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Abstract: LiNi0.5Mn1.5O4 (LNMO) cathodes offer a cobalt-free, high-voltage alternative to current state-of-the-art Li-ion battery cathodes, and are particularly well-suited for high-power applications due to their 3D lithium-ion pathways and structural stability. However, degradation of commercial electrolytes at high voltages exacerbates capacity decay, as instability at the cathode surface causes active material loss, surface reconstructions, thickening surface layers, and increases in internal cell resistance. Cationic substitution has been proposed to enhance surface stability, thus limiting capacity decay. Here, we demonstrate the stabilizing effect of Mg on the LNMO cathode surface, which is most evident during the early stages of cycling. This study indicates that improved O 2p-TM 3d hybridization in Mg-substituted LNMO, facilitated by Li-site defects, leads to the formation of a stable surface layer that is corrosion-resistant at high voltage. Examination of Fe-substituted and unsubstituted LNMO further confirms that the surface stability is uniquely enabled by Mg substitution. This work offers valuable insights into surface design for reducing degradation in high-voltage spinel cathodes.

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Abstract: The solid electrolyte interphase that forms on Li6PS5Cl argyrodite solid electrolytes has been reported to continually grow through a diffusion-controlled process, yet this process is not fully understood. Here, we use a combination of electrochemical and X-ray photoelectron spectroscopy techniques to elucidate the role of phosphorus in this growth mechanism. We uncover how Li6PS5Cl can decompose at potentials well above the full reduction to Li3P, forming partially lithiated phosphorus species, LixP. We provide evidence of a gradient of LixP species throughout the solid electrolyte interphase and propose a growth mechanism in which the rate-determining step is the diffusion of lithium through LixP. We predict continuous solid electrolyte interphase growth as long as metallic lithium is present and a LixP percolation pathway exists, highlighting the importance of understanding and engineering solid electrolyte interphase composition and nanostructure in solid-state batteries. We believe that this growth mechanism would apply to any solid electrolyte interphase that can contain partially lithiated phosphorus, or potentially any lithium alloy.

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Abstract: Exotic quasiparticle states have been proposed in mixed-valent compounds exhibiting valence transitions. However, clear spectroscopic evidence identifying these states has remained elusive. Using synchrotron-based hard x-ray and extreme ultraviolet photoemission spectroscopy, we have probed the Tm 3⁢𝑑 and 4⁢𝑓 emissions in TmSe1−𝑥⁢Te𝑥, where a Te concentration-dependent semimetal–insulator transition occurs alongside the valence transition. Our photoemission results, which are characteristic of the bulk, track this combined transition across the critical concentration (𝑥𝑐 =0.29). Notably, our results reveal a noninteger valence for the insulating phase and a novel quasiparticle excitation in the semimetallic phase: a Holstein polaron that extends beyond the standard periodic Anderson model.

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Abstract: Chemical vapour deposition enables large-domain growth of ideal graphene, yet many applications of graphene require the controlled inclusion of specific defects. We present a one-step chemical vapour deposition procedure aimed at retaining the precursor topology when incorporated into the grown carbonaceous film. When azupyrene, the molecular analogue of the Stone–Wales defect in graphene, is used as a precursor, carbonaceous monolayers with a range of morphologies are produced as a function of the copper substrate growth temperature. The higher the substrate temperature during deposition, the closer the resulting monolayer is to ideal graphene. Analysis, with a set of complementary materials characterisation techniques, reveals morphological changes closely correlated with changes in the atomic adsorption heights, network topology, and concentration of 5-/7-membered carbon rings. The engineered defective carbon monolayers can be transferred to different substrates, potentially enabling applications in nanoelectronics, sensorics, and catalysis.

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Abstract: This thesis explores the influence of close space sublimation (CSS) growth conditions on antimony selenide solar cells as well as the possible benefits of post-growth processing approaches including an assessment of protective layer annealing. Single and two-step growth approaches involving the use of seed layers to modify film coverage and grain structure were investigated as a way of improving the solar cell performance. Both isolated layers and complete device structures were fabricated to allow investigation of the interrelation of preferred ribbon orientation with device efficiency. It was identified that, whilst the use of a seed layer was an important step to achieve good film coverage and grain morphology, the ribbon orientation appeared to have minimal influence on performance. The developed CSS growth approaches were then expanded to produce antimony sulfoselenide films and devices for a single phase source material. It was demonstrated that the approach was feasible, allowing the formation of material with a notably higher bandgap than for the base selenide. This indicated that material did not completely degrade during sublimation with the resulting devices achieve >4% efficiency and with a notably higher open circuit voltage than selenide counterparts. There were however significant issues with the formation of large oxide phase regions within the absorber. These served to reduce the device performance with the cause being attributed to sulphur loss and reaction with oxygen, the growth ambient during deposition. Post growth annealing approaches to improve antimony selenide solar cell efficiency were systematically investigated. Air, selenium, and nitrogen environments were initially compared across a broad temperature range. The results highlighted the degree of sensitivity of the material to post growth annealing with bot air annealing and selenization causing minimal changes to film morphology but drastic performance loss. Nitrogen annealing appeared more favourable with some minor open circuit voltage increases, both again the overall trend was a decrease in cell performance. To overcome these limitations the nitrogen ambient annealing approach was expanded to a protective layer annealing approach. A series of capping layers CdS, ZnO and P2O5 were deposited on the back surface prior to the annealing process to protect the antimony selenide layers and then etched off prior to device completion. The CdS capping layer was found to protect the surface from oxidation but frustratingly still resulted in performance decreases. There was however one “outlier” device series which showed a marked improvement for all device parameters. This result was not reproducible despite many attempts but seemed to indicate the potential of the approach so other materials were investigated. ZnO was considered but it was quickly determined it was unsuited as a capping layer. P2O5 however was tested and despite the limited number of samples being able to be prepared, it was found to notably improve device performance even with short time and low temperature anneals. Secondary ion mass spectrometry analysis showed significant quantities of phosphorus had been incorporated in the film during annealing. This finding demonstrates there is high potential from the protective layer annealing approach and indicates additional work in this area could leave to improved device efficiencies.