Advanced characterisation of next generation battery cathode materials using X-rays

Abstract

In recent years, lithium-ion batteries have been the topic of focus in the automotive industry as an alternative source of power through electric vehicles. Research has focused on producing new cathode materials using a variety of transition metal oxides. However, we are still yet to understand their behaviour completely.

The focus of this Thesis was to investigate the changes that occur within the high-nickel cathode materials Li(Ni0.8Mn0.1Co0.1)O2 (NMC 811) and Li(Ni0.8Co0.15Al0.05)2 (NCA). This was achieved by developing a newly designed in situ electrochemical battery cell which enabled the materials to undergo X-ray analysis during a series of charge and discharge cycles at a variety of conditions.

High energy resolution fluorescence detection X-ray absorption near edge structure (HERFD-XANES) and X-ray emission spectroscopy (XES), were used to probe the oxidation state changes of nickel, manganese, and cobalt in NMC 811 and nickel oxidation state changes in NCA. From the changes documented in the HERFD-XANES studies, the oxidation state changes in high nickel cathode materials were quantified during a cell cycle. The oxidation state changes contained information on the behaviour of transition metal oxides during a charge and discharge cycle and provided a greater understanding of the reversibility of the electrochemically active nickel whilst charging to a variety of upper limit voltages. Features that were found from the quantification of HERFD-XANES was that the cobalt and manganese species within NMC 811 were more electrochemically active than previously reported. Within NCA, we report data on how a higher upper voltage limit can impact the reversibility of nickel species within the cathode materials during a first cycle despite reporting that the bulk of the material is reversible.

In situ and operando studies using X-ray diffraction (XRD) was able to probe the phase changes that NMC 811 and NCA undergo during a cell cycle when cycling to a range of various upper voltage limits. From these studies, we were able to quantify the lattice parameter changes in real time to understand the bulk changes of the material using XRD. Alongside the HERFD-XANES data which worked to understand the atomic level changes in elements using HERFD-XANES. From the high speed 2D XRD we were able to document a new feature in NMC 811 and NCA where we can see the expansion and contraction of the c-axis during a cell cycle and relaxation which helps us understand the behaviour of Ni4+ at high voltages and how the formation of Ni2+ occurs from the reduction of unstable nickel species.

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Identifiers

ORCID for Andrew Hector: orcid.org/0000-0002-9964-2163

ORCID for Andrea Russell: orcid.org/0000-0002-8382-6443

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