Structure and Electrochemistry of Modified Transition Metal Oxides as Electrodes for Metal-Ion Batteries

Abstract

FeMn-based P2 phases are attractive cathode materials for sodium ion batteries due to their combination of impressive electrochemical performance, environmental friendliness and low-cost reagents required for synthesis. In this thesis, K-modified P2-Na2/3Fe2/3Mn1/3O2 phases synthesised via the addition of stoichiometric amounts of K2CO3, up to 0.16 per formula unit, were investigated. A thorough characterisation of the structure, morphology and electrochemical performance of the K-modified P2 phase was undertaken using X-ray diffraction, neutron powder diffraction, scanning electron microscopy, energy dispersive spectroscopy experiments differential capacity analysis, in situ X-ray diffraction experiments under electrochemical cycling, and electrochemical measurements. New candidate Li-ion battery anode materials, monoclinic Li4Ti5O12, and Mn-doped monoclinic Li4-yNayMn0.5Ti4.5O12, synthesised by ion-exchange from M-Na4Ti5O12 and M-Na4Mn0.5Ti4.5O12 phases, have been structurally characterised and electrochemically investigated as lithium ion battery anode materials. Results indicated complete ion-exchange for the Li4Ti5O12 phase, while some Na remained in the M-Li4-yNayMn0.5Ti4.5O12 phase, such that the final stoichiometry was Li3.75Na0.25Mn0.5Ti4.5O12. Electrochemically, M-Li4Ti5O12 was found to afford a reversible specific capacity of 65 mAh/g after 50 cycles and display a promising rate capability, with stable electrochemical performance at rates of C/20 and 2C. Ex situ X-ray powder diffraction data collected at various points during electrochemical cycling and inspection of voltage profiles revealed that phase changes in Li4Ti5O12 proceed via a two-phase mechanism, in contrast to M-Na4Ti5O12 which proceeds via a solid-solution mechanism.