crystals
Article
The New Method of XRD Measurement of the Degree
of Disorder for Anode Coke Material
Zhuo Zhang 1,2 and Qi Wang 1, *
1
2
*
The Key Laboratory of Chemical Metallurgy Engineering, University of Science and Technology Liaoning,
Anshan 114051, China; [email protected]
School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China
Correspondence: [email protected]; Tel.: +86-0412-592-9537; Fax: +86-0412-592-953
Academic Editor: Helmut Cölfen
Received: 6 November 2016; Accepted: 24 December 2016; Published: 18 January 2017
Abstract: Quantitative analysis by X-ray powder diffraction of two cokes (pitch coke and petroleum
coke) shows that their crystal structure changed with increasing temperature. The crystal data
processing of the crystallization degree of disorder is used with further improvement of the proposed
microcrystalline-stacking fault calculation method. With this improvement it is now possible to obtain
the degree of stacking disorder of two cokes applied as anode materials at different graphitization
temperatures. Raman spectroscopy verified the accuracy of this method, which is more reliable
than the crystal structure refinement using the d002 method. This paper provides the theoretical
analysis and interpretation of the relationship between the microstructure model of the material and
quantitative data, discharge capacity, and the first charge-discharge efficiency.
Keywords: degree of disorder; X-ray diffraction; microstructure; anode material
1. Introduction
Graphite-2H [1] is a popular lithium-ion battery anode material, owing to its excellent
electrochemical properties, such as charge and discharge capacity and first charge efficiency. According
to materials science, the properties of materials are decided by their crystal microstructures.
An important indicator of artificial graphite microstructure is the stacking disorder of graphite carbon
atoms (P), which must be determined in material production units and battery production units.
There are many ways to characterize the degree of disorder of carbon anode materials, such as
the whole powder pattern fitting method [2], d002 method [3], and the newer Langford method [4–6].
Shi et al. [2] proposed the “single” and “double” models, which rely on full spectrum fitting. The
program calculates the degree of disorder of the carbon material, producing more reliable results.
Unfortunately, the calculation is very complicated and requires a custom computer program that is not
easily implemented in a production facility that requires immediate results. Based on the previous
method, Michio Inagaki et al. [3] proposed that d002 is an important parameter to characterize the
stacking disorder degree. The d002 calculated process is relatively simple, but its credibility is low, with
only an auxiliary reference value. Li et al. [5] proposed an improved Langford method to calculate
degree of disorder of graphitization materials. The method is easier than d002 method due to a simpler
calculation procedure. However, meaningful quantitative results for graphite materials were only
acquired using this method at high temperature (2800 ◦ C). The study found that it is difficult to obtain
reliable results for graphite in coke anode materials over a wider temperature range. Graphitization
under different temperature is very important to research for the mechanism and properties of high
performance carbon materials. Therefore, we believe that the principle of the method is justifiably
correct, but it is necessary to improve the method for a wider range of materials and for different
graphite preparation processes.
Crystals 2017, 7, 5; doi:10.3390/cryst7010005
www.mdpi.com/journal/crystals
Crystals 2017, 7, 5
2 of 10
In this paper, the method is improved and the results of this improved method can be used
to adjust the carbon anode materials processing parameters, which are needed to achieve higher
performance. This paper reports the results of quantitative X-ray diffraction (XRD) analyses. This study
aims to determine the degrees of microcrystalline structure disorder of petroleum coke and pitch coke
at the graphitization temperature range of 1800–2800 ◦ C.
Langford used the current method to calculate the stacking degree of disorder of the carbon anode
materials, but was unable to obtain meaningful results. To obtain reasonable results, the Langford
method needs to be improved. In order to determine the appropriate improvement method, the
characterization of the anode materials was compared with Raman spectra and the d002 methods. The
results show that the improved Langford method is consistent with the calculated results of Raman
spectroscopy and the d002 methods qualitatively, but more reliable and accurate than the latter two.
Changes in electrochemical properties verify the accuracy of the proposed method.
2. Experimental
2.1. Sample Preparation
The samples selected for investigation are petroleum coke (PC) and pitch coke (TC). The samples
were heat-treated from 1800 ◦ C to 2800 ◦ C for 1 h in pure Ar flow. The experimental scheme and
respective sample numbers are shown in Table 1.
Table 1. The different temperatures list and the post-processing number.
Sample
Numbers
Raw Material
1800 ◦ C
2000 ◦ C
2200 ◦ C
2400 ◦ C
2600 ◦ C
2800 ◦ C
Petroleum coke
pitch coke
PC18
TC18
PC20
TC20
PC22
TC22
PC24
TC24
PC26
TC26
PC28
TC28
2.2. Characterization
2.2.1. XRD Technique
X-ray powder diffraction analysis was performed using Rigaku Dmax2200PC diffractometer
(Rigaku Corporation, Tokyo, Japan) and Cu Kα-radiation. The X-ray intensity was measured in the
range of 5◦ ≤ 2θ ≤ 90◦ with a scan speed of 2◦ ·min−1 . The peak position of the 002 coke peak was
measured. Using Bragg’s law, the interlayer d-spacing was culated. The improved Langford method
was used to calculate the stacking disorder degree, P.
2.2.2. Raman Technique
The wavelength of the laser was 514.5 nm. The detection range of wave number is 500–3000 cm−1 .
The full widths were measured at the half maxima of the peak D and peak G to give a quantitative
analysis of the microstructure. The intensity ratio of the D and G peak was measured to indirectly
show the graphitization degree.
2.2.3. Electrochemical Measurement
The electrochemical performance test used multi-channel Aibin half-cell test systems (Arbin
Instruments, College Station, TX, USA). It tests discharge capacity and the first charge-discharge
efficiency, which measures the electrochemical properties of anode material at different temperatures.
