The importance of exploring new solid electro-lytes for all-solid-state batteries has led to significant interest in NASICON-type materials. Here, the Sc3+-substituted NASICON compositions (NaScZr2-x)-Sc-3-Zr-x(SiO4)(2-x)(PO4)(1+x) (termed N3) and Na2ScyZr2-y(SiO4)(1-y)(PO4)(2+y) (termed N2) (x, y = 0-1) are studied as model Na+-ion conducting electrolytes for solid-state batteries. The influence of Sc3+ substitution on the crystal structures and local atomic environments has been characterized by powder X-ray diffraction (XRD) and neutron powder diffraction (NPD), as well as solid-state Na-23, P-31, and Si-29 nuclear magnetic resonance (NMR) spectroscopy. A phase transition between 295 and 473 K from monoclinic C2/c to rhombohedral R (3) over barc is observed for the N3 compositions, while N2 compositions crystallize in a rhombohedral R (3) over barc unit cell in this temperature range. Alternating current (AC) impedance spectroscopy, molecular dynamics (MD), and high temperature Na-23 NMR studies are in good agreement, showing that, with a higher Sc3+ concentration, the ionic conductivity (of about 10(-4) S/cm at 473 K) decreases and the activation energy for ion diffusion increases. Na-23 NMR experiments indicate that the nature of the Na+-ion motion is two-dimensional on the local atomic scale of NMR although the long-range diffusion pathways are three-dimensional. In addition, a combination of MD, bond valence, maximum entropy/Rietveld, and van Hove correlation methods has been used to reveal that the Na+-ion diffusion in these NASICON materials is three-dimensional and that there is a continuous exchange of sodium ions between Na(1) and Na(2) sites.

Metal halide perovskites are exceptional candidates for inexpensive yet high-performing optoelectronic devices. Nevertheless, polycrystalline perovskite films are still limited by nonradiative losses due to charge carrier trap states that can be affected by illumination. Here, in situ microphotoluminescence measurements are used to elucidate the impact of light-soaking individual methylammonium lead iodide grains in high-quality polycrystalline films while immersing them with different atmospheric environments. It is shown that emission from each grain depends sensitively on both the environment and the nature of the specific grain, i.e., whether it shows good (bright grain) or poor (dark grain) luminescence properties. It is found that the dark grains show substantial rises in emission, while the bright grain emission is steady when illuminated in the presence of oxygen and/or water molecules. The results are explained using density functional theory calculations, which reveal strong adsorption energies of the molecules to the perovskite surfaces. It is also found that oxygen molecules bind particularly strongly to surface iodide vacancies which, in the presence of photoexcited electrons, lead to efficient passivation of the carrier trap states that arise from these vacancies. The work reveals a unique insight into the nature of nonradiative decay and the impact of atmospheric passivation on the microscale properties of perovskite films.

Metal halide perovskite solar cells have rapidly emerged as leading contenders in photovoltaic technology. Compositions with a mixture of cation species on the A-site show the best performance and have higher stability. However, the underlying fundamentals of such an enhancement are not fully understood. Here, we investigate the local structures and dynamics of mixed A-cation compositions. We show that substitution of low concentrations of smaller cations on the A-site in formamidimium lead iodide (CH(NH2)2PbI3) results in a global “locking” of the PbI6 octahedra tilting. In the locked structure the octahedra tilt at a larger angle but undergo a much reduced amplitude of rocking motion. A key impact of this feature is that the rotational or tumbling motion of the CH(NH2)2+ molecular ion in a locked cage is severely restricted. We discuss the impact of locking on the photovoltaic performance and stability.

The silicate compounds Li2MSiO4 (where M = Mn, Fe, Co) have received significant attention recently as Li intercalation electrodes. Overwhelmingly they exhibit relatively poor kinetics of ion intercalation. By synthesizing Li‐rich solid solutions of the form Li2+2xFe1−xSiO4 (with 0 ≤ x ≤ 0.3), the structural requirements for fast ion transport and hence relatively fast intercalation have been identified. Specifically the presence of additional Li+ in interstitial sites, not normally occupied in the stoichiometric Li2FeSiO4 compound, enhances ion transport by more than two orders of magnitude. The results, obtained by combining electrochemical measurements, with powder X‐ray and neutron diffraction and atomistic modeling of the ion dynamics, provide valuable guidance in designing future intercalation electrodes with high Li‐ion transport and, hence, fast electrode kinetics.

Halide perovskites offer low cost and high efficiency solar cell materials but serious issues related to air and moisture stability remain. In this study we show, using UV-vis, fluorescence and time of flight secondary ion mass spectrometry (ToF-SIMS) techniques, that the degradation of methylammonium lead iodide solar cells is significantly accelerated when both air and moisture are present in comparison to when just air or moisture is present alone. Using ab initio computational techniques we identify the thermodynamic driving force for the enhanced reactivity and highlight the regions of the photoexcited material that are the most likely reaction centres. We suggest that water catalyses the reaction by stabilising the reactive superoxide species, enabling them to react with the methylammonium cation.

