Volume Changes of Graphite Anodes Revisited: A Combined

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C: Energy Conversion and Storage; Energy and Charge Transport

Volume Changes of Graphite Anodes Revisited: A Combined Operando X-Ray Diffraction and In Situ Pressure Analysis Study Simon Schweidler, Lea de Biasi, Alexander Schiele, Pascal Hartmann, Torsten Brezesinski, and Jürgen Janek J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01873 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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The Journal of Physical Chemistry

Volume Changes of Graphite Anodes Revisited: A Combined Operando X-Ray Diffraction and In Situ Pressure Analysis Study Simon Schweidler,† Lea de Biasi,*,† Alexander Schiele,† Pascal Hartmann,†,‡ Torsten Brezesinski,*,† and Jürgen Janek*,†,∥ †

Battery and Electrochemistry Laboratory, Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡

∥Institute

BASF SE, 67056 Ludwigshafen, Germany

of Physical Chemistry & Center for Materials Research, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany

ACS Paragon Plus Environment

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Abstract Lithium intercalation into graphite is one of the electrochemically best studied solid state reaction, and its application in lithium-ion batteries was a pioneering step in the development of advanced electrochemical storage devices. Therefore, one might expect that virtually any aspect of this important reaction has been examined both qualitatively and quantitatively. All the more it is surprising that there are only few experimental studies on the volume expansion of graphite, especially under cycling conditions. To the best of our knowledge, there exists no comprehensive set of structural data as a function of lithium content. Here, we present this missing information using combined results from electrochemical testing and operando X-ray diffraction. The changes in lattice parameters and unit cell volume are examined and related to the different intercalation stages and phase transition regimes. A total volume expansion (from space-groupindependent evaluation) of 13.2% is observed when C6 is fully lithiated to a composition of LiC6, of which approximately 5.9% occur in the early dilute stages. The remaining expansion of approximately 7.3% is due to transition from stage 2 to stage 1. These findings are corroborated by in situ pressure measurements on prelithiated Li4Ti5O12/graphite cells. Collectively, our data provide valuable information about one of the most important electrode materials for lithium-ion batteries and clearly demonstrate that even partially lithiated graphite experiences considerable crystallographic strain.


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Introduction Since the beginning of the age of rechargeable lithium-ion batteries (LIBs) in 1991, their rapidly increasing demand in the range of consumer electronics and electric vehicles required continuous technological progress.1 While first LIBs used LiCoO2 as the cathode active material (CAM), immense efforts have been devoted on developing and designing new CAMs.2-4 The current state-of-the-art CAMs are layered LiMO2 compounds (with M being mainly a mixture of Ni, Co, Mn, and/or Al), which are capable of delivering specific capacities of >200 mAh/g.4 In contrast, graphite has been constantly used as the anode active material since it was originally suggested by Rüdorff and Hofmann for anion intercalation and later for the intercalation of other species including lithium.2,4-8 With its high practical specific capacity of around 350 mAh/g (theoretical capacity of 372 mAh/g) and low and flat working potential (between 250 and 50 mV vs Li+/Li), it is still by far the most commonly used commercial anode material for LIBs. Various causes for capacity loss of LIBs have been proposed such as (i) dissolution of transition metal ions from the cathode and their migration and incorporation into the solid electrolyte interphase (SEI) of the graphite-based negative electrode,9 (ii) oxidative decomposition of electrolyte by highly reactive Ni species,10,11 and (iii) phase transformations and crystallographic volume changes both in the layered cathode material12 and the graphite,13 to name a few. In particular, volume changes of the active materials lead to strain, fracture of secondary particles,14,15 and generation of fresh reactive surface. Thereby, more and more active Li is consumed by parasitic reactions, such as incorporation in the SEI on graphite and cathode electrolyte interphase as well as electrolyte decomposition.16 For the monitoring of the responsible volume changes, operando X-ray diffraction (XRD) is a powerful technique and numerous studies were conducted in the past to investigate the crystallographic changes in


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layered cathode materials during cycling operation.14,17-19 In particular, for the understanding of capacity fading, caused by the interplay of cathode and graphite anode, diffraction studies currently focus more and more on full-cells.20,21 From the electrochemical point of view, the intercalation of lithium into graphite has been extensively described in recent years by Winter et al., Taminato et al., and Senyshyn et al.22-24 Surprisingly, literature reports provide only few data on phase transitions and lattice parameter changes under cycling conditions or, in other words, as a function of state of charge (SOC).23-26 Mostly, the change in interlayer distance is quoted to be about 10%,13,27 but there are hardly any studies presenting the change in unit cell volume and total volume of graphite vs SOC.21 However, this information is essential for the analysis of fullcells, which prompted us to investigate the volume changes systematically and quantitatively and to provide a convenient set of data for a broad range of compositions in the LixC6 system. In summary, the volume changes of graphite (for the different stage transitions during lithiation and delithiation) were investigated via operando X-ray diffraction (XRD) combined with Rietveld refinement analysis and in situ pressure measurements.

