Selective removal of nanopores by triphenylphosphine treatment on the natural graphite anode

Electrochimica Acta 326 (2019) 134993

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Selective removal of nanopores by triphenylphosphine treatment on the natural graphite anode Jaewon Kim a, Kimin Park a, Hyungsub Woo a, Bumjin Gil a, Yoon-Soo Park b, Il Seok Kim b, Byungwoo Park a, * a

Department of Materials Science and Engineering and Research Institute of Advanced Materials, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea Samsung SDI, 130 Samsung-ro, Yeongtong-gu, Suwon, Gyeonggi-do, 16678, Republic of Korea

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 July 2019 Received in revised form 18 August 2019 Accepted 30 September 2019 Available online 1 October 2019

Artificial graphite (AG) and natural graphite (NG) have different physical properties, which should have significant effects on their unique electrochemical characteristics: NG is known to have superior specific capacity and low cost, whereas AG shows superior cyclability and low-swelling characteristics. Herein, we present a straightforward strategy using triphenylphosphine to improve NG, enabling its physical properties to resemble those of AG and thereby improving its cyclability and swelling characteristics. The volume of a-few-nanometer-sized pores of NG is significantly reduced by triphenylphosphine treatment, which is similar to that of the AG. As a result, triphenylphosphine-treated NG (TPP) anode shows improved cyclability (83.6% after 300 cycles) compared to that of NG, confirming the correlation between nanopore volume and cyclability. Furthermore, the expansion of single-layer pouch cell prepared with TPP, as monitored by in situ thickness measurement, is suppressed to 9% after 55 cycles, which is superior to the pristine NG showing 14% expansion after the same number of cycles. Electrochemical impedance analyses of symmetric cells reveal that the changes of ionic and solid-electrolyte interface (SEI) resistance are accompanied by the reduction of nanopores, confirming that the formation of decreased SEI layer is a crucial factor for realizing less-swelling and highly durable anodes based on the cost-effective NG materials. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Natural graphite Nanopores Triphenylphosphine Swelling Cyclability

1. Introduction Graphite has been considered as an excellent commercial anode material for Li-ion battery since Sony first commercialized it in 1991. Although various electrochemically active anode materials have been developed, such as Si, Sn and Li4Ti5O12, graphite is still attractive in commercial terms [1e3]. Compared to Li-alloy based materials such as Si and Sn, graphite has relatively small initial capacity because of the intrinsic character of intercalation reaction up to LiC6 [4e6]. However, since the volume expansion of graphite (~13%) is less than other anode materials, the reversible capacity during the charge/discharge processes is usually better than other anodes [7]. Moreover, graphite shows excellent electronic conductivity compared to other materials and high energy density due to its low intercalation voltage which is close to Li [8e10].

* Corresponding author. E-mail address: [email protected] (B. Park). https://doi.org/10.1016/j.electacta.2019.134993 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

Meanwhile, graphite particles are basically plate-like shaped due to the unique 2D structure, which can be stacked with each other forming secondary particles. As the graphite particles can be either randomly stacked or orderly stacked, forming different internal structures, the mechanical and kinetic properties of graphite anodes greatly vary with the raw graphite materials and the postprocessing such as physical deformation or pitch coating. Thus, to meet the specific needs of a battery, such as high-power or extrastability, it is important to design the graphite anodes through careful selection of raw materials and the post-treatment [11,12]. Graphite can be categorized into artificial graphite (AG) and natural graphite (NG). AG is usually manufactured by annealing amorphous carbons at a high temperature, or by calcination of petroleum coke or coal tar pitch which can be graphitized [13,14]. For this reason, it is known that AG usually possesses high purity, ordered internal structure and excellent cycle performance. However, since the AG requires high-temperature heat treatment, the price of AG is twice as expensive as that of NG. Unlike AG, NG has a randomly-ordered internal structure, but has a high degree of

