Cheracterisation of Battery Materials Using Surface Science Techniques

Abstract

LiNixMnyCozO2 (NMC) layered oxide cathodes are commonly used in lithium-ion batteries (LIBs) for their high capacity and energy density, particularly with increased nickel content. However, nickel-rich NMC cathodes exhibit severe interaction with electrolyte that leads to formation of cathode electrolyte interphase (CEI) and rapid capacity fade. Comparably, the graphite anode, commonly used for its high capacity and stability, forms a solid electrolyte interphase (SEI) during cycling. These passivation layers act as protective barriers but are not ideal, which undergo dissolution, breakage, and repair throughout cycling, impacting battery performance. The further deployment of the LIBs requires a deeper understanding of the dynamic chemical process occurring on the electrode surface. This thesis employed Secondary Ion Mass Spectrometry (SIMS) and Hard X-ray Photoelectron Spectroscopy (HAXPES) to investigate the surface and subsurface of materials related to NMC cathode. In this context, in Chapter 4, we examine the effect of C60, Arn, (CO2)n and (H2O)n primary ion beams on MnO, MnO2, Co3O4 and NiO metal oxides. Results indicate that gas cluster ion beams (GCIBs) containing CO2 and H2O enhance metal oxide and metal hydroxide secondary ions yields compared to those with Ar only, providing insights into ion-beam interactions. In Chapter 5, we explore the application of HAXPES for analyzing the 1s orbital of first row transition metals (FRTMs) in cobalt, manganese and nickel compounds. By measuring the separation between transition metal (TM) satellite-main peaks and TM satellite-O 1peaks, we develop a reliable approach to differentiate oxidation states, providing a precise determination for each metal oxide. Chapter 6 compares the CEI layer across various cycling states for NMC111, NMC532 and NMC811 cathodes. The results reveal that the CEI begins forming in the not-charged (N.C.) and thickens in the single-charged (S.C.) state, with thickness increasing with nickel content. A detailed analysis of the CEI on the NMC111 electrode across N.C., S.C., single-charge-discharged (S.C.D.) and end-of-life (E.O.L.) states show that the CEI layer reaches its maximum thickness in the S.C. state before decreasing in subsequent states. SIMS analysis indicates that the CEI layer comprises a dual-layer structure. Additionally, the SEI layer on the graphite anode shows that electrolyte degradation starts in the N.C. state, peak in the S.C. state, and then decreases during subsequent cycles. Collectively, the work reported in this thesis demonstrates improved understanding of the methodology of electrode characterization using SIMS and HAXPES and through their application, the dynamic chemical processes occurring at LIB electrodes.