Experimental Methods Materials and Electrochemical Testing Graphite electrodes having 95.7 wt% active material and a loading of around 7.1 mg/cm2 were obtained from BASF SE. LP472 (1M LiPF6 in 3:7 by weight ethylene carbonate and diethyl carbonate and 2 wt% vinylene carbonate) was used as electrolyte (BASF SE). For operando Xray diffraction, pouch cells were assembled in a dry room with dew point < −50 °C. They comprised a graphite electrode (40 × 20 mm2, rounded flake-like particles of average diameter 10-20 µm), a GF/A glass fiber separator (50 × 30 mm2, WhatmanTM), a Li foil anode (44 × 24


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mm2, Albemarle Germany GmbH), and 1.5 mL of electrolyte. The cells were discharged and charged at 25 °C and at constant currents of 1.43 and 1.33 mA (corresponding to rates of approximately C/7 to C/8). Coin cells were assembled in an Ar-filled glovebox and cycled at 25 °C and at a rate of C/10. For in situ pressure analysis, a graphite electrode (synthetic graphite from Hitachi; provided by Thin Film Technology, Karlsruhe Institute of Technology) having >93 wt% active material and a loading of 2.5 mg/cm2 was cycled against a prelithiated Li4Ti5O12 electrode (MTI Corporation) with 90 wt% active material and a loading of 15 mg/cm2. Electrodes of 40 mm in diameter were punched (having a 4 mm hole in the middle for both proper gas exchange and the reference electrode), and GF/A glass fiber filter paper and 600 µL of LP472 served as separator and electrolyte, respectively. The pressure cells were cycled at 25 °C and at a rate of C/10. Instrumentation XRD patterns were collected on a custom laboratory diffractometer with a rotating Mo-Kα1,2 anode and a fast area detector. More details of the diffractometer and the overall setup are provided elsewhere.28 2D diffraction patterns were obtained in transmission geometry in the range of 7.1-44.2° 2θ with an exposure time of 90 s. The intensity of two consecutive patterns was added up and corrected for spike noise generated from cosmic radiation. The sample-todetector distance, the detector tilt angle, and the instrumental resolution were determined using CeO2 powder as a reference material. The instrumental resolution function was described by a Thompson-Cox-Hastings pseudo-Voigt profile function. Rietveld refinement of XRD data was done using the software TOPAS-Academic V5.


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Results and Discussion Both the observed diffraction pattern of a graphite half-cell prior to cycling and the calculated pattern from Rietveld refinement are depicted in Fig. 1a.

Figure 1. (a) Observed XRD pattern and calculated pattern from Rietveld refinement of a graphite half-cell prior to cycling. The major reflections originate from both Al foil of the pouch bag and the Cu current collector. (b) Evolution of the 002 reflection (equivalent to 002 for LiC12 and 001 for LiC6) during the second cycle.

The lithium-free graphite structure can be described by a model with space group P63/mmc (no. 194), with carbon atoms present in a layered hexagonal arrangement (AB stacking) as shown in Fig. 2.24 The refinement yields lattice parameters a = 2.464 Å and c = 6.716 Å, in good agreement with the literature.21 During lithium intercalation, graphite undergoes a series of phase transitions. Depending on the Li concentration, various phases, also referred to as stages, are known to exist. Their lattice structures can be distinguished according to Li ordering and shift in the 2D stacking sequence to AA of the graphene layers.21,23 In the dilute region (x(Li) < 0.25 in LixC6) of stages 1L, 4L, 3L, and 2L, lithium intercalation is assumed to have no in-plane order.29


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Phases that are present at higher degrees of lithiation (LiC12 and LiC6) are assigned to space group P6/mmm (Fig. 2). While alternating stacks are shifted by (1/3a, 2/3a, 0) with respect to each other in graphite (AB stacking),30 they are stacked in a non-shifted way in LiC12 and LiC6 (AA stacking).7 In this symmetry, the unit cell axes are defined differently compared to those of pure graphite, resulting in unit cell parameters a = 4.288(2) Å and c = 7.065(20) Å for LiC12 and a = 4.305(1) Å and c = 3.706(1) Å for LiC6.21

Figure 2. Crystal structure of (a) graphite (space group P63/mmc) and the two major lithium intercalation compounds (b) LiC12 and (c) LiC6 (space group P6/mmm). Different stacking of graphene layers with (d) AB sequence in graphite and (e) AA sequence in LiC12 and LiC6.

