Scholars Journal of Engineering and Technology (SJET) ISSN 2321-435X (Online)

Scholars Journal of Engineering and Technology (SJET) ISSN 2321-435X (Online)
Scholars Journal of Engineering and Technology (SJET)
Sch. J. Eng. Tech., 2014; 2(6A):828-836
ISSN 2321-435X (Online)
ISSN 2347-9523 (Print)
©Scholars Academic and Scientific Publisher
(An International Publisher for Academic and Scientific Resources)
www.saspublisher.com
Research Article
In-situ Deposition Route Synthesis of Graphene Oxide-Sulfur Composites as
Rechargeable Lithium-Sulfur Battery Cathode
Xu Meiling, Wang Xinhui, Ren Yinzhe*
College of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, Shanxi, China
*Corresponding author
Ren Yinzhe
Email:
Abstract: Na 2 S2 O3 was served as a source of sulfur, and graphene oxide-sulfur composites were synthesized by liquid
phase in-situ deposition method. In order to observe surface morphology, phase structure and electrochemical properties
of the composites, the prepared products had been characterized by X-ray diffraction (XRD), fourier transform infrared
spectroscopy (FT-IR), scanning electron microscopy(SEM), charge-discharge measurements and electrochemical
impedance spectroscopy(EIS). Comparing properties of composites synthesized by liquid phase in-situ deposition
method and conventional method, the results showed that the former exhibited better crystal structure, more successful
combination between sulfur and graphene oxide and good electrochemical performance. The in-situ composite was with
first discharge capacity of 589mAh/g at the current density of 50mA/g, maintaining 241mAh/g after 20 cycles. And the
reversible capacity retention was 40.9%.
Keywords: Lithium-sulfur battery cathode, Graphene Oxide-Sulfur Composites, Liquid phase in-situ deposition route
INTRODUCTION
As increasing concern about climate change, the
environment, limited global energy supply, the demand
of clean and efficient energy storage devices is also
growing steadily. Lithium ion batteries with high
energy density have become the main force of portable
electronic devices market. At present, compared to the
specific capacity of anode materials (graphite: 370
mAh/g, Si: 4200 mAh/g), the low specific capacity of
cathode materials (layered oxide: 150 mAh/g,
LiFePO4:170 mAh/g) is still an important factor to limit
battery energy density [1].With low cost, environment
friendly, high theoretical specific capacity ,sulfur has
become a promising cathode material. However, its low
electronic conductivity, soluble intermediate polysulfide
and volume expansion lead to short cycle life, low
specific capacity and bad energy utilization of lithiumsulfur battery, which restrict the commercial
applications [2]. In this paper, on one hand, the
traditional sublimed sulfur was improved, we produced
sulfur with smaller size by reacting with as sulfur
source; On the other hand, using in-situ deposition
method ensured that the sulfur and graphene oxide
combined more evenly, thus the electrochemical
performance of the battery was obviously improved.
EXPERIMENTAL SECTION
Preparation of graphite oxide
Graphite oxide was prepared by modified Hummers
method [3], divided into three stages.
Low temperature stage: first, weigh 200 ml of
concentrated sulfuric acid in drying beaker, place it in
the ice water bath to cool to about 2˚C, add 3.0g of
graphite powder and 6.0g of in turn, mix well. 40 min
later, add 9.0 g of slowly, keep reacting for 5 h in the
ice water bath with continuous magnetic stirring, in the
process, the color of solution changes from black to
green.
Medium temperature stage: warm the solution to
35˚C, last for 90 min.
High temperature stage: continue to heat up to 75˚C,
keep for 1 h, with bubbles generated in the process, the
solution changes from green to brown.
Return the solution to room temperature, stir and
slowly add a certain amount of deionized water. Then,
add 30% slowly, the solution turned into bright yellow
and there are a lot of air bubbles generated, stop adding
until no bubbles turn up. The obtained solution was
washed with 5 wt % and deionized water , until the PH
of solution was close to neutral ,the brown colloidal
substance was obtained, dry in the drum wind drying
oven under constant 60 ˚C. At last, we got sheet
graphite oxide.
