Enhanced Li-ion dynamics in trivalently doped Lithium Phosphidosilicate Li2SiP2: A candidate material as a solid li electrolyte

Oxide and sulphide solid electrolyte materials have enjoyed significant interest in the solid-state battery community. Phosphide materials however are relatively unexplored despite the potential for being high lithium containing systems. This work reports on the phosphidosilicate system Li2SiP2 , one of many systems in the Li-Si-P phase diagram. The phosphidosilicates display complex structures and very large unit cells, which present challenges for ab-initio simulations. We present the first computational report on the theoretical ionic conductivity and related diffusion mechanisms of the material Li2SiP2 , selected due to it’s unusual supertetrahedral framework which is a recurrent motif amongst the phosphidosilicates. Group 13 dopants have also been introduced into Li2SiP2 showing preference for the silicon site over the lithium site, with Al0 Si doping showing extremely low defect incorporation energies of 0.05 eV, with no increase in defect energy up to concentrations of 10% Al0 Si. Furthermore, clustering of Al0 Si has been found to be unfavourable, in line with trends seen in oxide zeolite structures. Ab-initio molecular dynamics (AIMD) simulations indicate high ionic conductivity in pure Li2SiP2 of up to 3.19 × 10−1 S.cm-1 at 700 K. Doping with 10% Al0 Si and associated Li• i compensating defects leads to higher ionic conductivities at lower temperatures when compared to pure Li2SiP2 . The activation energies to lithium diffusion were found to be low at 0.30 eV and 0.24 eV for pure and 10% Al0 Si doped Li2SiP2 respectively, in line with previous experimental observations of pure Li2SiP2 . Multiple lithium migration pathways have also been extracted, with some mechanisms displaying activation energies as low as 0.05 eV. Furthermore, our calculated intercalation voltages suggest that these materials are stable against lithium metal and therefore could be very attractive in stabilising the electrode/electrolyte interface

A computational study of doped olivine structured Cd2GeO4: local defect trapping of interstitial oxide ions

Computational modelling techniques have been employed to investigate defects and ionic conductivity in Cd2GeO4. We show due to highly unfavourable intrinsic defect formation energies the ionic conducting ability of pristine Cd2GeO4 is extremely limited. The modelling results suggest trivalent doping on the Cd site as a viable means of promoting the formation of the oxygen interstitial defects. However, the defect cluster calculations for the first time explicitly suggest a strong association of the oxide defects to the dopant cations and tetrahedral units. Defect clustering is a complicated phenomenon and therefore not trivial to assess. In this study the trapping energies are explicitly quantified. The trends are further confirmed by molecular dynamic simulations. Despite this, the calculated diffusion coefficients do suggest an enhanced oxide ion mobility in the doped system compared to the pristine Cd2GeO4

Structure and lithium-ion dynamics in fluoride-doped cubic Li7La3Zr2O12 (LLZO) garnet for Li solid-state battery applications

The lithium-stuffed garnet Li7La3Zr2O12 (LLZO), when suitably doped, is a promising candidate material for use as a solid-state electrolyte within advanced Li-ion batteries. It possesses the thermal and mechanical stability of many inorganic ceramics, while exhibiting high Li+ ionic conductivities often associated with conventional liquid electrolytes, making it an ideal component for large-scale energy storage. However, only the high-temperature cubic phase has any meaningful Li-ion conductivity. Typically the formation of this phase is achieved through cation doping (e.g., Al3+ on the Li site) to lower the Li content and so disrupt Li ordering. However, Li-site doping, in particular, may potentially lead to some disruption of the Li-ion conduction pathways and suboptimal ionic conductivities. Consequently, other novel doping strategies involving the anion site are gaining traction, for example, F– for O2– as an alternative strategy to lower the Li content without directly blocking the lithium-diffusion pathways. For the first time, classical potential-based simulations have been employed to simulate the incorporation of fluoride anions into LLZO. Low incorporation energies have been calculated, suggesting fluoride anions are stable on the oxygen sites with a compensating lithium-ion vacancy defect. Molecular dynamics calculations suggest a definitive phase transition to the more desirable cubic phase of LLZO when doped with fluoride at temperature significantly lower than that for the tetragonal–cubic phase transition found for pure LLZO. Remarkably, the lithium-ion transport properties are shown to improve in the fluoride-doped samples particularly at low temperatures due to the stabilization of the cubic phase, suggesting anion doping of garnet systems may be a compelling alternative route to optimize the ionic conductivity

