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<p>  </p> <p>Fossil fuel has dominated the global energy market for centuries, and the world is undergoing a great energy revolution from fossil fuel energy to renewable energies, given the concerns on global warming and extreme weather caused by the emission of carbon dioxide. Lithium ion batteries (LIBs) play an irreplaceable role in this incredible energy transition from fossil energy to renewable energy, given their importance in energy storage for electricity grids and promoting the mass adoption of battery electric vehicles (BEVs). Extreme fast charging (XFC) of LIBs, aiming to shorten the charging time to 15 minutes, will significantly improve their adoption in both the EV market and grid energy storage. However, XFC has been significantly hindered by the relatively sluggish Li+ transport within LIBs.</p> <p>Herein, effects caused by increasing charging rates (from 1C, 4C to 6C) on LiNi0.6Mn0.2Co0.2O2 (NMC622) || graphite cell were systematically probed via various characterization methods. From electrochemical test on their rate/long term cycling performance, the significant decrease in available capacity under high charging rates was verified. Structural evolutions of cycled NMC622 cathode and graphite anode were further probed via ex-situ powder diffraction, and it was found that lattice parameters <em>a</em> and <em>c</em> of NMC622 experience irreversible evolution due to loss of active Li+ within NMC622; no structural evolution was found for the graphite anode, even after 200 cycles under 6C (10 minutes) high charging rates. The aging behavior of liquid electrolyte was further analyzed via inductively coupled plasma-optical emission spectrometry (ICP-OES) and gas chromatography-mass spectrometry (GC-MS), increased Li+ concentration under higher charging rates and show-up of diethyl carbonate (DEC) and dimethyl carbonate (DMC) caused by transesterification both suggest faster aging/degradation of liquid electrolyte under higher charging rates.  </p> <p>Given the structural evolution of NMC622 caused by irreversible Li+ loss after long term cycling, the structural evolution of both NMC622 cathode and lithiated graphite anode were further studied via operando neutron diffraction on customized LiNi0.6Mn0.2Co0.2O2 (NMC622) || graphite cell. Via a quantitative analysis of collected Bragg peaks for NMC622 and lithiated graphite anode, we found the rate independent structural evolution of NMC622: its lattice parameters <em>a</em> and <em>c</em> are mainly determined by Li+ contents within it (<em>x</em> within Li<em>x</em>Ni0.6Mn0.2Co0.2O2) and follow the same evolution during the deintercalation process, from slowest 0.27 C charging to the fastest 4.4 C charging. For graphite intercalated compounds (GICs) formed during Li+ intercalating into graphite, the sequential phase transition from pure graphite → stage III (LiC30) → stage II (LiC12) → stage I (LiC6) phase under 0.27 C charging is consistent with previous studies. This sequential phase transition is generally maintained under increasing charging rates, and the co-existence of LiC12 phase and LiC6 was found for lithiated graphite under 4.4 C charging, mainly due to the large inhomogeneity under these high charging rates. Meanwhile, for the stage II (LiC12) → stage I (LiC6) transition, which contributes half the specific capacity for the graphite anode, quantitative analysis via Johnson-Mehl-Avrami-Kolmogorov (JMAK) model suggests it to be a diffusion-controlled, one-dimensional transition, with decreasing nucleation kinetics under increasing charging rates. </p> <p>Based on the LiC12 → LiC6 transition process, strategies to improve the Li+ transport properties were further utilized. Various cosolvents with smaller viscosity, from dimethyl carbonate (DMC), ethyl acetate (EA), methyl acetate (MA) to ethyl formate (EF), were further tested by replacing 20% (weight percent) ethyl methyl carbonate (EMC) of typical 1.2 M LiPF6 salt solvated in ethylene carbonate (EC)/EMC solvents (with a weight ratio of 30:70). From the measurement of their ion conductivity, the introduction of these cosolvents indeed enhanced the Li+ transport properties. This was further verified by improved rate performance from 2C, 3C to 4C charging for liquid electrolytes using these cosolvents. Both X-ray absorption spectroscopy (XAS) and X-ray powder diffraction (XRD) indicated the increase of Ni valence state and structural evolution of NMC622, all resulting from the irreversible loss of active Li+ within the NMC622 cathode. From long term cycling performance and further analysis of interfaces formed between electrode and anode, the best performance of electrolyte using DMC cosolvent was attributed to the most stable solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) formed during the cycling. </p>

