Nanostructured LiNi_0.5Mn_1.5O₄ cathode material synthesized by polymer-assisted co-precipitation method with improved rate capability

Abstract

The polymer-assisted oxalate co-precipitation method has been developed to synthesize nanostructured LiNi_0.5Mn_1.5O₄. In this method, polyacrylic acid (PAA) as a dispersant agent was applied in the co-precipitation process to achieve the well-dispersed and homogenous precursor route. The obtained powder possesses small particle around 50–100 nm as compared to the one synthesized by traditional co-precipitation method. Besides, the porous texture was also found in as-fabricated LiNi_0.5Mn_1.5O₄. The X-ray diffraction reveals the well-crystallized cubic spinel structure without the presence of undesired impurities, such as NiO or Li_xNi_1−xO, commonly found in other fabrication method. An excellent cycling stability is obtained with the capacity retention over 90% after 100 cycles in 1 C at room temperature. Moreover, the superior rate capability was also derived by nanostructured LiNi_0.5Mn_1.5O₄, in which the discharge capacity in 5 and 7 C is around 110 and 100 mAhg⁻¹, respectively. The greatly enhanced rate capability is attributed to the significantly shortened Li-ion diffusion pathway, associating with the nano-textured particle morphology.

Introduction

Recently, the Ni-doped Manganese spinel oxide – LiNi_0.5Mn_1.5O₄ has become the most promising and attracting cathode material for lithium ion battery, due to its good electrochemical properties and high working potential around 5 V [1], [2]. A variety of synthetic methods for preparation of LiNi_0.5Mn_1.5O₄ have been developed, including solid state reaction [3], sol–gel [4], co-precipitation [5], [6], spray pyrolysis [7], [8], electrophoretic deposition [9], pulsed laser deposition [10], molten salt method [11] and emulsion drying [12]. In general, co-precipitation method has been widely used to synthesize LiNi_0.5Mn_1.5O₄ owing to the homogenous mixing of precursors to obtain well-crystallized spinel powders [13], [14], [15], [16]. Besides, the method is relatively low cost. However, particle aggregations and the lack of size uniformity are still observed in the resulting powder due to the undispersed mixture and the high temperature sintering process. In this study, in order to improve the size uniformity of LiNi_0.5Mn_1.5O₄ as well as to reduce particle aggregations, polymer-assisted oxalate co-precipitation method was applied. Polyacrylic acid (PAA) is used as chelating agent to accomplish the nanostructured LiNi_0.5Mn_1.5O₄ with uniformed particle size.