Cycling Behavior of Silicon-Containing Graphite Electrodes, Part B

Article Cite This: J. Phys. Chem. C 2017, 121, 25718-25728

pubs.acs.org/JPCC

Cycling Behavior of Silicon-Containing Graphite Electrodes, Part B: Effect of the Silicon Source Tiphaine Schott,† Rosa Robert,† Sergio Pacheco Benito,‡ Pirmin A. Ulmann,‡ Patrick Lanz,‡ Simone Zürcher,‡ Michael E. Spahr,‡ Petr Novák,† and Sigita Trabesinger*,† †

Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Imerys Graphite & Carbon, CH-6743 Bodio, Switzerland

‡

S Supporting Information *

ABSTRACT: Silicon (Si) is a promising candidate to enhance the specific charge of graphite electrode, but there is no consensus in the literature on its cycling mechanism. Our aim in this study was to understand Si electrochemical behavior in commercially viable graphite/Si electrodes. From the comparison of three types of commercial Si particles with a producerdeclared particle sizes of 30−50 nm, 70−130, and 100 nm, respectively, we identified the presence of micrometric Si agglomerates and the Si micro- and mesoporosity as the main physical properties affecting the cycling performance. Moreover, ex situ SEM, XRD, and Raman investigations allowed us to understand the lithiation/delithiation mechanism for each type of Si particles. For nanoscale Si particles, the entire Si particle is utilized, resulting in high specific charge, and the stress induced by the formation of Li15Si4 alloy upon deep lithiation is well managed within the Si mesoporosity. This leads to reversible cycling behavior and, thus, to good cycling stability. On the other hand, micrometric Si aggregates undergo a two-phase lithiation mechanism with early Li15Si4 formation in the particle shell. This leads to stress-induced core disconnection during the first lithiation, and shell pulverization during the following delithiation, resulting in overall low specific charge and rapid performance fading.

■

INTRODUCTION Silicon (Si), with its very high specific charge (3580 mAh g−1), is a promising candidate to enhance the specific charge of graphite electrodes, aiming at higher energy density Li-ion batteries. In the first part of this study,1 we demonstrated that the utilization of Si, the lithiation depth, and the cycling stability of graphite electrodes containing 5 wt % of Si additive are dependent on both the lithiation cutoff conditions and the silicon nature itself, suggesting a relationship between the lithiation mechanism and the physical properties of Si. Earlier studies have shown that reducing the particle size of Si from several micrometers (>10 μm) to a few micrometers significantly improves the cycling performance.2−4 When the Si particle size is further decreased down to 70−130 nm, this results in even lower performance fading4−7 due to lower volume changes and it prevents Si particle pulverization, which occurs for micrometric particles.5 Actually, Liu et al.8 demonstrated, by an in situ transmission electron microscopy © 2017 American Chemical Society

(TEM) investigation, that there is a critical particle size of 150 nm, below which the Si particle will expand without fracture, while above that the particle will crack during the first lithiation. This is due to the tensile hoop stress created at the two-phase interface between the inner crystalline Si core and the outer amorphous Li−Si alloy shell. Zeng et al.9 have recently determined by in situ Raman spectroscopy that the core of 100 nm Si particles indeed endured a compressive stress of ∼0.3 GPa during the formation of the Li−Si alloy in the outer layer, the latter being then subject to tensile stress and, thus, to cracking. Since the thickness of the lithiated alloy shell is reported to be the determining factor for particle fracture,8 researchers focused their efforts to develop synthetic methods to obtain Received: August 24, 2017 Revised: October 25, 2017 Published: October 25, 2017 25718

DOI: 10.1021/acs.jpcc.7b08457 J. Phys. Chem. C 2017, 121, 25718−25728

Article

The Journal of Physical Chemistry C

electrochemical performance and the physical properties of commercially available Si powders.

