Elucidating the energy storage mechanism of ZnMn2O4 as promising anode for Li-ion batteries

Abstract

Tetragonal spinel ZnMn₂O₄ provides extremely high capacity as an anode for Li-ion batteries owing to a conversion-type mechanism. In this work, nanoparticle-composed layered ZnMn₂O₄ 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 ZnMn₂O₄ 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 1^st cycle. An intermediate phase, tetragonal spinel LiZnMn₂O₄, is formed and coexists with the original ZnMn₂O₄ during the 1^st lithiation. Electrochemical impedance spectroscopy applied at varying potentials during the 1^st 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 Li₂CO₃, which can contribute additional capacity. In addition, an Mn(II)/Mn(III) redox reaction appearing during the 60^th to 100^th 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 90^th 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 ZnMn₂O₄ conversion anodes in Li-ion batteries are clarified. This work can shed light on an understanding of other conversion-type electrode materials.