Abstract
This paper reports on the electrochemical lithium intercalation/deintercalation of single-crystalline 
nanowires that were synthesized by a two-step approach including a hydrothermal process and a post-calcinating treatment. Techniques of X-ray diffraction, field-effect scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy were used to characterize the structure, morphology, and composition of the as-prepared nanowires. It was found that 
nanowires were of high purity with the diameter of 
and the length up to 
. The electrochemical lithium intercalation/deintercalation of the 
nanowires was investigated by cyclic voltammograms and galvanostatic charge-discharge methods. The results showed that the single-crystalline 
nanowires exhibited high initial discharge capacity of 
, indicating their potential application as the cathode materials of rechargeable lithium-ion batteries.
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Lithium-ion rechargeable batteries have been widely used in personal portable electronics such as in the areas of cell phones, laptops, and digital cameras.1 Among the various candidates for Li-ion intercalations, vanadium pentaoxide 
has received extensive attention due to its layered structure and distinctive redox behavior.2–8 It has been found that the electrodes made from one-dimensional (1D) nanostructured 
possess high surface areas and short 
diffusion lengths, which greatly improve the discharge capacities and enhance the reversible Li intercalation/deintercalation kinetics.9–12 For example, the initial discharge capacities of both amorphous 
nanotube and layered-lamellar 
nanorolls were about 
,3, 10 which are much higher than that of the corresponding bulk. In addition, it was found that the crystallinity of nanostructured 
has a great effect on the electrode properties. The noncrystalline or low-crystalline 
showed higher initial intercalation capacity than that of crystalline 
but with poor structural stability.13, 14 Recently, the hydrothermal process has been used to prepare 1D nanostructrured vanadium oxides such as wires,15, 16 ribbons,17 rods,18 belts,19 and tubes.5, 20 Herein we report on high-purity synthesis of single-crystalline 
nanowires by a simple hydrothermal method and their lithium intercalation/deintercalation properties.
Experimental
Synthesis
The single-crystalline 
nanowires were prepared via a two-step approach, which involves the precursor preparation of 
nanowires via hydrothermal treatment of polycrystalline 
and the post-calcination of the as-synthesized 
to obtain 
nanowires. In the first step, a 
aqueous suspension of commercial polycrystalline 
(
, size of 
) and the nonionic surfactant Triton X-100 (t-octylphenyl-
, 
, 
) was stirred for 
to form an orange slurry (pH 7) that was later treated in an autoclave at 
for 
to prepare 
nanowires. In the second step, the as-prepared 
nanowires were post-calcinated at 
for 
in air to engender the target product 
nanowires
Characterization
The structure and morphology of the samples were analyzed by powder X-ray diffraction (XRD, Rigaku D/max 2500 X-ray generator with Cu 
radiation), scanning electron microscopy (SEM, Philips XL-30 and JEOL JSM-6700F Field Emission), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) (Philips Tecnai F20, 
).21
Electrochemical measurements
Electrochemical properties of the as-prepared 
nanowires were studied using two-electrode test cells with lithium metal as the counter and reference electrodes. The working electrode was fabricated by compressing a mixture of the active material (
nanowires, 
), the conductive material (acetylene black, 
), and binder (polytetrafluoroethylene, PTFE, 
) into a nickel foam. The cell assembly was similar to our previous work22 and the electrolyte solution was 
dissolved in a mixture of ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) with the volume ratio of 
. The cyclic voltammograms (CV) were measured using a Solartron SI 1287 Electrochemical Interface with a scan rate of 
at ambient temperature. The capacity and charge-discharge cycling performance of the electrode were investigated by a galvanostatic discharge-charge method recorded at the current density of C/5 
and C/20 
in the potential range of 
.
Results and discussion
Structure and morphology characterization
Figure 1a shows the XRD pattern of the as-prepared 
nanowires. The peaks can be indexed to an orthorhombic crystalline phase 
(JCPDS no. 85-2401) with calculated lattice constants 
, 
, and 
. Figure 1b shows the XRD pattern of the as-prepared 
nanowires, which can be indexed to an orthorhombic phase of 
(JCPDS no. 41-1426) with lattice constants 
, 
, and 
. Thus, the pure phase 
was obtained via the thermal decomposition of 
.
Figure 1. XRD patterns of the as-synthesized (a) precursor 
nanowires and (b) product 
nanowires.
The FESEM image in Fig. 2a shows that the as-prepared 
consists of uniform nanowires with the lengths up to 
. The typical diameter of the nanowires was found to be 
at a higher magnification (Fig. 2b). Note that one of the nanowires did not break up even when it bent to about 90°, indicating the good flexibility of the nanowires. The FESEM images of the resultant 
were shown in Fig. 2c and 2d. It can be observed that the morphology of the 
is similar to the precursor 
, which is reasonably attributed to the shape memory effect during the thermal decomposition.
Figure 2. The representative FESEM images of (a, b) 
nanowires and (c, d) 
nanowires at different magnifications. (d) Taken from the rectangle marked in (c).
Further insight into the morphology and microstructure of the nanowires was investigated by TEM and HRTEM (Fig. 3). Figure 3a shows the typical TEM image of the as-synthesized 
nanowires. Figure 3c exhibits the HRTEM image of the edge of a single nanowire shown in Fig. 3b. It is observed that the interplanar spacing is 
, which corresponds to the (200) planes of 
. The shape of 
nanowires, as shown in Fig. 3d, 3e and 3f, is similar to that of 
nanowires. A single nanowire is exhibited in Fig. 3e, and the HRTEM image of its body part marked with square is demonstrated in Fig. 3f. The observed interplanar spacing of 
agrees well with the value between the (200) planes of orthorhombic 
crystals. The clear lattice structure was observed extending to the edges (not shown), indicating the single-crystalline structure of 
nanowires.