nonagglomerated nanoparticles with controlled particle size, e.g., by varying the surfactant nature and treatment temperature.10 However, even for particles smaller than the reported critical size (150 nm), the cycling mechanism was found to be particle-size-dependent. Indeed, it was reported that 10 nm sized particles lithiate to form a mixed amorphous and crystalline phase and recover their pristine size after a full cycle, while 20 nm sized particles convert to an amorphous phase and do not recover their original morphology after delithiation.10 In addition to controlling the nanoparticle size, some researchers tailored the shape of the nanomaterials to enhance the cycling performance. For example, Xiao et al.11 obtained better cycling stability using Si nanospheres with a mesoporous shell surrounding a hollow core than using commercial Si nanoparticles. This enhanced performance was attributed to the ability of the mesopores in the shell to buffer the volume changes and to the low overall expansion due to the shell expanding not outward but toward the hollow center. Interestingly, these hollow amorphous nanomaterials undergo a wave-propagation lithiation mechanism,11 by contrast to the core−shell mechanism reported for monolithic particles. Indeed, McDowell et al.12 found that, also for amorphous Si nanoparticles, the first lithiation occurs through a two-phase reaction mechanism. However, the critical thickness above which the amorphous Si particle cracks upon lithiation is higher than that reported for crystalline Si,8 and that the subsequent delithiation of amorphous Si and further cycling proceed through a single phase reaction mechanism.12 There is no consensus in the literature reports on the lithiation/delithiation mechanism of Si depending on its particle size, its morphology, and its crystallinity. Moreover, the suggested mechanisms are mostly based on in situ TEM studies performed on single Si particles,13 which raises questions about the resulting applicability to commercially available Si within viable electrode formulations, i.e., when high loadings of active material in the presence of both binder and carbon additive are used. Using electrodes with three types of commercially available Si particles, Yoon et al. showed that the cycling behavior was similar for both 50 nm and 700 nm sized Si particles.14 The performance fading observed for these electrodes was reported to be due to an incomplete delithiation, which is caused by increased electrode resistance with particle shrinkage rather than particle pulverization occurring during lithiation. The critical size proposed by Liu et al.8 also seems not to be relevant in a recent study by Tranchot et al.15 where better cycling stability was obtained for electrodes containing Si with a particle size of 230 nm than with 85 nm. By ex situ scanning electron microscopy (SEM), in situ dilatometry, and acoustic emission studies, they demonstrated that, while the bigger Si particles are enduring a progressive reversible expansion with lithiation, the smaller particles suffer from abrupt expansion and further electrode degradation while cycling. This lack of cohesion was attributed to an insufficient amount of binder to bond the Si with higher surface area15 and shows that the performance degradation of standard electrodes may not be directly linked to the resistance to cracking of the particles themselves. The aim of the present study is to better understand the lithiation/delithiation mechanism of commercially viable graphite/Si composite electrode formulations by studying the relationship and its evolution during cycling between the

■

EXPERIMENTAL METHODS Electrode Preparation and Electrochemical Characterization. The procedure followed to prepare electrodes is reported in the first part of this study (including composition of electrodes and reference cycling data for electrodes without Si),1 leading to the following electrode composition: 5 wt % Si, 90 wt % graphite, KS6L (Imerys Graphite & Carbon); 1 wt % conductive carbon additive, Super C65 (Imerys Graphite & Carbon); 4 wt % binder, carboxymethyl cellulose (CMC):poly(acrylic acid) (PAA) (1:1). In addition to Si 30−50 (30−50 nm, 98%, Nanostructured & Amorphous Materials) and Si 70− 130 (70−130 nm, >99%, Nanostructured & Amorphous Materials), a third type of Si was investigated: Si with a particle size of 100 nm, 99.9%, Hongwu, further referred to as Si 100. The electrochemical measurements were carried out with a standard battery cycler (Astrol Electronics AG, Switzerland) at 25 °C using half-cells (with metallic lithium counter electrode), as described in the first part of this study.1 The standard cycling procedure was a constant current−constant voltage (CCCV) protocol as follows: the first constant current (CC) cycle was performed at slow rate (20 mA g−1), while further cycles were performed at 50 mA g−1 for the lithiation, and at 186 mA g−1 for the delithiation. After each CC step, the cutoff potential (5 mV or 50 mV for lithiation, 1.5 V for delithiation) was maintained until the current dropped below 5 mA g−1 (CV step). For additional tests, some of the cells were investigated using a constant current rate of (i) 125 mA g−1 for the lithiation and 465 mA g−1 for the delithiation (2.5 times faster than standard cycling), or (ii) 5 mA g−1 for the lithiation and 18.6 mA g−1 for the delithiation (10 times slower than standard cycling), using 50 mV as the lithiation cutoff potential. In this study, the potential is given vs Li+/Li and the specific charge and cycling current are denoted per mass of the whole coating. In the cycling performance plots, only the specific charge obtained during lithiation is shown for clarity. Characterization of Si Particles and Composite Electrodes. The particle size distribution of the pristine Si powders was evaluated by laser diffraction in a water/surfactant mixture, and their morphology was studied by SEM using a Zeiss scanning electron microscope operated at an accelerating voltage of 5 kV (for Si 30−50) or 1 kV (for Si 70−130) in the secondary electron detection mode. The morphology and size dispersion of the Si particles and agglomerates in the pristine and cycled composite electrodes were also investigated by SEM using a Carl Zeiss UltraTM 55 scanning electron microscope operated at an accelerating voltage of 3 kV in the secondary electron detection mode. After being rinsed by dimethyl carbonate (DMC) to remove electrolyte salt, cycled electrodes were dried and transferred from the glovebox to the SEM in a vacuum-sealed specimen transfer holder to avoid air exposure. In order to investigate the internal morphology of the electrodes, cross sections of electrodes were prepared by broad-beam argon-ion milling (Hitachi IM4000). For this purpose, the electrodes were milled for 4 h at 4 kV with an ion current of ca. 120 μA, before being transferred to the SEM for analysis. Phase composition and crystallinity of the pristine Si powders, composite electrodes, and cycled electrodes were determined ex situ by X-ray diffraction (XRD) and Raman spectroscopy. XRD experiments were performed at room 25719


