Mechanisms for Lithium Nucleation and Dendrite Growth in Selected

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Mechanisms for Lithium Nucleation and Dendrite Growth in Selected Carbon Allotropes Xin Su,† Fulya Dogan,† Jan Ilavsky,§ Victor A. Maroni,† David J. Gosztola,‡ and Wenquan Lu*,† †

Chemical Sciences and Engineering Division, ‡Center for Nanoscale Materials, and §Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439-4837, United States S Supporting Information *

ABSTRACT: Using information gained from parallel investigations on lithiated/over-lithiated graphite and hard carbon, we propose two different processes for lithium nucleation and dendrite growth in the carbon allotropes based on their different crystal structures. In the case of graphite, lithium nucleation and initial growth of lithium dendrites occur inhomogeneously on the graphite surfaces during the over-lithiation process, which exposes lithium nucleation to the electrolyte causing formation of a large amount of electrolyte degradation products (EDPs) and knob-like dendrites covered with a thick lithium electrolyte interphase (LEI). However, in the case of hard carbon, lithium nucleation and initial growth occur inside the hard carbon nanopores, which limits side reactions, provides a higher capacity (∼550 mA h/g vs ∼370 mA h/g for graphite), and generates dendrites with smooth clean surfaces during the over-lithiation process. These findings could profoundly influence the overall electrode design, the improvement of performance, and the inherent safety of carbon-based electrodes for lithium ion batteries.

1. INTRODUCTION

over-lithiation of two carbon allotropesgraphite and hard carbon. Wide angle X-ray scattering (WAXS), Raman spectroscopy, and solid-state nuclear magnetic resonance (NMR) have proven to be powerful tools for studying the structure and chemical status of carbon-based and lithium-containing materials, respectively.9,11,13−17,23 In the study presented here we use scanning electron microscopy (SEM), WAXS, ex situ Raman mapping, electrochemical analysis, and NMR spectroscopy to probe lithiated/over-lithiated graphite and hard carbon electrodes in order to investigate the impacts of their crystal structure on lithium insertion and on the nucleation and growth of lithium dendrites during lithiation/over-lithiation. The goal of this research is to increase our understanding of the response of carbon materials in lithium batteries in ways that lead to performance improvement and safety enhancement.

Carbon is the most common anode host material employed today in lithium ion batteries.1−10 Two major kinds of carbon are used in the fabrication of lithium ion battery anodes. One is graphite, the theoretical capacity of which is ∼370 mA h/g with relatively low irreversible capacity;2,11 the second is hard carbon (nongraphitizable), which can offer higher capacity (550 mA h/ g) but exhibits a larger initial irreversible capacity loss compared to graphite.2,4 Since the graphite and hard carbon are allotropes, the only difference between them is their respective crystal structures. Numerous prior research reports have explored (1) the lithium storage capabilities of disordered carbon and graphite through nuclear magnetic resonance (NMR)11−16 spectroscopy and (2) carbon structure evolution during lithtiation/de-lithiation by in situ Raman spectroscopy.17,18 However, when carbon allotropes are lithiated beyond their capacity limit, how they respond in terms of lithium nucleation and dendrite growth is still unclear. Some results have been reported concerning formation of lithium dendrites on lithium/copper anodes and on graphite under abuse conditions.6,19−22 Nonetheless, it remains a challenge to elucidate the details of the lithiation/over-lithiation mechanism(s) by focusing on only one type of carbon anode or utilizing a single technology, since the operating current density, temperature, and electrode structure all have impacts on the formation of lithium dendrites. To get more detailed information about the issues mentioned above and elaborated on by numerous other investigators,2,4,6,11,13−15,17,18 we have undertaken a comprehensive systematic study of the lithiation/ © 2017 American Chemical Society

2. RESULTS 2.1. Pristine Carbon Electrodes. The pristine carbon electrodes were first characterized by SEM and Raman spectroscopy measurements. The results of these measurements are presented in the Supporting Information. In summary, the SEM images reveal that the particle sizes of the graphite and hard carbon in the electrodes are quite similar, ranging from a few micrometers to around 20 μm (see Figure S1a,c in the Supporting Information). The corresponding EDS Received: January 6, 2017 Revised: July 7, 2017 Published: July 10, 2017 6205