Figure 3. Typical TEM and HRTEM images of (a-c) 
nanowires and (d-f) 
nanowires. (c) Taken from the square marked in (b). (f) Taken from the square marked in (e).
Electrochemical performance
Figure 4 shows the CVs of the electrode made from 
nanowires at a scan rate of 
. It can be seen that during the cathodic scanning in the first cycle, two distinctive peaks are observed at 2.65 and 
vs 
, being assigned to a complicated multistep lithium intercalation process.10 In the following anodic scanning, two peaks were observed at around 2.76 and 
(vs 
), corresponding to the lithium extraction processes. The 
intercalation and deintercalation process can be expressed by
Figure 4. CVs of the electrodes made by 
nanowires. (a) The first cycle (solid line), (b) the second cycle (dash line), and (c) the third cycle (dot line), with the scan rate of 
.
As lithium ions were inserted into the layers of 
, the phase transformation consecutively occurred from 
to 
, 
, 
, and 
.9, 12 Among various phases of 
, 
can be recovered to the pristine 
through lithium deintercalation, while 
and 
(rock-salt type structure) were formed irreversibly. However, for battery application, 
and 
can be reversibly cycled.23, 24
Note that the cathodic pattern in the second cycle is remarkably distinct from that in the first cycle. The main difference is the two-peaks shifting from 
and 
, suggesting that irreversible phase transformation occurred during the lithium insertion process.11 In the following CV cycles, there are no substantial changes in the peak potential and curve shape, which remain similar to those in the second cycle, indicating the reversibility of electrochemical lithium insertion/extraction.
Figure 5 illustrates the first three and the tenth discharge-charge cycles of the electrodes made from 
nanowires at a constant current density of C/20 
at room temperature. There are three obvious voltage plateaus in the first discharge curve, which is the typical characteristic of crystalline orthorhombic 
discharge pattern.11, 25 The initial discharge capacity was found to be 
, which corresponds to an intercalation of about 
into 
, revealing the formation of 
. Note that the second discharge curve exhibits an obvious indistinct discharge plateau which is different from that in the first cycle, indicating a irreversible phase transformation of 
. As a result, the discharge capacity dropped to 
in the second cycle. The third and the following discharge-charge cycles (such as the tenth cycle) are reversible and similar to the second one with less loss of capacities, which confirms the reversibility of lithium intercalation/deintercalation in 
.23 In a word, the results of CV and galvanostatic measurement are consistent not only in the occurrence of irreversible phase transformation in the first cycle, but also in the reversibility of lithium intercalation/deintercalation in the second and the following cycles.
Figure 5. Electrode discharge-charge curves of 
nanowires at the first cycle (black solid line), the second cycle (dash line), the third cycle (dotted line), and the tenth cycle (gray solid line) recorded at C/20 
.
As the current density increased from C/20 
to C/5 
, the initial capacity decreased from 
. It is reported that the theoretical discharge capacities of 
for 
and 
for 
are rarely reached for the crystalline 
.10 In our experiments, however, high initial discharge capacities were obtained with 
nanowire electrodes, indicating the formation of 
at both two current densities.
Figure 6 shows the curves of discharge capacity vs cycle number for the electrodes made from 
nanowires at two different charge-discharge current densities (C/20 and C/5). The capacities decreased with the increasing cycle number, and a rapid capacity loss is observed in the second cycle at both current densities. After the second cycle, the capacity decay was measured. Thus, the lithium ions in 
can be reversibly inserted/extracted. In the case of the current density of C/5, the capacity decays faster than that of C/20, indicating that the electrode cycle life of 
nanowires still needs improvement. It was reported that the doped 
with Mn and Mo were found of better electrochemical performance than the undoped ones.26, 27 Thus, the doped 
nanowires might be a solution to further enhance the electrochemical properties of vanadium oxides nanowires.
Figure 6. Variation of discharge capacities vs cycle numbers for the electrodes of 
nanowires at the discharge/charge current densities of (a) C/20 
and (b) C/5 
.
Figure 7 displays the SEM of the cathode mixture which was removed from the nanowire electrode after 30 discharge-charge cycles at the current density of C/5. It can be observed that the nanowire morphology of the active materials can still be preserved during the numerous charge-discharge cycles. The diameter of the nanowires is a little wider and the surface is rougher than that of the as-synthesized 
nanowires, which might be due to the coating of the binder (polytetrafluoroethylene, PTFE). In addition, a lot of chunky material is visible in Fig. 7. An explanation follows. The cathode mixture after cycling experiments was prepared by ultrasonic vibrating the electrode in ethanol. As a result, the active material of 
nanowires was taken out accompanied by the chunky conductive material (acetylene black, ATB) and the binder. The SEM observation indicates the rigid structural feature of 
nanowires, which can be tolerant of the repeated lithium intercalation/deintercalation processes, and thus is beneficial to holding the capacity of the battery.
Figure 7. SEM of 
nanowires after 30 discharge/charge cycles at C/5 
.
Conclusions
In summary, single-crystalline 
nanowires were fabricated via a two-step route including a hydrothermal process and a post-calcination treatment, which can be extended to the large-scale synthesis of 
nanowires. The electrochemical measurements showed that the electrodes made by the as-synthesized orthorhombic 
nanowires exhibited a high initial discharge capacity of 
. The improved electrochemical properties are attributed to the intrinsic structure feature of single-crystalline 
nanowires.
Acknowledgment
This work was supported by National Key Basic Research Program (2005CB623607), the Ministry of Education (2004164), and Tianjin City Project (05YFJMJC00300).
Nankai University assisted in meeting the publication costs of this article.