Abstract
Potassium-ion batteries (PIBs) are considered as a viable alternative to lithium-ion batteries in large scale stationary energy storage. However, the low cost, high rate and long life anode still remains great challenge for the commercialization of PIBs. Herein, high performance carbon anodes for PIBs have been successfully prepared from low-cost and abundant coal through ball-milling, acid washing, oxidation and carbonization. We discover that the potassium storage performances of coal-based carbons are closely related to the coal rank. A middle-rank bituminous coal-based carbon anode delivers a high reversible capacity of 260 mAh g—1 at 0.05 A g—1 and maintains 118 mAh g—1 after 1200 cycles at 1 A g—1. Moreover, at higher current densities of 0.1, 0.5, and 5.0 A g—1, it maintains high storage capacities of 230, 176, and 88 mA h g—1, respectively. The low cost and high performance coal-based carbon anodes are supposed to accelerate the development of PIBs toward future large-scale applications.
1. Introduction
Lithium-ion batteries (LIBs) have been successfully used in portable electronic devices and electric vehicles. However, their applications in large scale stationary energy storage, e.g., power grids, are severely limited by the rarity of lithium. Potassium-ion batteries (PIBs) have been considered as a promising alternative to replace the state-of-the-art LIBs due to a number of prospective advantages: (1) The natural abundance and low cost of potassium (1.5 wt% in the earth's crust). (2) The standard redox potential of K/ K⁺ ( 2.92 V vs. SHE) is comparable to that of Li/Li⁺ ( 3.04 V vs. SHE), indicating a high cell voltage of PIBs. (3) The conductivity of K⁺ electrolyte is higher than that of Li⁺ electrolyte [1]. Therefore, PIBs have attracted tremendous interests in application of large- scale energy storage systems. Nevertheless, the commercial application of PIBs is still severely limited by their poor cycling stability and inferior rate performance because of the large ionic radius of K⁺ (1.38 Å) compared to that of Li⁺ (0.76 Å). Graphite was first investigated as PIB anode in consideration of the thermodynamic stability of stage I intercalation compound (KC8) and exhibited a high reversible capacity of 273 mAh g—1 at a very small current density of C/40 (1 C 279 mA g—1) [2]. However, its capacity dropped dramatically at larger current density, only 80 mAh g—1 retained at 1 C. Meanwhile, the graphite displayed an inferior cycling performance with retention of ca 50% after 50 cycles. The poor rate performance and inferior cycling stability can be related to the dramatic volume expansion of graphite as a result of the large ionic radius of K⁺. To address this problem, many attempts have been made to synthesize various carbon materials with low crystallinity [2e4] or delicate nanostructures [5e7] for PIB anodes. Most of them, however, suffer from tedious synthesis procedure and unaffordable raw materials cost, which dramatically increase the cost of PIBs. Furthermore, their K⁺ storage performances do not yet meet the demand for stationary storage purposes. Therefore, the fabrication of high performance and low cost carbon anodes for PIBs is urgently needed.
Coal is an abundant solid fossil fuel with wide geographic distribution. Besides heat generation, coal with high carbon content has been extensively investigated as an important raw material for producing functional carbon materials [8e16]. Recently, coal-based carbon anodes for LIBs and sodium-ion batteries have exhibited encouraging electrochemical performances [17e19]. However, to the best of our knowledge, few literatures have focused on the application of coal-based carbon anodes in PIBs.
Herein, we report on the K⁺ storage performances of micron- sized coal-based carbon materials for PIB anodes. Three kinds of coals with progressively rising coal ranks were selected as pre- cursors. The micron-sized particles significantly increase the electrode compaction density compared to that of nanomaterials. We investigated the effect of the coal ranks on the microstructures of coal-based carbons, and their correlation to the electrochemical performances through structural and electrochemical characterizations coupled with kinetics analysis. Due to the relatively ordered carbon clusters with enlarged interlayer spacing, the middle- rank bituminous coal-based carbon anode exhibits high reversible capacity (260 mAh g—1 at 0.05 A g—1), outstanding rate performance (88 mAh g—1 at 5 A g—1) and excellent cycling stability (118 mAh g—1 at 1 A g—1 after 1200 cycles with retention of 68.6%).
