Online Digital Holographic Method for Interface Reaction Monitoring in

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An On-Line 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 J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09920 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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The Journal of Physical Chemistry

An On-line 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, China. ‡

Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2

3QZ, UK §

College of Chemistry and Chemical Engineering, Xuzhou University of Technology.

Corresponding author:

*

E-mail: [email protected]

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ABSTRACT Understanding the reaction mechanisms at the interface of electrode and electrolyte is both fundamental interest and essential to improve the lithium-ions battery (LIBs) performance. Herein, we report an on-line 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 microscope and X-ray photoelectron spectroscopy. In particular, the time resolution of digital holographic method is 0.04 s and fast enough to distinguish detail reduction process of ethylene carbonate (EC), for which EC will be firstly reduced to generate lithium alkyl carbonates, and then the reduction product is Li2CO3 to form a stable SEI films. To our best of knowledges, this is the first report on the reduction order of EC solvent and can act as an effective complement to understanding the formation mechanisms of SEI films.


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The search is on for enhancing the energy density and the 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, high-voltage 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 electrode and electrolyte, mainly about solid electrolyte interphase (SEI) films, is prerequisite to address 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.710,16

The formation of SEI films on graphite anodes has been widely characterized by various ex-

situ techniques, such as x-ray photoelectron spectroscopy (XPS), Fourier Transform infrared spectroscopy (FTIR), Raman, scanning electron microscope (SEM), transmission electron microscope (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 Microscope (AFM) imaging was also used to present real-time views of the morphological evolution of SEI films.21,22 However, due to the complexity of the


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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 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 MachZehnder interferometer. A light beam with the wavelength of 632.8 nm was generated by a HeNe 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


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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.

Scheme 1. Schematic diagram of the optical setup of the digital holography system used in testing a lithium-ion battery. (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. The principle of this experiment is based on the relationship between the solution concentration (∆C), the solution refractive index (∆n), and the phase difference (∆Φ), which was formulated as ∆C = k×∆n = (kλ0 /2πd) ∆Φ (1) 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, semi-quantitative analysis can be achieved by use of holography. Additionally, it is worth


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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.

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.


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The electrochemical performance of the LiFePO4/Graphite full cell tested in a transparent mold (Fig. S1) is firstly presented in Fig. 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. Fig. 1b shows reconstructed phase maps from the initial interferograms (Fig. S2) according to equation 1, with each map corresponding to the selected potentials labeled in Fig. 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 Fig. 1c. Any change in the dissolved species can lead to a shift in the phase difference (∆Φ) at the interface, where ∆Φ>0 signifies an increase in concentration, and ∆Φ

Online Digital Holographic Method for Interface Reaction Monitoring in

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