Atomistic insights of multiple stacking faults in CdTe thin-film photovoltaics: A DFT study

Stacking faults in CdTe were studied using DFT simulations. Twin and tetrahedral stacking fault energies are significantly lower than previously suggested, strongly correlating with their high density observed experimentally. No long range ordering was found for tetrahedral stacking faults while a resistance for polytype clustering was calculated. All experimentally observed faults were shown to be electronically benign when considered in isolation but increased density may produce shallow electron trap states

Mechanism of carbon dioxide and water incorporation in Ba2TiO4: A joint computational and experimental study

© 2017 American Chemical Society. CO 2 incorporation in solids is attracting considerable interest in a range of energy-related areas. Materials degradation through CO 2 incorporation is also a critical problem with some fuel cell materials, particularly for proton conducting ceramic fuel cells. Despite this importance, the fundamental understanding of the mechanism of CO 2 incorporation is lacking. Furthermore, the growing use of lower temperature sol gel routes for the design and synthesis of new functional materials may be unwittingly introducing significant residual carbonate and hydroxyl ions into the material, and so studies such as the one reported here investigating the incorporation of carbonate and hydroxyl ions are important, to help explain how this may affect the structure and properties. This study on Ba 2 TiO 4 suggests highly unfavorable intrinsic defect formation energies but comparatively low H 2 O and CO 2 incorporation energies, in accord with experimental findings. Carbonate defects are likely to form in both pristine and undoped Ba 2 TiO 4 systems, whereas those based on H 2 O will only form in systems containing other supporting defects, such as oxygen interstitials or vacancies. However, both hydroxyl and carbonate defects will trap oxide ion defects induced through doping, and the results from both experimental and modeling studies suggest that it is primarily the presence of carbonate that is responsible for stabilizing the high temperature α′-phase at lower temperatures

Carbon dioxide and water incorporation mechanisms in SrFeO3−δ phases: a computational study

With a higher propensity for low temperature synthesis routes along with a move toward lower solid oxide fuel cell operating temperatures, water and carbon dioxide incorporation in strontium ferrite is of importance. Despite this, the mechanisms are not well understood. In this work, classicalpotential-based computational techniques are used to determine the favourability of water and CO2 incorporation mechanisms in both SrFeO3−δ and SrFeO2.5. Our studies suggest that intrinsic Frenkel and Schottky type defects are unlikely to form, but that water and carbon dioxide incorporation are favourable in both phases. Water incorporation is likely for both the cubic and brownmillerite phases, with hydroxyl ions preferring to sit on octahedral oxygen sites in both structures, causing slight tilting of the shared octahedra. Interstitial hydroxyl ions are only likely for the brownmillerite phase, where the hydroxyl ions are most stable between adjacent FeO4 tetrahedral chains. Carbon dioxide incorporation via carbonate defects is most favourable when a carbonate molecule exists on an iron site, preferring the iron site with lower oxygen coordination. This involves formation of multiple oxygen vacancies surrounding the iron site, and thus we conclude that carbonate can trap oxygen vacancies. </p

The role of excited-state character, structural relaxation, and symmetry breaking in enabling delayed fluorescence activity in push-pull chromophores

Thermally activated delayed fluorescence (TADF) is a current promising route for generating highly efficient light-emitting devices. However, the design process of new chromophores is hampered by the complicated underlying photophysics. In this work, four closely related donor-π-acceptor-π-donor systems are investigated, two of which were synthesised previously, with the aim of elucidating their varying effectiveness for TADF. We outline that the frontier orbitals are insufficient for discriminating between the molecules. Subsequently, a detailed analysis of the excited states at a correlated ab initio level highlights the presence of a number of closely spaced singlet and triplet states of varying character. Results from five density functionals are compared against this reference revealing dramatic changes in, both, excited state energies and wavefunctions following variations in the amount of Hartree-Fock exchange included. Excited-state minima are optimised in solution showing the crucial role of structural variations and symmetry breaking for producing a strongly emissive S1 state. The adiabatic singlet-triplet gaps thus obtained depend strongly on the range separation parameter used in the hybrid density functional calculations. More generally, this work highlights intricate differences present between singlet and triplet excited state wavefunctions and the challenges in describing them accurately