2. Experimental
2.1. Preparation of coal-based carbons
Three kinds of Chinese coals with progressively rising coal ranks, i.e. a lignite (denoted as L) and two bituminous coals (denoted as B₁ and B₂), were selected as precursors for the synthesis of anode materials. In a typical run, 10 g of raw coal was ball- milled for 4 h. The obtained coal powders were treated in 300 mL of mixed acid of hydrochloric acid and hydrofluoric acid at 60 °C for 24 h to remove the mineral matters. The obtained products were collected by filtration and washed with deionized water to keep neutral. The proximate and ultimate analyses of the raw coal and acid washed coal samples are listed in Table 1. The acid washed coal powders were oxidized in air flow at 300 °C for 1 h with a heating rate of 1 °C min—1, followed by carbonization at 900 °C for 1 h under nitrogen at 2 °C min—1. The obtained products were collected after natural cooling and denoted as L-A-900, B₁-A-900 and B2-A-900, where A represents the acid washing and 900 stands for the carbonization temperature. For comparison, the B₁-900 was synthesized under the same conditions using ball-milled B₁ as pre- cursor without acid washing.
2.2. Characterizations of coal-based carbons
The morphology of coal-based carbons was investigated by a FEI QUANTA 450 scanning electron microscope (SEM) and a FEI Tecnai F30 transmission electron microscope (TEM). X-ray diffraction (XRD) patterns of coal-based carbons were recorded on a D/MAX- 2400 diffractometer using the Cu K𝞪 X-ray radiation source (𝞴= 0.154056 nm, 5-9°, 10° min—1). Raman spectra were analyzed using a Thermo Fisher Scientific DXR with an argon ion laser excitation wavelength of 532 nm at room temperature. The nitrogen adsorption and desorption isotherms were measured at 196 °C on a Micromeritics ASAP2020 automatic adsorption instrument. Specific surface areas and pore size distributions were calculated by the Brunauer-Emmett-Teller (BET) and the non-liner density functional theory (NLDFT), respectively.
Table 1 Proximate and ultimate analyses of coal samples.
2.3. Electrochemical measurements of coal-based carbon anodes
The working electrodes were prepared by mixing 70 wt% coal- based carbons, 20 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solvent. The mixture slurry was casted onto the Cu foil current collector, then dried at 120 °C in vacuum for 12 h. After drying, the Cu foil was punched out as 14 mm diameter electrodes. Potassium foil was used as the counter electrode. The electrolyte was 0.8 mol L—1 KPF₆ dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 in volume). Polypropylene microporous film (Whatman GF/A 47 mm) was used as the separator. CR2016 coin-type half-cells were assembled in an argon-filled glove box (MBRAUN UNIlab) with moisture and oxygen concentrations below 0.1 ppm. The galvano- static charge/discharge tests were performed on a Land BT2000 battery test system (Wuhan, China) at a voltage ranged from 0.01 to 2.50 V versus K⁺/K at various current densities. Cyclic voltammetry
(CV) was recorded using a Bio-logic VMP-300 electrochemical workstation in the range from 0.01 to 2.50 V at various scan rates. Electrochemical impedance spectroscopy (EIS) was carried out using the same electrochemical workstation with an amplitude of 5.0 mV over the frequency ranging from 100 kHz to 10 mHz.
3. Results and discussions
As shown in Table 1, with the progressive rise of the coal rank, the volatile and the content of oxygen decrease, while the content of carbon increases. By elevating the heat treatment temperature, the volatile will be removed as small molecule hydrocarbons, CO, CO2, H2 and H2O through pyrolysis reaction and consequently form porous structure. So, the decreased volatile and increased carbon content of the high-rank coal are expected to reduce the porosity of the obtained coal-based carbons. After acid washing, the ash yield of B₁ reduced from 7.64 wt% to 0.75 wt%, indicating an effective removal of the mineral matters in coals.
B₁, a middle-rank bituminous coal, was selected to study the effect of mineral matters on the electrochemical performances of coal-based carbon anodes. It can be seen from Fig. 1 that the initial discharge and charge capacities of B₁-900 (without acid washing) are 569 and 176 mAh g—1, respectively, corresponding to an initial
Fig. 1. Initial charge and discharge curves of B₁-A-900 and B₁-900 at 0.05 A g—1. (A colour version of this figure can be viewed online.)
Coulombic efficiency (ICE) of 31.0%. The mineral matters of coal mainly consist of Mg, Al, Si, Ca, Fe and their compounds, which may form alloys with K during the first discharge to increase the irreversible capacity. After acid washing, the detrimental mineral matters can be effectively removed. As a result, the ICE of B₁-A-900 increases to 43.2%.
