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Cite This: J. Phys. Chem. C 2017, 121, 24733-24739

Online Digital Holographic Method for Interface Reaction Monitoring in Lithium-Ion Batteries Chao Lai,† Boyu Yuan,† Hongliu Dai,† Kai Xi,‡ Christopher J. Harris,‡ Chao Wang,*,†,§ and R. Vasant Kumar‡ †

School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou, Jiangsu 221116, P. R. China Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom § College of Chemistry and Chemical Engineering, Xuzhou University of Technology, Xuzhou, Jiangsu 221000, P. R. China ‡

S Supporting Information *

ABSTRACT: Understanding the reaction mechanisms at the interface of electrode and electrolyte is both of fundamental interest and essential to improve lithium-ion battery (LIB) performance. Herein, we report an online digital holographic method to in situ observe the entire interface change between electrode and electrolyte in lithium-ions batteries. The accuracy of this technology is well verified in LiFePO4/graphite full-cell systems, graphite/Li half-cell systems in EC-based and PC-based electrolyte, respectively, and supported by the characterized results of conventional instruments, including scanning electron microscopy and X-ray photoelectron spectroscopy. In particular, the time resolution of the digital holographic method is 0.04 s and fast enough to distinguish detail reduction process of ethylene carbonate (EC), for which EC will be first reduced to generate lithium alkyl carbonates, and then the reduction product is Li2CO3 to form a stable SEI films. To our best of knowledge, this is the first report on the reduction order of the EC solvent and can act as an effective complement to understanding the formation mechanisms of SEI films.

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photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman, scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning electrochemical microscopy (SECM), electrochemical impedance spectroscopy (EIS), nuclear magnetic resonance (NMR), and mass spectroscopy (MS).7−10,17−21 Recently, in situ atomic force microscopy (AFM) imaging was also used to present realtime views of the morphological evolution of SEI films.21,22 However, due to the complexity of the interactions between electrode and electrolyte, issues such as detailed reaction process, composition, stability, and influence on battery performance of the SEI films are still open for discussion.7 Furthermore, since SEI films are mechanically fragile and highly sensitive to degradation in air, further damage could happen under the highly energetic beam spot employed in techniques such as XPS, SEM, AFM, and TEM. Therefore, an experimental technique that can monitor the SEI formation in situ without causing severe structural damage is needed. Recently, digital holography based on sensing the variations in refractive index of a certain medium has been used for the in situ monitoring of various dynamic processes, including

he search is on for enhancing the energy density and safety of the battery beyond the traditional Li-ion setup involving graphite anode and LiCoO2 cathode for potential applications in large-scale energy storage, hybrid electric vehicles (HEVs), and electric vehicles (EVs).1,2 Thus, various high-energy materials, such as silicon-based anodes, highvoltage cathodes, and sulfur-based cathodes have been developed. While all these approaches showed enhanced specific capacity and energy density, the battery suffers from poor cycle performance and safety.3−6 Minimizing the reactions between electrodes and electrolytes, as well as keeping smooth transport pathways, are key to ensuring batteries with long cycle life, high safety, and high energy after cycling.7−16 Understanding the reaction mechanisms at the interface of the electrode and the electrolyte, mainly about solid electrolyte interphase (SEI) films, is prerequisite to addressing the above challenges, while this is highly dependent on the developing of novel in situ monitoring technologies. Solid electrolyte interphase (SEI) films, arising from the reduction of electrolyte on electrodes (e.g., graphite, carbon, Li metal, Si), are widespread in different battery systems.7−16 SEI film is an electronic insulator, but a good lithium-ion conductor and stable SEI film are mandatory for the safety, calendar life, and cycle life of batteries, especially during fast cycling.7−10,16 The formation of SEI films on graphite anodes has been widely characterized by various ex-situ techniques, such as X-ray © 2017 American Chemical Society

Received: October 6, 2017 Revised: October 18, 2017 Published: October 18, 2017 24733

DOI: 10.1021/acs.jpcc.7b09920 J. Phys. Chem. C 2017, 121, 24733−24739

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The Journal of Physical Chemistry C Scheme 1. Diagram of the Optical Setup of the Digital Holography System Used in Testing a Lithium-Ion Batterya

a M: mirror; BS: beam splitter; SF: spatial filter; L: lens; CCD: charge coupled device; O: observation electrode; C: counter electrode. The laser beam only passes through one side of the electrode−electrolyte interface.