DOI: 10.1021/acs.chemmater.7b00072 Chem. Mater. 2017, 29, 6205−6213

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Chemistry of Materials

change of the graphite, is supported by the optical image of Figure S2a.19,26 The lithiated graphite with specific capacity of 250 mA h/g had nonuniform color, consisting of red areas, which were attributed to LiC12, and gold areas, which were attributed to LiC6. Further lithiation resulted in lithium nucleation accompanied by the knob-like structure seen in Figure 1d,e, a structure that has also been observed previously on graphite electrodes subjected to abuse conditions.6 This knob-like structure remains for the de-lithiated sample, which can be seen from the SEM images in Figure S3 in the Supporting Information. These dendrites will detrimentally affect the reversibility of a graphite electrode. As shown in the voltage profile of Figure 3b, the lithiation voltage drops quickly right after full lithiation and reaches a −40 mV plateau at which potential the nucleation and growth of lithium dendrites occur simultanenously. This is supported by the 7Li NMR peak at 265 ppm for lithium metal seen in Figure 7a, the dQ/dV peak at 0.038 V vs Li/Li+ for stripping lithium dendrites in Figure 3c, and the optical image for lithiated graphite at 450 mA h/g in Figure S2b. The optical image of lithiated graphite with a specific capacity of 450 mA h/g consisted of various gold and white areas that were assigned to LiC6 and metallic lithium dendrites, respectively.19,26 Based on the series of observations above, it is obvious that the lithiation of graphite is nonuniform, which could be associated with the nonuniform resistance of the graphite electrode caused by impeded lithium ion diffusion and/or reduced electronic conductivity. In the case of hard carbon, the morphology of the hard carbon did not change until the first lithiation capacity reached 550 mA h/g, at which point white protrusions created during lithium nucleation, such as the ones shown in Figure 2d, began to appear. These point white protrusions disappear and leave no trace of electrode degradation products (EDPs) on the delithiated hard carbon electrode as shown in SEM images in Figure S4 in Supporting Information. As shown in Figure 2e, lithium dendrites with smooth clean surfaces eventually grew

spectra in Figure S1a,c confirm that both carbon allotropes are pure materials. The Raman spectrum of the graphite electrode in Figure S1b shows that there is a sharp and strong G band at 1580 cm−1, which demonstrates the high crystalline quality of the graphite due to its well-aligned layers. However, the Raman spectrum of the hard carbon electrode, as shown in Figure S1d, indicates that the D band intensity at 1302 cm−1 is higher than the G band intensity at 1591 cm−1. The D band is attributable to the graphitic grain boundary phonon, and the G band is the vibrational mode involving symmetric bond stretching within the graphite plane. The intensity ratio IG/ID of hard carbon can be used to determine the crystalline size of carbon grains.17,24 So, in a sense, the IG/ID ratio also gives an indication of the disordered nature of the hard carbon due to its random arrangement of small layers with interconnecting nanopores. 2.2. Morphology Evolution of the Carbon Electrodes during Lithium Nucleation and Dendrite Growth. In order to investigate the nucleation and dendrite growth of lithium in graphite and hard carbon, both materials were lithiated to a series of specific capacities ranging from 250 to 650 mA h/g. The graphite and hard carbon electrodes were then examined at various states of lithiation by SEM after dip rinsing with electrolyte solvent (ethyl−methyl carbonates) as previous works, which can remove the residual lithium salt without changing the surface morphology of the electrode.4,6,25 As shown in the SEM images in Figure 1a−d and the optical

Figure 1. SEM images of the graphite electrodes lithiated at 0.1 C to the specific capacities of (a) 0, (b) 250, (c) 370, (d) 550, and (e) 650 mA h/g.

