During the last decade, many research efforts have been made to develop alloy-type anode materials for lithium ion batteries (LIBs), because of their much higher storage capacity compared to graphite (372 mAh/g) (1). Sn is one of the alloy-type materials and it has theoretical capacity of 994 mAh/g (for the charged Li4.4
Sn phase) (2). Sn is also non-toxic, abundant and inexpensive. However, the major challenge in the development of Sn anode is the high volume change (about 300%) involved in the reaction scheme, which could result in particle fracture and electrode delamination from the current collector, thereby leading to rapid loss of specific capacity(2). Therefore, it is essential to understand the dynamic morphology change of Sn electrode in LIB cycling processes. To this end, several in-situ studies have conducted to investigate the morphology change of Sn electrode. For instance, Ebner et al. used X-ray tomography to visualize and quantify the origins and evolution of electrochemical and mechanical degradation of tin oxide electrode(3). Sun et al. used in-situ synchrotron radiography to investigate the lithiation and delithiation mechanisms of multiple Sn particles in a customized flat radiography cell(4). Wang et al. studied the microstructural evolutions of a Sn electrode using in-situ synchrotron X-ray nanotomography(5). However, the dynamic morphology change of Sn particles in the size range between 1 µm and 10 µm has not been investigated. In this study, we investigated the morphology evolution of Sn electrode made of smaller particles (~5 µm in diameter) via in-operando Synchrotron transmission X-ray microscopy (TXM). In addition, we also studied effects of particle size and original morphology on the dynamic morphology change during (de)lithiation processes.
To investigate the morphological evolution of Sn particles, anode electrode was fabricated from a 25:45:30 (weight%) mixture of Sn, super-P carbon black, and PVDF. The slurry was coated on carbon paper and the electrolyte utilized was 1M LiPF6 in EC/DEC solution. The Sn electrode, separator and Li counter electrode were assembled in standard 2016 coin cell with holes on both sides sealed by Kapton tapes. A synchrotron TXM with a spatial resolution of 40 nm was employed to obtain morphological data of the electrodes. The TXM temporarily located at the Beamline 8-BM-B of Advanced Photon Source is operated by the National Synchrotron Light Source-II (NSLS-II) through a NSLS-II transition program. The 2D in-operando TXM images were used to visualize and quantify morphology change of Sn particles during (de)lithiation processes. As shown in Fig. 1, volume expansion of Sn particles are more likely to take place at the surface with larger curvature, infiltrating from outside to the inside. The morphology change of Sn particles with different sizes start and end at almost the same time.
Acknowledgments: This work was supported by US National Science Foundation under Grant No. 1335850.
Figure 1. Morphology change of several Sn particles during a 0.1 C lithiation process at different time steps.
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