Visualization of Mg2+ Electrochemical Insertion/Deinsertion into MnO2: Ex-Situ TEM Study

Abstract

Magnesium-ion (Mg-ion) rechargeable batteries are perhaps one of the most promising candidates that could potentially replace the current lithium ion battery technology. While the low cost and stability of magnesium make the Mg-ion battery an attractive technology, the divalent charge of Mg2+ ion brings in several significant scientific questions: How the divalent charge of Mg2+ affects their insertion/deinsertion behaviors? How far can Mg2+ ions penetrate into the bulk of the electrode material? One of the best ways to answer these questions is to visualize the Mg2+ insertion/deinsertion to/from the electrode material.In this work, we visualized the process of the electrochemical insertion and deinsertion of Mg2+ into a MnO2 nanowire electrode by utilizing transmission electron microscopy (TEM) technique and electron energy loss spectroscopy (EELS). MnO2 nanowires were electrochemically grown within an aluminum anodized oxide (AAO) template. The obtained free-standing MnO2 nanowire array showed well-ordered, uniform structure and excellent contact with the current collector. These features of electrodeposited MnO2 nanowire electrode are beneficial for various imaging analyses as they would provide consistent results throughout the measurements. Moreover, this MnO2 nanowire electrode does not require conductive additives or a polymer binder solution to fabricate the electrode, therefore clear images of the nanowire samples can be obtained with no interferences from such additives.From the ex-situ TEM energy dispersive X-ray spectroscopy (EDS) analysis, we found that the Mg2+ insertion is mostly confined to the surface of MnO2 nanowires. In order to observe the distribution of Mg2+ ions within the MnO2 nanowire in more detail, the discharged MnO2 nanowire array was sliced via microtome and its cross-section was analyzed by EELS. The EELS analysis on the cross-section of the discharged MnO2 nanowire showed that Mg2+ is also found at the center of the MnO2 nanowire, but at a lower degree than the surface. Moreover, we found that almost all of the inserted Mg2+ is retracted after charging the MnO2 electrode. This work not only presents interesting Mg2+ insertion/deinsertion behavior with MnO2, but also provides a systematic method that can be applied for the analysis of other cation/electrode systems.