Direct Observation of Inhomogeneous Solid Electrolyte Interphase on

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Direct Observation of Inhomogeneous Solid Electrolyte Interphase on MnO Anode with Atomic Force Microscopy and Spectroscopy Jie Zhang,†,§,# Rui Wang,‡,# Xiaocheng Yang,†,∥,# Wei Lu,*,† Xiaodong Wu,† Xiaoping Wang,§ Hong Li,‡ and Liwei Chen*,† †

i-LAB, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China § Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026, China ∥ Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China ‡

S Supporting Information *

ABSTRACT: Solid electrolyte interphase (SEI) is an in situ formed thin coating on lithium ion battery (LIB) electrodes. The mechanical property of SEI largely defines the cycling performance and the safety of LIBs but has been rarely investigated. Here, we report quantitatively the Young’s modulus of SEI films on MnO anodes. The inhomogeneity of SEI film in morphology, structure, and mechanical properties provides new insights to the evolution of SEI on electrodes. Furthermore, the quantitative methodology established in this study opens a new approach to direct investigation of SEI properties in various electrode materials systems. KEYWORDS: Lithium-ion battery, solid electrolyte interphase, SEI, atomic force microscopy, force spectroscopy

S

SEI film in both morphology and mechanical properties is quantitatively identified for the first time. Single-layered and double-layered SEI films with similar Young’s modulus distribution coexist on the surface of discharged electrodes. This result provides new insights about previously widely accepted double-layer SEI model.1 Furthermore, the quantitative method established here for the measurement of SEI mechanical properties is widely applicable to various electrode and electrolyte material systems and thus paves an important initial step for rational design of SEI films with desired mechanical properties. MnO films with thickness of 130 nm were prepared on Ti substrates by pulsed laser deposition.10 Five Swagelok-type two-electrode cells were constructed using the same batch of MnO films as the working electrodes and lithium foil as the counter electrodes. The electrolyte was 1 M LiPF6 dissolved in 1:1 volume ratio mixture of ethylene carbonate (EC) and propylene carbonate (PC) (Novolyte Technologies, Inc. Suzhou, China). The five cells were assembled in an argonfilled glovebox (H2O < 1 ppm, Vigor Co., Suzhou, China) and cycled between 0.01 and 3 V using an automatic battery tester (Land Co., Wuhan, China) at a constant current density of 6 μA/cm2 (∼0.11 C). During the first cycle, the discharging of our cells was cutoff at 0.8, 0.3, 0.1, and 0.01 V, respectively and

olid electrolyte interphase (SEI) is known as an electronic insulating but ionic conducting film formed on the surface of the anode and cathode in Li-ion batteries when the insertion potential of the anode is lower than 1.2 V versus Li+/Li and the lithium extraction potential of the cathode is beyond 4.2 V versus Li+/Li.1−5 It is widely accepted that the physical and chemical properties of the SEI film have significant impacts on the electrochemical performances of Li-ion batteries.1−5 Many techniques including optical spectroscopy, electron microscopy, X-ray diffractometry and thermoanalysis have been used for analyzing the composition and microstructure of SEI films.3,6 Great efforts have been paid to modify the mechanical properties of SEI films by adding additives in electrolyte and/ or modifying electrode surfaces, aiming at the formation of elastic and flexible SEI films to accommodate large volume variation during charging/discharging cycles.5 This is especially important for high capacity alloy or conversion reaction type anode materials in order to achieve excellent cycling performance and high Coulombic efficiency. 7,8 However, the mechanical properties of SEI films have been rarely investigated. Recently, surface contact stiffness of the SEI on Si nanowires was measured by Xu et al. using atomic force microscopy (AFM) nanoidentation technique and it was found to increase from 0.43 to 0.96 N m−1 upon addition of alkoxysilane additive in the electrolyte.9 This was the only measurement on mechanical property of SEI films so far. In this work, we investigate the SEI film on MnO thin film anodes using AFM imaging and force spectroscopy. Inhomogeneity of © 2012 American Chemical Society

Received: February 10, 2012 Revised: February 29, 2012 Published: March 2, 2012 2153

