Influence of graphite edge crystallographic orientation on the first lithium intercalation in Li-ion battery

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Influence of graphite edge crystallographic orientation on the first lithium intercalation in Li-ion battery Ph. Bernardo a,b, J.-M. Le Meins a,d,*, L. Vidal a, J. Dentzer a, R. Gadiou P. Nova´k b, M.E. Spahr c, C. Vix-Guterl a,d

a,d

, W. Ma¨rkle

b,1

,

a

Institut de Science des Mate´riaux de Mulhouse, LRC CNRS, F-68057 Mulhouse, France Paul Scherrer Institut, Electrochemistry Laboratory, CH-5232 Villigen PSI, Switzerland c Imerys Graphite & Carbon SA, CH-6743 Bodio TI, Switzerland d Re´seau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR3459, 33 Rue Saint Leu, 80039 Amiens Cedex, France b

A R T I C L E I N F O

A B S T R A C T

Article history:

Graphite is a widely used anode electrode material for Li-ion batteries. When the anode is

Received 12 February 2015

polarized to a low potential, the electrolyte reduction products form a film called SEI (Solid

Accepted 4 May 2015

Electrolyte Interphase) playing a crucial role in the reversibility of the electrochemical

Available online 9 May 2015

cycling. This SEI layer is preferentially formed through the carbon edge planes that can be divided into two main categories with different reactivity: zigzag and armchair edges. Main objective of this work is to experimentally study the influence of the carbon edge atoms configuration on SEI formation and consequently on the electrochemical performance of the electrode. In this aim, a preferential etching method based on oxygen or water gasification was first developed to control zigzag and armchair edge concentration. A range of graphite samples having different reactivity could be prepared and electrochemically tested. Experimental results point out that the carbon edge atom configuration is an important parameter controlling the reactivity of the graphite toward the electrolyte. In particular, a carbon material having a high concentration of zigzag edge carbon atoms is not recommended for the formation of an ad hoc SEI. As a result, the co-intercalation of solvent is enhanced leading to an exfoliation tendency of the electrode. Ó 2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Recent developments in the field of portable electrical devices have been the main driving force behind the search for lithium-ion batteries with high energy density and power density, without compromising the safety aspects. Despite considerable efforts to find other materials with higher

specific charge, almost all present lithium-ion batteries contain a graphite negative electrode due to its low cost, good cyclability, reliability, and non-toxicity [1]. Many researchers have focused their works on the optimization of the graphite electrode regarding the reduction of the specific charge loss (irreversible capacity) during the first electrochemical Li+ insertion. This irreversible capacity is due to the electrolyte

* Corresponding author at: Institut de Science des Mate´riaux de Mulhouse, LRC CNRS, F-68057 Mulhouse, France. E-mail address: [email protected] (J.-M. Le Meins). 1 Present address: Daimler AG, D-70546 Stuttgart-Untertu¨rkheim, Germany. http://dx.doi.org/10.1016/j.carbon.2015.05.001 0008-6223/Ó 2015 Elsevier Ltd. All rights reserved.

