Development of the Ni-Rich Li1+XNi0.905Co0.043Al0.052O2 High-Energy-Density Cathode Materials for Li-Ion Cells: Operando XRD Studies, Surface Modification, and Electrochemical Properties

Abstract

With an increasing pressure to address major energy and environmental issues associated with consumption of the fossil fuels, the public is more and more aware of the need for electrification of the transportation sector. However, the rapid development of electric vehicles (EV) industry has put forward much higher and strict requirements for the energy storage density and safety of the applied batteries. Obviously, the accessible to the driving range per charge, depend on the characteristics of the EV batteries, which in turn depend mainly on the reversible capacity of the cathode and anode materials. To meet the demand of more than 500 kilometers driving range, development of high-energy-density cathode materials is required. Currently, the Ni-rich layered oxides (LiNiO2-type) are considered as primary candidates for a significant boosting of the properties, due to the very high reversible discharge capacity (>200 mAh g-1), high operating voltage (~3.7 V vs. Li/Li+), and relatively low costs. Nevertheless, such oxides still suffer from severe issues, such as surface sensitivity, poor structure with Li/Ni mixing effect, and insufficient thermal stability, which hinder their practical application. In particular, presence of LiOH/Li2CO3 lithium residuals in the active material, which originate from the synthesis process, has deleterious effect on the rate performance and may affect cycling stability of the cells.In this work, a simple vibratory dry mixing method was adopted to synthesize high-Ni-content Li1+xNi0.905Co0.043Al0.052O2 (NCA90) cathode materials with varying lithium excess used during the preparation route. The Ni-rich [Ni0.905Co0.043Al0.052](OH)2 precursor was used in the synthesis. The obtained samples maintained the desired ball-like morphology of the precursor, as well as layered R-3m crystal structure. To quantify the lattice volume changes during charge and discharge, operando XRD experiments were also conducted. A custom-made cell, having a Be window was used for the measurements. Two cycles were measured, with the first one up to 4.3 V, and the second up to 4.5 V. The registered charge/discharge curves, obtained using the operando cell, were nearly the same as those obtained using the normal 2032 coin-type half cells. Analysis of the XRD operando data showed that the a lattice parameter decreases monotonically as the charging proceeds, whereas the c parameter initially increases, and then begins to contract, for voltages higher than ca. 4.0 V. Above ca. 4.2 V, the c parameter abruptly decreases, which can be related to the so called H2 → H3 phase transition. It is worth noting that this transition is responsible for the capacity fading. During charge and discharge, the observed behavior of the 003 reflection was reversible, but showing small changes in the recorded intensity and position of the peak. Overall, the generated profiles of the structural changes of the Ni90 cathode materials are almost symmetrical, with only small asymmetric behavior, especially visible during the first cycle. The results indicate that the operando XRD tests are important for realizing the real-time monitoring and provide on-site information of the structural evolution and phase transitions occurring for the cathode materials. Also, the electrochemical performance of the NCA90 cathodes was characterized using 2032 coin-type half cells with a Li metal anode and 1 M LiPF6 in EC:DEC 1:1 vol. ratio electrolyte. The cycling tests were performed between 2.7 V and 4.3-4.5 V at different current densities. One of the best performing Li1.05Ni0.905Co0.043Al0.052O2 cathode delivered an initial discharge capacity of 234.5 mAh g-1 at 0.05 C, with a capacity retention of 70.7% after 100 cycles. Finally, an attempt was done to introduce surface coating of the obtained cathode materials, in order to enhance the capacity retention.