Hybrid materials composed of different functional structural units offer the possibility of tuning both the thermal and electronic properties of a material independently. Using quantum mechanical calculations, we investigate the change in the electronic and thermoelectric transport properties of graphene and hydrogen-terminated carbon nanoribbons (CNRs) when these are placed on the SrTiO3 (001) surface (STO). We predict that both p-type and n-type composite materials can be achieved by coupling graphene/CNR to different surface terminations of STO. We show that the electronic properties of graphene and CNR are significantly altered on SrO-terminated STO but are preserved upon interaction with TiO2-terminated STO and that CNRs possess distinct electronic states around the Fermi level because of their quasi-one-dimensional nature, leading to a calculated Seebeck coefficient much higher than that of a pristine graphene sheet. Moreover, our calculations reveal that in the Ti-02-SrTiO3/CNR system there is a favorable electronic level alignment between the CNR and STO, where the highest occupied molecular orbital of the CNR is positioned in the middle of the STO band gap, resembling n-type doping of the substrate. Our results offer design principles for guiding the engineering of future hybrid thermoelectric materials and, more generally, nanoelectronic materials comprising oxide and graphitic components.

Halide perovskites offer low cost and high efficiency solar cell materials but serious issues related to air and moisture stability remain. In this study we show, using UV-vis, fluorescence and time of flight secondary ion mass spectrometry (ToF-SIMS) techniques, that the degradation of methylammonium lead iodide solar cells is significantly accelerated when both air and moisture are present in comparison to when just air or moisture is present alone. Using ab initio computational techniques we identify the thermodynamic driving force for the enhanced reactivity and highlight the regions of the photoexcited material that are the most likely reaction centres. We suggest that water catalyses the reaction by stabilising the reactive superoxide species, enabling them to react with the methylammonium cation.

Lithium superionic conductor (LISICON)-related compositions Li4±xSi1-xXxO4 (X = P, Al, or Ge) are important materials that have been identified as potential solid electrolytes for all solid state batteries. Here, we show that the room temperature lithium ion conductivity can be improved by several orders of magnitude through substitution on Si sites. We apply a combined computer simulation and experimental approach to a wide range of compositions (Li4SiO4, Li3.75Si0.75P0.25O4, Li4.25Si0.75Al0.25O4, Li4Al0.33Si0.33P0.33O4, and Li4Al1/3Si1/6Ge1/6P1/3O4) which include new doped materials. Depending on the temperature, three different Li+ ion diffusion mechanisms are observed. The polyanion mixing introduced by substitution lowers the temperature at which the transition to a superionic state with high Li+ ion conductivity occurs. These insights help to rationalize the mechanism of the lithium ion conductivity enhancement and provide strategies for designing materials with promising transport properties.

Metal halide perovskites are generating enormous excitement for use in solar cells and light-emission applications, but devices still show substantial non-radiative losses. Here, we show that by combining light and atmospheric treatments, we can increase the internal luminescence quantum efficiencies of polycrystalline perovskite films from 1% to 89%, with carrier lifetimes of 32 ms and diffusion lengths of 77 mm, comparable with perovskite single crystals. Remarkably, the surface recombination velocity of holes in the treated films is 0.4 cm/s, approaching the values for fully passivated crystalline silicon, which has the lowest values for any semiconductor to date. The enhancements translate to solar cell power-conversion efficiencies of 19.2%, with a near-instant rise to stabilized power output, consistent with suppression of ion migration. We propose a mechanismin which light creates superoxide species fromoxygen that remove shallow surface states. The work reveals an industrially scalable post-treatment capable of producing state-of-the-art semiconducting films.

Organic-inorganic halide perovskites are attracting extraordinary attention in the field of energy materials. The reaction of hybrid lead halide perovskites with Li metal has recently been proposed for a number of potential applications. However, the mechanisms for Li uptake in such materials, such as intercalation and conversion, are still unknown. Using a combination of density functional theory and electrochemical and diffraction techniques, we consider Li intercalation and conversion reactions in CH3NH3PbI3, CH3NH3PbBr3, and CH3NH3PbCI3. Our simulations suggest that conversion reactions with Li are far more energetically preferable in these materials than Li intercalation. Calculations confirm the formation of Pb metal as a result of Li conversion in all three materials, and this is supported by X-ray diffraction analysis of CH3NH3PbBr3. The results of this study provide fresh insights into lithium and halide perovskite reactions that will hopefully drive further exploration of these materials for a wider variety of energy applications.