Article


Figure 1. (a, b) Cycling performance (specific charge, squares; Coulombic efficiency, stars) of various graphite electrodes containing 5 wt % Si with (a) 5 mV CCCV or (b) 50 mV CCCV as lithiation cutoff conditions.

protocols, leading to a practical specific charge significantly lower than that for Si 30−50, and to a negligible gain compared to graphite after long-term cycling (Figure 1); (ii) a higher cutoff potential does not affect the Si lithiation depth. Indeed, in electrodes with Si 100, in the same manner as with Si 70− 130,1 the crystalline Li15Si4 alloy seems to be formed with a cutoff potential as high as 50 mV when a CV step is used, as identified by the delithiation plateau16 in the first cycle voltage profile (Figure S1) and by a delithiation peak at ca. 0.44 V in the corresponding differential capacity plot (Figure 2). These

temperature using a PANalytical Empyrean diffractometer equipped with a linear detector (X’Celerator). Diffraction patterns were recorded using Cu Kα radiation in either reflection or transmission mode over the 2θ range of 15° ≤ 2θ ≤ 135°. Pristine Si powders were measured in reflection mode. XRD measurements on composite electrodes and cycled electrode materials were performed on capillaries in transmission mode. Cycled electrodes were first rinsed with DMC to remove the residues of salt and electrolyte from the surface. After that, the electrode films were removed from the Cucurrent collector and placed in a 0.5 mm diameter capillary. Raman spectroscopy was carried out at room temperature using a Horiba HR800 microscope, equipped with a He−Ne laser (632.8 nm). A typical spectrum was recorded in the range of 200−800 cm−1 using a 50× objective, accumulating 3 scans with an acquisition time of 30 s per scan. Multiple areas of pristine Si powder and ion-milled pristine and cycled composite electrodes were analyzed. Gas-adsorption measurements were carried out at 77 K using Micromeritics ASAP 2020 equipment. Full N2 isotherms (adsorption and desorption) were recorded of samples degassed in vacuum for 6 h at 300 °C. The BET specific surface area (SSABET) was determined applying the BET theory, while the micro-, the meso-, and the macroporosity and the pore-size distribution were determined from the t-plot method, BJH theory, and nonlocal density functional theory (NLDFT), respectively. The electrochemically active surface areas (EASA) of the electrodes were determined experimentally from cyclic voltammograms at various scan rates in the potential window [3.15; 3.35 V] where both the electrolyte and electrode material are electrochemically inert, i.e., where no redox reactions are occurring. The EASAs were calculated from the obtained capacitive currents, assuming a double layer capacitance of 50 mF m−2, as is typical for the investigated nonaqueous electrolyte systems.

Figure 2. Differential capacity plots of the first cycle of graphite electrodes with 5 wt % Si 100 with 5 mV CCCV or 50 mV CCCV lithiation cycling conditions.

similarities suggest that the size of the Si nanoparticles is crucial for defining the cycling behavior: a particle size around 100 nm according to the supplier (Si 70−130, Si 100) leads to significantly worse performance than a particle size around 40 nm (Si 30−50). Although the lithiation mechanism of Si was reported to be size- rather than rate-dependent,8 a possible cause of the differences, when cycling Si with different particle size, is the applied current per Si surface area. Si 70−130 (as well as Si 100) has a geometrical particle surface area 6.25 times lower than Si 30−50. In order to cycle them with similar current densities, electrodes containing Si 30−50 were cycled at higher rate, and electrodes containing Si 70−130 at lower rate, using the 50 mV CCCV cycling protocol to identify the effect of the cycling rate on the delithiation plateau of different size Si particles. Figure 3 shows that the potential profiles of electrodes containing Si 30−50 recorded at a 2.5 times higher rate are very

