Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
www.acsaem.org
Elucidating the Li-Ion Battery Performance Benefits Enabled by Multifunctional Separators Hanshuo Liu,† Anjan Banerjee,§ Baruch Ziv,§ Kristopher J. Harris,‡ Nicholas P. W. Pieczonka,∥ Shalom Luski,§ Gianluigi A. Botton,† Gillian R. Goward,‡ Doron Aurbach,§ and Ion C. Halalay*,∥ †
Department of Materials Science and Engineering and ‡Department of Chemistry, McMaster University, Hamilton, Ontario L8S 4K1, Canada § Department of Chemistry, Bar-Ilan University, Ramat-Gan 5290002, Israel ∥ Global Research & Development, General Motors, Warren, Michigan 48092-2031, United States S Supporting Information *
ABSTRACT: The dissolution of transition metal ions from positive electrodes and loss of (both electroactive and transport) Li+ ions seriously impair the durability of lithium ion batteries. We show herein that the improvement in the cycle life of lithium manganate spinel-graphite cells effected by multifunctional separators results from smaller interfacial resistances at both positive and negative electrodes, that can in turn be traced back to thinner, more uniform, and chemically different surface films, due to lessened parasitic reactions and a decreased accumulation of parasitic reaction products at electrode surfaces, as evidenced by HR-SEM, FIBSEM, EDX, 19F MAS NMR, and ICP-OES data. KEYWORDS: Li-ion batteries, Mn dissolution, LixMn2O4, graphite, functional separator, ion-exchange resin, SEI, FIB-SEM
T
We selected cells with lithium manganate spinel (LixMn2O4, also known as LMO) positive and graphite negative electrodes as model system in our studies, since this electrode couple is most severely affected by Mn dissolution. We reported previously on the benefits for cell performance effected by multifunctional separators (MFSs) consisting of a polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) matrix and a modified commercial cross-linked styrene-divinylbenzene copolymer functionalized with disodium iminodiacetate groups, called poly(IDANa2) henceforth, as filler.28 Herein we delve into the reasons for the observed behavior, using highresolution scanning electron microscopy (HR-SEM), focused ion beam-scanning electron microscopy (FIB-SEM), energydispersive X-ray spectroscopy (EDX), 19F magic-angle spinning nuclear magnetic resonance (MAS NMR), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and Fouriertransform infrared spectroscopy (FTIR) as analytical tools subsequent to cell disassembly. In contrast to our previous work, we use an LMO material with high Mn dissolution rate in our present investigation, to ensure that the thickness of the composite mosaic-like surface film (also known as solid electrolyte interphase or SEI31) on the graphite electrodes from all cells is large enough to allow
he dissolution of transition metal (TM) ions from positive electrodes and the loss of Li+ ions are two main degradation modes for lithium ion batteries’ performance. The former is most severe in materials with Mn3+ ions and spinel phases,1 while the latter is dominant in cells with positive electrode materials not affected by TM ion dissolution.2 The TM ions’ dissolution from positive electrodes initiates the socalled dissolution−migration−deposition−catalytic reactions performance degradation mechanism in Li-ion batteries (LIBs). Mn2+ and Mn3+ ions3 dissolved from the positive electrode migrate through the electrolyte solution and deposit at the negative electrode, where they catalyze decomposition reactions of solvent molecules and anions, with consumption of Li+ ions, generation of gases, and passivating film thickening. All of this results in a decreased LIB power and a shortened life.4−7 Several mitigation measures were proposed for this LIB performance degradation mode.1,8−17 Each, however, is far from 100% effective, and only several coordinated measures are likely to maximize LIB durability. In recent years, we and others have investigated materials that can trap transition metal ions and/or scavenge HF, and demonstrated their benefits for LIB performance.18−30 While many of these materials can be incorporated into separators, coated onto separators and electrodes, or used as electrode binders, separators are an ideal test platform during proof-of-concept experiments, due to the ease of analysis and clarity in interpretation they offer during the post-disassembly diagnostics of cell components. © XXXX American Chemical Society
Received: March 19, 2018 Accepted: May 15, 2018 Published: May 15, 2018 A
