Hollow MnCo2O4 submicrospheres with multilevel interiors: From mesoporous spheres to yolk-in-double-shell structures

We present a general strategy to synthesize uniform MnCo2O4 submicrospheres with various hollow structures. By using MnCo-glycolate submicrospheres as the precursor with proper manipulation of ramping rates during the heating process, we have fabricated hollow MnCo2O4 submicrospheres with multilevel interiors, including mesoporous spheres, hollow spheres, yolk-shell spheres, shell-in-shell spheres, and yolk-in-double-shell spheres. Interestingly, when tested as anode materials in lithium ion batteries, the MnCo2O4 submicrospheres with a yolk-shell structure showed the best performance among these multilevel interior structures because these structures can not only supply a high contact area but also maintain a stable structure

Simple synthesis of yolk-shelled ZnCo2O4 microspheres towards enhancing the electrochemical performance of lithium-ion batteries in conjunction with a sodium carboxymethyl cellulose binder

Mixed metal oxides have been attracting more and more attention recently because of their advantages and superiorities, which can improve the electrochemical performance of single metal oxides. These advantages include structural stability, good electronic conductivity, and reversible capacity. In this work, uniform yolk-shelled ZnCo2O4 microspheres were synthesized by pyrolysis of ZnCo-glycolate microsphere precursors which were prepared via a simple refluxing route without any precipitant or surfactant. The formation process of the yolk-shelled microsphere structure during the thermal decomposition of ZnCo-glycolate is discussed, which is mainly based on the heterogeneous contraction caused by non-equilibrium heat treatment. The performances of the as-prepared ZnCo2O4 electrodes using sodium carboxylmethyl cellulose (CMC) and poly-vinylidene fluoride (PVDF) as binders are also compared. Constant current and rate charge–discharge testing results demonstrated that the ZnCo2O4 electrodes using CMC as the binder had better performance than those using PVDF as the binder. It was worth pointing out that the electrode using CMC as the binder nicely yields a discharge capacity of 331 mA h g−1 after 500 cycles at a current density of 1000 mA g−1, which is close to the theoretical value of graphite (371 mA h g−1). Furthermore, the obtained synthetic insights on the complex hollow structures will be of benefit to the design of other anode materials for lithium ion batteries

MnO@Carbon core-shell nanowires as stable high-performance anodes for lithium-ion batteries

A facile method is presented for the large-scale preparation of rationally designed mesocrystalline MnO@carbon core-shell nanowires with a jointed appearance. The nanostructures have a unique arrangement of internally encapsulated highly oriented and interconnected MnO nanorods and graphitized carbon layers forming an external coating. Based on a comparison and analysis of the crystal structures of MnOOH, Mn2O3, and MnO@C, we propose a sequential topotactic transformation of the corresponding precursors to the products. Very interestingly, the individual mesoporous single-crystalline MnO nanorods are strongly interconnected and maintain the same crystallographic orientation, which is a typical feature of mesocrystals. When tested for their applicability to Li-ion batteries (LIB), the MnO@carbon core-shell nanowires showed excellent capacity retention, superior cycling performance, and high rate capability. Specifically, the MnO@carbon core-shell nanostructures could deliver reversible capacities as high as 801 mA h g -1 at a high current density of 500 mA g-1, with excellent electrochemical stability after testing over 200 cycles, indicating their potential application in LIBs. The remarkable electrochemical performance can mainly be attributed to the highly uniform carbon layer around the MnO nanowires, which is not only effective in buffering the structural strain and volume variations of anodes during repeated electrochemical reactions, but also greatly enhances the conductivity of the electrode material. Our results confirm the feasibility of using these rationally designed composite materials for practical applications. The present strategy is simple but very effective, and appears to be sufficiently versatile to be extended to other high-capacity electrode materials with large volume variations and low electrical conductivities

High Electrochemical Performance of Monodisperse NiCo₂O₄ Mesoporous Microspheres as an Anode Material for Li-Ion Batteries

Binary metal oxides have been regarded as ideal and potential anode materials, which can ameliorate and offset the electrochemical performance of the single metal oxides, such as reversible capacity, structural stability and electronic conductivity. In this work, monodisperse NiCo₂O₄ mesoporous microspheres are fabricated by a facile solvothermal method followed by pyrolysis of the Ni_0.33Co_0.67CO₃ precursor. The Brunauer–Emmett–Teller (BET) surface area of NiCo₂O₄ mesoporous microspheres is determined to be about 40.58 m² g^–1 with dominant pore diameter of 14.5 nm and narrow size distribution of 10–20 nm. Our as-prepared NiCo₂O₄ products were evaluated as the anode material for the lithium-ion-battery (LIB) application. It is demonstrated that the special structural features of the NiCo₂O₄ microspheres including uniformity of the surface texture, the integrity and porosity exert significant effect on the electrochemical performances. The discharge capacity of NiCo₂O₄ microspheres could reach 1198 mA h g^–1 after 30 discharge–charge cycles at a current density of 200 mA g^–1. More importantly, when the current density increased to 800 mA·g^–1, it can render reversible capacity of 705 mA h g^–1 even after 500 cycles, indicating its potential applications for next-generation high power lithium ion batteries (LIBs). The superior battery performance is mainly attributed to the unique micro/nanostructure composed of interconnected NiCo₂O₄ nanocrystals, which provides good electrolyte diffusion and large electrode–electrolyte contact area, and meanwhile reduces volume change during charge/discharge process. The strategy is simple but very effective, and because of its versatility, it could be extended to other high-capacity metal oxide anode materials for LIBs

Hollow MnCo₂O₄ Submicrospheres with Multilevel Interiors: From Mesoporous Spheres to Yolk-in-Double-Shell Structures

We present a general strategy to synthesize uniform MnCo₂O₄ submicrospheres with various hollow structures. By using MnCo-glycolate submicrospheres as the precursor with proper manipulation of ramping rates during the heating process, we have fabricated hollow MnCo₂O₄ submicrospheres with multilevel interiors, including mesoporous spheres, hollow spheres, yolk–shell spheres, shell-in-shell spheres, and yolk-in-double-shell spheres. Interestingly, when tested as anode materials in lithium ion batteries, the MnCo₂O₄ submicrospheres with a yolk–shell structure showed the best performance among these multilevel interior structures because these structures can not only supply a high contact area but also maintain a stable structure