Montse Casas-Cabanas

Microstructural analysis of nickel hydroxide: Anisotropic size versus stacking faults

Two different approaches for studying sample’s contributions to diffraction-line broadening are analyzed by applying them to several nickel hydroxide samples. Both are based in the refinement of powder diffraction data but differ in the microstructural model used. The first one consists in the refinement of the powder diffraction pattern using the FAULTS program, a modification of DIFFaX , which assigns peak broadening mainly to the presence of stacking faults and treats finite size effects by convolution with a Voigt function. The second method makes use of the program FULLPROF , which allows the use of linear combinations of spherical harmonics to model peak broadening coming from anisotropic size effects. The complementary use of transmission electron microscopy has allowed us to evaluate the best approach for the Ni(OH) 2 case. In addition, peak shifts, corresponding to reflections 10 l ( l ≠0) were observed in defective nickel hydroxide samples that can be directly correlated with the degree of faulting.

Atomic defects during ordering transitions in LiNi0.5Mn1.5O4 and their relationship with electrochemical properties

Decoupling the relevant parameters determining the electrochemical performance of spinel-type LiNi0.5Mn1.5O4 would contribute to promote its commercialization as cathode material for Li-ion batteries with high energy density. These parameters mainly comprise Ni/Mn ordering and non-stoichiometry, but their drivers and individual contribution to electrochemical performance remain to be fully ascertained. A series of samples annealed at different temperatures in the vicinity of an ordering transition have been thoroughly characterized by means of neutron powder diffraction to accurately establish composition–structure–property relationships in this material. The analysis revealed that deviations from a perfectly ordered crystal are possible through two different types of defects with significantly different effects on properties. These structural defects are in addition to previously described compositional defects, involving the creation of Mn3+ in the spinel lattice and Ni-rich rock salt secondary phases. Among the two types, the formation of antiphase boundaries is detrimental to transport, leading to poor rate performance of the electrode. In contrast, Ni/Mn mixing in an ordered framework can lead to behavior competitive with fully disordered samples, even at much lower Mn3+ contents that theoretically impart enhanced electronic conductivity. This work establishes design guidelines for fast transport in materials close to full stoichiometry, avoiding deleterious effects of rock salt impurities and Mn3+ dissolution.

FAULTS: a program for refinement of structures with extended defects

The FAULTS program is a powerful tool for the refinement of diffraction patterns of materials with planar defects. A new release of the FAULTS program is herein presented, together with a number of new capabilities, aimed at improving the refinement process and evolving towards a more user-friendly approach. These include the possibility to refine multiple sets of single-crystal profiles of diffuse streaks, the visualization of the model structures, the possibility to add the diffracted intensities from secondary phases as background and the new DIFFaX2FAULTS converter, among others. Three examples related to battery materials are shown to illustrate the capabilities of the program.

Order and disorder in NMC layered materials: a FAULTS simulation analysis

The program FAULTS has been used to simulate the X-ray powder diffraction (XRD), neutron powder diffraction (NPD), and electron diffraction (ED) patterns of several structural models for LiNi 1/3 Mn 1/3 Co 1/3 O 2 , including different types of ordering of the transition metal (TM) cations in the TM slabs, different amounts of Li + /Ni II+ cation mixing and different amounts of stacking faults. The results demonstrate the relevance of the structural information provided by NPD and ED data as compared with XRD to characterize the microstructure of NMC (LiNi 1− y-z Mn y Co z O 2 ) compounds.

The nickel battery positive electrode revisited: stability and structure of the β-NiOOH phase

The crystal structure of the nickel battery positive electrode material, β-NiOOH, is analyzed through a joint approach involving NMR and FTIR spectroscopies, powder neutron diffraction and DFT calculations. The obtained results confirm that structural changes occur during the β-Ni(OH)2/β-NiOOH transformation leading to a metastable crystal structure with a TP2 host lattice. This structure involves two types of hydrogen atoms both forming primary and secondary hydrogen bonds. The formation of TP2 NiOOH as opposed to the more stable P3 host type during β-Ni(OH)2/β-NiOOH transformation has a kinetic origin that can be understood by a lower strain penalty involved in the transformation.