KCl-Modified Graphite as High Performance Anode Material for

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KCl-Modified Graphite as High Performance Anode Material for Lithium-Ion Batteries with Excellent Rate Performance Yan Wu, Liying Wang, Yifan Li, Zhenyang Zhao, Longwei Yin, Hui Li, and Yu-Jun Bai J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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The Journal of Physical Chemistry

KCl-modified Graphite as High Performance Anode Material for Lithium-ion Batteries with Excellent Rate Performance Yan Wu, Li-Ying Wang, Yi-Fan Li, Zhen-Yang Zhao, Long-Wei Yin, Hui Li*, Yu-Jun Bai * Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, People's Republic of China. E-mail: [email protected] (H. L.); [email protected] (Y.-J. Bai) Abstract: Electrochemical properties of a graphite negative electrode of lithium-ion batteries have been enhanced by treating raw graphite with alkali molten salt. Here, the KCl-modified graphite is fabricated via a simple and low-cost technique that graphite particles and potassium chloride are simply mixed with deionized water and sintered. Compared to the raw material, the rate capability of the graphite modified by potassium is enhanced significantly to 269.7 mAh g-1 at 1 C after 200 cycles which effectively solves the main problem to limit the application of the graphite anode for Li-ion batteries in the field of hybrid vehicles. First-principle calculations have been done to analyze the effect of K-ions on the lithium storage capacity of the graphite. This study provides a scientific basis for adopting the K-modified technique to enhance the electrochemical properties of the negative electrode materials.

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1. Introduction Since the commercialization1 of lithium ion rechargeable batteries with a carbon negative material was announced by Sony at 1991, it shows great potential to fulfill the continuously surging demand in both small-scale electronic equipments and large-scale applications, such as renewable power plants, electric vehicles (EV) and stationary energy storage2-4. However, the lithium battery technology for these applications is restricted by some unresolved problems such as costs, safety5, low temperature performance and materials availability. To deal with those issues, the development of novel Li ion batteries mainly aims at the commercial anode and cathode materials that are continued to mature. Graphite as the most commercialized lithium-ion battery carbon negative electrode material6 shows a high specific capacity cycle stability, low irreversible capacities, low discharge potentials close to lithium metal and constant discharge capacities but there are two problems that the capacity of the graphite-based Lithiumion batteries (LIBs) is much lower than other newly fabricated materials7-9 and their poor rate capability10 needed to be solved. In order to enhance its electrochemical properties as negative materials in future LIBs, some key studies have involved introducing ion-functional groups11, metal oxide12, carbon composite13-14 and doping with foreign ions15. Park et al.13 showed that the rate performance, first charge-discharge efficiency, and cycling performance could be enhanced by the addition of the natural graphite/carbon nanofiber composites; Han et al.14 proved that the charge-transfer resistance on the electrode–electrolyte interface could be


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decreased by coating amorphous carbon on the negative graphite; Herstedt et al.16 showed that rate performance could be significantly improved by the modification of surface properties of graphite. In the same time, numerous studies to date have shown that the current density and ratio discharge property can be improved by altering the internal or surface structure of the graphite, along with decreased electrode destruction during the charging and discharging or increased the number of micropores and nanometer channels7, 17-19. Recently, the alkali metal compound has been found to has an significant function in improving the electrochemical properties of the cathode20 and anode21-25 materials. Jung et al.21 proved that a higher amount of potassium in iron oxides would lead to a higher capacity during cycling; Groult et al.22 showed that the correlation could be built between the electrochemical properties of the carbon materials and surface modification with the alkaline cations; Komaba et al.23 reported that the irreversible electroreduction capacity at the initial discharge/charge was reduced by coating the graphite powder with alkali chloride solid; Zuo et al.24 demonstrated that the silicon/carbon powders could be treated by KCl aqueous solutions to improve the electrochemical properties of the composite as the negative materials for lithium ion batteries. By surveying these previous reports, the primary studies about alkali salts in anode materials were the modification and improvement of the solid electrolyte interface (SEI) film26 and low-rate circulation property. However, there was little focus on the impact of KCl doping on the high-rate discharge property of the graphitic negative electrode active material.



