mA g-1, it has the charge 20
capacities of 192.6, 182.1, 176.9 and 172.1 mAh g-1 in the 100th, 200th, 300th and 400th cycles, respectively. It also exhibits a capacity retention as high as 69 % after 1000 cycles.
Owing
to
the
outstanding
electrochemical
properties,
a
LiMn1.5Ni0.5O4/K6Nb10.8O30 full cell is fabricated to power the LEDs at the first time.
Acknowledgement This work is sponsored by National Natural Science Foundation of China (U1632114), Ningbo Key Innovation Team (2014B81005), and K.C. Wong Magna Fund in Ningbo University.
Supporting Information Supplementary data associated with this article can be found in this submission, including Rietveld refinement profiles, SEM images, ex situ XRD patterns, in situ XRD evolution, changes of lattice parameters, Li content x versus voltage, in situ TEM video, Nb 3d XPS spectra, charge/discharge curves, cycling performances, comparative capacities, electrochemical kinetic parameters and ICP-AES data of K6Nb10.8O30 GNB and K6Nb10.8O30 NW.
Video S1. In situ TEM of K6Nb10.8O30 GNB upon discharge.
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Figure 1. The formation process of K6Nb10.8O30 GNB and K6Nb10.8O30 NW.
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Figure 2. (a) SEM, (b, c) TEM, (d) HRTEM, (e) SAED and (f-i) EDS elemental mappings characterizations of K6Nb10.8O30 GNB.
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Figure 3. (a, b) SEM images of K6Nb10.8O3 GNB, and (c) corresponding sketch illustration of three-type groove nanobelts; (d-f) SEM images of K6Nb10.8O3 NW, and (g-i) corresponding EDS elemental mapping images of K, Nb and O elements.
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Figure 4. Schematic diagrams and ex situ TEM observations of electrochemical process of K6Nb10.8O3 GNB. (a, d) 1 cycle, (b, e) after 100 cycles, and (c,f) after 400 cycles.
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Figure 5. (a) Schematic illustration of lithium storage in K6Nb10.8O3; (b) HRTEM images of K6Nb10.8O3 GNB at pristine, lithiated and delithiated states; (c) Schematic illustration of in situ experimental setup inside a TEM; (d) In situ TEM observation of electrochemical process of K6Nb10.8O3 GNB in the initial stage, (e, f, g, h) in the lithiated stages.
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Figure 6. (a) Crystal structure and XRD pattern of K6Nb10.8O3 GNB; (b) Overall in situ XRD pattern with its charge/discharge curve; (c) Local colormap image in 20-24.5°; (d) Local colormap image in 26.5-31°; (e) Local colormap image in 31-35.5°.
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Figure 7. (a) Crystal structure of K6Nb10.8O30 viewed along the c-axis; (b) Three types of channels for lithium ions transportation in K6Nb10.8O30; (c) Possible sites for lithium ions storage in K6Nb10.8O30.
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Figure 8. (a) Cyclic voltammograms of K6Nb10.8O30 GNB recorded at different scan rates (0.1, 0.2, 0.3, 0.5, 1.0 and 2.0 mV s-1); (b) The 1st, 2nd, 100th, 200th and 400th charge/discharge curves of K6Nb10.8O30 GNB; (c) Tong-term cycling and capacity retention stability of K6Nb10.8O30 GNB at a current density of 100 mA g-1; (d) The comparison of reversible capacity and cycling stability between K6Nb10.8O30 and previous reported anode materials. 35
Figure 9. The Nb 3d XPS spectra of K6Nb10.8O30 GNB at different discharge/charge states. (a) Pristine state; (b) Lithiation to 0.5 V; (c) Delithiation to 3.0 V.
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Figure 10. (a) Rate capability of K6Nb10.8O30 NW; (b) Rate capability of K6Nb10.8O30 GNB; (c) Long-term cycle performance of K6Nb10.8O30 NW at 100 mA g-1; (d) Long-term cycle performance of K6Nb10.8O30 GNB at 100 mA g-1; (d) An LEDs array powered by a LiMn1.5Ni0.5O4/K6Nb10.8O30 full cell. 37
Haojie Zhu received his B.E. degree in Chemical Engineering and Technology from Jiaxing University in 2016. Now he is pursuing his M.S. degree at Ningbo University. His research interest is focused on nanomaterials for rechargeable lithium/sodium ion batteries.
Xing Cheng received her B.S. degree in Chemistry from Xinyang Normal University in 2016 and she is currently a Master candidate at Ningbo University. Her current research focuses on electrospun nanomaterials for rechargeable lithium-ion batteries.
Haoxiang Yu received his B.E. degree in Applied Chemistry from College of Science & Technology Ningbo University in 2012 and he is now pursuing his M.S. degree at Ningbo University. His current research focuses on advanced materials for rechargeable batteries. 38
Wuquan Ye received his B.S. degree in Chemistry from Quzhou University in 2016. He is currently a Master candidate at Ningbo University. He is interested in synthesized niobium oxide nanoparticles for energy conversion and storage.
Na Peng received her B.S. degree in Chemistry from Ningbo University in 2017 and she is now pursuing her M.S. degree at Ningbo University. Her current research focuses on nanomaterials for rechargeable lithium-ion batteries.
Runtian Zheng received his B.S. degree in Chemistry from Xinyang Normal University in 2017 and he is now pursuing her M.S. degree at Ningbo university. His current research focuses on the design and synthesis of nanomaterials, and investigation of their fundamental properties and applications in energy storage devices. 39
Tingting Liu received her B.S. degree from Mudanjiang Normal University in 2017 and she is now pursuing her M.S. degree at the Ningbo University. Her current research focuses on metal sulfides for rechargeable alkali-ion batteries.
Shui Miao received his Ph.D degree in Chemistry from Zhejiang University in 2002. He was a visiting scholar at University of Ottawa in 2010-2011. Now he is an associate professor at Ningbo University. His research interest is focused on energy storage materials.
Jie Shu is currently an associate professor at Ningbo University. He received his Ph.D degree from Institute of Physics, Chinese Academy of Sciences in 2007. He ever worked as a postdoctoral researcher at National Institute 40
of Advanced Industrial Science and Technology (Japan), Université de Picardie Jules Verne and Centre national de la recherché scientifique (France). His research interest is focused on energy storage materials. He has published more than 100 papers, 2 book chapters, and 30 patents.
Highlights
K6Nb10.8O30 groove nanobelts are prepared by electrospinning.
K6Nb10.8O30 groove nanobelts show superior performance to nanowires.
A charge capacity of 161.5 mAh g-1 is remained after 1000 cycles.
Structural evolution of K6Nb10.8O30 is analyzed by in situ XRD and TEM.
Cycling stability of K6Nb10.8O30 is demonstrated by ex situ TEM and XPS.
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