Self-template synthesis of Li1.13Ni0.30Mn0.57O2 anothorn spheres and nanorods as high-performance cathode materials for lithium-ion batteries

Self-template synthesis of Li1.13Ni0.30Mn0.57O2 anothorn spheres and nanorods as high-performance cathode materials for lithium-ion batteries

Accepted Manuscript Self-template synthesis of Li1.13Ni0.30Mn0.57O2 anothorn spheres and nanorods as high-performance cathode materials for lithium-io...

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Accepted Manuscript Self-template synthesis of Li1.13Ni0.30Mn0.57O2 anothorn spheres and nanorods as high-performance cathode materials for lithium-ion batteries Yan Jiang, Ze Yang, Fei Mei, Yuanming Zhou, Jinxia Xu, Yunhui Huang PII:

S0925-8388(15)31559-0

DOI:

10.1016/j.jallcom.2015.11.003

Reference:

JALCOM 35858

To appear in:

Journal of Alloys and Compounds

Received Date: 9 September 2015 Revised Date:

30 October 2015

Accepted Date: 2 November 2015

Please cite this article as: Y. Jiang, Z. Yang, F. Mei, Y. Zhou, J. Xu, Y. Huang, Self-template synthesis of Li1.13Ni0.30Mn0.57O2 anothorn spheres and nanorods as high-performance cathode materials for lithium-ion batteries, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.11.003. 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 proof before it is published in its final 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.

ACCEPTED MANUSCRIPT

Self-template synthesis of Li1.13Ni0.30Mn0.57O2 anothorn spheres and nanorods as high-performance cathode materials for lithium-ion batteries

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Yan Jianga,b,c, Ze Yangc, Fei Meia,b, Yuanming Zhoua,b, Jinxia Xua,b, and Yunhui Huang*c

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a Hubei Collaborative Innovation Center for High-efficient Utilization of Solar Energy, Wuhan 430068, China b School of Electrical & Electronic Engineering, Hubei University of Technology, Wuhan 430068, China c State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

Abstract: Commercial cathode materials for lithium-ion batteries require both excellent electrochemical and processing performance. Nanoscaled cathode materials with high surface area usually have low compacted density and low loading of active materials on the electrode because of high-content usage of conductive agent and

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binder, which is expected to be solved by incorporating one-dimensional (1D) particles into dense three-dimensional (3D) structure. In this work, a topochemical method

is

used

to

synthesize

high-capacity

Li-rich

cathode

material

Li1.13Ni0.30Mn0.57O2 with controllable 3D and 1D hybrid structure containing

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nanothorn spheres and nanorods by using γ-MnO2 and β-MnO2 as templates. Structural and electrochemical lithium insertion/desertion properties are investigated.

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Experimental results show that Li1.13Ni0.30Mn0.57O2 with well-designed 3D structure exhibits superior electrochemical performance especially higher volumetric capacity than 1D material.

Keywords: Li-ion battery; Li-excess cathode material; Template method; High performance.

* Corresponding author. Tel. & fax: 86-27-87558241 1

ACCEPTED MANUSCRIPT E-mail address: [email protected]

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560

mA 15

480 400

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640 -1

g

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g

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g mA 15

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-1 g mA g g mA mA 30 A 60 0 0m 10 20 -1

0 50

240

80 0

0

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Ag 0m 0 10

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160

g mA

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320

3D 1D

10 20 30 40 50 60 70 80 90 100

Cycle number

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-3 Volumetric capacity (mAh cm )

Graphical abstract

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ACCEPTED MANUSCRIPT 1. Introduction Since lithium-ion batteries (LIBs) were first commercialized by Sony Corporation in 1990, they have been extensively used as powers for 3C (computer, communication, consumer-electronics) applications.[1] With rapid development of electronic devices, electric and hybrid electric vehicles, energy storage systems call for superior LIBs

