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ELECTROSPUN TiO2-BASED NANOMATERIALS FOR

ENERGY STORAGE

XIAOYAN LI

Ph.D

The Hong Kong Polytechnic University

2015

The Hong Kong Polytechnic University

Department of Mechanical Engineering

Electrospun TiO2-based Nanomaterials for Energy

Storage

Xiaoyan LI

A thesis submitted in partial fulfillment of the requirement for the

degree of Doctor of Philosophy

May 2015

CERTIFICATE OF ORIGINALITY

I hereby declare that this thesis is my own work and that, to the best of my

knowledge and belief, it reproduces no material previously published or written,

nor material that has been accepted for the award of any other degree or

diploma, except where due acknowledgement has been made in the text.

(Signed)

Xiaoyan LI (Name of Student)

I

Abstract

Energy storage is of great importance in supporting the wide application of two

renewable clean energy sources including solar and intermittent wind energy.

Batteries, such as Ni-MH, Ni-Cd, lead-acid, and lithium ion batteries (LIBs), become

the leading candidates for electrical vehicles (EV) and possible for practical

applications to store electrical energy in the form of chemical energy. Among of these

batteries, LIBs outperform others due to their high volumetric and gravimetric energy

densities, low self-discharge, wide temperature working window, long cycle life and

no memory effect. These benefits make them gain remarkable success in portable

electronics, such as notebooks, mobile phones, and digital cameras. However, in order

to satisfy the demand of new markets in EVs and hybrid electric vehicles (HEV),

LIBs are required with higher energy and power densities, higher safety, longer

durability and lower cost. Titanium dioxide (TiO2) has emerged as a promising anode

material for LIBs due to its low cost, environmental friendliness, and structural

stability during lithium insertion/deinsertion processes. However, poor electron

transport limits its practical application, especially at high current densities. This

thesis focuses mainly on developing novel TiO2-based anodes with excellent

electrochemical performance in terms of high capacity, excellent rate capability and

long cycle life by using electrospinning technique.

By a simple coaxial electrospinning technique combined with subsequent

calcination treatment, one-dimensional porous TiO2-carbon composite nanofibers

(ODPTCNs) with plentiful pores as storage regions and high conductivity for the

rapid transportation of both electrons and lithium ions was developed. The novel

II

ODPTCNs show a remarkable specific reversible capacity of ~806 mAh g-1 and a

high volumetric capacity of ~1.2 Ah cm-3, and maintain the capacity of ~680 mAh g-1

after 250 cycles at a current density of 100 mA g-1 and exhibit an exceptional

discharge rate capability of 5 A g-1 while retaining a capacity of ~ 260 mAh g-1 after

1600 cycles.

In order to increase the energy density of TiO2 materials, MnO2 with a theoretical

capacity approximately triple higher than that of the graphite anode was decorated

with TiO2 to form a core/shell TiO2-MnO2/MnO2 heterostructures by combining an

electrospinning technique with a hydrothermal reaction. The large surface area of the

resulting materials offers sufficient electrode/electrolyte interface to promote the

charge-transfer reactions, which yields a better rate capability. The porous structure of

TiO2-MnO2/MnO2 nanofibers not only facilitates Li-ion access, but also

accommodates large volumetric expansion during the charging/discharging processes,

resulting in an excellent cycle performance. This material delivers a high reversible

capacity of 891 mAh g-1 at the first cycle and maintains the capacity of 888 mAh g-1

after 50 cycles at the current density of 0.1 A g-1; it also shows a remarkable rate

capability of 2 A g-1 while retaining a capacity of 185 mAh g-1 after 500 cycles.

Nanotube structure is also a promising structure for LIBs due to its typical

advantages such as one-dimensional (1D) and hollow structures, which can improve

the electrochemical performance of the electrodes. A novel nanoarchitecture

constructed by Co3O4 nanoparticles encapsulated in the porous binary Co3O4-TiO2

nanotubes was exploited by simple electrospinning and hydrolysis method, followed

by a calcination. The prepared hybrid nanomaterials show enhanced electrochemical

performance in terms of remarkable specific reversible capacity of ~1007 mAh g-1 at

III

the current density of 0.1 A g-1 after 160 cycles, and an extraordinarily stable capacity

retention of 673 mAh g-1 after 2000 cycles at a current density of 3 A g-1.

A novel Sn-nanoparticle in-situ insertion approach by lithiation was exploited to

address the problem of large expansion of Sn and thus achieve improved performance.

Sn nanoparticles are inserted into the pores of highly stable titanium dioxide-carbon

(TiO2-x-C) nanofiber substrates by lithiation, which can effectively localize the Sn

nanoparticles. During lithiation, Sn reacts with Li to form Li4.4Sn alloy accompanying

by large volume change, and thus insert into the pores surrounding the initial Sn

nanoparticles in the TiO2-x-C nanofibers. However, the Li4.4Sn alloy cannot recover to

the original Sn nanoparticle with diameter about 150 nm after delithiation due to the

surface absorption force between Sn nanoparticle and TiO2-x-C substrate, separating

many smaller Sn nanoparticles in the pores of TiO2-x-C nanofiber. These detached Sn

nanoparticles remained in the pores, which were able to accommodate their expansion,

thus yielding a very long cycle life. The lithiation-induced size reduction of the Sn

nanoparticles leads to their uniform distribution in the fibers, which enhances the

electrochemical activation. Batteries containing these porous TiO2-x-C-Sn nanofibers

exhibit a high specific capacity of 957 mAh g-1 after 200 cycles at 0.1 A g-1 and can

cycle more than 10,000 times at 3 A g-1 while retaining more than 82.3 % of their

capacity (0.177 % decrease per 100 cycles), which represents the longest cycling life

of Sn-based anodes for LIBs so far.

IV

Publications

Journal papers

1. Xiaoyan Li, Yuming Chen, Limin Zhou, Yiu-Wing Mai, Haitao Huang.

Exceptional electrochemical performance of porous TiO2-carbon nanofibers

for lithium ion battery anodes. J. Mater. Chem. A, 2014, 2, 3875-3880.

2. Xiaoyan Li, Yuming Chen, Haimin Yao, Xiangyang Zhou, Juan Yang, Haitao

Huang, Yiu-Wing Mai and Limin Zhou. Core/shell TiO2-MnO2/MnO2

heterostructure anodes for high-performance lithium-ion batteries. RSC Adv.,

2014, 4, 39906-39911.

3. Xiaoyan Li, Yuming Chen, Hongtao Wang, Haimin Yao, Haitao Huang, Yiu-

Wing Mai and Limin Zhou. Inserting tin nanoparticles into the pores of TiO2-

x-C nanofibers by lithiation. In process.

4. Xiaoyan Li, Yuming Chen, Jing Hu, Haimin Yao, Haitao Huang, Yiu-Wing

Mai and Limin Zhou. One-dimensional nanoarchitectures constructed by

Co3O4 nanoparticles encapsulated in the porous binary Co3O4-TiO2 nanotubes

as anode materials with superior lithium-ion storage performance. In process.

5. Yuming Chen, Xiaoyan Li, Xiangyang Zhou, Haimin Yao, Haitao Huang,

Yiu-Wing Mai and Limin Zhou. Hollow-tunneled Graphitic Carbon

Nanofibers Through Ni-diffusion-induced Graphitization as High-performance

Anode Materials. Energy Environ. Sci., 2014, 7, 2689-2696.

6. Yuming Chen, Xiaoyan Li, Kyu-Sung Park, Jianhe Hong, Jie Song, Limin

Zhou, Yiu-Wing Mai, Haitao Huang, and John B. Goodenough. Sulfur

V

Encapsulated in Porous Hollow CNTs@CNFs for High-Performance Lithium-

Sulfur Batteries. J. Mater. Chem. A, 2014, 2, 10126-10130.

7. Yuming Chen, Xiaoyan Li, Kyusung Park, Jie Song, Jianhe Hong, Limin

Zhou, Yiu-Wing Mai, Haitao Huang, and John B. Goodenough. Hollow

Carbon-Nanotube/Carbon-Nanofiber Hybrid Anodes for Li-Ion Batteries. J.

Am. Chem. Soc., 2013,135, 16280-16283.

Book Chapter

Yuming Chen, Xiaoyan Li, Limin Zhou, Yiu-Wing Mai, and Haitao Huang. High-

Performance Electrospun Nanostructured Composite Fiber Anodes for Lithium-ion

Batteries, Chapter 21 in MultiFunctionality of Polymer Composites: Challenges and

New Solutions, Editors by Klaus Friedrich and Ulf Breuer, Elsevier, 2015, 662-689.

Conference presentation

1. Xiaoyan Li, Yuming Chen, Hongtao Wang, Haimin Yao and Limin Zhou. Tin

Nanoparticles Encapsulated in the Porous TiO2-C Nanofibers as Long-term

Anodes for Lithium-ion Batteries. EMRS Spring, 2015, Lille.

2. Xiaoyan Li, Yuming Chen and Limin Zhou. TiO2-C Nanofibers with Nano-

Sn Particles for Lithium Ion Batteries. ICNN, 2015, Greece.

3. Xiaoyan Li, Yuming Chen and Limin Zhou. Electrospun Nanostructured

Composite Fiber Anodes for Li-ion Batteries. EMN, 2015, Turkey.

