Electrode Interfaces

Deformation and Chemomechanical Degradation at Solid Electrolyte−Electrode Interfaces Xin Su,†,§ Kai Guo,† Teng Ma,† Prabhakar A. Tamirisa,‡ Hui Ye,‡ Huajian Gao,† and Brian W. Sheldon†,* †

School of Engineering, Brown University, Providence, Rhode Island 02912, United States Medtronic Energy and Component Center, 6700 Shingle Creek Parkway, Brooklyn Center, Minnesota 55430, United States § Chemical Science and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439-4837, United States ‡

S Supporting Information *

ABSTRACT: Solid electrolytes in batteries are inevitably subjected to mechanical strains when the active materials undergo chemically induced volume changes. It is difficult to probe these effects in complex battery structures. Thus, we developed a new in situ method to monitor mechanical deformation during electrochemical cycling, using simplified thin-film structures. This approach was applied to polymer electrolytes on V2O5−x thin-film electrodes. Analysis of these deflection measurements was performed with a finite element model. The results indicate that the electrolyte compliance is rate-dependent and that it varies with the polymer molecular weight. Our approach was also employed to investigate interactions between chemical and mechanical changes at the solid electrolyte−electrode interface. Here, in situ stress studies were combined with impedance spectroscopy and ex situ peel tests. These results show that interfacial chemistry changes during electrochemical cycling lead to a significant decrease in the electrolyte−electrode adhesion energy.

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mechanical integrity of these interfaces has not been previously studied. In conjunction with the PEO we employed thin-film V2O5−x cathodes. This family of materials exhibits high reversible capacities (up to 294 mAh/g) in the voltage range of 2.0−4.0 V.21−23 Lithium polymer batteries with vanadium pentoxide cathodes have attracted attention for various applications.24,25 The expansion and contraction of vanadium oxide electrodes during lithiation−delithiation cycling will lead to deformation in the PEO electrolyte. As noted above, the electrochemical and mechanical properties of the interface between electrode materials and PEO electrolyte are expected to be critical.4,10−12,26,27 The thin-film approach that we present here makes it possible to study these properties simultaneously. Finite element analysis was also employed to fully interpret the results obtained from the new in situ methodology that we are presenting here.

olid electrolytes offer promising solutions for addressing the safety issues that are associated with the liquid electrolytes that are widely used in Li ion batteries.1−6 The resulting interfaces with active electrode materials are inevitably subjected to mechanical strains as lithiation and delithiation lead to volume changes in the active materials. The corresponding mechanical degradation of these interfaces presents substantial challenges; however, it is difficult to directly investigate these effects in practical electrodes with complex architectures. In the work presented here, we have probed mechanical deformation at cathode−solid electrolyte interfaces with a novel in situ approach. A model thin film geometry is employed for this work, which greatly facilitates interpretation of the measurements. Poly(ethylene oxide) (PEO) was selected for this initial investigation, largely because it exhibits both low elastic modulus and high ionic conductivity.3−5,7−17 In these materials, it is also well-established that the conductivity across the electrode−solid electrolyte interfaces is a critical property that can degrade under some conditions.17−20 In spite of the extensive prior work on PEO, the relationship between the conductivity and the © XXXX American Chemical Society

Received: June 3, 2017 Accepted: June 26, 2017

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DOI: 10.1021/acsenergylett.7b00481 ACS Energy Lett. 2017, 2, 1729−1733