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with higher energy and power densities, which accordingly brings the higher demand for the cathode materials. LiCoO2, the most successful cathode material for the first-generation LIBs, cannot meet the demand of the next-generation batteries due to

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the limit of gravimetric energy density, toxicity, safety and cobalt resource.[2] Therefore, a series of alternative cathode materials with high capacity, such as

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LiNixMnyCo1-x-yO2,[3-5] LiMPO4 (M = Fe, Co, Mn, etc.) and Li3V2(PO4)3,[6-9] LiMn2O4,[10-12] LiNi0.5Mn1.5O4,[13, 14] xLi2MnO3·(1-x)LiMO2 (M = Ni, Mn, Co, Fe, etc.),[15-22] have been investigated. Among them, Li-rich materials xLi2MnO3·(1-x)LiMO2 have attracted much attention due to high capacity and high operation voltage.[23-29]

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For cathode materials, fabrication of nanostructure is helpful to achieve superior electrochemical performance due to the shortened Li-ion diffusion distance. For example, 1D nanostructure is effective for xLi2MnO3·(1-x)LiMO2 to improve the rate

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capability.[30, 31] However, such structure usually gives rise to low tap density and hence low volumetric energy density because a large amount of conducting agent and

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binder are needed to compensate the conductivity and improve the processability. Cathode material with 3D micro-spherical morphology is preferred for electrode processing in LIBs due to high compacted density, and thus high volumetric capacity can be attained. However, micro-scaled cathode materials usually suffer from lower rate capability as compared with 1D nanostructured materials. In order to achieve ideal rate capability and processing performance, it is desirable to combine the advantages from both 1D and 3D materials. Therefore, fabrication of 3D microspheres assembled by 1D nanostructured particles is an efficient strategy for development of high-performance cathode materials.[32, 33] 3

ACCEPTED MANUSCRIPT In

this

work,

Li-rich

cathode

material

with

nominal

formula

of

0.3Li2MnO3·0.7LiNi0.5Mn0.5O2 (written as Li1.13Ni0.30Mn0.57O2) containing 1D nanorods and 3D nanothorn spheres was designed and fabricated via a simple self-template method. Tunable morphology and high electrochemical performance

excellent rate capability and high volumetric capacity. 2. Experimental

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2.1 Materials synthesis

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were attained. The 3D nanothorn spheres assembled by 1D nanorods exhibited

MnO2 prepared by hydrothermal method was used as template.[34, 35] Shortly, 16

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mmol MnSO4⋅H2O and 16 mmol (NH4)2S2O8 were added into 50 mL distilled water under magnetic stirring for 1 h to achieve a pink solution. The solution was transferred into a teflon-lined stainless steel autoclave, sealed, and heated at 120 °C or 180 °C for 12 h. After hydrothermal reaction, the product was filtered, washed for several times with distilled water and ethanol, and dried at 80 °C overnight. The obtained black powder (MnO2) was collected as template for the designed sample.

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Stoichiometric MnO2 template (0.7824 g) and Ni(NO3)2⋅6H2O (1.4930 g) were dispersed in ethanol by ultrasonic vibration for 2 h. LiOH (0.8812 g) solution was added into the foregoing mixed system drop by drop with continuous stirring at room

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temperature. The obtained suspension was dried at 100 °C for 12 h, ground, and

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calcined at 700 °C for 10 h in air. The synthetic route is illustrated in Scheme 1. 2.2 Materials characterization The phase of the samples was examined by X-ray diffraction (XRD) on X’Pert

Pro diffractometer (X’Pert Pro, PANalytical B.V.) using Cu-Kα radiation (λ = 1.5406 Å). The morphology of the products was observed by field-emission scanning electron microscopy (FE-SEM, FEI, Sirion 200). The microstructure was further analyzed with transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) on a JEOL-2010. 2.3 Electrochemical measurements 4

