Understanding Li-storage mechanism and performance of MnFe₂O₄ by in situ TEM observation on its electrochemical process in nano lithium battery

In this work, we fabricated an all-solid-state nano lithium battery MnFe₂O₄/graphene-Li₂O-Li to understand the electrochemical Li-storage mechanism and performance of MnFe₂O₄ using in situ transmission electron microscopy (TEM) technique. We found that single-crystalline MnFe₂O₄ is converted into polycrystalline Li₂O/Mn/Fe with large volume expansion upon discharge and subsequently into polycrystalline MnO/Fe₃O₄ with volume shrinkage upon charge. Reversible conversion between MnO/Fe₃O₄ and Li₂O/Mn/Fe occurs during the following cycles with small volume changes. We also found that both MnO/Fe₃O₄ and Li₂O/Mn/Fe can be tightly confined by graphene despite the volume change and particle pulverization, and that free space that buffers the volume changes still exists even at deep lithiation state. In situ TEM characterization also indicates that graphene is a good conductor for both Li ion and electrons. The combined conducting, buffering and confining effects of graphene revealed by in situ TEM characterization can well explain the role it plays in improving the electrochemical properties of MnFe₂O₄.