Elucidating the energy storage mechanism of $\mathrm{ZnMn_{2}O_{4}}$ as promising anode for Li-ion batteries

Tetragonal spinel ZnMn$_2$O$_4$ provides extremely high capacity as an anode for Li-ion batteries owing to a conversion-type mechanism. In this work, nanoparticle-composed layered ZnMn$_2$O$_4$ is synthesized using a co-precipitation method. Calcination parameters are optimized through thermal gravimetric analysis and in situ high temperature synchrotron X-ray powder diffraction. The ZnMn$_2$O$_4$ shows an initial lithiation capacity of ∼1400 mA h g$^{−1}$ and a high reversible capacity of ∼900 mA h g$^{−1}$ at a specific current of 0.5 A g$^{−1}$. In situ synchrotron X-ray powder diffraction reveals phase evolution during the 1st cycle. An intermediate phase, tetragonal spinel LiZnMn$_2$O$_4$, is formed and coexists with the original ZnMn$_2$O$_4$ during the 1st lithiation. Electrochemical impedance spectroscopy applied at varying potentials during the 1st cycle provides evidence of the high Li$^+$ diffusion coefficient and low resistance of the electrode in the lithiated state, which enables a high rate performance with 810 mA h g$^{−1}$ at 1 A g$^{−1}$ and 580 mA h g$^{−1}$ at 2 A g$^{−1}$. X-ray photoelectron spectroscopy reveals that the solid-electrolyte interphase is mainly composed of LiOH and Li$_2$CO$_3$, which can contribute additional capacity. In addition, an Mn(II)/Mn(III) redox reaction appearing during the 60th to 100th cycles is reported for the first time and could be another reason for the capacity increase upon cycling (the maximum capacity is ∼1250 mA h g$^{−1}$ at the 90th cycle). This redox reaction is facilitated by the increase in the electronic conductivity upon cycling. Based on these investigations, fundamental insights into the energy storage mechanism of ZnMn$_2$O$_4$ conversion anodes in Li-ion batteries are clarified. This work can shed light on an understanding of other conversion-type electrode materials