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

Abstract

A novel spherical concentration-gradient material with an average composition of LiMn_1.87Ni_0.13O₄ 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 LiMn_1.87Ni_0.13O₄ is LiMn_1.5Ni_0.5O₄. The physicochemical and electrochemical performances of the spherical LiMn_1.87Ni_0.13O₄ 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 LiMn_1.87Ni_0.13O₄ sample has a typical Fd3m spinel structure. It can be found from the cross-sectional SEM images and EDXS analysis that the LiMn_1.87Ni_0.13O₄ particles are quite homogeneous without any apparent gap between the inner core and the outer concentration-gradient layer. Especially, the LiMn_1.87Ni_0.13O₄ 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 LiMn_1.87Ni_0.13O₄ sample shows excellent capacity stability even at an elevated temperature, i.e. 55 °C, where a traditional LiMn₂O₄ sample inevitably fails. Thus, the LiMn_1.87Ni_0.13O₄ 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.