■

RESULTS AND DISCUSSION Effect of the Si Source on the Cycling Performance. Aiming to understand the role of Si particle size on the cycling performance, the cycling of electrodes containing Si 100 was evaluated using two different lithiation cutoff potentials and compared to electrode compositions with other Si particle sizes, as evaluated in detail in part A.1 Interestingly, the electrochemical behavior of Si 100 is very similar to Si 70−130: (i) the performance fading is pronounced with both cycling 25720


Article


properties affecting the cycling behavior, the morphological and structural properties of the different Si sources were investigated before and after cycling. Importance of Si Physical Properties. The morphology and quality of dispersion of the different types of Si nanoparticles in the pristine electrodes were investigated by SEM, and representative images of both top view and crosssectional view are shown in Figure 4, Figure 5, and Figure 6 for Si 30−50, Si 70−130, and Si 100, respectively. SEM images of the pristine powders before electrode preparation can be found in Figure S3 and Figure S4. Si 30−50 consists of spherical particles, mostly in the range of 30−70 nm in diameter, in agreement with supplier data, with a few particles up to 200 nm in size (Figure 4). These nanoparticles are bonded to each other forming micron-sized agglomerates, which are rather well dispersed between the graphite flakes. By contrast, Si 70−130 is composed of a mixture of dense spheres of 1−2 μm size, and of 50−200 nm nanoparticles, and some short nanowires (Figure 5). The particle size distribution is, therefore, much wider than expected from the supplier data, the true particle size being well above the given value. Furthermore, this Si is not able to be homogeneously distributed in the electrode. Micrometric Si agglomerates are also observed in Si 100 (Figure 6), although they are macroporous and not dense as in the case of Si 70− 130. Additionally, no filaments are observed in this third silicon powder (Figure 6). Gas-adsorption measurements were performed to investigate the porosity of the Si powders and to determine their SSABET. The obtained N2-adsorption/desorption isotherms (Figure S5) correspond to a type II isotherm with a weak hysteresis loop, matching powder samples with a wide pore-size distribution. The pore-size distribution obtained from the NLDFT model (Figure 7a) shows that the three Si materials have a bimodal mesopore distribution with a large portion of mesopores between 250 Å and 500 Å, but still a significant portion below 250 Å. However, some differences between the various Si sources can be observed. Si 30−50 contains the largest amount of small mesopores among the studied powders (Figure 7a), as well as a rather large amount of macropores according to BJH method analysis (Figure 8). Thus Si 30−50 exhibits the largest cumulative pore volume and SSABET, combined with a relatively low average pore width (Table 1). Si 70−130 contains a smaller

Figure 3. First cycle potential profiles of graphite electrodes with 5 wt % Si 30−50 and Si 70−130 with 50 mV CCCV as lithiation cutoff condition and at various cycling rates.

similar to those obtained at the standard cycling rate. In particular, none of them revealed the presence of a delithiation plateau, indicating that Li15Si4 is still not formed. The two differences observed with higher cycling rate for Si 30−50 are that the overpotential is slightly higher and the specific charge of the first cycle is slightly lower, however, without an impact on the cycling stability, which was identical for both cycling rates (Figure S2). Cycling the electrodes containing Si 70−130 at much lower rate had no effect on the lithiation depth either: despite the large difference in cycling rate (factor of 10), the plateau corresponding to the delithiation of the crystalline Li− Si alloy is present on the voltage profiles for both tested rates (Figure 3). Furthermore, similar specific charge values are obtained. The only difference with Si 70−130 was a more pronounced electrolyte reduction at lower rate during the first lithiation. These different-cycling-rate experiments excluded an effect of the applied current density per Si surface area as the cause of the different electrochemical behavior of the various Si particle sizes, and the actual reasons still needed to be clarified for (i) differences in initial specific charge, pointing to different Si utilization; (ii) differences in relationship of the voltage profile to the cycling protocol, indicating different potential of Li−Si alloying; and (iii) differences in fading rate, suggesting different failure mechanisms. In order to get more insight into the Si

Figure 4. Representative (a−c) top view and (d−f) cross-sectional SEM images of pristine graphite electrodes containing 5 wt % Si 30−50. 25721


Article


Figure 5. Representative (a−c) top view and (d−f) cross-sectional SEM images of pristine graphite electrodes containing 5 wt % Si 70−130.

Figure 6. Representative (a−c) top view and (d−f) cross-sectional SEM images of pristine graphite electrodes containing 5 wt % Si 100.