Coalification, analogous to carbonization process, is a geological process involving the removal of heteroatoms and the increase of carbon content. Here, a lignite and two bituminous coals were selected to study the effects of coalification (coal rank) on the electrochemical performances of coal-based anodes. As shown in Fig. 2aec, after ball-milling, the particle sizes of coal-based carbons are mostly less than 4 mm. Proper particle size facilitates high compaction density (1.13 g cm—3) and outstanding electrochemical performances. The microstructures of coal-based carbons were further analyzed by TEM (Fig. 2def) and HRTEM (Fig. 2gei). In the case of L-A-900, a disordered microstructure consisting of nano- sized carbon vesicles (Fig. 2d) was formed with large numbers of wrinkled carbon layers. Lignite, as a typical low-rank coal derived from plant debris, retains the molecular structural characteristic of its precursors, i.e., high heteroatom content and low aromaticity. Therefore, L-A-900 exhibits hard carbon feature with high sp3 hybridized carbon proportion and oxygen content, which would destroy the hexagonal lattices of the graphene layers resulting in an extremely disordered porous microstructure. With the rise of the coal rank, the aromaticity of raw coal molecules increases while the content of heteroatoms decreases. This evolution of coal molecular structure contributes to the formation of more ordered and developed carbon clusters (Fig. 2e and f) with soft carbon nature.
Therefore, regardless of the very similar interlayer spacings of B₁-A-900 and L-A-900 (both about 0.39 nm, as shown in Fig. 2g and h), B₁-A-900 shows a more ordered turbostratic microstructure with certain orientation, which is expected to boost the rapid diffusion and intercalation of K⁺. B₂ coal has the highest coal rank among the three coals. As expected, B₂-A-900 exhibits the most ordered microstructure, accompanied by a decreased interlayer spacing of 0.38 nm (Fig. 2i), which inevitably increases the diffusion resistance of K⁺.
The XRD patterns in Fig. 3a show two broad weak diffraction peaks at around 26° and 43°, corresponding to the diffraction of carbon layer 002 and 100 planes, respectively. According to the calculated results from 002 diffraction peaks based on Bragg equation, the interlayer spacings (d002) of L-A-900, B₁-A-900 and B₂-A-900 are 0.393, 0.388 and 0.380 nm, respectively. This result indicates that the interlayer spacings of coal-based carbons gradually decrease with the progressive rise of the coal rank, which is consistent with HRTEM results. Fig. 3b shows the Raman spectra of coal-based carbons, which present two independent characteristic peaks, D band centered at 1343 cm—1 and G band centered at 1589 cm—1. The relative intensity ratio of D band to G band (ID/IG) is usually used to qualitatively evaluate the degree of crystallinity of carbon materials. The value of ID/IG of L-A-900, B₁-A-900 and B₂-A- 900 are 0.993, 0.992 and 0.987, respectively, indicative of the reduced defect density with the progressive rise of the coal rank.
Pore structure is critical to the electrochemical performances of carbon anodes. A proper pore structure in carbon anodes can provide diffusion channels and active sites for K⁺ storage. Nevertheless, an excess surface area will lower the ICE as well as decrease the compaction density of anode [20,21]. The pore structures of the coal-based carbons were investigated using nitrogen adsorption and desorption isotherms. As shown in Fig. 3c, all the three materials show similar adsorption-desorption isotherms. The sharp increase of adsorption volume at low relative pressure (P/ P0 0e0.01) is attributed to the nitrogen filling in micropores below 2 nm, indicating the existence of abundant micropores in coal-based carbons. The indistinct hysteresis loop in medium relative pressure indicates that there are mesopores in coal-based carbons. The slight adsorption uptake at high relative pressure (close to P/P0 1.00) indicates the existence of macropores. According to the pore size distribution curves shown in Fig. 3d, there are mainly micropores less than 1 nm and some mesopores and macropores in coal-based carbons.
Fig. 2. (aec) SEM images of coal-based carbons. Inset: corresponding particle size distributions of coal-based carbons. (def) TEM images of coal-based carbons. (gei) HRTEM images of coal-based carbons. Inset: schematic description of the effect of the carbons' structure on the K⁺ diffusion. (Yellow balls represent K⁺, black lines represent carbon layers). (A colour version of this figure can be viewed online.)
Fig. 3. Structure analysis of coal-based carbons: (a) XRD patterns. (b) Raman spectra. (c) N2 adsorption-desorption isotherms and (d) pore size distribution curves. (A colour version of this figure can be viewed online.)