DOI: 10.1021/acsaem.8b00436 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Letter
ACS Applied Energy Materials reliable quantitative comparisons of their cross-sections by FIBSEM and EDX, as well as to keep the duration of our study manageable. Small 3-electrode LMO-graphite pouch cells were filled with a 1 M LiPF6 solution in a binary mixture of ethylene carbonate and dimethyl carbonate (1:1 v/v) and were subjected to 4 weeks of constant current cycling at 55 °C, with 5 h charging and discharging durations. We used an additive-free electrolyte solution in our study, to test the efficacy of the MFSs under the most drastic cell operating conditions. Electrochemical ac impedance spectroscopy (EIS) data were recorded at 30 °C immediately after cell “formation,” and then at weekly intervals throughout the duration of the constant current cycling tests. Two separators with differing loadings of poly(IDANa2), denoted MFS 1−1 and MFS 1−2, were used in the present study. (1−1 and 1−2 are the mass ratios of the PVDF-HFP matrix to poly(IDANa2) filler; see Table S1 for further details). Cells with commercial polypropylene (PP) or PVDF-HFP separators served as comparison baselines. Results from cell cycling tests are shown in Figure 1a. Data from duplicate cells are shown, to illustrate the reproducibility of our electrochemical cycling results. After 4 weeks of cycling at 55 °C, the cells with MFSs retain ∼43% of their initial capacity, compared to ∼25% for cells with the PP separator. While capacity losses in all cells are large, as many details of electrode fabrication and cell construction still need optimization, it is nevertheless clear that the cells with MFS 1−1 have 17 mAh g−1 more capacity left at EOT than the cells with the PP separator, which represents 19% of the initial capacity of the cells. The 0.8% difference in Faradaic efficiency is also noteworthy, as it indicates a reduction in the irreversible reactions during the cell charging, for cells with MFS 1−1. The capacity retention and Faradaic efficiency correlate with the Mn amounts found in the graphite electrodes (173 μg for the cell with MFS 1−1 and 279 μg for the cell with the PP separator). Electrochemical impedance spectroscopy (EIS) data (Figure 1b−e) provide some insights into the reasons for the improvements in cell performance effected by the MFSs. While the data from panels b and c, d and e, look pairwise almost identical, the factor of 3 decrease in the axes’ ranges between the data from the cell with the PP separator and the cell with the MFS indicates a reduction in interfacial resistances by a similar factor. Most noteworthy, after 4 weeks of cycling, the interfacial resistances of both the LMO and graphite electrodes in the cell with the MFS are equal to or smaller than the respective resistances in the cell with the baseline separator after only 1 week of cycling. Even though at the end-of-test (EOT) there exists a difference by a factor of 3 in the interfacial resistances of both electrodes from the cell with the PP separator and the respective electrodes from the cell with MFS 1−1, SEM micrographs show significant surface film growth on the graphite electrodes from both types of cells (Figure 2b,c, also Figure S1 for high-magnification images). In the pristine electrode (Figure 2a), the active material phase is shown as the particles in medium gray, while the binder plus carbon black mixture is the crinkly, sponge-like structures shown in light gray. Note that the binder plus carbon black phase is no longer visible at EOT (in either panels b or c of Figure 2) and is covered by the surface film on both graphite electrodes. On the other hand, neither significant film growth nor any other major changes can be discerned by electron microscopy in the LMO electrodes from cycled cells, when compared to their pristine state (see Figure 2d−f, also Figure S2 for low-magnification
Figure 1. (a) Discharging capacity and Faradaic efficiency as a function of cycle number for LMO-graphite cells cycled at 0.2 C rate and 55 °C temperature. The inset shows the Mn amounts found in the graphite electrodes at EOT. Nyquist plots of EIS data for individual electrodes, collected after formation and during 4 weeks of cycling: graphite electrode with (b) PP and (c) MFS 1−1 separators; LMO electrode with (d) PP and (e) MFS 1−1 separators. Plot identifier key: F = formation, number = test week. Note the factor of 3 change in the Z′ and −Z′′ axes ranges from b to c and from d to e.
