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Letter
Observing Framework Expansion of Ordered Mesoporous Hard Carbon Anodes with Ionic Liquid Electrolytes via In-Situ Small-Angle Neutron Scattering Craig A. Bridges, Xiao-Guang Sun, Bingkun Guo, William T. Heller, Lilin He, Mariappan Parans Paranthaman, and Sheng Dai ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00472 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017
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Observing Framework Expansion of Ordered Mesoporous Hard Carbon Anodes with Ionic Liquid Electrolytes via In-Situ Small-Angle Neutron Scattering Craig A. Bridges,†,* Xiao-Guang Sun,† Bingkun Guo ,† William T. Heller,‡ Lilin He, ‡ Mariappan Parans Paranthaman,† Sheng Dai† †ChemicalSciencesDivision,OakRidgeNationalLaboratory,OakRidge,Tennessee37831, USA ‡BiologyandSoftMatterDivision,OakRidgeNationalLaboratory,OakRidge,Tennessee 37831, USA AUTHOR INFORMATION Corresponding Author Craig Bridges email:
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ABSTRACT
The reversible capacity of materials for energy storage, such as battery electrodes, is deeply connected with their microstructure. Here, we address the fundamental mechanism by which hard mesoporous carbons, which exhibit high capacities versus Li, achieve stable cycling during the initial “break-in” cycles with ionic liquid electrolytes. Using in-situ small angle neutron scattering we show that hard carbon anodes that exhibit reversible Li+ cycling typically expand in volume up to 15% during the first discharge cycle, with only relatively minor expansion and contraction in subsequent cycles after a suitable solid electrolyte interphase (SEI) has formed. While a largely irreversible framework expansion is observed in the first cycle for the 1-methyl1-propypyrrolidinium bis-(trifluoromethanesulfonyl)imide (MPPY.TFSI) electrolyte, reversible expansion is observed in the electrolyte lithium bis(trifluoro-methanesulfonyl)imide (LiTFSI)/1ethyl-3-methyl-imidazolium
bis(trifluoromethanesulf-onyl)imide
(EMIM.TFSI)
related
to
EMIM+ intercalation and de-intercalation before a stable SEI is formed. We find that irreversible framework expansion in conjunction with SEI formation is essential for the stable cycling of hard carbon electrodes.
TOC GRAPHICS
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Carbonaceous anodes continue to play a key role in the development of new and higher performance reversible energy storage devices, as ever more demanding cell voltages and current densities are required at lower cost for applications in consumer electronics and electric vehicles.1-4 Hard carbons have shown promise for high capacity Li+ ion, as well as new Na+ and K+ ion based batteries.5-6 The maximum achievable capacity of an electrode is related to intrinsic factors such as the atomic structure and chemical potential, and to extrinsic factors such as particle morphology, strain, and interfacial reactions. Understanding how to optimize the extrinsic factors is crucial for the successful development of any given battery chemistry. Interfacial reactions of an electrolyte at the anode surface to form a solid electrolyte interphase (SEI) have received a great deal of attention because of their impact on battery safety in addition to irreversible capacity losses.7-10,11-12 Ordered mesoporous hard carbon is a useful model anode material with a high surface area, and is relevant to various storage technologies in which interfacial reactions, morphology and stress can all affect performance.13,14-16,17-19,20-22 Prior structural studies have shown that Li-ion intercalation into graphitic layers in hard carbon electrodes is largely reversible, but studies performed without using ordered mesopores are not able to quantitatively study the changes in carbon framework expansion during cycling.23,14, 24 This expansion can be monitored using in-situ neutron scattering methods, which are complimentary to other spectroscopic (e.g., Raman) or microstructure sensitive (e.g., electron microscopy) methods.8,14, 25-33 SANS has proven to be useful in the characterization of porous materials, where the contrast variation provided by neutrons can give additional information.34-36 In-situ small angle neutron scattering (SANS) is are particularly well-suited to observing changes at the electrode-electrolyte interface and in the bulk microstructure during battery cycling, due to its sensitivity to hydrogen and lithium content. Furthermore, neutrons are highly penetrating,
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which enables the use of fully assembled, functional batteries. Here we report the first direct observation of framework expansion and SEI formation in mesoporous hard carbon electrodes with ionic liquid electrolytes, using in-situ electrochemical SANS for MPPY.TFSI and EMIM.TFSI based electrolytes in Li-ion batteries. Ordered mesoporous hard carbon was prepared via a hard template method from ordered mesoporous silica.37-40 In-situ studies were conducted on the EQ-SANS beamline at the SNS41 using CR2032 cells assembled with slurry-coated hard carbon electrodes, 7Li metal anodes, Celgard separator, and ionic liquid electrolyte. Analysis of the in-situ data was performed with the SASView software.42 Our analysis focuses largely on the changes that occur in the Gaussian peak near Q = 0.08 Å-1 (Fig. 1(b)), as the peak is well