World Journal of Engineering
LAYERED OXIDE NANOWIRES FOR HIGH-ENERGY LITHIUM ION
BATTERIES
Liqiang Mai1,2,* , Lin Xu1, Xu Xu1, Chunhua Han1, Yanzhu Luo1
1. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of
Technology, Wuhan, 430070, China
2. Department of Chemistry and Chemical Biology, Harvard University,
Cambridge, Massachusetts 02138, USA
*E-mail address: mlq@cmliris.harvard.edu
Introduction
lithiation results in a drastic improvement in the cycling stability of the
nanowires.
Among the cathodes for lithium or lithium ion batteries, the layered oxides
such as vanadium oxides, molybdenum oxides and their derivatives [1-4],
with outstanding structure of intercalation compounds and tunable various
valent states, have the highest specific capacities compared with other
counterparts. Unfortunately, vanadium oxides and molybdenum oxides
have long suffered from serious capacity fading issues upon cycling, which
limit their commercialization. Recently, nanowire structures have attracted
great interest and have been used as electrodes of lithium batteries because
they have a short Li-ion insertion/extraction distance, facile strain
relaxation upon electrochemical cycling, and very large surface to volume
ratio to contact with the electrolyte, which can improve the capacity and
cycle life of lithium batteries. However, in the process of battery fabrication,
owing to the high surface energy, nanomaterials are often self-aggregated,
which reduces the effective contact areas of active materials, conductive
additives and electrolyte. How to avoid self-aggregation of nanowire
cathodes/anodes, as well as understanding the mechanism of fast capacity
fading is still a challenge and of great importance.
Fig. 2a shows the curves of discharge capacity versus the cycle number for
the non-lithiated and lithiated MoO3 nanowires at a current density of
30mA/g. The discharge capacity of MoO3 nanowires is 301 mAh/g in the
first cycle, and 264 mAh/g after 5 cycles. However, the discharge capacity
of bulk MoO3 is 249 mAh/g in the first cycle, and drops dramatically to 27
mAh/g after 5 cycles. This is likely to be due to shape and size effects of
the nanobelts, with increased surfaces, edges, and corners shortening the
diffusion lengths of Li ions. We note that the lithiated MoO3 nanowires
exhibit a initial discharge capacity slightly smaller than that of the nonlithiated MoO3 nanowires, because some Li+ ions introduced during the
secondary hydrothermal lithiation process occupy some sites that are
electrochemically active for Li-storage. For the non-lithiated MoO3
nanobelts, the discharge capacity decreased to 180mAh/g after 15 cycles,
corresponding to a capacity retention of 60%. However, the discharge
capacity of the lithiated MoO3 nanobelts decreased to 220 mAh/g after
15cycles, corresponding to a capacity retention of 92% showing the
stability and enhanced performance of the lithiated nanobelts.
This paper will review the development of nanowire cathodes for highenergy lithium batteries based on the resent research in our group [5-8],
which covers from homogeneous nanowire cathodes to hierarchical
nanowire cathodes for reducing nanowire aggregation, and single nanowire
cathode for nanoscale battery diagnosis.
Fig.2: (a) The discharge capacity as a function of the cycle number for the
MoO3 nanowires before and after lithiation via secondary hydrothermal
reaction. (b) I–V transport measurements of single nanowire fabricated
devices using the samples before and after lithiation. [6]
Homogeneous nanowires cathodes
Homogeneous VO2 short nanowires with length of several micrometers and
diameter of 50 nm were synthesized by hydrothermal reaction (Fig. 1a) [5].
Briefly, V2O5 powders were mixed in aqueous media together with an
organic templating agent such as cetyltrimethyl ammonium bromide
(CTAB) and the mixture stirred for 48 hours in air. The resulting
rheological suspension was transferred into a Teflon-lined autoclave with a
stainless steel shell. The autoclave was kept at 180 °C for about a week.
The final product was washed with distilled water and dried at 80 °C. The
charge/discharge voltage plateau for the VO2 nanowires is 2.75/2.5 V, and
the specific charge and discharge capacity are 254 and 247 mAh/g,
respectively. The first cycle efficiency exceeds 97.5%. The discharge
specific capacity arrives at 180 mAh/g after 30 cycle (Fig. 1b). In contrast
to normal VO2 crystal material whose reversible capacity is about 160
mAh/g, VO2 nanowires possess better electrochemical property which is
due to stable structure and high surface activity.
