<p>The formation of surface films on electrodes in contact with non-aqueous electrolytes in lithium-ion batteries has a vital impact on battery performance. A basic understanding of such films is essential to the development of next-generation power sources. The surface chemistry, morphology and thermal stability of two typical anode and cathode materials, graphite and LiNi_0.8Co_0.2O₂, have here been evaluated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction, scanning electron microscopy and differential scanning calorimetry, and placed in relation to the electrochemical performance of the electrodes. </p><p>Chemical and morphological information on electrochemically formed graphite surface films has been obtained accurately by combining XPS measurements with Ar⁺ ion etching. An improved picture of the spatial organisation, including thickness determination of the surface film and characterisation of individual component species, has been established by a novel sputtering calibration procedure. The stability of the surface films has been shown to depend strongly on temperature and choice of lithium salt. Decomposition products from elevated-temperature storage in different electrolyte systems were identified and coupled to effects such as capacity loss and increase in electrode resistance. Different decomposition mechanisms are proposed for surface films formed in electrolytes containing LiBF₄, LiPF₆, LiN(SO₂CF₃)₂ and LiCF₃SO₃ salts.</p><p>Surface film formation due to electrolyte decomposition has been confirmed on LiNi_0.8Co_0.2O₂ positive electrodes. An overall surface-layer increase with temperature has been identified and provides an explanation for the impedance increase the material experiences on elevated-temperature storage. </p><p>Surface phenomena are clearly major factors to consider in selecting materials for practical Li-ion batteries.</p>

The formation of surface films on electrodes in contact with non-aqueous electrolytes in lithium-ion batteries has a vital impact on battery performance. A basic understanding of such films is essential to the development of next-generation power sources. The surface chemistry, morphology and thermal stability of two typical anode and cathode materials, graphite and LiNi0.8Co0.2O2, have here been evaluated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction, scanning electron microscopy and differential scanning calorimetry, and placed in relation to the electrochemical performance of the electrodes. Chemical and morphological information on electrochemically formed graphite surface films has been obtained accurately by combining XPS measurements with Ar+ ion etching. An improved picture of the spatial organisation, including thickness determination of the surface film and characterisation of individual component species, has been established by a novel sputtering calibration procedure. The stability of the surface films has been shown to depend strongly on temperature and choice of lithium salt. Decomposition products from elevated-temperature storage in different electrolyte systems were identified and coupled to effects such as capacity loss and increase in electrode resistance. Different decomposition mechanisms are proposed for surface films formed in electrolytes containing LiBF4, LiPF6, LiN(SO2CF3)2 and LiCF3SO3 salts. Surface film formation due to electrolyte decomposition has been confirmed on LiNi0.8Co0.2O2 positive electrodes. An overall surface-layer increase with temperature has been identified and provides an explanation for the impedance increase the material experiences on elevated-temperature storage. Surface phenomena are clearly major factors to consider in selecting materials for practical Li-ion batteries.

<p>This thesis presents an investigation into the structure, composition and performance of naturally formed surface films on AZ alloys as a function of pH and alloyed Al concentration. STEM verified the film structure was bi-layer, consisting of an inner barrier layer, which was visibly deteriorated, and an outer porous layer. EDS SmartMaps™ coupled with the Inca™ software package determined the inner barrier layer was predominantly composed of MgO, whereas the outer layer was primarily Mg(OH)₂. However, both layers appeared to posses mixed oxide/hydroxide components according to ToF-SIMS analysis.</p> <p>Environmental pH had the largest effect on the structure and composition of the surface film. The near-neutral sample showed significant breakdown within the inner layer, which was attributed to natural hydration of MgO to Mg(OH)₂. This favourable hydration reaction is slower in alkaline environments and as such, the stability of the inner barrier layers of the pH 14 samples were noticeably improved. The effect of alloyed Al concentration was less significant however; increased enrichment of Al into the surface film structure appeared to cause a reduction in the thickness of the corrosion film itself.</p> <p>Drastic differences in corrosion performance were observed between the near-neutral and alkaline environments. Significantly better corrosion resistance to anodic dissolution was present in the alkaline environment coupled with a noticeably lower corrosion rate. The absence of breakdown potentials along with the presence of mass transport controlled anodic kinetics signified that the improved stability of the inner barrier layer was responsible for improved corrosion performance. In contrast, severe pitting and a narrow range of anodic stability were present for the near-neutral samples where the inner barrier layer was significantly compromised. This deterioration was deemed responsible for accelerated cathodic kinetics as well as minimal impedance to aggressive Cl^- ions from initiating wide scale electrochemical breakdown of the surface film.</p> / Master of Applied Science (MASc)