Abstract
The electrochemical performance as anodes for lithium-ion batteries of graphite-like materials that were prepared from anthracites and unburned carbon concentrates from coal combustion fly ashes by high temperature treatment was investigated by galvanostatic cycling of lithium test cells. Some of the materials prepared have provided reversible capacities up to ~ 310 mA h g-1 after 50 discharge/ charge cycles. These values are similar to those of oil-derived graphite (petroleum coke being the main precursor) which is currently used as anodic material in commercial lithium-ion batteries. Larger reversible lithium storage capacities were obtained by using those materials with higher degree of graphitic structural order as evaluated from X-ray diffraction and Raman spectroscopy. In this context, reasonably good linear correlations between the battery reversible capacity and the structural parameters of the electrode- forming materials were found. Furthermore, all materials prepared showed excellent retention of the charge capacity along the cycling as well as low values of the irreversible capacity. Apparently, both the high degree of structural order and the irregular morphology of the particles appear to contribute to the good electrochemical performance as anode in lithium-ion batteries of these materials, thus making feasible their utilization to this application.
1. Introduction
Lithium-ion batteries (LIBs) are currently the energy source for most of the portable electronic gadgets such as cellular phones, laptops, digital cameras, work tools, etc. In 2012, the world market of LIBs reached a value of 11.7·10⁹ $ USA which can explain the interest, both scientific and technologic, in this kind of batteries. The performance of LIBs is based on the use of intercalation materials as electrodes. During the charge, the lithium ions are de-intercalated from the cathode, they move through the electrolyte and, finally, they are intercalated in the anode. In the discharge, the opposite process occurs spontaneously. The compensation of the charge goes through an external electrical circuit (Figure 1).
Figure 1. Lithium-ion cell performance: charging and discharging.
Generally, commercially available LIBs use lithium mixed oxides (mainly LiCoO2), solutions of lithium salts in organic solvents and synthetic graphite as cathode, electrolyte and anode, respectively. The choice of graphite as anode is due to its relatively high specific capacity, high cycling efficiency and low irreversible capacity [1-4]. Currently, petroleum coke is the main precursor material in the manufacturing of synthetic graphite [5]. However, it presents the inconvenience of being obtained from an energy source whose reserves are limited. Moreover, a large amount of them are located in countries having serious social and political problems, even immersed in wars. Therefore, other alternative precursors such as coal and coal-derived products have been investigated [6- 17]. Among the different classes of coals, anthracites were found to graphitize at temperatures above 2000 ºC [6,8,10]. In this context, graphite with structural characteristics comparable to those of commercially available oil-derived synthetic graphite was prepared from anthracites and unburned carbon concentrates from coal combustion fly ashes by high temperature treatment [11, 12,14-17].
This work is focused on the electrochemical performance as negative electrode in lithium-ion batteries of synthetic graphite that were prepared by high temperature treatment (HTT) of Spanish anthracites and unburned carbon concentrates (UCCs) from coal combustion fly ashes. Specifically, two anthracites denoted AF (91.00 of C wt.%, daf basis) and ATO (93.13 of C wt.%, daf basis), from Villablino in the north-west of Spain were selected for this study. A representative sample of both anthracites was ground to size < 20 μm for the heat treatment experiments. On the other hand, three UCCs that were obtained from A or B pulverized coal combustion fly ashes (mainly fed with anthracites) by screening out the ≤ 80 μm fraction (A/CVP) and following an oil agglomeration methodology described previously [18] by using a waste vegetable oil at concentrations of 1 wt.% (B/CIQ1) and 5 wt.% (B/CIQ5) were also heat treated. Unburned carbon contents of 54.64, 78.35 and 68.02 wt.% were determined for A/CVP, B/CIQ1 and B/CIQ5, respectively. The HTT of the anthracites and UCCs were carried out at the temperature interval of 1800-2800 ºC in a graphite electrical furnace for 1 h under argon flow. The heating rates were 25, 20 and 10 ºC min-1 in the ranges: room temperature-1000 ºC, 1000-2000 ºC and 2000-prescribed temperature, respectively. The materials thus prepared were identified by the precursor and the treatment temperature such as ATO/2600 and B/CIQ5/2400.