Operando XRD measurements were performed during cycling of a graphite half-cell in the potential range between 5 mV and 1.2 V over a period of three cycles. In order to verify the proper electrochemical performance of the pouch cell, graphite electrodes were also cycled at the same conditions against lithium in standard coin cells. The comparison reveals similar results for


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both cell types (Fig. S1). The profiles of the initial and second charge/discharge cycle for the operando cell are depicted in Figs. 3a and b (see also Fig. S2 for all three cycles).

Figure 3. Graphite potential vs lithium content for (a) the initial and (b) second cycle. The different lithium intercalation stages and phase transition regimes in the second cycle are indicated in part c.


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The shape of the charge/discharge curves for both cycles is characterized by the typical lithium (de-)intercalation plateaus, corresponding to two-phase regimes of the different lithiation stages of graphite as indicated in Fig. 3c.31,32 The number of each stage (namely, 1L, 4L, 3L, 2L, 2, and 1)31,33 specifies the number of unoccupied graphene layers between two occupied layers.22,34,35 The different stages and stage transitions have already been studied in detail and will not be discussed any further in this work.22,31,36 However, we note that the phase separation of 4L, 3L, and 2L is still subject of ongoing debate.23 In the first cycle, the half-cell delivers a specific capacity of about 380 mAh/g (lithiation of graphite), corresponding to a lithium content x(Li) = 1.02 (per formula unit LixC6), which exceeds the theoretical capacity by 13 mAh/g. In the second cycle, a specific capacity of 353 mAh/g is achieved (x(Li) = 0.95). The irreversible capacity on the first cycle is typically ascribed to SEI formation.22,37 In the present study, the irreversible capacity (23 mAh/g) corresponds to x(Li) = 0.073, equivalent to a lithium loss of around 7.3% in the first cycle. It was shown that the first cycle irreversible capacity strongly depends on the surface morphology and surface chemistry of the respective graphite material.38 However, we note that for full-cells using industrial graphite electrode tapes, operando neutron diffraction detected a lithium loss of about 7% (based on the specific capacity) in the initial cycle due to SEI formation, which is in good agreement with our data.13 Over the entire duration of the experiment, 3312 diffraction patterns were taken with a time resolution of 3 min. For each diffraction pattern, a detailed evaluation of the diffraction data was performed by the use of Rietveld refinement. A full set of refinement data for all cycles is provided in Fig. S3. However, because of the irreversible capacity in the first cycle, the


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calculation of lithium content is not representative of the actual amount of lithium intercalated into the host structure. Therefore, in the following, the lattice parameter changes and phase transformations of graphite will be discussed in detail only for the second cycle. Fig. 1b shows a contour plot of diffraction intensity in the 2θ region of the 002 reflection of graphite, and Fig. 4 depicts the Rietveld refinement results for the a and c lattice parameters and the conversion of the latter into interlayer spacing dC-C (distance between two neighboring graphene layers), all plotted as a function of lithium content. The conversion into dC-C allows for symmetryindependent comparison of the expansion of lithium-graphite phases during stage transformation. This also facilitates the evaluation of refinement data within a larger context of present literature.23,24,35 Analogous to the cell potential, both the position and intensity of the graphite 00l reflection undergo several changes during the course of lithium intercalation as different stages and twophase regions are passed. In the range between the compositions C and LiC 12, a shift to lower 2θ values is observed that is accompanied by significant variations in intensity and full width at half maximum, indicating the appearance of dilute stages 1L, 4L, 3L, and 2L. During the transformation of stage 2L to stage 2 (LiC12), the intensity of the 002 reflection strongly increases, which might reflect the higher degree of order with higher Li content. During the LiC12-to-LiC6 transformation, the 002 and 001 reflections of both phases coexist over a wider range of lithium intercalation, while the 002 reflection of LiC12 becomes weaker until only the 001 reflection of LiC6 remains.


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Figure 4. Rietveld refinement results for the second cycle: (a) a-axis, (b) c-axis, and (c) interlayer spacing dC-C vs lithium content.