2.3. Review of the Stacking Degree Disorder of 2H-Graphite Method Proposed by Li et al.
To illustrate our calculation method, a brief review of the method of calculation by Li et al.
is provided. Hexagonal grids of graphite are defined by ABCABC... or ABAB... stacking rules.
The former is the 2H-graphite hexagonal structure and part of the P63/mmc (No. 194) space group,
2.3. Review of the Stacking Degree Disorder of 2H-Graphite Method Proposed by Li et al.
To illustrate our calculation method, a brief review of the method of calculation by Li et al. is
provided. Hexagonal grids of graphite are defined by ABCABC... or ABAB... stacking rules. The
Crystals 2017, 7, 5
3 of 10
former
is the 2H-graphite hexagonal structure and part of the P63/mmc (No. 194) space group, the
latter is 3R-graphite diamond or rhombohedral structure and is part of the R3 (No. 146) space group
[7].the
Structure
are shown
in Figure
1a,b.
latter ismodels
3R-graphite
diamond
or rhombohedral
structure and is part of the R3 (No. 146) space
group [7]. Structure models are shown in Figure 1a,b.
A
A
B
B
A
C
B
A
(a) Hexagonal graphite
(b) Rhombohedral graphite
Figure 1. Crystal structures for two modifications of graphite. (a) Hexagonal graphite; (b)
Figure 1.
Crystal structures for two modifications of graphite.
(a) Hexagonal graphite;
Rhombohedral
graphite.
(b) Rhombohedral
graphite.
If stacking webs have no regularity (i.e., neither ABAB... nor ABCABC..., which are periodically
If stacking webs have no regularity (i.e., neither ABAB... nor ABCABC..., which are periodically
arranged parallel stacking modes) then this is considered a stacking fault. The samples were heatarranged parallel stacking modes) then this is considered a stacking fault. The samples were
treated from 1800 °C to 2800
°C. 2H-graphite still has non-ideal ABAB.... stacking, a stacking fault.
heat-treated from 1800 ◦ C to 2800 ◦ C. 2H-graphite still has non-ideal ABAB.... stacking, a stacking
The stacking fault will result in XRD diffraction line broadening, an effect known as stacking fault
fault. The stacking fault will result in XRD diffraction line broadening, an effect known as stacking
broadening.
fault
broadening.
The
grain size also affects the diffraction line width, called the microcrystalline width. Thus, the
The
size Formula
also affects
diffraction
line Full
width,
calledatthe
microcrystalline
width. Thus,
relationshipgrain
among
busthewidth,
i.e., the
Width
Half
Maximum (FWHM)
β,
the
relationship
among
Formula
bus
width,
i.e.,
the
Full
Width
at
Half
Maximum
(FWHM)
microcrystalline broadening, βC and layer fault broadening, βP, can be expressed in Equation
(1) [5]: β,
microcrystalline broadening, βC and layer fault broadening, βP , can be expressed in Equation (1) [5]:
(1)
β = βc + βp
β = βc + βp
(1)
For a close-packed hexagonal crystal structure, assuming that structures in the samples are
nearly equiaxed
crystal, we
can use the
following
Equations:
For a close-packed
hexagonal
crystal
structure,
assuming that structures in the samples are nearly
equiaxed crystal, we can use the following
β =Equations:
βc + Mcosфz
h − k =β3n
orc hk0
M z= 0
=β
+ Mcosφ
(2)
(2)
h – k = 3n ± 1, l even M = 3P/(2c)
(3)
h − k = 3n or hk0 M = 0
hh–- k == 3n
MM
= P/(2c)
(3)
3n ±±1,1,l lodd
even
= 3P/(2c)
h - k = 3n ± 1, l odd M = P/(2c)
where c is the lattice constant of the C-axis direction, and Φz is the angle between the diffraction
surface
surface
(001).
Li etdirection,
al. [5] places
formula
and Equation
(3)
whereand
c ishexagonal
the latticebase
constant
of the
C-axis
andthe
φz Scherrer
is the angle
between
the diffraction
into
Equation
and creates
the
following
equation
on FWHM,
β, and formula
the diffraction
angle, θ,(3)
surface
and(2)
hexagonal
base
surface
(001).
Li et al.based
[5] places
the Scherrer
and Equation
which
meets the(2)
conditions
of the
of the diffraction
of 2H-graphite[5]:
into Equation
and creates
thecalculation
following equation
based onlines
FWHM,
β, and the diffraction angle, θ,
which meets the conditions of the calculation of
thecosф
diffraction lines of 2H-graphite [5]:
.
101 cosθ101
=
+
(4)
2


β101cos θ101
D
0.89
cos φz101 cos θ101
=
+
(4)
λ
2c
λ
D
where P is the degree of disorder, λ is wave length of the X-ray, D is the average diameter of the
crystallite,
βhkldegree
and θhkl
the FWHM
and Bragg
angle
of the
hklDplane,
where P and
is the
ofare
disorder,
λ is wave
length
of the
X-ray,
is therespectively.
average diameter of the
In 2H-graphite,
−kθ
= hkl
3n are
± 1;the
when
l = odd,
peak
appears,
= even, 102 and 022
crystallite,
and βhklhand
FWHM
and101
Bragg
angle
of theand
hkl when
plane,lrespectively.
(or 202)Inpeaks
do not appear
they are
orpeak
too weak.
In and
this when
case, Li
al. [5]
2H-graphite,
h − k =because
3n ± 1; when
l = extinct
odd, 101
appears,
l =eteven,
102further
and 022
assumed
the
shape
to
be
a
polyhedral
crystallite
or
near
equiaxed.