above the background, and for these coin cells it is challenging to produce a sample suitable for quantitative background subtraction. The peak position is related to the separation between the pores and the peak area is sensitive to differences between the scattering length density (SLD) of the pores and the SLD of the hard carbon framework (see the supplemental information (SI) for more details), which is referred to as the contrast (SLD). The SLD is a measure of the strength of scattering of a material. In the pore, the SLD of SEI components (e.g., Li2S2O4, LiSO2CF3) is different from the SLD of the electrolytes (Fig. 1(a)), such that there is a change in the average SLD of the pore during SEI formation. The SLD of the framework changes during cation intercalation. Therefore, the SANS experiment measures the changes in pore-framework contrast (peak area) and framework expansion (peak position) that occur during SEI formation and cation intercalation. Ex-situ cell cycling was carried out to examine the cycling behavior of the hard carbon electrode in ionic liquid electrolytes. The electrolytes EMIM.TFSI and MPPY.TFSI have been selected as they are well known, relatively stable electrolytes with large electrochemical
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windows.43-44 It is known that the imidazolium and pyrrolidinium cations may suffer from cointercalation into the graphene planes of graphite, and may undergo reduction upon cycling, but this has not been demonstrated for mesoporous hard carbon anodes. As shown in Fig. S5(a, b), the reversible capacities of the hard carbon are above 200 mAh g-1 in both ionic liquid electrolytes. However, the initial SEI formation cycles are dramatically different for the two electrolytes (Fig. 2a, b). For MPPY.TFSI based electrolyte, there is a continual reduction below 1.0 V for SEI formation, resulting in discharge (Li intercalation) and charge (Li de-intercalation) capacities of 983 and 283 mAh g-1, respectively with a low coulombic efficiency of 28.5 % (Fig. S5a). Nonetheless, for MPPY.TFSI the coulombic efficiency immediately increases to 90 % in the second cycle, and by the 5th cycle the reversible capacity is relatively stable near 98%.
Figure 1. (a) Calculated SLDs of various components of the cell. The difference in SLD between the pore and carbon framework (SLD) is correlated with peak area in the SANS data. SEI components shown in green should decrease peak area, and those shown in red should increase
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peak area. (b) An example fit to SANS data, using equation [S1] in the supplemental information, is shown for a cell with the EMIM.TFSI electrolyte and a mesoporous ordered hard carbon electrode. The inset contains the fit for MPPY.TFSI electrolyte. For EMIM.TFSI based ionic liquid, there are well-defined plateaus during the first few discharge and charge cycles. Fig. S6 shows similar discharge-charge plateaus of a graphite electrode in the same ionic liquid electrolyte, though the hysteresis between charge and discharge plateaus is smaller. Previously, Novak et al observed a similar discharge plateau for graphite electrodes in the same ionic liquid electrolyte, but no charge plateau was observed and they simply attributed the plateau to electrolyte reduction.
45-46
However, these plateaus are largely
due to the intercalation/de-intercalation of the EMIM cation, as was observed by Ogumi et al on the intercalation/de-intercalation of TMHA cation in graphite electrodes.47 Unlike the cointercalation of solvent molecules such as propylene carbonate (PC) in carbonate electrolyte causing ex-foliation of the graphite electrode,48 the EMIM cation can be reversibly intercalated/de-intercalated into/from the graphitic domains within the hard carbon framework without causing exfoliation; however it is also being reduced at the same time, as the initial low columbic efficiency of 54% reflects (Fig. S5b). With cycling, the discharge plateau gradually changes to a slope while the charge plateau gradually increases, indicating the increased energetic barrier to remove the intercalated ionic liquid cation. After the fourth cycle, the EMIM cation is not intercalated. Instead, it is mainly being reduced on the electrode surface where it forms SEI layers and enables reversible lithium intercalation/de-intercalation. This process results in a relatively unusual trend in coulombic efficiency (CE) in the initial cycles - the mechanically poor SEI that initially forms is disrupted by the intercalation/de-intercalation of the EMIM cation, leading to a drop in CE during the third and fourth cycles as more SEI is formed
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during discharge. After a more stable SEI is formed, the observed coulombic efficiency of higher than 100% in the latter cycles for EMIM.TFSI based electrolyte may relate to some of the EMIM cation species trapped between graphitic layers in the initial cycles, and later reduced; these species might still be released at higher potentials during the charge process, though based upon the coulombic efficiency this is a minor effect. There is no such phenomenon observed for MPPY.TFSI base electrolyte. Nonetheless, these ex-situ cycling data suggest SEI formation and ionic liquid cation intercalation, but do not provide quantitative information on the microstructure that the SANS data contain.