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decomposition when the anode is polarized to a low potential resulting in the formation of a surface film called the Solid Electrolyte Interphase (SEI) [2]. This SEI film is ideally a Liion conductor and electrical insulator stopping further electrolyte decomposition and solvent co-intercalation. Its presence was evidenced many times through different characterization techniques [1,3–7]. SEI plays a crucial role in the reversibility of the electrochemical cycling [8]. Actually, its formation during the first cycle, leading to an unavoidable specific charge loss, is necessary to protect graphite from electrochemical exfoliation [1,8–10]. It is generally agreed that to a certain extent the solvent cointercalation occurs and it is at least a part of the process related to the formation of the SEI. If the co-intercalation is too massive, graphite exfoliation takes place [11–13]. Menachem et al. claimed in 1997 that the improvement of the battery performance is attributed to the formation of the SEI chemically bonded to the carboxylic groups of the surface at the zigzag and armchair edges, Other papers confirm this hypothesis by indicating that the intercalation process mainly occurs through the carbon edge planes [1,14]. However, some experimental evidences were missing to support these conclusions. Over the last years, we have working to study the influence of the graphite characteristics on the SEI layer formation process. In particular, we have proposed experimental methodologies to modify one graphite characteristic without changing significantly the others [7,15–18]. The aim of these previous works was to experimentally identify the key factors that govern the formation of an effective SEI layer during the first electrochemical cycle and consequently the irreversible capacity and the exfoliation tendency. Hence, we studied the effect of the total surface area, crystallographic structure, surface chemistry and structural order on the graphite electrochemical performance. The experimental results reported in previous papers [7,15–18] clearly highlighted the following important points: (a) the active surface area (ASA) which is an intrinsic characteristic of the graphite is the determining parameter that controls the irreversible capacity and the exfoliation tendency during the first electrochemical cycle, (b) the presence of oxygen groups at the surface is critical for the formation of an efficient SEI layer for a graphite sample having a ASA value high enough to avoid the exfoliation phenomenon; however the type and the amount of surface (C–O) complexes are not the predominant parameters, (c) the total surface area and the crystallographic structure do not govern the irreversible capacity and the exfoliation tendency. More specifically, we experimentally proved that the graphite tends to exfoliate if the graphite ASA is below a certain threshold value [15–17]. This result has been interpreted as follows: a significant solvent co-intercalation will occur when edge atoms are not reactive enough to form an appropriate SEI layer. As a consequence, edge atom reactivity determines the edge plane SEI formation, which controls solvent cointercalation tendency by acting as a physical barrier. It must be noted that we also demonstrated that this exfoliation could not be avoided by adding oxygen complexes at the graphite surface having an ASA below this threshold value. This result confirms the key role playing by the graphite active sites. It was the first time that a quantitative relationship

between the active surface area (ASA) of the graphite which is an intrinsic characteristic, the exfoliation tendency and irreversible capacity could be experimentally established [9,15,17]. For a graphite sample having an ASA value high enough to hinder exfoliation, we experimentally showed that the surface oxygen-containing groups are useful electrochemically reducible groups that act as nucleation sites for an effective SEI formation during the first reduction process [15,17]. We also experimentally observed that (a) the graphite tends to exfoliate when its C(O) surface groups are substituting by C(H) groups without changing the ASA value of the graphite. (b) The irreversible capacity measured is much lower compared to a graphite with a ASA value lower than the threshold one which drastically exfoliates. Hence, the presence of C(H) bonds promotes a partial exfoliation. These results pointed out that (a) the active sites seem to be no longer available leading to a decrease of the reactivity between the electrolyte and the graphite surface, avoiding the formation of an efficient SEI layer, (b) an additional parameter to the ASA one related to the reactivity of the edge planes has to be taken into account, (c) hydrogen stabilizes only a fraction of the actives sites since the graphite is partially exfoliated. These results suggest that the topology of the carbon edge planes (zigzag {1 0 0} or armchair {1 1 0}) as represented in Fig. 1 could influence the graphite-electrolyte interaction since it is known that the zigzag surface is more reactive compared to the armchair one. Therefore, one may ask the following questions: (a) is the SEI formation on zigzag and armchair surfaces different? (b) Can we experimentally measure different electrochemical behaviors for a graphite electrode containing particles with a majority of zigzag edge carbon atoms or a majority of armchair edge atoms? To bring enough experimental evidences to answer to these questions, it was necessary to prepare graphite samples having different armchair and zigzag carbon edges concentration. Changing this ratio is not obvious and we have had to first succeed in developing an experimental method to adjust this ratio. This experimental method based on selective treatments by oxygen or water vapor allowed us to perform a systematic electrochemical study on graphite samples for which the edge carbon atoms configuration could be tailored. New insights into edge plane

{100} zigzag planes {110} armchair planes

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Zigzag edge atoms with two neighbors Armchair edge atoms with two neighbors

b a

Fig. 1 – Miller indices of the so-called zigzag and armchair edge planes (projection into (0 0 1) plane, please note that the (hki‘) notation is not used for this work). Circles and squares represent different types of edge atoms. In a zigzag or armchair edge, the two-neighbor atoms are the most reactive (unpaired electron). (A color version of this figure can be viewed online.)

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reactivity toward electrolyte in regard to its crystallographic orientation could be obtained from the experimental results as will be described in this paper.

2.

Experimental section

2.1.