ACCEPTED MANUSCRIPT Charge-discharge performance was measured on 2032 type coin cells assembled in argon-filled glove box. The cathode film was made from a mixture of Li1.13Ni0.30Mn0.57O2, carbon black and polyvinylidene fluoride (PVDF) binder in a weight ratio of 80:10:10 on a substrate of Al foil. The diameter of the electrode film

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was 8 mm and the thickness was controlled to be 55 µm (including 20 µm Al foil). In the coin cells, metallic lithium foil was used as the counter and reference electrode; Celgard 2400 served as the separator, and the electrolyte was 1 mol L–1 LiPF6 in a mixed solvent of ethylene carbonate and diethyl carbonate (EC/DEC, 1:1, V/V). The

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galvanostatic charge-discharge tests were carried out on a battery measurement system (LAND Electronics Co., Ltd., China) within a voltage range of 2.0–4.8 V vs.

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Li/Li+ at room temperature. The electrochemical impedance spectroscopy (EIS) was carried out on an electrochemical workstation (Solartron, UK). 3. Results and discussion

XRD patterns of the MnO2 templates and the final Li1.13Ni0.30Mn0.57O2 samples

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are shown in Fig. 1. It is clear that the product prepared at 120 °C is of γ-MnO2 phase (JCPDS No. 14-0644). For the product prepared at 180 °C, all diffraction peaks are in good agreement with the pure β-MnO2 phase (JCPDS No. 24-0735). The broad diffraction peaks of γ-MnO2 indicate smaller crystalline size and lower crystallinity

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than β-MnO2, which is due to the low reaction temperature.[34, 36, 37] XRD patterns of Li1.13Ni0.30Mn0.57O2 products can be well indexed to a layered R-NaFeO2-type

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structure (R-3m symmetry) in which the Fe sites are occupied by Mn and Ni ions, and the Na sites by Li ions (Fig. 1b). Two weak reflection peaks appeared between 21° and 25° agree well with the honeycomb ordering of Mn and Li ions in the transition metal layers in Li2MnO3 (space group C2/m).[38, 39] Fig. 2 shows the SEM images of the MnO2 templates and the Li1.13Ni0.30Mn0.57O2 samples. Typical γ-MnO2 nanothorn spheres with diameter of 5–7 µm are observed in Fig. 2a. The individual γ-MnO2 nanothorn sphere is comprised of numerous nanorods with diameter of 20–40 nm and length of about 2 µm. With γ-MnO2 as template, the 5

ACCEPTED MANUSCRIPT obtained Li1.13Ni0.30Mn0.57O2 particles remain almost the same spherical appearance as the template except for the increased diameter of nanorods. As shown in Fig. 2c, β-MnO2 particles have a rod-like morphology with diameter of 30–70 nm and length of about 3 µm. The Li1.13Ni0.30Mn0.57O2 particles prepared from β-MnO2 also show the

the MnO2 template incorporated with Ni and Li ions.

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nanorod morphology with increased diameter, which is because of the calcination of

Morphology and structure of the templates and the aimed products were further examined by TEM and HRTEM (Figs. 3 and 4). For γ-MnO2, TEM image further

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confirms the hedgehog-like morphology (Fig. 3a); while HRTEM shows clear lattice fringes, indicative of high crystallinity (Fig. 3c). The atomic fringe distance of 0.26

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nm in the HRTEM image can be fit to the (031) lattice spacing of γ-MnO2. For β-MnO2, nanorod morphology is observed (Fig. 3d, 3e); while HRTEM also reveals the highly crystalline structure, in which the fringe spacing of 0.21 nm corresponds to the (111) planes of β-MnO2. Fig. 4 shows the TEM and HRTEM images of the final Li1.13Ni0.30Mn0.57O2 product. It is observed that the morphology of the template is well

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reserved during the formation of Li1.13Ni0.30Mn0.57O2 (Fig. 4a and 4d). Close view of the nanorods in both 3D and 1D products shows that the diameter obviously increases (Fig. 4b and 4e), and the microstructure of the nanorods has been partially destroyed,