As the refinement shows, at first, the C-to-2L transformation (0 ≤ x(Li) ≤ 0.25) is characterized by a quasi linear behavior of the c lattice parameter. However, closer inspection reveals marginal


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changes in slope of the c-axis evolution. Comparison with the plateaus in the potential curve in Fig. 3c indicates that these features can be assigned to the 1L-to-4L, 4L-to-3L, and 3L-to-2L phase transformations. The increase in c-axis (or interlayer spacing dC-C) is, on the one hand, affected by the amount of intercalated Li. On the other hand, it can be assigned to the changes in graphene sheet stacking that take place in LiC12,7 where Li ions are accommodated in the center of adjacent hexagons in every second interlayer, leading to higher repulsion between the layers. During the 2L-to-2 transformation, the interlayer spacing is rarely affected, as rather in-plane Li ordering than occupation of new interlayers dominates the intercalation mechanism in this region. After the overall composition of stage 2 (x(Li) = 0.5, LiC12) is reached and the system passes the coexistence region of stage 2 and stage 1 (LiC6), the c-axis of stage 2 levels off. In the newly formed stage 1, lithium is intercalated into every interlayer, leading to a significant increase in dC-C.23 According to other studies, the interlayer spacing increases from 3.35 Å for x = 0 to 3.52 Å for the end of stage 2 and 3.7 Å for x = 0.74 to 0.95 for stage 1, which is in good agreement with our results.23,32,36 Interestingly, a slight increase in a-axes or, in terms of a spacegroup-independent description, in intralayer C-C distance is also detected as shown in Fig. 4a. Despite the two-phase reaction from stage 2 to stage 1, which should typically be reflected in a flat voltage plateau, a slight decrease in cell potential is seen before pure stage 1 is achieved. This may be due to kinetic limitations and therefore compositional inhomogeneity in the electrode. As a side note, we also like to point out that the structural evolution in the beginning of the first cycle slightly varies from that of the second and third cycle (Fig. S4). The cell parameters virtually do not change until a potential of around 0.5 V is reached (0 < x(Li) < 0.013), suggesting that Li is not intercalated into graphite and charge is rather consumed by SEI formation.


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Table 1. Rietveld refinement results for different lithium intercalation stages and phase transition regimes of graphite. Values are given for the beginning and end of each regime. For a spacegroup-independent calculation of the relative volume change per phase transition regime, the unit cell metric of graphite (P63/mmc) is used as reference. Stage

x(Li)

c-axis / Å

Unit cell volume / Å3

Volume change / %

Solid solution

0-0.05

6.716(3)-6.788(4)

35.31(2)-35.61(2)

0.8

1L-4L

0.05-0.1

6.788(4)-6.868(5)

35.61(2)-36.11(4)

1.4

4L

0.1-0.12

6.868(5)-6.896(4)

36.11(4)-36.26(2)

0.4

4L-3L

0.12-0.18

6.896(4)-6.959(4)

36.26(2)-36.59(2)

0.9

3L

0.18-0.2

6.959(4)-6.972(4)

36.59(2)-36.67(2)

0.2

3L-2L

0.2-0.23

6.972(4)-7.013 (5)

36.67(2)-36.87(3)

0.5

2L

0.23-0.25

7.013(5)-7.023(5)

36.87(3)-36.92(3)

0.1

2L-2

0.25-0.44

7.023(5)

LiC24: 7.03(7)

36.92(3)

LiC24: 37.0(2)

LiC12: 7.037(3) 2

0.44-0.5

2-1

0.5-0.85

7.041(2)-7.043(4) 7.043(1)

LiC12: 7.04(1)

LiC12: 37.4(2) 37.4(2)-37.4(2) 37.464(3)

LiC6: 7.40(1) 1

0.85-0.95

7.40(1)-7.414(6)

LiC24: 0.4 LiC12: 1.2 0.2

LiC12: 37.7(2)

LiC12: 0.6

LiC6: 39.85(9)

LiC6: 6.3

39.85(8)-39.95(7)

0.2

Tab. 1 shows the refined c lattice parameter and the calculated unit cell volume of graphite at the beginning and end of each two-phase regime. The continuous evolution of the unit cell volume and phase fraction of each stage is also depicted in Figs. 5a and b. In addition to the unit cell volume of the individual phases (stages), the overall volume change of the lithium-graphite system was determined as shown in Fig. 5c (see also Tab. S1 for numerical values of the lattice parameters a and c, the unit cell volume V, and the total volume change of graphite vs lithium