Preparation of the SLX 50 graphite samples

One type of highly crystalline, synthetic graphite material: TIMREXâ SLX50 (TIMCAL SA, Bodio, Switzerland) was selected as pristine material for the negative electrode. The particle size distribution (PSD) as determined by laser diffusion (Malvern) is: d10 = 12 lm, d50 = 28 lm et d90 = 53 lm. The SLX-50 samples are put into contact with oxygen or water vapor to modify the zigzag/armchair configuration. These experiments are performed in a horizontal fused silica furnace. For oxygen partial gasification, SLX50 graphite powder is put into fused silica crucible. The sample is heattreated up to 600 °C under synthetic air flow (20 l h1) and maintain at this temperature for different times. For steam partial gasification, the sample is heat-treated up to 950 °C under argon flow passing through a hermetically sealed bubbling round-bottom flask partly filled with distilled water. The sample is maintained at 950 °C for different times. The justification of the choice of the oxidative etching agents and the experimental proof of the effectiveness of this process to tailor the carbon edges configuration are set forth in the results and discussion section. All the graphite samples prepared are listed in Table 1 with their preparation conditions.

2.2.

Material characterizations

2.2.1. Determination of the carbon edge configuration (zigzag or armchair) Two etched graphite samples were analyzed by TEM (Philips CM200 microscope working at 200 kV) to determine the crystallographic orientation of the edges: one belonging to the oxygen-etched graphite family (SLX50 O2-600 26%) and the other belonging to the steam-etched graphite family (SLX50 H2O-950 34%). As the two sample families have two distinct electrochemical behaviors (as shown in the Section 3); the study of the two samples belonging to different families has been considered to bring forth enough clues about the

subject. The SAED image rotation depending on the magnitude applied was calibrated using the crystal a-MoO3 as a reference. According this method, the image rotation correction to be applied for each magnitude setting has been established (see Supplementary Material_2). Knowing the relative rotation between image and diffraction pattern, one can determine the orientation of any crystal plane. Once this calibration is done, knowing the distance of reticular planes and comparing the directions of the edges with those of the corresponding reciprocal vectors allows to assign an edge in the {1 0 0} family or the {1 1 0} one, provided that it is sufficiently long and straight.

2.2.2.

Graphite samples characterization

The graphite samples were characterized to determine their surface chemistry, active surface area and total surface area (TSA). The surface chemistry was studied by temperatureprogrammed desorption (TPD). TPD was performed in a vacuum system equipped with mass spectrometer at a maximum pressure of 104 Pa. Before the experiment, the mass spectrometer was calibrated for H2, H2O, CO, N2, O2, CO2 and NO. The graphite sample was deposited on a fused silica crucible and heat-treated up to 950 °C with a linear heating rate range of 5–10 °C min1. During the experiment, the gas phase was continuously analyzed quantitatively by the mass spectrometer. The quantitative determination of functional groups was based on H2, H2O, CO, CO2 and NO as the amount of O2 and N2 was found to be negligible. The total pressure evolved during the heat treatment was measured as a function of temperature using a Bayard-Alpert gauge. The total gas pressure was compared to the pressure calculated from the sum of partial pressures of the gas species deduced from the mass spectrometry analysis of the gas stream. The difference between these two pressures was ascribed to the presence of hydrocarbons. The ASA was determined by using a method based on dioxygen chemisorption, developed by Walker and co-workers [19]. The graphite samples are first outgassed under vacuum (104 Pa) at 950 °C for 1 h to remove all of surface complex which was the result of their prior oxidation. Subsequently, oxygen is chemisorbed at 300 °C over a period of 10 h (with an initial oxygen pressure of 66.5 Pa), leading to the formation of surface oxygenated complex. The amount of CO and CO2 desorbed from the sample (determined by mass spectrometry), resulting from the thermal

Table 1 – Treatment conditions of the graphite samples (all the samples were put in contact with ambient air after the treatment). Sample labels (including weight loss)

Nature of etchant

Temperature (°C)

Time (h)

Weight loss (%)

SLX50 SLX50 SLX50 SLX50 SLX50 SLX50 SLX50 SLX50 SLX50

N/A Oxygen Oxygen Oxygen Oxygen Steam Steam Steam Steam

N/A 600 600 600 600 950 950 950 950

N/A 6 10 20 30 100 120 220 250

0 7 13 26 46 19 21 34 38

O2-600 7% O2-600 13% O2-600 26% O2-600 46% H2O-950 19% H2O-950 21% H2O-950 34% H2O-950 38%

decomposition of oxygen complex, is measured by performing a TPD step between 300 and 950 °C. Knowing the number of mole for each gas desorbed, and taking the area of an edge carbon site (lying in the (1 0 0) plane) that chemisorbed an oxygen atom as 0.083 nm2, the surface area occupied by chemisorbed oxygen can be determined. The TSA of graphite was determined by nitrogen gas adsorption at 196 °C and calculated by applying the Brunauer–Emmett–Teller (BET) method [20] Before nitrogen gas adsorption was performed, the graphite samples were outgassed at 300 °C for 12 h.