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which can be ascribed to the incorporation of Li and Ni ions into MnO2 template. In the HRTEM images, clear lattice fringes indicate the crystalline structure, in which

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(104) and (003) planes of rhombohedral LiMO2 (M = Ni, Mn, Co) planes can be identified according to the fringe spacing of γ-MnO2 and β-MnO2 derived products, respectively (Fig. 4c and 4f). Typical electrochemical performance of the Li1.13Ni0.30Mn0.57O2 electrode is

shown in Fig. 5. The cells were tested at a current density of 15 mA g−1 within the voltage range from 2.0 to 4.8 V. Two obvious voltage regions during the initial charge process are observed (Fig. 5a and 5b). For the first region, the voltage increases monotonically to about 4.5 V with a smooth slope. The second region is a plateau from 4.5 to 4.8 V with a rapid increase in capacity. Each voltage region originates 6

ACCEPTED MANUSCRIPT from different electrochemical reactions corresponding to the lithium extraction process. The first process is attributed to removal of Li ions from the cathode with concomitant oxidation of Ni2+ to Ni4+, while the second process is ascribed to removal of Li2O from the cathode.[40, 41] Further investigation of rate capability for 3D

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nanothorns and 1D nanorods conducted at different current densities is displayed Fig. 5. Fig. 5a shows the typical discharge curves of 3D nanothorn spheres at current densities ranging from 15 to 1000 mA g−1. The 3D nanothorn spheres deliver specific discharge capacities of 287, 270, 258, 246, 238, 158 and 109 mAh g−1 at current

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densities of 15, 30, 60, 100, 200, 500 and 1000 mA g−1, respectively. Fig. 5b depicts that the 1D nanorods also exhibit high discharge capacities of 275, 257, 248, 229, 209,

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131 and 69 mAh g−1 at discharge current densities of 15, 30, 60, 100, 200, 500 and 1000 mA g−1, respectively. Both the 3D nanothorn spheres and 1D nanorods show high reversible capacity at different current densities.

As shown in Fig. 6a and b, the 3D nanothorn spheres show a slight enhancement in specific capacity as compared with 1D nanorods, but we can see that their rate

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capability is improved. To give further comparison between the 3D nanothorns spheres and the 1D nanorods cathode materials, rate performance based on the volumetric capacity was investigated. The volumetric capacity was converted from

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gravimetric capacity and compacted density. The measured compacted densities are about 1.68 and 1.16 g cm−3 for nanothorn microspheres and nanorods, respectively,

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under the same electrode thickness of 55 µm.[42] As shown in Fig. 6c, the 3D microspheres show a remarkable improvement in volumetric capacity compared with 1D nanorods. The volumetric capacity of the 3D microspheres electrode is almost 1.5 times higher than that of 1D nanorods electrode under all discharge currents. To evaluate the structure stability of the cathode materials, the cells were charged/discharged at different current densities from 15 to 1000 mA g−1 each for 10 cycles, and then run back at 15 mA g−1 for 30 cycles. Obviously, the specific capacity can be recovered well to the initial capacity when the current density is tuned back to 15 mA g−1 for both 3D and 1D cathode materials, indicative of stable structure over 7

ACCEPTED MANUSCRIPT cycling. Fig. 6d depicts the outstanding cyclability of the nanothorn spheres and nanorods. At current densities of 15, 100 and 200 mAh g−1, the capacity retention ratios after 100 cycles are 93.2%, 93.3% and 95.6%, respectively, for the nanothorn spheres, whereas the corresponding ratios are 91.4%, 92.8% and 93.6% for the

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nanorods. The former shows better cyclability than the latter. Fig. 7a demonstrates the differential capacity dQ/dV curves of first cycle for both 3D nanothorn spheres and 1D nanorods electrodes to check clearly the redox peaks. For 3D nanothorn spheres electrode, two peaks at 3.7 and 4.5 V, corresponding to

