A high-rate and long cycling life cathode for rechargeable lithium-ion batteries: hollow LiNi0.5Mn0.5O2 nano/micro hierarchical microspheres
Introduction
Recently, rechargeable lithium-ion batteries (LIBs) have gained enormous attention for applications in high power and energy density devices [1], [2], [3]. Since layered-structural LiNi0.5Mn0.5O2 (LNMO) was firstly reported by Ohzuku and Makimura [4], it is considered as a prospective candidate of LiCoO2 cathode owing to its high reversible capacity, abundant availability, better chemical stability, environmental benignancy and low cost [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Although LNMO exhibits several merits in its electrochemical performance, it suffers from poor intrinsic rate capability and poor cycle stability.
For the past few years, nanostructured layer cathode materials have been widely investigated by several research groups to improving their rate capability [13], [14], [15], [16], [17], [18], [19]. Nanoscale materials demonstrate shorter diffusion paths and higher electrolyte/electrode contact areas, which result in higher capacity and better rate capability. However, types of LNMO nanostructures reported are rather limited, which is mainly due to undesirable particle growth during the essential high-temperature sintering process. Therefore, morphologically controlled synthesis of LNMO nanostructures remains a great challenge. In addition, to identify the process of Li+ ions insertion and extraction, various characteristic technologies have been used to investigate the charge and discharge procedures, especially the first cycle [6], [13], [20]. Recently, D. Buchholz reported an investigation in to the first lithiation/delithiation of the high capacity layered cathode through X-ray absorption spectroscopy [20].
Recently, monodispersed hollow nano/micro hierarchical structures have attracted more and more attentions in high power batteries due to their characteristic features, such as smaller primary particle size and large inter void space. Many research groups have focus on the hollow nano/micro hierarchical structures to improve the electrochemical performances of cathodes, such as LiNi0.5Mn1.5O4 [1], LiMn2O4 [21], LiNi1/3Co1/3Mn1/3O2 [22], ect. The primary nanoparticles can effectively reduce the diffusion distances for Li+-ions, leading to better rate capability. The secondary micrometer structures can guarantee a good cycle life and easy fabrication of electrodes in commercial process. More importantly, the interior void space can buffer the volume change during repeated Li+-ions insertion/extraction, causing improved cycling life. However, the long cycling life has been rarely investigated for the hollow nano/micro hierarchical structures.
In this work, uniform hollow microspheres of LNMO (LNMO-HS) controlled synthesis are reported. LNMO-HS was characterized by XRD, SEM, TEM, XPS and with the electrochemical measurements. Furthermore, we paid close attention to the long cycling life of LNMO-HS cathode at high current density (15 C). It was exciting that LNMO-HS cathode delivers a high discharge capacity (134.6 mAh g−1) and the capacity retention (95.5% during 1000 charge and discharge cycles) at 15 C.
Section snippets
Preparation of LNMO-HS
LNMO-HS was synthesized by a simple impregnation process: Firstly, MnCO3 microspheres with about 1 μm in diameter were prepared through the method reported by our group [22]. Secondly, MnCO3 microspheres were dried and decomposed at 400 °C to obtain MnO2 microspheres. Then, stoichiometric amounts of the MnO2 microspheres, Ni(NO3)2·6H2O and LiOH·H2O were mixed to form a suspension in ethanol. Stirring was continued until the ethanol evaporated out at room temperature. Finally, the mixture was
Results and discussion
The XRD patterns of as-prepared MnCO3, MnO2 and LNMO-HS samples are shown in Fig. 1. All the peaks of MnCO3 can be readily indexed as a hexagonal structure with a space group of R-3c corresponding to MnCO3 (JCPDS No. 44-1472). After calcining MnCO3 at 400 °C, the XRD peaks were assigned to MnO2 (JCPDS No. 24-0735). Due to the large weight loss and release of CO2 during the thermal decomposition, the obtained MnO2 are highly porous.
To prepare LNMO-HS, a simple impregnation method is applied to
Conclusions
In conclusion, LiNi0.5Mn0.5O2 hollow nano/micro hierarchical microspheres (LNMO-HS) are synthesized through an impregnation strategy. The hollow microspherical morphology of LNMO-HS is controlled by as-prepared MnCO3 microspheres. LNMO-HS consists of approximately 100 nm nanoparticles and has large interior void space. As cathode for LIBs, LNMO-HS displays a high-rate capability and excellent cyclability, especially long cycling life. After 1000 cycles at 15C, LNMO-HS can maintain 95.5% of the
Acknowledgement
This work was supported financially by Natural Science Program of Henan Provincial Department of Education (No. 15A430034).
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