The
grain
size,
D002,
D100,
and
(or 202) peaks do not appear because they are extinct or too weak. In this case, Li et al. [5] further
assumed the shape to be a polyhedral crystallite or near equiaxed. The grain size, D002, D100, and
Crystals 2017, 7, 5
4 of 10
Crystals 2017, 7, 5
D004 of 002, 100, and 004 planes normal are substantially equal, and as such the following equation
D004 of 002, 100, and 004 planes normal are substantially equal, and as such the following equation
can be calculated as an average particle size:
can be calculated as an average particle size:
D + D100 + D004
D=
D = 002
3
(5)
(5)
The
The calculations
calculations proposed
proposed by
by Li
Li et
et al.
al. [5]
[5] are
are successful
successful under
under their
their experiment
experiment conditions,
conditions,
◦ C to
however,
our
experiment
conditions
are
different
(e.g.,
our
temperature
range
is
from
however, our experiment conditions are different (e.g., our temperature range is from 1800
1800 °C
to
◦
◦
2800
°C,
while
Li’s
temperature
is
consistent
2800
°C).
We
do
not
make
sure
whether
Li’s
method
2800 C, while Li’s temperature is consistent 2800 C). We do not make sure whether Li’s method is
is
suitable
for calculating
calculating the
thedisorder
disorderdegree
degreeofofanode
anodematerial
material
under
experiment
conditions,
so
suitable for
under
ourour
experiment
conditions,
so we
we
do something
for calculations
of anode
material
Li’ method;
if necessary,
the method
willwill
do something
for calculations
of anode
material
by Li’by
method;
if necessary,
the method
would
would
be
improved.
be improved.
3. Results
Results and
and Discussion
Discussion
XRD Spectra
Spectra and
and Data
Data Analysis
Analysis of Two Kinds of Graphitized Coke
3.1. XRD
XRD results
results of
of petroleum
petroleum coke
coke and
and pitch
pitch coke
coke anode
anode materials
materials at
at different heat treatment
treatment
XRD
in Figure
2. XRD
datum fitting
microcrystalline
structure calculations
temperatures are
areshown
shown
in Figure
2. analysis
XRD analysis
datum
fitting microcrystalline
structure
are
provided
in
Table
2.
Figure
2
shows
that,
after
high-temperature
treatment,
2θ
angles
of
(002)
peak
calculations are provided in Table 2. Figure 2 shows that, after high-temperature treatment, 2θ angles
◦
petroleum
coke
and pitchcoke
cokeand
move
to the
right,
twotocokes
at 2200
both appear,
(101)
of
(002) peak
petroleum
pitch
coke
move
the right,
twoCcokes
at 2200 namely
°C both the
appear,
peak and
2H-graphite,
can be seen
from
Table
2. The
value
of 2.
d002
namely
the(112)
(101)peak
peakofand
(112) peak which
of 2H-graphite,
which
can
be seen
from
Table
Thegradually
value of
approaches
theapproaches
theoretical value
of standardvalue
graphite
(3.354 Å) with
increasing
but the
d
002 gradually
the theoretical
of standard
graphite
(3.354 temperature,
Å) with increasing
minimum value
d remains
3.377
Å. It of
is therefore
difficult
a completely
ordered
graphite
temperature,
butofthe
minimum
value
d remains
3.377 to
Å.achieve
It is therefore
difficult
to achieve
a
structure inordered
either ofgraphite
the cokes.
completely
structure in either of the cokes.
002
100
002
004
101
004
110
112
100
110
101
112
PC26
PC24
TC28
Intensity(a.u.)
Intensity(a.u.)
PC28
TC26
TC24
PC22
TC22
PC20
TC20
TC18
PC18
20
40
2θ(°)
60
80
10
20
(a)
30
40
50
2θ(°)
60
70
80
90
(b)
Figure
Figure 2.
2. XRD
XRDdiffraction
diffractionpatterns
patternsof
oftwo
twokinds
kindsof
ofcoke
cokeanode
anodematerials
materialsunder
underdifferent
differenttemperatures.
temperatures.
(a)
XRD
diffraction
patterns
of
petroleum
coke
under
different
temperatures;
(b)
XRD
(a) XRD diffraction patterns of petroleum coke under different temperatures; (b) XRD diffraction
diffraction
patterns
temperatures.
patterns of
of pitch
pitch coke
coke at
at different
different temperatures.
Table
Table 2.
2. Microcrystalline
Microcrystalline structure
structure analysis
analysis of
of two
two kinds
kinds of
ofcoke
cokeanode
anodematerials
materialsat
atdifferent
differenttemperatures.
temperatures.
Categories
2θ002 (°)
2θ002 (◦ )
PC18 Categories
25.947
PC18
25.947
PC20
25.934
PC20
25.934
PC22 PC22 26.282
26.282
PC24 PC24 26.328
26.328
26.357
PC26 PC26 26.357
PC28
26.378
PC28 TC18 26.378
25.928
26.171
TC18 TC20 25.928
TC22
26.298
TC20 TC24 26.171
26.354
26.370
TC22 TC26 26.298
26.366
TC24 TC28 26.354
TC26
26.370
TC28
26.366
2θ101 (°)
2θ101 (◦ )
44.276
44.276
44.212
44.212
44.301
44.301
44.337
44.337
44.031
44.212
44.031
44.287
44.288
44.212
44.306
44.287
44.288
44.306
d002 (Å)
d3.433
002 (Å)
3.433
3.432
3.432
3.388
3.388
3.382
3.382
3.378
3.378
3.376
3.376
3.434
3.402
3.434
3.386
3.402
3.379
3.377
3.386
3.377
3.379
3.377
3.377
D002 (nm)
D00224.598
(nm)
24.598
25.371
25.371
33.923
33.923
37.209
37.209
36.538
36.538
39.391
39.391
21.697
24.772
21.697
28.441
24.772
29.373
28.851
28.441
30.716
29.373
28.851
30.716
D100 (nm)
D004 (nm)
D112 (nm)
D100 (nm)
10.391 D004 (nm)13.775D112 (nm)
10.391
10.944
10.944
20.909
20.909
21.386
21.386
25.846
25.846
26.748
3.86426.748
2.8383.864
15.789
22.3052.838
21.731
15.789
28.482
22.305
21.731
28.482
13.775
14.108
18.910
25.026
28.599
29.364
13.721
18.130
20.799
23.025
24.363
26.322
14.108
18.910 8.647
25.026 8.788
28.599 10.325
12.479
29.364
13.721 2.254
6.151
18.130 8.339
20.799 9.802
23.025 10.812
24.363
26.322
8.647
8.788
10.325
12.479
2.254
6.151
8.339
9.802
10.812
Crystals 2017, 7, 5
5 of 10
Three kinds of stacking disorder degrees can be seen in Table 3 below, P1 , P2 , and Pd002 . Pd002 is
obtained by d002 , P1 is calculated by D1(Li’s method [4–6]), and P2 is obtained by D112 (our method).