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Figure 1. (a) Voltage profiles of Sample I, II, III, IV and V, which were discharged to 0.8, 0.3, 0.1, and 0.01 V and charged to 3.0 V, respectively. (b− g) AFM images (2 × 2 μm2 scan size) of MnO electrode surface of the samples discharge/charge to different voltages during the first cycle at 6 μA/ cm2 in 1 M LiPF6/EC+PC (1:1). Note that the height color scales in (e) and (f) are different from other figures.

formation and growth of the SEI film and excessive lithium storage at the formed Li2O/Mn grain boundary regions.11 During the charging process back to 3.0 V versus Li+/Li, a part of the formed Mn/Li2O converts back into MnO and the SEI film are partially decomposed.11 Therefore, the variation of the surface morphology and roughness should be caused by the conversion reaction and/or the formation and decomposition of SEI. The calculated volume expansion ratio of MnO when fully converted into Li2O/Mn is 169%. This is the maximum possible value assuming a 100% reaction depth. Therefore, the drastic increase of grain size from ∼20 to ∼200 nm and the variation of the surface roughness from 12 to 280 nm after the first discharging should be mainly related to the formation and decomposition of the SEI film. Further evidence of the SEI formation on the MnO electrode after discharging comes from the following “scraping” experiment. The AFM images of the same surface area on MnO electrode of the Sample IV before and after a scraping test are recorded using AC mode, as shown in Figure 2. After the first

the fifth cell was discharged to 0.01 V and then charged to 3.0 V as shown in Figure 1a. Although the five films were prepared in the same batch, slight variations in thickness, density, and discharging reactivity may still exist, which caused the discharging rates of the five cells to be slightly different from each other. The MnO thin film electrodes at different states, hereafter named as Samples I−V, were then dissembled from the cells and washed by anhydrous dimethyl carbonate solvent several times and dried in the vacuum antechamber of the Vigor glovebox. The samples were then transferred to an Agilent 5500 AFM (Agilent Technologies, Santa Clara, CA) equipped with a homemade vacuum glovebox specially designed for oxygen and moisture free AFM environment. Si3N4-coated silicon AFM tips (NSC19/Si3N 4, Mikromasch, Estonia) with a resonant frequency of about 80 kHz and spring constant of about 0.6 nN/nm were used in imaging and force spectroscopy measurements. Figure 1b−g shows atomic force micrographs of the MnO films at different discharging/charging states. As-deposited MnO film exhibits granular morphology with ∼12 nm surface roughness (Figure 1b). Samples I and II, which are MnO electrodes discharged to 0.8 and 0.3 V (Figure 1c,d), show similar morphology with slightly varied surface roughness, ∼17 and 14 nm, respectively. When the MnO thin film is discharged to 0.1 V (Sample III), surface grain size starts to increase and the surface roughness significantly increases to 65 nm (Figure 1e). When the MnO electrode is discharged to 0.01 V (Sample IV), the surface presents ∼200 nm granular deposits and the surface roughness increases to ∼280 nm (Figure 1f). For Sample V, after charging to 3.0 V the large granular deposits disappear and the morphology similar to the original MnO surface appears again with a surface roughness of 11 nm (Figure 1g). The cutting-off voltages of different samples are chosen to reach different lithiation status of the electrodes. The reaction mechanism of MnO with lithium has been elucidated previously.11 For the first cycle, the formation of the SEI in EC-based electrolyte starts at 0.8 V versus Li+/Li. At low voltage range of 0.3−0.01 V versus Li+/Li, lithium is stored via three different mechanisms simultaneously, including conversion reaction of MnO into Li2O/Mn nanocomposite, further

Figure 2. Surface scraping test on MnO electrode discharged to 0.01 V (Sample IV) with a force of 150 nN. AC mode images of Sample IV (a) before and (b) after several contact mode scans over the central 5 × 5 μm2 area under 150 nN normal force. The scale bars are 5 μm.