Figure 7. (a) Pore-size distribution of pristine Si powders using the NLDFT model; (b) enlarged plot in the microporosity and small mesoporosity region.

quantity of large mesopores, as compared to the other Si

theless, Si 70−130 exhibits a larger pore surface area than Si 100 as it contains a larger amount of small mesopores. All Si powders also contain micropores, with some differences (Figure 7b). The smallest Si particles, Si 30−50, do not contain any ultramicropores but only micropores above 13 Å as well as small mesopores. This leads to a negligible

powders (Figure 7a), while Si 100 presents the largest amount of macropores (Figure 8), as also observed by SEM analysis. Si 100 also has the largest average pore size and a higher cumulative pore volume than Si 70−130 (Table 1). Never25722


Article


Figure 8. (a) Pore volume and (b) pore surface area distribution of pristine Si powders using the BJH theory.

(Table 2). The SSABET of the studied electrodes are very similar since the SSABET of Si is dominated by that of graphite. The

Table 1. Gas Adsorption Analysis Data for Pristine Si Powders in Comparison to Supplier Data Si properties

Si 30−50

Si 70−130

Si 100

Cumulative pore volumea (cm3 g−1) Cumulative pore surface areaa (m2 g−1) Average pore widtha (Å) t-plot micropore areab (m2 g−1) Measured SSABET (m2 g−1) Supplier’s SSABET (m2 g−1) Average particle size from SSA (nm) Supplier’s average particle size (nm)

0.115 40.3 114 0 37.9 70−80 158 30−50

0.072 21.5 133 2.4 25.1 22 239 70−130

0.083 15.2 215 1.0 15.4

Table 2. SSABET and EASA of the Electrodes short name

electrode composition

Si 30−50

5% Si 30−50 + 90% graphite + 1% SuperC45 + 4% CMC/PAA 5% Si 70−130 + 90% graphite + 1% SuperC45 + 4% CMC/PAA 5% Si 100 + 90% graphite + 1% SuperC45 + 4% CMC/PAA 95% graphite + 1% SuperC45 + 4% CMC/PAA

Si 70−130 390 100

Si 100

a

BJH theory valid between 17 Å and 3000 Å pore width. bt-plot method valid for pore width below 20 Å.

Graphite

micropore volume as calculated by the t-plot method (Table 1). By contrast, both Si 70−130 and Si 100 contain a bimodal micropore distribution, with a large part of the micropores above 8 Å and a smaller part below 8 Å (Figure 7b). However, Si 70−130 has larger microporosity, resulting in a larger micropore volume (Table 1). Together with the larger amount of small mesopores, this also leads to a higher SSABET than for Si 100 (Table 1). The SSABET measured for Si 70−130 is actually very close to that given by the supplier despite poor fit between the quoted and observed particle sizes, and is lower than the SSABET of Si 30−50. For the latter Si source, the obtained value is, however, half of that given by the supplier, which might be explained by the nanoparticle agglomeration as found by SEM. The average particle sizes, calculated from gas adsorption and data and density of Si, are higher than the ones given by the supplier (Table 1), in agreement with the SEM observations. They are, however, much lower than the ones obtained by laser diffraction (Figure S6), which reflects agglomerate size rather than particle size. Moreover, while Si 100 has the highest particle size according to gas adsorption results, laser diffraction data suggest that Si 70−130 contains a larger portion of bigger particles and Si 100 contains a larger amount of smaller particles, making clear the differences between dense aggregates observed in Si 70−130 and open/porous aggregates of Si 100. Nevertheless, Si 30−50 has the smallest particles according to both techniques, as expected. The electrochemically active surface area (EASA) of the electrodes was determined by cyclic voltammetry (Figure S7, Table S1) and compared to the SSABET of the electrodes calculated from the SSABET of each electrode component

SSABET (m2 g−1)

EASA (m2 g−1)

20.3

6.7

19.7

8.5

19.2

6.1

19.4

6.7

electrode EASA is, however, 2−3 times lower than the SSABET of the same electrode. This is due to the fact that the electrolyte cannot access all pores in the electrode and, consequently, only part of the electrode’s surface is wetted by electrolyte. Surprisingly, the EASA is the highest, even if not by a large margin, for electrodes containing Si 70−130, while electrodes containing Si 30−50 and Si 100 have almost identical EASA. This may be explained by the fact that Si 70−130 contains Si nanofilaments as well as fewer large pores than other Si types, which are easier to access for electrolyte. After evaluating the physical properties of the studied Si types and comparing their electrochemical behavior, it can be concluded that the cycling mechanism is not affected by (i) the cumulative pore volume and pore surface area; (ii) the quantity of macropores and large mesopores (although they are expected to buffer the volume expansion); (iii) the specific and electrochemically active surface area (SSABET and EASA), which also explains why the electrochemical performance was insensitive to the current density; and (iv) the overall particle size distribution. Indeed, all these properties are different for Si 70−130 and Si 100, despite the fact that they behave identically while cycling. However, our results suggest that the presence of micrometric Si agglomerates and the presence of ultramicropores and a lower amount of larger micropores and small mesopores are the properties leading to poor cycling performance of electrodes containing larger Si particles (Si 70− 130 and Si 100). SEM analysis was performed on cycled electrodes to evaluate volume changes and detect either electrode or Si particle cracking, as well as possible Si disconnection from the electrode’s conductive network, all of which may explain the 25723