One might see that some of the P1 values exceed 100%, which should not happen because P1 is the
ratio. Therefore, the results are hard to be accepted (i.e., Li’s method [5] might be not suitable for
our condition).
Table 3. The degree of stacking disorder of two kinds of coke anode materials.
Categories
PC18
PC20
PC22
PC24
PC26
PC28
P1 (%)
P2 (%)
Pd002 (%)
102.22
115.22
76.58
60.41
31.61
27.76
22.70
19.59
71.27
61.74
22.99
15.86
10.95
10.50
Categories
TC18
TC20
TC22
TC24
TC26
TC28
P1 (%)
P2 (%)
Pd002 (%)
309.85
157.74
118.02
80.21
67.50
44.88
27.28
21.17
19.32
18.26
73.23
38.22
20.92
15.92
13.49
11.81
3.2. Improved Calculation Method
In order to provide more meaningful experimental data, the Li et al. method [5] needs to be
re-evaluated. A detailed analysis is as follows:
We believe that the formula which describes equiaxed crystals, Equation (5), needs to be amended.
The formula uses (002) and (004) crystal planes to calculate normal direction grain size. However,
the two planes belong to the same families of planes. They are linearly dependent in mathematics
with both expressing the grain size of c-axis direction. In Equation (5), although the (100) crystal
plane is orthogonal to the (002) crystal plane, there is no data for the third independent crystal plane,
so D1 cannot accurately express the grain size of polyhedron. Therefore, the formula needs to be
amended. Since graphite is typically anisotropic in nature, the orientations of the (002) and (100) planes
vary widely. It is challenging to express graphite grain size using the average value of their crystal
orientation. This paper shows that a different set of crystal planes could be a more direct representation
of graphite grain size. The crystal face in 2H graphite is the (112) surface, and in 3R graphite it is the
(113) surface.
From Equation (3), the half width of the crystal surface is not subject to the influence of the
stacking fault (it satisfies h − k = 0). In addition, this crystal surface is neither parallel to the a-axis nor
parallel to the crystal plane, c-axis. The grain size of the graphite crystallites may represent the actual
average grain size. This paper uses the D2 designation.
D2 = D112
(6)
The geometric arrangement of graphite planes can be described as follows: in 2H graphite,
the structure of the arrangement shown in Figure 3, the (112) plane, can be expressed as a triangle.
The sizes of the three sides are nearly equal at approximately 0.4159, 0.4263, and 0.4263 nm, respectively.
In 3R graphite, the planes do not change, but their index changes to (113), and the diffraction line is
still present. An important improvement of the Li et al. method [5] is the fact that Equation (6) replaces
Equation (5).
In this paper D2 is placed into Equation (4) which when calculated, as seen in Table 2, the degree
of stacking disorder in the sample decreases with increasing temperature. At 2800 ◦ C, the disorder of
two cokes are 19.59% and 18.26%, respectively, in line with the temperature gradient that improves the
degree of order of the coke anode material. This gradual increase of the degree of graphitization is the
actual law resulting in absurd results greater than 100%.
Based on the stacking degree of disorder (P2, in Table 3), the changing amounts of microcrystalline
structure are different for pitch coke and petroleum coke at 2600 ◦ C and 2800 ◦ C, respectively.
The degree of disorder for petroleum coke decreased by 3.11%, and pitch coke decreased only by
Crystals 2017, 7, 5
6 of 10
1.08%, however, the degree of disorder of petroleum coke obtained by the d002 method is insignificant,
Crystals 2017, 7, 5
only
0.45%,
the degree of disorder of pitch coke is reduced by 1.68%.
Crystals
2017, 7,while
5
c
1 1 0
1 1 0
c
A
A
B
B
b
b
A
A
a
a
1 1 2
1 1 2
Figure
Figure3.3.Graphite
Graphitelattice
latticearrangement
arrangementpicture.
picture.
Figure 3. Graphite lattice arrangement picture.
The stacking
stacking disorder
disorder (P2)
shown
in
Table
When
the stacking
disorder
was calculated,
the
The
(P2)isis
isshown
shownin
inTable
Table3.3.3.
When
stacking
disorder
calculated,
The stacking
disorder (P2)
When
thethe
stacking
disorder
waswas
calculated,
the
◦
(112)
peak
of
petroleum
coke
appeared
at
a
temperature
of
2200
°C.
The
(112)
peak
of
pitch
coke
the
(112)
peak
of petroleum
coke
appeared
a temperatureofof2200
2200°C.C.The
The(112)
(112)peak
peak of
of pitch
pitch coke
(112)
peak
of petroleum
coke
appeared
at at
a temperature
coke
appeared at
at aa temperature
temperature of
of 2000
2000 ◦°C.
It can
can be
beseen
seenthat
thatdifferent
differentsamples
sampleshave
havedifferent
differentlattice
lattice
appeared
C.
It
appeared at a temperature of 2000 °C. It can be seen that different samples have different lattice
transformation criticalpoints.
points. Thesepoints
points arean
an importantresult
result ofthis
this paper.
transformation
transformation critical
critical points. These
These points are
are an important
important result of
of this paper.
paper.
