Spherical concentration-gradient LiMn1.87Ni0.13O4 spinel as a high performance cathode for lithium ion batteries

A novel spherical concentration-gradient material with an average composition of LiMn1.87Ni0.13O4 is successfully synthesized via a co-precipitation route, in which the homogeneous LiMn2O4 core is encapsulated by a continuously Ni increasing concentration-gradient layer, and the composition of the outmost layer of the spherical LiMn1.87Ni0.13O4 is LiMn1.5Ni0.5O4. The physicochemical and electrochemical performances of the spherical LiMn1.87Ni0.13O4 sample are investigated by X-ray diffraction (XRD) and electrochemical tests, and using a scanning electron microscope (SEM) with an energy-dispersive X-ray spectroscope (EDXS). The results show that the LiMn1.87Ni0.13O4 sample has a typical Fd3m spinel structure. It can be found from the cross-sectional SEM images and EDXS analysis that the LiMn1.87Ni0.13O4 particles are quite homogeneous without any apparent gap between the inner core and the outer concentration-gradient layer. Especially, the LiMn1.87Ni0.13O4 sample has excellent performance at an elevated temperature. It delivers a discharge capacity of 108.2 mA h g−1 between 3.0 and 4.4 V vs. Li/Li+ with a retention of 90.2% over 200 cycles at a rate of 0.5 C (74 mA g−1) at 55 °C. Besides, it has an exceptional capacity of 129.1 mA h g−1 between 3.0 and 4.9 V with a retention of 91.9% over 100 cycles at a rate of 0.5 C at 55 °C. Apparently, the LiMn1.87Ni0.13O4 sample shows excellent capacity stability even at an elevated temperature, i.e. 55 °C, where a traditional LiMn2O4 sample inevitably fails. Thus, the LiMn1.87Ni0.13O4 sample with a homogeneous LiMn2O4 core material and an isotropy concentration-gradient outer layer shell will be a promising cathode material for advanced lithium ion batteries.