Figure 2. Charge-discharge profiles for ex-situ half-cells containing ordered mesoporous hard carbon in (a) 0.5 M LiTFSI/MPPY·TFSI and (b) 0.5 M LiTFSI/EMIM·TFSI electrolyte under a current density of 25 A cm-2. The arrows in (b) indicate the general trend in capacity with increasing cycle number. The process of surface passivation is connected with changes in microstructure and strain during cycling, especially in cases where the SEI could prevent electrolyte co-intercalation. The SEI layer formed with carbonate electrolytes is typically thought to consist of a mixture of organic components (e.g., ROLi) and inorganic components (Li2CO3, LiF, LiOH, etc.).49 The SEI formed in typical carbonate electrolytes differs from that in high voltage ionic liquid based
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electrolytes, and it is known that both cation and anion components of these ionic liquids can be reduced at cathodic potentials on the graphite electrodes.50-51 Typical decomposition products are presented in Fig. 1(a) and Figs. S3-S4. We use the SANS data to understand how the reversible capacity correlates with SEI formation and microstructural changes that are dominated by the choice of ionic liquid. The results of the SANS data analysis for the MPPY.TFSI and EMIM.TFSI are shown in Figure 3. The data have been separated into discharge (grey) and charge (yellow) regions. The peak area, which is correlated to the contrast between the framework and the pore, displays significantly different behavior depending upon which electrolyte is used. The cell with MPPY.TFSI (Fig. 3(c,e)) shows an increase in peak area (increase in contrast) above 1.5 V during the initial discharge stage. This initial voltage drop from an open cell voltage of 2.5-2.8V to ~1.0V, is related to the formation of an electric double layer (EDL) before desolvation and intercalation of the Li ion. A similar, though less prominent, increase in peak area is seen for the EMIM.TFSI cell (Fig. 3(d,f)). Such behavior has been previously noted to occur with carbonate electrolytes by in-situ SANS, and the increase is related to the higher concentration of Li+ ions in the pores during adsorption of solvated Li+ on the surface that decreases the SLD of the pore.28 Further discharge of the cells leads to a mixture of Li+ or ionic liquid cation intercalation in conjunction with expansion of the carbon framework.
For MPPY.TFSI, the initial cation
adsorption stage is followed immediately by a decrease in peak area, and then by a continuous increase in the peak area until the end of the discharge stage; for EMIM.TFSI, the cell exhibits a continuous increase in peak area with discharge. Comparison with Figures S3 and S4 indicates that lithium intercalation and decomposition of the ionic liquid would likely cause a decrease in peak area, which can reasonably explain the decrease for MPPY.TFSI after the adsorption stage.
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The estimates of SLD given in Figures S3 and S4 suggest that the peak area increase could be related to deposition of Li2O or hydrogenated phases such as polyethylene, LiOCH3 or LiOH in the mesopores. The presence of moisture could lead to the formation of LiOH, but the electrolytes are dried and do not contain carbonate electrolyte to form LiOCH3. Therefore, these compounds are unlikely to produce this trend. There are two remaining scenarios to explain this increase in peak area. The increase is may be related to an increase in the density of the framework itself, or to an increase in pore volume associated with the volume expansion. If there is partial ionic liquid intercalation and reduction in conjunction with Li+ intercalation, with the accompanying slight volume expansion, then the density of the framework may increase; this is possible due to the fact that the hard carbon is known to contain nanosized voids, which may therefore be filled by ionic liquid rather than Li+ as is typically observed in carbonate electrolytes.52 The resulting change in the SLD of the framework would become dominant relative to the changes in the SLD of the pore during SEI deposition on the pore surface. Alternatively, as the intensity is proportional to the pore volume, when the framework expands the larger pore volume could contribute to an increase in the peak area. Given the evidence from electrochemical data for the intercalation and decomposition of the ionic liquids, and from the SANS data for an expansion of the framework, we expect that both of these mechanisms may contribute to the observed scattering during the initial discharge, and their contributions cannot yet be quantitatively separated. We argue that these contributions dominate over the likely contributions of ionic liquid decomposition (SEI) on the surface during the initial discharge in this case. For MPPY.TFSI, the changes in peak area beyond the first discharge do not directly follow the trend in pore-pore spacing - the increase in peak area during charging and decrease during discharging is in agreement with expectation for Li+ ion intercalation and removal.
