Selective oxidation of primary and secondary alcohols with chromium trioxide in ionic liquid

Article abstract of DOI:10.1016/j.jpowsour.2005.03.076

The oxidation of primary and secondary alcohols to corresponding aldehydes and ketones by CrO₃ in room temperature ionic liquids is an efficient and low flammability method with fast rate, facile procedure and high yields.

Full text of DOI:10.1016/j.jpowsour.2005.03.076

Journal of Power Sources 146 (2005) 753–757
Capacity degradation of lithium rechargeable batteries
J.P. Zhenga,∗, P.L. Mossa, R. Fub, Z. Mab, Y. Xinb, G. Auc, E.J. Plichtac
a
Department of Electrical and Computer Engineering, Florida A&M University and Florida State University, Tallahassee, FL 32310, USA
b
National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA
c
U.S. Army Communications-Electronics Command, Ft. Monmouth, NJ 07703, USA
Available online 25 April 2005
Abstract
Li rechargeable cells made with structural arrangement Li/membrane/LixV2O5 were examined under different charge states using ac
impedance, environmental scanning electron microscope (ESEM), transmission electron microscope (TEM) and the high-resolution nuclear
magnetic resonance (NMR). These states include charged, discharged, and over cycled. The lowest internal resistance was obtained from the
cell at charged state; the resistance increased when the cell was discharged; and the highest resistance was obtained from the cell at over-cycled
state. From the ESEM and TEM studies, it was found that the surface of cathode electrode was porous initially; however, it was coated with
an amorphous film and porous features had also disappeared from the cell at over cycled state. In addition, higher concentration of aluminum
was found at the surface of the cathode electrode in over-cycled cells. From NMR studies, Li ion signals, which correspond to Li ions in the
liquid electrolyte, on the surface of LixV2O5 cathode electrode, and inside the LixV2O5 cathode, were obtained. The mechanisms for capacity
degradation and cycle lifetime of the cell are discussed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Capacity degradation; LiV2O5 cathode; Li rechargeable batteries
1. Introduction
cess would result in depletion of the solvent in the electrolyte
and deposition of Li [8] and other films [7] on the electrode
surfaces. The depletion of the solvent and deposition of thin
films developed in the batteries contributes toward the in-
crease of battery impedance and ending of battery life. It was
also found that the charge and discharge cycling of cathodes
caused microstructural damage and cation disorder [9,10]. It
was believed that breakdown of the oxide cathode material
due to repeated lattice contraction and expansion appears to
be an important failure mode in the oxide electrodes [10].
It was also reported that the dendritic electrodeposition of
Li had been obtained in Li-polymer cells [11,12]. The Li
dendritic growth would eventually short the battery.
A Li rechargeable battery is formed from three active
components that include a metallic lithium (Li) anode, a
lithium-ion-conducting electrolyte, and a lithium insertion
cathode (such as LixMyOz; M = V, Mn, Co, Ni) [1–3].
During cell discharge and charge, Li is inserted into and
extracted from the host structures of the cathode electrodes,
respectively. The ideal electrode material and electrolyte for
Li rechargeable batteries should have outstanding electro-
chemical performance and stable capacity upon extended
cycle life. However, degradation on the anode, cathode
electrodes, and electrolyte always occurs during the charge
and discharge cycling. Various mechanisms that caused the
capacity degradation and decreased cycle life of the battery
were reported by different groups. For example, the decom-
position of organic electrolytes with gas generation during
charge and discharge cycling was obtained [4–7]. This pro-
2. Experimental
Panasonic® vanadium pentoxide (V2O5) Li-rechargeable
batteries were used as experimental samples. The button
cell batteries were made with Li (anode)/membrane/LixV2O5
(cathode), and were rated at 30 mA h. The aluminum (Al)
∗
Corresponding author. Tel.: +1 850 410 6464; fax: +1 850 410 6479.
E-mail address: zheng@eng.fsu.edu (J.P. Zheng).
0378-7753/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jpowsour.2005.03.076