The porosity parameters of the coal-based carbons are listed in Table S1. The micropore proportions of L-A-900, B₁-A-900 and B₂-A-900 are 83.9%, 76.8% and 71.8%, respectively. While the mesopores proportions of L-A-900, B₁-A-900 and B₂-A-900 are 14.3%, 21.0% and 25.2%, respectively. Macropores can function as electrolyte reservoir, mesopores can boost the diffusion of K⁺ [22] and micropores can provide active storage sites for K⁺. Thus, hierarchically porous structure improves the electrochemical performances of coal-based carbons. The porous structure is due to the volatilization of small molecules during carbonization. It can be seen from Table 1 that the volatile yield decreases gradually with the progressive rise of the coal rank, causing a decrease of BET specific surface areas of coal-based car- bons. As expected, the specific surface areas of L-A-900, B₁-A-900 and B₂-A-900 calculated based on BET method are 519, 392 and 264 m2 g—1, respectively.
Fig. 4a shows the first charge-discharge curves of coal-based carbon anodes at 0.05 A g—1. The ICE of L-A-900, B₁-A-900 and B₂- A-900 are 29.4%, 43.2% and 41.7%, respectively. The irreversible capacity is attributed to the formation of solid electrolyte interface (SEI) and the irreversible intercalation of K⁺ in the first discharge cycle, which is closely related to the specific surface areas and the surface chemistry of carbons. The ICEs of coal-based carbon anodes were comparable to those of the porous carbon PIB anodes in previous literatures [23e25], which could be attributed to their large specific surface area. The initial six CV curves of B₁-A-900 at 0.1 mV s—1 are shown in Fig. 4b. The broad reduction peak below 2.0 V in the first cycle is mainly related to the formation of initial SEI on the electrode surface. Remarkably, this peak disappears in the subsequent cycles, suggesting the stable SEI film has formed during the first cycle. Furthermore, the relatively sharp oxidation peak centered at 0.32 V corresponds to K⁺ extraction from the micropores and defects of B₁-A-900. More importantly, the CV curves in the subsequent cycles almost overlap, demonstrating the excellent reversibility of coal-based carbon anodes. As shown in Fig. 4c, B₁-A- 900 maintains the highest capacity of 118 mAh g—1 at 1 A g—1 after 1200 cycles with the capacity retention of 68.6% (compared to the 2nd cycle capacity of 172 mAh g—1). Besides, the Coulombic efficiency of B₁-A-900 nearly approaches 100% during cycling.
The rate performances of coal-based carbon anodes are shown in Fig. 5a. The B₁-A-900 anode achieves a high capacity of 320 mAh g—1 in the second cycle and retains 260 mAh g—1 after 10 cycles at 0.05 A g—1. In addition, B₁-A-900 delivers the highest reversible capacities of 230, 207, 176, 147, 121 and 88 mAh g—1 at 0.1, 0.2, 0.5, 1, 2 and 5 A g—1, respectively. When the current density switches back to 0.05 A g—1, the capacity recovers to 232 mAh g—1 with the capacity retention as high as 89.2% (compared to the 10th cycle ca- pacity of 260 mAh g—1), indicating a good electrochemical reversibility of B₁-A-900.
The EIS technology is widely used to study the processes occurring at electrode/electrolyte interfaces and K⁺ intercalation/ de-intercalation in electrodes. The EIS curves of coal-based carbon anodes are shown in Fig. 5b. All the Nyquist plots of coal-based carbon anodes are made of a depressed semicircle in the high- frequency zone and a sloping line in the low-frequency zone. The former corresponds to the charge-transfer process at the electrode/ electrolyte interfaces, and the latter represents the Warburg impedance associated with K⁺ diffusion in coal-based carbon anodes [26]. B₂-A-900 shows the smallest diameter of the semicircles, indicating the lowest charge-transfer resistance. The low charge- transfer resistance can be attributed to its highest crystallinity resulting from the highest coal rank, which is consistent with the XRD and Raman results as shown in Fig. 3. With that in mind, a medium charge-transfer resistance of B₁-A-900 which is slightly higher than that of B₂-A-900 is not unexpected.
The K⁺ diffusion properties were evaluated by calculating the K⁺ diffusion coefficients (DþK ) with following equation [27]:
Fig. 4. Electrochemical performances of coal-based carbon anodes: (a) Initial charge and discharge curves at 0.05 A g—1. (b) The initial six CV curves at 0.1 mV s—1. (c) Cycling performance along with Coulombic efficiency at 1 A g—1. (A colour version of this figure can be viewed online.)
