Microstructure Evolution and Conversion Mechanism of Mn₃O₄ under Electrochemical Cyclings

Probing the microstructure evolution, phase change, and fundamental conversion mechanism of anodes for lithium ion batteries (LIBs) during lithiation–delithiation cycles is important to gain insights into understanding how the electrode works and thus how it can be improved. The electrochemical reaction and phase evolution of Mn₃O₄ during lithiation–delithiation cycles remain unknown. To observe the real-time electrochemical behaviors of Mn₃O₄ during lithiation–delithiation cycles, a nanosized LIB was constructed inside a transmission electron microscope (TEM) using an individual Mn₃O₄/graphene moiety as the anode. Upon the first lithiation, Mn₃O₄ nanoparticles are lithiated into the crystallized Mn nanograins embedded within the Li₂O matrix. However, Mn and Li₂O cannot be recovered to the original Mn₃O₄ phase but to MnO after the first full delithiation, which results in an irreversible phase transformation. Such incomplete conversion reaction accounts for the huge capacity fading during the first cycle of Mn₃O₄-based LIBs. Excellent cyclability between Mn and MnO is also established during the subsequent lithiation–delithiation cycles, which is beneficial to the capacity retention in real battery. It provides an in-depth understanding of the phase evolution and conversion mechanism of Mn₃O₄ during lithiation–delithiation and holds the promise of improving the capacity for the development of durable, high-capacity, and high-rate anodes for LIBs.