images in Figure S2a,b, the morphology of the graphite electrode changed immediately when it was lithiated to just 250 mA h/g, as shown in Figure 1b. In sharp contrast to the morphology of the pristine graphite in Figure 1a, we observed many protrusions on the surface of the charged graphite, which are likely caused by the expansion of the graphite particles due to lithium insertion. This morphology feature becomes prominent for the fully lithiated graphite at 370 mA h/g, as shown in Figure 1c. The increase of the graphite interlayer distance caused by the inserted lithium, which results in a color

Figure 2. SEM images of the hard carbon electrodes lithiated at 0.1 C to the specific capacities of (a) 0, (b) 250, (c) 450, (d) 550, and (e) 650 mA h/g. 6206


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Figure 3. (a) Voltage profiles of the graphite electrodes lithiated and de-lithiated at a series of specific capacities of 250, 370, 450, 550, and 650 mA h/g. Corresponding enlarged voltage profiles (b) and dQ/dV for de-lithiation (c) of the graphite electrodes. (d) Voltage profiles of the hard carbon electrodes lithiated and de-lithiated at a series of specific capacities of 250, 350, 450, 550, and 650 mA h/g. Corresponding enlarged voltage profiles (e) and dQ/dV for de-lithiation (f) of the hard carbon electrodes. Note in the inset in panel e the inflection point at 550 mA h/g for the 650 mA h/g electrode.

Figure 4. (a) WAXS of the lithiated graphite with the specific capacities of 0, 250, 450, and 650 mA h/g. (b) WAXS of the lithiated hard carbon with the specific capacities of 0, 250, 450, and 650 mA h/g.

indicate two different lithium storage processes. Above 0 V the cell capacity can be attributed to lithium insertion between the graphene layers of the graphite; below 0 V the lithium starts to nucleate in the nanopores. However, the cell voltage starts to increase when the discharge capacity is more than 550 mA h/g. The voltage rise after this inflection point (seen in Figure 3e) is due to the overpotential for lithium growth being reduced when

out of the hard carbon when the lithiation capacity of the hard carbon increased from 550 to 650 mA h/g. The lithium nucleation and the growth of lithium dendrites can be further confirmed by the expanded voltage profiles in Figure 3e at the end of the lithiation process. We noticed that there are two different voltage decay rates above and below 0 V vs Li/Li+, respectively. We believe that the two voltage decay rates 6207


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Figure 5. (a and b) Raman spectra of lithiated graphite electrodes with the specific capacities of 250, 370, 550, and 650 mA h/g. (c) Raman spectra of the hard carbon electrodes lithiated to the specific capacities of 0, 250, 450, 550, and 650 mA h/g. (d) IG/ID ratio for the corresponding Raman spectra of the lithiated hard carbon electrodes.

Figure 6. (a) Optical image of the graphite electrode lithiated to a specific capacity of 450 mA h/g for the Raman mapping measurement. (b) Raman spectra of spots 1 and 2 on the optical image. Corresponding SEM images of the yellow area (c) for spot 1 and white area (d) for spot 2, indicated by the dashed squares labeled with c and d in the optical image of panel a, respecitively.

2.3. Structure Evolution of the Carbon Electrodes during Lithium Insertion and Nucleation. To gain further understanding of the structure and morphology changes during the lithiation and over-lithiation of the hard carbon and graphite electrodes, WAXS and ex situ Raman mapping measurements were carried out to explore how their respective structures evolved during lithium insertion and nucleation. WAXS can explore the crystalline structure (e.g., interlayer distance) of carbon allotropes during lithiation/over-lithiation. The WAXS patterns in Figure 4a show that the interlayer distance in the pristine graphite was 0.335 nm, which is consistent with other reports.27,28 When the graphite was lithiated to 250 mA h/g, two different interlayer distances were

the lithium grows out from the surface of the hard carbon.The lithium nucleates and the dendrites consist of metallic lithium, which was stripped during the de-lithiation process, as can be seen from the first dQ/dV peak in Figure 3f for the hard carbon electrodes with 650 and 550 mA h/g lithiation capacity. Additional evidence of lithium nucleation and dendrite growth can be obtained from the 7Li NMR shift of lithium metal at 264 ppm, as shown in Figure 7b for lithiated hard carbon at a capacity of 550 mA h/g. The dendrites observed on the surface of the hard carbon electrode with a capacity of 650 mA h/g exhibit a smooth surface that is noticeably different from the lithium on the graphite electrode as described above. 6208