The
above
conclusion
is
supported
by
high-resolution
transmission
electronmicroscopy
microscopy(TEM)
(TEM)
The
above
conclusion
is
supported
by
high-resolution
transmission
electron
The above conclusion is supported by high-resolution transmission electron microscopy (TEM)
experiments. Figure
4a–c
show the
the petroleum
petroleum coke
coke structure
structure at
at 2000
2000 ◦°C,
2200 ◦°C,
and 2800
2800 ◦°C,
experiments.
Figure 4a–c
4a–c show
show
C, 2200
2200
C, and
and
C,
experiments. Figure
the petroleum
coke structure
at 2000
°C,
°C,
2800 °C,
◦
respectively.
At
2000
°C
graphite
sheets
are
arranged
disorderly
and
crosslinked
to
each
other.
At
respectively.
At2000
2000°CCgraphite
graphitesheets
sheets
arranged
disorderly
crosslinked
to each
other.
respectively. At
areare
arranged
disorderly
andand
crosslinked
to each
other.
At
◦
2200
°C,
the
graphite
sheet
stacking
appears
partially
ordered,
but
not
very
clear
with
some
At
2200
graphitesheet
sheetstacking
stackingappears
appearspartially
partiallyordered,
ordered,but
but not
not very
very clear
clear with
with some
2200
°C, C,
thethegraphite
some
◦
disorderly
structures
remaining.
At
2800
°C,
graphite
sheets
appear
to
be
orderly
and
distinct.
This
disorderly
At 2800
2800 °C,
C, graphite
graphite sheets
sheets appear
appear to
to be
disorderly structures
structures remaining.
remaining. At
be orderly
orderly and
and distinct.
distinct. This
This
strongly
supports
the
XRD
results.
strongly
strongly supports
supports the
the XRD
XRD results.
results.
(a) TEM image of PC20
(a) TEM image of PC20
(b) TEM image of PC22
(b) TEM image of PC22
(c) TEM image of PC28
(c) TEM image of PC28
Figure 4. Transmission electron microscopy (TEM) microstructure analysis of petroleum coke under
Figure 4. Transmission electron microscopy (TEM) microstructure analysis of petroleum coke under
different temperatures. (a) TEM image of PC20; (b) TEM image of PC22; (c) TEM image of PC28.
of PC22;
PC22; (c)
(c) TEM
TEM image
image of
of PC28.
PC28.
different temperatures. (a) TEM image of PC20; (b) TEM image of
3.3. Raman Spectral Data Analysis in Two Kinds of Cokes after Graphitization
3.3. Raman
in Two
Two Kinds
Kinds of
of Cokes
Cokes after
after Graphitization
Graphitization
3.3.
Raman Spectral
Spectral Data
Data Analysis
Analysis in
Figure 5a,b represent Raman spectra of petroleum coke and pitch coke, respectively, acquired at
Figure 5a,b
Raman spectra
ofofpetroleum
coke
and pitch
coke,
respectively,
acquired
at
Figure
5a,b represent
represent
spectra
petroleum
coke
pitch
coke,
respectively,
acquired
different
temperatures.
It Raman
can be seen
from
Figure 5 that
theand
D peak
height
decreases and
G peak
different
temperatures.
It
can
be
seen
from
Figure
5
that
the
D
peak
height
decreases
and
G
peak
at
differentintemperatures.
It can be
seen
from Figure
5 that the D
peakmay
height
and
G peak
increases
the two materials
with
increasing
temperature,
which
bedecreases
due to the
structural
increases in
inthe
thetwo
twomaterials
materials
with
increasing
temperature,
which
may
be
due
to
the
structural
increases
with
increasing
temperature,
which
may
be
due
to
the
structural
defects
defects reducing and the increase of sp22 hybridized bonds.
2
defects
reducing
and
the
increase
of
sp
hybridized
bonds.
reducing
increase
hybridized
The and
peakthe
area
ratio of
of sp
D and
G, R (R bonds.
= ID/IG) can estimate the degree of graphitization. The
The peak
peak area
area ratio
ratio of
of D
D and
and G,
G, RR(R
(R==ID/IG)
ID/IG) can
can estimate
estimate the
the degree of
of graphitization.
graphitization. The
The
The
smaller the R value, the higher the degree
of graphitization
[8–10].degree
Table 4 shows
that with the
smaller the
the R
R value,
value, the
the higher
higher the degree
degree of graphitization
graphitization [8–10].
[8–10]. Table
4 shows
with the
the
smaller
Table
shows that
that
with
increasing temperature
from 1800the
°C to 2800 of
°C, the R value of petroleum
coke4decreases
from
27.78%
◦
◦
increasing
temperature
from
1800
°C
to
2800
°C,
the
R
value
of
petroleum
coke
decreases
from
27.78%
increasing
C to
2800decreases
C, the Rfrom
value26.61%
of petroleum
coke decreases from 27.78%
to 10.33%,temperature
while the R from
value1800
of pitch
coke
to 9.97%.
to 10.33%,
10.33%, while
while the
the R
R value
value of
of pitch
pitch coke
coke decreases
decreases from
from 26.61%
26.61% to
to 9.97%.
9.97%.
to
However, the R value can only characterize the qualitative change of the degree of
However, the R value can only characterize the qualitative change of the degree of
graphitization, so Shi [11] proposed the degree of disorder PR (PR = ID/(ID + IG)) to determine its
graphitization, so Shi [11] proposed the degree of disorder PR (PR = ID/(ID + IG)) to determine its
crystal structure. As can be seen from Table 3, the degree of disorder of petroleum coke, PR, dropped
crystal structure. As can be seen from Table 3, the degree of disorder of petroleum coke, PR, dropped
from 21.76% to 9.36% with increasing temperature, and the degree of disorder of pitch coke, PR,
from 21.76% to 9.36% with increasing temperature, and the degree of disorder of pitch coke, PR,
dropped from 21.01% to 9.07%.
dropped from 21.01% to 9.07%.
