CHEMELECTROCHEM HIGHLIGHTS DOI: 10.1002/celc.201300267

Watching Microstructures in Action in Lithium-Ion Batteries Jun Wang*[a]

Synchrotron-based X-ray imaging techniques are non-destructive, sensitive to material density, and can reveal internal structures of a specimen. When combined with the tenability of an X-ray wavelength, synchrotron-based imaging is also sensitive to elemental distribution and chemical states. These techniques are ideally suited for in situ studies of a variety of materials and systems. Tomography capability leads to three-dimensional characterization, which is critical to obtain a full understanding of complex micro- and nano-structural information. Comprehensive quantitative analysis that is available through X-ray imaging, together with its temporal resolution and spectral imaging, render this technique unique and powerful in research and development across many disciplines of science and engineering today, including energy storage, nanoporous material functions, microelectronics, as well as heterogeneous systems in environmental and biological sciences. Lithium-ion batteries (LIBs), as major energy-storage devices, have been widely used in many applications from portable electronics to transportation vehicles. However, the performance cycle lifetime, capacity, stability, and safety are often degraded, which is associated with mechanical failure in the hosting electrode material. The insertion-based electrochemical reaction of LIBs introduces strains during the lithiation and delithiation process, leading to microstructural evolution in the electrodes. Large volume changes have been reported in high energy-density negative electrodes, such as silicon- and tinbased compounds, resulting in mechanical fracture and pulverization, which can lead to the loss of battery capacity and cycle life.[1, 2] To better understand the factors causing the performance degradation, it is critical to have a full picture of the mechanism of the microstructural evolution in three dimensions with the relevant spatial resolution as cycling proceeds in an operating battery cell. In recent years, various in situ characterization techniques have been applied for studying battery materials, such as nuclear magnetic resonance (NMR),[3] scanning electron microscopy (SEM),[4] transmission electron microscopy (TEM),[5] atomic force microscopy (AFM),[6] and X-ray diffraction and spectroscopy.[7] However, it is still challenging to directly visualize microstructural evolution and chemical composition distribution changes within individual particles and throughout the elec-

[a] Dr. J. Wang Photon Science Directorate, Brookhaven National Laboratory 75 Brookhaven Avenue, BLDG 744, Upton, NY 11973 (USA) E-mail: [email protected]

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trode with three-dimensional meso-scale resolution when a real battery cell is in operation. Recently Ebner et al. applied synchrotron-based X-ray tomography to a LIB with SnO as the anode material, and directly observed and quantified the electrochemical and mechanical degradation in three-dimensional structures with micrometer spatial resolution.[8] SnO is a model material that typically undergoes a conversion reaction. In this work, tomographic data sets were collected every 15 min during the lithiation and delithiation processes. The three-dimensional structures observed at each time point were reconstructed to show the structural evolution, and quantitative analysis was performed to extract chemical information to correlate with that structural evolution. By analyzing the contrast of the images through the attenuation coefficient, which is determined by the composition and mass density of a material, the lithiation and delithiation processes were monitored and quantitatively characterized. During the lithiation process, a core–shell structure was clearly observed, showing the conversion reaction in which the SnO becomes Sn (Figure 1, in which the color represents the attenuation coefficient), and the following alloying reaction of Sn with lithium (LixSn, 0 < x < 4.4) was demonstrated by the decreasing attenuation coefficient (Figure 1, in which the color changes from yellow–green to dark green). The dealloying reaction was indicated with an increasing attenuation coefficient during the delithiation process (shown in Figure 1). Volume expansion and cracks appeared during the lithiation process and pulverization occurred during delithiation, as also demonstrated in Figure 1. The electrochemical cell was designed and operated as a fully functional battery, so that electrochemical data could be collected in situ whilst cycling, which enabled correlation with the structural evolution with micrometer resolution. This is a powerful and unique capability provided by synchrotron-based X-ray tomographic microscopy (SRXTM). The ability to observe microstructures in three dimensions in a non-destructive manner is the key to enabling the in situ study of a real battery cell, which will provide the structural and chemical information that is critical for battery research. The work by Ebner et al.[8] demonstrated this powerful technique on a LIB with a spatial resolution of approximately 2 mm. There is tremendous interest in the energy-storage research community to go much beyond micrometer-scale observations and into the nanoscale, because many of the early degradation effects occur on scales of just a few tens of nanometers. Furthermore, research to enhance the electrode performance has indicated that nanostructured electrodes often provide improved characteristics and a better balance between ion diffuChemElectroChem 2014, 2, 329 – 331