2.3.

1.4

Cirreversible

10 mA.g-1 in 1M LiPF6, EC:DMC (1:1 by weight) electrolyte

1.2 Creversible

1.0 0.8 0.6

SLX50: no exfoliation SLX50 O2-600 26%: exfoliation

0.4 0.2

Electrochemical measurements

Graphite negative electrodes were prepared by doctor-blading slurries constituted of graphite active material (90 wt.%) and poly(vinylidene fluoride) binder (10 wt.%, SOLEF 1015, Solvay SA) dispersed in N-methylpyrrolidinone (NMP), onto a copper foil. The electrodes were vacuum dried at 120 °C for 12 h. After drying, the electrode disks (1.3 cm2) were punched for test cell assembly in the argon-filled glove box. A particular attention was paid to the density of the electrode since we showed in a previous work that the electrode mass loading and thickness influence the irreversible capacity [15]. In this work and for each working electrode, the mass loading was typically fixed around 8–10 mg of active material per cm2 with a thickness in the range of 90–100 lm. Hermetically sealed laboratory test cells were used in which the working and lithium counter electrodes were slightly pressed together against a glass fiber separator soaked with 500 ml 1 M LiPF6 in EC:DMC (Ethylene Carbonate:Dimethyl Carbonate) 1:1 electrolyte (Ferro). Both the oxygen and water contents were less than 1 ppm in the argon-filled glove box. Galvanostatic measurements were performed at a specific current of 10 mA g1 (related to the mass of graphite) in order to complete the SEI formation in the first electrochemical Li+ insertion. When a potential of 5 mV vs. Li+/Li was reached, the Li-insertion (reduction, ie the insertion of lithium and SEI formation) step was continued until the specific current dropped below 5 mA g1. The Li-removal step (oxidation, i.e. the de-insertion of lithium) was performed at specific current of 10 mA g1 until a cut-off potential of 1.5 V vs. Li+/Li was reached. The electrochemical measurements were carried out in a temperature controlled chamber at 25 °C. The irreversible capacity corresponding to the first electrochemical cycle was calculated using the following equation: Cirr ¼

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Potential (V versus Li+/Li)

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QC  QD  100 QC

where QC and QD correspond to the specific capacity of the 1st Li+ insertion and the 1st Li+ removal processes, respectively (Fig. 2). The capacity is normalized to 372 Ah/kg corresponding to the theoretical reversible capacity of graphite, according to the formula LiC6. Values of x > 1 are due to sidereactions ascribed to the SEI formation (cf. the encircled plateau at 0.8 V in Fig. 2). The exfoliation tendency of the graphite can be deduced from the galvanostatic curves and post-mortem SEM analysis of the electrode. If exfoliation takes place, a distinct additional potential plateau can be observed at 0.5 V vs. Li+/Li on the initial galvanostatic Li-insertion curve (Fig. 2). The

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

x in LixC6 Fig. 2 – First electrochemical Li+ insertion/removal into SLX50 graphite negative electrode and SLX50 O2-600-26%. The irreversible and reversible capacities are evidenced for SLX50. SEI formation occurs at 0.8 V (encircled dotted plateau). For SLX50 O2-600-26% exfoliation is deduced from the presence of a plateau at 0.5 V (dashed ellipse) and the morphology of post-mortem exfoliated graphite particle (Fig. 11). (A color version of this figure can be viewed online.)

graphite exfoliation generally results in a drastic enlargement of the surface area in contact with the electrolyte (cf. Fig. 11 showing the particular morphology of post-mortem exfoliated particle of SLX50 O2 600 26% obtained after oxygen treatment at 600 °C). This leads to ongoing electrolyte decomposition reactions related to the SEI formation on this newly formed surface. The consequence is additional charge consumption as indicated by the potential plateau at about 0.5 V vs. Li+/Li [7,10]. As a result, the irreversible capacity sharply increases. From the galvanostatic curve and SEM observations, it clearly appears that graphite sample modified by the oxygen treatment promotes the formation of an inappropriate SEI allowing massive solvent co-intercalation responsible of the exfoliation.

3.