4. Weiqun Li, Xiaoyan Li, Jingjing Tang, Haimin, Yao and Limin Zhou. Optimal

Si-nanoparticle-based Nanocomposite Structure with Long-term Stability for

http://pubs.acs.org/action/doSearch?action=search&author=Chen%2C+Y&qsSearchArea=authorhttp://pubs.acs.org/action/doSearch?action=search&author=Li%2C+X&qsSearchArea=authorhttp://pubs.acs.org/action/doSearch?action=search&author=Park%2C+K&qsSearchArea=authorhttp://pubs.acs.org/action/doSearch?action=search&author=Song%2C+J&qsSearchArea=authorhttp://pubs.acs.org/action/doSearch?action=search&author=Hong%2C+J&qsSearchArea=authorhttp://pubs.acs.org/action/doSearch?action=search&author=Zhou%2C+L&qsSearchArea=authorhttp://pubs.acs.org/action/doSearch?action=search&author=Zhou%2C+L&qsSearchArea=authorhttp://pubs.acs.org/action/doSearch?action=search&author=Mai%2C+Y&qsSearchArea=authorhttp://pubs.acs.org/action/doSearch?action=search&author=Huang%2C+H&qsSearchArea=authorhttp://pubs.acs.org/action/doSearch?action=search&author=Goodenough%2C+J+B&qsSearchArea=author

VI

Li-ion Batteries. AESP7, 2015, Madrid.

5. Xiaoyan Li, Yuming Chen, Haitao Huang, Yiu-Wing Mai and Limin Zhou.

High-performance Core-shell TiO2-MnO2 Composite Nanofibers Lithium Ion

Battery Anodes. ICNN, 2014, Hong Kong.

6. Xiaoyan Li, Yuming Chen, Yiu-Wing Mai and Limin Zhou. Electrospun

Carbon-based Nanofiber/Nanotube Anode Materials for Lithium-ion Batteries.

PSE, 2014, Beijing.

VII

Acknowledgements

I would like to express my deepest appreciation to my supervisor, Prof. Limin Zhou,

who led me into an interesting research topic and provided constant guidance,

constructive suggestion, and patient help. Without his continuous support and

encouragement during the past three years of my PhD study, I could not complete this

thesis.

I would like to thank Prof. Yiu-Wing Mai in the University of Sydney, Prof. Haitao

Huang and Prof. Haimin Yao in The Hong Kong Polytechnic University for their kind

supports, constructive discussions, and comments on my research.

I would like to thank Prof. Xiaodong Chen in the Nanyang Technological

University for being my temporary supervisor to guide and support me during my

visit under the exchange program, which greatly helps me a lot in my current and

further research.

Besides, my genuine thanks also go to all the past and present group members

including Dr. Zhouguang Lu, Dr. Keyu Xie, Dr. Min Guo, Dr. Yuming Chen, Miss

Jing Hu, Miss Jingjing Tang, Mr. Weiqun Li and Mr. Qian Wang for their advices,

inspirations, encouragements and helps. Their friendships are one of the treasures that

I have got during the past few years.

I would also like to thank The Hong Kong Polytechnic University for its financial

support to my study.

Finally, I am heartily thankful to my beloved families for their unconditional love,

forever understanding, encouragements, and continuous supports.

VIII

This thesis is dedicated to all of them.

IX

Table of Contents

Abstract ........................................................................................................................................ I

Publications ............................................................................................................................... IV

Acknowledgements .................................................................................................................. VII

Table of Contents ....................................................................................................................... IX

List of figures ............................................................................................................................ XI

List of tables .........................................................................................................................XVIII

List of Abbreviations .............................................................................................................. XIX

Chapter 1 Introduction ................................................................................................................. 1

1.1 Electrospinning ................................................................................................................ 1

1.1.1 The principle of electrospinning ............................................................................... 2

1.1.2 Influence parameters of spinning technology on fiber formation ................................ 2

1.2 Lithium-ion batteries ....................................................................................................... 6

1.2.1 Cathode ................................................................................................................... 10

1.2.1.1 Hybrid cathode based on carbon and olivine compound ................................. 10

1.2.1.2 Hybrid cathode based on carbon and silicate compound ................................. 15

1.2.2 Anode ..................................................................................................................... 18

1.2.2.1 Carbonaceous anode ........................................................................................ 18

1.2.2.2 Hybrid metal oxide anode ............................................................................... 29

1.2.2.3 Hybrid alloy anode .......................................................................................... 46

1.3 Objectives and outline ................................................................................................... 53

Chapter 2 Porous TiO2 nanofibers with conductive carbon....................................................... 57

2.1 Introduction ................................................................................................................... 57

2.2 Coaxial electrospinning technique ................................................................................ 58

2.3 Characterization of the ODPTCNs ................................................................................ 60

2.4 Li-ion storage in ODPTCNs .......................................................................................... 69

2.5 Summary ....................................................................................................................... 77

Chapter 3 A core/shell TiO2-MnO2/MnO2 heterostructure as LIB anodes ................................ 78

3.1 Introduction ................................................................................................................... 78

3.2 Preparation of core/shell TiO2-MnO2/MnO2 heterostructure ........................................ 79

X

3.3 Characterisation of core/shell TiO2-MnO2/MnO2 heterostructure ................................. 80

3.4 Advantages of core/shell TiO2-MnO2/MnO2 heterostructure anodes ............................ 89

3.5 Summary ....................................................................................................................... 96

Chapter 4 Porous binary Co3O4-TiO2 nanotubes as anodes for LIBs ........................................ 98

4.1 Introduction ................................................................................................................... 98

4.2 Formation of Co3O4@TiO2-Co3O4 nanoarchitectures ................................................... 99

4.3 Characterization of Co3O4@TiO2-Co3O4 nanoarchitectures ....................................... 101

4.4 Li-ion storage in Co3O4@TiO2-Co3O4 nanoarchitectures ........................................... 107

4.5 Summary ..................................................................................................................... 116

Chapter 5 In-situ insertion of Sn nanoparticles into pores of TiO2-x-C nanofibers .................. 118

5.1 Introduction ................................................................................................................. 118

5.2 Fabrication and characterizations of TiO2-x-C-Sn nanofibers ...................................... 119

5.3 In-situ TEM observation during charging/discharging processes ............................... 127

5.4 Merits of size decrease of Sn nanoparticles in the TiO2-x-C nanofibers ...................... 129

5.5 Virtues of TiO2-x-C-Sn anodes .................................................................................... 131

5.6 Summary ..................................................................................................................... 140

Chapter 6 Conclusion and Further work ................................................................................. 142

6.1 Conclusion ................................................................................................................... 142

6.2 Future work ................................................................................................................. 144

6.2.1 Core/shell C-MOx/C-TiO2 composite nanofibers ................................................. 146

6.2.2 C/TiO2-MOx-CNT core/shell composite nanofibers ............................................. 148

6.2.3 C-TiO2 coaxial nanofibers decorated with MOx nanorod arrays .......................... 150

Bibliography ............................................................................................................................ 152

XI

List of figures

Figure 1.1 Schematic illustration of a typical electrospinning setup.

Figure 1.2 Schematic illustration of a typical Li-ion battery.

Figure 1.3 (a) Schematic of the synthesis of LiFePO4/CNT/C composite nanofibers,

(b) SEM image of the prepared composite nanofibers, and (c) cycling performance of

LiFePO4/CNT/C composite nanofibers, LiFePO4/C composite nanofibers and

LiFePO4 at 0.05C.

Figure 1.4 (a) TEM image of Li2Mn0.94Cr0.06SiO4/C composite nanofibers. (b)

Cycling performance of Li2MnSiO4 powder, Li2Mn0.94Cr0.06SiO4 powder, and

Li2Mn0.94Cr0.06SiO4/C composite nanofibers at a current density of C/20.

Figure 1.5 (a) Schematic of material processing. (b-f) TEM images of the typical step

of Ni diffusion. (g) The charge/discharge profile of the resulting materials at 0.1 A g-1.

Figure 1.6 (a) Schematic of the synthesis of activated hollow CNT-CNF hybrid

material. (b) SAED pattern, TEM image, line profile of d-spacing of the prepared

hybrid carbons. (c) The electrochemical performance of the resulting hybrid materials.

Figure 1.7 (a) Schematic illustration of the synthesis process of the porous

LTO/carbon (PLTO/C) composite nanofibers, (b) cycling performance of PLTO/C

hybrid, LTO/C composite and LTO, and (c) SEM image of the PLTO/C composite.

Fig. 1.8 Structure of (a) anatase TiO2, (b) rutile TiO2 and (c) TiO2(B).

XII

Figure 1.9 (a) SEM image of indicate the cross-section of the carbon nanofibers. (b)

TEM image of 3D-TiO2/C samples after calcination; the insets are the magnified

images and the SAED patterns. (c) Discharge-charge curves of the 3D-TiO2/C

electrode at various rates (1 C (4th cycle), 2 C (9th cycle), 5 C (14th cycle), 10 C (19th

cycle), 20 C (24th cycle) and 30 C (29th cycle)). (d) Cycling performance of the 3D-

TiO2/C electrode at various rates (1-30 C). (e) Cycling performance of the 3D-TiO2/C

electrode at the rate of 30 C.

Figure 1.10 (a) HRTEM image taken from the outer edges of a TiO2:RuO2 sphere; (b)

corresponding schematic illustration of the self-wired path of deposited RuO2

nanoparticles. (c-f) Rate performance. Variation of discharge (■) charge (●) capacities

versus cycle number for different anatase electrodes cycled at different rates between

voltage limits of 1 and 3 V.

Figure 1.11 (a) Schematic showing the fabrication process of the composite of Co3O4

nanoparticles in PCNFs, (b) TEM image of Co3O4/PCNF composite, and (c) cycling

performance of Co3O4/PCNF composite at 0.1 A g-1.

Figure 1.12 (a) SEM image and its corresponding element mapping (orange for

carbon, green for iron, yellow for oxygen and green for silver) of the Ag-Fe2O3/CNF

composite. (b) Cycling performance of the Ag-Fe2O3/CNF composite.

