Suchergebnisse

High-rate lithium ion batteries can play a critical role in decarbonizing our energy systems both through their underpinning of the transition to use renewable energy resources such as photovoltaics and electrification of transport. Their ability to be rapidly and frequently charged and discharged can enable this energy storage technology to play a key role in facilitating future lowcarbon electricity networks and thereby limit emissions that may result from transport electrification if fossil fuels are required for battery production and charging. This decarbonizing transition will require lithium ion technology to provide increased power and longer cycle lives at reduced cost. Rate performance and cycle life are ultimately limited by the materials used and the kinetics associated with the charge transfer reactions, ionic and electronic conduction. We review materials strategies for electrode materials and electrolytes that can facilitate high rates and long cycle lives and explore the new opportunities that may arise in embedded distributed storage via devices that blur the distinction between supercapacitors and batteries. ; This work has been supported by the Australian Research Council (ARC) through grants DP170103219 and FT170100447 (Future Fellowship – Alison Lennon). Yu Jiang and Charles Hall acknowledge the support of the Australian Government through their Research Training Program Scholarships. Kent J. Griffith acknowledges funding from the Winston Churchill Foundation of the United States and a Herchel Smith Scholarship. Kent J. Griffith and Clare P. Grey thank the EPSRC for a LIBATT grant (EP/M009521/1). The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for any information or advice contained herein.

A continuous structural change during the (de)lithiation of lithium-ion battery material, anatase TiO 2 , which undergoes a crystal symmetry change, was not found even at high rates. ; This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. This work was supported by funding from European Union FP7- 265368 via the Eurolion Project and the Cambridge Overseas Trust (materials preparation, XRD), and as part of the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Awards DE-SC0012583 (data analysis). We thank Dr Tao Liu for help with the SEM, Dr Simon Clarke and Dr Michael Carpenter for their helpful comments and discussions, and Dr Kamila Wiaderek, Dr Olaf Borkiewicz, Dr Karena Chapman and Dr Peter Chupas for their help with the setup for XRD measurement. ; This is the final version of the article. It first appeared from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C6TA00673F

The field of paramagnetic NMR has expanded considerably in recent years, both in solution and the solid state. This review addresses both the theoretical description of paramagnetic NMR, and the way in which it is currently practised. We provide a review of the theory of the NMR parameters of systems in both solution and the solid state. Here we unify the different languages used by the NMR, EPR, quantum chemistry/DFT, and magnetism communities to provide a comprehensive and coherent theoretical description. We cover the theory of the paramagnetic shift and shift anisotropy in solution both in the traditional formalism in terms of the magnetic susceptibility tensor, and using a more modern formalism employing the relevant EPR parameters, such as are used in first-principles calculations. In addition we examine the theory first in the simple non-relativistic picture, and then in the presence of spin-orbit coupling. These ideas are then extended to a description of the paramagnetic shift in periodic solids, where it is necessary to include the bulk magnetic properties, such as magnetic ordering at low temperatures. The description of the paramagnetic shift is completed by describing the current understanding of such shifts due to lanthanide and actinide ions. We then examine the paramagnetic relaxation enhancement, using a simple model employing a phenomenological picture of the electronic relaxation, and again using a more complex state-of-the-art theory which incorporates electronic relaxation explicitly. An additional important consideration in the solid state is the impact of bulk magnetic susceptibility effects on the form of the spectrum, where we include some ideas from the field of classical electrodynamics. We then continue by describing in detail the solution and solid-state NMR methods that have been deployed in the study of paramagnetic systems in chemistry, biology, and the materials sciences. Finally we describe a number of case studies in paramagnetic NMR that have been specifically chosen to highlight how the theory in part one, and the methods in part two, can be used in practice. The systems chosen include small organometallic complexes in solution, solid battery electrode 3 materials, metalloproteins in both solution and the solid state, systems containing lanthanide ions, and multi-component materials used in pharmaceutical controlled-release formulations that have been doped with paramagnetic species to measure the component domain sizes. ; This work was supported by the People Programme (Marie Curie Actions Initial Training Networks (ITN)) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA grant agreement no. 317127, the "pNMR" project. AJP received funding from the Swedish Research Council (Vetenskapsrådet) (2016-03441).

Improving electrochemical energy storage is one of the major issues of our time. The search for new battery materials together with the drive to improve performance and lower cost of existing and new batteries is not without its challenges. Success in these matters is undoubtedly based on first understanding the underlying chemistries of the materials and the relations between the components involved. A combined application of experimental and theoretical techniques has proven to be a powerful strategy to gain insights into many of the questions that arise from the "how do batteries work and why do they fail" challenge. In this Review, we highlight the application of solid-state nuclear magnetic resonance (NMR) spectroscopy in battery research: a technique that can be extremely powerful in characterizing local structures in battery materials, even in highly disordered systems. An introduction on electrochemical energy storage illustrates the research aims and prospective approaches to reach these. We particularly address "NMR in battery research" by giving a brief introduction to electrochemical techniques and applications as well as background information on both in and ex situ solid-state NMR spectroscopy. We will try to answer the question "Is NMR suitable and how can it help me to solve my problem?" by shortly reviewing some of our recent research on electrodes, microstructure formation, electrolytes and interfaces, in which the application of NMR was helpful. Finally, we share hands-on experience directly from the lab bench to answer the fundamental question "Where and how should I start?" to help guide a researcher's way through the manifold possible approaches. ; This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 655444 (O.P.). K.J.G. thanks the Winston Churchill Foundation of the United States and the Herchel Smith Scholarship for financial support.

