Researchers at the University of Akron have developed hierarchical porous Mn₃O₄/C nanospheres as anode materials for Li-ion batteries. These nanospheres exhibited a high reversible specific capacity (1237 mAh/g at 200 mA/g), excellent ratability (425 mAh/g at 4 A/g), and extremely long cycle life (no significant capacity fading after 3000 cycles at 4A/g) as an anode in a Li-ion battery. A paper on their work is published in the Journal of Power Sources.

Transition metal oxides (MO_x, where M is Mn, Co, Ni, Cu or Fe etc.) are promising anode candidate materials, owing to their high theoretical capacity and low cost. Among those materials, Mn₃O₄ has been intensively investigated as one of the most promising anode materials due to its abundance, low oxidation potential and competitive electrochemical performance. However, several issues hamper the utilization of transition metal oxides as anode materials in LIBs: First, the poor intrinsic electrical conductivity of metal oxides limits the electron transfer throughout the electrode, leading to poor active materials utilization and low ratability. Second, the large volume expansion and shrinkage of the metal oxides during the lithiation and delithiation can result in electrode pulverization that promotes capacity fading during cycling. It has been well recognized that nano-engineering and carbon hybridization are effective ways to overcome or limit these issues

… In this work, a facile method to synthesize highly porous Mn₃O₄/C nanospheres with hierarchical structure was achieved by self-assembly to form spherical Mn based MOC, followed by a thermal annealing process. The Mn₃O₄/C nanospheres consisted of homogeneously distributed Mn₃O₄ nanocrystals with a conformal carbon coating. Such a hierarchical, porous structure provided both good electrical conductivity and volume changes accommodation capability, which were desired for transition metal oxide based conversion reaction type electrode. These characteristics led to high specific capacity, excellent ratability and ultra-long cycle life in a lithium-ion half-cell for the Mn₃O₄/C nanosphere electrode.

—Liu et al.

Schematic illustration of the fabrication procedure for the Mn₃O₄/C nanospheres. Credit: ACS, Liu et al.

The team synthesized a self-assembled manganese (Mn)-based metal organic complex (Mn-MOC) with a spherical structure via a solvothermal reaction. The researchers then converted the Mn-MOC precursor materials to hierarchical porous Mn₃O₄/C nanospheres through a thermal annealing treatment.

Electrochemical performance of Mn3O4/C nanosphere as anode in LIBs. a) Cyclic voltammogram of Mn3O4/C nanosphere performed at a scan rate of 0.2 mV/s within the voltage range of 0.005–3 V (vs. Li/Li+). b) Galvanostatic charge and discharge profiles for the 1st, 10th, 50th, 100th, 190th cycles at a current density of 200 mA/g (Voltage vs. Li/Li+). c) Cycle performance at a current density of 200 mA/g d) Specific capacities at current densities of 200 mA/g, 500 mA/g, 1 A/g, 2 A/g and 4 A/g. e) Long cyclability test at a current density of 4 A/g. Click to enlarge. Credit: ACS, Liu et al.

They attributed the lithium storage capacity to the unique porous hierarchical structure of the nanospheres, which consist of homogeneously distributed Mn₃O₄ nanocrystals with thin carbon shells. Such a nanostructure not only provides large reaction surface area and enhanced electrical conductivity, but also promotes the formation of a stable solid electrolyte interphase (SEI) and accommodates the volume change of the conversion reaction type electrode.

Resources

• Kewei Liu, Feng Zou, Yuandong Sun, Zitian Yu, Xinye Liu, Leyao Zhou, Yanfeng Xia, Bryan D. Vogt, Yu Zhu (2018) “Self-assembled Mn₃O₄/C nanospheres as high-performance anode materials for lithium ion batteries,” Journal of Power Sources, Volume 395, Pages 92-97 doi: 10.1016/j.jpowsour.2018.05.064