Jingfa Li

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 NiCo(2)O(4) mesoporous microspheres are fabricated by a facile solvothermal method followed by pyrolysis of the Ni(0.33)Co(0.67)CO(3) precursor. The Brunauer-Emmett-Teller (BET) surface area of NiCo(2)O(4) mesoporous microspheres is determined to be about 40.58 m(2) g(-1) with dominant pore diameter of 14.5 nm and narrow size distribution of 10-20 nm. Our as-prepared NiCo(2)O(4) products were evaluated as the anode material for the lithium-ion-battery (LIB) application. It is demonstrated that the special structural features of the NiCo(2)O(4) microspheres including uniformity of the surface texture, the integrity and porosity exert significant effect on the electrochemical performances. The discharge capacity of NiCo(2)O(4) 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 NiCo(2)O(4) 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.

Spinel Mn1.5Co1.5O4 core–shell microspheres as Li-ion battery anode materials with a long cycle life and high capacity

Transition metal oxides are important functional materials that have gained enormous research interest in recent years. In this work, porous cubic manganese cobalt spinel Mn1.5Co1.5O4 core–shell microspheres were first prepared via a urea-assisted solvothermal route followed by pyrolysis of the carbonate precursor. The microsphere is composed of the shell of 400 nm thickness and the core with a 2.5 μm diameter. Nitrogen sorption isotherms show that this structure possesses a high surface area of 27.0 m2 g−1 with an average pore diameter of 30 nm. Compared with a simple spherical nanopowder, such a core–shell like porous structure is expected to improve the electrochemical performance, due to its higher resistance against separation or isolation during the electrochemical reaction. The as-prepared Mn1.5Co1.5O4 core–shell microspheres show an excellent cyclic performance at high current density with more than 90% capacity retention in a testing range of 300 cycles when used as an anode material for lithium ion batteries (LIBs), which can be attributed to the appropriate pore size and unique core–shell structures. Therefore, the Mn1.5Co1.5O4 core–shell microspheres prepared by the present synthetic route could be identified as a potential anode candidate for the near future development of LIBs.

Mesoporous NiO ultrathin nanowire networks topotactically transformed from α-Ni(OH)2 hierarchical microspheres and their superior electrochemical capacitance properties and excellent capability for water treatment

Low-cost controlled strategies for the synthesis of mesoporous nickel oxide materials are highly desirable owing to its significant applications for power storage and other fields. In this contribution, we develop a novel hydrothermal route to synthesize α-Ni(OH)2, in which urea has not only been utilized to produce hydroxyl anions, but also to organize ultrathin nanowires/nanosheets into a network-like hierarchical assemblage. The morphological evolution process of this organized product has been investigated by examining different reaction intermediates during the synthesis. The growth and thus final assemblage of α-Ni(OH)2 can be finely tuned by selecting preparative parameters such as the molar ratio of starting chemicals. Based on the topotactic transformation from α-Ni(OH)2, various mesoporous NiO hierarchical microspheres from ultrathin nanowires/nanosheets self-assembly have been prepared via thermal decomposition in an air atmosphere. The electrochemical performances of the typical nickel oxide products are evaluated. It is demonstrated that tuning of the surface texture and the pore size of the NiO products is very significant for electrochemical capacitor and water treatment applications. The mesoporous NiO network-like hierarchical microspheres exhibit excellent cyclic performance with nearly 100% capacity retention at a current density of 10 A g−1 in a testing range of 2000 cycles. Moreover, the mesoporous NiO network-like hierarchical microspheres have excellent ability to remove organic pollutants from wastewater by their wonderful surface adsorption ability.

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, Mn2 O3 , 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.