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graphitization, so it has advantages in terms of initial discharge capacity and energy density [15,16]. However, the flake structure of NG can be disadvantageous in kinetic properties. In addition, cycle performance of NG is generally inferior to that of AG, because of the larger surface area and the exposed (intact) edge surface which causes more irreversible reactions to form uneven and thick solidelectrolyte interface (SEI) layers during lithiation [17,18]. Since the long-cycle- and power-performance characteristics of electronic devices are generally preferred, many studies have been carried out to improve the cyclability and rate capability of NG. Instead of directly using plate-like NG, the “spheroidization” method, which mechanically modifies graphite to provide isotropic properties, is commonly utilized (spherical natural graphite). In addition, studies are underway to improve the electrochemical characteristics of NG through surface modification methods [19,20], chemical or air oxidation [21e23], partial cathodic preexfoliation process [24e26], and polymer or silicon coatings [27e29]. Most of the studies have focused on improving the electrochemical performance of NG and AG separately, and there are some lack of understanding in fundamental characteristics of the two kinds of graphite [27e31]. In this study, we studied the differences in the properties of NG and AG by classifying and quantifying through analytical methods, in order to understand the factors that affect the electrochemical properties of NG. One of such factors is nanopores (1 e 4 nm) existing on the surface of graphite, as revealed by pore analysis [32e34]. To reduce the volume of nanopores present in most of the graphite, phosphorous-based triphenylphosphine (C18H15P) was straightforwardly utilized to modify the graphite, instead of using common carbon-coating source such as pitch [35e37]. In addition, triphenylphosphine treatment was conducted to induce phosphorous-based bonds on the graphite surface as the PeO and PeC bonds on the graphite surface are known to prevent the penetration of the solvated PF
6 , which helps forming chemically stable SEI layers [38,39]. Through a simple solid-state reaction using triphenylphosphine, nanopores were effectively reduced to form a stable SEI layer [39,40], which thereby improves the cyclability and expansion characteristics of NG. To further understand the observed full-cell phenomena, we examined symmetric cells with impedance spectroscopy to analyze different kinetics of each graphite anode. 2. Experimental procedure 2.1. Preparation of triphenylphosphine-treated natural graphite Natural graphite (NG) powders with an average particle size of 20 mm were used as the raw materials. The triphenylphosphinetreated NG (TPP) was synthesized by impregnating the NG powders with triphenylphosphine (NG:triphenylphosphine ¼ 1:1 by weight) followed by mixing and calcining under Ar atmosphere. To control nanopores in NG, this study used two-step annealing processes. In the first step, NG-triphenylphosphine mixture was annealed at 180  C for 60 min. In the second step, the reaction temperature was raised to 800  C and then held for 60 min. The derivative thermogravimetric (DTG) analysis of triphenylphosphine shows that the starting point appears at around 180  C (Fig. S1), and the nanopores can be effectively reduced by the two-step heat treatment at this temperature [32]. For comparison, commercial artificial graphite (AG) was also investigated.

