X-Ray Nanotomography of Cu6Sn5 Anode Material for Li-Ion Batteries

Abstract

In recent years, great attention has been paid to tin as an alternative to graphite as a negative electrode material in Li-ion batteries due to the high theoretical capacity of its fully lithiated phase, Li4.4Sn (994 mAh∙g-1). However, tin suffers from large volume expansion during lithiation and delithiation causing capacity to drop quickly as a function of cycle number. Significant volume expansion of the negative electrode has severe effects that reduce the cycle lifetime of the electrode such as material pulverization, separation from the current collector, and instability of the solid electrolyte interphase. One way to reduce this expansion is to create an inert matrix to help support the Sn during electrochemical cycling such as the use of copper in the form of the intermetallic alloy Cu6Sn5. As the issues stemming from this severe volume expansion all occur on a microstructural level, x-ray nanotomography (XNT) is used to image the anode material at a resolution of 60 nm. XNT provides the opportunity to non-destructively obtain the 3D structure of heterogeneous materials and provide a series of high resolution digital images for microstructural analysis and computational modeling. In this current study, Cu6Sn5 was synthesized by sintering copper and tin in the stoichiometric proportion to form the alloy anode material. XNT measurements were taken of the resulting non-lithiated material at 8.8 KeV over a 180° range of rotation. Single component and mixed samples containing the bronze alloy, Cu, and Sn were imaged. Based on absorption contrast regions of Cu, Sn, and Cu6Sn5 were differentiated throughout multiple samples. The capability to distinguish the different materials within mixed samples suggests that microstructure and composition changes resulting from lithiation and delithiation in Cu6Sn5 can be observed and better understood with 3D x-ray imaging methods. These methods may also be applicable to other intermetallic alloy electrode systems.