images). The surfaces of the graphite electrodes are clearly more affected by surface film growth due to Mn dissolution than the LMO electrode surfaces. Slight differences in the LMO particles’ surface morphologies are visible at high magnification, both between the pristine (Figure 2d) and cycled electrodes, as well as between the electrodes cycled with the PP separator (Figure 2e) and MFS 1−1 (Figure 2f). The pristine LMO particle has a smooth and clean surface, as well as sharp edges, B
DOI: 10.1021/acsaem.8b00436 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
statistical distribution of the SEI thickness for 30 and 18 μm long FIB cross-sections on the two electrodes cycled, respectively, with the PP separator and MFS 1−1. The average SEI thickness on the two electrodes is ∼60 and ∼160 nm for the graphite electrodes from the cells with MFS and PP separator, respectively. The SEI on the graphite cycled with the MFS is thus 2.7× thinner and has a ∼2× narrower thickness variation range than on the graphite cycled with the PP separator. A small (1%) fraction of the SEI on the graphite cycled with the PP separator has a thickness smaller than 50 nm, indicating the existence of localized regions (pinholes or perhaps narrow canyon-like trenches) where the resistance is significantly smaller than in their surroundings, which are therefore susceptible to localized Li plating. In contrast, no such thinned area fraction is observed in the SEI of the graphite electrode cycled with MFS 1−1. An MFS thus enables a more homogeneous current distribution in a LIB than a PP separator. Nevertheless, note that the SEI on the graphite electrodes from cells with both types of separators also comprises pockets with much larger than average cross-section dimensions that are filled with an inhomogeneous mixture of carbon-black filler, binder, and SEI (Figure 2j and Figure S5a), likely resulting from a nonoptimal electrode fabrication process. EDX analyses within the FIB-SEM cross-sections (Figure 2k,l, also Figure S5b,c) indicate F:P ratios of {1.6, 3.1} and {2.6, 5.6} at two locations on graphite electrodes cycled with a PP separator and MFS 1−1, respectively. F:P ratios closer to 6 for the SEI formed in the presence of MFS 1−1 suggest that the PF6− anions are more stable than in the presence of a PP separator. Formation of a qualitatively different SEI is also suggested by the ICP analysis of harvested graphite electrodes, which shows the presence of Na+ ions in the graphite electrode from a cell with MFS 1−2 (see Table S2), which were shown to impart beneficial properties to the graphite SEI.32 The deficit of F relative to P in the SEI may be in part accounted for by the formation of LiF and its precipitation inside MFS 1−2, as shown by the 19F MAS NMR data from Figure 3a. The 19F MAS NMR data indicate that MFS 1−2 contains 5 times more LiF than the PVDF-HFP separator, in excellent agreement with the results for the Li amounts in the harvested separators determined by ICP-OES (see Table S2). This clearly means deposition of degradation products inside separator pores, away from the electrode surfaces. Note also that no NaF was detected by MAS NMR in the MFS harvested from the cycled cell containing an MFS. Since the MFS also contains significant amounts of trapped Mn2x+ and Mn3+ ions, the small but perceptible (∼15 cm−1) shift of the two strong overlapping IR bands near 1600 cm−1 in the FTIR spectrum of the poly(IDANa2) from the used MFS toward higher wavenumbers (Figure 3b) clearly indicates a substantial substitution of Na+ ions in the poly(IDANa2) of the MFS by Li+ ions during the cell cycling. (The heavier manganese ions coordinated by the IDA groups would cause a shift of the 1600 cm−1 band toward lower wavenumbers, yet the opposite is observed, despite the relatively large Mn amounts found in the MFS from the cycled cell, see Table S2.) This interpretation is also corroborated by the presence of Na+ ions in the graphite electrodes and Li+ in the MFS from the cells with MFSs (see Table S2). In conclusion, we delved herein into the underpinnings for the improvements in Li-ion battery performance enabled by multifunctional, chemically active separators. At the macroscopic (phenomenological) level, the benefits are described by
Figure 2. Low-magnification SEM images for graphite electrode surfaces in (a) pristine state, and after cycling for 4 weeks at 0.2 C rate and 55 °C in LMO-graphite cells with (b) a PP separator or (c) MFS 1−1. High-magnification SEM images of LMO electrode surfaces in (d) pristine state and after cycling in cells with (e) a PP separator and (f) MFS 1−1. SEM images of FIB cross-sections of graphite electrodes from cycled cells containing (g) a PP separator and (h) MFS 1−1. (i) Statistical distribution of the SEI thickness on the graphite electrodes from cells with MFS 1−1 and a PP baseline separator. (j) Crosssectional SEM image displaying a pocket near the electrode surface that comprises a mixture of carbon black, PVdF-HFP binder, and SEI. (k, l) EDX spectra recorded at two locations marked with red circles in panel j.