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J.P. Zheng et al. / Journal of Power Sources 146 (2005) 753–757
mesh and stainless steel bottom case of the button cell were
used as the current collectors for anode and cathode elec-
trodes, respectively. The cells are 23 mm in diameter and
2.0 mm in thickness.
An Arbin battery test system was used for charge and
discharge cycling of cells. During the charge cycle, a con-
stant voltage mode of 3.4 V was used and during discharge
cycle, a constant current mode at 10 mA was used. Three dif-
ferent samples for ac impedance spectral, microscopic, and
NMR studies were prepared as follows: one charged cell was
charged to 3.4 V at a constant voltage mode; one discharged
cell was discharged to 2.1 V at a constant current (10 mA)
mode; and one over-cycled cell was continuously charged
and discharged until the cell had the capacity close to zero. A
Solartron electrochemical measurement unit (model 1280B)
was used to measure the ac impedance spectra in a frequency
range from 0.01 Hz to 20 kHz at room temperature. During ac
impedance measurements, a sinusoidal source with an ampli-
tude of 10 mV superimposed on a dc bias voltage was applied
to the cell. The dc voltages were 3.4 and 2.1 V for charged
and discharged cells, respectively. An integral step of 2 s was
also used to increase the accuracy at each frequency point
to offset the error due to the presents of noise in the lines
connecting the spectrometer to the battery.
Fig. 1. Capacities of the cell as a function of cycle numbers during discharge
process at 25 ◦C. Symbols of (ꢀ) and (ꢁ) represent charge and discharge
capacities.
the electrolyte/electrode interface and the linear portion of
this plot (0.01 to ∼0.794 Hz) represents the diffusion of Li
into the LixV2O5 structure or break of phase boundaries in
the structure. Upon closer examination it can be seen at a high
frequency of 20 kHz there was an electrolyte resistance of
about 3.1 ꢀ. The charge transfer resistance at the electrolyte
electrode interface can be determined by Zꢀ at 0.794 Hz,
and was approximately 12.2 ꢀ–3.1 ꢀ = 9.1 ꢀ. The low
frequency point (∼0.01 Hz) represents the diffusion of Li
in and out of the LxV2O5 structure. The Nyquist plot for
discharged cell shows an electrolyte resistance of the same
value as that of the charged cell. However, the charge transfer
resistance for discharged cell was greater than 100 ꢀ. The
impedance at low frequency (0.01 Hz) was also much greater
than that for charged cell. The increase in charge transfer
resistance and impedance at low frequency can be under-
stood by the fact that the concentrations of Li ions in the
After ac impedance spectral study, these cells were then
opened, and the LixV2O5 cathode electrode materials were
removed from cells in a glove box filled with argon gas for
microscopic and NMR studies.
An environmental scanning electron microscope (ESEM)
was used for imaging the surface morphology of electrodes.
The energy dispersive X-ray spectroscope (EDX) built into
the ESEM system was employed to analyze the chemical
composition of the electrode. A Jeol-2010 transmission elec-
tron microscope (TEM) operated at 200 kV and selected area
electron diffraction (SAED) built into the TEM system were
used to investigate the microstructures of the cathode materi-
als including particles morphology and crystalline structures.
The 7Li NMR measurements were performed on Bruker
7
DMX-600 spectrometers (B0 = 14.1 T) with Li NMR fre-
quency of 233.31 MHz. The 7Li spin-lattice relaxation times
were measured in a temperature range of 220–300 K.
3. Results and discussion
Fig. 1 shows the charge and discharge capacities as a func-
tion of number of cycle at about 25 ◦C. The battery was cycled
in a voltage range from 2.1 to 3.4 V. It can be seen that the
initial capacity of the cell was about 35 mA h; however, after
26 cycles, the capacity reduced to less than 15% of the initial
capacity.
Fig. 2 shows Nyquist plot of three cells at 25 ◦C. The
Nyquist plot for the charged cell shows a small semi circle in
the frequency range from 0.794 Hz to 20 kHz and a linear plot
atanangleof52◦ withfrequencyrangefrom0.01to0.794 Hz.
The semi circle in this plot represents the charge transfer at
Fig. 2. The Nyquist plot of cells at (a) charged, (b) discharged, and (c)
over-cycled states in the frequency range from 0.01 Hz to 20 kHz at room
temperature.

J.P. Zheng et al. / Journal of Power Sources 146 (2005) 753–757
755
electrode surface region and in LixV2O5 were high and as
a result the diffusion of Li ions in and out of the surface
region and the structure were low. The Nyquist plot from
over-cycled cell shows a dramatic increase in the electrolyte
resistance at 20 kHz which is directly related to a decrease in
ionic conductivities. This decrease in conductive indicates a
decrease in cycle life. At both high and low frequencies the
transport of Li ions flowing from the anode to the LixV2O5
structure was very low.
surface morphologies of LixV2O5 cathode electrodes at the
interface with membrane for cells from charged and over cy-
cled states, respectively. It can be seen from Fig. 3a that the
electrode was formed with particles in microsize; the bound-
ary between particles could clearly be seen. The voids and
grooves up to 10 ␮m could also be observed. Similar sur-
face morphology was obtained from the cathode electrode of
the discharge cell. However, from Fig. 3b for an over cycled
cell, the boundary between particles could not be identified.
In order to closely examine the cathode electrodes at dis-
charged states, ESEM system was used to study the surface
morphology and chemical composition. Fig. 3a and b show
Fig. 3. ESEM images of cathode surfaces from (a) charged and (b) over-
cycled cells. The surface faced to membrane in the cell.
Fig. 4. (a) TEM image of cathode material from a charged cell, and (b)
SAED pattern from the large particle.