Crystals 2017, 7, 5
7 of 10
However, the R value can only characterize the qualitative change of the degree of graphitization,
so Shi [11] proposed the degree of disorder PR (PR = ID/(ID + IG)) to determine its crystal structure.
As can be seen from Table 3, the degree of disorder of petroleum coke, PR , dropped from 21.76% to
9.36% with increasing temperature, and the degree of disorder of pitch coke, PR , dropped from 21.01%
to 9.07%.
Crystals
2017, 7, 5
G-band
(a)
G-Band
D-band
(b)
D-Band
PC26
PC24
TC28
Intensity(a.u.)
Intensity(a.u.)
PC28
TC26
TC24
PC22
TC22
PC20
TC20
PC18
1000
1500
2000
-1
Raman Shift [cm ]
TC18
1000
1500
2000
-1
Raman Shift [cm ]
Figure
spectra
of two
coke of
anode
at differentattemperatures.
(a) Raman
Figure 5.5.Raman
Raman
spectra
ofkinds
two of
kinds
cokematerials
anode materials
different temperatures.
spectra
of
petroleum
coke
at
different
temperatures;
(b)
Raman
spectra
of
pitch
coke
at
different
(a) Raman spectra of petroleum coke at different temperatures; (b) Raman spectra of pitch
coke at
temperatures.
different temperatures.
Table
Table4.4.Raman
Ramanstructural
structuralanalysis
analysisof
oftwo
twoanode
anodematerials
materialsat
at different
different temperatures.
temperatures.
Categories
PC18
Categories
PC20
PC18
PC22
PC20
PC24
PC22
PC26
PC24
PC28
PC26
PC28
ID
36996.633
ID
11402.538
36996.633
6191.185
11402.538
8125.216
6191.185
4499.386
8125.216
2483.245
4499.386
IG
132947.533
IG
42307.602
132947.533
36309.367
42307.602
57023.172
36309.367
37766.283
57023.172
24031.675
37766.283
2483.245
24031.675
R (%)
PR (%)
Categories
27.78
R (%)
PR21.76
(%)
TC18
Categories
26.95
21.22
27.78
17.05
26.95
14.24
17.05
11.91
14.24
10.33
11.91
10.33
21.76
14.56
21.22
12.47
14.56
10.64
12.47
9.36
10.64
9.36
TC20
TC18
TC22
TC20
TC24
TC22
TC26
TC24
TC28
TC26
TC28
ID
14633.458
ID
11162.159
14633.458
5100.839
11162.159
6090.779
5100.839
2294.976
6090.779
4954.759
2294.976
IG
54986.276
IG
49591.129
54986.276
29241.519
49591.129
36191.928
29241.519
17445.023
36191.928
49667.213
17445.023
4954.759
49667.213
R (%)
PR (%)
26.61
R (%)
21.01
PR
(%)
22.50
26.61
17.43
22.50
16.82
17.43
13.15
16.82
9.97
13.15
9.97
18.37
21.01
14.85
18.37
14.40
14.85
11.62
14.40
9.07
11.62
9.07
Figure 6 is a graph of the three methods used to calculate the degree of disorder at different heat
treatment temperatures. It shows that the degrees of disorder, obtained from the three methods, are
Figurewith
6 is a increasing
graph of thegraphitization
three methodstemperature.
used to calculate
the degree
of disorder
at different
consistent
However,
Raman
spectra
showed heat
the
treatment
temperatures.
It
shows
that
the
degrees
of
disorder,
obtained
from
the
three
methods,
are
2
stretching vibration of sp hybrid bond defects within the carbon lattice plane and degree of disorder
consistent
with
increasing
graphitization
temperature.
However,
Raman
spectra
showed
the
stretching
of d002 method is characterized by a (002) arrangement. Therefore, they only represent a single plane
vibration
ofdirection
sp2 hybrid
bond defects
withinnot
thethe
carbon
plane
and degree
of disorder
d002
and
a single
disorder,
respectively,
wholelattice
crystal
arrangement
of them.
On theofother
method
characterized
(002)
arrangement.
they
onlyrepresents
represent athe
single
planeofand
hand,
theisparameter
D112by
is ajust
a measure
of theTherefore,
stack faults
which
disorder
thea
single
direction
disorder,
respectively,
not
the
whole
crystal
arrangement
of
them.
On
the
other
hand,
crystal stack. That is why their disordered values are less than the present value of the degree of
the parameter
D112
justimproved
a measure
of the stack faults which represents the disorder of the crystal
disorder
obtained
byisour
methods.
stack. That is why their disordered values are less than the present value of the degree of disorder
obtained by our improved methods.
80
PC-P
TC-P
70
Figure 7 shows the relationships between the degree
of graphitization of petroleum
coke and
70
TC-P
PC-P
pitch coke
and
electrochemical
performances.
It
is
evident
from
Tables
2–4
and
Figure
7
that
the
degree
TC-P
PC-P
60
60
of graphitization (1-P) and first charge-discharge efficiency are directly related. It also proves that the
50
50
reduction of the degree of disorder (i.e., the increase degree of graphitization) is good for insertion or
40
40
deinsertion
of lithium ions in the graphite intercalation [12]
and decreasing the irreversible capacity of
30
the carbon
anode
[13].
30
2
d002
d002
R
R
P(%)
P(%)
2
20
20
10
10
1800
2000
2200
2400
HTT(C°)
(a)
2600
2800
1800
2000
2200
2400
2600
2800
HTT(C°)
(b)
Figure 6. The relationship between the degree of disorder results of three methods and the heat
treatment temperature graph. (a) The relationship between disordering degree of petroleum coke and
heat treatment temperature; (b) The relationship between disordering degree of pitch coke and heat
treatment temperature.