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<p>Thin metal films have found their use in many magnetic devices. They form pseudo two-dimensional systems, where the mechanisms for the magnetic interactions between the layers are not completely understood. Layered crystal structures have an advantage over such artificial systems, since the layers can be strictly mono-atomic without any unwanted admixture. In this study, some model systems of layered magnetic crystal structures and their solid solutions have been investigated by x-ray and neutron diffraction, Mössbauer and electron spectroscopy, heat-capacity and magnetic measurements, and first-principle electronic structure calculations, with the goal of deepening our understanding through controlled chemical synthesis.</p><p>The compounds TlCo₂S₂, TlCo₂Se₂ and their solid solution TlCo₂Se_2-xS_x, all containing well separated cobalt atom sheets, order with the moments ferromagnetically aligned within the sheets. In TlCo₂S₂, the net result is ferromagnetism, while TlCo₂Se₂ exhibits antiferromagnetism. The inter-layer distance is crucial for the long-range coupling, and it was varied systematically through Se-S substitution. The incommensurate helical magnetic structure found for TlCo₂Se₂ (x = 0) prevails in the composition range 0 ≤ x ≤ 1.5 but the pitch of the helix changes. The accompanying reduction in inter-layer distance on sulphur substitution varies almost linearly with the coupling angle of the helix. An additional competing commensurate helix (90°) appears in the medium composition range (found for x = 0.5 and 1.0).</p><p>The systems TlCo_2-xMe_xSe₂ show helical magnetic ordering for Me = Fe or Cu, while a collinear antiferromagnetic structure occurs for Me = Ni. Magnetic order is created by iron substitution for copper in the Pauli paramagnetic TlCu₂Se₂, but now with the moments perpendicular to the metal sheets.</p><p>TlCrTe₂ forms a quite different crystal structure, with intra-layer ferromagnetic alignment and net collinear antiferromagnetism. In contrast to the other phases, the values of the moments conform well to a localised model for Cr³⁺.</p>

Synthesis, nuclear structure, and magnetic properties of the perovskites: Nd0.7-xMgxSr0.3MnO3 (x=0.0, 0.1), Nd0.6Mg0.1Sr0.3Mn1-zMgzO3 (z=0.1, 0.2), LaCr1-yMnyO3 (y=0.0, 0.1, 0.2, 0.3) and La1-xNdxFe0.5Cr0.5O3 (x=0.1, 0.15, 0.2) have been studied. The structure of the samples was investigated by X-ray and Neutron powder diffraction and the magnetic properties were investigated by magnetization measurements using SQUID-magnetometry. All compounds have orthorhombic structure with spacegroup Pnma (No. 62). The compounds which had the composition Nd0.7Sr0.3Mn1-yMgyO3 by preparation, were found to attain the composition Nd0.7-xMgxSr0.3MnO3 and Nd0.6Mg0.1Sr0.3Mn1-zMgzO3. The x=0.0 and 0.1 compounds order in a pure ferromagnetic structure at about 200 K, but the Mn moments become slightly tilted and attain an antiferromagnetic component below 20 K. A ferromagnetic Nd moment also appears at low temperatures. The compounds with Mg substitution y=0.2 and 0.3 do not exhibit long range magnetic order, but local ferromagnetic correlations among the Mn moments appear below 200 K. At low temperature, also a local antiferromagnetic ordering of the Nd magnetic moments occurs. In these compounds, the Mn3+/Mn4+ ratio is reduced so that the double exchange interaction is suppressed and the antiferromagnetic superexchange interaction favoured. The samples of composition LaCr1-yMnyO3 have orthorhombic structure at room temperature and below. The magnetic properties of the system are markedly affected by Mn-substitution. The parent compound LaCrO3 is a pure G-type antiferromagnet with Néel temperature 290 K. With incresing Mn-substitution, a ferromagnetic component developes in the ordered phase bcause of canting of the magnetic moments. The degree of canting increases with increasing Mn-substitution and the magnitude of the antiferromagnetic component of the moment decreases. The system La1-xNdxFe0.5Cr0.5O3 has the same antiferromagnetic G-type structure as LaCrO3, but orders already at temperatures above 400 K and develops only a very weak ferromagnetic component of the magnetic moment at low temperatures.

Thin metal films have found their use in many magnetic devices. They form pseudo two-dimensional systems, where the mechanisms for the magnetic interactions between the layers are not completely understood. Layered crystal structures have an advantage over such artificial systems, since the layers can be strictly mono-atomic without any unwanted admixture. In this study, some model systems of layered magnetic crystal structures and their solid solutions have been investigated by x-ray and neutron diffraction, Mössbauer and electron spectroscopy, heat-capacity and magnetic measurements, and first-principle electronic structure calculations, with the goal of deepening our understanding through controlled chemical synthesis. The compounds TlCo2S2, TlCo2Se2 and their solid solution TlCo2Se2-xSx, all containing well separated cobalt atom sheets, order with the moments ferromagnetically aligned within the sheets. In TlCo2S2, the net result is ferromagnetism, while TlCo2Se2 exhibits antiferromagnetism. The inter-layer distance is crucial for the long-range coupling, and it was varied systematically through Se-S substitution. The incommensurate helical magnetic structure found for TlCo2Se2 (x = 0) prevails in the composition range 0 ≤ x ≤ 1.5 but the pitch of the helix changes. The accompanying reduction in inter-layer distance on sulphur substitution varies almost linearly with the coupling angle of the helix. An additional competing commensurate helix (90°) appears in the medium composition range (found for x = 0.5 and 1.0). The systems TlCo2-xMexSe2 show helical magnetic ordering for Me = Fe or Cu, while a collinear antiferromagnetic structure occurs for Me = Ni. Magnetic order is created by iron substitution for copper in the Pauli paramagnetic TlCu2Se2, but now with the moments perpendicular to the metal sheets. TlCrTe2 forms a quite different crystal structure, with intra-layer ferromagnetic alignment and net collinear antiferromagnetism. In contrast to the other phases, the values of the moments conform well to a localised model for Cr3+.