gas adsorption instrument. The specific surface areas and pore size distributions of the prepared samples were calculated by the Brunauer-Emmet-Teller (BET) and Barre-Joyner-Halenda (BJH) methods, respectively. The crystal structures of the prepared samples were characterized by x-ray diffraction (XRD, D8 Advance: Bruker), and the morphologies of synthesized materials were analyzed by field-emission scanning electron microscopy (FE-SEM, SIGMA: Carl Zeiss). The chemical states of the sample surfaces were investigated by x-ray photoelectron spectroscopy (XPS, AXIS-HSi: KRATOS), and thermogravimetric analysis (TGA) and DTG profiles were obtained using Discovery TGA (TA Instrument) between 50  C and 800  C at a heating rate of 5  C min
1 under N2 atmosphere. 2.3. Electrochemical measurements The electrodes were prepared with active materials (graphite), sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) binder at a weight ratio of 96.5:2.0:1.5. The slurry was uniformly mixed by homogenizer, coated onto the Cu foil using a doctor-blade method, and calendared by roll press. The electrodes were dried at 110  C in vacuum overnight. The thickness of the electrode was 98 mm with the electrode density of 1:55 g cm
3 . In order to assemble the cell, CR2032 (half cell and full cell) and pouch (full cell) type cells were utilized in an Ar-filled glove box and dry room, respectively. The 1.15 M LiPF6 in ethylene carbonate (EC)/ dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (2:4:4 by volume) with a 1.5 wt % of vinylene carbonate (VC) additive was used as an electrolyte (Panax Etec). Charge/discharge performance of the half cell was characterized with WBCS3000S tester (WonATech Co., Ltd. Korea) between 0.01 V (constant current (CC, 0.1 C) - constant voltage (CV, 0.01 V until the current decreased to 0.02 C) mode) and 1.5 V. In case of full cell, LiNi0.88Co0.10Al0.02O2 (NCA) was adopted as the cathode and the ratio of negative to positive electrode capacity (N/P ratio) was fixed to 1.1. The electrochemical test of the full cell was cycled at 0.1 C (4.3 mA cm
1) for two cycles (formation step) to form a SEI layer at the initial stage, and the assessments of cyclability were conducted at 0.2 C for 300 cycles using TOSCAT-3000 tester (Toyo System, Japan) between 2.5 and 4.2 V (CC e CV mode). An in situ thickness measurements were performed using a single layer pouch cell. The cell was secured to a flat Al stage, and a displacement sensor (Keyence GT2) was used to measure the changes in the cell thickness during charge/discharge cycling. The first 3 cycles of this test was a formation step (at 0.1 C), and the rest of the cycles were conducted at 0.2 C up to 55th cycle. Each charge/discharge cycling was conducted between 2.5 and 4.2 V. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted using a potentiostat (ZIVE MP1: WonATech Co., Ltd. Korea). The symmetric cells (graphite/graphite) were fabricated by employing two graphite electrodes with a state of charge (SOC) of 0%, ~1%, 25%, 50%, 75%, and ~100%, collected from two identical graphite/Li half cells. EIS measurement was carried out for symmetric cells at the open-circuit voltage (OCV). For cointype half cells, measurements were carried at 1.5 e 0.1 V (0.2-V interval) and SOC of 20 e 100% (5% interval). A relaxation time of 5 min was allowed before starting every EIS measurement. The frequency range was 100 kHz to 10 mHz with AC amplitude of 10 mV. 3. Results and discussion

2.2. Materials characterization Volumetric nitrogen adsorption isotherms (at 77 K) up to 1 bar were measured using a Micromeritics ASAP 2020 static volumetric

Nanopore distributions exhibit notable differences with NG and AG, with NG showing more pore volume and BET surface area than that of the AG (Fig. 1). The pore-volume distributions derived from

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Fig. 1. Characterization of nanosized pores in natural graphite (NG), triphenylphosphine-treated NG (TPP), and artificial graphite (AG). (a) Pore distributions (<10 nm) of graphite samples with the total pore distributions in the inset. (b) Integrated nanopore volume and BET surface area of the graphite samples. (c) SEM images of NG, TPP, and AG. (d) Schematic illustration of nanopore-passivation effect by triphenylphosphine treatment.

the pore diameter of graphite samples seem similar to each other, but the estimated values of pore volume of the samples show distinguishable differences (Fig. 1(a)). Notably, the volume of nanosized pores (1 e 4 nm) of NG (2.2  10
3 cm3 g
1) is higher than that of AG (0.7  10
3 cm3 g
1). Considering that the SEI growth is in the range of several nanometers, SEI layer of NG is expected to be formed much irregularly due to the larger contents of nanopores compared to AG. While the nanopore volume of triphenylphosphine-treated graphite (TPP) approaches to that of the AG, it shows ~40% reduction in the nanopore volume compared to that of the NG. Graphite materials for commercial Li-ion batteries usually possess low specific surface area (<10 m2 g
1) to minimize the irreversible capacity loss due to the SEI formation on the surface [16,17]. As shown in Fig. 1(b), NG (5.72 m2 g
1) has higher BET surface area than AG (1.27 m2 g
1), which has an adverse effect on the cyclability and expansion characteristics. (The SEI layer formation is described in more detail with a symmetric-cell impedance data in the following Fig. 5.) In order to clarify that the reduction of nanopores is caused only by triphenylphosphine, we examined the pore distribution of NG annealed at the same temperature without triphenylphosphine, as shown in Fig. S2. The overall shapes of all the graphite samples look similar with a diameter of ~20 mm, as characterized by SEM (Fig. 1(c)). The influence of metastable rhombohedral phase (ABCABC