AC mode scan at the 15 × 15 μm2 area (Figure 2a), contact mode scans were performed at a zoomed-in area of 5 × 5 μm2 in size. A normal force of 150 nN was applied during the contact mode scans. Then a second AC mode image was collected over the original 15 × 15 μm2 area (Figure 2b). A square-shaped recessed area of 5 × 5 μm2 in size is clearly observed. As shown in Figure 2b, the height of the square edges 2154

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increased significantly, indicating the accumulation of some species. Further scraping did not change the surface of inner square area anymore, which means that no further species can be moved under 150 nN force after cleaning out weakly adhered surface species. This result indicates clearly that some species adhere to the surface of MnO thin film electrodes after deep discharging and they can be scraped off. It is highly probable that these species are the SEI film. The scraping experiment also indicates that the mechanical strengths of AFM tip and cantilever are comparable to that of the SEI on MnO electrode. Therefore, AFM is suitable for not only ex situ imaging but also direct probing of the mechanical properties of the SEI. We access the structure, mechanical property, as well as the evolution of the SEI using the force spectroscopy method in the following sections. Actually, AFM force spectroscopy has been widely applied in studying mechanical properties of various materials.13 As shown in Figure 3a, in a force spectroscopy measurement the cantilever

calibrated, we can thus probe the mechanical properties of the sample through force spectroscopy experiments. Hundreds of force curves were obtained on random locations from each of the five samples. NSC19 AFM tips (Mikromasch, Estonia) were used in these experiments with a constant loading rate of 400 nm·s−1. Figure 3b,d show two representative force curves, which are obtained from the Samples III and IV, respectively. By converting the tip deflection signal to normal force F and deriving indentation depth δ from Δz and Δd, the two force curves in Figure 3b,d are transformed into indentation curves shown in Figure 3c,e, respectively. Qualitatively, the shape of the indentation curves exhibits two common features. First, the SEI responds to external mechanical force with initial elastic deformation followed by plastic yield. Using Figure 3c as an example, when the tip is first pushed beyond the contact point (δ = 0 nm), the SEI exhibits elastic deformation with a linear slope in the indentation curve. Upon further indentation, the slope decreases and the indentation curve shows a plateau at which stage the tip penetrates deeper into the SEI with relatively small increase in indentation force. This is the plastic yield region. After the tip penetrates through the entire SEI (∼δ = 43 nm in Figure 3c), the tip hits the solid electrode surface and the slope in the force curve drastically increases.12 The second feature of the indentation curves is that many of the curves exhibit layered structures. While Figure 3b,c shows the process of the tip penetrating a single layer, Figure 3d,e reveals an example of double-layered SEI. The major feature in Figure 3e is a linear elastic region and a plateaued yield region at about 40 < δ < 110 nm, but interestingly, another set of elastic and yield regions are clearly observed at about 0 < δ < 40 nm (Figure 3e inset). This suggests that the SEI at this particular location has two layers. The finding of the layered structure is consistent with previously proposed compactstratified layer (CSL) model14 or multilayered model.15 Quantitative information regarding the SEI, such as the Young’s modulus and the layer thickness, can also be extracted from the indentation curves. Because the indentation depth in our experiments is much larger than the AFM tip radius (∼10 nm), a modified Hertz model, Sneddon model, is employed to describe the indentation behavior.16,17 Approximating the indentation process as a conical tip pushing into a flat surface, the Sneddon model obtain the relationship between the loading force F and indentation depth δ in the elastic regime as F = (2/π)(E/[π(1 − ν2)]δ2 tan(α), in which E is the Young’s modulus, α = 15° is the half cone angle of the AFM tip, and ν is the Poisson ratio, which is set to be 0.5, assuming rubber elasticity for SEI films at the early stage of elastic region. Thus, Young’s modulus can be obtained from the elastic region of experimental indentation curves (see Supporting Information for details). The thickness of an SEI layer is the indentation depth from the beginning of the elastic region to the end of the yield region. For example, a Young’s modulus of 1.1 GPa and a thickness of 43 nm were obtained for the single-layered SEI in Figure 3b,c. For the double-layered force curve in Figure 3d,e, we obtain a modulus of 16 MPa and a thickness of 40 nm for the outer layer, and a modulus of 540 MPa and a thickness 70 nm for the inner layer. Previous AFM force spectroscopy studies reported that Young’s modulus of most polymeric materials is around tens of MPa,18 and that of inorganic materials is much larger. On the basis of the quantitative Young’s modulus values, we found that most of the inner layers

