Mapping chemical, ionic and electronic properties of high capacity cathode materials for lithium-ion batteries

Thesis

Nickel-rich layered cathode materials, such as Li(Ni_0.8Mn_0.1Co_0.1)O₂ (NMC811), are integral to commercial Li-ion batteries for portable electronics and electric vehicles. However, the elevated nickel content in NMC811 induces accelerated degradation, limiting battery cycle life and performance. Increasing the upper cut-off potential enhances energy density. However, it exacerbates irreversible degradation at the cathode electrolyte interface (CEI), resulting in oxygen release and transition metal dissolution that impacts long-term battery cycling.

This study initially employed spatially resolved synchrotron source X-ray photoemission electron microscopy (XPEEM) to resolve the distribution of degradation products in the CEI and transition metal oxidation states at cycled polycrystalline NMC811 surfaces. Characterisation revealed particle-to-particle variation in degradation mechanisms, offering insights into transition metal dissolution.

Secondly, the role of transition metal dissolution from the cathode and subsequent deposition at graphitic anode interfaces is examined using X-ray Absorption Near Edge Structure (XANES) and Near Edge X-ray Absorption Fine Structure (NEXAFS) techniques. XANES measurements revealed ion exchange pathways for dissolved cations (Ni and Mn) and probed the redox activity of Ni and Mn on graphite electrodes. This investigation sheds light on the mechanisms of transition metal incorporation onto the graphite solid-electrolyte interphase (SEI) and the redox activity of Ni and Mn on the electrode surface.

In the final results chapter, an environmental cell was developed to track the evolution of solid-electrolyte interphase (SEI) formation on Ni and MnO using operando NEXAFS techniques. This novel ultra-high vacuum-compatible electrochemical cell design, incorporating SiN_x membranes, enabled monitoring of electronic structure changes during potential dependent SEI evolution.

This thesis thus presents advancements in material characterisation and offers crucial insights into the role of transition metals at battery interfaces. High-resolution XPEEM enables precise spatial resolution of surface electrode changes. XAS was utilised as a powerful tool for elucidating cation exchange and redox processes of transition metal cations at the anode interface. Moreover, the development of environmental cells facilitates the acquisition of valuable potential-dependent data under realistic battery operating conditions.

Authors: Fraser, MW

• DOI: 10.5287/ora-zbdney8ew
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: solid electrolyte interphase; Near Edge X-ray Absorption Fine Structure; X-ray Absorption Near Edge Structure; lithium nickel manganese cobalt oxide; lithium-ion battery; operando; NMC811; transition metal dissolution
• Subjects: Batteries; X-rays; Materials; Lithium ion batteries