Figure 7 shows the relationships between the degree of graphitization of petroleum coke and
stretching vibration of sp hybrid bond defects within the carbon lattice plane and degree of disorder
of d002 method is characterized by a (002) arrangement. Therefore, they only represent a single plane
and a single direction disorder, respectively, not the whole crystal arrangement of them. On the other
hand, the parameter D112 is just a measure of the stack faults which represents the disorder of the
crystal stack. That is why their disordered values are less than the present value of the degree of
Crystals 2017, 7, 5
8 of 10
disorder obtained by our improved methods.
80
PC-P2
70
PC-PR
60
TC-Pd002
TC-PR
60
50
50
P(%)
P(%)
TC-P2
70
PC-Pd002
40
Crystals 2017, 7, 5
40
30
30
that the reduction
of the degree of disorder (i.e., the increase
degree of graphitization) is good for
20
20
insertion or deinsertion of lithium ions in the graphite
intercalation [12] and decreasing the
10
10
irreversible capacity of the carbon anode [13].
1800
2000
2200
2400
2600
2800
1800
2000
2200
2400
2600
2800
The degrees of disorder HTT
and(C°)lithium performances of petroleum coke and
coke at different
HTT(pitch
C°)
temperatures are shown in (a)
Table 5 and the charge-discharge capacity graph
(b)of lithium battery can be
referred to in Figure 1 of the supplementary data. It can be seen from Table 5 that with an increase in
Figure 6.
6.
relationship
between
degree
of disorder
disorder results
results
of three
three
methods
and the
the heat
heat
Figure
The
relationship
between
the
degreecharacterization
of
of
methods
and
temperature,
theThe
degree
of disorder
usingthe
different
methods
showed
a decreasing
trend.
treatment
temperature
graph.
(a)
The
relationship
between
disordering
degree
of
petroleum
coke
and
treatment
temperature
graph. (a)at
The
relationship
between and
disordering
cokePand
The degree
of disorder
determined
2600
°C by Langford
used bydegree
Li et of
al.petroleum
showed that
1 is still
heat treatment
treatment temperature;
(b) The
The relationship
relationship between
disordering degree
of pitch
pitch coke
and heat
heat
heat
(b)
greater
than
100%. temperature;
At temperatures
above
2800 between
°C, the disordering
treatment, degree
P1, is of
still
notcoke
lessand
than
60%.
treatment temperature.
temperature.
treatment
Microcrystalline structures cannot characterize the degree of disorder in actual experimental samples.
Figure 7 shows the relationships between the degree of graphitization of petroleum coke and
340
PC-(1-P )
pitch coke 320
and electrochemical
performances. It is evident
from
TC-(1-P ) Tables 2–4 and Figure 7 that the
320
PC-(1-P )
TC-(1-P )
300
degree of graphitization
efficiency
are directly related. It also proves
PC-(1-P ) (1-P) and first charge-discharge
300
TC-(1-P )
4
4
R
R
d002
280
discharge capacity(mAh/g)
discharge capacity(mAh/g)
d002
260
240
220
200
180
160
0.0
280
260
240
220
200
180
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
160
0.0
1.0
0.1
0.2
0.3
0.4
0.5
1-P
(a)
The first charge/discharge efficiency %
The first charge/discharge efficiency %
PC-(1-Pd002)
90
89
88
87
86
0.2
0.9
1.0
TC-(1-P2)
PC-(1-PR)
0.1
0.8
94
PC-(1-P2)
85
0.0
0.7
(b)
92
91
0.6
1-P
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
TC-(1-PR)
93
TC-(1-Pd002)
92
91
90
89
0.0
1-P
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1-P
(c)
(d)
Figure
7. The relationships between graphitization degree and electrochemical properties of
Figure 7. The relationships between graphitization degree and electrochemical properties of petroleum
petroleum
cokecoke.
and pitch
coke.
(a) Plots of graphitization
versus capacity
the discharge
capacitycoke;
for
coke and pitch
(a) Plots
of graphitization
degree versusdegree
the discharge
for petroleum
petroleum
coke;
(b)
Plots
of
graphitization
degree
versus
the
discharge
capacity
for
pitch
coke;
(b) Plots of graphitization degree versus the discharge capacity for pitch coke; (c) Plots of graphitization
(c)
Plots versus
of graphitization
degree versus theefficiency
first charge/discharge
petroleum
coke; (d)
degree
the first charge/discharge
for petroleumefficiency
coke; (d)for
Plots
of graphitization
Plots
of
graphitization
degree
versus
the
first
charge/discharge
efficiency
for
pitch
coke.
degree versus the first charge/discharge efficiency for pitch coke.
Table 5. The degree of disorder and lithium performance of petroleum coke and pitch coke at different
The degrees of disorder and lithium performances of petroleum coke and pitch coke at different
heat treatment temperatures.
temperatures are shown in Table 5 and the charge-discharge capacity graph of lithium battery can be
Categories
PR (%)
Pd002 (%)
P1 (%)
P2 (%)
Charge/Discharge Capacity (mAh/g)
The First Charge-Discharge Efficiency (%)
referred to in Figure S1 of the supplementary data. It can be seen from Table 5 that with an increase in
PC18
21.76
71.27
179.8/210.0
85.6
temperature,
the degree
of -disorder
characterization methods showed
a decreasing
PC20
21.22
61.74
- using different
190.3/220.8
86.2
◦ C by Langford and used by Li et89.8
PC22 The degree
14.56
22.99
102.22determined
31.61
trend.
of
disorder
at 2600 272.9/303.9
al. showed that P1
PC24
PC26
PC28
TC18
TC20
TC22
TC24
TC26
12.47
10.64
9.36
21.01
18.37
14.85
14.4
11.62
15.86
10.95
10.5
73.23
38.22
20.92
15.92
13.49
115.22
76.58
60.41
157.74
118.02
80.21
27.76
22.70
19.59
44.88
27.28
21.17
19.32
293.6/323.3
309.8/341.6
316.4/348.1
175.8/196.6
234.6/255.8
285.4/308.9
305.1/330.6
318.3/342.6
90.8
90.7
90.9
89.4
91.7
92.4
92.3
92.9
Crystals 2017, 7, 5
9 of 10
is still greater than 100%. At temperatures above 2800 ◦ C, the treatment, P1 , is still not less than 60%.