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Preparation of Graphene oxide / sulfur (GO/S)
composite
A. The liquid phase in-situ deposition method
synthesis
0.200 g of graphite oxide was dispersed into 200 ml
of deionized water, ultrasonic for 2 h to form 1 mg/ml
graphene oxide solution. Add isopropyl alcohol as
dispersants. Slowly add 300 ml of 0.3 mol/L solution
under ultrasonic. After 10 min, 100 ml of 0.3 mol/L was
added drop wise, quickly the solution changed from
brown to grayish white, continue ultrasound for 30 min.
Then transfer to the magnetic stirrer, stir for 24 h at
room temperature. At last, we heated the as-synthesized
samples in an argon (Ar) environment at 155˚Cfor 12 h.
The liquid phase in-situ composite was obtained.
B. The conventional method synthesis
By contrast, we synthesized pure sulfur by the same
method, the difference only lied in that the method first
drop into solution, after the complete reaction, add
graphene oxide solution. Herein, we prepared the
conventional
method
composite.
Structural characterization
The synthesized samples were examined by an X-ray
diffractometer (Tokyo Rigaku Ultima IV-185 type) with
Cu-Kα radiation( λ=0.15406nm) between 10°and 80°at
a scan rate of 5°/min. The microstructure and
morphology of the samples were characterized by
scanning electron microscopy(SEM,JSM-7500F).More
detailed structural information on the composites were
obtained by using a Fourier transform-infrared
spectrometer(FT-IR,Varian640).The sulfur content in
the composite was determined by thermogravimetric
analyzer (TGA,SETARAM LABSYS).
Cell assembly and electrochemical measurements
The composite was mixed with acetylene black
conductive agent and poly(vinylidene fluoride)(PVDF)
binder in a weight ratio of 80:10:10 in N-methyl
pyrrolidone(NMP) solution with a magnetic stirrer for
8h.Then, the well-mixed slurry was uniformly pasted
onto an Al foil with a blade and dried in a electrothermostatic blast oven at 60˚C for 12h,followed by
pressing with a roller under a pressure of 20MPa and
punching out circular electrodes of 1.1cm in diameter.
The cathode electrodes were dried in a vacuum oven at
60˚C for 3h before transferring into an Argon-filled
glove box. The active material loading density of the
electrode is ca. 3.0mg•cm-2 .The electrolyte was 1M
LiPF6 dissolved in a mixture of dimethyl carbonate
(DMC) and ethylene carbonate(EC)(1:1,v/v) .The
CR2016 coin cells were assembled with the prepared
cathode disks, the electrolyte ,the microporous
polypropylene separators(Celgard2300),nickel foam
current collectors and lithium sheets as counter
electrode and reference electrode in a glove box filled
with argon.
The charge-discharge profiles, cyclability and rate
capability were assessed with Land cell test system.
Parameter Settings are as follows:
Let stand for 30 s, voltage range from 1.0 V to 3.0 V,
galvanostatic charge/ discharge current density of
50mA/g (25 mA/g, 100 mA /g).
Electrochemical impedance spectroscopy (EIS) was
achieved with an amplitude of 10mV at the applied
frequency range from 1MHz to 10MHz on
electrochemical workstation (CHI660C).
RESULTS AND DISCUSSION
Structure analysis
Fig.1 compared the X-ray diffraction (XRD) patterns
of the graphite, graphite oxide, sublimed sulfur and
composite prepared by liquid phase in-situ deposition
method. The graphite showed sharp crystalline peak at
2θ=26.4°, the corresponding graphite interlayer spacing
was 0.3364nm. Graphite oxide, with original
characteristic diffraction peak of graphite disappearing,
a new wider diffraction peak appeared at 2θ= 11.9 °.The
corresponding interlayer spacing became 0.739 nm,
indicating that graphite was completely oxidated by
oxidant, the insertion of oxygen-containing functional
groups (carboxyl, carbonyl, hydroxyl group, epoxy
group) made the interlayer spacing increase. In the
composite, characteristic diffraction peak of graphite
oxide disappeared, maintain the characteristic
diffraction peak of sulfur, but the peak intensity
weakened, peak shape was not simple accumulation of
graphite oxide and sulfur diffraction peaks, graphite
oxide reacted with part of sulfur to form new chemical
bonds (the later FT-IR characterization also confirms
this).There were still a part of elemental sulfur scattered
on the graphite oxide. Dispersion effect of graphite
oxide prevented the accumulation of sulfur largely, thus
peak intensity was reduced, showing that elemental
sulfur was with smaller particle size.
