3D Microstructural Equilibrium in Na-Ion Batteries Revealed By in Situ Hard X-Ray Nanotomography

Abstract

In recent years, sodium-ion batteries (NIB) have attracted much attention due to their low cost and abundant resources. Because sodium shares the similar chemical properties and fundamental principles as lithium, many battery materials that function well in Li ion systems were therefore expected to work well with Na ion systems. Nevertheless, more and more recent studies indicate some discrepancies in the behavior of the reaction mechanism. As a result, some understanding and knowledge of the Li-ion system cannot be applied directly to the Na-ion system. Compared to comprehensively studied Li-ion batteries, Na-ion batteries are still largely unexplored and critically need more fundamental studies. In this talk, we present a non-destructive in situ 3D x-ray nanotomography study of Na-ion electrode by using a newly developed transmission x-ray microscopy (TXM) technique at National Synchrotron Light Source, Brookhaven National Laboratory. [1-5] The TXM possesses a few unique capabilities including large field of view, super resolution in 3D, local tomography and automated markerless image acquisition and alignment, enabling high quality 3D observation and analysis. The unique electrochemical cell developed in-house allows real electrochemical measurement, e.g. discharge/charge profile, to be correlated to the 3D tomography observation during the electrochemical reactions. Sn is a well-established model material for studying the alloy-dealloy process occurring in Metal-ion battery systems. For the first time we have tracked the 3D microstructural change of Sn alloy anode in a Na-ion battery, and quantitatively analyzed the 3D microstructural evolution during ten electrochemical cycles. By correlating the quantified 3D microstructural information to the chemical change, we observed a 3D microstructural equilibrium occurring after the first cycle. Contrary to our conventional wisdom at its Li-ion analogue, Sn exhibits excellent microstructural reversibility with negligible pulverization during Na-ion extraction. The observed unusual low-volume NaxSn phase considerably accommodates the Na-ion insertion-induced stress. 3D surface curvature analysis also explains the microstructural stability in NIB. These new findings bring fresh insight into the mechanism of the microstructural degradation and contribute to engineering advanced battery materials. This talk will also present a comparative study of Sn in NIB and LIB, and reveal the significant difference in microstructural/electrochemical equilibrium for these two battery systems. Details about in situ 3D X-ray imaging technology, our beamline, and its broaden application in energy studies will also be discussed.