Figure 1.13 (a) Schematic of the preparation process, (b) cycling performance, (c, d)

TEM images of the nanocavity-engineered Si/carbon composite.

Figure 1.14 The outline of research plan.

XIII

Figure 2.1 Schematic illustration of the typical coaxial electrospinning technique

used to prepare the PS/Ti(OiPr)4/PMMA composite nanofibers.

Figure 2.2 TGA analysis of (A) PS in Ar and (B) ODPTCNs in flowing air.

Figure 2.3 (A,B,C) FESEM, (D,E) TEM-SEI, (F) TEM, and (G) HRTEM images of

the ODPTCNs.

Figure 2.4 (A) X-ray diffraction patterns of the ODPTCNs. (B) Ti 2p, (C) C1s, and

(D) O 1s high-resolution XPS spectrum. (E) EELS of C-K, Ti-L2,3, and O-K edges

taken across the ODPTCNs interfaces. (F) Ramam spectrum of the ODPTCNs.

Figure 2.5 (A) Pore size distribution and (B) nitrogen adsorption-desorption

isotherms of the ODPTCNs.

Figure 2.6 Electrochemical properties of the ODPTCNs as anode electrodes for Li-

ion batteries. (A) Charge-discharge profiles of the ODPTCNs and TiO2 nanofibers at

a current density of 100 mA g-1 between 3 and 0 V versus Li+/Li. (B) Cycle

voltammograms of the ODPTCNs in LiPF6 with Li as counter and reference electrode

at a scan rate of 0.1 mV/s. (C) Cycling performance of the ODPTCNs at different

current densities between 3 and 0 V versus Li+/Li. (D) Cycling performance of TiO2

nanofibers at a current density of 100 mA g-1. (E) Rate capability of the ODPTCNs.

(F) Nyquist plots of the ODPTCNs and TiO2 electrodes with an amplitude of 5 mV in

the frequency range from 100 kHz to 10 mHz.

Figure 2.7 Charge-discharge curves of the ODPTCNs (A) and TiO2 nanofibers (B)

electrodes obtained at different current densities in the voltage range of 1-3V.

XIV

Figure 2.8 Schematic diagram illustrating the Li+ insertion and deinsertion processes

inside the ODPTCNs.

Figure 3.1 Schematic illustration of the preparation of the core/shell TiO2-

MnO2/MnO2 heterostructure.

Figure 3.2 (a, b, c, d) XRD patterns, XPS fully scanned spectra, EELS spectra, EDS

spectrum of core-shell TiO2-MnO2/MnO2 heterostructures. (e) XPS narrow scan

spectra of Mn 2p for TiO2-MnO2/MnO2 heterostructures. (f) EELS spectra of Mn-L

edge, and (g) N2 adsorption-desorption isotherms of TiO2-MnO2/MnO2

heterostructures and corresponding pore-size distribution.

Figure 3.3 (a) SEM image of porous TiO2-carbon nanofibers. (b) SEM and (c, d)

TEM-SEI images of core/shell TiO2-MnO2/MnO2 heterostructures.

Figure 3.4 (a, b) TEM and HRTEM images of TiO2-carbon nanofiber and (c, d, e)

TEM and HRTEM images of the core/shell TiO2-MnO2/MnO2 heterostructure.

Figure 3.5 (a) Dark-field TEM image and (b-d) corresponding scanning line of

core/shell TiO2-MnO2/MnO2 heterostructures.

Figure 3.6 Electrochemical properties of the core/shell TiO2-MnO2/MnO2

heterostructure anodes for Li-ion batteries. (a-c) The first three charge-discharge

curves of (a) TiO2, (b) MnO2, and (c) TiO2-MnO2/MnO2 at 0.1 A g-1. (d) CV curve of

the core/shell TiO2-MnO2/MnO2 at a scan rate of 0.1 mV/s. (e) The rate performance

at various current densities and (f) Cycling performance of core/shell TiO2-

MnO2/MnO2 heterostructure anodes at 0.1 and 2 A g-1. (g) Cycle performance of TiO2

and MnO2 at 0.1 A g-1.

XV

Figure 4.1 Schematic illustration of the formation of the porous Co3O4@TiO2-Co3O4

heterostructure. (a) Co-CNFs, (b) Co/CNFs-TiO2, (c) porous Co3O4@TiO2-Co3O4

heterostructure after annealing in air at 500 °C for 5 h.

Figure 4.2 (a,b) TEM and HRTEM images of Co-CNFs, (c) TEM-BF, (d) TEM-SEI,

(e) TEM, and (f) HRTEM images of Co3O4@TiO2-Co3O4 heterostructure.

Figure 4.3 (a) Dark-field TEM image and (b)-(d) corresponding scanning line of

Co3O4@TiO2-Co3O4 heterostructure.

Figure 4.4 (a) XRD pattern (b) pore size distribution of Co3O4@TiO2-Co3O4

heterostructure.

Figure 4.5 XPS spectra of Co3O4@TiO2-Co3O4 architecture: (a) full spectrum (b) Ti

2p spectrum (c) Co 2p spectrum (d) O 1s spectrum.

Figure 4.6 (a) Cyclic voltammograms at a scan rate of 0.1 mV s-1. (b)

Charge/discharge curves of Co3O4@TiO2-Co3O4 architecture at a current density of

0.1 A g-1. (c) Charge/discharge curves of Co3O4 nanoparticles at 0.1 A g-1. (d)

Cyclability and Coulombic efficiency of Co3O4@TiO2-Co3O4 architecture at 0.1 A g-1.

(e) Cycling performance of Co3O4 nanotubes at 0.1 A g-1. (f) The effect of Co(Ac)2

loading on the capacity of the resulting materials after 1000 cycles at a current density

of 3 A g-1. (g) Comparison of the rate capabilities of different Co3O4-based electrodes.

(h) Cycling performance of Co3O4@TiO2-Co3O4 architecture at 3 A g-1.

Figure 4.7 (a) Dark TEM image (b-d) mapping of Co, O and Ti of Co3O4@TiO2-

Co3O4 after 1000 cycles.

XVI

Figure 5.1 (a) The formation of highly stable porous TiO2-x-C-Sn composite

nanofibers and their participation in the charge-discharge process, (b) Morphological

changes of the TiO2-x-C-Sn nanofiber and Sn nanoparticle electrodes during cycling.

Figure 5.2 (a) FESEM (b) TEM-SEI (c) TEM-BF (d) and (e) TEM (f) HRTEM

images of TiO2-x-C-Sn nanofibers.

Figure 5.3 (a) Typical XRD pattern, (b) the pore size distribution, (c) Nitrogen

adsorption-desorption isotherm, (d) Raman spectrum, (e) an EDS spectrum of highly

stable porous TiO2-x-Sn-C composite nanofibers.

Figure 5.4 (a) TEM-BF image and (b) its corresponding EDS elemental line scanning

of the TiO2-x-C-Sn composite nanofiber.

Figure 5.5 (a) Typical XPS spectrum, (b-e) XPS of spectra of the porous TiO2-x-C-Sn

composite nanofibers (b) Sn 3d spectrum (c) Ti 2p (d) C 1s spectrum (e) O 1s

spectrum.

Figure 5.6 Typical morphological evolutions of Sn nanoparticles in the highly stable

porous TiO2-x-C nanofiber. (a) The first charging process (b) the first discharging

process (c) the second charging/discharging steps (d) the fifth charging/discharging

steps.

Figure 5.7 TEM and HRTEM images of the highly stable porous TiO2-x-C-Sn

composite nanofiber electrode. (a) and (c) after 1 and 1000 charge-discharge cycles,

(b) and (d) corresponding line scanning of (a) and (c).

XVII

Figure 5.8 Nyquist plots of the highly stable porous TiO2-x-C-Sn composite nanofiber

electrode after 1 and 1000 cycles in the frequency range from 100 kHz to10 mHz.

Figure 5.9 Electrochemical performance of the highly stable porous TiO2-x-C-Sn

composite nanofiber electrodes. (a) charge-discharge profile, (b) CV curves, (c)

cycling performance of TiO2-x-C-Sn, TiO2-C and TiO2 nanofibers at the rate of 0.1 A

g-1, (d) rate and cycling performance of the porous TiO2-x-C-Sn composite nanofibers,

(e) cycling performance of TiO2 at the current density of a current density of 3 A g-1.

Figure 6.1 Ragone plot showing the advantage of hybrid supercapacitors.

Figure 6.2 Schematic of fabricating shell/core C-TiO2/C-MOx composite nanofibers

with highly capacitive C-MOx encapsulated inside hollow and porous C-TiO2

nanofibers by co-electrospinning method. Here C refers to the residual carbon from

the thermal decomposition of the organic polymer phases and MOx refers to NiO,

Fe3O4 or their uniform composite NiO-Fe3O4.

Figure 6.3 Schematic showing the protocol for synthesis of C/TiO2-MOx-CNT

core/shell composite nanofibers.

Figure 6.4 Schematic showing the process for fabricating C-TiO2 coaxial-nanofibers

with MOx nanorod arrays decorated on the surface.

XVIII

List of tables

Table 1.1 Influence of injection system and geometry of collector on fiber

morphology.

Table 1.2 The function of some typical polymers on the resulting materials.

Table 1.3 Electrochemical capacity of carbon-based LIB cathodes.

Table 1.4 Electrochemical capacity of carbonaceous anodes for LIBs.

Table 2.1 Comparison of capacities for various TiO2-based anode electrodes.

Table 3.1 Weight and atomic concentrations of Mn and Ti elements in TiO2-

MnO2/MnO2 nanofibers.

Table 3.2 Comparison of the capacities for various MnO2-based electrodes.

Table 5.1 EDS composite profile of TiO2-x-C-Sn nanofiber.