Arsenic doping and diffusion in CdTe: a DFT study of bulk and grain boundaries

The doping of CdTe with As is a method which is thought to increase cell efficiency by increasing electron hole concentrations. This doping relies on the diffusion of As through CdTe resulting in AsTe substitution. The potential effectiveness of this is considered through kinetic and electronic properties calculations in both bulk and Σ3 and Σ9 grain boundaries using Density Functional Theory. In bulk zinc-blende CdTe, isolated As diffuses with barriers <0.5 eV and with similar barriers through wurtzite structured CdTe, generated by stacking faults, suggesting that As will not be trapped at the stacking faults and hence the transport of isolated As will be unhindered in bulk CdTe. Substitutional arsenic in bulk CdTe has little effect on the band gap except when it is positively charged in the AX-centre position or occurring as a di-interstitial. However in contrast to the case of chlorine, arsenic present in the grain boundaries introduces defect states into the band gap. This suggests that a doping strategy whereby the grain boundaries are first saturated with chlorine, before single arsenic atoms are introduced, might be more beneficial.</p

Combined experimental and computational study of Ce-doped La3Zr2Li7O12 garnet solid-state electrolyte

Li-containing garnet materials have been attracting considerable interest as potential solid-state electrolytes for Li ion batteries. In such Ln3M2LixO12 (Ln = lanthanide, alkaline earth; M = Zr, Hf, Sn, Nb, Ta, Sb, Bi, Te), the best Li ion conductivity is observed for Li contents, x, just below the maximum 7.0. The decrease in conductivity for x = 7.0 systems is related to Li ordering (cell changes from cubic to tetragonal) to prevent too short Li-Li interactions. In this work, we report a combined experimental and modeling study of Ce4+ doping in La3Zr2Li7O12. We show for the first time that Ce4+ can be doped onto the Zr4+ site in this material. This doping strategy results in a reduction in the tetragonal distortion as well as a lowering of the temperature of the tetragonal-cubic phase transition, attributed to the increase in cell size reducing Li-Li interaction strain. Coupled with these changes, the conductivity shows a significant (1.5 orders of magnitude) improvement. Furthermore, the Ce doping also reduces the interfacial resistance (388 ω cm2 for Li7La3Z1.75Ce025O12) in contact with Li metal, giving additional potential benefits to this doping strategy. The long-term cycling stability of a Li//garnet//Li symmetric cell over 190 h has been demonstrated

Atomistic simulation of helium diffusion and clustering in plutonium dioxide

This study uses molecular dynamics and barrier searching methods to investigate the diffusion and clustering of helium in plutonium dioxide. Such fundamental understanding of helium behaviour is required because radiogenic helium generated from the alpha decay of Pu nuclei can accumulate over time and storage of spent nuclear fuel needs to be safe and secure. The results show that in perfect PuO2, interstitial He is not mobile over nanosecond time scales at temperatures below 1500 K with the lowest diffusion barrier being 2.4 eV. Above this temperature O vacancies can form and diffusion increases. The He diffusion barrier drops to 0.6 eV when oxygen vacancies are present. High temperature simulations show that the key He diffusion mechanism is oxygen vacancy assisted inter-site hopping rather than the direct path between adjacent interstitial sites. Unlike oxygen vacancies, plutonium vacancies act as helium traps. However, isolated substitutional He at Pu sites can be easily ejected through displacement by neighbouring interstitial Pu atoms. High temperature MD simulations show that helium can diffuse into clusters with the majority of helium clusters which form over nanosecond time scales having a He : vacancy ratio below 1 : 1. Further static calculations show that a ∼3.5 : 1 He : vacancy ratio is the largest possible for an energetically stable helium cluster. Schottky defects act as seed points for He cluster growth and a high local concentrations of He can create such defects which then pin the growing He cluster.</p