Fig. 5. (a) Rate capacities at different current densities of coal-based anodes. (b) Nyquist plots of coal-based anodes for the fresh cells. (c) The relationship plot between Z' and u—1/ 2 at low-frequency of Nyquist plots. (d) K⁺ diffusion coefficients of L-A-900, B₁-A-900 and B₂-A-900. (A colour version of this figure can be viewed online.)
where R is the gas constant, T is the absolute temperature, A is the electrode surface area, n is the number of transferred electrons per atom in the electrochemical reaction, F is Faraday's constant, C is the concentration of K⁺ and 𝞼 is the Warburg coefficient that is associated with Z' and 𝟂—1/2 at low-frequency of Nyquist plots.
The K⁺ storage mechanism in B₁-A-900 was investigated by CV curves at different scans shown in Fig. 6a. The ratio of surface capacitive contribution and diffusion-controlled intercalation reaction can be quantified by the works of Dunn and co-workers [28]. The current response i at a specific potential V can be separated into surface capacitive behaviors and diffusion-controlled intercalation reactions:
Thanks to its proper pore structure and microstructure, the rate performance of B₁-A-900 is superior to those of several latest reported carbon-based anodes for PIBs (Fig. 7), such as the poly- nanocrystalline graphite [33], tire-derived carbon [34], activated graphite [35], graphite [36], expanded graphite [37], ordered mesoporous carbon [38] and reduced graphene oxide [39].
The pristine and cycled anodes with B₁-A-900 as active material were observed by SEM. As shown in Fig. 8, the cycled anode maintains integrity without obvious cracks on its surface, indicating its excellent structural stability. The cost-performance ($ Wh—1) is an important parameter for the development of practical PIBs. Commercial available graphite powder, the most widely used anode in LIBs, was compared to illustrate the cost advantage of coal-based carbon anodes. The rate and cycling performances of B₁- A-900 and graphite are shown in Fig. S2. The B₁-A-900 achieves higher specific capacities at high current densities than commercial graphite. Considering the material and production costs, the cost- performance of B₁-A-900 is about ten percent of that of commercial graphite at high current densities, indicating the promising future of coal-based carbon anodes for PIBs.
Fig. 6. (a) CV curves at different scan rates of B₁-A-900. (b, c) Surface capacitive contributions at scan rates of 0.1 and 2.0 mV s—1 for B₁-A-900. (d) The ratios of surface capacitive contribution and diffusion-controlled intercalation capacities at various scan rates of B₁-A-900. (A colour version of this figure can be viewed online.)
Fig. 7. Rate performance comparison of B₁-A-900 with some latest reported carbon- based anodes. (A colour version of this figure can be viewed online.)
Fig. 8. SEM images of B₁-A-900 electrodes before and after cycled at 1 A g—1: (a, b) The surface of B₁-A-900 electrodes. (c, d) The cross section of B₁-A-900 electrodes. (The thickness of the copper current collector is 10 mm).
4. Conclusions
In conclusion, low-cost and high-performance carbon anodes for PIBs have been successfully prepared from coal. The coal rank significantly affects the microstructures and electrochemical per- formances of coal-based carbons. A middle-rank bituminous coal- based carbon, B₁-A-900, exhibits the superior reversible capacity (260 mAh g—1 at 0.05 A g—1), outstanding rate performance (88 mAh g—1 at 5 A g—1) and excellent cycling stability (118 mAh g—1 at 1A g—1 after 1200 cycles with retention of 68.6%). The relatively ordered carbon clusters with enlarged interlayer spacing of B₁-A- 900 can improve its K⁺ diffusion coefficient, contributing to an excellent rate performance. These findings of the relationship between the microstructure and electrochemical performance will provide useful guidance for designing high performance carbon anodes for PIBs in the future.
Source: Nan Xiao a, *, Xiaoyu Zhang a, Chang Liu a, YuweiWang a, Hongqiang Li a, Jieshan Qiu a, b, **
a State Key Lab of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical Engineering, PSU-DUT Joint Center for Energy Research
Chemical Engineering, Dalian University of Technology
b College of Chemical Engineering, Beijing University of Chemical Technology
a State Key Lab of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical Engineering, PSU-DUT Joint Center for Energy Research
Chemical Engineering, Dalian University of Technology
b College of Chemical Engineering, Beijing University of Chemical Technology
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