Controllable Fabrication of CoMn2O4 Microflowers and Microspheres As High-Performance Lithium Ion Battery Anode Material
Authors: Jin Yuhong , Jiang Qianlei , Zhao Yujuan , Ji Changwei 
Published: February 2020

The growing electric vehicle requirements have attracted great interest in development of lithium ion batteries (LIBs) with large gravimetric and volumetric capacities, long cycle lifespan. To meet these applications, high quality electrode materials are needed. However, conventional bulk electrode materials cannot fully reach the increasing demands due to their inherent limits in performance. In recent years, nanostructured electrode materials with higher rate capability have been acknowledged by low lithium-ion diffusion distance and high electrode/electrolyte area. However, electrode made from nanostructured electrode materials meets these challenges. (1) Nanoparticles tend to self-aggregate and the side reaction with the electrolyte due to high specific surface area, which lead to low initial coulombic efficiency and poor cycling life. (2) Nanoparticles have a huge volume changes during Li+ insertion/ extraction that result in fracture of nanostructured particle, delaminating of the conductive coating and low capacity with cycle increasing. (3) Nanostructured particles have low tap density which results in a low volumetric energy density.Therefore, the electrode materials with optimal structure should be developed to deal with above-mentioned problems. Micro-/nanostructures with controlled size and morphology have attracted considerable attention as high-performance anode materials for next-generation LIBs. This micro-/nanostructure should be a porous microstructure composed of primary nanocrystallines tightly compacted to form 3D channels for ion diffusion. Moreover, this structure can exploit many advantages of nanostructure for lithium ion batteries due to the reduction of side reactions with electrolyte, leading to high density and long cycling life with high safety.

Some simple micro-/nano-structured metal oxides such as Mn-based oxides, Co-based oxides, Fe-based oxides and Sn-based oxides, have been recently reported for use in LIBs. Mn-based oxide electrodes (MnO, MnO2, Mn2O3, Mn3O4, ZnMn2O4, NiMn2O4 and CoMn2O4) are developed due to low operating voltages (1.3-1.5 V for lithium extraction) and high energy density. Among these Mn-based anodes, CoMn2O4 has been considered for substitution of the conventional graphite anode in LIBs. The shape and micro/nanostructure of CoMn2O4 material play an important role in the electrochemical performances.

In this paper, uniform hierarchical porous CoMn2O4 microspheres and microflowers were realized by adjusting the urea concentration through a solvothermal process followed by a post-annealing treatment. In the low urea concentration, the microflower with diameter size of around 10 μm is composed of porous nanosheets with a thickness of about 20 nm. In the high urea concentration, a large amount of uniform CoMn2O4 microspheres is obtained with average diameter of about 8 μm. CoMn2O4 microsphere is highly porous and composed of numerous primary nanoparticles. As expected, the distribution of the O, Mn and Co elements is homogeneously distributed on the surface for CoMn2O4 microflower and microsphere. The actual molar ratios of Mn and Co for CoMn2O4 microflower and microspheres obtained from energy-dispersive X-ray (EDX) analysis are close to 2, which are consistent with the ratio of that in the precursor solution. The chemical compositional and crystalline structural changes of as-prepared samples were characterized using X-ray diffraction (XRD) and thermogravimetric analysis (TGA). These results show the precursor in the low urea concentration is a MnCo-complex structure (including CoMn-layered double hydroxide and CoMn-glycolates or alkoxide derivatives) in an alkaline environment under this solvothermal conditions. Meanwhile, in the high urea concentration, the precursor Co1/3Mn2/3CO3 sample is formed. The possible formation mechanism of as-prepared CoMn2O4 microsphere and microflower precursors is explained due to the presence of CO2 pressure into the reaction solution. When used as anode materials for lithium-ion batteries, the porous CoMn2O4 microflower and microspheres exhibited good long-life cycling performance at high rate density. At a current density of 100 mA g-1, the CoMn2O4 microflower and microspheres exhibit an initial capacity of 1200 and 1225 mA h g-1 and the capacity is maintained at 710 and 220 mAh g-1 after 50 cycles. Excellent rate capabilities (~813 mAh g-1 at 200 mA g-1 and ~612 mAh g-1 at 600 mA g-1) are observed for CoMn2O4 microspheres.

Based on these results, the good electrochemical properties can result from the porous CoMn2O4 microspheres. On the one hand, the micro-/nano porous structured CoMn2O4 microspheres could significantly enhance the electrode integrity by buffering the volume changes with repeating Li+ insertion/ extraction, which is contributed to good cycling stability. On the other hand, the micro-/nano porous structured CoMn2O4 microspheres provide extra active position for Li+ storage and short Li+ ion diffusion length, contributing to the high specific capacity and better rate capability.

The CoMn2O4 hierarchical porous microspheres, which have good electrochemical performance and ease of fabrication, hold the great promise as the anode materials for high-performance LIBs.

Figure 1