Formation of quasi-mesocrystal ZnMn2O4 twin microspheres via an oriented attachment for lithium-ion batteries

At present, transition metal oxides (TMOs) have generally been fabricated via annealing carbonates pre-obtained by a precipitation/solvothermal route. We noted that researchers mainly focused on how to get the expected TMOs through calcination. However, study about the formation process of the corresponding precursors is rarely investigated. Instead, it is of much importance for the development of materials chemistry. Herein, as an example, for the first time, we devise a facile polyol-based method to synthesize the quasi-mesocrystal ZnMn2O4 porous twin-microspheres. Formation chemistry and electrochemical properties of the twin spheres have been investigated in detail. A distinctive oriented attachment accompanied by Ostwald ripening is proposed to understand the formation of the 3D carbonate twin microspheres, providing a new research opportunity for investigating the formation of novel micro/nanostructures. Benefitting from the many unique structural advantages, including quasi-mesocrystal architecture, 3D hierarchical porous microstructure, and lithium alloying reaction, the as-prepared ZnMn2O4 twin spheres represent remarkable lithium storage properties when evaluated as anode materials for lithium-ion batteries (LIBs), with high capacity, long cycle life and remarkable rate capability. The 3D porous hierarchical structures demonstrate great potential as anode materials for high-performance LIBs.

General synthesis of xLi2MnO3·(1 − x)LiNi1/3Co1/3Mn1/3O2 (x = 1/4, 1/3, and 1/2) hollow microspheres towards enhancing the performance of rechargeable lithium ion batteries

Well-crystallized and high-performance xLi2MnO3·(1 − x)LiNi1/3Co1/3Mn1/3O2 (x = 1/2, 1/3, 1/4) hollow microspheres are prepared through an in situ self-sacrificial template route. By precisely adjusting y values in the starting precursor, CoyMn3−yO4 porous microspheres, uniform xLi2MnO3·(1 − x)LiNi1/3Co1/3Mn1/3O2 hollow microspheres are formed. Property testing of lithium ion batteries shows that an appropriate x value plays an important role in their electrochemical behavior: a higher x value leads to a higher specific capacity, but a worse cycling capability, and vice versa; when x = 1/3, the xLi2MnO3·(1 − x)LiNi1/3Co1/3Mn1/3O2 electrode shows a high specific capacity and a high capacity retention rate. In particular, the hollow microspheres with x = 1/3 achieved a reversible capacity as high as 203 mA h g−1 at 0.25C and 181 mA h g−1 at 1.0C over 200 cycles. A rate capacity as high as 191 mA h g−1 at 5.0C is obtained after 5 cycles stepwise from 0.25C to 5.0C. This work provides a general approach based on the use of an in situ self-sacrificial template to synthesize xLi2MnO3·(1 − x)LiMO2 (0 < x < 1, M = Ni, Co, Mn, etc.) at various x values and other targeted materials in a morphology with hollow interiors, which is inaccessible through a wet chemistry route.

General Approach to Prepare 0.33Li2MnO3 · 0.67LiNi1/3Co1/3Mn1/3O2 Hollow Microspheres for High Performance Lithium Ion Batteries

Li-excess manganese-based oxide layered structures, have drawn increasing interests as the promising cathodes to succeed the conventional LiCoO2 in lithium ion batteries (LIBs). It could deliver a higher energy density and output potential, as well as the nature of environment benign and low cost. Pristine Li-excess manganese-based oxides however suffer from poor rate capacity and voltage fading after cycling, and their inherent capacity limits of bulk size in performance. Micro-/Nanostructured electrode materials are considered to hold the key to overcome these thresholds. This paper reports a general approach to prepare 0.33Li2MnO3 · 0.67LiNi1/3Co1/3Mn1/3O2 microspheres with pores and void space, which benefits improving both the capacity and cyclability. The electrode made of hollow 0.33Li2MnO3 · 0.67LiNi1/3Co1/3Mn1/3O2 microspheres exhibits a 224 mAh g-1 discharge capacity over 200 cycles at 0.25 C rate, and 195 mAh g-1 at 5.0 C rate. These results indicated good perspective of hollow microspheres for practical battery applications.