Methylammonium lead halide perovskites are attracting intense interest as promising materials for next-generation solar cells, but serious issues related to long-term stability need to be addressed. Perovskite films based on CH3NH3PbI3 undergo rapid degradation when exposed to oxygen and light. Here, we report mechanistic insights into this oxygen-induced photodegradation from a range of experimental and computational techniques. We find fast oxygen diffusion into CH3NH3PbI3 films is accompanied by photo-induced formation of highly reactive superoxide species. Perovskite films composed of small crystallites show higher yields of superoxide and lower stability. Ab initio simulations indicate that iodide vacancies are the preferred sites in mediating the photo-induced formation of superoxide species from oxygen. Thin-film passivation with iodide salts is shown to enhance film and device stability. The understanding of degradation phenomena gained from this study is important for the future design and optimization of stable perovskite solar cells.

Lattice strain is considered a promising approach to modulate the structural and functional properties of oxide materials. In this study we investigate the effect of lattice strain on doping, hydration and proton transport for the family of scheelite-type proton conductors using both atomistic and DFT computational methods. The results suggest that tensile strain improves the dopant solubility and proton uptake of the material. The anisotropic proton pathways change from being within the a-b plane to being in the a-c plane. However, the predicted reduction in the migration barrier suggests that improvements in ionic conductivity due to lattice strain effects will be limited, in contrast with the work on oxide ion conduction. Such results are rationalized in terms of structural changes and differences in migration steps between oxide ions and protonic species.

The recently discovered lithium-rich cathode material Li7Mn(BO3)3 has a high theoretical capacity and an unusual tetrahedral Mn coordination. Atomistic simulation and density functional theory (DFT) techniques are employed to provide insights into the defect and redox chemistry, the structural changes upon lithium extraction and the mechanisms of lithium ion diffusion. The most favourable intrinsic defects are Li/Mn anti-site pairs, where Li and Mn ions occupy interchanged positions, and Li Frenkel defects. DFT calculations reproduce the experimental cell voltage and confirm the presence of the unusually high Mn redox state, which corresponds to a theoretical capacity of nearly 288 mA h g . The ability to reach the high manganese oxidation state is related to both the initial tetrahedral coordination of Mn and the observed distortion/tilting of the BO3 units to accommodate the contraction of the Mn–O bonds upon oxidation. Molecular dynamics (MD) simulations indicate fast three-dimensional lithium diffusion in line with the good rate performance observed.

Solar cells based on organic-inorganic halide perovskites have recently shown rapidly rising power conversion efficiencies, but exhibit unusual behaviour such as current-voltage hysteresis and a low-frequency giant dielectric response. Ionic transport has been suggested to be an important factor contributing to these effects; however, the chemical origin of this transport and the mobile species are unclear. Here, the activation energies for ionic migration in methylammonium lead iodide (CH3NH3PbI3) are derived from first principles, and are compared with kinetic data extracted from the current-voltage response of a perovskite-based solar cell. We identify the microscopic transport mechanisms, and find facile vacancy-assisted migration of iodide ions with an activation energy of 0.6 eV, in good agreement with the kinetic measurements. The results of this combined computational and experimental study suggest that hybrid halide perovskites are mixed ionic-electronic conductors, a finding that has major implications for solar cell device architectures

Solid electrolytes that are chemically stable and have a high ionic conductivity would dramatically enhance the safety and operating lifespan of rechargeable lithium batteries. Here, we apply a multi-technique approach to the Li-ion conducting system (1-z)Li4SiO4-(z)Li3PO4 with the aim of developing a solid electrolyte with enhanced ionic conductivity. Previously unidentified superstructure and immiscibility features in high-purity samples are characterized by X-ray and neutron diffraction across a range of compositions (z = 0.0-1.0). Ionic conductivities from AC impedance measurements and large-scale molecular dynamics (MD) simulations are in good agreement, showing very low values in the parent phases (Li4SiO4 and Li3PO4) but orders of magnitude higher conductivities (10(-3) S/cm at 573 K) in the mixed compositions. The MD simulations reveal new mechanistic insights into the mixed Si/P compositions in which Li-ion conduction occurs through 3D pathways and a cooperative interstitial mechanism; such correlated motion is a key factor in promoting high ionic conductivity. Solid-state (6)Li, (7)Li, and (31)P NMR experiments reveal enhanced local Li-ion dynamics and atomic disorder in the solid solutions, which are correlated to the ionic diffusivity. These unique insights will be valuable in developing strategies to optimize the ionic conductivity in this system and to identify next-generation solid electrolytes.