where k is the concentrative refractivity; λ0 is the wavelength of the laser light; d means the geometrical path length where a refractive index variation exists.29 In an electrochemical system, semiquantitative analysis can be achieved by use of holography. Additionally, it is worth mentioned that this method enabled the easy realization of in situ observation of the entire process of SEI film by merely using the conventional electrodes; while special treatments of electrodes are always necessary in other in situ technologies, such as in situ TEM.8,9,30 The direct information concerning the electrode/electrolyte interface obtained by such means can provide new insights into the mechanisms of many electrochemical processes happening on the electrode/electrolyte interface. The electrochemical performance of the LiFePO4/Graphite full cell tested in a transparent mold (Figure S1 of the Supporting Information, SI) is first presented in Figure 1a. LiPF6 (1 M) dissolved in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) with a volume ratio of 1:1:1 was used as the electrolyte, designated as EC/EMC/DMC electrolytes. As shown, two typical peaks centered at 3.87 and 2.87 V can be observed, corresponding to the Fe2+/Fe3+ redox couple reaction.31,32 As compared to previous reports, the shift of peaks during the anodic and cathodic process can be attributed to the polarization resulting from the larger distance between electrodes in contrast to those coin cells. Figure 1b shows reconstructed phase maps from the initial interferograms (Figure S2) according to eq 1, with each map corresponding to the selected potentials labeled in Figure 1a. In each phase map, the left part corresponds to the electrode, and the right part, represents the electrolyte. The solid-solution interface is plotted in Figure 1c. Any change in the dissolved species can lead to a shift in the phase difference (ΔΦ) at the interface, where ΔΦ > 0 signifies an increase in concentration, and ΔΦ < 0 equates to a decrease. Such a shift is observed as a color change in Figure 1b, as a result of the ongoing electrochemical reaction. It can be observed that at the

electrochemical reactions.24−28 Digital holography can probe electrochemical reactions at the electrode/electrolyte interface by measuring the optical path length (OPL) distribution, which allows transparent samples to be described with diffraction limited transverse resolution and a subwavelength axial accuracy. The advantages of this modern full-field optical method are the nondestructive working principle, fast response, and advanced system performance.24−28 Therefore, the digital holographic method can be expected to in situ detect the entire process of SEI formation and provides detailed information that was not available before. Scheme 1 illustrates the optical setup of the digital holography method based on a Mach−Zehnder interferometer. A light beam with the wavelength of 632.8 nm was generated by an He−Ne laser. It was split into two beams by the beam splitter: the one passed through L3, cell and L2 acted as object beam and the other through L1 as reference beam. The distances between the transparent cell and two lenses (L2, L3) are about 245 mm (L2) and 180 mm (L3), respectively. The object beam transmitted the electrolyte near by the working electrode in the electrochemical cell, which finally combined with the reference beam after a beam splitter cube. Changes in concentration caused by the electrochemical reaction at the electrode/electrolyte interface lead to changes in the refractive index and thus variations in the phase of the object beam as it passes through the surface of the electrode, as shown in Scheme. 1. This phase change can be encoded in holograph form after being recorded via a CCD image sensor. The actual spatial resolution is 1.7 μm, and the time resolution is 0.04 s in the experiment. The principle of this experiment is based on the relationship between the solution concentration (ΔC), the solution refractive index (Δn), and the phase difference (ΔΦ), which was formulated as follows: ΔC = k × Δn = (kλ 0 /2πd)ΔΦ

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DOI: 10.1021/acs.jpcc.7b09920 J. Phys. Chem. C 2017, 121, 24733−24739

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Figure 1. (a) Cyclic voltammogram of a LiFePO4/graphite full-cell tested in a transparent mold at a scan rate of 0.1 mV s−1; (b) the corresponding distributions of the optical difference at various times relative to the initial time and corresponding to different CV potentials. The observation electrode is graphite.