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Figure 1. A series of cross-sections through two particles demonstrates a core–shell process, volume expansion, and particle fracture during the initial reduction. Particle re-densification is then observed during subsequent oxidation. Adapted from ref. [8]

sional mapping is critical for a better understanding of the electrochemical reaction mechanism associated with mechanical degradation. X-ray absorption near-edge structure (XANES) spectroscopy combined with SRXTM and TXM is a powerful tool to meet those needs. Currently, two-dimensional chemical mapping through this spectroscopic TXM imaging technique has been reported.[11] Chemical mapping in three dimensions is another potential capability in this field, which may lead to valuable electrochemical–structural information in in situ battery research. Looking forward, it is vital for battery materials research to reveal structural and chemical evolution during electrochemical cycling. This is because the battery is in a realistic operating state at the characteristic length scales relevant to the ion diffusion in an electrode. There is no doubt that both microtomography and nano-tomography will play an important role in future battery research, as well as in many other research areas.

Acknowledgements Figure 2. Three-dimensional reconstruction of a LiVO2 electrode, with a field of view of 40 mm and a voxel size of 20 nm, showing internal cracks before oxidation.

sion and structural integrity, leading to longer cycle life. With X-ray lens-based transmission X-ray microscopes (TXMs) now available at several synchrotron beamlines, it would be very interesting to conduct in situ three-dimensional X-ray tomography on a LIB at a spatial resolution approaching 30–50 nm.[9] Figure 2 is a reconstructed three-dimensional image of a LiVO2 electrode before oxidation with a voxel size of 20 nm. Stressinduced internal cracks with sizes of 30–240 nm, caused by processing of the electrode, were observed and measured. The evolution of internal cracks, before and after oxidation in the same particle, was studied and a comprehensive quantitative analysis was applied, based on a high-quality three-dimensional reconstruction at the nanometer scale,[10] indicating the potential to perform three-dimensional synchrotron nano-tomography in operando. Chemical information, such as oxidation state, chemical composition, and elemental distribution, from three-dimen 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0298CH10886. Keywords: batteries · electrochemistry · microstructures · synchrotron · X-ray tomography [1] J. M. Tarascon, M. Armand, Nature 2001, 414, 359 – 367. [2] B. Dunn, H. Kamath, J. M. Tarascon, Science 2011, 334, 928 – 935. [3] R. Bhattacharyya, B. Key, H. Chen, A. S. Best, A. F. Hollenkamp, C. P. Grey, Nat. Mater. 2010, 9, 504 – 510. [4] C. M. Lpez, J. T. Vaughey, D. M. Dees, J. Electrochem. Soc. 2009, 156, A726 – A729. [5] J. Y. Huang, L. Zhong, C. M. Wang, J. P. Sullivan, W. Xu, L. Q. Zhang, S. X. Mao, N. S. Hudak, X. H. Liu, A. Subramanian, H. Fan, L. Qi, A. Kushima, J. Li, Science 2010, 330, 1515 – 1520. [6] R. Wen, M. Hong, H. R. Byon, J. Am. Chem. Soc. 2013, 135, 10870 – 10876. [7] I. A. Courtney, J. R. Dahn, J. Electrochem. Soc. 1997, 144, 2045 – 2052; D. Takamatsu, Y. Koyama, Y. Orikasa, S. Mori, T. Nakatsutsumi, T. Hirano, H. Tanida, H. Arai, Y. Uchimoto, Z. Ogumi, Angew. Chem. Int. Ed. 2012, 51, 11597 – 11601.

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CHEMELECTROCHEM HIGHLIGHTS [8] M. Ebner, F. Marone, M. Stampanoni, V. Wood, Science 2013, 342, 716 – 720. [9] J. Wang, Y.-C. K. Chen, Q. X. Yuan, A. Tkachuk, C. Erdonmez, B. Hornberger, M. Feser, Appl Phys Lett. 2012, 100, 143107 – 4. [10] Y.-C. K. Chen-Wiegart, P. Shearing, Q. X. Yuan, A. Tkachuk, J. Wang, Electrochem. Commun. 2012, 21, 58 – 61.

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Received: December 23, 2013 Published online on January 21, 2014

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