Investigating Substitutional Point Defects in Nickel-Rich Layered Oxides through Ion Exchange Synthesis

Abstract

The primary aim of this doctoral project was to comprehensively understand the capacity loss in the initial charge/discharge cycle of layered LiNiO2 (LNO) and related LiNiaCobMncO2 (NCM) cathode materials and to untangle the specific influences of material characteristics, such as particle size, composition and defect density. The project led to an innovative method of synthesizing LNO and Ni-rich NCM, diverging from the commonly used solid-state synthesis. This method produces layered oxides devoid of Ni_Li^• substitutional defects by creating sodium analogs of LNO and Ni-rich NCM, namely NaNiO2 and NaNiaCobMncO2. The larger size of sodium ions, compared to lithium ions, facilitates the formation of perfectly layered phases in these sodium analogs. Subsequently, such phases can be transformed into well layered LNO and NCM through an exchange of sodium ions with lithium ions. This approach enabled the examination of perfectly layered LNO and Ni-rich NCM for the first time. Three sets of monolithic LNO particles with differing grain sizes were synthesized using the developed ion exchange method, allowing the selective study of the impact of particle size on the initial capacity loss without contributions from Ni_Li^• defects. The study reconstructed the influence of Ni_Li^• substitutional defects on conventional LNO and validated the findings by introducing magnesium to the lithium site using a unique dual ion exchange approach. The absence of Ni_Li^• defects led to faster lithium diffusion, but resulted in material degradation at high potentials, thus highlighting the ambivalent role of Ni_Li^• substitutional defects, which contribute to stabilization at high states of charge, but also hinder diffusion. Additionally, the role of nickel content in the initial capacity loss was studied on ion-exchanged NCM materials with variable nickel content. Similar to the effects observed for Ni_Li^• defects, the study revealed a complex interplay between stability (thermodynamics) and diffusion (kinetics). Lower nickel contents were found to stabilize the material at high potentials, whereas higher nickel contents mitigated polarization during discharge. Throughout the investigation, a trade-off between material stability and lithium diffusion was observed. Materials with enhanced diffusion tended to be less stable and vice versa. The significant instability of LNO, even at low cut-off potentials of 4.3 V vs. Li+/Li, was observed in ion-exchanged materials for the first time. Literature known material was inherently stabilized due to the presence of Ni_Li^•, which obscured this property. This raises the question what the optimal concentration and the ideal properties of lithium ion substituents are.