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J.P. Zheng et al. / Journal of Power Sources 146 (2005) 753–757
The voids and grooves had also disappeared. The lines on the
surface are traces of membrane surface.
TEM was also used to image particles in the cathode elec-
trode. Fig. 4 shows a TEM image of the cathode material
from a charged cell. From TEM images, we can see that two
different groups of materials were found from the electrode.
The majority material was formed with large particles in the
order of ␮m. The SAED patterns indicated that large particles
were single crystalline phase of LixV2O5, close to the crys-
tal structure of V2O5. Amorphous regions containing small
particles were also present, these amorphous regions have
high amount of Al from the EDX analysis. From both TEM
and EDX results, there is no ascertainable difference in the
general features among three cells at different charged states
including over-cycled state. It indicates that no microstruc-
tural crash or damage occurred in V2O5 crystals.
The chemical compositions at the cathode surface were
analyzed using EDX. The system can only detect elements
that are heavier than oxygen; therefore, Li and other com-
ponents from the electrolyte could not be identified. It was
found that the intensity of Al signals from the over-cycled cell
was higher than that from charged cell. The result indicated
that the Al was deposited on the surface of cathode during
the charge and discharge cycle; the only source for Al in
the cell is the current collector from the Li anode electrode.
The Al must have been dissolved in the electrolyte before
the deposition on the cathode electrode. The Al distribution
inside the cathode electrode was also investigated by map-
ping the Al signal on a cross-section of cathode electrode.
It was found that in general the Al concentration inside the
electrode was much lower than that on the surface of elec-
trode, and was quite uniformly distributed inside electrode.
This observation is reasonable, since the deposition would
occur on surface and along the boundary of particles. There-
fore, where the voids and grooves existed, it would have high
concentration of Al.
Fig. 5 shows the 7Li NMR spectra for the LixV2O5 sam-
ples at the different charge states. Clearly, there exists a broad
peak at −0.4 ppm in the spectra. For discharged state, the res-
onance at −23.6 ppm has much greater intensity than that at
−0.4 ppm and such can be attributed to the excess Li ions
inserted into the LixV2O5 cathode electrode during the dis-
charge process. For the charged state, the peak intensity at
−17.6 ppm is on the same order of that at −0.4 ppm. For
the over cycled state, no resonance signal is observed except
one at −0.4 ppm. From temperature dependence of 7Li NMR
spectra as well as the spin–lattice relaxation times measure-
ments, it was found that there were two components in the
resonance at −17.0 to −23.0 ppm as shown in Fig. 6. The sig-
Fig. 5. The 7Li NMR spectra of LixV2O5 cathode materials from three
different charge states: (a) discharged state, (b) charged state, and (c) over
cycled state. The asterisks indicate the spinning sidebands.
Fig. 6. Temperature dependence of the 7Li chemical shifts for the LixV2O5
cathode materials at (a) the discharged and (b) the charged states.

J.P. Zheng et al. / Journal of Power Sources 146 (2005) 753–757
757
structure was obtained from the cathode electrode of over
cycled cell. After considering all experimental observations,
we suggested that the capacity degradation and cycle life of
Li–LixV2O5 cells are due to the decomposition of organic
electrolyte during charge and discharge cycling. This pro-
cess would result in depletion of the solvents in the electrolyte
and also deposition of Li, Al, and other materials on the sur-
face of the cathode electrode, which reduced the porosity of
the electrode. The depletion of the solvent and deposition
of thin film on the cathode electrode caused the increase of
cell impedance and ending of the battery life. The degrada-
tion mechanism for Li–LixV2O5 cells is similar to the for
Li–MoS2 cells [6].
Fig. 7. Static 51V NMR spectra of different LixV2O5 cathodes at room tem-
perature.
Acknowledgements
nal at −17.0 is almost temperature independent as shown in
Fig. 6, for both the charged and discharged states. This reso-
nance peak cannot be from Li-metal because the Knight shift
for metallic lithium is typically in the range of 250–360 ppm
[13,14]. While the signal at −17.0 ppm has similar intensity
with respect to the peak intensity at −0.4 ppm for both the
charged and discharged states, it is thus attributed to the Li
ions on the surface of the V2O5 host, since it is reasonable
to believe that the V2O5 hosts in the different charge states
have similar surface area.
This work was partially supported by US Army
Communications–Electronics Command. RF thanks Pro-
gram Enhancement Grant (PEG) (Project #550240537) from
Florida State University. XY thanks NHMFL funded by NSF
under cooperative agreement DMR-0084173 and the State of
Florida.
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7
from the high-resolution Li NMR spectra, two groups of
Li were obtained from the cathode electrode of charged and
discharged cells, they were located inside and outside of the
V2O5 structure; however, only the Li located outside of V2O5