This work is focused on the electrochemical performance as negative electrode in lithium-ion batteries of synthetic graphite that were prepared by high temperature treatment (HTT) of Spanish anthracites and unburned carbon concentrates (UCCs) from coal combustion fly ashes. Specifically, two anthracites denoted AF (91.00 of C wt.%, daf basis) and ATO (93.13 of C wt.%, daf basis), from Villablino in the north-west of Spain were selected for this study. A representative sample of both anthracites was ground to size < 20 μm for the heat treatment experiments. On the other hand, three UCCs that were obtained from A or B pulverized coal combustion fly ashes (mainly fed with anthracites) by screening out the ≤ 80 μm fraction (A/CVP) and following an oil agglomeration methodology described previously [18] by using a waste vegetable oil at concentrations of 1 wt.% (B/CIQ1) and 5 wt.% (B/CIQ5) were also heat treated. Unburned carbon contents of 54.64, 78.35 and 68.02 wt.% were determined for A/CVP, B/CIQ1 and B/CIQ5, respectively. The HTT of the anthracites and UCCs were carried out at the temperature interval of 1800-2800 ºC in a graphite electrical furnace for 1 h under argon flow. The heating rates were 25, 20 and 10 ºC min-1 in the ranges: room temperature-1000 ºC, 1000-2000 ºC and 2000-prescribed temperature, respectively. The materials thus prepared were identified by the precursor and the treatment temperature such as ATO/2600 and B/CIQ5/2400.
2. Structural characteristics of the graphite- like materials prepared from anthracites and unburned carbon concentrates from coal combustion fly ashes.
All materials prepared were characterized by means of X-ray diffraction (XRD), and Raman spectroscopy following the methodology previously described in other works [12,14,15,19]. For comparative purposes, a petroleum-based graphite, named SG, was also characterized, this carbon material being commercialized to be employed as anode in the manufacturing of lithium-ion batteries.
The mean interlayer spacing, d₀₀₂, and crystallite sizes, Lc and La, and the relative intensity of the Raman D-band (ID/It where It = IG + ID + ID') of the materials that were prepared from the anthracites (AF and ATO), and the UCCs (A/CVP, B/CIQ1 and B/CIQ5) by HTT (1800-2800 ºC) are summarized in Table 1. Data corresponding to SG synthetic graphite are also reported.
Table 1. d₀₀₂, Lc, La, and ID/It parameters of the materials prepared from ATO and AF anthracites, A/CVP, B/CIQ1 and B/CIQ5 UCCs by HTT and of SG synthetic graphite of reference.
The materials structural data shows that, as the treatment temperature of the precursor is rising, d₀₀₂, and ID/It decrease whereas the crystallites sizes, Lc and La, grow gradually. These facts are associated with the improvement of the degrees of the structural order and crystalline orientation as well as the removal of the surface defects of the materials as a consequence of the development of a three- dimensional graphite structure [6-17]. Furthermore, a comparative analysis of these results leads to the conclusion that the degree of crystallinity of the materials depends on the precursor. For example, materials with interlayer spacing of 0.3387 nm and 0.3370 nm were prepared from ATO and AF anthracites, respectively, by heating at 2800 ºC (Table 1). This significant difference in the ability to graphitize of both the anthracites and the UCCs has been previously discussed attending to their characteristics (composition, microstructure, mineral matter/ash contents) [6,10,12,15]. Among them, the mineral matter was found to act as a graphitization catalyst, thus explaining the more graphite-like materials obtained from AF anthracite as compared to ATO anthracite, or from A/CVP as compared to B/ CIQ1 and B/CIQ5 (see ash contents of the precursors in Table 2).
Table 2. Ash contents of ATO, AF, A/CVP, B/CIQ1 and B/CIQ5.
