Publication News

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news-zphase

25th July 2019

Nature of the “Z”-phase in layered Na-ion battery cathodes

Research by the Peter Bruce Group and collaborators at the Paul Scherrer Institut, Argonne National Laboratory and Uppsala University, as reported in Energy & Environmental Science explains that the Li-ion battery cell chemistry has a relatively high cost and suffers from sustainability issues, particularly surrounding the use of cobalt. The Na-ion battery represents a promising alternative as it avoids these problems and possesses several other advantageous characteristics (e.g. use of Al negative current collector). For high capacity layered oxide cathodes, the larger ionic radius of Na+ compared with Li+ grants a wider choice of transition metals that can be utilised and opens an avenue towards low cost, Co-free batteries. At the same time, the larger radius of Na+ enables a richer variety of stable layered configurations (O3, P2, P3 etc.), which can lead to multiple changes in crystal structure as the battery is charged and discharged. In this paper, they focus on the P2 layered oxide Na2/3[Ni1/6Mn1/2Fe1/3]O2 and unravel its complex structural changes at low Na contents (previously termed the “Z”-phase). Specifically, we identify this phase as a continuously evolving intergrowth structure between trigonal prismatic and octahedral stacking. This work can be reasonably extended to many other P2-type cathodes and will also have relevance to additional layered oxide cathode types (e.g. Li-ion Ni-rich cathodes) that exhibit high voltage structural transitions.

Polaron orbital in Li2O2

23rd June 2019

Polarons from First Principles, without Supercells

Weng Hong Sio and colleagues from the Materials Modelling and Design Group led by Professor Giustino have recently published papers in Physical Review Letters and Physical Review B  which report a new formalism for modelling polarons. The new method combines elements of idealized mathematical models and numerical methods based on density-functional theory. A Physics viewpoint highlights that by reducing the calculations to within a single primitive crystal cell this new method opens up exciting prospects for studying more complex materials beyond the reach of previous models.

MWCNT interaction with solvents

21st June 2019

The application of the surface energy based solubility parameter theory for the rational design of polymer-functionalized MWCNTs

Carbon nanotubes (CNTs) are found at the boundary between the molecular and the macroscopic scale granting them the ability to interact strongly with molecules through non-covalent interactions. As reported in Physical Chemical Chemistry Physics researchers in the Nanomaterials By Design group have applied surface energy based solubility parameters theory to model the degree of polystyrene-functionalisation of MWCNTs in six different organic solvents. The experimental characterization of the polymer-functionalized MWCNTs is consistent with the predictions of this model providing a breakthrough towards the rational design of functionalized MWCNTs based on thermodynamic parameters.  This paper was selected as one of 2019 PCCP HOT Articles.

Oxygen Loss

6th June 2019

What Triggers Oxygen Loss in Oxygen Redox Cathode Materials?

Research by the Peter Bruce Group and collaborators at Uppsala University, the University of Kent and  Paul Scherrer Institut as reported in Chemistry of Materials clarifies It is possible to increase the charge capacity of transitionmetal (TM) oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen. However, oxygen loss often occurs. To explore what affects oxygen loss in oxygen redox materials, they compared two analogous Na-ion cathodes, P2-Na0.67Mg0.28Mn0.72O2 and P2- Na0.78Li0.25Mn0.75O2. On charging to 4.5 V, >0.4e − are removed from the oxide ions of these materials, but neither compound exhibits oxygen loss. Li is retained in P2-Na0.78Li0.25Mn0.75O2 but displaced from the TM to the alkali metal layers, showing that vacancies in the TM layers, which also occur in other oxygen redox compounds that exhibit oxygen loss such as Li[Li0.2Ni0.2Mn0.6]O2, are not a trigger for oxygen loss. On charging at 5 V, P2-Na0.78Li0.25Mn0.75O2 exhibits oxygen loss, whereas P2-Na0.67Mg0.28Mn0.72O2 does not. Under these conditions, both Na+ and Li+ are removed from P2-Na0.78Li0.25Mn0.75O2, resulting in underbonded oxygen (fewer than 3 cations coordinating oxygen) and surface-localized O loss. In contrast, for P2-Na0.67Mg0.28Mn0.72O2, oxygen remains coordinated by at least 2 Mn4+ and 1 Mg2+ ions, stabilizing the oxygen and avoiding oxygen loss.

