The release of the lattice oxygen due to the thermal degradation of layered lithium transition metal oxides is one of the major safety concerns in Li-ion batteries. The oxygen release is generally attributed to the phase transitions from the layered structure to spinel and rocksalt structures that contain less lattice oxygen. Here, a different degradation pathway in LiCoO2 is found, through oxygen vacancy facilitated cation migration and reduction. This process leaves undercoordinated oxygen that gives rise to oxygen release while the structure integrity of the defect-free region is mostly preserved. This oxygen release mechanism can be called surface degradation due to the kinetic control of the cation migration but has a slow surface to bulk propagation with continuous loss of the surface cation ions. It is also strongly correlated with the high-voltage cycling defects that end up with a significant local oxygen release at low temperatures. This work unveils the thermal vulnerability of high-voltage Li-ion batteries and the critical role of the surface fraction as a general mitigating approach.

Temperature- and coverage-dependent studies of the Au(1 1 1)-supported spin crossover Fe(II) complex (SCO) of the type [Fe(H2B(pz)2)2(bipy)] with a suite of surface-sensitive spectroscopy and microscopy tools show that the substrate inhibits thermally induced transitions of the molecular spin state, so that both high-spin and low-spin states are preserved far beyond the spin transition temperature of free molecules. Scanning tunneling microscopy confirms that [Fe(H2B(pz)2)2(bipy)] grows as ordered, molecular bilayer islands at sub-monolayer coverage and as disordered film at higher coverage. The temperature dependence of the electronic structure suggest that the SCO films exhibit a mixture of spin states at room temperature, but upon cooling below the spin crossover transition the film spin state is best described as a mix of high-spin and low-spin state molecules of a ratio that is constant. This locking of the spin state is most likely the result of a substrate-induced conformational change of the interfacial molecules, but it is estimated that also the intra-atomic electron-electron Coulomb correlation energy, or Hubbard correlation energy U, could be an additional contributing factor.

Quantitative and well-targeted design of modern alloys is extremely challenging due to their immense compositional space. When considering only 50 elements for compositional blending the number of possible alloys is practically infinite, as is the associated unexplored property realm. In this paper, we present a simple property-targeted quantitative design approach for atomic-level complexity in complex concentrated and high-entropy alloys, based on quantum-mechanically derived atomic-level pressure approximation. It allows identification of the best suited element mix for high solid-solution strengthening using the simple electronegativity difference among the constituent elements. This approach can be used for designing alloys with customized properties, such as a simple binary NiV solid solution whose yield strength exceeds that of the Cantor high-entropy alloy by nearly a factor of two. This study provides general design rules that enable effective utilization of atomic level information to reduce the immense degrees of freedom in compositional space without sacrificing physics-related plausibility.

International audienceVariable-temperature studies of the electronic structures of four different Fe(II) spin crossover molecules, [Fe(H2B(pz)2)2(bipy)] (pz = pyrazol-1-yl, bipy = 2,2′-bipyridine), [Fe(H2B(pz)2)2(phen)], [Fe(phen)2(NCS)2] (phen = 9,10-phenantroline), and [Fe(PM-AzA)2(NCS)2] (PM-AzA = 4-phenyldiazenyl-N-(pyridin-2-ylmethylene)aniline) by X-ray absorption spectroscopy (XAS), combined with electrical properties studies of the [Fe(PM-AzA)2(NCS)2] single crystal are presented. We show that both the XAS signature of the spin state of powdered samples and the dielectric permittivity of the [Fe(PM-AzA)2(NCS)2] single crystal change at significantly lower temperatures than the magnetometry, structure, and resistivity indicators of a spin crossover transition. The changes in electronic structure are in agreement with the expectations from density functional theory (DFT) results for the different molecular electronic structures associated with the high-spin and low-spin states. These findings suggest that the electronic structure phase ordering process does not simply follow the spin transition

Whilst it has long been known that disorder profoundly affects transport properties, recent measurements on a series of solid solution 3d-transition metal alloys reveal two orders of magnitude variations in the residual resistivity. Using ab-initio methods, we demonstrate that, while the carrier density of all alloys is as high as in normal metals, the electron mean-free-path can vary from ~10 Å (strong scattering limit) to ~10 3 Å (weak scattering limit). Here, we delineate the underlying electron scattering mechanisms responsible for this disparate behavior. While site-diagonal, spin dependent, potential scattering is always dominant, for alloys containing only Fe, Co, and Ni the majority spin channel experiences negligible disorder scattering, thereby providing a short circuit, while for Cr/Mn containing alloys both spin channels experience strong disorder scattering due to an electron filling effect. Somewhat surprisingly, other scattering mechanisms -including displacement, or size effect, scattering which has been shown to strongly correlate with such diverse properties as yield strength -are found to be relatively weak in most cases 4 component equiatomic fcc solid solutions: NiPd, NiCo, NiFe, NiFeCo, NiCoCr, NiCoMn, NiCrCoMn, NiFeCoMn and NiFeCoCr. This set of alloys combined with NiFeCoCrMn and NiFeCoCrPd (here collectively referred to as Cantor-Wu alloys), constitute a rich playground for comprehensive studies of the role of maximal disorder on the properties of multi-component alloys by controlling both the number (increasing configurational entropy) and types (chemical specificity) of alloying elements 4,5,8 . Of interest here are the results of recent residual resistivity measurements 5,8 of a subset of Cantor-Wu alloys that show, rather than increasing monotonically with increasing numbers of components, values of r0 break into two subgroups of low (r0 <10 µW•cm) and high (r0 >75 µW•cm) resistivity alloys. In addition, two entropically identical alloys, NiCoFe (r0 = 1.7µW•cm) and NiCoCr (r0 =92.7 µW•cm), fall into different resistivity groupings. Remarkably, the least and most resistive alloys differ by almost two orders of magnitude, r0(NiCo)=1.3µW•cm; r0(NiFeCoCrPd)=124.8µW•cm. Interestingly, the low resistivity group have r0 values typical of dilute weak scattering alloys in which there are clearly defined host (solvent) and impurity (solute) elements. In such alloys, r0 arises from the scattering of a low Fermi energy DOS of nearly-freeelectron sp-states with large λ e [ε F ] and r0 generally obeys both Nordheim's relation (r0∝c((1-c);where c is impurity concentration) 9 and Linde's "law" (r0 ∝ (DZ) 2 ; where DZ is the valence difference between host and impurity atoms) 10 . (see Ref. 11 for a discussion) This, despite the fact that, in equiatomic alloys, the concept of host and impurity elements is lost and the Fermi energy falls in the high density of state (DOS) d-bands 5 . At the other extreme, high-r0 NiFeCoCrPd is