LiMn2O4–yBryNanoparticles Synthesized by a Room Temperature Solid-State Coordination Method
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LiMn2O4–yBrynanoparticles were synthesized successfully for the first time by a room temperature solid-state coordination method. X-ray diffractometry patterns indicated that the LiMn2O4–yBrypowders were well-crystallized pure spinel phase. Transmission electron microscopy images showed that the LiMn2O4–yBrypowders consisted of small and uniform nanosized particles. Synthesis conditions such as the calcination temperature and the content of Br−were investigated to optimize the ideal condition for preparing LiMn2O4–yBrywith the best electrochemical performances. The optimized synthesis condition was found in this work; the calcination temperature is 800 °C and the content of Br−is 0.05. The initial discharge capacity of LiMn2O3.95Br0.05obtained from the optimized synthesis condition was 134 mAh/g, which is far higher than that of pure LiMn2O4, indicating introduction of Br−in LiMn2O4is quite effective in improving the initial discharge capacity.
KeywordsLiMn2O4–yBry Nanoparticles Room temperature solid-state coordination method Lithium–ion battery
Development of the cathode materials for lithium–ion battery is vital to meet the demands of portable devices, power tools, e-bikes, future usages of electric vehicles, and so on . Among three promising candidates for cathode materials (LiCoO2, LiNiO2, and LiMn2O4), lithium manganese oxides (LiMn2O4)  are inexpensive cathode materials with a high energy density, environmental acceptability, and are more abundant in nature. In spite of these advantages, LiMn2O4 has the problem of severe capacity fading during charge and discharge cycles [3, 4], which makes it unsuitable for commercial application. Intensive research has particularly focused on the mechanism of capacity fading and has suggested numerous solutions.
Among these projects, doping [5, 6] is considered to be an effective path to improve the electrochemical performance of spinel LiMn2O4, so several attempts have been made for improving the lithium manganese spinels by doping various metals ions [7, 8, 9, 10]. Although such substitutions often result in enhancing the stability of spinel, the first discharge capacity of them is considerably lower than that of the parent compound. The reduction in the first discharge capacity is mainly due to the fact that the substituent ions do not contribute to the discharge capacity. In 1999, Amatucci et al.  and Palacin et al.  reported that the introduction of the anion in spinel structure can reduce the Mn oxidation state and then increase the first discharge capacity. These interesting results derived from the anion doping stimulated our research interest to investigate the effect of other anions doping. To the best of our knowledge, up to now, no Br−-doped cathode materials (LiMn2O4–yBry) have been reported.
It is believed that single-phase, homogeneity, uniform particle morphology with nanometer size distribution is the desired feature for achieving a higher electrode activity . Nanometer-scale structured electrode materials are of great interest as potential building blocks for future generation electronic devices with greatly reduced size , because they show higher capacity and better cycling performance than conventional electrodes composed of this kind of materials . There have been increased interests in synthesizing nanostructures and their derivative compounds for their diverse physicochemical properties and potential applications as cathode materials for lithium–ion batteries . Moreover, it is well known that the preparation methods and post-treatment techniques could influence the structure and electrochemical performance of materials significantly [17, 18, 19]. Further, the subtle variation of chemical composition [20, 21] will bring great changes in the electrochemical performance of products. Room temperature solid-state coordination method [22, 23] possesses the advantages of simple manipulation and better prospects for commercialization as compared to the conventional solid-state method . More meaningfully, it achieves homogeneous mixture of the starting components, a low synthesis temperature and small grain size of the powders. Besides this, several nanomaterials have been synthesized using this method [25, 26, 27]. In this study, LiMn2O4–yBry nanoparticles were synthesized successfully for the first time by a room temperature solid-state coordination method. The structures, morphologies, and electrochemical properties of these materials were also investigated.
An X-ray diffractometer (XRD) (MXP18AHF, Mac, Japan) with Cu Kα radiation (λ = 1.54056 Å) was used for the identification of the crystalline phases of the powders. The morphological characteristics of the products were investigated using transmission electron microscope (TEM, H-600, Hitachi, Japan).
The cells consisted of a LiMn2O4-based composite as the positive electrode, a Li disk as the negative electrode, and an electrolyte of 1 M LiPF6in a 1:1 (volume ratio) mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC). The cathode was formed by mixing the active material with acetylene black and PVDF binder in 85:10:5 ratio inN-methyl-pyrrolidone (NMP). NMP acts as the solvent for the binder. The paste was applied to an aluminum foil current collector using a blade. The film was dried at 60 °C in air for 1 h and then was vacuum dried at 120 °C for 4 h. Celgard 2300 membrane was used as the separator. The cells were assembled in an argon-filled glove box. All the electrochemical tests were carried out at room temperature. Cyclic voltammetry (CV) (CHI660B Electrochemical Workstation Chenhua Co. of Shanghai, China) experiments were conducted from 3.2 to 4.35 V at a scan rate of 0.1 mV/s, and a Li metal disk served as both counter and reference electrode. Charge/discharge tests were performed at a constant current density of 0.30 mA/cm2within the potential range of 3.0 and 4.35 V.
Results and Discussion
Lattice constants calculated from the XRD spectra
Calcination temperature (°C)
Lattice parameter (a/nm)
In this work, LiMn2O4–yBrynanoparticles were synthesized by a room temperature solid-state coordination method for the first time. The powders have homogeneous morphology, small particles, and high crystallinity. The CV and charge/discharge test revealed that Br−doping improves the initial discharge capacity of the samples. The LiMn2O3.95Br0.05calcined at 800 °C has an initial discharge capacity of 134 mAh/g.
This work was partially supported by the Nature Science Foundation of Xinjiang Province (grant nos. 200821121 and 200721102) and the National Nature Science Foundation of China (grant nos. 20666005 and 20661003).