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The nonuniform lithium nucleation and dendtrite growth on graphite electrodes at different capacities could be due to varying local resistances for diffusion of lithium and for electron conduction, which has been observed by optical and SEM images. Such resistances could affect the high rate and extended cycling performance of graphite electrodes. However, as shown by the Raman results in Figure 5c for hard carbon at several lithiation capacities, there was almost no change of the G band at 1590 cm−1 and the D band at 1308 cm−1 in terms of peak position and intensity from pristine to lithiated hard carbon up to 450 mA h/g capacity. With further lithiation, the value of the IG/ID ratio started to decrease, while the peak position and intensity of the D band stayed the same. As stated previously, the D band is attributable to the graphitic grain boundary phonon and the G band arises from symmetric bond stretching within the graphite plane. The intensity ratio IG/ID of hard carbon is in fact proportional to the crystallinity of the hard carbon. Therefore, the decrease of the IG/ID ratio in Figure 5c,d indicates that the ordered zone of hard carbon is disturbed by the plated lithium metal around it as shown in Figure 8b. The IG/ID ratio decreased further as the lithium dendrites grew out from the lithiated hard carbon when it was further charged to 650 mA h/g. Meanwhile, no other Raman bands were detected on lithiated hard carbon even when the hard carbon was lithiated to 650 mA h/g at which point it was covered with dendrites. This was probably because the nucleation and initial growth of the lithium dendrites occurred inside the hard carbon, which minimized side reactions between the lithium and the electrolyte. 2.4. Lithium Status during Insertion and Nucleation in the Carbon Electrodes. To gain additional information about the lithium charge status in lithiated graphite and hard carbon, solid-state magic angle spinning 7Li NMR spectroscopy was utilized to explore in greater detail the impact of carbon structure on lithium storage, nucleation, and dendrite growth. The graphite sample lithiated to 250 mA h/g showed a principal 7Li resonance at ∼44 ppm, whereas the hard carbon sample at the same state of charge showed a principal 7Li resonance at 11 ppm (Figure 7a,b). The resonance at 44 ppm observed in the graphite sample is due to the presence of reversible lithium stored in graphite, which is consistent with the literature.11,12,14,31,32 The resonance at ∼11 ppm in the hard carbon can be assigned to reversibly intercalated lithium in the hard carbon due to the ionic character of the lithium stored in the small layer structure of the aromatic rings. When graphite was lithiated to 450 mA h/g, apart from the resonance at ∼44 ppm, a new resonance started to form at ∼266 ppm due to formation of lithium metal. This is consistent with the lithium dendrites observed by SEM on lithiated graphite with a specific capacity of 450 mA h/g. Compared to the resonance at 264 ppm for the lithium dendrites on hard carbon, the shift of the peak for the lithium dendrites on graphite was probably caused by a different morphology of the dendrites on graphite vs hard carbon.33 But no 7Li resonance for semi-metallic lithium was observed when the specific capacity of lithiated graphite increased from 250 to 650 mA h/ g. Based on the results from NMR, the dQ/dV peaks for delithiated graphite, and SEM images of the lithiated graphite in Figure 2 and Figure 6, nucleation of lithium dendrites is presumed to happen directly on the surface of the graphite. When we looked at the hard carbon sample with a specific capacity of 450 mA h/g, both in the lithiated and de-lithiated state, a pronounced 7Li resonance was observed at 0.5 ppm.