stacking instead of ABAB) on the electrochemical properties is still controversial [41e45]. However, recent studies based on the ethylene carbonate (EC) electrolytes show drawbacks of deteriorated irreversible capacity due to the rhombohedral structure [46,47]. X-ray diffraction confirms the presence of rhombohedral phase in NG, but not in AG (Fig. 2(a)), which may be considered as the influence of nanopores. In the case of TPP sample, the rhombohedral peaks are reduced compared to NG, probably since bonding of P and C atoms (from C18H15P) to the carbon matrix has changed the metastable phase (Fig. 2(b)). To confirm the effect of triphenylphosphine incorporation on the reduced rhombohedral structure, XRD patterns of only heat-treated NG samples were also measured, as shown in Fig. S3. The lattice constants and local strains along the c-axis direction of graphite powders are shown in Fig. 2(b). As is generally known, AG (6.735 Å) has a larger lattice constant compared to NG (6.721 Å), whereas TPP showed slightly increased lattice constant of 6:727 A: In addition, compared to the initial state of nonuniform distribution of local strain in NG (0.01%), AG (0.21%) has a large local strain along the [001] direction. Triphenylphosphine has also induced a larger local strain (0.10%) than NG because of phosphorus affecting carbon layers. Since the local strain along the [001] direction and large lattice constant of the graphite can accelerate the Liþ diffusion [48,49], electrochemical characteristics of AG and TPP are expected to be superior to those of NG. The local strains after 300 cycles

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Fig. 2. Structural characterization of NG, TPP, and AG. (a) XRD patterns of the graphite samples. (b) Contents of rhombohedral phase in each sample (top). Lattice constants (middle) and local strains (bottom) along the [001] direction, for the pristine and after 300-cycled samples. (c) XPS spectra of phosphorous (P 2p) of NG and TPP.

become more relaxed from the AG and TPP powders (even though the local strain of NG looks increased with large error bars). From the XPS (Figs. 2(c) and S4), it can be seen that phosphorus is incorporated into the graphite framework through the PeC bonding (~130.3 eV), and the PeO bonding ð 132:5 eVÞ is indicated through the reaction between triphenylphosphine and a small amount of functional groups present in graphite [50,51]. As the presence of PeC and PeO bonding on the graphite surface can contribute to the formation of a chemically stable SEI layer, such phosphorous-based bondings are expected to improve the cyclability and expansion characteristics of NG [38,39]. The apparent content of phosphorus in TPP was estimated by XPS (~0.85 at. %) and EDS analysis (~0.20 at.% in Fig. S5). EDS mapping shows that the phosphorus atoms are evenly distributed throughout the surface of the graphite (Fig. S5). The cycle performance and Coulombic efficiency of full cell are shown in Fig. 3(a). AG (85.3%) has the highest capacity retention after 300 cycles, as expected by the trends of nanopores and rhombohedral phase, as discussed above. Likewise, TPP (83.6%) also exhibits improved cyclability compared to NG (81.3%). The half-cell tests also show that the electrochemical characteristics of three samples are different (Fig. S6). It can be seen that NG suffers from higher polarization than other electrodes (voltage profile, dQ/dV) and has more constant-voltage than constant-current region. The cyclic voltammetry reveals that the formation of SEI in NG is more prominent near ~0.6 V than that of the other samples. In order to understand the relationship between the side reaction (SEI formation) and the nanopore content, the thickness changes of the electrodes after 300 cycles were measured (Fig. 3(b and c)). All the electrodes were calendared to have thicknesses of 98 mm. After 300 cycles, the thicknesses of AG and TPP electrodes are slightly increased by ~105 and ~110 mm, respectively. However, in the NG electrode, the thickness increases significantly to ~140 mm, indicating that more side reactions occur in NG.