Letter

http://pubs.acs.org/journal/aelccp

Letter

ACS Energy Letters

Supporting Information, in the liquid electrolyte, Fnom provides a direct measure of the average stress in the oxide film according to the Stoney equation (eq 1 in Methods in the Supporting Information). The roughly linear relationship between time and stress implies that the product of the elastic modulus and the partial molar volume of Li in this amorphous material is essentially constant. To our knowledge, an independent measurement of the elastic constants for the V2O5−x films is not available. However, with a rough estimate of M ∼ 100 GPa, the measured stress of V2O5−x film in liquid electrolyte in Figure F 2a (⟨σ ⟩ = nom = 0.4 GPa) corresponds to an elastic strain of h ∼0.4% and 0.3 μmole of Li added. The results in Figure 2a compare the stress responses in PEO and the liquid electrolyte. Parallel measurements like this were conducted to compare Fnom at roughly the same state of charge (SOC). While both experiments were run at the same current, the higher conductivity in the liquid leads to a slightly smaller overpotential and a corresponding increase in capacity (i.e., because the experiment runs longer to reach the same final potential). To account for this, the results with the liquid electrolyte were normalized to show Fnom values at roughly equivalent SOC. This adjustment consists of multiplying the capacity and Fnom values by the same factor (typically between 0.8 and 0.9), so that the voltage traces in the first cycle are precisely aligned on the horizontal axis. This simple approach provides an accurate comparison here, largely because the Fnom versus capacity response of the V2O5−x cathode films is linear. In Figure 2a it is important to note that at a given SOC the stress in the electrode film should be the same in both cases. Thus, the smaller Fnom values obtained with the PEO are consistent with the idea that the solid electrolyte limits the amount of bending that occurs. As noted in the Supporting Information, for the liquid case eq 1 leads to Fnom = ⟨σ⟩ h (via the Stoney equation). When PEO is employed instead, the stress in the film applies the same membrane force f, but the solid electrolyte restricts the bending

Batteries of V2O5−x/liquid electrolyte/Li and V2O5−x/PEO electrolyte/Li were assembled for in situ stress measurement at room temperature according to the configurations in Figure 1. The as-deposited V2O5−x films employed here are amorphous and featureless (X-ray diffraction and scanning electron microscopy evidence are shown in Figure S1a,b).

Figure 1. Schematics of 50 nm V2O5−x film in liquid electrolyte (a) and PEO electrolyte (b) for in situ stress measurement during the charge−discharge cycling test.

In situ stress measurements provide information about the mechanical response of these materials during electrochemical cycling. Results during galvanostatic cycling at 1 μA are shown in Figure 2a. The measured values of Fnom in Figure 2a show compressive stress as Li is added and an equivalent tensile stress when Li is removed. This reversible response is indicative of elastic deformation in the film. As noted in Methods in the

Figure 2. (a) In situ stress evolution of V2O5−x film in PEO electrolyte (5000 kDa, red curve) and liquid electrolyte (normalized, black curve); (b) peeling test of PEO electrolyte to V2O5−x film and (c) impedance measurement at 50% SOC of V2O5−x film in PEO electrolyte before and after the cycling test (inset: zoom in high-frequency region); (d) impedance measurement at 50% SOC of V2O5−x film in liquid electrolyte before and after the cycling test (inset: zoom in high-frequency region). 1730

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Figure 3. In situ stress evolution of V2O5−x film in PEO electrolyte with molecular weight of 5000 kDa in red curve (a) and 100 kDa in red curve (b) during the charge−discharge cycling test of the solid-state lithium battery; (c) impedance of V2O5−x film (0% SOC) in PEO electrolyte with different molecular weights of 5000 kDa (□),100 kDa (△), and liquid electrolyte(▲) before the cycling tests.

of the film−substrate system. Direct evidence for this additional constraint is seen in Figure 2a, where the magnitude of Fnom during cycling is reduced. Here the 5000 kDa PEO leads to bending which is less than one-third of that observed in the liquid electrolyte (i.e., decreasing to ∼6 GPa·nm from ∼20 GPa·nm) after normalizing charged−discharged lithium to the same amount. Analysis of these Fnom measurements is presented below. Basic measurements for the adhesion energy of the PEO electrolyte and V2O5−x electrode before and after electrochemical cycling were conducted with peel tests. The results in Figure 2b show that cycling led to a significant decrease in adhesion. The adhesion energy of PEO electrolyte to the vanadium oxide decreases from 6.8 to 1.6 J/m2 after the cycling test for in situ stress measurements. This decrease indicates that significant mechanical degradation of the interface has been induced by cycling. Corresponding electrochemical impedance spectroscopy (EIS) measurements before and after the cycling tests are also reported in Figure 2c,d. In these Nyquist plots there is evidence for only one depressed semicircle, which is commonly attributed to surface processes which consist of both charge transfer and other interfacial phenomena such as transport through a surface passivation film (i.e, the solid electrolyte interphase). In addition to this “surface resistance”, the intercept value was also used as a measure of “contact resistance” (with the PEO, this is expected to include a substantial contribution from the electrolyte conductivity). In the current study, these resistance values were used only for relative comparisons. The contact resistance and surface resistance of the EIS for V2O5−x electrode/PEO electrolyte/lithium are ∼400 Ω and ∼6 kΩ, respectively. These are similar to reported values for the interface between cathode and PEO electrolyte. All of the surface resistance values we obtained are much larger than the expected resistance values for the lithium metal interfaces.28,29 This implies that the measured surface resistances should be dominated by the V 2 O 5−x

