Revisiting Inverse Spinels: Structure, Electrochemical Properties and Li-ion Diffusion Mechanisms in Li2+xNi2-2xCrxV2O8 (0 ≤ x ≤ 1)

Materials based on the normal spinel structure have attracted significant interest due to the wide number of elements that can be incorporated and the interesting electrical properties which they display. This, as well as the open three-dimensional framework that they contain, has led to the successful commercialisation of several Li-based spinel materials e.g. LiMn2O4 and Li4Ti5O12 in Li-ion batteries. Materials based on the inverse spinel structure have also been investigated. However, their continued development and optimisation has been restricted by the lack of a complete understanding of how Li-ions diffuse through such materials.
This thesis explores the synthesis, structural and electrical characterisation as well as the electrochemical properties of a series of novel inverse spinels with the formula Li2+xNi2-2xCrxV2O8 (0 ≤ x ≤ 1) for potential Li-ion battery applications. In Chapter 1, an introduction to the theory that underpins battery materials is outlined. Chapter 2 details a comprehensive review of the literature to date regarding spinel structures, as well as other common structure types of interest for Li-ion battery applications. Chapter 3 reports the experimental methods utilised within this work, including synthesis and characterisation techniques. The synthesis of Li2+xNi2-2xCrxV2O8 (0 ≤ x ≤ 1) via solid-state and citric acid sol-gel routes is investigated in Chapter 4. In analysis of combined X-ray and neutron powder diffraction refinements, the materials adopt an inverse spinel structure with no evidence of cation ordering on octahedral sites. Meanwhile, impedance measurements indicate that the total conductivity increases with increasing Li/Cr content. The determined activation energies are found to be comparable to those in normal spinels. In Chapter 5, the Li-ion diffusion is determined to be via a 16c-8a-16c pathway using variable-temperature neutron powder diffraction measurements. Alongside this, muon spectroscopy is used as a complementary technique to probe the local Li-ion diffusion kinetics. The electrochemical properties of the solid solution as potential cathode and anode materials are investigated in Chapter 6 using galvanostatic cycling and in-situ cycling XANES. The cathodic performance is shown to be limited due to the irreversible capacity loss caused by oxidation of Cr3+ to Cr6+. The anodic performance, however, shows promising capacity retention after 30 cycles for samples excluding Ni. Finally, the main findings of this work, as well as potential future avenues, are summarised in Chapter 7.