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

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.