above, it is clear that digital holography is a nondestructive, responsive, and effective method to investigate the interfacial reactions of LIBs. In particular, digital holography is employed to investigate the formation of an SEI film on the graphite to further illustrate its advantages as an in situ monitoring technology. The recorded CVs (Figure 2a) are in agreement with those reported in the literature for graphite electrode. During the cathodic process, there are three distinct processes can be observed in the first cycle at 0.790, 0.770, and 0.525 V vs Li+/Li (Figure 2d,f,h). These peaks are associated with three sudden reaction steps depleting the dissolving species in the electrolyte. As the electrochemical reaction proceeds, a continuous and gradual consumption of interfacial ions can be observed, due to the intercalation of Li-ions into the graphitic lattice, as shown in Figure 2j,k. The phase maps recorded at the second cycle (Figure S5) shows almost no differences between the electrolyte and graphite interfaces in the range of 0.815− 0.500 V, suggesting that a stable SEI film has been produced

beginning of the anodic process, there is a significant increase in concentration due to the electro-migration of electrolyte ions (Figure 1d). When lithium-ions are intercalated into the graphite anode, the consumption of dissolved ions at the interface can be observed in Figure 1e and the resulting decrease can be observed in Figure 1f, due to the massive extraction of ions at the peak of the cyclic voltammagram. During the cathodic process, a similar phenomenon can be observed, for which the decrease of dissolved species can be observed in Figure 1h due to the migration of ions to cathode, while the increase of interfacial concentration is presented resulting from the massive deintercalation of lithium ions and other matching ions at the graphite anode. In order to further verify the digital holographic method, we have also observed the LiFePO4 cathode, and results are given in Figure S3. The results match well with the change in graphite anode. The rise and fall in the interfacial concentrations can be displayed more directly in the three-dimensional phase maps presented in Figure S4 for graphite anode. On the basis of all the results 24735

DOI: 10.1021/acs.jpcc.7b09920 J. Phys. Chem. C 2017, 121, 24733−24739

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Figure 2. (a) Cyclic voltammogram of a graphite/lithium half-cell at a scan rate of 0.5 mV s−1; (b) the corresponding distribution of the optical difference at varying potentials relative to the initial potential (point a) and corresponding to different CV potentials. The observation electrode is graphite.

six components at 284.4, 285.4, 286.4, 287.5, 288.7, and 289.9 eV, which may be attributed to the CC bonds in graphite, in addition to C−C, C−O, CO, COO, and CO3 bonds, respectively.8,9,13,14,19,23 The peak at 289.9 eV is typically designated as the carbonate species in Li2CO3. It is observed that the Li2CO3 content initially increases after cycling to 0.650 V and is then unchanged after further cycling to 0 V, suggesting that the second reaction (point f) may relate to a reduction of the carbonate solvent to generate Li2CO3. Such a hypothesis may be further confirmed by results of the Li 1s spectra. Figure 3b,d,f shows a clear increase in Li2CO3 content. The other two peaks at 55.6 and 54.9 eV can be designated to Li−F from precipitates of LiF, and Li−OC originating from lithium alkyl carbonates or their polymers, respectively.8,9,19,23 It is also observed that the content of lithium alkyl carbonates continues to increase as the electrochemical reaction proceeds, indicating that the reduction of the carbonate solvent into lithium alkyl carbonates is conducted first and followed by similar reduction reactions throughout the entire electrochemical process. The

during the initial cycle, which then blocks the further reduction of electrolyte and electrolyte salt. The real-time changes of the ions concentration at the interface of the graphite anode are visualized in the recorded video with the consecutive holograms collected during the first cycle (Movies S1 and S2 in the SI). It can be observed from the video that the reduction of electrolyte commences at 0.815 V and proceeds throughout the whole electrochemical process until 0.460 V to form a stable SEI film. Meanwhile, there are three rapid electrolyte consuming processes similar to those shown in Figure 2b (points d, f, and h). To gain more insights about the three reactions that cause the rapid electrolyte depletion, the surface morphology and composition of the graphite anode at different cutoff potentials were investigated by scanning electron microscopy (SEM) and XPS analysis. As given in Figure S6, Li, C, O, F, and P can be detected at different cutoff potentials in the XPS survey scans. Figure 3 shows the C 1s and Li 1s spectra of a graphite anode cycled in EC/EMC/DMC electrolytes. The C 1s core spectrum shows 24736

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Figure 3. C 1s and Li 1s spectra of the SEI film on the surface of the graphite anode after discharge to point e (a,b), point g (c,d), and point k (e,f) in Figure 2, respectively.

