Influence of Ni/Mn Ordering and Oxygen Deficiency on Structure, Magnetic and Electrochemical Properties of LiNi0.5Mn1.5O4

Abstract

LiNi0.5Mn1.5O4 (LNMO) is known as a 5 V class of cathode material for lithium-ion secondary batteries. LNMO adopts the spinel structure with two different space groups (Fd3-m and P4332) depending on the arrangement of Ni and Mn cations, and the difference of Ni/Mn arrangement influences their electrochemical properties. The Ni/Mn disordered phase exhibits a small step at 4.0 V attributable to Mn3+/4+ redox couple and two distinguishable plateaus around 4.7 V attributable to Ni2+/3+ and Ni3+/4+ redox couples in charge/discharge curves. Conversely, Ni/Mn ordered phase shows a single plateau around 4.7 V attributed to Ni2+/4+ redox couple and the low voltage plateau around 4.0 V is hardly observed. However, disordered phase (Fd3-m) has oxygen deficiencies and contains an impurity phase. Thus, the predominant factors of their electrochemical properties are not clear. Moreover, there are a few reports on magnetic properties of this system, in which the samples included an impurity of a rock salt phase. In this study, we synthesized Ni/Mn disordered phase of LNMO without oxygen deficiency and investigated the magnetic properties and the electrochemical properties. LNMO sample was synthesized from precursor of Ni and Mn oxalate prepared by coprecipitation method to obtain single-phase LNMO at low temperature. Mn/Ni ratio of the obtained precursor was confirmed to be the target composition by ICP analysis. Synthesis conditions of LNMO samples are summarized in Table 1. LNMO samples were characterized by powder X-ray diffraction measurements, Raman spectroscopy, iodometry, magnetic measurements and charge/discharge measurements. Powder X-ray diffraction measurements indicated that sample B, C and D were single-phase, while sample A slightly contained the rock salt phase. Raman spectroscopy revealed that Ni/Mn arrangement in sample A and B is disordered and that in sample C and D is ordered, indicating that annealing at 600 ˚C is necessary to obtain Ni/Mn ordered phases. Iodometry indicated that only sample A has the oxygen deficiencies. Conversely, sample B does not have oxygen deficiencies, thus we succeeded in synthesis of Ni/Mn disordered phase of LNMO without oxygen deficiency. The lattice constant of sample A is greater than other samples due to reduction from Mn4+ to Mn3+ with introducing the oxygen deficiencies. The saturated magnetization of sample A is lower than those of other samples due to the antiferromagnetic superexchange interaction between Mn4+ and Mn3+ via oxygen. The saturated magnetization is also influenced by Ni/Mn ordering. Moreover, Sample A and B exhibited different electrochemical properties. The sample A showed a small step at 4.1 V attributable to Mn3+/4+ redox couple, while sample B did not show any steps around 4.1 V. The discharge capacity of sample B decreases largely compared to sample A. We will discuss the influences of Ni/Mn ordering and oxygen deficiencies on structure, and magnetic and electrochemical properties of LNMO. Figure 1