In order to observe the swelling of an electrode during charging/ discharging in real time, the thickness changes of the pouch cells were recorded (Fig. 4(a)). The variations of both thickness and voltage over time are shown in Fig. 4(bed). As shown in previous reports [52,53], the expansion of the pouch cell is somewhat irreversible as the cycle progresses. Such irreversible change can be attributed to the side reactions inside the pouch cell such as fracture and/or SEI formation. Fig. 4(c) shows the discharge state of the full cell (i.e., thickness of the delithiated graphite electrode) vs. cycling, which indicates the degree of expansion of the graphite electrode. NG showed larger swelling of 13.5% expansion after 50 cycles than that of AG (4.3%) and TPP (9.3%). Therefore, it is possible to effectively suppress the side reaction of NG by triphenylphosphine treatment, which may result in the improved cycle performance. Generally, cathode electrolyte interface (CEI) formation can occur through side reactions in the cathode [54e57]. In order to confirm that the side reactions affecting the expansion are mostly due to the graphite electrode, the change in thickness of the cathode electrode for 55 cycles was also observed (Fig. S7). It can be seen that, unlike graphite electrodes (Fig. 3(c)), the cathode electrode shows negligible change during 55 cycles, which indicates that the main factor influencing the expansion is due to the side reaction of the graphite electrode. Electrochemical impedance spectroscopy (EIS) using a half cell has a disadvantage because the impedances of the working electrode (graphite) and the counter electrode (Li metal) are observed at the same time, making it difficult to separate the effect of each electrode (Fig. 5(a)) [58e60]. In order to clearly observe the exact impedance, many research groups have utilized impedance analysis using a symmetric cell [61e63]. In Fig. 5(b), the resistance of Liion (Rion) on the graphite surface is compared cell in the pristine (delithiated) state [61,62]. In the case of delithiated graphite, charge transfer does not occur so that only solution and ionic resistance behaviors are observed, while lithiated-graphite symmetric cells

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Fig. 3. Electrochemical properties of full cells using NG, TPP, and AG as anodes. (a) Cycle performances and Coulombic efficiencies of the NG, TPP, and AG (potential range of 4.2 e 2.5 V vs. Li/Liþ for 300 cycles with charge/discharge current density of 4.3 mA cm
2 (0.1 C)). Cross-sectional SEM images of the graphite electrodes (b) before and (c) after 300 cycles.

Fig. 4. Swelling analysis of the electrode by in situ thickness measurement of the NG, TPP, and AG pouch full cells (3  4 cm2). (a) Schematic diagram of the experimental setup, and (b) the thickness variations of single-layer pouch cells during lithiation/delithiation over time. (c) Changes of thicknesses (initial electrode thickness: 98 mm) in the delithiated graphite up to 55th cycle, and (d) the corresponding voltage vs. time profiles.

exhibit charge transfer behavior (Fig. S8) [61,62]. The resistance values of Rion are 27.1 U cm2 (AG), 71.9 U cm2 (NG), and 42.6 U cm2 (TPP), respectively. The tortuosity (t) of the electrodes indicating the effective transport properties of Liþ in the porous electrode can be obtained by Refs. [64e66]:

t¼


Rion Akε ; 2d

(1)

where A is the cross-sectional area of the electrode, k is the

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Fig. 5. Electrochemical impedance spectroscopy from the symmetric cells using two same graphite electrodes. (a) Schematic cell configuration of symmetric vs. asymmetric cells. (b) Impedance spectra at the pristine state (without lithiation). The intercept at the x-axis indicates Rion/3. (c) Tortuosity of graphite electrodes. The tortuosity (t, unitless) is a parameter that accounts for the morphological effects of the electrode on the effective Liþ transport properties (from Eq. (1)). (d) Impedance spectra at 0.1-V cell voltage (50% lithiation). The dashed line predominantly arises from the SEI resistance (with details in Fig. S8).