XIX

List of Abbreviations

1D One dimensional

3D Three dimensional

BET Brunauer Emmett and Teller

C Carbon

CNFs Carbon nanofibers

CNTs Carbon nanotubes

CVD Chemical vapor deposition

CV Cyclic voltametric

DMF N,N-Dimethylmethanamide

DMC Dimethyl carbonate

EC Ethylene carbonate

EV Electronic vehicles

EIS Electrochemical impedance spectroscopy

EDS Electron diffraction spectroscopy

EELS Electron energy loss spectroscopy

HEV Hybrid vehicles

HRTEM High Resolution Transmission electron microscopy

KMnO4 Potassium permanganate

LIBs Lithium ion batteries

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XX

LiFePO4 Lithium iron phosphate

LTO Li4Ti5O12

ODPTCNs One dimensional porous TiO2-carbon nanofibers

PS Polystyrene

PVDF Polyvinylidene fluoride

PAN Polyacrylonitrile

PVA Polyvinylalcohol

PVP Polyvinyl pyrrolidone

PEO Poly (ethylene oxide)

PLLA Poly-L-lactic acid

PMMA Poly (methyl methacrylate)

SEI Solid electrolyte interphase

SAED Selected area electron diffraction

SEM Scanning electron microscopy

TEM Transmission electron microscopy

TGA Thermogravimetric analysis

TBT Tributyltin

TBOT Tetrabutylorthotitanate

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

1

Chapter 1 Introduction

1.1 Electrospinning

Electrospinning is an advanced, powerful, and simple technology to synthesis 1D

nanostructured materials with fiber diameters of tens of nanometers up to

micrometers.1 Electrospinning technique that was first exploited in 1934 has been

shown to fabricate a number and variety of uniform 1D nanomaterials, such as

polymers, organic and inorganic composites and their hybrids.2 Polymer nanofibers

can be produced by electrospinning corresponding polymer precursors which are

dissolved in suitable solvent.3 Ceramic or metal nanofibers can be prepared by heating

electrospun composite precursors that contain organometallic products or metal atoms

in an oxidizing or reducing atmosphere. Carbon nanofibers can be synthesized from

the conversion of hydrocarbon nanofibers such as polyacrylonitrile (PAN) and

polyvinylalcohol (PVA) under high temperature in an inert atmosphere. Their hybrid

such as metal-carbon and ceramic-carbon can also be obtained from their

corresponding mixed precursors followed by a one-step or multi-step heat treatment.

To date, a wide variety of electrospun materials have been prepared by

electrospinning technique including metal oxides/ceramics1b, 1c, 3-4 such as TiO2, NiO,

CuO, Ag, La2O3, Y2O3, and LaOCl, mixed metal oxides5 such as LiCoO3, NiFe2O4,

Li4Ti5O12 and LiMn2O4, hybrids6 such as carbon-Co3O4, carbon-SnOx, carbon-TiO2,

MnO2-TiO2, Nylon-6/gelatin, and polymers7 such as PVA, PAN, and polyvinyl

pyrrolidone (PVP). Electrospinning shows a strong and extreme advantage in

fabricating many types of hierarchical nanostructures that are difficult to be produced

through other approaches.

2

1.1.1 The principle of electrospinning

The electrospinning technique is a unique method based on electrostatic forces to

synthesize fibers from polymer solutions or melts with larger surface areas as

compared with conventional spinning process.8 An electrospinning unit generally

consists of three major parts that include a high voltage power supply, a spinneret, and

a collector (Figure 1.1).9 The typical process of electrospinning is as follows. First,

when a high electrical potential is applied between a grounded collector and a

spinneret, a liquid droplet of a polymer solution or melt held at the end of a capillary

tube is electrified and the surface of the droplet is covered by the induced charges. A

conical object that is also called as the Taylor cone will be obtained under the effect

of electrostatic forces. Second, when the force caused by the applied voltage potential

overcomes the surface tension of the polymer solution or melt, a charged jet will be

ejected from the tip of the needle, followed by a complex stretching and whipping

process due to repulsive forces of the carried charges to achieve a long thread. Third,

as the jet further elongates to be longer and thinner accompanied by an evaporation of

the solvent in jet, the solidified fibers can be prepared.10 These prepared nanofibers

show ample advanced characteristics including high surface-to-volume ratio,

controllable fiber diameter and surface morphology and porous structure. A wide

variety of fiber morphologies can be developed by adjusting the parameters of the

polymer solution such as conductivity, viscosity, and surface tension, and the

electrospinning process parameters such as the electric potential, flow rate, and

distance between the capillary and collector.11

1.1.2 Influence parameters of spinning technology on fiber formation

3

Spinning technology which depends on a spinneret extrusion process can be

categorized as melt spinning, solution spinning, and emulsion spinning. The

formation of the resulting product by electrospinning technique is based on the

specific process and parameters adopted. Melt spinning is advantageous in high-

throughput rate and high process safety. Solution spinning provides unique

advantages in lower energy consumption, larger variety of electrospun fiber materials,

high mechanical integrity as well as superior optical and electrical property of the

resulting materials. Emulsion spinning is available for these polymers that show high

melting point and for the fabrication of flame-retardant fibers. Of three types of

spinning technology, solution spinning is mainly used to prepare 1D nanostructured

fibers due to the advantages above-mentioned as well as very easy to obtain proper

mixed precursor solution. Major parameters that influence the structure of electrospun

fibers include: (1) the viscosity of electrospinning solution which bases on the

concentration of polymer and the molecular weight of the polymer is very important

to determine whether the long and continuous fibers can be achieved by

electrospinning. With low viscosity of the solution, the fiber cannot be formed and

only particles are obtained, which is due to low polymer chain entanglements.

However, high viscosity solution will lead to the unstable flow rate of solution; (2) the

electric field is an important process factor for electrospinning. Generally, when the

electric potential increases, the fiber diameter will decrease. At low voltage, beads in

the fiber will occur. The flow rate of the solution is another key issue, which affects

the jet velocity and the rate of solution transfer.12 Decrease in feeding rate causes the

formation of smaller-diameter fiber, whereas at high feeding rate fibers with spindle-

like beads are formed; (3) there are several spinneret collectors including foil, metal

4

and rotating drum. Low-cost available aluminium foil that connects with a grounded

electrode to provide a path for current is most widely used as the collector. The

modification of geometry of collector that includes flat plat, parallel plate or stripe,

rotating drum/disc, and conveyor also plays an important role in the morphology of

the resulting fibers. For example, aligned fibers can be achieved by adjusting

electrostatic interactions which can be realized by using two parallel static collecting

stripes or plates which are separated by a well-designed gap.1a However, the major

disadvantage of this design is low ability to scale-up and poor capability to produce

high-density and highly-aligned fibers as it over-depends on the width and property of

the gap. Using a rotating drum or disc collector, highly aligned flexible fibrous

products with high density are available;13 (4) two types of solvent removal process

are dry and wet. In the dry spinning, solvent in polymer solution is evaporated in air

or an inert gas accompanied by the formation of solidified fibers.14 Factors that

include the solvent system, atmosphere temperature, and blowing condition can also

affect the morphology of the resulting electrospun fibers. In the wet spinning, the

solvent in electrospun solution is removed in liquid bath in which the spinning solvent

can miscible with liquid and polymer that cannot dissolve in liquid remains and

precipitates.15 The defects and voids can be obtained by controlling the solidification

process occurred in the polymeric solution-nonsolvent interface; (5) recent

tremendous work has been made in developing injection system. For example, to

increase the rate of production or the volume of production multi needle components

that allow parallel multi-processing are successfully deigned.16 Coaxial

electrospinning has been regarded as a deep development in electrospinning

technique.17 A spinneret used in coaxial electrospinning consists of two coaxial

5

capillaries storing two viscous solutions which can form a core-shell jet under the

effect of an electric potential and then a solidified core-shell fiber mat can be

produced. It is a simple, advanced, and attractive method to prepare core-shell fibers

with very long fiber up to several centimeters as compared with other approaches.

Polymer, organic, inorganic, or their composite core-shell or hollow materials can be

obtained using corresponding proper solutions, with or without a post treatment such

as calcination and solvent extraction. The critical factor for the formation of core-shell

structure in core-shell technology is how to control the immiscibility of the core and

sheath liquids during the electrospinning process. Very recently, a novel triple-coaxial

electrospinning has also been developed in which a spinneret that consists of three

coaxial capillaries is stored with three viscous fluids.11b Parameters affecting the core-

Figure 1.1 Schematic illustration of a typical electrospinning setup.

6

shell electrospinning will also influence the triple-coaxial electrospinning and the

resulting fibers.

In general, electrospun composite nanomaterials with targeted structure and

morphology can be designed by carefully adjusting and optimizing various parameters

such as electric potential, injection system and geometry of collector. Table 1.1 shows

the influence of injection system and geometry of collector on fiber morphology. It is

clear that nonaligned or aligned composite fibers or tubes with core-shell structure can

be obtained. In addition, we also briefly describe the function of some typical

polymers on the resulting materials shown in Table 1.2.