whereas the LMO particles from electrodes cycled in both types of cells display visible indications of surface roughening due to the electrochemical cycling. Thus, even though some changes are detected in the surface morphology of the LMO electrodes after cycling, it cannot be concluded directly from these features that a cathode−electrolyte interface layer exists on the positive electrode. Nevertheless, the aggregate EIS and SEM data suggest that the presence of the MFS also leads to thinner films on positive electrode surfaces, when compared to cells with a PP separator. Cross-sections of cycled graphite electrodes were examined by FIB-SEM, to evaluate the thickness of their SEIs. Figure 2g (also Figure S3) shows that the SEI on the graphite electrode from the cell with the PP separator is considerably thicker than the SEI on the graphite electrode from the cell containing MFS 1−1, see Figure 2h (also Figure S4). Figure 2i displays the C
DOI: 10.1021/acsaem.8b00436 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
ACS Applied Energy Materials
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hanshuo Liu: 0000-0001-7745-5407 Kristopher J. Harris: 0000-0003-4205-7761 Gianluigi A. Botton: 0000-0002-8746-1146 Gillian R. Goward: 0000-0002-7489-3329 Ion C. Halalay: 0000-0003-0307-8463 Author Contributions
The manuscript was written through the contributions of all authors. All authors contributed to the interpretation of the data. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Partial support for this work was obtained from the Israel Science Foundation in the framework of the INREP project and from the National Science and Engineering Research Council of Canada, Grant CRDPJ494074-16.
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Figure 3. (a) Direct polarization F MAS NMR spectra for a baseline PVdF-HFP separator and MFS 1−2 after high-temperature cycling for 4 weeks at 55 °C in LMO∥graphite cells. (b) FTIR spectra of (1) pristine poly(IDANa2) powder, (2) used MFS 1−2, and (3) used PVdF-HPF baseline separator. The two overlapping bands located between 1550 and 1700 cm−1, and the very weak band at 1500 cm−1, are characteristic for acetate groups.
an increased capacity retention during operational life (mimicked in the lab through electrochemical charging− discharging cycling) as well as reductions of the electrodes’ interfacial resistances early in cell life and a slowing-down of their growth during subsequent battery use. At the microscopic level, we show that the observed performance improvements enabled by MFS are due to lessened parasitic reactions, as reflected in quantitatively dif ferent (thinner) surface films on electrodes (due to fewer degradation products deposited at electrode surfaces) and in their qualitatively dif ferent nature (as shown both by the distribution of local thickness and by the differences in the chemical makeup of the surface films). The lower interfacial resistances enabled by MFSs increase the capacity utilization and improve the power performance of LIBs. Furthermore, the more uniform SEI created in the presence of MFSs enables a more homogeneous current distribution, thus the avoidance of local overcharging and overdischarging of the electrodes during LIB operation. In aggregate, all these effects lead to an improved power performance and an increased LIB life.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00436. Multifunctional separators’ properties; electrode preparation; cell assembly; electrochemical test procedures; and HR-SEM, FIB-SEM, EDX, and ICP-OES data (PDF) D
DOI: 10.1021/acsaem.8b00436 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsaem.8b00436 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