According to a mechanism proposed for the catalytic graphitization of hard carbons [20], the active constituents of the mineral matter of the carbon material (Al, Fe, Si, etc) would preferentially react with disordered carbons located in the edges of the turbostratic domains to form the corresponding carbide. Subsequently, this carbide would decompose in graphitic carbon at high temperature by which the size of the already-existing graphite layers would be increased. Because of this catalytic effect, graphite materials showing structural and textural characteristics comparable to those of the petroleum-based graphite of reference (SG) which is currently used in energy applications were prepared from the anthracites and UCCs studied in this work.
3. Electrochemical performance of the graphite- like materials
The electrochemical study of the materials prepared was carried out using two-electrodes Swagelok- type cells which were assembled in a dry box under argon atmosphere and water and oxygen contents below 1 ppm (Figure 2). Working electrodes (WE) were prepared by mixing the active material and polyvinylidene fluoride (PVDF), which acts as binder, in a proportion of 92:8 wt.%, respectively. All of the active materials were ground to 20 μm top size prior the electrode preparation. This mixture or slurry was deposited on a copper foil of 12 mm diameter by airbrushing and dried at 120 ºC for ~ 24 h. Thus, a thin and uniform surface of slurry coating the copper was obtained. Metallic lithium disc of 12 mm diameter was used as counter-electrode (CE). A 1M LiPF6 (EC:DEC, 1:1, w/w) solution impregnating two separator glass micro-fiber disks acts as electrolyte.
The cells were subjected to galvanostatic cycling in the 2.1-0.003 V vs Li/Li+ voltage range during 50 cycles at a constant current of C/10 corresponding to attain the theoretical graphite capacity of 372 mA h g-1 in 10 h, i.e., to form the LiC intercalation compound, being one lithium ion per six carbon atoms the maximum amount of Li+ that graphite can be inserted in its bulk [21].
Figure 2. Two-electrode Swagelok-type cell.
3.1. Lithium intercalation/de-intercalation mechanism
The voltage curves of the first discharge-charge cycle and the second discharge of the lithium cells using A/CVP/2700, B/CIQ5/2600, B/CIQ1/2600, AF/2800 and ATO/02800 as WEs are shown in the Figure 3. For comparison, the voltage profile of the reference graphite, SG, was also included in the same figure. As seen, the mechanism of lithium ions intercalation/ de-intercalation in the bulk of these materials is much the same to that of SG. At the beginning of the discharge, the voltage drops quickly to ~ 0.8 V (vs Li/Li+). At this point, a short plateau is observed which is attributed to the electrolyte decomposition causing the formation of solid electrolyte interface (SEI) on the graphite surface and an irreversible consumption of lithium ions [22]. Subsequently, the voltage drops gradually to ~ 0.2 V vs Li/Li+. Below this point, the lithium intercalation into the material starts as shown by the appearance of three plateaus at ~ 0.18 V, ~ 0.10 V and ~ 0.06 V. These plateaus which correspond to the different stages of lithium ions intercalation in graphite-like materials [23] can be better appreciate in the graph insert in Figure 3.
Figure 3. Voltage (V vs Li/Li+) vs x in Li C during the 1st discharge-charge cycle and 2nd discharge of A/CVP/2700, B/CIQ5/2600, B/ CIQ1/2600, AF/2800, ATO/02800 and SG materials.
3.2. Reversible capacity and capacity retention along cycling
As an example, the results of the galvanostatic cycling of the graphite-like materials prepared from the UCCs A/CVP and B/CIQ5 together with that of the SG graphite are presented in the Figure 4 (discharge capacity along cycling plots). Firstly, it is worth to mention that some of the materials prepared in this work, specifically A/CVP/2700 and B/CIQ5/2600, have provided reversible capacities similar to SG graphite (310 mA h g-1 after 50 cycles). Furthermore, all of them show a remarkable stable capacity along cycling with capacity keeping values in the range of 90-99 % after 50 cycles. Similar results were attained by cycling the graphite-like materials prepared from AF and ATO anthracites [16].
Figure 4. Extended galvanostatic cycling of (a) A/CVP/1800-2700 and SG materials, and (b) B/CIQ5/1800-2600 and SG materials.