Polyanionic phosphates have the potential to act as low-cost cathodes and stable framework materials for Na ion batteries. The mixed phosphates Na4M3(PO4)(2)P2O7 (M = Fe, Mn, Co, Ni) are a fascinating new class of materials recently reported to be attractive Na ion cathodes which display low-volume changes upon cycling, indicative of long-lifetime operation. Key issues surrounding intrinsic defects, Na ion migration mechanisms, and voltage trends have been investigated through a combination of atomistic energy minimization, molecular dynamics (MD), and density functional theory simulations. For all compositions, the most energetically favorable defect is calculated to be the Na/M antisite pair. MD simulations suggest Na+ diffusion extends across a 3D network of migration pathways with an activation barrier of 0.20-0.24 eV, and diffusion coefficients (D-Na) of 10(-10)-10(-11) cm(2) s(-1) at 325 K, suggesting good rate capability. The voltage trends indicate that doping the Fe-based cathode with Ni can significantly increase the voltage, and hence the energy density.

The recently discovered lithium-rich cathode material Li7Mn(BO3)3 has a high theoretical capacity and an unusual tetrahedral Mn(2+) coordination. Atomistic simulation and density functional theory (DFT) techniques are employed to provide insights into the defect and redox chemistry, the structural changes upon lithium extraction and the mechanisms of lithium ion diffusion. The most favourable intrinsic defects are Li/Mn anti-site pairs, where Li and Mn ions occupy interchanged positions, and Li Frenkel defects. DFT calculations reproduce the experimental cell voltage and confirm the presence of the unusually high Mn(V) redox state, which corresponds to a theoretical capacity of nearly 288 mA h g(-1). The ability to reach the high manganese oxidation state is related to both the initial tetrahedral coordination of Mn and the observed distortion/tilting of the BO3 units to accommodate the contraction of the Mn-O bonds upon oxidation. Molecular dynamics (MD) simulations indicate fast three-dimensional lithium diffusion in line with the good rate performance observed.

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Lithium transition metal silicates, Li2MSiO4(M=Fe, Mn, Co)[1], have attracted a lot of interest recently as positive electrodes for Li-ion batteries due to their potentially high theoretical capacities and to their rich crystal chemistry as they undergo complex polymorphism. One of the drawbacks of the silicates is their modest rate capability, particularly at room temperature. The present work addressed this problem. The polymorphs of Li2MSiO4can be classified into low (β) and high (γ) temperature forms, which differ in the ordering/distribution of cations within tetrahedral sites of an hcp based packing of oxygen. β forms exhibit only corner sharing and γ polymorphs both corner and edge sharing. It has been shown by us and by others that Li2FeSiO4 exists in at least 3 different polymorphic forms and that. in the case of Li2MnSiO4two stable polymorphs are observed [1-3]. The silicates are analogous to the LISICON materials (Lithium SuperIonic Conductors), of the form Li2+2xZn1-xGeO4, which adopt the γ structure with interstitial and highly mobile lithium. Therefore, stabilization of the γ polymorph to room temperature represents a potentially attractive route to enhancing the rate performance thus overcoming one of the major barriers to adoption of Li2MSiO4 cathodes. With appropriate substitutions we have been able to stabilise the γ polymorph for both Li2FeSiO4 and Li2MnSiO4systems. We will report the effects on structure and electrochemistry of the various substitutions. In the iron system good cycling stability and improved rate capability are observed (fig. 2). References [1] a) A. Nytén, A. Abouimrane, M. Armand, T. Gustafsson and J.O. Thomas, Electrochem. Comm., 7, (2005) 156; b) M. S. Islam, R. Dominko, C. Masquelier, C. Sirisopanaporn, A. R. Armstrong and P. G. Bruce, J. Mater. Chem., 21 (2011) 9811. [2] S. I. Nishimura, S. Hayase, R. Kanno, M. Yashima, N. Nakayama, A. Yamada, J. Am. Chem. Soc., 130 (2008) 13212. [3] C. Sirisopanaporn, A. Boulineau, D. Hanzel, R. Dominko, B. Budic, A. R. Armstrong, P. G. Bruce, C. Masquelier, Inorg. Chem., 49 (2010) 7446.

The search for high voltage cathodes for lithium-ion batteries has led to recent interest in the monoclinic Li2Fe(SO4)(2) material which has a voltage of 3.83 V vs. lithium, the highest recorded for a fluorine-free iron-based compound. Here we investigate the defect, surface and lithium migration properties of the Li2M(SO4)(2) (M = Fe, Mn, Co) materials using combined atomistic modelling and density functional theory (DFT) techniques. All intrinsic defect types including Li/M antisite disorder are found to be of high energy, suggesting insignificant concentrations. Low activation energies are found for lithium migration along the a-axis channels giving rise to long-range 1D diffusion, which are supported by molecular dynamics (MD) simulations. For the crystal morphology a significant surface area is exposed to these 1D diffusion channels, which would allow facile Li insertion and extraction. Using DFT simulations we reproduce the high voltage of the Li2Fe(SO4)(2) material in accord with electrochemical data and also examine local structural distortions on lithium extraction.