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content in steps of 0.05 x(Li) for the first three cycles). In order to calculate the total volume change based on a consistent space-group-independent volume unit over the whole range of lithium intercalation, a conversion of cell metrics of stage 2 and stage 1 was included with regard to that of graphite (P63/mmc). Therefore, the distance between every second C atom in one hexagon was taken as the a-axis dimension. The c-axis dimension of stage 2 and graphite are identical and, for stage 1, the c-axis can be transformed by 2 × dC-C. Until the beginning of the phase transition regime 2L-to-2, a continuous, almost uniform increase in volume is observed for each transition region. Then, in the final phase separation from stage 2 to stage 1, a strong increase in volume is noted. Please note that the discontinuous jumps at approximately x(Li) = 0.6 and 0.86 are artefacts of the refinement. With regard to the starting volume at x(Li) = 0, the overall volume increases by 6.1% for stage 2 (x(Li) = 0.50) and 13.2% for stage 1 (x(Li) = 0.95). As the refinement of phase fractions reveals, only stage 1 is present at the end of the cycle, thus confirming virtually complete lithiation of the graphite electrode.


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Figure 5. (a) Unit cell volume, (b) phase fraction, and (c) total volume change of graphite vs lithium content. The sudden, discontinuous jumps in total volume change at around x(Li) = 0.6 and 0.86 are artefacts from the calculation since contributions from stage 2 and stage 1 are taken into account only if their weight fraction exceeds approximately 20 wt% as depicted in part b.


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In order to evaluate the influence of the graphite expansion on a cell level, in situ volume analysis of prelithiated Li4Ti5O12 (LTO)/graphite cells was performed by use of a previously reported pressure measurement system.11,39 In essence, the volume change of the cell leads to a pressure change of a small constant gas volume on top of the cell. Because of its zero-strain properties, (de-)lithiation of LTO does not affect the cell volume significantly.11,40 Thus, during cycling, the total volume change of the cell results only from changes in volume of the graphite anode and gas formation, for example, due to SEI formation, reduction of trace H2O, and electrolyte decomposition. By curve fitting and subtracting the gas evolution from the pressure data (see Appendix in the Supporting Information for more details and Figs. S5-S7 for cycling and pressure data), the graphite contribution to the pressure changes can be extracted and converted to volume changes as shown in Fig. 6. In the volume change vs time curve, a periodic increase and decrease is noted, which correlates well with the (de-)intercalation of the graphite electrode. However, as is evident, the resolution is limited, which is why the curve is more or less featureless (compare to Fig. 5c). Nevertheless, the relative change in volume is in excellent agreement with the operando XRD results.


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Figure 6. In situ pressure analysis of prelithiated Li4Ti5O12/graphite cells (3-electrode geometry) for the third to fifth cycles. (a) Volume change of graphite and (b) graphite potential vs Li reference. The volume change curve smoothed with an FFT filter using 150 data points is shown in red. The blue lines indicate the total volume change determined by operando XRD.

Conclusions The results from operando XRD and in situ pressure analysis provide clear evidence that occupation of only half of the intercalation sites by lithium already causes considerable crystallographic changes and expansion of graphite. With regard to full-cells, this means that even incomplete lithiation, which typically exists for anode/cathode balancing reasons, leads to


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substantial strain in the lattice, which almost certainly affects the long-term stability of graphite. Cycling in a limited range of SOC, e.g., from 20 to 80%, may therefore be beneficial to protect the anode, thereby improving cycle life. The complete set of volume data reported here allows better approximation of the volumetric impact arising at the graphite side and should be taken into account in the analysis of degradation processes of rechargeable lithium-ion cells.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Comparison of specific capacities of graphite pouch and coin cells, graphite potential vs lithium content and full set of refinement data for the first three cycles (including numerical values in steps of 0.05 x(Li)), details on pressure measurements, cycling and pressure evolution data for prelithiated LTO/graphite cells.

Author Information Corresponding Authors *Phone: +49 721 60826870, E-mail: [email protected] *Phone: +49 721 60828827, E-mail: [email protected] *Phone: +49 641 9934500, E-mail: [email protected]


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Notes The authors declare no competing financial interest.

Acknowledgements We thank Dr. Toru Hatsukade for proofreading and language editing. This study is part of the projects being funded within the BASF International Network for Batteries and Electrochemistry.

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Volume Changes of Graphite Anodes Revisited: A Combined

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