Article


lithiation, the micrometric particles of Si 70−130 are pulverized (Figures 11a, 11b) and/or disconnected from the particle core

differences in cycling behavior between the smaller Si particles (Si 30−50) and the larger ones (Si 70−130 and Si 100). For this analysis, a 5 mV CCCV lithiation cycling protocol was chosen to get maximum electrode lithiation and, thus, maximum morphological changes. First of all, we found that the morphology of Si 30−50 is not significantly affected by the first cycle. Only a slight expansion of the spherical particles, which leads to more compact agglomerates between the graphite flakes, was observed after the first lithiation (Figure 9). After the following delithiation,

Figure 11. Representative (a, b) top view and (c, d) cross-sectional SEM images of graphite electrodes containing 5 wt % Si 70−130 after the first lithiation with 5 mV CCCV cycling protocol.

(Figures 11c, 11d), which is surrounded by a shell-like porous matrix. Particle pulverization and core disconnection from the conductive network are also observed after the following delithiation (Figure 12). Thus, these morphological changes are Figure 9. Representative (a, b) top view and (c, d) cross-sectional SEM images of graphite electrodes containing 5 wt % Si 30−50 after the first lithiation with 5 mV CCCV cycling protocol.

i.e., after the first cycle, the particles shrank back to their initial size and morphology. This demonstrates that the particle expansion occurring during the lithiation is reversible (Figure 10, to be compared to pristine electrodes in Figure 4) for this Si

Figure 12. Representative (a, b) top view and (c, d) cross-sectional SEM images of graphite electrodes containing 5 wt % Si 70−130 after the first delithiation with 5 mV CCCV cycling protocol.

irreversible, and may explain the low specific charge and the early performance fading observed for these electrodes. After longer cycling, the particle core remains unutilized, while the surrounding shell becomes thicker and more porous (Figure S10, Figure S11). However, Si 70−130 is still electrochemically active (if only partially) even after 50 cycles as Si-particle morphology after the 50th lithiation (Figure S10) is different from that after the 50th delithiation (Figure S11) and these electrodes deliver capacity higher than that of only graphite after the same number of cycles (Figure 1). Interestingly, the overall agglomerate size is not significantly affected by the charge/discharge reactions and, thus, does not lead to the cracking of the electrode itself. The absence of cracks is also a

Figure 10. Representative (a, b) top view and (c, d) cross-sectional SEM images of graphite electrodes containing 5 wt % Si 30−50 after the first delithiation with 5 mV CCCV cycling protocol.

particle size. Long-term cycling leads, however, to irreversible morphological changes (Figure S8, Figure S9) with an agglomeration of nonspherical particles between the graphite flakes, although this does not significantly affect the cycling performance (Figure 1). Different behavior has been observed for the electrodes containing Si 70−130. Already after the first 25724