observedsome interlayer spacings expanded from 0.335 to 0.348 nm, while other interlayer spacings compressed from 0.335 to 0.310 nm, as shown in Figure 8b. This finding supports the Rudorff model in that the lithium alternatively inserts into the layers of graphite without significant structural distortions.17 When graphite was lithiated to a capacity above 450 mA h/g, only one interlayer distance was observed, i.e., an expansion from 0.335 to 0.369 nm. However, in the case of hard carbon (Figure 4b) no additional interlayer spacing between 0.3 and 0.4 nm was observed. This is because the random arrangement of the loose layers of hard carbon is not decernable using WAXS. To obtain more information, Raman measurements were carried out to explore the structure and composition evolution of the graphite and hard carbon during lithiation/overlithiation. When the graphite was lithiated to 250 mA h/g and even further lithiated to 370 mA h/g, the observed Raman spectra of the lithiated graphite, shown in Figure 5a,b, exhibit none of the features seen for the pristine material. Instead an intense broad band centered near 1400 cm−1 appears, which may be due to the strong impact of inserted lithium on symmetric bond stretching within the graphite plane due to its smaller interlayer distance compared to hard carbon. When the graphite was lithiated to 550 or 650 mA h/g, three new Raman bands centered near ca. 516, ca. 1023, and ca. 1850 cm−1 appeared, as seen in Figure 5b. The broad bands around 516 and 1023 cm−1 can be assigned to EDPs that might include Li2CO3, oxygenated organic species, and Li−O−C- moieties; the peak around 1850 cm−1 is attributed to the -CC group of X−CC−X, where X can be Li and/or an organic group in accord with the work of Naudin et al. and Tian et al.29,30 The Li2CO3 and X−CC−X are believed to be products of reactions between lithium and the electrolyte during the nucleation and growth of lithium dendrites on the graphite. The side reactions between the lithium dendrites on the graphite and the electrolyte can cause significant capacity fading of graphite electrodes under abuse conditions, as we reported previously.6 To confirm this, micro-Raman mapping was carried out on the lithiated graphite charged to a specific capacity of 450 mA h/g. The microscope image for the Raman scattering in Figure 6a demonstrates that the lithiated graphite consists of about three-fourths gold area (attributed to LiC6) and about onefourth white area (attributed to lithium plating/dendrites). The corresponding SEM images in panels c and d of Figure 6, from the pinpointed spots in the dashed squares labeled with c and d in panel a of Figure 6, confirm that lithium dendrites were mostly grown on the white area but were barely apparent on the gold area. The corresponding Raman spectra from the gold area and the white area are shown in panel b of Figure 6. The Raman spectrum of the gold area is similar to what we observed for lithiated graphite with specific capacities of 250 and 370 mA h/g. On the white area with lithium dendrites there are again three new Raman bands at 516, 1023, and 1850 cm−1, which we again assign to EDPs, including the X−CC−X type, as discussed above. The appearance of these bands confirms that the EPDs are products caused by reaction between lithium and electrolyte during lithium nucleation and dendrite growth on the graphite anode. This probably occurred because the lithium nucleation and initial growth of lithium dendrites happened on the graphite surface where the anode comes in contact with the electrolyte and where side reactions during the lithium nucleation and dendrite growth would be expected to occur. 6209


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which is roughly twice that of graphite (∼10%). This is because the hard carbon has more attached hetero-elements than graphite. When the hard carbon was lithiated to a capacity of 550 mA h/g, formation of new peaks at 264 ppm for Li metal and at 105 ppm for semi-metallic Li were observed. The semimetallic lithium is lithium whose charge is between that of lithium metal and of LiCx. Based on what we have observed in SEM images of lithiated hard carbon with a capacity of 550 mA h/g, as seen in Figure 1d, the dQ/dV peaks for de-lithiating hard carbon, the Li metal, and the semi-metallic Li are a consequence of lithium nucleation inside nanopores of the hard carbon.