Results and discussion

3.1. Control of the edge crystallographic orientation of graphite particles by selective etching In order to study the influence of graphite edge crystallographic orientation on the SEI formation during the first electrochemical Li+ insertion, the zigzag/armchair edge atoms ratio of SLX50 was modified by partially gasified the graphite with oxygen or water. The method development was based on the following theoretical background. It is known that zigzag and armchair carbon edge atoms have different reactivity [21,22] and that zigzag surface is less stable thermodynamically than the armchair one [23]. In addition, from simple knowledge of carbon edge atom reactivity, we can show that zigzag planes are a priori etched the fastest. Graphite edge atoms with two neighbors are the most reactive because they possess one unpaired available electron (Fig. 1). During etching, carbon atoms are gasified from the edge to the center of the particle. In order to be as clear as possible, let us

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simplify and consider a rectangular particle with same amount of zigzag and armchair two-neighbor edge atoms, respectively noticed Cz and Ca (Fig. 2). When one row of two-neighbor zigzag atoms is gasified, one row of carbon atoms with one neighbor appears (Fig. 3). When one row of two-neighbor armchair atoms is gasified, one row of two-neighbor armchair atoms appears again. Since atoms with one neighbor are highly reactive, they are likely to be etched instantly. As a result, for each row of etched two-neighbor armchair atoms, there are two rows of etched two-neighbor zigzag atoms. If the etching process presented in Fig. 3 is repeated several times, we observe that the fraction of zigzag edge atoms increases (Fig. 4). As a consequence, gasification of a rectangular graphite particle with equal number of zigzag and armchair edge atoms leads to the increase of zigzag edge atom concentration. So, without any consideration of the intrinsic relative stability of zigzag and armchair edge atoms, their crystallographic orientation implies difference of reactivity. To be used as electrode in Li-ions battery, the particle size distribution (PSD) of the graphite sample has to be around 30 lm. To obtain such PSD, a milling process is applied in order to break big particles into small ones until the wanted PSD is reached. The morphology of the particle edges after milling is not a priori well defined (Fig. 5). In average, it is expected that the graphite-milled particles in the powder have an equal amount of armchair and zigzag edge atoms. Although the global morphology is totally random, the edges structure of these milled particles remains similar to the ideal one locally. Indeed, the edges are locally built from a random succession of zigzag or armchair units of a few atoms. Therefore, during etching, the simple gasification scheme shown on Fig. 4 can still be applied for anticipating the evolution of zigzag edge atoms fraction. Gasification of a particle with a random shape, containing in equal proportion zigzag and armchair edge atoms, is likely to increase the atomic fraction of zigzag edge atoms because they are etched the fastest. Oxygen gasification at low temperature (600 °C) is chosen for increasing the fraction of zigzag edge atoms. At this temperature, graphite particles are etched slowly from the edge to the center. As explained previously, zigzag two-neighbor

1

2

28 Cz and 28 Ca

atoms will always be etched the fastest. As a consequence, to decrease the fraction of zigzag edge atoms, a way is to deactivate zigzag two-neighbor atoms while gasifying armchair two-neighbor atoms. Steam gasification is chosen for this capability. Actually, according to Yang [24], the steam gasification mechanism is as follows: C þ H2 O ¢ CðOÞ þ H2

ðAÞ

1 C þ H2 ¢ CðHÞ 2

ðBÞ

First, there is formation of oxygen surface complex: C(O) and hydrogen release (Reaction (A)), then C(O) decomposes into CO. Finally, the hydrogen produced can partly chemisorb on active sites (Reaction (B)). The steam gasification reaction must occur at high temperature (>950 °C) to be efficient. It is expected that hydrogen chemisorption would take place preferentially at zigzag edge atoms since they are the most reactive. Because, C(H) complexes can be thermally decomposed only at temperature up to 1500 °C [25], we expected that steam gasification at 950 °C would etch preferentially armchair atoms while the zigzag ones would be deactivated by the molecular hydrogen continuously formed. To check etching methods effectiveness, the graphite samples partly gasified by oxygen or steam were analyzed by TEM. Despite the fact that numerous edges in both samples SLX50 O2-600 26% and SLX50 H2O-950 34% could not be indexed into {1 0 0} or {1 1 0}, one observes that all the indexed edges of graphite particles gasified by oxygen are 100% zigzag, whereas the indexed edges of graphite particles modified by steam are nearly all armchair (90%) (cf. Supplementary Material_2). Fig. 8 shows three zigzag edges of a graphite particle belonging to the sample SLX50 O2-600 26% (Table 3) whereas two armchair edges belonging to the sample SLX50 H2O-950 34% (Table 4) can be observed in Fig. 9. The surface chemistry of the graphite samples was analyzed by TPD to get information on the type and amount of surface complexes formed during the etching treatments. During the TPD, the release of the following four main gases was observed: H2, H2O, CO and CO2. The amounts desorbed are reported in Table 2 with the values of the as-pristine graphite sample.