Demands for new, energy-efficient appliances have greatly increased in response to our growing need for a more environmentally friendly society. Magnetic refrigeration is a technique that utilizes the magnetocaloric effect, with possible energy savings of up to 30% compared to commercial gas compression refrigerators. A material appropriate for commercial magnetocaloric devices should be both cheap and non-toxic; it should also exhibit a first-order magnetic transitions close to room temperature. The magnetic properties of Fe2P-related materials can be relevant in this context, since their magnetic properties can be finely tuned through the substitution of Fe by Mn and P by Si, As, Ge or B to meet the general requirements for a magnetocaloric device. An in-depth study has therefore here been made of the structural and magnetic properties of the (Fe,Mn)2(P,Si)-system. The phase diagram of the FeMnP1-xSix-system has been carefully re-examined. It is found to contain two single-phase regions: an orthorhombic Co2P-type structure (x &lt; 0.15) and a hexagonal Fe2P-type structure (0.24 ≤ x &lt; 0.50). Selected compounds within the Fe2P-type region of the phase diagram have been shown to exhibit potential for use in magnetic refrigeration applications. Neutron powder diffraction has here been used to determine the magnetic structures of selected crystalline compositions within the FeMnP1-xSix-system to gain a better understanding of its magnetic properties. The Fe2P-type region is mainly ferromagnetic, but an incommensurate antiferromagnetic structure has also been identified close to the Co2P/Fe2P-type phase border for x ≈ 0.25. The so-called ''virgin effect'' in the Fe2P-type region of the FeMn(P,Si) phase diagram is found to be accompanied by an irreversible structural phase transition induced by magnetostriction. This new phase is found to be preserved during successive cooling-heating cycles. Furthermore, the magnetic properties of the substituted Fe2P-type structure changes significantly for metal:non-metal ratios away from 2:1. Such deviations could well explain the apparently conflicting structure-property relationships described in earlier literature for the FeMnP1-xSix-system.

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<p>Materials with new or improved properties are crucial for technological development. To provide the foundation for future successful products, it is important to prepare and characterise new chemical compounds that could show unusual properties. The properties of magnetic materials are closely related to their crystal, magnetic and electronic structures. This thesis focuses on the novel synthesis and structural characterisation of a number of new ternary or <i>pseudo</i>-ternary silicides and germanides of manganese with iridium, cobalt or palladium. To provide a more complete picture of the complex magnetic properties, crystal and magnetic structure refinements by the Rietveld method of X-ray and neutron powder diffraction data are complemented by single-crystal X-ray diffraction, electron diffraction, magnetisation measurements and Reverse Monte Carlo simulations of magnetic short-range order. The experimental results are corroborated by first-principles electronic structure and total energy calculations. </p><p>A commensurate non-collinear antiferromagnetic structure is found for most compounds of the solid solution Mn₃Ir_1-yCo_ySi_1-xGe_x. The non-collinearity is a result of geometric frustration in a crystal structure with magnetic Mn atoms located on a three-dimensional network of triangles. The close structural similarity to the β-modification of elemental manganese, which does not order magnetically, inspired a closer theoretical comparison of the Mn₃Ir_1-yCo_ySi_1-xGe_x propertieswith β-Mn.</p><p>Magnetic frustration is also observed for Mn₄Ir_7-xMn_xGe₆, and is an important factor underlying the dramatic change from commensurate antiferromagnetic order to spin glass properties induced by a small variation in Mn concentration. Magnetic short-range order with dominant antiferromagnetic correlation is observed for Mn₈Pd₁₅Si₇, and results from a random distribution of Mn atoms in-between the geometrically frustrated magnetic moments on the Mn octahedra. </p><p>An incommensurate cycloidal magnetic structure, observed for IrMnSi, is stabilised by an electronic structure effect, which also accounts for the non-collinearity of the Mn₃IrSi type magnetic structure.</p>