Multi‐Step Phase Transitions of Mn₃O₄ During Galvanostatic Lithiation: An In Situ Transmission Electron Microscopic Investigation

Abstract For study of electrochemical reaction mechanisms at nanoscale, in situ electrochemical transmission electron microscopy (EC‐TEM) exceeds many other methods due to its high temporal and spatial resolution. However, the limited amount of active materials used in previous in situ TEM studies prevents the model EC cells to operate in the constant‐current (galvanostatic) charge/discharge mode that is required for accurate control of electrochemical processes. Herein, a new in situ EC‐TEM technique is developed to investigate multi‐step phase transitions of Mn₃O₄ electrodes under the galvanostatic charge/discharge mode and constant‐voltage discharge mode. In galvanostatic mode, the lithiation of Mn₃O₄ undergoes multi‐step phase transitions following a reaction pathway of Mn₃O₄ + Li⁺ → LiMn₃O₄ + Li⁺ → MnO + Li₂O → Mn + Li₂O. It is also found that lithium ions prefer to enter Mn₃O₄ along the {101} direction to form LiMn₃O₄ with the help of transitional boundary phase of Li_xMn₃O₄. These results are in sharp contrast to that obtained under a constant‐voltage discharge mode, where only a single‐step lithiation process of Mn₃O₄ + Li⁺ → Mn + Li₂O is observed.