Li short-circuit

6th June 2019

Microstructural Growth of Lithium in All-Solid-State Batteries

Research by the Peter Bruce Group and collaborators at the University of Cambridge as reported in Chemistry of Materials explains All-solid-state batteries potentially offer safe, high-energy-density electrochemical energy storage, yet are plagued with issues surrounding Li microstructural growth and subsequent cell death. The researchers use 7Li NMR chemical shift imaging and electron microscopy to track Li microstructural growth in the garnet-type solid electrolyte, Li6.5La3Zr1.5Ta0.5O12. They follow the early stages of Li microstructural growth during galvanostatic cycling, from the formation of Li on the electrode surface to dendritic Li connecting both electrodes in symmetrical cells, and correlate these changes with alterations observed in the voltage profiles during cycling and impedance measurements. During these experiments, they observe transformations at both the stripping and plating interfaces, indicating heterogeneities in both Li removal and deposition. At low current densities, 7Li magnetic resonance imaging detects the formation of Li microstructures in cells before short-circuits are observed and allows changes in the electrochemical profiles to be rationalized

Anisotopic Fracture

31st May 2019

Anisotropic Fracture Dynamics Due to Local Lattice Distortions

A brittle material under loading fails by the nucleation and propagation of a sharp crack. Combining annular dark-field scanning transmission electron microscopy and empirical molecular dynamics that are validated by first-principles calculations, researchers at MIT, Oxford and ePSIC have shown in ACS Nano that local atomic distortions from different types of atoms causes a propagation anisotropy.  Conventional  conditions for brittle failure such as surface energy, elasticity, and crack geometry cannot account for this anisotropy because this kind of anisotropy has not been considered as a parameter in traditional theories.

Writing NV Qubits in Diamond

18th May 2019

Laser writing of individual nitrogen-vacancy defects in diamond with near-unity yield

Professor Jason Smith and his fellow NQIT colleagues based in Engineering Science and at the University of Warwick demonstrate, in a paper published in Optica, the success of the new method to create particular defects in diamonds known as nitrogen-vacancy (NV) colour centres. These comprise a nitrogen impurity in the diamond (carbon) lattice located adjacent to an empty lattice site or vacancy. The ability to write NV centres into diamond with a high degree of control is an essential first step towards being able to engineer atomic defect structures for the development of a new generation of quantum information technologies.  Further news articles are available on the Oxford News and EurekAlert.

Nanosims of Se in CdTe

16th May 2019

Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells

Electricity produced by cadmium telluride (CdTe) photovoltaic modules has one of the lowest-costs in the solar industry, and is now much cheaper than fossil fuel-based sources in many regions of the world. Recent substantial efficiency gains have been achieved by alloying selenium into the CdTe absorber, improving efficiency from 19.5% to the current record value of 22.1%. This paper in Nature Energy reports directly correlated cathodoluminescence and NanoSIMS results showing that selenium passivates critical defects in the bulk of the absorber layer, and this understanding provides promising directions for further efficiency improvements.

news-opticalneuralnetwork

9th May 2019

All-optical spiking neurosynaptic networks with self-learning capabilities

Researchers from University of Munster, University of Oxford and University of Exeter report in Nature an all-optical version of a neurosynaptic system, capable of supervised and unsupervised learning. They exploit wavelength division multiplexing techniques to implement a scalable circuit architecture for photonic neural networks, successfully demonstrating pattern recognition directly in the optical domain. Such photonic neurosynaptic networks promise access to the high speed and high bandwidth inherent to optical systems, thus enabling the direct processing of optical telecommunication and visual data. This report is also discussed further in a Nature news article highlighting the potential future impact should photonic neural accelerators offering high energy efficiencies eventually emerge. This paper also featured in MPLS news.

news-ba2AgIO6-cover

30th April 2019

Oxide Analogs of Halide Perovskites and the New Semiconductor Ba2AgIO6

The past few years witnessed the rise of halide perovskites as prominent materials for a wide range of optoelectronic applications. George Volonakis and Feliciano Giustino  from the Materials Modelling and Design Group report in The Journal of Physical Chemistry Letters a novel concept of perovskite analogs, which has led to identifying a new semiconductor, Ba2AgIO6, which is a cubic oxide double perovskite with a direct low gap in the visible range at 1.9 eV, opening new opportunities in perovskite-based electronics optoelectronics and energy applications. This paper has been selected to feature on the front cover of the journal.

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