By comparing the galvanostatic cycling results (Figure 4) and the structural parameters (XRD and Raman) of the materials prepared (Table 1), it is clear that those with higher degree of graphitic structural order provide larger lithium storage capacity. For example, battery reversible capacity values of 308 mA h g-1 and 185 mA h g-1 were measured after 50 cycles by using A/CVP/2700 and A/CVP/2200 materials with crystallite size, Lc, of 28 nm and 13 nm, respectively.
In fact, reasonably good linear correlations between the reversible capacity and the structural parameters of the electrode-forming materials were found. For example, R2 coefficients values of 0.954 and 0.973 were calculated for the interlayer spacing, d₀₀₂, and the thickness of the crystallite, Lc, of the graphite-like materials obtained from AF and ATO anthracites [16].
The dependence of the electrochemical intercalation of lithium ions in well-ordered carbon materials on their crystal structure has been previously studied by other authors [1,24,25]. The crystal thickness, Lc, was reported to be the most important factor affecting the extent of the reversible capacity provided by a specific material in the electrode. As in the work discussed here, a tendency of the capacity to increase with the material Lc was observed. Nevertheless, no specific correlation between the electrode capacity and the crystal thickness or other crystalline parameters of the materials was established. However, when graphite-like materials of high degree of crystallinity obtained from different precursors were considered, this tendency was not followed at all and larger capacities were delivered by materials with lower or similar Lc values. In this context, as mentioned above, A/CVP/2700 and B/CIQ5/2600 have delivered reversible capacities similar to SG graphite with a much higher Lc crystallite size (Table 1). Therefore, the good electrochemical behaviour of these graphite-like materials that were prepared from the UCCs should be related to other non structural factors. Among them, the influence of the graphite morphology on its electrochemical performance has been widely studied in previous works [1,26-31]. In an attempt to clarify this point, the morphology of A/CVP/2700, B/CIQ5/2600 and SG was studied by SEM and the corresponding micrographs are shown in the Figure 5. SG graphite shows the presence of flakes. Unlike SG, A/CVP/2700 and B/CIQ5/2600 materials have an irregular particle shape which has been suggested to improve the electrode performance due to the formation of voids between particles, thus allowing a good percolation of the electrolyte solution to reach the electrode active mass, i.e., favouring the Li intercalation in the bulk of the graphite-like material [28].
Figure 5. SEM images of a) A/CVP/2700, b) B/CIQ5/2600 and c) SG materials.
3.3. Irreversible capacity
The irreversible capacity losses during the first discharge-charge of the graphite-like materials studied are reported in Table 3. Generally, the majority of the materials show irreversible capacity percentages similar to that of SG (~ 25 %). Although other side reactions and phenomena may contribute to the irreversible consumption of lithium ions [32- 34], it can be considered that the irreversible capacity is mainly due to the formation of the solid electrolyte interface on the surface of graphite electrode [35]. Because of the SEI film covers the electrode surface exposed to the electrolyte solution it is easy to conclude that irreversible capacity has been related to the surface area of the electrode material [22,32-34,36]. In fact, proportionality with the BET specific surface area was found in graphite materials belonging to the same family. In this work, the values of the BET specific surface area of the graphite- like materials prepared are < 10 m2 g-1, being the typical values for petroleum-based graphites used as anodes in commercial LIBs [37]. Therefore, as expected, the values of irreversible capacity of these materials and SG are comparable. However, there is no dependence between these two parameters. In previous works, it has been found a relation between the irreversible capacity and the active surface area (ASA) of the carbon materials [33,34]. The ASA is defined as the cumulated area of the different type of defects present on the carbon surface (stacking faults, dislocations and vacancies). Therefore, the ASA can be considered as indirect estimation of the degree of structural order of the material. But any relation has been found between the development of the three-dimensional graphite structure and the BET specific surface area or ASA (indirectly estimated from the structural parameters). Taking into account that no exfoliation has been observed during the first discharge, other factors different than the order and microcrystal orientation should influence on the irreversible charge loss.
Table 3. First cycle irreversible capacity of the materials prepared from ATO and AF anthracites, and A/CVP, B/CIQ1 and B/CIQ5 UCCs by HTT and of the SG synthetic graphite of reference.
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