conductivity of the electrolyte, ε is the porosity of electrode, and d is the electrode thickness. The tortuosity in Fig. 5(c) indicate that the Li-ion transfer at the electrode is fast in the order of AG > TPP > NG. Combining these with the nanopore data, it can be said that the more abundant the nanopores are on the graphite surface, the more adversely affected the Li-ion diffusion is. The apparent Li-ion diffusivity is estimated through EIS to further confirm the differences in the behavior of Li-ions on the graphite surfaces [67,68]. The lowest Li-ion diffusivity in NG (Fig. S9) again exhibits the same tendency with Rion (Fig. 5(b)). The Nyquist plot of a symmetric cell with 50% lithiated graphite electrodes is shown in Fig. 5(d). Considering the relaxation frequency of AG (150 Hz), TPP (199 Hz), and NG (250 Hz), the SEI resistance is dominant compared to the bulk charge transfer resistance (Fig. S8) [69,70]. The SEI resistance values were 18.6 kU cm2 (AG), 51.5 kU cm2 (NG), and 23.0 kU cm2 (TPP), respectively, with the consideration of BET area of each sample and the mass of graphite coated on the electrode (21.62 mg) (Fig. S8) [71,72]. The SEI resistance values of the graphite samples with different Li contents are shown in Fig. S10. In accordance with the trend of electrode expansion, the vigorous SEI formation in NG

exhibits higher than those of the AG and TPP in all the regions. Furthermore, in the case of the NG electrode, the SEI resistance continues to increase, which is different from the other cases: AG and TPP samples. Therefore, a higher level of nanopores on the NG surface hinders the formation of a stable SEI layer, which also affects the electrochemical properties of graphite electrodes. 4. Conclusions In this work, electrochemical properties of graphite were categorized with its structural properties and nanopore contents. Since the NG possesses three times more nanopores than AG, the cyclability and expansion characteristics are rapidly deteriorated due to the formation of unstable SEI layer in the nanopores. In order to cover the nanopores on NG, a facile treatment with triphenylphosphine was used. Compared to the previous methods of carbon coating, our triphenylphosphine treatment is revealed to have some additional effects of controlling the nanopores on the surface and stability enhancement of the SEI layer by incorporating phosphorus to the graphite matrix. The cycle performance of TPP exhibited 83.6% retention after 300 cycles, and the full cell using