1.2 Lithium-ion batteries

Because of the decrease of fossil fuel and the demand for a clean and secure energy,

there has been an increasing demand for developing a new generation of electric

energy storage for powering low or zero emission electric vehicles and hybrid electric

vehicles.18 Among all types of batteries, LIBs has gained a continuously growing

scientific interest because their theoretical energy/power density is the highest for all

solid electrodes.19 It, however, is very difficult to be used in industrial area due to the

internal short circuiting caused by the formation of dendritic lithium, resulting in a

catastrophic failure and unsatisfactory cycling life of battery. When metallic lithium is

replaced by a lithium-insertion or allying compound that serves as negative electrode

materials, LIBs, a family of rechargeable batteries, is developed, which not only can

enhance the cycling life and rate capability, but also address the safety issue.20 Figure

1.2 shows a typical LIB cell consisting of an anode, a cathode, a liquid electrolyte

7

(e.g., lithium hexafluorophosphate in ethylene carbonate and dimethyl carbonate

mixture), and an electrolyte-filled separator.21

Table 1.1 Influence of injection system and geometry of collector on fiber

morphology

Single or multi needle Coaxial needle Triple-axial needle

Flat plate Nonaligned fibers Nonaligned core-shell

fibers or nonaligned

tubes

Nonaligned triple core-

shell fibers or

nonaligned core-shell

tubes

Parallel

plate/stripe

Aligned fibers

Aligned fibers

Aligned fibers or tubes Aligned triple core-shell

fibers or aligned core-

shell tubes

Rotating

drum/disc

Aligned fibers

Aligned fibers

Aligned fibers Aligned triple core-shell

fibers or aligned core-

shell tubes

The basic mechanism of LIB bases on the flowing of Li ions between the anode and

the cathode, which enables the conversion and storage of chemical energy into

electric energy. The reaction of anode during charge/discharge process is

Table 1.2 The function of some typical polymers on the resulting materials

8

Polymer Carbon resource

(Function)

Sacrificed

component

(Function)

Achievement Ref.

PAN √ √ Carbon matrix

Porous structure

22

PVP √ √ Carbon matrix

Porous structure

23

PS √ √ Carbon matrix

Porous structure

24

PLLA ˟ √ Porous structure 25

PVA √ √ Carbon matrix

Porous structure

26

PMMA √ √ (C2H2) The growth of

CNT

Porous structure

11a

PEO √ √ Carbon matrix

Porous structure

27

9

LixC6↔6C+xLi++xe-(0

10

batteries for a wide range of applications.28 The performances of LIB (e.g.,

energy/power density, cycling life, cost, and safety) depend largely on the

characteristics of the electrode materials (e.g., composition, structure, morphology,

and size). For the high energy density, the optimized properties of the electrodes

materials should include high cell voltage which means large difference between the

redox potentials of the cathode and anode, and high specific capacity. For the long

cycle life, the electrode materials possess long-term stability and durability of

materials during cycling that only allows for very small change in crystal structure

and morphology of the components. To achieve high power density, the prepared

materials should have proper nanostructured architecture and porous structure to

enhance the charge transfer. Therefore, designing the electrode materials with desired

structure, morphology and composition is very important for developing a new

generation of high-performance battery.

1.2.1 Cathode

1.2.1.1 Hybrid cathode based on carbon and olivine compound

1.2.1.1.1 LiFePO4/carbon hybrid

Ordered-olivine-structure lithium iron phosphate (LiFePO4) shows good thermal

stability and can maintain structure integrity in N2 or O2 up to 350 ºC, which is due to

the strong covalent P-O bonds in the (PO4)3- polyanionic clusters.29 In addition, there

are not any potentially disadvantaged reactions between LiFePO4 cathode and

electrolyte during the charging/discharging process at the high temperature up to 85

ºC, avoiding the limitation of working temperature and the loss of oxygen that makes

the material unsafe.30 The theoretical capacity of LiFePO4 ↔ FePO4 + Li+ + e is 170

11

mAh g-1 with a flat V of 3.45 V versus x in LixFePO4 according to the Gibbs phase

rule.29 However, the low conductivity of LiFePO4 (~1 × 10–9 S cm–1) leads to the poor

rate capability.31 Since its discovery in 1997, tremendous progresses have been made

in improving the performance of LiFePO4 electrodes through various approaches such

as metal ions doping, designing nanocrystal, conductive coating, and conductive

LiFePO4 hybrid.29 Incorporation of carbonaceous materials such as CNFs and CNTs

is the most efficient way to enhance the electronic conductivity. Chen et al. prepared

electrospun LiFePO4/carbon (C) fiber composite by a sol-gel electrospinning method

using LiNO3, NH4H2PO4, Fe(NO3)3·9H2O, citric acid and PVP as precursor

materials.32 The prepared LiFePO4/C nanofibers with fiber diameter of 270 nm to 372

nm consisted of polycrystalline nanoparticles with particle diameter of about 30 nm

and an amorphous carbon layer with a thickness of ~ 2.6 nm. As a cathode, this

material with high surface area of 167.7 m2/g exhibited the initial discharge capacity

of 166 mAh g-1 at 0.1 C (17 mAh g-1). After 500 cycles at 5C, the discharge capacity

retention of the electrode was over 93 %, with a very high Coulombic efficiency of ~

100 %, showing a good cycling performance. Another typical work was carried out by

Wang’s group who showed LiFePO4 nanoparticles decorated with CNTs and CNFs

synthesized by sol-gel-based electrospinning and heating treatment.33 As shown in

Figure 1.3a, a mixture of PAN, functionalized CNTs, and LiFePO4 precursor (lithium

acetate, phosphoric acid, and iron(II) acetate) in DMF solvent was used as spinning

solution. The electrospun LiFePO4 precursor/PAN/CNTs composites were further

calcined to prepare the CNT/C/LiFePO4 composite nanofibers (Figure 1.3b). CNF

matrix not only provides a conductive network throughout the whole electrode, but

has high specific surface area and porous structure, which shorten Li-ion diffusion

12

pathway and then increase the electrode reaction kinetics. Small-size LiFePO4

particles encapsulated in CNF matrix enable higher reversible capacity. Furthermore,

the introduction of CNTs into the composite creates a conducting bridge between

LiFePO4 particles to enhance the electrochemical efficiency. As a result, the

CNT/C/LiFePO4 composite cathode delivered a high capacity of 169 mAh g–1 after 50

cycles at 0.05 C, with 99 % of the theoretical capacity of LiFePO4 (Figure 1.3c). At

rates of 0.05, 0.1, 0.2, 0.5, 1, and 2 C, the composite cathode also had 169, 165, 158,

148,134, and 121 mA h g–1, respectively, showing a good rate capability.33

1.2.1.1.2 LiMnPO4/carbon hybrid

Fe in LiFePO4 can be replaced by other transition metal ions (e.g., Mn, Co and Ni)

to form other olivine structures such as LiMnPO4, LiCoPO4 and LiNiPO4 with a high

working potential of 4.1, 4.8, and 5.1 V, respectively. Among three olivine samples,

LiMnPO4 attracts intensive interesting as the working potential of current electrolyte

is compatible with that of LiFePO4, while the working potentials of the other two

samples are high, raising the serious safety issues. In addition, LiMnPO4 shows

enhanced capacity that is about 20 % higher than that of LiFePO4. However, several

key disadvantages including poor cycle stability caused by the large volumetric

change and low electronic conductivity are reported for LiMnPO4 cathodes. Therefore,

achieving high-energy and high-power LiMnPO4 cathodes still remains a great

challenge. Decreasing size of LiMnPO4 into nanostructure, coating with carbon, and

decorating with other elements are major approaches to address these drawbacks. Lu

et al. synthesized electrospun C/LiMnPO4 composite nanofibers from the mixture of

poly(ethylene oxide) (PEO), Mn(NO3)2·4H2O, and LiH2PO4 in de-ionized water.27 In

13

Figure 1.3 (a) Schematic of the synthesis of LiFePO4/CNT/C composite nanofibers,

(b) SEM image of the prepared composite nanofibers, and (c) cycling performance of

LiFePO4/CNT/C composite nanofibers, LiFePO4/C composite nanofibers and

LiFePO4 at 0.05C.33

(b)

(a)

(c)

(b)

14

addition, C/LiMg0.05Mn0.95PO4 hybrid nanofibers also can be obtained by adding

Mg(NO3)2·6H2O into the mixed solution. The resulting C/LiMg0.05Mn0.95PO4

nanofiber cathodes showed a capacity of ~141 mAh g-1 at the first cycle between 2.5

and 4.5 V, which is higher than that of C/LiMnPO4 at 0.1 C. In addition, the capacity

of the C/LiMg0.05Mn0.95PO4 nanofiber cathode was ~ 107 mAh g-1 at a high current

density of 5 C. Furthermore, no capacity fading for 200 cycles can be observed with a

~ 100 % of Coulombic efficiency. The high performance would be due to the

introduction of a small amount of Mg substitution which can reduce the volume and

structural mismatch during the extraction/insertion process and carbon coating which

enhances electrochemical kinetics, electron/ion transport rate and mechanical integrity.

1.2.1.1.3 LiFe1-yMnyPO4/carbon hybrid

An important breakthrough in olivine structure is self-doped LiFe1-yMnyPO4 solid

solution materials in which the replacement of Fe with Mn increases the cell voltage

and the energy density as compared with LiFePO4.34 In addition, the ion transfer

kinetics of LiMnPO4 can be enhanced by replacing some of Mn with Fe. More

importantly, the doping can accommodate the mechanical stress caused by pulse

charging and discharging, thereby yielding a long cycle life. A typical LiFe1-

yMnyPO4/carbon hybrid nanofiber synthesized through electrospinning of a mixture of

LiOH·H2O, FeSO4·7H2O, H3PO4 MnSO4·1H2O and PVP was demonstrated by Hagen

et al in 2014.35 LiFe1-yMnyPO4 crystallites synthesized by reaction of their

corresponding precursors at high temperature are surrounded by carbon matrix. The

size of crystallites is found as a function of the ratio of Fe and Mn. When used as

cathodes these hybrid nanofibers with high surface area of ~ 111.3 m2/g delivered the

15

discharge capacity of 125 mAh g-1. At higher current densities, the composite sample

exhibited only slight fading of capacity.