To understand the superior performance of lithiated nanobelts for Li +
storage, we measured the electrical transport through individual MoO3
nanobelt before and after lithiation (Fig. 2b). Before lithiation, the I-V
characteristics of the nanobelt show asymmetric Schottky barriers at the
two ends (the solid curve), as created between semiconductor MoO3 (with a
bandgap of 3.1eV) and Au/Pt electrodes, and the transported current is in
the order of ca.300pA at ca.2V. After lithiation, the I-V curve shows ohmic
behavior (the dashed curve), and the transported current is of the order of
10 nA at a bias of ca.2V. This result suggests that the Li+ ions introduced
during lithiation effectively converted the MoO3 nanobelts from
semiconductor to metallic behavior. According to the measured resistance,
the effective length, and cross section of the nanobelt, the conductivity was
evaluated to be approximately10-4 S cm-1 and10-2 S cm-1 before and after
lithiation, respectively. Therefore, the conductivity was increased by close
to two orders of magnitudes via lithiation. Because the nanobelt grows
along [001], the increase of conductivity along the nanobelt implies an
increase of carrier density in the MoO6 octahedral layers. This suggests that
Li+ ions have been introduced as interstitials into the layers. During the
electrochemical cycling, interlayer spacing of MoO3 continues to be
widened/narrowed due to lithium ion insertion/extration reaction. Therefore,
lithiation can enhance structural stability of MoO3 electrode during lithium
ion insertion/extration process. The Li+ ions, first introduced during
lithiation and later remaining in the lattice, enhance the electrical
conductivity, which may assist the transport of the Li + ions to be inserted
and extracted in future charge-discharge processes.
Fig.1: TEM image (a) and initial charge/discharge curve (b) of VO2
nanowires (the inset is the curves of their specific capacity and efficiency vs.
cycle number). [5]
MoO3 nanowires, as another promising layered oxide cathode with high
specific capacity, were prepared by hydrothermal method [6]. The samples
showed a long, beltlike morphology with widths of 80–400 nm and lengths
of 5–10 μm, and a rectanglelike cross section. Secondary hydrothermal
reaction was used for chemical pre-lithiation to improve the cycling
stability of the nanowires. Briefly, to attain the lithiated MoO3 nanowires,
0.20 g MoO3 nanowires were stirred with 0.29 g LiCl in deionized water for
2 days, and the resultant light-blue solution was transferred to a Teflonlined autoclave and kept at 180 °C for 24 h. Next, the autoclave was left to
cool down in air and the solid precipitate was filtered out and washed with
deionized water. The resulting slurry was allowed to dry at 100 °C. The
Hierarchical nanowires cathodes
Hierarchical nanostructured materials such as hollow nanospheres, porous
nanostructures, nanotubes, nanowire-on-nanowire structures, and kinked
nanowires, etc. can ensure the surface remains uncovered to keep the
effective contact areas large even if a small amount of inevitable selfaggregation occurs. Moreover, if one dimension of the nano-crystallites is
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World Journal of Engineering
up to a few hundred microns or even at millimeter scale, such as ultralong
nanowires or nanobelts, self-aggregation of the nanomaterials can be
effectively prevented. Therefore, to some extent, ultralong hierarchical
nanowire is one of the most favorable structures as cathode/anode materials
for high performance lithium batteries.
Fig.5: Schematic diagram of single nanowire electrode device design. [8]
Conductivity of the single vanadium oxide nanowire was recorded during
battery operation. Fig. 6 shows the transport properties of the same single
nanowire at different charge/discharge status. Initially, the vanadium oxide
nanowire was highly conductive (Fig. 6a), agreeing with its original intact
crystal structures. Along with lithium ion intercalation by shallow discharge
with 100 pA for 200 seconds, the nanowire conductance was decreased
over two orders (Fig. 6b). The conductance change can be restored to
previous scale upon Li+ ion deintercalation with shallow charge with -100
pA for 200 seconds (Fig. 6c) indicating reversible structure change.
However, when the battery device was deeply discharged with 100 pA for
400 seconds, the nanowire conductance dropped for over five orders (Fig.
6d). This change was permanent and couldn’t be recovered even after deep
charging with -100 pA for 400 seconds, indicating that permanent structure
change happens when too much lithium ions were intercalated into the
vanadium oxide layered structures. Here, the material electrical properties,
crystal structure change and electrochemical charge/discharge status are
clearly correlated on the single nanowire electrode platform.