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oxidation of Ni2+ and removal of Li2O, appear in oxidation process.[43] The peaks between 3.5 and 4.5 V in reduction process can be ascribed to reduction of Ni4+,[43]

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while the peak centered at 3.3 V is assigned to reduction of Mn4+ to Mn3+.[43] The 1D nanorods electrode shows similar peaks but a slight polarization as compared with the 3D electrode, suggesting that the 3D electrode exhibits better intercalation kinetics. For Li-rich cathode materials, medium discharge voltage variation is an important parameter during cycling due to phase transformation from layered to spinel structure,

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which indicates the rate of formation of spinel phase.[44-46] The values of medium discharge voltage are compared in Fig. 7b. The two electrodes both show voltage decay, and the shifts of medium discharge voltage are 0.272 V and 0.247 V,

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respectively. The comparable voltage shifts demonstrate that 1 D and 3D morphologies do not affect the phase transformation during cycling.

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Fig. 8 shows the EIS curves for 3D nanothorn spheres and 1D nanorods electrodes.

Both curves consist of one semicircle at high-to-medium frequency and an inclined line at low frequency. In the equivalent circuit), Rs is the internal resistance of the cell, Rct indicates related charge-transfer resistances at the electrode/electrolyte interface, CPE represents a non-ideal double-layer capacitor of non-homogeneous interface of electrolyte and cathode, and W is the diffusion-controlled Warburg impedance at the low frequency.[47, 48] Table 1 displays the fitted impedance parameters according to the equivalent circuit. The values of Rs, Rct and σw for 3D nanothorn sphere electrode are 12 Ω cm-2, 188 Ω cm-2 and 66 Ω cm-2 s-1/2, whereas the values for 1D nanorod 8

ACCEPTED MANUSCRIPT electrode are 8 Ω cm-2, 201 Ω cm-2 and 63 Ω cm-2 s-1/2. The lower Rct and higher σw further confirm that the 3D nanothorn spheres electrode exhibits better intercalation kinetics, which is in accordance with the dQ/dV results. Our electrochemical results demonstrate that the 3D nanothorn spheres are more

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favorable to act as commercial cathode material than the 1D nanorods. Actually, the 3D nanothorn spheres are comprised of numerous nanorods. On one hand, they exhibit fast Li-ion diffusion like typical nanosized cathode materials; on the other hand, they have high compacted density like the common bulk materials. Therefore,

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both the rate capability and the capacity especially volumetric capacity are desirable, which means that the Li-rich cathode materials with microstructure of 3D nanothorn

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spheres are promising for commercial application. 4. Conclusions

Lithium-rich and cobalt-free Li1.13Ni0.30Mn0.57O2 3D nanothorn spheres and 1D nanorods have been successfully synthesized via a self-template method with

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hydrothermally synthesized γ-MnO2 nanothorn spheres and β-MnO2 nanorods as templates. The obtained Li1.13Ni0.30Mn0.57O2 products can well reserve the morphology of MnO2 templates. Electrochemical measurements show that both 3D and 1D cathode materials have excellent rate performance and cyclability. As

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compared with the 1D nanorods, the 3D microspheres electrode exhibits better rate capability and remarkably higher volumetric capacity, indicating that the 3D

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structured Li1.13Ni0.30Mn0.57O2 is more favorable for commercial application in Li-ion batteries. The present work not only develops a topochemical method that can be used to synthesize controllable nanostructured oxide materials, but also provide a feasible strategy

to

design

commercial

Li-rich

cathode

materials

with

excellent

electrochemical performance.

