Synthesis and Electrochemical Properties of LiNi_0.5Mn_1.5O₄ for Li-Ion Batteries by the Metal–Organic Framework Method

Abstract

A LiNi_0.5Mn_1.5O₄ cathode material with high surface orientation was prepared via a complexing reaction coupled with the elevated-temperature solid-state method. First, a bimetal-organic framework containing Ni²⁺ and Mn²⁺ ions was synthesized via a self-assembly route using pyromellitic acid (PMA) as a dispersant and complexing agent. This step was followed by calcination with lithium acetate using PMA as a structure-directing agent. The resulting LiNi_0.5Mn_1.5O₄ (M-LNMO) cathode material was investigated using X-ray diffraction, transmission and scanning electron microscopies, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and charge/discharge tests. For comparison, LiNi_0.5Mn_1.5O₄ samples were prepared by coprecipitation and the solid-phase method under the same conditions. M-LNMO was highly crystalline with low impurity, uniform grain size, and a preferred orientation in the (111) and (110) planes. Owing to these advantages, the M-LNMO cathode material exhibited overwhelmingly high cyclic stability and rate capability and M-LNMO delivered a capacity of 145 mAh g^-1 at a discharge rate of 0.1C and a discharge capacity retention of 86.6% at 5C after 1000 cycles. Even at an extremely high discharge rate (10C), the specific capacity was 112.7 mAh g^-1, and 78.7% of its initial capacity was retained over 500 cycles. The superior electrochemical performance, particularly during a low-rate operation, was conferred by improved crystallinity and the crystal orientation of the particles.