Fig. 2 showed the FT-IR graph of graphite, graphite
oxide and the composite prepared by liquid phase insitu deposition method. In graphite oxide spectrum, we
found that a weak absorption peak was at 1720 cm-1, it
corresponded to C=O stretching vibration of carboxyl
and carbonyl. Absorption peak at 1390 cm–1 attributed
to the C- OH stretching vibration. Strong absorption
peak at 1080 cm-1 corresponded to the C-O stretching
vibration. All above indicated polar groups significantly
increase after the graphite oxidation, the surface of
graphite oxide did exist a large number of oxygencontaining functional groups. This provided favorable
conditions for the preparation of composite using
graphene oxide to fix sulfur later [4]. There was an
obvious absorption peak at 1206 cm–1 for composite,
which did not appear in graphite oxide spectrogram. It
agreed with C-S absorption peaks, this also proved that
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the composite was not only the mixture of graphite
oxide and pure sulfur but also they reacted chemically
simultaneously. Sulfur replaced part of oxygen on
functional groups, forming C - S bond [5].
Fig.1 XRD patterns of the graphite (a), graphite oxide (b), sublimed sulfur(c) and the composite(d)
Fig. 2: FT-IR graph of graphite(a), graphite oxide(b) and the composite(c)
Fig. 3a and b illustrated the morphology of graphite
and graphite oxide, typical lamellar structure can be
seen in graphite. After oxidation, the layer spacing was
bigger. This result indicated that oxygen-containing
functional groups inserted interlayer successfully under
the action of oxidants [6]. Fig. 3c and d showed the
surface microstructure of the sublimed sulfur and newmade sulfur by 0.3mol/L and 0.3mol/L. Particle size
distribution of sublimed sulfur was very uneven from a
few microns to dozens of microns. Compared with
sublimed sulfur, the particle size of new-made sulfur
with chain structure decreased greatly and was uniform
between 2 ~ 3 microns, some sulfur tended to reunite.
Fig. 3e and f showed the SEM image of the as-prepared
GO/S composite before and after heat treatment
respectively. Before heat treatment, there were a lot of
sulfur on the surface of the composite, the sulfur
particles were fairly evenly distributed on the composite
surface with less than 100nm diameter, indicating that
graphene oxide can restrain reunion of new-made
sulfur, the formation of small sulfur molecules was
more advantageous to improve the electrochemical
performance of composite. After heat treatment, As part
of sulfur evaporate and the other part was fused into
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Xu Meiling et al., Sch. J. Eng. Tech., 2014; 2(6A):828-836
graphene oxide interlamination, sulfur on the surface of
the composite significantly reduced [7].
Fig. 3: SEM image of (a) graphite; (b) graphite oxide; (c) sublimed sulfur ;(d) new-made sulfur by
0.3M Na 2 S2 O3 and 0.3M HCl ; liquid phase in-situ GO/S composite before (e) and after( f) heat treatment in Ar
environment for 12h at 155˚C.
Fig.4 was thermogravimetric analysis (TGA) curve of
graphite oxide (a) and in-situ GO/S composite(b) after
heat treatment. As shown in the figure, the weight of
graphite oxide after calcination in the tube furnace
reduced by about 5% in the TGA temperature range,
considering moisture loss, mass loss caused by the
decrease of functional group was less. Besides, graphite
oxide would not burn in the temperature range. Some
oxygen-containing functional groups in composite were
replaced by sulfur, ignoring the mass loss caused by the
decrease of functional groups, the result indicates that
sulfur burning leads to a decrease of composite mass.