Acknowledgements This work was supported by Hubei Provincial Natural Science Foundation 9

ACCEPTED MANUSCRIPT (Grant Nos. 2014CFB588), and the Natural Science Foundation of China (Grant Nos. 11304092, 11305056, 51371079), and Open Foundation of Hubei Collaborative Innovation Center for High-efficiency Utilization of Solar Energy (Grant Nos. HBSKFMS2014015), and Talents of high level scientific research foundation of

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Hubei University of technology (Grant Nos. BSQD13031); In addition, the authors thank Analytical and Testing Center of Huazhong University of Science and Technology for XRD, TG-DTA, SEM and TEM measurements.

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ACCEPTED MANUSCRIPT [47] A. Y. Shenouda and H. K. Liu, J. Electrochem. Soc. 157 (2010) A1183-A1187. [48] Y. S. Bai, X. Y. Wang, X. Y. Zhang, H. B. Shu, X. K. Yang, B. N. Hu, Q. L. Wei, H. Wu and Y. F. Song,

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Electrochim. Acta 109 (2013) 355-364.

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ACCEPTED MANUSCRIPT Captions of figures Scheme 1 Schematic illustration for preparation of Li1.13Ni0.30Mn0.57O2 3D nanothorn spheres and 1D nanorods. Fig. 1 XRD patterns for (a) MnO2 templates and (b) Li1.13Ni0.30Mn0.57O2 products (1D

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nanorods and 3D nanothorn spheres). Fig. 2 SEM images for (a) γ-MnO2, (b) Li1.13Ni0.30Mn0.57O2 nanothorn spheres prepared from γ-MnO2, (c) β-MnO2, (d) Li1.13Ni0.30Mn0.57O2 nanorods prepared from β-MnO2.

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Fig. 3 TEM and HRTEM images for (a, b, c) the as-prepared γ-MnO2, and (d, e, f) β-MnO2.

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Fig. 4 TEM and HRTEM images of Li1.13Ni0.30Mn0.57O2: (a, b, c) nanorods prepared from β-MnO2, and (d, e, f) nanothorn spheres prepared from γ-MnO2. Fig. 5 Charge-discharge curves of Li1.13Ni0.30Mn0.57O2: (a) 3D nanothorn spheres and (b) 1D nanorods.

Fig. 6 Specific capacities over cycling at various current densities for 3D nanothorn

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spheres and 1D nanorods: (a, b) gram capacities; (c, d) volumetric capacities. Fig. 7 (a) dQ/dV curves of first cycle, and (b) comparison of medium discharge voltage for 3D nanothorn spheres and 1D nanorods electrodes.

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Fig. 8 EIS curves for 3D nanothorn spheres and 1D nanorods electrodes.

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Scheme 1 Jiang et al.

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Figure 1 Jiang et al.

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Figure 2 Jiang et al.

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Figure 3 Jiang et al.

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Figure 4 Jiang et al.

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Figure 5 Jiang et al.

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Figure 6 Jiang et al.

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ACCEPTED MANUSCRIPT 1200 800

3D 1D

400

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dQ/dV (mAh/g V)

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0

-400

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3D 1D

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3.0 3.5 4.0 4.5 + Potetial (V vs. Li/Li )

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20 30 Cycle Number

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Medium dsicharge voltage (V vs. Li/Li

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Figure 7 Jiang et al.

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-300 Rct

Rs

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CPE

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3D 1D

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-Z`` (Ω)

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Figure 8 Jiang et al.

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ACCEPTED MANUSCRIPT Table 1

Rs (Ω cm-2)

Rct (Ω cm-2)

σw (Ω cm-2 s-1/2)

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188

66

1D

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201

63

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Samples

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ACCEPTED MANUSCRIPT Highlights • A simple self-template method to fabricate 3D nanothorn spheres and 1D nanorods Li-rich cathode material 0.3Li2MnO3·0.7LiNi0.5Mn0.5O2 was developed.

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• The 3D microspheres electrode exhibits better rate capability and remarkably higher volumetric capacity than 1D nanorods, indicating that the 3D structured 0.3Li2MnO3·0.7LiNi0.5Mn0.5O2 is more favorable for commercial application in

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LIBs..