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Herein, we reported a feasible processing method to obtain the potassium chloride-modified graphite as anode material for Li-ion battery with high ratio discharge property. First-principle methods27-28 calculations have been done to analyze the effect of potassium on the performance of lithium-storage property of graphite. 2. Fabrication and test 2.1. Materials Graphite was a sort of commercial graphite with a spherical structure from Shandong province, other reagents, such as 1-methyl-2-pyrrolidone (NMP), potassium chloride (KCl) and Polyvinylidene fluoride (PVDF) were analytical pure which were produced by Sinopharm Chemical Reagent Co. Ltd. 2.2. Fabrication of graphite@KCl There were two simple steps to achieve the KCl-modified graphite particles. Firstly, weighing the raw material with the weight percent of 1 : 0.01 (graphite : KCl) and putting them into a 50 ml ceramic crucible to mix with deionized water for 0.5 h under magnetic stirring, and the mixture was dried in an oven at 105°C for 12 h under the atmospheric pressure. Secondly, in the Komaba’s research23 the KCl-modified graphite prepared by simply drying at 100 ºC, in this paper, the dried mixture was sintered in a quartz tubular furnace at 800°C, 850°C and 1000°C for 5 h, respectively, under nitrogen atmosphere. Based on different treating temperature, the sample was named G@K800, G@K850 and G@K1000, respectively. 2.3. Characterization


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The degree of graphitization of carbon composite was characterized by Raman spectra on the Renishaw confocal Raman microspectroscopy (Renishaw Co. Ltd., Gloucestershire, U.K.) system (the laser excitation wavelength is 780nm). The X-ray diffraction (XRD) patterns of the samples were performed using a Rigaku Dmax-2500 diffractometer (Cu kα radiation, λ = 0.1542 nm) at a scanning rate of 4 min-1. The micromorphology was tested by a JSM-6700F field emission scanning electron microscopy (FESEM) and JEOL-2100F high-resolution transmission electron microscope (HRTEM) with Energy Dispersive Spectrometer (EDS). 2.4. Electrochemical measurement Half-cell using lithium foil as the reference electrode was used to test the electrochemical properties of the graphite@KCl composites. After dispersing the material sample, acetylene black and the binder containing 9.0 wt% polyvinylidene fluoride with a weight ratio of 80:10:10 in 1-methyl 2-pyrrolidone (NMP, Sigma Aldrich) solvent, the resulting homogeneous slurry was coated on Cu foil current collectors and then dried at 120oC for 12 h in a vacuum oven. Subsequently, the electrodes were cut to 14 mm in diameter that the weight of active materials on each electrode was about 3.0 mg, and CR2025

button cells were assembled in an

glove-box filled with inert argon gas using the celgard 2300 as separator, nickel foam as current collector and 1M LiPF6 solved in equal volume of ethylene carbonate/dimethyl carbonate as electrolyte. Galvanostatic static cycle properties of the half-cells were tested by a Land CT2001A battery testing system from 0.02 to 3.0 V (vs. Li/Li+) at a constant 25oC. An IviumStat electrochemistry workstation was used



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to test the electrochemical impedance spectroscopy (EIS) and cyclic voltammogram (CV). The scanning rate of CV was 0.1 mV s−1 between 0.02 and 3.0 V, and the EIS was measured in a range of frequency from 100 KHz to 0.01 Hz with AC signal amplitude of 5 mV at open-circuit voltage. 2.5. Modeling of graphite The density functional theory (DFT) calculations were conducted by the Dmol3 package29. The generalized gradient approximation (GGA) functional with formulas of Perdew, Burke, and Ernzerhof (PBE) was used in this work to determined the exchange-correlation function. The k-points for the Brillouin zone sampling were 8 × 8 × 1 for optimization of the structures. The density of states (DOS) was determined by a 4 × 4 × 2 mesh. 3. Results and Discussion 3.1. Structure and morphology of KCl-modified graphite

Fig. 1. XRD patterns of the KCl-modified and control graphite; the inset is the magnified image at 2θ range from 26 to 27 degree. The XRD patterns of control graphite and the samples that are prepared with


