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. 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