Impact of Nb Substitution on Crystal Structures and Electrode Reversibility of LiNiO2

Abstract

Introduction Lithium-ion batteries with high energy density have been currently used for transportation applications such as electric vehicles. As a positive electrode material, layered LiNiO2 shows high reversible capacity, good rate characteristics, and low material cost when compared to LiCoO2. However, a complicated phase transitions during charge and discharge proceed for LiNiO2, which influences long-term cyclability.[1] In this study, impact of Nb substitution on crystal structures and electrode reversibility of LiNiO2 is examined, and its origin of improved reversibility as electrode materials is discussed in detail. Experimental Nb-substituted LiNiO2 samples were synthesized from LiOH・H2O, Ni (OH)2, and Nb2O5 used as starting materials, and mixtures were calcined at 650 oC. Crystal structures of the samples were analyzed by synchrotron X-ray diffraction (XRD), and the particle morphology was observed by using a scanning electron microscope (SEM). Results and Discussion Synchrotron XRD data were collected for both LiNiO2 with or without Nb substitution. as shown in Figure 1. Although Nb substitution (5 mol% of Nb) for LiNiO2 clearly results in lowered crystallinity, successful synthesis of a rocksalt layered oxide, which is isostructural with LiNiO2, is noted from synchrotron XRD data.Figure 2 shows results of electrochemical properties of LiNiO2 with or without Nb substitution. Although LiNiO2 without Nb ions delivers an initial discharge capacity of over 200 mA h g-1, the increase in polarization and reduction of reversible capacities are observed on continuous electrochemical cycles. On the other hand, the increase in polarization was effectively suppressed for the Nb-substituted sample, leading to good reversibility.Based on these results, we will discuss factors affecting electrode reversibility of LiNiO2 and the possibility of cobalt-free high-capacity and long-cycle life positive electrode materials. Reference [1] U.-H.Kim et.al., and Y.-K.Sun, Energy Environ. Sci., 11, 1271 (2018). Figure 1