Microcrystalline structures cannot characterize the degree of disorder in actual experimental samples.
Table 5. The degree of disorder and lithium performance of petroleum coke and pitch coke at different
heat treatment temperatures.
Categories PR (%)
PC18
PC20
PC22
PC24
PC26
PC28
TC18
TC20
TC22
TC24
TC26
TC28
21.76
21.22
14.56
12.47
10.64
9.36
21.01
18.37
14.85
14.4
11.62
9.07
Pd002 (%)
P1 (%)
P2 (%)
Charge/Discharge
Capacity (mAh/g)
The First Charge-Discharge
Efficiency (%)
71.27
61.74
22.99
15.86
10.95
10.5
73.23
38.22
20.92
15.92
13.49
11.81
102.22
115.22
76.58
60.41
157.74
118.02
80.21
67.5
31.61
27.76
22.70
19.59
44.88
27.28
21.17
19.32
18.26
179.8/210.0
190.3/220.8
272.9/303.9
293.6/323.3
309.8/341.6
316.4/348.1
175.8/196.6
234.6/255.8
285.4/308.9
305.1/330.6
318.3/342.6
321.7/344.8
85.6
86.2
89.8
90.8
90.7
90.9
89.4
91.7
92.4
92.3
92.9
93.3
4. Conclusions
(1). This paper improves the method for calculating stack faulting of graphite by XRD. It provides
an improved formula, which was used to calculate the degree of disorder of two cokes and has a wide
processing temperature range. The two kinds of anode materials have different structure transition
temperatures obtained by the improved method. The transition temperatures of pitch coke and
petroleum coke are 2000 ◦ C and 2200 ◦ C, respectively.
(2). Qualitative TEM and Raman spectroscopic results support the layer fault calculation for
anode materials.
(3). XRD analysis provided a complete calculation of the degree of disorder for the carbon anode
materials of lithium. It proposed a stacking model and data processing method, which is based on
using (112) planes to calculate the degree of stacking disorder. The analytical method to calculate the
degree of disorder of 2H-graphite correlated well with electrochemical properties and supported the
results proposed by this paper.
(4). The proposed method for calculation of stacking disorder degree of hexagonal graphite
can provide credible results when credible FWHM data of (112) crystal plane and other parameters
are obtained.
Supplementary Materials: The following are available online at www.mdpi.com/2073-4352/7/1/5/s1, Figure S1:
The parameters and the results of calculations, Table S1: 1st charge-discharge voltage vs. specific capacity profile
for each category.
Acknowledgments: The support of The National Natural Science Foundation of China (project U1361212) is
greatly acknowledgement.
Author Contributions: Zhou Zhang and Qi Wang conceived and designed the experiments; Zhuo Zhang
performed the experiments; Zhuo Zhang and Qi Wang analyzed the data; Zhuo Zhang contributed materials;
Zhuo Zhang wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
Franklin, E.R. The structure of graphitic carbons. Acta Crystallogr. 1951, 4, 253–261. [CrossRef]
Shi, H.; Reimers, J.N.; Dahn, J.R. Structure-refinement program for disorderd carbon. J. Appl. Cryst. 1993, 26,
827–836. [CrossRef]
Inagaki, M.; Shiraishi, M. The evaluation of graphitization degree. Carbon Tech. 1951, 5, 165–175.
Yang, C.Z.; Jiang, C.H. Line profile analysis and microstructure characterization of diffraction line broaden.
PTCA Part A Phys. Test 2014, 50, 658–667.
Crystals 2017, 7, 5
5.
6.
7.
8.
9.
10.
11.
12.
13.
10 of 10
Li, H.; Yang, C.Z.; Liu, F. Novel method for determining stacking disorder degree in hexagonal graphite by
X-ray diffraction. Sci. China Chem. 2009, 52, 174–180. [CrossRef]
Li, H.; Yang, C.Z.; Liu, F. Determining graphitization and disordered degrees in 2H-Graphite by X-Ray
diffraction methods. J. Test Meas. Technol. 2009, 23, 161–167.
Langford, J.I.; Bouitif, A. The use of pattern decomposition to study the combined X-ray diffraction effects of
crystallite size and stacking faults in ex-oxalate zinc oxide. J. Appl. Crystallogr. 1993, 26, 22–33. [CrossRef]
Inagaki, M.; Feiyu, K. Carbon Materials Science and Engineering: From Fundamentals to Applications; Tsinghua
University Press: Beijing, China, 2006.
Wang, H.J.; Wang, H.F. The effect of graphitization temperature on the microstructure and mechanical
properties of carbon fibers. New Carbon Mater. 2005, 20, 158–163.
Zerda, W.T.; Xu, W. High pressure Raman and neutron scattering study on structure of carbon black particles.
Carbon 2000, 38, 355–361. [CrossRef]
Yang, S. Studies on the Microstructure and Properties of Carbon Fibers by Raman Spectroscopy.
Master’s Thesis, Donghua University, Shanghai, China, 2010; pp. 29–36.
Niu, P.X.; Wang, Y.L.; Zhan, L. Electrochemical Performance of Needle Coke and Pitch Coke Used as Anode
Material for Li-ion Battery. J. Mater. Sci. Eng. 2011, 29, 204–209.
Zhang, B.; Guo, H.; Li, X.; Wang, Z.; Peng, W. Mechanism for effects of structure and properties of carbon on
its electrochemical characteristics as anode of lithium ion battery. J. Cent. South Univ. (Sci. Technol.) 2007, 38,
454–460.
© 2017 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).