1.2.1.2 Hybrid cathode based on carbon and silicate compound

Silicate materials (Li2MSiO4; M = Mn, Fe, and Co) are of particular interest for LIB

cathode because of their low cost, environmentally benign nature, and high theoretical

capacity. Among these, Li2MnSiO4 has been proven to be an attractive cathode

material when compared with Fe that has only two oxidation states, the Mn allows for

reduction from MnIV to MnII, leading to a complete lithiation to form Li2MnSiO4

which enables the insertion/extraction of two Li ions per transition metal and then

show a theoretical capacity of 333 mAh g-1. The main challenges for the commercial

application of the Li2MnSiO4 bulk cathode are the fast capacity fading, large

irreversible capacity, and poor rate capability due to low electronic conductivity

(~10−16 S cm−1 at room temperature) and structural instability caused by the Jahn-

Teller effect resulting from the Mn3+ at the octahedral sites. Several strategies, such as

developing nanostructured Li2MnSiO4, decorating with conductive additives, and

doping techniques, have been investigated to solve the capacity degradation. It is well

known that the nanostructured materials can shorten Li-ions diffusion pathways and

enhance the interaction between the electrode and electrolyte, resulting in enhanced

electrochemical performance. Li2MnSiO4/C composite nanofibers prepared by

electrospinning from a mixture of lithium acetate, manganese acetate, tetraethyl

orthosilicate, and PEO were reported by Park et al.36 The resulting Li2MnSiO4/C

composite nanofiber cathode delivered an initial discharge capacity of ~209 mAh g-1

at a rate of 0.05 C with the retention of 77 % over 20 cycles. Zhang et al. also

16

developed a Li2MnSiO4/C composite nanofiber showing a discharge capacity of 185

mAh g-1 with low capacity retention of 54 % after 20 cycles.37 Although several

approaches such as designing 1D nanostructure and carbon decorating have been

devoted to improve the performance of Li2MnSiO4, they are still not very effective

Figure 1.4 (a) TEM image of Li2Mn0.94Cr0.06SiO4/C composite nanofibers. (b)

Cycling performance of Li2MnSiO4 powder, Li2Mn0.94Cr0.06SiO4 powder, and

Li2Mn0.94Cr0.06SiO4/C composite nanofibers at a current density of C/20.38

(a)

(b)

17

in addressing unaccepted issues such as low capacity and poor cycle performance,

which are mainly attributed to the collapse of crystal structure during the lithium

extraction process. Therefore, developing a way to stabilize the crystal structure of

Li2MnSiO4 is urgently required. Recently, a Li2Mn(1−x)CrxSiO4/carbon composite

nanofibers were reported with enhanced cycling performance.38 The electrospinning

solution was first prepared by mixing PAN, lithium acetate dehydrate, chromium

nitrate nonhydrate, manganese acetate, tetraethyl orthosilicate, and citric acid in

water/ethanol/ethylene glycol solvent. Next, Li2Mn(1−x)CrxSiO4/carbon composite

nanofibers were prepared by calcination of their corresponding precursor composite

Table 1.3 Electrochemical capacity of carbon-based LIB cathodes

Electrode type Current

density

Capacity (mAh g-1) Ref.

LiFePO4/carbon 0.1 C 166 32

LiFePO4/carbon 0.05 C 169 33

LiMg0.05Mn0.95PO4/carbon 0.1 C 141 27

LiFe1-yMnyPO4/carbon C/2 125 35

Li2MnSiO4/carbon 0.05C 209 36

Li2MnSiO4/carbon C/20 185 37

Li2Mn(1−x)CrxSiO4/carbon C/20 314 38

18

nanofibers (Figure 1.4a). This designed structure showed a high discharge capacity of

314 mAh g-1 at the 5th cycle. After 20 cycles, a capacity of 273 mAh g-1 still remains

(Figure 1.4b), which is much better than that of Li2MnSiO4. The authors attribute the

improvement of capacity to two reasons: (1) a formed high conductive carbon fiber

matrix which enables fast ion transport and good mechanical behavior, and (2) Cr

doping which increases the cell volume and induce defects in the lattice, thereby

enhancing the structure stability. We summarized the electrochemical capacity of

carbon-based LIB cathodes in Table 1.3.

1.2.2 Anode

1.2.2.1 Carbonaceous anode

Carbon is one of most abundant elements on the earth with a number of structural

forms such as graphite, graphene, diamond, fullerene, carbon nanotube (CNT), carbon

nanofiber (CNF), and other carbons (soft carbon, hard carbon, or diamond-like carbon

and graphitic carbon). Carbon has two types of bond which are sp2 and sp3

hybridization. Since the discovery in 1989 by SONY Corporation, graphite is current

commercial anode materials for LIBs because of its low cost, long cycle life and

environmental friendliness. However, graphite suffers from a low theoretical capacity

of 372 mAh g-1 (LiC6 composition),21 limited rate performance, and internal short

circuiting resulting from the growth of dendritic lithium caused by its low working

potential of around 0 V versus Li+/Li, which limit the use in electric energy storage.

In order to overcome these problems, great efforts have been made to develop 1D

electrospun nanocarbons, such as CNFs, CNTs, and hybrid carbon, to increase the

power and energy densities of LIBs. The following sub-sections focus on some

19

typical novel high-performance carbon nanomaterials by electrospinning, which were

summarized in Table 1.4.

1.2.2.1.1 CNFs

CNFs, which are mainly prepared by CVD approach, hydrothermal method, and

electrospinning technique, have been widely studied as very effective anodes for

Table 1.4 Electrochemical capacity of carbonaceous anodes for LIBs

Electrode type Current density

(mA g-1)

Capacity (mAh g-1) Ref.

CNF 30 450 39

Freestanding CNF 100 555 40

porous CNFs 50 556 25

Activated CNFs 50 533 41

Hybrid CNFs 50 750 22

Hybrid CNFs 50 983 42

Activated hollow

graphitic CNFs

100 1560 43

Hybrid CNTs 50 969 11b

CNF/CNT Hybrid 100 1150 11a

20

LIBs.44 A unique merit of electrospun CNFs is very long and continuous fiber

network that can provide mechanical and electrical interconnect and also a way of

free-standing anodes. In addition, all of defects such as lattice and surface defects in

CNFs are chemically active for intercalation of Li ions. Electrochemical test results

showed that electrospun CNF anode delivered a reversible intercalation capacity of

~450 mAh g-1 at 30 mA g-1.39 The electrochemical performance can be greatly

enhanced by directly using the electrospun CNF films as free-standing anodes,

without carbon black and binder. The free-standing film anode has an improved

capacity up to 555 mAh g-1 at 100 mA g-1 after 105 cycles.40

Extensive research has been done to evaluate the effect of pore structure and degree

of graphitization on the performance of carbonaceous anodes. A higher BET surface

area, larger pore volume, higher degree of graphitization increase the Li ion diffusion,

provide additional sites for Li ion storage, giving rise to the improvement of battery

performance. A traditional method is to use scarified polymer that can be decomposed

during calcination, such as poly-L-lactic acid (PLLA) and poly(methyl methacrylate)

(PMMA),25, 45 or activating agent that can activate the carbon, such as ZnCl2 and

KOH,41 to produce pores in the resulting CNFs. The prepared porous CNFs showed

enhanced capacities ranging from 533 to 556 mAh g-1 as compared with nonporous

counterparts (410 mAh g-1). To address the issue of difficult control of pore number

and diameter in traditional approach, an interesting method based on a combination of

transition metal-based temple such as Ni and Fe and electrospinning have been

developed.22, 42 In-situ formed transition metal nanoparticle formed by the

decomposition of transition metal precursor can turn the amorphous carbon on their

surface into graphitic carbon, producing the transition metal nanoparticles

21

encapsulated in graphitic carbon nanospheres in electrospun CNFs. After acid

treatment, transition metal nanoparticles were dissolved and thus amorphous CNFs

with hollow graphitic carbon nanospheres were obtained. The prepared hybrid carbon

showed high specific gravimetric capacities of ~750-983 mAh g-1 and good cycling

stability up to several hundred cycles. However, there are still some amorphous

carbons in these prepared carbon materials, which affect the degree of graphitization

and thus influence the rate performance of the resulting materials. A novel Ni-induced

graphitization approach through the Ni diffusion was exploited to obtain hollow-

tunnelled graphitic CNFs (Figure 1.5a).43 The diffusion of Ni particle in the

electrospun amorphous carbon can be controlled by a combination of heating

treatment and pumping. The graphitic nanosphere formed by the effect of Ni

nanoparticle can be cracked by a large stress induced by suddenly pumping at high

temperature, which allows Ni particles to diffuse out of the graphitic nanosphere via

the crack and then undergo an elongation-contract process, yielding a hollow tunnel

structure and turning the amorphous carbon into graphitic carbon (Figure 1.5b-f).

Activated hollow graphitic CNFs can be prepared by chemical activation and acid

treatment (Figure 1.5a). In addition, the addition of PMMA also would induce more

pores in the resulting materials. This interesting and rational approach leads to high

specific capacity of 1560 mAh g-1 at 0.1 A g-1 (Figure 1.5g), outstanding rate

capability, and long cycling life up to 2000 cycles, which are promising

characteristics for high-performance carbon anodes.