Article

The Journal of Physical Chemistry C sign that the binder nature and the Si−binder−carbon surface ratio in our electrodes is adequate.15,17 Electrode Composition and Si Crystallinity. The structural properties of the electrode materials were investigated before and after cycling to explain the cycling behavior observed for different Si sizes and to correlate them to the morphological changes. The Si powders were investigated by Raman and XRD to obtain the reference data. For both Si 30− 50 and Si 70−130, a sharp and symmetrical Raman Si peak is observed at 521 cm−1 (Figure S12a). This suggests that neither Si oxide, characterized18,19 by a peak around 515 cm−1, nor amorphous Si, characterized20,21 by a broad and asymmetric peak around 465 cm−1, is present on the surface of the nanoparticles in detectable amounts in crystalline Si22 samples. Raman data demonstrate that possible surface modification by Si amorphization or oxidation cannot explain the differences in cycling behavior or first cycle efficiency due to the fact that the spectra are identical for both Si types. XRD analysis on asreceived silicon powders (Figure S12b) confirms that both Si 30−50 and Si 70−130 consist of highly crystalline cubic Si (space group Fd3̅m). The crystallite size, calculated according to the Scherrer equation, is in good agreement with the supplier data. However, while no crystalline impurities were detected for Si 30−50, Si 70−130 presented around 10 wt % of secondary phases, identified as the α and β forms of Si3N4. This does not agree with the powder purity given by the supplier (>99%), but it explains to a limited extent the lower specific charge reached by the corresponding electrodes as the amount of available Si is lower than expected. Before and after the cycling, using different cycling protocols, ex situ XRD analysis was performed on electrodes containing either Si 30−50 or Si 70−130 (Figure 13). The XRD patterns of the pristine electrodes can be indexed based on two major phases, cubic Si (space group Fd3̅m) and hexagonal C (space group P6̅m2), while the shoulder at ∼24° corresponds to amorphous carbon (SuperC45 carbon additive) (Figure 13, black). After the first lithiation down to 50 mV, a small amount of unreacted crystalline Si is left, suggesting that it is not fully converted yet, and Li15Si4 is only detected for electrodes with Si 70−130 (Figure 13, green). When the lithiation is continued down to 5 mV CCCV, no crystalline Si is detected anymore in electrodes with both Si types, all Bragg reflections corresponding to LiC6 and Li15Si4 (Figure 13, red). Interestingly, this proves that nanoparticles of Si 30−50 can be lithiated to a crystalline Li−Si alloy, by contrast to a previous study where only amorphous alloy was detected in nanoscale Si.23 The XRD pattern for Si 70−130 at 5 mV CCCV also reflects a fully lithiated Si, although, as we observed by SEM (Figures 11c, 11d), Si 70−130 at this potential presents a morphology in which the particle’s core is disconnected from the shell. In order to determine the phase composition of the disconnected core, ex situ Raman analysis was performed on cross sections of the lithiated electrodes containing Si 70−130, with the laser focused on the particle core. The sharp peak of crystalline Si observed in the pristine electrodes has completely disappeared after the first lithiation (Figure 14). Instead, a broad peak at ∼ 465 cm−1 is observed, corresponding to amorphous Si.20,21 This demonstrates that the entire particle is reacting during the first lithiation: the particle shell is converted to the crystalline Li−Si alloy as detected by XRD (Figure 13b), while Raman analysis showed that the particle core is only converted  before the disconnection occurs  from crystalline to amorphous Si, despite the very low cutoff voltage.

Figure 13. Normalized XRD patterns of graphite electrodes containing 5 wt % (a) Si 30−50 or (b) Si 70−130 during the first cycle.

Figure 14. Normalized Raman spectra of graphite electrodes containing 5 wt % Si 70−130 before and after the first lithiation, using 5 mV CCCV as cutoff conditions. The laser was focused on the particle core of the ion-milled cross section of the lithiated electrode.

After the following delithiation, no remaining lithiation products, i.e. LiC6 and Li15Si4, are detected by XRD, suggesting a full reconversion of the Li−Si alloys to Si for both Si sources (Figure 13, blue). Crystalline Si peaks are hardly visible after 1 cycle. This shows that most of the Si is in the amorphous state after the delithiation, in agreement with the literature.16,24 It is also important to note that the Si3N4 impurities present in Si 70−130 seem to be electrochemically inert as they are still detectable after the first cycle (Figure S13), while the graphite structure is slightly affected with the disappearance of the rhombohedral reflections at around 43.5° in both electrodes. The XRD results, therefore, demonstrate that the utilization of the crystalline Si, the nature of the lithiation products 25725


Article


also was observed for Si nanospheres with a mesoporous shell.11 The presence of mesopores helps to release the stress induced by Li15Si425 formation. During the following delithiation, the whole crystalline alloy is delithiated to an amorphous phase, which leads to a plateau in the voltage profile. This delithiated phase is expected to be pure amorphous Si as the full theoretical specific charge is reached. Importantly, the morphology and size of the delithiated Si particles after one cycle are very similar to the pristine ones, showing that they are not significantly affected by the volume expansion, even when the crystalline Li15Si4 phase is formed. Moreover, although long-term cycling results in some irreversible changes regarding the morphology of Si, the electronic connection with the graphite flakes is still ensured thanks to appropriate electrode and electrolyte formulation, and up to 90% of the Si can still contribute to the specific charge of the electrode. That is the reason why this type of Si nanoparticle demonstrates good cycling stability. For Si 70−130, the cycling behavior is significantly different from the smaller particle response. Initially, this Si powder is highly crystalline, it contains around 10% of Si3N4 impurities, and the average particle size is much larger than declared by the producer, despite the mean crystallite size being 90 nm. Some nanowires and very small particles, responsible for the higher EASA, are also observed, but the electrode behavior is dominated by the micrometric Si agglomerates. When the electrode is lithiated down to 50 mV, Si is partially converted to Li15Si4, while some crystalline Si (and probably amorphous phase too) is remaining. The formation of the crystalline alloy leads to a potential plateau during the following delithiation already with 50 mV cutoff potential. This phase is formed earlier in Si 70−130 than in Si 30−50, maybe because solid state reactions are slower in nanoparticles,23 although this behavior seems to be independent of the cycling rate in our study. When the lithiation is continued down to 5 mV, all crystalline Si is converted: the particle shell is composed of crystalline Li15Si4 while the core is mostly amorphous Si that does not alloy with lithium, despite the low potential. This explains, together with the presence of inactive impurities, why the specific charge is limited to ∼70% of the theoretical value already in the first cycle. These Si 70−130 results demonstrate that a two-phase reaction mechanism is occurring in our electrodes during the first lithiation. However, by contrast to the literature reports, the failure of the larger Si particles is not due to particle pulverization induced by the stress between the outer amorphous shell and the inner crystalline core8,9 but is rather due to the disconnection of the particle core from the particle shell, and a fortiori from the conductive network, making further Si alloying with lithium impossible. This disconnection is caused by the stress between the crystalline Li15Si4 shell and the amorphous Si core formed after the first lithiation of crystalline Si. Formation of Li15Si4 is known to induce stress within Si material due to crystal volume expansion with lithium intercalation, and this stress is better released in the presence of small mesopores than micropores.25 Therefore, Si 70−130 is not able to manage the stress as well as Si 30−50, which has a larger amount of mesopores and lower microporosity, and in which the crystalline alloy is formed later. During the following delithiation, the Si core, as it was disconnected from the shell during the lithiation, does not contribute anymore to the specific charge and remains amorphous. The particle shell is delithiated from crystalline