electrode−electrolyte interface. Thus, the comparisons in Figure 2 show that the surface resistance of the PEO−cathode interface increases significantly after cycling, whereas the surface resistance with the liquid electrolyte remained the same or decreased slightly after the same cycling test. On the basis of these observations, we believe that the increased impedance at the V2O5−x electrode−PEO interface is likely to be related to the mechanical degradation that was observed. In situ stress measurements were also performed with lower molecular weight PEO. As shown in Figure 3a,b, the lower molecular weight (and hence lower stiffness) of the PEO led to a significant increase in the Fnom amplitude. Here the full cycle amplitude is ∼15 GPa·nm, which is close to that in the case of a liquid electrolyte. As expected, the softer PEO allows the substrate/V2O5−x film system to bend much more easily, compared to stiffer electrolytes. Impedance spectroscopy was also used to characterize the electrochemical properties of electrode−PEO interface. The results in Figure 3c show that the contact resistance from EIS for the V2O5−x electrode in liquid electrolyte is the smallest (15 Ω), as expected. The initial contact resistance from EIS for the V2O5−x electrode in PEO electrolyte increases from 234 to 1075 Ω as the PEO molecular weight increases from 100 to 5000 kDa (i.e., PEO electrolyte has lower conductivity than the liquid electrolyte, and PEO electrolyte with lower molecular weight of PEO has higher conductivity, as expected). It was not possible to perform peel tests with the much softer 100 kDa PEO. On the basis of the smooth and highly reversible stress response measured with the solid electrolytes, the electrode− electrolyte interface appears to be mechanically intact during cycling. This implies that the sharp decrease in adhesion energy that was produced by cycling was caused by the electrochemical changes at the interface (i.e., as opposed to the reverse situation, where mechanical degradation led to increased impedance). In considering this interpretation, it should also be noted that the observed stiffness of the PEO electrolyte was quite low. The PEO 1731