J. Kim et al. / Electrochimica Acta 326 (2019) 134993

TPP showed a 40% reduction in expansion compared to NG after 55 cycles. According to the EIS analysis using symmetric cells, such improved performance can be related to the lower SEI resistance which is related to the lower nanopore contents of the samples. The AG, NG, and TPP samples are used to confirm whether the reduction of nanopores has a close effect on the SEI formation and electrochemical properties. Based on the results, nanopore control of the graphite materials can remarkably enhance the overall performance of the Li-ion battery. Acknowledgments This research was supported by the Samsung SDI Co. and by the National Research Foundation of Korea (21A20131912052 (BK21PLUS SNU Materials Division for Educating Creative Global Leaders)). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134993. References [1] J.-K. Yoo, J. Kim, M.-J. Choi, Y.-U. Park, J. Hong, K.M. Baek, K. Kang, Y.S. Jung, Extremely high yield conversion from low-cost sand to high-capacity Si electrodes for Li-ion batteries, Adv. Energy Mater. 4 (2014) 140062. [2] J.W. Choi, D. Aurbach, Promise and reality of post-lithium-ion batteries with high energy densities, Nat. Rev. Mater. 1 (2016) 1e16. [3] H. Woo, J. Kang, J. Kim, C. Kim, S. Nam, B. Park, Development of carbon-based cathodes for Li-air batteries: present and future, Electron. Mater. Lett. 12 (2016) 551e567. [4] G. Yoon, H. Kim, I. Park, K. Kang, Conditions for reversible Na intercalation in graphite: theoretical studies on the interplay among guest ions, solvent, and graphite host, Adv. Energy Mater. 7 (2016) 1601519. [5] H. Woo, S. Wi, J. Kim, J. Kim, S. Lee, T. Hwang, J. Kang, J. Kim, K. Park, B. Gil, S. Nam, B. Park, Complementary surface modification by disordered carbon and reduced graphene oxide on SnO2 hollow spheres as an anode for Li-ion battery, Carbon 129 (2018) 342e348. [6] M.M. Atabaki, R. Kovacevic, Graphene composites as anode materials in lithium-ion batteries, Electron. Mater. Lett. 9 (2013) 133e153. [7] S. Schweidler, L. de Biasi, A. Schiele, P. Hartmann, T. Brezesinski, J. Janek, Volume changes of graphite anodes revisited: a combined operando X-ray diffraction and in situ pressure analysis study, J. Phys. Chem. C 122 (2018) 8829e8835. [8] Q. Liu, S. Li, S. Wang, X. Zhang, S. Zhou, Y. Bai, J. Zheng, X. Lu, Kinetically determined phase transition from stage II (LiC12) to stage I (LiC6) in a graphite anode for Li-ion batteries, J. Phys. Chem. Lett. 9 (2018) 5567e5573. [9] Y. Oh, S. Nam, S. Wi, J. Kang, T. Hwang, S. Lee, H.H. Park, J. Cabana, C. Kim, Effective wrapping of graphene on individual Li4Ti5O12 grains for high-rate Liion batteries, J.Mater.Chem.A 2 (2014) 2023e2027. [10] S. Nam, S.J. Yang, S. Lee, J. Kim, J. Kang, J.Y. Oh, C.R. Park, T. Moon, K.T. Lee, B. Park, Wrapping SnO2 with porosity-tuned graphene as a strategy for highrate performance in lithium battery anodes, Carbon 85 (2015) 289e298. [11] N.C. Gallego, C.I. Contescu, H.M. Meyer III, J.Y. Howe, R.A. Meisner, E.A. Payzant, M.J. Lance, S.Y. Yoon, M. Denlinger, D.L. Wood III, Advanced surface and microstructural characterization of natural graphite anodes for lithium ion batteries, Carbon 72 (2014) 393e401. [12] S.J. An, J. Li, C. Daniel, D. Mohanty, S. Nagpure, D.L. Wood III, The state of understanding of the lithium-ion-battery graphite solid electrolyte interphase (SEI) and its relationship to formation cycling, Carbon 105 (2016) 52e76. [13] C. Mao, M. Wood, L. David, S.J. An, Y. Sheng, Z. Du, H.M. Meyer III, R.E. Ruther, D.L. Wood III, Selecting the best graphite for long-life, high-energy Li-ion batteries, J. Electrochem. Soc. 165 (2018) A1837eA1845. [14] T. Ishii, Y. Kaburagi, A. Yoshida, Y. Hishiyama, H. Oka, N. Setoyama, J.-I. Ozaki, T. Kyotani, Analyses of trace amounts of edge sites in natural graphite, synthetic graphite and high-temperature treated coke for the understanding of their carbon molecular structures, Carbon 125 (2017) 146e155. [15] T. Iijima, K. Suzuki, Y. Matsuda, Electrodic characteristics of various carbon materials for lithium rechargeable batteries, Synth. Met. 73 (1995) 9e20. [16] C.-L. Fan, H. He, K.-H. Zhang, S.-C. Han, Structural developments of artificial graphite scraps in further graphitization and its relationships with discharge capacity, Electrochim. Acta 75 (2012) 311e315. [17] T. Placke, V. Siozios, R. Schmitz, S.F. Lux, P. Bieker, C. Colle, H.-W. Meyer, S. Passerini, M. Winter, Influence of graphite surface modifications on the ratio of basal plane to “Non-Basal plane” surface area and on the anode performance in lithium ion batteries, J. Power Sources 200 (2012) 83e91.

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