Fig.3: (a) FESEM images of electrospun NH4VO3/PVA composite
nanowires. (b) FESEM images of the ultralong hierarchical V2O5 nanowires
after annealing. (c) TEM image of the ultralong hierarchical V2O5
nanowires after annealing. (d) HRTEM image and (e) FFT patterns of a
single nanorod on the hierarchical V2O5 nanowires. [7]
To this end, ultralong hierarchical vanadium oxide nanowires with diameter
of 100-200 nm and length up to several millimeters were synthesized using
the low-cost starting materials by electrospinning combined with annealing
(Fig. 3) [7]. Briefly, PVA solution (10 wt%) mixed with NH4VO3 was
delivered into a metallic needle, which was connected to a high-voltage
power supply, and a piece of grounded aluminum foil was placed 20 cm
below the tip of the needle. As a high voltage of 20 kV was applied, the
precursor solution jet accelerated towards the aluminum foil, leading to the
formation of NH4VO3/PVA composite nanowires. The composite
nanowires was then annealed at 480 °C in air for 3 h to obtain ultralong
hierarchical vanadium oxide nanowires.
The hierarchical nanowires were constructed from attached vanadium oxide
nanorods of diameter around 50 nm and length of 100 nm. The initial and
50th discharge capacities of the ultralong hierarchical vanadium oxide
nanowire cathodes are up to 390 mAh/g and 201 mAh/g when the lithium
battery cycled between 1.75 and 4.0 V. When the battery was cycled
between 2.0 and 4.0 V, the initial and 50th discharge capacities of the
nanowire cathodes are 275 mAh/g and 187 mAh/g. Compared with selfaggregated short nanorods synthesized by hydrothermal method, the
ultralong hierarchical vanadium oxide nanowires exhibit much higher
capacity. This is due to the fact that self-aggregation of the unique nanorodin-nanowire structures have been greatly reduced because of the attachment
of nanorods in the ultralong nanowires, which can keep the effective
contact areas of active materials, conductive additives and electrolyte large
and fully realize the advantage of nanomaterial-based cathodes. This
demonstrates that ultralong hierarchical vanadium oxide nanowire is one of
the most favorable nanostructures as cathodes for improving cycling
performance of lithium batteries.
Fig.6: Single vanadium oxide nanowire transport properties at different
charge/discharge states: (a) initial state; (b) after Li ion intercalation
(shallow discharge with 100 pA for 200 s); (c) after Li ion deintercalation
(shallow charge with 100 pA for 200 s); (d) after deep discharge with 100
pA for 400 s; (e) after deep charge with 100 pA for 400 s. [8]
Conclusion
Layered oxides such as vanadium oxides and molybdenum oxides are
promising cathodes for high-energy lithium ion batteries, but limited by fast
capacity fading. To some extent, applying nanowire structures as the
electrodes can improve the battery life. Remarkably, hierarchical nanowires
and/or pre-lithiated nanowires show much better cycling stability than the
ordinary homogeneous nanowires with self-aggregation and poor
conductivity. Furthermore, we have reported a study of vanadium oxide
based cathode at single nanowire level and demonstrated that single
nanowire electrode can work as a versatile platform to study the correlation
between material structure changes, transport property and electrochemical
property.
ACKNOWLEDGEMENT
This work was partially supported by the National Nature Science
Foundation of China (50702039, 51072153), the Research Fund for the
Doctoral Program of Higher Education (20070497012), the Fundamental
Research Funds for the Central Universities (2010-II-016), and Scientific
Research Foundation for Returned Scholars, Ministry of Education of
China (2008-890). We express our deep thanks to Professor C. M. Lieber of
Harvard University, Prof Z. L. Wang of Georgia Institute of Technology
and Prof W. Chen of Wuhan University of Technology for careful
supervisions, strong supports, and stimulating discussions. We also thank
Dr. Y.J. Dong and Professor Y. Shao of Massachusetts Institute of
Technology for collaboration and helpful discussion.
Fig.4: (a, b) Charge/discharge curves of hierarchical vanadium oxide
nanowires at voltages of 2-4 and 1.75-4 V, respectively. (c, d) Capacity vs
cycle number, and Coulombic efficiency vs cycle number of the ultralong
hierarchical vanadium oxide nanowires. [7]
Single nanowire cathode
Until now, the fundamental mechanisms of capacity fading and the direct
relationship between electrical transport, structure and electrochemistry of
vanadium oxide and silicon nanowire electrode materials, remain largely
unexplored. This limitation is particularly critical for the development and
optimization of high-energy density Li ion battery electrode materials.
Here, we report fabrication of the smallest all-solid electrical energy storage
devices using a simple design with just one nanowire. We will take it as a
unique platform for in situ probing the direct correlation of electrical
transport, structure and electrochemistry to push the fundamental limits of
the nanowire materials for energy storage applications.
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