3. DISCUSSION Based on the observations reported above, we propose the lithium insertion, lithium nucleation, and lithium dendrite growth mechanisms for graphite and hard carbon described below. As shown in the schematic picture of Figure 8a, graphite has a well-aligned, tight layered structure. The graphite surface morphology and interlayer distance change instantaneously when lithium is inserted into graphite based on the SEM images and WAXS patterns for lithiated graphite. In addition, the Raman spectra indicate that the inserted lithium impacts the carbon bond stretching in the graphite planes because of the strong physical and chemical interactions between lithium and the graphite layers. The inserted lithium atoms are wellordered in lithiated graphite. Thus, the specific capacity of graphite exhibits a fixed value of 370 mA h/g. Once graphite is over-lithiated, lithium nucleation occurs instantaneously on the surface of the graphite accompanied by dendtrite growth under conditions where the graphite is surrounded by electrolyte, as shown in Figure 8a. We assume that the freshly nucleated lithium is much more reactive with electrolyte than bulk lithium would be. Therefore, there should be vigorous side reactions between the freshly nucleated lithium and the electrolyte around it, which leads to the thick lithium electrolyte interphase (LEI) on nucleated lithium. Further lithium plating takes place on the tip of the nucleated lithium, which causes the LEI to break and further react with electrolyte to form more LEI. This explains the large amount of EDPs, generated by side reactions and readily detected by Raman at specific locations on the lithium dendrites. The side reactions accompanying lithium

Figure 7. (a) 7Li MAS NMR of the lithiated graphite with specific capacities of 250, 450, 550, and 650 mA h/g. (b) NMR of the lithiated hard carbon with specific capacities of 250, 450 (lithiated and delithiated), and 550 mA h/g.

This resonance can be due to the presence of lithiumcontaining EDPs or lithium attachments to the hard carbon generated by the electrochemical reaction between lithium ions and hetero-elements (e.g., H, O, or N) on the surface of the hard carbon.11,12,31,34 Since EDPs were present on the lithiated/de-lithiated hard carbon, the 0.5 ppm resonance was assigned to the irreversible lithium stored in lithiated hard carbon due to the strong ionic binding between the heteroelements derivitized to the hard carbon and the lithium around them. According to the voltage profiles for lithiation/delithiation of graphite and hard carbon seen in Figure 3a,d, there appears to be an ∼20% irreversible capacity for hard carbon,

Figure 8. Schematic process for lithium insertion and nucleation in (a) graphite and (b) hard carbon during the lithiation/over-lithiation. 6210


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Chemistry of Materials nucleation and dendrite growth on lithiated graphite determine the knob-like structure covered with a thick LEI as observed by SEM. Lithium nucleation and dendrite growth on hard carbon are in sharp contrast to the case of graphite, as illustrated in Figure 8a,b, due to their different stuctures. Hard carbon exhibits a random arrangement of small loose layers infused with nanopores. Based on SEM images and Raman spectra of lithiated hard carbon, the hard carbon surface morphology and crystal structure retain the character of the pristine material when the charging capacity is less than 550 mA h/g. However, further lithiation results in lithium nucleation taking place inside the hard carbon structure according to the SEM images, voltage profiles, dQ/dV peaks, and NMR spectra. We believe that the defects in the hard carbon nanopores induce nucleation of lithium inside the hard carbon,35,36 which also accounts for the larger specific capacity of hard carbon when compared to that of graphite. At less than 550 mA h/g, nucleation of lithium takes place in the nanopores of the hard carbon, where there is little or no electrolyte for side reactions between lithium and the electrolyte. This provides an explanation for why in the case of hard carbon no EDPs are formed by side reactions between lithium and electrolyte. Furthermore, EDPs are still not detected by Raman spectroscopy measurements on the hard carbon even after over-liathiation to 650 mA h/g, at which point the lithium has grown out of the nanopores and should come in contact with the electrolyte. We believe that the failure to detect EDPs on hard carbon could be due to its different lithium plating mechanism. When closely checking the plated lithium metal on the surface of hard carbon in Figure 2e, it appears as solid, large diameter needles, which is totally different from the small, tube-like features on the surface of graphite (Figure 1d,e). As proposed in Figure 8b, the additional lithium plating after 550 mA h/g will continue on top of the lithium nucleates formed within the pores, which will lead to large size solid needle-like lithium metal. This needle-like lithium metal has a smaller surface area that seemingly results in much reduced formation of EDPs. Furthermore, for 650 mA h/ g lithiated hard carbon, only about 100 mA h/g equivalent lithium metal is plated outside of the pores, which is another reason for less EDPs. The different lithium nucleation and dendrite growth mechanisms on hard carbon and graphite are also supported by the different 7Li NMR peak shapes for lithium dendrites on over-lithiated hard carbon and graphite (a sharp peak for hard carbon vs broader peak for graphite with a shoudler), the difference being caused by differences in the morphology of the dendrites.33