Therefore, we can directly read the sulfur content of
GO/S composite from the TGA [5]. And at calcination
temperature of 155 ˚C, the mass of the composite had
no change, showing that excess sulfur and part of
oxygen-containing functional groups have been
removed in the previous tube furnace burning and the
composite is stable. The composite began
weightlessness at 220 ˚C and sulfur combusts
completely at about 360 ˚C, after that, the mass
remained constant. It was concluded that sulfur content
of the composite was 74.4%.
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Fig. 4: TGA curve of graphite oxide (a) and in-situ GO/S composite (b) after heat treatment recorded in N2 with a
heating rate of 5˚C/min.
Electrochemical performance
The first discharge profiles of liquid phase in-situ
deposition composite, conventional method composite
and pure sulfur cathodes at a current density of 50mA/g
were shown in Fig.5. The first discharge plateau of insitu composite was at around 2.26V and relatively
stable, specific capacity reached 589mAh/g. The initial
discharge plateau of conventional method composite
was at around 2.21V and falls faster, at the end of the
discharge, exhibiting specific capacity of 429mAh/g. In
contrast, the first discharge plateau of pure sulfur
cathode was slant, the average voltage was 2.15 V or so,
the first discharge specific capacity was only
385mAh/g, which was lower than the previous two
methods composite. Discharge platform voltage was
closely related to the battery discharge mechanism [8].
Discharge plateau differences between in-situ
deposition composite and the conventional method
composite might result from different crystal shapes and
particle sizes of sulfur synthesized by the two methods
and the combination of graphene oxide and sulfur in
different ways and so on. Pure sulfur cathode utilized
sublimed sulfur with larger particle size and uneven
distribution as sulfur source directly, only add the
acetylene black as conductive agent, no graphene oxide
served as conductive and support role. As shown in the
figure, its first discharge capacity was the lowest of the
three. Thus new-made sulfur based on the aqueous
reaction can improve battery performance greatly [9],
and in-situ deposition reaction based on aqueous
solution can produce smaller sulfur molecules which
were better dispersed on the graphene oxide with
excellent performance, even interacted with graphene
oxide to form strong chemical bonds. These can not
only improve the conductivity of sulfur but also fix
intermediate polysulfides to prevent them dissolve in
the electrolyte.
Fig.6 displayed the second charge/discharge profile
of liquid phase in-situ composite and pure sulfur
cathode (50mA/g). As seen from the figure, the second
discharge specific capacity of liquid phase in-situ
composite was 476mAh/g, representing a 80.8%
capacity retention. Charging to a certain stage, the
voltage rose steeply. The second discharge specific
capacity of pure sulfur cathode was 278mAh/g with
capacity retention of 72.2%.And when the charging
voltage reached a certain value (3.2 V), the voltage
never rose steeply, the battery would never reach a full
charge state [10]. This was due to the fact that sulfur
and intermediates had not been well fixed, so
polysulfide ions dissolved in electrolyte, the battery
appeared more serious shuttle effect. The shuttle current
formed by shuttle effect exceeded the charging current,
which lead to severe polarization phenomenon in the
battery.
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Fig. 5: The first discharge curve of liquid phase in-situ deposition GO/S composite (a); conventional method
composite (b) and pure sulfur(c) cathodes at a current density of 50 mA/g
Fig. 6: The second galvanostatic charge/discharge profiles of liquid phase in-situ GO/S composite (a) and pure
sulfur (b) cathode (50mA/g)
During charging, thermodynamics process of
oxidizing high-order polysulfide into elemental sulfur
was very slow. So at the end of the charging, a large
number of active substances existed in the electrolyte in
the form of high-order polysulfide. Therefore, from the
second discharge on, discharge active substances had a
big difference with that of the first time, only a small
amount of active substances began to discharge from
the elemental sulfur, other active substances discharged
from (4≤n≤8). Discharge end-products and
were
insoluble in the electrolyte, but deposited on the surface
of the cathode structure, they themselves were insulated
and can not contact with conductive materials
adequately, making this part of sulfur inactive, leading
to capacity irreversible decay in Li-S batteries [11-14].