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potassium chloride aqueous solutions are shown in Fig. 1. The intense diffraction peaks of graphite indicate the high crystallinity of the natural graphite. From inset, the Bragg peaks of KCl-treated graphite shows a small shift to smaller 2θ angles compared to the untreated sample, indicating a increase in the average interlayer spacing of the changed crystal structure by the embedding of potassium ions with large radius30. Raman spectroscopy as a very sensitive technique is chosen to illustrate the variation of carbon structure. As shown in Fig. 2, there are four band shifts such as the so-called G band appearing at 1582 cm−1 , the D band at about 1328 cm−1, the D′ band at about 1618 cm−1 and the G′-band at about 2686 cm−1, in which D and D′ bands are the first order Raman spectra (fundamental vibrational modes) of the well-ordered graphite, in the same time, the G and G′′ bands are the second order Raman spectra (harmonic and combination modes) of the disordered carbonaceous material31. Theoretically, the degree of graphitization of the carbon material can be represented by the integrated intensity ratio ID/IG that is 0.2, 0.6, 0.5, 0.3, respectively, which indicates the improvement of disordered degree. This loose structure can improve the electrochemical properties of LIBs by providing more storage space for Li-ions18 .



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Fig. 2. Laser Raman spectra of samples.

Fig. 3. SEM images at different magnifications (a, b, c); TEM images of G@K850 (d) and their potassium (e) and, chlorine (f) elemental mapping images. SEM images of different magnifications (Fig. 3a, b, and c) demonstrate the morphology of the commercial graphite, as exemplified by G@K850, is a typical micron spherical graphite. It can be also seen that the graphite surface has many defects which can provide more storage space for Li-ions, conducive to improving the electrochemical performance. To further confirm the distribution of the potassium and chlorine elemental in the graphite, the mapping image corresponding to the regions in


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Fig. 3d (as shown in Fig. 3e and f) is investigated by EDS analysis, suggesting the uniform distribution of the elementals in graphite particles, which is mainly caused by the sol-gel process used in sample preparation, making the modified sample more uniform and thus have more stable performance. 3.2. Electrochemical performance Motivated by these structural features, we evaluate the electrochemical performance in half-cell configuration for the potential of the KCl-modified graphite as an negative material for LIBs.

Fig. 4. Cycling performance at 0.1C (a), corresponding Coulombic efficiency (b), rate capability at various current densities from 0.1C to 2C (c), and cycling performance at 1C (d) for G, G@K800, G@K850 and G@K1000. Fig. 4a shows the charge and discharge curves of the bare graphite particles and



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G@K composites at 0.1C for 60 cycles. With the exception of discharge capacity for the first time, the average discharge capacity of the G@K850 is 437.6 mAh g-1, that is slightly higher than the untreated-graphite sample with discharge capacity of 411.7 mAh g-1, and significantly higher than G@K800 (296.2 mAh g-1) and G@K1000 (282.4 mAh g-1), indicating that too high or too low treated temperature would cause a negative effect on the material electrochemical properties. As shown in Fig. 4b, despite the large reduction of capacity due to forming the solid electrolyte interface (SEI) films in the initial several cycles, the capacity tends to stabilize and almost close to 100%, demonstrating the higher cycling stability. It is obvious that the G@K850 and the G exhibit similar performance at the beginning of the fifty cycles (Fig. 4c) but when the density is recovered to 0.5 C, the reversible capacity of the former one is reached to 306.8 mAh g-1 which is better than that of the G (232.8 mAh g-1). This phenomenon encourages us to further test the capacity of the sample at high current charging and discharging . In the Komaba’s research work23, the KCl-modified graphite succesfully prepared without high temperature sintering, but it showed excellent initial capacity at 175 mA g-1 and maintained this value during the initial 20 cycles. However, other main properties such as long-term cycling stability were not provided. In this study, as shown in Fig. 4d, 200 charge/discharge cycles at 1C (372 mA g-1) are tested. Results show that the average discharge capacity for G@K850 is 269.7 mAh g-1 that is significantly higher than 100.8, 93.6, and 159.7 mAh g-1 for G, G@K800, and G@K1000, respectively. Compared to the superior cycle stability of G@K850, from


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the second cycle onwards, the capacity of G, G@K800 and G@K1000 increases gradually from 70.9 to 177 mA h g-1, 64.6 to 164 mA h g-1 , 75.9 to 236.4 mA h g-1 within 200 cycles after the capacity decreases within the initial twenty cycles. On one hand, this increase in capacity may be due to the electrode materials activation during charging and discharging process32, on the other hand, it may be due to the reversible growth of a polymeric gel-like film33.