variety of electrolyte degradation products and knob-like dendrites. (2) In addition, the observed nonuniform lithiation of the graphite electrode makes the electrode susceptible to dendrite growth. A graphite electrode with improved conductivity should be developed to address this issue. (3) When hard carbon is over-lithiated, the nanopores of the hard carbon offer space for lithium nucleation and initial growth inside the hard carbon structure, thereby minimizing instantaneous side reactions between lithium and the electrolyte, leading to reduced depletion of electroactive lithium and consequently higher reversible capacity. (4) In addition, the hetero-elements (e.g., H, O, and N) on the hard carbon surfaces offer sites for stronger ionic binding of lithium which contributes to the larger irreversible capacity of hard carbon compared to graphite. (5) Finally, the nanopores of hard carbon can accommodate extra lithium and thus provide higher capacity. The different lithium nucleation and dendrite growth mechanisms exhibited by the two carbon allotropes could give hard carbon a safety edge compared to graphite where lithium plating is a concern.

5. EXPERIMENTAL SECTION 5.1. Electrode Preparation. The preparation of the graphite and hard carbon electrodes was as follows: First the active materials graphite powder (CPreme A12, Phillips 66) for graphite electrode and hard carbon powder (Carbotron P, Kureha Battery Materials Japan) for hard carbon electrode, respectively, and carbon black (Timcal C45)were added to a solution of poly(vinylidene difluoride) (PVDF) dissolved in N-methy-2-pyrrolidone. After the electrode slurry was stirred at 2000 rpm for 3 min using a Thinky mixer (ARE310), it was coated onto Cu foil. Then the electrode was dried at 75 °C for 4 h, followed by additional drying at 75 °C in vacuum overnight. To remove any residual moisture, the electrodes were dried at 120 °C under vacuum for over 4 h before being assembled into 2032 type coin cells. The active composition was 92% for both the graphite and hard carbon electrodes. The total loading and area of both the graphite electrode and the hard carbon electrode were the same: 8.8 mg and 1.6 cm2, respectively. Lithium metal was used as the counter electrode, and a solution of 1.2 mol/L LiPF6 in ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (3/7 by weight) was used as the electrolyte in the all cells. 5.2. Lithiation of Graphite and Hard Carbon Electrodes. The graphite electrodes were lithiated to a series of specific capacities, i.e., 250, 370, 450, 550, and 650 mA h/g, at a rate of 0.1 C. The hard carbon sss electrodes were lithiated to a similar series of specific capacities, i.e., 250, 350, 450, 550, and 650 mA h/g, at a rate of 0.1 C. The current densities for 0.1 C at specific capacities of 250, 370, 450, 550, and 650 mA h/g are 0.13, 0.19, 0.23, 0.28, and 0.33 mA h/cm2, respectively. After lithiation, the cells were disassembled. The electrodes were gently dipped into EMC and dried in a glovebox for 2 h before any of the ex situ SEM, WAXS, Raman, and NMR measurements were performed. 5.3. SEM/EDS Characterization of Graphite and Hard Carbon Electrodes. The surface morphologies and compositions of the pristine graphite and hard carbon electrodes were characterized using a scanning electron microscope (SEM, Hitachi S-4700 at 10 kV) equipped with energy dispersive spectroscopy (EDS). After extraction, the lithiated graphite and hard carbon electrodes were gently dipped into EMC and dried in an Ar glovebox for 2 h. They were then loaded into the sample holders and directly transferred into the SEM chamber with Ar gas protection to prevent exposure of the electrodes to ambient moisture and oxygen. 5.4. Raman Characterization of Graphite and Hard Carbon Electrodes. Raman spectroscopy measurements on the pristine and lithiated electrodes were carried out using a Renishaw inVia Raman Microscope operated with 785 nm excitation. The samples were

