Steinar Birgisson

A P2-NaxCo0.7Mn0.3O2 (x ≈ 1.0) cathode material for Na-ion batteries with superior rate and cycle capability

The development of sodium ion batteries is of key importance in future energy technology, but so far no materials are known with sufficient rate and cycle capability for real applications. Here we report on a P2-NaxCo0.7Mn0.3O2 (x ≈ 1.0) cathode material with outstanding properties composed of layered hexagonal nanosheets synthesized using a simple and scalable co-precipitation and solid state reaction method. The material has excellent cycle capability with 84% capacity retention after 225 cycles at 1C rate, and an extraordinary rate capability with ∼75% of the 1C capacity maintained even at a very high charge/discharge rate of 30C. The unique properties may be attributed to a combination of the layered structure and the nanosheet morphology that mitigates the stress caused by Na ion diffusion during cycling. Moreover, the nanosheets are highly (001) oriented, leading to a large active surface of (010)/(100) planes, which could facilitate easy Na ion extraction/intercalation.

Crystal structure, microstructure and electrochemical properties of hydrothermally synthesised LiMn2O4

The hydrothermal synthesis method offers an environmentally benign way of synthesizing Li-ion battery materials with strong control of particle size and morphology, and thereby also the electrochemical performance. Here we present an in depth investigation of the crystal structure, microstructure and electrochemical properties of hydrothermally synthesized LiMn2O4, which is a widely used cathode material. A range of samples were synthesized by a simple, single-step hydrothermal route, and the products were characterized by elaborate Rietveld refinement of powder X-ray diffraction data, electron microscopy and electrochemical analysis. A distinct bimodal crystallite size distribution of LiMn2O4 was formed together with a Mn3O4 impurity. At high LiOH concentration the layered LixMnyO2 phase was formed. The crystallite sizes and impurity weight fractions were found to be highly synthesis dependent, and the amount of spinel impurity phase was found to correlate with deterioration of the electrochemical performance. The Mn3O4 phase can be very difficult to quantify in standard powder X-ray diffraction and due to peak overlap and X-ray fluorescence impurity levels of more than 10% are easily hidden. Furthermore, the spinel LiMn2O4 phase can easily be mistaken for the layered LixMnyO2 phase. The present study therefore highlights the importance of thorough structural characterization in studies of battery materials.

Towards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO

Varying atomic short-range order is correlated with the ratio of the monoclinic (m) to tetragonal (t) phase in ZrO2 nanoparticle formation by solvothermal methods. Reactions from Zr oxynitrate in supercritical methanol and Zr acetate in water (hydrothermal route) were studied in situ by X-ray total scattering. Irrespective of the Zr source and solvent, the structure of the precursor in solution consists of edge-shared tetramer chains. Upon heating, the nearest-neighbor Zr-O and Zr-Zr distances shorten initially while the medium-range connectivity is broken. Depending on the reaction conditions, the disordered intermediate transforms either rapidly into m-ZrO2, or more gradually into mixed m- and t-ZrO2 with a concurrent increase of the shortest Zr-Zr distance. In the hydrothermal case, the structural similarity of the amorphous intermediate and m-ZrO2 favors the formation of almost phase-pure m-ZrO2 nanoparticles with a size of 5 nm, considerably smaller than the often-cited critical size below which the tetragonal is assumed to be favoured. Pair distribution function analysis thus unravels ZrO2 phase formation on the atomic scale and in this way provides a major step towards understanding polymorphism of ZrO2 beyond empirical approaches.

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In situ powder X-ray diffraction (PXRD) is a powerful characterization tool owing to its ability to provide time-resolved information about phase composition, crystal structure and microstructure. The application of high-flux synchrotron X-ray beams and the development of custom-built reactors have facilitated second-scale time-resolved studies of nanocrystallite formation and growth during solvothermal synthesis. The short exposure times required for good time resolution limit the data quality, while the employed high-temperature–high-pressure reactors further complicate data acquisition and treatment. Based on experience gathered during ten years of conducting in situ studies of solvothermal reactions at a number of different synchrotrons, a compilation of useful advice for conducting in situ PXRD experiments and data treatment is presented here. In addition, the reproducibility of the employed portable in situ PXRD setup, experimental procedure and data analysis is evaluated. This evaluation is based on repeated measurements of an LaB6 line-profile standard throughout 5 d of beamtime and on the repetition of ten identical in situ synchrotron PXRD experiments on the hydrothermal formation of γ-Fe2O3 nanocrystallites. The study reveals inconsistencies in the absolute structural and microstructural values extracted by Rietveld refinement and whole powder pattern modelling of the in situ PXRD data, but also illustrates the robustness of trends and relative changes in the extracted parameters. From the data, estimates of the effective errors and reproducibility of in situ PXRD studies of solvothermal nanocrystallite formation are provided.