14

1 2

15

3

ETCHING

14

28 15

28

Carbon atom with one neighbor

Fig. 3 – Rectangular graphene containing 28 zigzag (Cz) and 28 armchair (Ca) two-neighbor carbon atoms, etched homogeneously along the edges. Etching one row of Cz necessary leads to one row of one-neighbor edge atoms which are highly reactive (right side). After their immediate etching, a new two-neighbor Cz row appears. In the same time, only one row of two-neighbor Ca succeeds to the etching of one row of Ca. (A color version of this figure can be viewed online.)

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Fig. 4 – Etching successive steps of a rectangular graphene containing 28 zigzag (Cz) and 28 armchair (Ca) two-neighbor carbon atoms. The fraction of zigzag atoms increases and goes from 0.5 to 0.52, 0.55, 0.58 . . .. (A color version of this figure can be viewed online.)

5e-9 SLX50-O2-600-26% H2

4e-9

Desorption (mol/s/g)

H2O CO CO2

3e-9

2e-9

1e-9

0 0

200

400

600

800

1000

Temperature (°C)

Fig. 5 – Typical particles of SLX50 graphite obtained after milling. (A color version of this figure can be viewed online.)

As an illustration, TPD curves of the samples SLX50 O2 600 26% and SXL50 H2O 950 34% are shown in Figs. 6 and 7. It must be noted that the TPD curve forms are similar for a given series of graphite. As expected, oxygen treatment promotes a surface mainly constituted of thermally stable surface oxygen-containing groups such as lactone, ether, phenol [26] which decompose into mainly CO above 600 °C. After water etching at 900 °C, the graphite surface is mainly constituted of very thermally stable C(O) complexes and C(H) bonds as confirmed by the hydrogen desorption rate curve and the total amount of hydrogen desorbed (Table 2). The values in Table 2 also confirm that the total amounts of CO and CO2 desorbed are much higher for the graphite treated in oxygen. Moreover, the TPD results confirm the graphite/H2O reaction mechanism proposed by Yang [24].

Fig. 6 – Desorption rates of H2, H2O, CO and CO2 gases from the surface of SLX50 O2-600-26% sample.

The experimental evidences provided by the TPD and TEM results confirm the etching process efficiency. Oxygen etching of graphite particles with random edge morphology increases the fraction of zigzag atoms whereas steam gasification causes the opposite by armchair atom preferential etching. Therefore, we succeed by a selective etching method to modify and control the concentration of the edge carbon atoms configuration. By this way, it was then possible to prepare a set of graphite samples to study their electrochemical behavior as reported in Supplementary Material_2.

3.2.

Electrochemical behavior of etched graphite samples

The graphite materials listed in Table 1 were characterized and electrochemically tested before and after etching. For all the samples, the values of the active surface area (ASA),

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4e-9 SLX50-H2O-950-34%

Desorption (mol/s/g)

3e-9

H2 H2O CO CO2

2e-9

1e-9

0 0

200

400

600

800

1000

Temperature (°C)

Fig. 7 – Desorption rates of H2, H2O, CO and CO2 gases from the surface of SLX50 H2O-950-34% sample.

Table 2 – Total amount of gas desorbed during the TPD analysis (lmol/g).