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Figure 3. In-situ SANS data for cycling of half-cells containing ordered mesoporous hard carbon cathodes, 7Li anodes and 0.5 M LiTFSI/MPPY·TFSI (a,c,e) or LiTFSI/EMIM·TFSI (b,d,f) electrolyte. Darker background for discharge regions, lighter background for charged regions. The trend in peak area for the SANS data on EMIM.TFSI is similar to MPPY.TFSI for the discharge stage, in which peak area increases by 76% during discharge. However, the data for the first charge cycles differ. The MPPY data show an increase indicative of de-intercalation of Li+ from the hard carbon anode, whereas the peak area decreases 25% in the EMIM data due to de-intercalation of ionic liquid cation. The pore-pore spacing for the MPPY.TFSI cell increases continuously over the entire initial discharge stage, which is followed by a slight overall increase over the second discharge stage (Fig. 3(a)). A minimal overall decrease in the spacing occurs in the charging regions. This initial
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increase results from a combination of electrolyte intercalation and Li+ ion intercalation, and the minimal overall reversibility in the pore-pore spacing suggests that some of the intercalated species remain in the framework after the charge stage, which prevents the framework from relaxing back to the initial state. The change in pore-pore spacing for EMIM-TFSI shows a remarkably different behavior from other electrolyte chemistries investigated with this approach, including carbonate28 and MPPY.TFSI. There is a continuous expansion of the mesoporous framework from a 74.3 Å spacing to 75.8 Å, which is followed by a contraction to 73.5 Å during charging. This reversible change in pore-pore spacing during cycling has not been observed previously in hard-carbon samples. Furthermore, we interpret the drop from 74.3 Å (fresh electrode) to 73.5 Å (charged, de-intercalated electrode) as a slight shrinking of the porous structure. This can be compared against examples of porous oxide electrodes, in which the microstructure changes during cycling versus lithium have involved full reversibility (TiO2), significant loss of mesostructure (Co3O4), and complete collapse of the mesostructured (SnO2).53 None of these cases shows evidence for a large, irreversible increase in framework volume as observed for MPPY.TFSI. The lack of contraction of the hard carbon in cases where lithium has intercalated may in part relate to incomplete de-intercalation of Li+ during charging, and the presence of an SEI layer that may prevent relaxation of the framework. To further understand the effect of intercalation upon the graphitic layers, we have cycled graphite electrodes with the EMIM.TFSI and MPPY.TFSI electrolytes, and collected the diffraction data shown in Figure S7. Due to the amorphous nature of the hard carbon graphitic layers, the use of crystalline graphite electrodes was necessary. These diffraction data contain information in both the broadening of the peaks, and the peak position, as summarized in Table
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S1. The graphite (001) peak broadens more after cycling in MPPY.TFSI than in EMIM.TFSI, with almost no broadening observed in the standard EC/DEC/DMC electrolyte. The results suggest that the intercalation of ionic liquids disrupts the interlayer stacking much more than the intercalation of Li+ (in the EC/DEC/DMC electrolyte), with the greater broadening of the MPPY.TFSI peaks suggesting that the bulkier MPPY+ cation causes a greater disruption of the layer stacking than does the flatter EMIM+ cation. Furthermore, there is a greater average peak shift to larger d-spacing for the bulkier MPPY+ cation than the EMIM+ cation. The changes in graphite layer spacings and peak broadening are consistent with the MPPY having a larger impact on the framework than the EMIM, which could lead to the irreversible expansion in the initial cycle for the hard carbon electrodes. These results demonstrate that the cases in which a stable SEI is formed in the initial cycle and significant Li+ intercalates, the lattice of the hard carbon framework exhibits a typical expansion of roughly 4 Å (~5% in this case) during the initial stage of discharge, and thereafter only minor expansion and contraction of 0.3-0.5 Å (