Fig. 5. The Nyquist plots of G, G@K800, G@K850 and G@K1000 (a) after initially 5 cycles; the relationship between Zre and ω-1/2 in the low frequency region (b); the corresponding equivalent circuit (c). To provide more information of the improved electrochemical performance by KCl, the change in the different treatment conditions can be clearly observed as Nyquist plots shown in Fig. 5. Figure 5a shows typical impedance spectra obtained at same potential by cells that are activated and stabilized by five galvanostatic charge/discharge cycles. There are two partially overlapped semicircles in the high



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and middle frequency regions and an inclined line in the low frequency region. The semicircles are related to the impedance of the charge-transfer reaction at the graphite/electrolyte interface and to the contact between graphite powder and current collector, respectively, while the straight slopping line to the Warburg impedance resulted from the Li+ diffusion at the electrode–electrolyte interfaces. Such a pattern of EIS is fitted by an equivalent circuit34 as shown in Fig. 5c, in which Rs is the ohmage, RSEI and Rct are resistances of SEI film and charge-transfer reaction, respectively, QSEI and Qct are capacitances for the SEI film and the double layer, respectively, and QD is the Warburg impedance. It is obvious that two partially overlapped semicircles in the high and middle frequency region of KCl-modified samples is smaller than that of the bare graphite. The change of the R value after modification demonstrates the increase of electronic conductivity. Another approach for Li+ diffusion coefficient is also given in Fig. 5b that is the relationship between Zre and ω-1/2 in the area of low frequency, which could be evaluated by the following formula in published work35 : Zre = Rs + Rct + σω-1/2

(1)

D = R2T2/2AF4C2σ2

(2)

Where Zre is the real component of impedance, RS is the ohmage of electrolyte, σ and D are the coefficients of Warburg impedance and Li+ diffusion, respectively, ω is the angular frequency in the area of low frequency, R is the gas constant, T is the kelvin temperature, F is the Faraday constant, A is the contact surface area between the electrolyte and the electrode, and C is the molar concentration of Li+.


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From Fig. 5b, the smaller slope of the straight line denotes the higher Li+ diffusion kinetics in comparison with the similar big slope of the bare graphite sample, so the G@K composites reveal higher rate discharge performance than the raw graphite by virtue of both the improved Li+ diffusion coefficient and the enhanced electronic conductivity.

Fig. 6. Discharge/charge plots for the 1st cycle at 0.1C, 2th, 100th and 200th cycles at 1C. (a) G, (b) G@K850. The discharge/charge curves for selected cycles at 0.1C (1st cycle) and 1C (2th, 100th and 200th cycles) are shown in Fig. 6. The discharge/charge capacities in the 1st charge-discharge are 521.8/402.8 mAh g−1 for G@K850 with a Coulombic efficiency of 77.2 % which are better than that for G (495.1/373.9 mAh g−1 with a Coulombic efficiency of 75.5 %). After the activation by the first galvanostatic discharge and charge, the performance was further at tested 1C. It is obvious that the potential interval (∆V) between the discharge/charge plateaus is smaller for G@K850 than that for G, illustrating better reversibility as depicted in Fig. 4d, corresponding well to the subsequent CV analysis. As the cycle number increases, the cycling stability and capacity of G significant decrease while those of G@K850 reveals



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excellent stability.