SLX50 SLX50 SLX50 SLX50 SLX50 SLX50 SLX50

O2-600 7% O2-600 13% O2-600 26% O2-600 46% H2O-950 21% H2O-950 34%

H2

H2O

CO

CO2

0.57 0.35 0.45 0.48 0.90 1.1 1.4

4.5 1.3 1.7 1.5 1.4 2.3 1.8

7.9 8.7 6.6 6.0 7.8 2.2 2.6

3.2 1.8 1.2 1.3 1.9 0.24 0.29

a

total surface area (TSA), density of active sites (dASA) and irreversible capacity (Cirr) are reported in Table 5. Because etching induces particles size distribution (PSD) variation, comparing ASA values is not entirely relevant since edge plane reactivity change can only be deduced if the edge plane surface area is constant. Unfortunately, this parameter is not known. Nevertheless, we used the TSA value for normalizing the ASA values. Despite the fact that it is a rough parameter, giving the whole surface area (basal and edge plane surface area), we found that oxygen and steam etching of SLX50 graphite display a similar value of active sites density: 0.05 (Table 5). In addition, the density of active sites after etching is higher than the SLX50 as-pristine graphite, showing that the edge planes became more reactive. The results summarize in Table 5 indicate that the irreversible capacity values are higher for oxygen-etching graphite samples compared to the steam-etching ones whereas the density of active sites remains almost constant whatever the etching process. Furthermore, SLX50 H2O samples have Cirr values close to the ones of the as-pristine graphite. The galvanostatic curves of all the samples reported in Fig. 10 and in the Supplementary Material_1 clearly indicate that exfoliation occurs only for the SLX50 O2 samples. Scanning Electron Microscopy (SEM) observations confirm also the same trend. An example is given in Fig. 11. All the experimental results are consistent and point out without doubt that oxygenetched samples exfoliate and steam graphite samples do not. TPD results correlated to the electrochemical measurements support the fact that the surface chemistry is not the parameter responsible of the exfoliation. In fact, it can be observed that the graphite sample treated in oxygen exfoliates whereas its surface chemistry is similar to the one of the as-pristine sample (see Fig. 6 and Table 2). In the same

b A B C

c

[001] zone axis

a* b*

(a): hidden by beam-stop, estimated value. Corresponding angle is proposed according to the symmetry (ie: adding 60° to the last measured spot)

Fig. 8 – TEM study of SLX50 O2-600 26%. (a) Studied particle, (b) image of the region (showing three straight edges A, B, C) used for SAED (c) SAED image and a table gathering the measured angles for the different equivalent zigzag and armchair reciprocal directions (cf. Supplementary Material_2). (A color version of this figure can be viewed online.)

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Table 3 – Edges orientation according to TEM measurements for SLX50 O2-600 26% (for more information, see Supplementary Material_2). Edges

Measured angle (°)

Corrected angle (°) according to magnitude of TEM

Corresponding angle for [hk0]* (°)

[hk0]* measured angle (°)

Edge orientation

A B C

78 134 195

115 171 232

205 261 322

202 ± 5 262 ± 5 322 ± 5

Zigzag Zigzag Zigzag

Table 4 – Edges orientation according to TEM measurements for SLX50 H2O-950 34% (for more information, see Supplementary Material_2). Edges

Measured angle (°)

Corrected angle (°) according to magnitude of TEM

Corresponding angle for [hk0]* (°)

[hk0]* measured angle (°)

Edge orientation

A B C

359 120 60

29 150 90

119 240 180

124 ± 5 243 ± 5 184 ± 5

Armchair Armchair Armchair

a

b C B A

c

[001] zone axis

a* b* (a): hidden by beam-stop, estimated value. Corresponding angle is proposed according to the symmetry (ie: adding 60° to the last measured spot)

Fig. 9 – TEM study of SLX50 H2O-950 34%. (a) studied particle, (b) image of the region (showing three straight edges A, B, C) used for SAED (c) SAED image and a table gathering the measured angles for the different equivalent zigzag and armchair reciprocal directions (cf. Supplementary Material_2). (A color version of this figure can be viewed online.)

way, the type and amount of surface complexes for the SLX50 H2O graphites are really different compared to as-pristine graphite and neither exfoliates. These experimental evidences confirm our previous results [15,17]. Since the role of the surface chemistry is not predominant, the only characteristic difference between the SLX50 O2 and SLX50 H2O samples is the carbon edge atoms configuration. It can then be concluded that armchair edge atoms configuration is preferable to avoid graphite exfoliation during the first lithium insertion cycle. On the contrary, graphite having a high concentration of zigzag edge carbon atoms promotes solvent co-intercalation and tends to exfoliate. This can be

related to the fact that the zigzag edge structure is more reactive than the armchair one. Because each carbon atom of the zigzag edge has an unpaired electron, reactants are more easily combined on zigzag edge through the interaction with active unpaired electrons, whereas the carbon atoms of the armchair edge side are more stable in chemical reactivity because of a triple covalent bond between the two open edge carbon atoms of each edge hexagonal ring. Therefore it is important for the prediction of the cell performance to take into account for a given graphite its active surface area and the carbon edge atom configuration. In summary, all the experimental results highlight that the graphite