4. CONCLUSION Through coordinated, systematic investigations on lithiated graphite and hard carbon using scanning electron microscopy, wide angle X-ray scattering, ex situ Raman mapping, and nuclear magnetic resonance spectroscopy, we explored the two different paths for lithium insertion, nucleation, and dendrite growth exhibited by hard carbon and graphite. The following is what we conclude in terms of developing effective approaches for improving the performance of carbon in lithium ion battery anodes. (1) When the graphite is over-lithiated, lithium nucleation and the accompanying growth of lithium dendrites will immediately take place on the surface of graphite; the freshly produced lithium will react with the electrolyte to produce a 6211


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Chemistry of Materials loaded into a Teflon sample holder fitted with O-ring seals to prevent contact of the electrodes with ambient atmosphere. The electrodes were pressed between a BaF2 window (10 mm in diameter and 2 mm thick) and a stainless steel backing disk. Raman spectra were recorded through the BaF2 window using a 50× focusing/collection optic. The mapping capability of the inVia Raman instrument was employed to collect spectra from different spots on the electrode surface. 5.5. WAXS Characterization of Graphite and Hard Carbon Electrodes. The crystal structure and composition of the pristine/ lithiated graphite and hard carbon electrodes were also investigated by WAXS as follows. The electrodes were rinsed, dried in an Ar glovebox, and sealed with Kapton tape. Crystal structures were characterized using a pinhole WAXS camera located at beamline 9-ID of the Advanced Photons Source. For these experiments we used a Pilatus 100k pixel-array detector located 206 mm from the sample. Samples were measured in transmission geometry using X-rays with a wavelength of 0.689 A (18 keV). These X-rays were monochromatized by use of a Si 111 monochromator, the beam size was 0.2 mm × 0.2 mm, and harmonic rejection was provided by use of Si 220 crystal optics. Data were reduced using the Nika data reduction package.37 5.6. NMR Characterization of Graphite and Hard Carbon Electrodes. To check the lithium charge status in the lithiated carbon electrodes charged to different capacities, the lithiated graphite and hard carbon were examined by solid-state NMR spectroscopy as follows. The carbon electrodes with different capacities were thoroughly rinsed with EMC and died in an Ar box. After that, the lithiated graphite and hard carbon powders were scratched from the current collectors and packed into rotors in preparation for the 7Li MAS NMR measurements. The experiments were performed at 7.02 T (300 MHz) on a Bruker Avance III HD spectrometer operating at a Larmor frequency of 116.64 MHz, using a 3.2 mm magic angle spinning probe. A rotor synchronized Hahn echo pulse sequence was used at a spinning frequency of 10 kHz. A π/2 pulse width of 3.5 μs was used with pulse recycle delays of 4 s. All data were collected at room temperature, and spectra were referenced to 1 M LiCl at 0 ppm.

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Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DEAC02-06CH11357.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00072. SEM/EDS of pristine graphite and hard carbon, optical images of lithiated graphite and hard carbon, and SEM of graphite and hard carbon after lithiation and de-lithiation (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +1(630)-252-3704. Fax: 630-252-4176. E-mail: [email protected]. ORCID

Xin Su: 0000-0002-1615-2856 Fulya Dogan: 0000-0001-7997-266X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the U.S. Department of Energy’s (DOE) Energy Efficiency & Renewable Energy (EERE) Vehicle Technologies Office. Part of this work was performed at the Electron Microscopy Center for Materials Research and the Center for Nanoscale Materials at Argonne National Laboratory, a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC0206CH11357. This research used resources of the Advanced 6212


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