Fig. 7. Cyclic voltammograms plots of all samples at a scanning rate of 0.1 mv s-1, after initial five cycles. The CV curves of all composites after 5 cycles from 0.02 V to 3 V and a scanning rate of 0.1 mV s-1 at ambient temperature are plotted in Fig. 7. All samples have two cathodic peaks and two anodic peaks during the scanning, that are generated through the Li+ insertion into or extraction from different graphite layers36-37. Generally speaking, at a very low scanning rate (such as 4 µV s-1), there are four couples of redox peaks representing to the number of Li+ intercalated into the adjacent graphene layers at different potentials38. LiC72 + Li⇌2LiC36

(1)

3LiC72 + Li⇌4LiC27

(2)

4LiC27 + 5Li ⇌9LiC12 (3) LiC12 + Li ⇌ 2LiC6

(4)

However, at a high scan rate of 0.1 mV s-1, the peaks overlap mutually. In other words, in the potential range (0.02~3 V), the reversible formation of subsequent


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stages in LixC6 is the main reaction. The potential difference between the oxidation and reduction peaks of G@K composites is less than that of the bare graphite revealing the reduced polarization39. It might be because the distance between the graphite layers is expanded by the slightly intercalation of the potassium ions with larger atomic radius than that of lithium ion during the high temperature sintering and the first charge/discharge, resulting in the enhanced Li+ diffusion in the interlayer space and lower resistance. 3.3. Calculation of the graphite structure In this study, first principle calculations have been performed to further investigate the role of the potassium for the lithium storage properties in graphite material. The high-symmetry Li positions at the four-layer graphite structure that we have constructed are three types: two carbon atoms (A); the bond center of a carbon-carbon bond (B); an atom and its corresponding center in the hexagon (H), between the upper and lower layers. Through calculation and analysis, the optimized C–C bond length of graphite is 1.42 Å that is similar to the previous theory calculation (1.422 Å ) and experimental data (1.420 Å)40-41. And when Li in the H position, the structure is the most stable because of the lower energy than that of Li in the A position (∆E=0.29 eV) and B position (∆E=0.97 eV). Consequently, when a potassium accesses into the H position to optimize the structure, the distance of the graphite layer is found to increases significantly (4.7 Å) (as shown in Fig. 8a). The DOS of graphite in Fig. 8b illustrates that graphite has zero bandgap and the potassium insertion on the center of



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the hexagon push up the Fermi level to the conduction band. Through the deeply Mulliken population analysis, the K ion transfers about 1.288 e charge to the graphite layer to form KCx.

Fig. 8. (a) Atomic model of a 4 × 4 × 2 mesh with a potassium ion in H position after optimization; (b) The density of states (DOS) of the graphite and the graphite with 1 K in H position. The inset is the magnified images near the Fermi level. Based on this result and some research work40, we choose the position as shown in Fig. 9 to further study the effect of potassium ions on the lithium storage performance of graphite.


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Fig. 9. Atomic model of a 4 × 4 × 2 mash with two ions in H position. (a): top view; (b): bottom view.

This calculations also shows that, when we placed a lithium and a potassium at positions (1) and (2), its ∆E is -6.91 eV which is lower than that when two lithium ions (-4.74 eV) or two potassium ions (-4.60 eV) are simultaneously placed, indicating that the presence of potassium in the graphite layers has a positive effect on the lithium storage ability of graphite. 4. Conclusions In this paper, KCl-modified graphite particle is fabricated simply by a solution mixing and sintering method. The composite powders show good electrochemical performance when used as negative material for LIBs, in which the sample sintered at 850 °C for 5 h has the best rate discharge performance (the discharge capacity is 269.7 mAh g-1 at 1 C). The impact of potassium on the lithium storage properties of graphite is investigated by first principles calculations. The result shows that the insertion of the potassium ion in the graphite layers improves the conductivity as well



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as the lithium storage.

AUTHOR INFORMATION *Corresponding Author. Email: [email protected] (H. L.); [email protected] (Y.-J. Bai)

Notes The authors declare no competing financial interest.

Acknowledgements: The authors would like to acknowledge the support from the National Natural Science Foundation of China (Grant No.51671114). This work is also supported by the Special Funding in the Project of the Taishan Scholar Construction Engineering.

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KCl-Modified Graphite as High Performance Anode Material for

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