466

CARBON

9 1 ( 2 0 1 5 ) 4 5 8 –4 6 7

Table 5 – Surface characteristics and 1st cycle irreversible capacity of SLX50 etched graphite samples. The dASA parameter is the ratio ASA/TSA. Sample labels

ASA (m2 g1)

TSA (m2 g1)

dASA

1st cycle Cirr (%)

SLX50 SLX50 SLX50 SLX50 SLX50 SLX50 SLX50 SLX50 SLX50

0.10 0.18 0.17 0.15 0.16 0.14 0.13 0.10 0.09

3.4 3.6 3.3 3.2 3.1 2.8 2.8 2.1 2.3

0.029 0.050 0.052 0.047 0.052 0.050 0.046 0.048 0.039

12 ± 3 21 ± 3 27 ± 3 26 ± 3 30 ± 3 18 ± 3 16 ± 3 11 ± 3 15 ± 3

O2-600 7% O2-600 13% O2-600 26% O2-600 46% H2O-950 19% H2O-950 21% H2O-950 34% H2O-950 38%

10 0 mA A.g-1 in 1M M LiP PF6, EC C:D DMC C (1::1 by b weig w ht) elec ctro olyte e

Potential (V vs. Li+/Li)

1 1.4 .4 1 1.2 .2

SLX S X50 0 S X50 SLX 0 O2-60 - 00 2 26%

1 1.0 .0

S X50 SLX 0 H2O-9 O 950 03 34% % 0 0.8 .8

particular the carbon edge atoms configuration. To our knowledge, it is the first time that the effect of carbon edge atoms configuration on the total cell performance of a Li-ions battery has been experimentally shown. Further experimental and theoretical investigations are in progress to better understand the SEI mechanism formation on these two types of edges.

0 0.6 .6

4.

0 0.4 .4 0 0.2 .2 0 0.0 .0 0.0

0.2

0.4

0.6

0.8

1.0

1. 1.2

1. 1.4

1. 1.6

x in L LixC6

Fig. 10 – First electrochemical Li+ insertion/de-insertion into as-pristine, oxygen-etched and steam-etched SLX50 graphite negative electrodes. Oxygen-etched graphite sample exfoliation is deduced from the presence of a plateau at 0.5 V (dotted ellipse).

Fig. 11 – Morphology of an exfoliated graphite particle from the sample SLX50 O2-600 26% after a first electrochemical Li+ insertion/de-insertion at 10 mA g1 in 1 M LiPF6, EC:DMC (1:1 by weight) electrolyte.

surface/electrolyte interaction leading to the SEI formation is controlled by the carbon edge atoms reactivity and in

Conclusion

We demonstrate that by adjusting the carbon edges atoms configuration it is possible to minimize the tendency of the graphite to exfoliate during the first electrochemical cycle. We also show that it is possible to control the zigzag or armchair edge concentration by tailoring the experimental conditions of the gasification. By TEM analysis, we point out that the oxygen etching leads to the increase of the zigzag edge concentration whereas the steam etching causes just the opposite. The electrochemical measurements clearly indicate that all the graphite samples etched in oxygen exfoliate while those etched in steam do not. This electrochemical behavior discrepancy underlines that the SEI formation mechanism and in fine the SEI properties also depends on the carbon edges configuration. Experimental results indicate that zigzag edge atoms are not recommended for the formation of an ad hoc SEI since oxygen-etched graphite particles are prone to exfoliation. Therefore, this work presents new highlights on the role played by zigzag and armchair edge atoms of synthetic graphite particles in the first cycle electrolyte decomposition in lithium-ion cell. To our knowledge, it is the first time that the effect of the carbon edge plane configuration on the electrochemical performance of a Li-ions battery was experimentally demonstrated. This original result brings undoubtedly new insights on the mechanism of SEI formation during the first Li-ions insertion cycle.

Acknowledgments The Imerys Graphite & Carbon Company, the CNRS, and the Re´gion Alsace are gratefully acknowledged for their financial support. The RS2E (Re´seau sur le Stockage Electrochimique de l’Energie) network is acknowledged for the financial support of this work through the ANR project Storex (ANR-10LABX-76-01).

CARBON

9 1 (2 0 1 5) 4 5 8–46 7

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2015.05.001.

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