1.2.2.1.2 CNTs

22

Since the discovery of 1D CNTs, considerable attention has been paid to CNTs

because of their wide applications, such as thermal conductor, catalyst support,

biological application, energy storage, which is due to their high surface-to-volume

ratios, exceptional mechanical properties, high chemical stability, and surface

activities.46 Tremendous research efforts have been made in understanding their

electrochemical properties such as the mechanism of the deintercalation/intercalation

of Li with CNTs and electrochemical capacity.47 It was accepted that space between

pseudo-graphitic layers of CNTs can store Li ions and both outside and inside tubes

that are chemically active are also available for intercalations of Li ion. Several key

factors, such as structure, length and diameter, defect, the degree of graphitization of

(a)

23

(b)

(c)

24

(d)

(e)

(f)

25

Figure 1.5 (a) Schematic of material processing. (b-f) TEM images of the typical step

of Ni diffusion. (g) The charge/discharge profile of the resulting materials at 0.1 A g-

1.43

CNTs, and surface functional group, play important roles in specific capacity and

cycling life of CNTs. For example, defects, pores, and functional groups in CNTs can

enhance the intercalation of Li in the voltage range above 1 V, which increase the

capacity performance.48 Short and small-diameter CNTs have a better performance as

compared with that of long and large-diameter CNTs due to their faster

insertion/extraction of Li ions and enhanced electronegative property.49 In addition,

higher degree of graphite crystallinity yields a better rate performance.50 As a result,

CNTs possess high reversible capacities of 450-1116 mAh g-1 (Li1.2C6-LiC2), which is

much higher than that of graphite (372 mAh g-1, LiC6). Electrospinning is an unusual

method to prepare polymer-based CNTs. However, there is only one work on

electrospun CNTs anode reported by Chen et al. in 2012 who synthesized a novel

architecture consisting of amorphous CNTs and hollow graphitic carbon nanospheres

(HGCNs) by a novel triple-coaxial electrospinning.11b Three different viscous liquids

(g)

26

that are mineral oil (inner), PAN solution (middle), and PVP/Ni(Ac)2 composite

(outer) were stored in three metallic capillaries. By controlling the mixture of outer

two liquids coaxial PAN/PVP/Ni(Ac)2@oil composite precursors were prepared,

followed by calcination and acid treatment to achieve amorphous CNTs with HGCNs.

When used as carbon anodes the synthesized electrospun carbon possesses unique

advantages that include many hollow structure (sphere and tube) and defects, high

conductivity, and good mechanical integrity. As a result, this structure delivered a

high capacity ~969 mAh g-1 at 50 mA g-1 and good cycling stability up to 650 cycles

and outstanding rate capability. However, the disadvantage that is the existence of

amorphous carbon need to be tacked.

1.2.2.1.3 CNF/CNT hybrid

Despite the improvements in capacity and cycling life, there is one severe

drawback of low volumetric capacity for the use of electrospun CNTs or CNFs as

electrode for LIBs because their volumetric density is low, which is ascribed to low

nanofiber or nanotube packing density. To be used in the industry-level LIBs, this

issue needs to be addressed. One notable development in this area is the work by

Chen et al., where CNTs were formed on the surface of electrospun CNFs by a novel

in situ CVD method, as shown in Figure 1.6a.11a PMMA was added into the mixture

of PAN and Ni(Ac)2 to obtain the PAN/Ni(Ac)2/PMMA composite nanofibers. During

the heating in the H2/N2 atmosphere, PAN and Ni(Ac)2 were changed to carbon and

Ni, whereas the PMMA was completely decomposed to produce pores in carbon/Ni

composite nanofibers, accompanied by a formation of C2H2 that was used as a carbon

source for the growth of CNTs under the effect of Ni nanoparticles. The well-

27

designed process creates a CNT/CNF/Ni hybrid material. After a combination of

chemical activation and acid treatment, an activated hollow CNT/CNF hybrid with a

very high surface area of 1840 m2 g-1 and a total pore volume of 1.21 m3 g-1 can be

achieved (Figure 1.6b). The hybrid carbon electrode was shown to exhibit a high

discharge capacity of ~ 1150 mAh g-1 at 0.1 A g-1 after 70 cycles and exceptional

cycling stability of over 3500 cycles while having more than 80 % capacity at a

current density of 8 A g-1 (Figure 1.6c). The remarkable improvement on the

electrochemical performance is due to the novel structure of the activated hollow

CNT/CNF hybrid materials: 1) many defects, pores and hollow spheres, and the large

d-spacing of graphene sheets offer more extra spaces for storing Li ions, significantly

enhancing the capacity performance; 2) high surface area and CNTs on the surface of

CNFs enable a better contact between active material and electrolyte. Furthermore,

growing CNTs between CNFs can enhance the volumetric capacity; 3) the porous and

hollow structure and high conductivity promote Li ion access and transportation,

(a)

28

Figure 1.6 (a) Schematic of the synthesis of activated hollow CNT-CNF hybrid

material. (b) SAED pattern, TEM image, line profile of d-spacing of the prepared

hybrid carbons. (c) The electrochemical performance of the resulting hybrid

materials.11a

thereby a better rate performance; 4) the N-doping in the composite electrode further

enhance the electrochemical performance.

(b)

(c)

29

1.2.2.2 Hybrid metal oxide anode

1.2.2.2.1 Electrospun Ti-based hybrid anode

Ti-based oxides by the insertion reaction mechanism have been widely studies as

attractive anode materials for LIBs because of their low cost, exceptional stability,

high working potential and environmental benign.51 In particular, Li4Ti5O12 (LTO)

has attracted much attention due to its advantages including excellent cycle

performance arising from zero-strain and no volume change during

insertion/extraction process, high voltage potential to improve safety by minimizing

the decomposition of liquid electrolyte and the formation of solid electrolyte interface

(SEI) film, and low cost.52 However, LTO suffers from low Li ion diffusion

coefficient and poor electronic conductivity.53 Many studies have demonstrated that

these disadvantages can be addressed by a combination of reducing the LTO bulk into

nanoscale and a carbon coating.54 Electrospinning technique is a unique and simple

way to prepare 1D carbon/LTO hybrid materials. Zhang’s group used a mixture of

tetrabutyltitanate, lithium acetate, and PVP in isopropyl alcohol as electrospinning

solution to prepare the carbon/LTO composite nanofibers.55 The synthesized hybrid

showed good rate capability: 150 mAh g-1 at C/5, 140 mAh g-1 at C/2, 130 mAh g-1 at

1C, and 120 mAh g-1 at 2 C. Huang’s group also developed conformal electrospun

carbon-coated LTO composite fibers which displayed a high capacity of 152 mAh g−1

at 0.5 C and 118 mAh g−1 at 10 C.56 Porous hybrid nanofibers based on the LTO and

carbon have been also explored to improve the performance of LTO anode. Though

porous structure is effective for increasing the electrode/electrolyte interfacial area, a

more efficient ion transport can be achieved, yielding a better rate capability. For

30

example, a small amount of amphiphilic triblock copolymer F127 was added into

LTO precursor solution. After electrospinning, solvent was removed. This leads to

greatly improve the concentration of F127 in LTO precursor nanofibers, which is

higher than its critical micelle concentration. High concentration of F127 will induce

the formation of mesopores in hydroxyl LTO precursor through self-assembly process.

Mesoporous LTO/carbon hybrid nanofibers were obtained after calcination of the

composite precursor. As anodes, this porous hybrid with high surface area of 212

m2/g exhibited a high capacity of 127.4 mAh g-1 and remained 122.7 mAh g-1 after

200 cycles at 5C.57 Very recently, Huang’s group also developed a highly porous

LTO/carbon (PLTO/C) composite nanofiber by a combination of electrospinning and

a controlled two-step annealing treatment.23 As shown in Figure 1.7a, LTO/C

composite nanofibers were first prepared by calcination of PVP/titanium

isopropoxide/lithium acetylacetonate composite precursors at 800 °C in a 5 % H2/Ar

atmosphere. Next, the synthesized LTO/C composite products were further heated at

350 °C in air to partially burn off the carbon matrix, which creates a large number of

nanopores in LTO/C hybrid. The obtained PLTO/C hybrid with diameter of ~200 nm

shows very rough surface (Figure 1.7c). A hierarchically porous structure can be

clearly observed in the PLTO/C hybrid with individual LTO crystal encapsulated in

amorphous carbon layer. Such a novel 1D nanostructured porous hybrid yields

outstanding rate performance and long cycling life. For example, at 0.5, 2, 10, and 30

C, the capacities of 161, 154, 150, and 143 mAh g−1 can be achieved. When the

current rate backs to 0.5C, the structure also delivered a high capacity of 159 mAh g−1.

The cycling performance in Figure 1.7b shows that no obvious capacity fading can be

31

Figure 1.7 (a) Schematic illustration of the synthesis process of the porous

LTO/carbon (PLTO/C) composite nanofibers, (b) cycling performance of PLTO/C

hybrid, LTO/C composite and LTO, and (c) SEM image of the PLTO/C composite.21

(a)

(b)

32

observed after 300 cycles with a high Coulombic efficiency close to 100 % after a

first few cycles, demonstrating the excellent cycling life.

TiO2 is also an important and interesting anode material with higher theoretical

capacity of 335 mAh g-1 when compared to LTO.58 TiO2 has eight polymorphs,

including anatase, rutile, brookite, TiO2-B (bronze), TiO2-R (ramsdellite), TiO2-H

(hollandite), TiO2-Ⅱ(columbite) and TiO2-Ⅲ (baddeleyite). Among them, anatase,

rutile and TiO2-B are considered as suitable and promising anodes for LIBs. The

structure of anatase TiO2 in Fig. 1.8a shows the stacking of 1D zigzag chains, which

are composed of adjacent edge-sharing TiO6 octahedrals. Along [100] and [010]

directions, a stacking with empty zigzag channels allows Li-insertion in the interstices

of TiO6 octahedrals to form LixTiO2.59 The possible positions of lithium ions are

displayed by the purple spheres. When X is larger than 0.05, the tetragonal anatase

phase is subjected to a phase transition with an orthorhombic distortion as a result of

the strong interaction of Li-Li. As the whole distortion of the atom positions is very

small, the volume change of the unit cell is negligible. For the rutile crystal, TiO6

octahedral shares the edges in the c-direction and corners in the ab planes. It is well

accepted that the preferential sites for lithium ion insertion are the oxygen octahedral

(Fig. 1.8b). In order to reach the potential positions, lithium ions need to migrate

through the tetrahedral site, which is near to the octahedral site in the ab-plane.