respectively. The element mappings of the graphite anode after discharge to 0 V are also given in Figure 4e, and it is obvious that the SEI film contains carbon, fluorine, and oxygen element, well consistent with the results of XPS. In EC/EMC/DMC electrolytes, EC is preferably reduced due to its high polarity and dielectric constant.8,16,34 Thus, combined with the results of digital holographic tests and XPS, the first-step reaction at 0.790 V (point d) can be attributed to the reduction of EC into lithium alkyl carbonates,8−10 as illustrated in eq 2:

third obvious consuming reaction occurs at 0.525 V, which can be attributed the reduction of electrolyte salts (LiPF6) and can thus explain the increasing content of LiF after cycling to 0 V.33 The formation of SEI films can be more clearly shown in the SEM images in Figures 4a−d and S7. When the cutoff potential is 0.780 V (point c in the first cycle), a thin layer has begun to form on the surface of the graphite electrodes, indicating that the SEI film is wrapping the graphite particles at this early stage of electrochemical scanning. This coating becomes thicker and denser when the cutoff potentials are set at 0.650 and 0 V, 24737

DOI: 10.1021/acs.jpcc.7b09920 J. Phys. Chem. C 2017, 121, 24733−24739

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PC does not produce an effective passive surface layer.8,9,35 Such a phenomenon is well in accordance with previous reports, further confirming the feasibility of digital holography as a means to investigate the electrochemical mechanisms of lithium-ion batteries. In summary, an online digital holographic method has been developed and used to record real-time images of the electrochemical processes that take place on the interface of LIBs. LiFePO4/graphite full-cell systems, graphite/Li half-cell systems in EC-based and PC-based electrolyte are investigated, respectively, and SEM and XPS testing are conducted to support our hypothesis. Especially, the dynamic formation of the SEI layer is recorded on video allowing us to draw a comprehensive mechanism of the formation. The SEI film formation is taking place on three steps in an EC-based electrolyte: (1) the reduction of EC to generate lithium alkyl carbonates, (2) the reduction of EC to produce Li2CO3, and (3) the decomposition of the electrolyte salts, which takes place slightly before the lithium-ion intercalation. Our results primarily present a detailed reduction mechanism of EC solvent in batteries, and this new insight will open the door for better understanding the formation of SEI films. This technique also can be applied to other fields, such as supercapacitors and catalysts.

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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/acs.jpcc.7b09920. Experimental sections and characterizations (PDF) In situ video (Movie S1) (AVI) In situ video (Movie S2) (AVI) In situ video (Movie S3) (AVI)

Figure 4. SEM images of the pristine graphite electrode (a) and the electrode after discharging to point e (b), point g (c), and point k (d) in Figure 2; SEM images and element mapping of cycled electrode after discharging to point k (e) in Figure 2.

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*E-mail: [email protected] (C.W.).

2EC + 2e− + 2Li+ → (CH 2OCO2 Li)2 + CH 2CH 2

ORCID

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Chao Lai: 0000-0002-6021-6343

As EC solvent is consumed, the content of EC at the interface will dramatically decrease, after which the reduction of EC is conducted by another reaction,8−10 as eq 3: EC + 2e− + 2Li+ → Li 2CO3 + CH 2CH 2

AUTHOR INFORMATION

Corresponding Author

Author Contributions

C.L., B.Y., and H.D. contributed equally to this work. Notes

The authors declare no competing financial interest.

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After cycling to 0.525 V, the reduction of electrolyte salts begins.8,33,34 However, due to the formation of a passive layer on the surface of the graphite electrode at high potential, the transfer of electrons from the electrode to the salt molecule is blocked, and thus the formation of a SEI film may mainly be generated from the reduction of solvent. Accordingly, it is obvious that the digital holographic method is fast response and sensitivite enough to distinguish the detailed reaction process during the formation of SEI films. To further verify the accuracy of the digital holographic method, the graphite/Li half-cell was also studied in a PC/EC/DMC electrolyte. As presented in Figure S8 and Movie S3, the constant consumption of dissolving species in the electrolyte can be observed, accompanied by a significant production of gas after cycling to 0.260 V. This is a common occurrence due to solvent cointercalation and exfoliation of the graphite electrode in the PC-based electrolyte, with the result being that the reduction of

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 21473081, 51572116, and 51401094) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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