Figure 3. (a) Schematic illustration of AFM force spectroscopy measurements. When the tip is pushed to the sample beyond the contact point with Δz distance, the tip may indented into the sample with δ depth and the cantilever may deflect upward for Δd displacement, where Δz = δ + Δd. (b,d) Typical force curves showing mechanical response of single-layered and double-layered SEI films, respectively. (c,e) Indentation curves converted from (b,d), respectively. The curves have been shifted horizontally to make z = 0 at the contact point, and shifted vertically to make d = 0 when tip is far away from surface.12.

deflection signal, d, is monitored as a function of z-piezo position, z, while the z-piezo drives the cantilever toward the sample surface. A flat line is observed in the force curve until the AFM tip touches the surface of the sample. Further advancement in z-piezo position results in both cantilever deflection (Δd) and tip indentation into the sample (δ), with the relationship Δz = Δd + δ. For a certain sum of Δz, the relative partition between Δd and δ is determined by the mechanical properties of the cantilever and the sample as well as the tip. Since the cantilever stiffness can be accurately 2155

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in the double-layered SEIs are inorganic materials and the soft outer layers are likely to be organic materials in nature. Systematic investigation on the sets of force spectroscopy curves from the Samples I−V reveals interesting information about the evolution of SEI on MnO anode. First of all, it is important to notice that the distribution of SEI on MnO surface is highly inhomogeneous. For all Samples I−V, significant portion of the force curves do not show the characteristic elastic deformation-plastic yield response of SEI (see Supporting Information Table S1 for detailed statistics). This indicates that the MnO electrode surfaces are only partially covered by SEI films at all discharging/charging states during the first cycle. Second, Samples I, II, and V display thin and hard SEI; on the other hand, Samples III and IV have developed much thicker and softer SEI (Supporting Information Figure S1, Table S1). It was previously proposed that1,19 the formation of SEI films on anodes during discharging is related mainly to the following processes: solvent molecules get electrons at the surface of the anode and are reduced into anions or free-radicals, which react with free lithium ions in the electrolyte, forming insoluble deposition on the surface of the anode. It was reported that for graphite anode, the two electron reduction process for carbonate solvents tends to form Li2CO3 at high voltage (>0.8 V) and single electron reduction process tends to form lithium alkyl carbonate, lithium oligomer, and lithium polymer at low voltage (1 GPa. This indicates that the soft organic SEI layer on the MnO electrode surface could decompose after charging back to 3.0 V while the inorganic SEI layer partially remains. This suggests that the soft SEI films formed at 0.1 to 0.01 V are unstable, which agrees with TEM observation.11 Unstable SEI could reduce the cycling performance of anodes.22 In summary, AFM topographical imaging and force spectroscopy measurements are employed to examine the morphology, structure, mechanical property, and the evolution of SEI on MnO anode. For the first time, we demonstrated quantitatively the inhomogeneity of SEI films in both morphological and mechanical properties. The methodology established in this study is generally applicable to the investigation of SEI in lithium ion battery systems. Combining both mechanical indentation and spatial mapping capabilities at the nanometer scale, AFM imaging and spectroscopy techniques are uniquely suited to evaluate the effect of different solvents, salts, and additives in the electrolyte on the properties of the SEI film on various electrode materials, as well as the mechanical properties of active electrode materials and binders during electrochemical charging and discharging.

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ASSOCIATED CONTENT

S Supporting Information *

Description of Sneddon model for the calculation of Young’s modulus, statistical data on SEI morphology, and film thickness. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (L.C.) [email protected]; (W.L.) wlu2008@ sinano.ac.cn. 2156

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Author Contributions #

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by National Basic Research Program of China (2010CB934700), National Natural Science Foundation of China (Nos. 20973122, 10904105), and Knowledge Innovation Program (No.KJCX2-YW-H21) of Chinese Academy of Sciences. L.C. thanks the Chinese Academy of Sciences Hundred Talents program.

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REFERENCES

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