(excluding the disconnected core) and the lithiation/ delithiation reversibility after the first cycle are similar for both electrodes containing Si 30−50 and Si 70−130, when 5 mV CCCV is used as limiting lithiation condition. Si Cycling Mechanism. The cycling mechanisms of Si 30− 50 and Si 70−130, summarizing the results from our study (including part A1), are shown in Figure 15 and Figure 16,

Figure 15. Schematic representation of the cycling mechanism for Si 30−50.

Figure 16. Schematic representation of the cycling mechanism for Si 70−130.

respectively. Calculations of the lithiated volume and thickness of the Si particles are given in Table S2 and Figure S14. The cycling mechanism of Si 100 is considered to be similar to the one of Si 70−130. Initially, the Si 30−50 particles have the expected size with high crystallinity and purity. After the first lithiation down to 50 mV, the crystalline Si is partially converted to an amorphous phase: amorphous Si or amorphous Li−Si alloy. Some crystalline Si is remaining, but crystalline Si−Li alloy is not formed yet. The absence of Li15Si4 explains the slightly lower specific charge than the theoretical value obtained using 50 mV CCCV as cycling protocol. It also explains why the plateau corresponding to the delithiation of Li15Si4 is not observed in the voltage profiles during the following delithiation using this cutoff potential. After deeper lithiation, down to 5 mV, all the Si is converted to the crystalline Li−Si alloy, resulting in clearly visible particle expansion. Since the theoretical value of the specific charge is reached for such lithiation condition, it means that the entire Si particle is converted to Li15Si4. The formation of this phase does not lead to particle pulverization because the induced volume expansion is partially buffered by the mesoporosity, determined by gas adsorption (Figure 7), as 25726


Article


on obtaining the relevant properties of Si and Si/C composite additives to get minimal performance fading of Si-containing electrodes.

alloy to amorphous phase, and suffers from pulverization due to volume contraction. Therefore, only a small amount of Si is participating in the lithiation/delithiation processes in further cycles, and the electrode’s performance fading cannot be attributed to continuous volume changes. Instead, the Si disconnection, by worsening the electrical contact and thus increasing the overpotential, causes a decrease in the lithiation degree of the still-connected Si: after some cycles, crystalline Li−Si alloy is not formed anymore, which can be clearly identified by the disappearance of the delithiation plateau. In conclusion, the major cause of the performance fading observed for Si 70−130 is the loss of connectivity of Si with the conductive network upon cycling, and this effect is especially significant because it becomes influential already during the first cycle.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08457. Electrode cycling performance, physical properties, and lithiated volume and thickness of Si particles (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +41563105775.

■

ORCID

CONCLUSIONS Our study showed that the cycling mechanism of graphite/Si electrodes is strongly dependent on the nature of the silicon material itself, and that the effects of the pristine-Si morphology on the electrode electrochemical behavior do not exactly correspond to findings in previous studies reported in the literature. For nanoscale Si with a crystallite and particle size around 40 nm (Si 30−50) we found that the crystalline Li15Si4 alloy is formed, but only upon deep lithiation (

Cycling Behavior of Silicon-Containing Graphite Electrodes, Part B

Recommend Documents