DOI: 10.1021/acsenergylett.7b00481 ACS Energy Lett. 2017, 2, 1729−1733

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ACS Energy Letters

however, as the electrolyte modulus increases, the profile changes significantly. The in situ optical technique employed here does not provide a precise measurement of this profile (i.e., the lateral resolution is limited by spacing between adjacent laser spots on the detector). However, a comparison between the spot spacing near the center and that near the edge of the wafer is possible. With liquid electrolyte there is no discernible difference, in agreement with the FEM analysis. With the 5000 kDa PEO the stress-thickness measured near the center of the sample is nearly uniform, while there are significant variations close to the clamped side or near the opposite edge. These boundary effects are similar to the FEM predictions shown in Figure S2, based on the long-term modulus of 0.01 MPa. The agreement between the experiment and the FEM-predicted profiles verifies that the low long-term modulus for the PEO is fully consistent with the experimental observations. Here, it is important to note that other long-term modulus values lead to FEM predictions which are substantially different from the curvatures at the center and edge of the specimen that were observed in the actual experiments. Beyond the work reported in this Letter, the modified MOSS technique developed here should be useful for other types of investigations. If the mechanical properties of the electrolyte are accurately known, the FEM can be used to evaluate stress evolution in the electrode material (i.e., analogous to conventional MOSS). This is essentially the inverse of the approach used in the current Letter, where the solid electrolyte properties were probed (i.e., by comparing results obtained with solid and liquid electrolytes). In summary, we have also developed a new in situ method to monitor the deformation of a battery stack during cycling. This was employed to explore the mechanical and electrochemical degradation of the interface between the electrode and polymer electrolyte. These measurements demonstrate that the PEO electrolyte is essentially elastic at the relatively long time scales used for these experiments. Fitting the measurements with a finite element model indicates that the long-term modulus is 0.02 MPa for 5000 kDa PEO and that lower molecular weights lead to even lower moduli. The mechanical response of the PEO that was observed also has potentially important implications, because it shows that the stiffness of the electrolyte can change substantially over the time scales that are relevant for different charge and discharge rates. Even with the low long-term modulus, the strain evolution of the V2O5−x electrodes in 5000 kDa PEO is significantly confined, whereas PEO with lower molecular weight exhibits a response which is similar to that of a liquid electrolyte. The results also demonstrate that electrochemical cycling leads to substantial changes in the properties of the PEO/V2O5−x interface. The significant decreases in adhesion that were measured are potentially problematic in batteries that employ these materials. The careful wafer curvature measurements that were performed indicate that debonding does not occur during the gentle cycling conditions that were employed, in spite of the low adhesion energy. Thus, the corresponding increase in the resistance of these interfaces appears to be primarily driven by chemical changes, which then lead to the decreased adhesion.

used in our experiments are gel-like materials. Previously reported direct nanoindentation of the 5000 kDa PEO gives a Young’s modulus close to 1 GPa.16 However, the time scale of each charging−discharging half cycle (about 7 h) is much longer than that in the nanoidentation tests. Thus, it is important to consider the viscoelastic response of the PEO electrolytes. The multibeam optical stress sensor (MOSS) measurements made it possible to obtain this type of information under the conditions that were used for the actual cycling experiments. At this slow strain rate of 0.06% per hour, the mechanical response of the viscoelastic PEO electrolyte is governed by its “long-term modulus”. Consequently, in the numerical simulations the PEO electrolyte was modeled as an elastic material with a low Young’s modulus, referred to as the long-term modulus of PEO electrolyte. Calculated Fnom for full lithiation of V2O5−x films in PEO electrolyte are plotted in Figure 4b, for different values of

Figure 4. (a) Structure and boundary conditions of the FEM model. Yellow area denotes the region where stress-thickness was measured in MOSS experiment. (b) The stress-thickness’s amplitude of the V2O5−x film’s center during the stress evolution in PEO electrolyte vs long-term modulus of PEO electrolyte using the finite element simulations.

the long-term modulus. When the PEO has a very low long-term modulus, Fnom is similar to the measured values obtained with a liquid electrolyte. This also matches the in situ stress measurements with 100 kDa PEO in Figure 3b. As the long-term modulus increases, the predicted Fnom amplitude increases from the liquid electrolyte value. The experimentally measured value of Fnom in 5000 kDa PEO is also highlighted in Figure 4b. This shows that a long-term modulus here of 0.02 MPa accurately describes the measurements. Note that this is several orders of magnitude lower than the instantaneous modulus obtained with nanoindentation. To verify this interpretation, a finite element method (FEM) analysis was also performed to predict the bending of the entire specimen. The FEM for several examples are shown in Figure S2. Here the clamped edge can produce some asymmetry in the curvature profile. In liquid electrolyte this effect is minimal;

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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/acsenergylett.7b00481. 1732

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Descriptions of the methods employed for the experiments and for the finite element modeling, including more detailed characterization data for the as-synthesized vanadium oxide films and more detailed FEM results (PDF)

AUTHOR INFORMATION

Corresponding Author

*182 Hope Street, Providence, RI 02912, United States. Tel: +1 (401) 863 2866. E-mail: [email protected]. ORCID

Xin Su: 0000-0002-1615-2856 Brian W. Sheldon: 0000-0002-9593-891X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from Medtronic Inc. and NSF (Award DMR-1410946). REFERENCES

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DOI: 10.1021/acsenergylett.7b00481 ACS Energy Lett. 2017, 2, 1729−1733