Therefore, lithium ion diffusion in rutile is anisotropic. The diffusion along the c-

direction is much faster than that in the ab planes, which results in the 1D lithium

diffusion in rutile. Further lithium insertion along c-direction would be prevented by

the repulsive Li–Li interactions in c-direction and trapped Li-ion pairs in the ab

planes.60,61,62 The structure of TiO2 (B) is composed of the corner-sharing and edge-

33

sharing TiO6 octahedral, which is the same as a derivative of the ReO3-type structure.

(Fig. 1.8c) TiO2 (B) shows a 1D infinite channel following the [001] direction,

providing many paths for lithium ion and accommodating volume change with

ignorable distortion of the structure.63,64

Early work on electrospun TiO2 nanofibers showed that the capacities ranged

from 175 mAh g-1 to ~192 mAh g-1.65 In order to improve the battery performance,

several methods have been developed, such as incorporating with other high-capacity

Fig. 1.8 Structure of (a) anatase TiO2, (b) rutile TiO2 and (c) TiO2(B).66

a

b

c

34

materials, decorating with carbonaceous materials, N-doping, and introducing porous

structure.67 Among these, carbon decoration is one of effective strategy to improve the

TiO2 performance.68 For example, composite nanofibers consisting of TiO2

nanoparticles and CNT prepared from a solution of TIP and CNTs were investigated

as anode materials for LIBs. The prepared composite nanofibers give rise to a

remarkably stable cycle ability with a very low capacity loss of about 8 % from 10 to

800 cycles.6d Incorporation of graphene as an additive to active TiO2 anodes is a

promising approach to form conductive networks in the composite electrode.

TiO2/graphene hybrid nanofibers were synthesized using an electrospinning method

followed by calcination.69 Nanostructured TiO2/graphene hybrid anodes exhibited a

high Li storage capacity of 185 mAh g-1 at 33 mA g-1 with a high capacity of 153

mAh g-1 after 100 cycles and high Coulombic efficiency of 99 %, showing good cycle

stability. Another work on electrospun TiO2-carbon hybrid showed a stable high

reversible capacity of 206 mAh g-1 after 100 cycles at a current of 30 mA g-1 and a

Coulombic efficiency closed to 100 %, displaying long cycling life.70 Recent work

has been proven that the introduction of porous structure into TiO2/carbon composite

nanofibers can further greatly improve their capacity performance. This enables

various advantages, including (a) good rate capability arising from improved the

kinetics associated with lithium and shorter lithium diffusion length, (b) stable cyclic

ability due to better accommodation of mechanical strains from the Li

charging/discharging process, and (c) high energy density resulting from porous

structure which provides additional space to store more Li ions.

1.2.2.2.2 Non-electrospun Ti-based hybrid anode

35

Cao et al. have developed CNTs/TiO2 core-shell nanocables by controlled

hydrolysis. CNTs/TiO2 showed the excellent rate capability compared with pure

CNTs and TiO2. The capacity of CNTs/TiO2 only decreased from 265 mAh g-1 to 238

mAh g-1 (90 % maintained) when the current density increased from 800 mA g-1 to

5000 mA g-1, while the value of CNTs fell from 201 mAh g-1 to 62 mAh g-1 (30 %

maintained) and nearly less than 5 mAh g-1 maintained for TiO2 under the same

current densities. The high specific capacity of CNTs/TiO2 nanocomposite resulted

from the synergism of the two materials. On one hand, CNTs facilitates the

transportation of electrons. On the other hand, the storage of Li for CNTs can be

improved by the assist of nanoporous TiO2 due to the thin layer on TiO2, making the

rapid access of Li ions from electrolyte.71 A 3D porous architecture composed of TiO2

nanotubes connected with a carbon nanofiber matrix was developed by Zhao group.72

TiO2 nanoparticles embedded in the carbon matrix was synthesized by electrospinning

and heat treatment. During hydrothermal treatment, TiO2 nanoparticles react with

NaOH solution and go out around the carbon nanofiber matrix to form a 3D porous

nanotube/nanofiber architecture as shown in Figure 1.9a and b. By using the prepared

materials as anodes for lithium-ion batteries without the addition of any conductive

agent or binder, high initial capacity and excellent rate performance were obtained.

Furthermore, the electrode showed stable cycling performance without any decay

after 1000 cycles at a high rate of 30 C (Figure 1.9c-e).

TiO2/SnO2 composites have been prepared for the anode materials in LIBs owing to

high capacity of SnO2 (781 mAh g-1). SnO2 nanocrystals on self-organized TiO2

nanotube arrays by solvothermal method have been prepared. The capacity of

SnO2/TiO2 anode is more than twice than that of TiO2. What is more, SnO2/TiO2

36

composite showed outstanding cycling performance with 70.8 % capacity maintained

after 100 cycles. That is because TiO2 nanotubes alleviate the volume changes of

SnO2 during charging/discharging processes.73 Roginskaya et al fabricate thin film of

TiO2/SnO2 through thermohydrolytic decomposition. They demonstrated that the

more stable capacity of the composites was obtained when the content of TiO2 in the

composites is higher, which can be varied from 0 to 20 %. The reason is that the

growth of β Sn was hindered by the interface of TiO2/SnO2, preventing the decay of

SnO2 during charge and discharge processes.74 Maier and co-workers developed

nanostructured mesoporous anatase TiO2 through efficient hierarchical mixed

conducting 3D networks to achieve superior electrochemical performance.75 As

shown in Figure 1.10 a and b, RuO2, providing extremely conducting paths for

electrons in a 3D network, distributes within the porous TiO2 spheres. This kind of

nanoscopic network leads to negligible insertion kinetics, increased local conductivity,

faster diffusion of Li+ and e-, and thus appeared to be the pivotal factor to achieve

high power density demanding for anode materials. Whereas the commercial

application of valuable metal or its oxides is hindered by expensive cost and toxicity,

combination of conductive additives at the nanoscale seems viable for remarkable

performance enhancement. The specific capacity at a rate of C/5 is as high as 214

mAh g-1, which is lowered to 190, 147, and 125 mAh g-1 at 1, 5, and 10 C,

respectively. The specific charge capacity of around 91 mAh g-1 is obtained even at

the greatly high rate of 30 C, which is two times higher than that of 5 nm anatase (48

mAh g-1) and nine times higher than that of mesoporous anatase spheres (10 mAh g-1).

37

a

b

c

38

Figure 1.9 (a) SEM image of indicate the cross-section of the carbon nanofibers. (b)

TEM image of 3D-TiO2/C samples after calcination; the insets are the magnified

images and the SAED patterns. (c) Discharge-charge curves of the 3D-TiO2/C

electrode at various rates (1 C (4th cycle), 2 C (9th cycle), 5 C (14th cycle), 10 C (19th

cycle), 20 C (24th cycle) and 30 C (29th cycle)). (d) Cycling performance of the 3D-

TiO2/C electrode at various rates (1-30 C). (e) Cycling performance of the3D-TiO2/C

electrode at the rate of 30 C.65

In addition, when the rate is lowered to C/5, the capacity of 210 mAh g-1 is regained

(Figure 1.10 c-f), demonstrating excellent reversibility.

d

e

39

a b

cc

d

40

Figure 1.10 (a) HRTEM image taken from the outer edges of a TiO2:RuO2 sphere; (b)

corresponding schematic illustration of the self-wired path of deposited RuO2

nanoparticles. (c-f) Rate performance. Variation of discharge (■) charge (●) capacities

versus cycle number for different anatase electrodes cycled at different rates between

voltage limits of 1 and 3 V.68

1.2.2.2.3 Transitional metal oxides/carbon hybrid

Several metal oxides (MOx, M= Co, Fe, Mn, etc.) following the conversion reaction

mechanism are converted to a metallic state accompanied by the formation of Li2O

during the electrode reactions, and then reversibly turned back to their initial products

after the delithiation.76 Since the electron that uses in the conversion reaction is more

than one due to the accommodation of Li in the vacant sites of these metal oxides,77

these metal oxides as anodes possess high specific reversible Li storage capacities

ranging from 400 to more than 1000 mAh g-1 with an average working voltage of 1.8-

2.0 V vs. Li/Li+.78 However, a number of disadvantages still remain in these metal

oxides, such as unstable SEI films caused by large volume expansion, large potential

e f

41

hysteresis between charge and discharge, and low Coulombic efficiency at the initial

cycle, giving rise to a great decrease of power efficiency of the electrodes.79 To solve

these issues, 1D electrospun nanostructured metal oxides/carbon fibrous hybrids have

been studied.80 Below we present several typical metal oxide/carbon hybrids by

electrospinning, such as iron oxide/carbon composite, cobalt oxide/carbon composite,

and manganese oxide/carbon composite.

Cobalt oxide. Cobalt oxide, such as Co3O4 and CoO, is one of the typical

conversion reaction-based anode materials for LIBs with the theoretical capacities

varying from 715 to 890 mAh g-1.81 Bulk cobalt oxides-based anodes suffer from poor

cycling stability because of the large volumetric expansion and unstable SEI film,

causing the poor capacity retention.82 1D cobalt oxide nanofibers have been shown to

alleviate these problems with improved Li storage capacity of 816-1336 mAh g-1 at

the initial cycle.83 However, severe aggregation of cob