d.100mA/g) and pure sulfur cathode (a.50 mA/g) were
shown in Fig.7.As previously encountered, the
discharge specific capacity of liquid phase in-situ
composite cathode decreased with increasing current
density. But they were all much higher than that of pure
sulfur cathode under the current density of 50 mA/g,
representing liquid phase in-situ composite with
excellent rate and cycle performance. On one hand,
liquid phase in-situ method can not only synthesize
small molecule sulfur with uniform particle size but
also obtain more homogeneous composites. On the
other hand, the addition of graphene oxide can prevent
sulfur reuniting, suppress the migration of elemental
sulfur and soluble intermediates, adapt to the volume
changes of sulfur during the cycling process and
maintain the structural integrity of the cathode [14-18].
The cycle life plots of liquid phase in-situ composite
cathode at various current density (b.50mA/g c.25mA/g
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Fig. 7: The cycle performance of liquid phase in-situ composite cathode at various current density(b.50mA/g
c.25mA/g d.100mA/g) and pure sulfur cathode(a.50 mA/g)
To
understand
the
reason
for
superior
electrochemical performance of liquid phase in-situ
composite, EIS measurements were carried out with
coin cells. The Nyquist profiles of liquid phase in-situ
composite, conventional method composite and pure
sulfur cathode and the equivalent circuits were shown in
Fig.8. Rel: resistance of electrolyte; Rsl: total resistance
of the surface layers on the sulphur and lithium
electrodes; CPEsl: distributed capacitance of the surface
layers of both the sulphur and lithium electrodes; Rer:
resistance to charge transfer on the sulphur electrode;
CPEer: a double layer capacitance distributed on the
surface of the pores in the sulphur electrode; W: the
Warburg impedance. Comparing the impedance
spectrum of three kinds of materials cathode before
cycling, we can find that diameter of the semicircle
increases in turn, the difference between the slope of the
straight line was not obvious, the intersection point of
the left side of semicircle with the horizontal axis was
basically the same. The above results showed that the
charge transfer resistance of liquid phase in-situ
composite cathode was the least, which should be
related that the method can synthesize uniform
composite, graphene oxide and sulfur combines firmly
to form a strong interaction, graphene oxide provided
convenient electronic transmission channel for
insulating sulfur. This was also consistent with the
result that in-situ composite had the highest first
discharge capacity in later cycle. And the lithium ion
diffusion degree of three kinds of materials in the
beginning was alike. The electrolyte impedance of three
kinds of cathodes was basically the same, illustrating
the composition and property of the electrolyte was
stable in the process of battery assembly [19, 20].
Fig. 9 was the electrochemical impedance spectra
of liquid phase in-situ composite before and after 20
cycles. As the figure shown that charge transfer
impedance of the composite cathode increases, the
charge transferred through electrode and electrolyte
interface became difficult, but the added value was
relatively small. The result showed that electrode
structure was damaged to a certain extent during
cycling, but generally, the structural integrity was
still relatively good. The slope of the straight line
decreased, indicating that irreversible deposited on
the electrode and hindered the diffusion of lithium
ions with the charge/discharge processing [21].
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Fig. 8: EIS spectrum of liquid phase in-situ GO/S composite (a); conventional method composite (b) and
pure sulfur (c) cathode before cycling and the equivalent circuits
Fig. 9: EIS spectra of liquid phase in-situ composite before (a) and after (b) 20 cycles
CONCLUSION
In short, we used chemical in-situ deposition method
in aqueous solution to prepare GO/S composite with the
excellent performance, which not only offered a lowcost and controlled approach for large-scale production
but also produced high-purity active material.
Compared with the conventional method composite and
pure sulfur cathode, the in-situ composite possessing
uniform and stable sub-micron structure exhibited a
higher specific capacity and cyclic stability. The initial
discharge specific capacity was 589mAh/g under the
current density of 50mA/g, the value of conventional
method composite and pure sulfur cathode were
respectively 429mAh/g and 385mAh/g, the specific
capacity of in-situ composite after 20 cycles was also
the highest. Thus, this new method improved the
electrochemical performance of the Li-S battery to
some extent.
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