This work reports a detailed chemical looping investigation of strontium ferrite (SrFeO3-δ), a material with the perovskite structure type able to donate oxygen and stay in a nonstoichiometric form over a broad range of oxygen partial pressures, starting at temperatures as low as 250°C (reduction in CO, measured in TGA). SrFeO3-δ is an economically attractive, simple, but remarkably stable material that can withstand repeated phase transitions during redox cycling. Mechanical mixing and calcination of iron oxide and strontium carbonate was evaluated as an effective way to obtain pure SrFeO3-δ. In situ XRD was performed to analyse structure transformations during reduction and reoxidation. Our work reports that much deeper reduction, from SrFeO3-δ to SrO and Fe, is reversible and results in oxygen release at a chemical potential suitable for hydrogen production. Thermogravimetric experiments with different gas compositions were applied to characterize the material and evaluate its available oxygen capacity. In both TGA and in-situ XRD experiments the material was reduced below δ=0.5 followed by reoxidation either with CO2 or air, to study phase segregation and reversibility of crystal structure transitions. As revealed by in-situ XRD, even deeply reduced material regenerates at 900°C to SrFeO3 δ with a cubic structure. To investigate the catalytic behaviour of SrFeO3-δ in methane combustion, experiments were performed in a fluidized bed rig. These showed SrFeO3-δ donates O2 into the gas phase but also assists with CH4 combustion by supplying lattice oxygen. To test the material for combustion and hydrogen production, long cycling experiments in a fluidized bed rig were also performed. SrFeO3-δ showed stability over 30 redox cycles, both in experiments with a 2-step oxidation performed in CO2 followed by air, as well as a single step oxidation in CO2 alone. Finally, the influence of CO/CO2 mixtures on material performance was tested; a fast and deep reduction in elevated pCO2 makes the material susceptible to carbonation, but the process can be reversed by increasing the temperature or lowering pCO2. ; EPSRC grant no. EP/K030132/1. European Union's Horizon 2020 Marie Skłodowska–Curie grant agreement No. 659764

Operando $^{23}$Na solid-state NMR and pair distribution function analysis experiments provide insights into the structure of hard carbon anodes in sodium-ion batteries. Capacity results from "diamagnetic" sodium ions first adsorbing onto pore surfaces, defects and between expanded layers, before pooling into larger quasi-metallic clusters/expanded carbon sheets at lower voltages. ; J. M. S. acknowledges funding from EPSRC and the European Commission under grant agreement no. 696656 (Graphene Flagship). P. K. A. acknowledges the School of the Physical Sciences of the University of Cambridge for funding through an Oppenheimer Research Fellowship and a Junior Research Fellowship from Gonville and Caius College, Cambridge. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 655444 (O. P.).

Three studies explored amateur and professional users' compliance with pesticide warning labels. Professionals were classified as people working in a profession in which the use of pesticides is a necessary part of their job. Amateurs used pesticides only in their leisure time. The first study showed that the wording used affected perception of the appropriateness of hazard statements, one of the most effective variations being the use of the personal pronoun (statements beginning "You should..."). The location of warning information was also found to affect actual compliance: Compliance increased when warning information was presented in the directions for use section. A supplemental directive increased compliance only for professional users. In a final study, "best-case" and "worst-case" linguistic variations were combined with best-case and worst-case locations for safety information. Instruction statements using the personal pronoun and presented in the directions for use section resulted in the highest levels of compliance. The differences in compliance between amateur and professional users are interpreted within the framework of Rasmussen's (1986) distinction among skill-, rule-, and knowledgebased behavior. Actual or potential applications of this research include the design of warning labels and safety information.

Alluaudite sodium iron sulfate Na$_{2+2x}$Fe$_{2−x}$(SO$_4$)$_3$ is one of the most promising candidates for a Na-ion battery cathode material with earth-abundant elements; it exhibits the highest potential among any Fe$^{3+}$/Fe$^{2+}$ redox reactions (3.8 V vs Na/Na$^+$ ), good cycle performance, and high rate capability. However, the reaction mechanism during electrochemical charging/discharging processes is still not understood. Here, we surveyed the intercalation mechanism via synchrotron X-ray diffraction (XRD), $^{23}$Na nuclear magnetic resonance (NMR), density functional theory (DFT) calculations, X-ray absorption near edge structure (XANES), and Mössbauer spectroscopy. Throughout charging/discharging processes, the structure undergoes a reversible, single-phase (solid solution) reaction based on a Fe$^{3+}$/Fe$^{2+}$ redox reaction with a small volume change of ca. 3.5% after an initial structural rearrangement upon the first charging process, where a small amount of Fe irreversibly migrates from the original site to a Na site. Sodium extraction occurs in a sequential manner at various Na sites in the structure at their specific voltage regions. ; The present work was financially supported from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the "Element Strategy Initiative for Catalysts & Batteries" (ESICB) project. The synchrotron XRD experiments were performed under KEK-PF User Program (No. 2013G670). Crystal structures and the Fourier difference maps were drawn by VESTA.65 G.O. acknowledges financial support from JSPS Research Fellowships under "Materials Education Program for the Future Leaders in Research, Industry, and Technology" (MERIT) project. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 655444 (O.P.). R.P. gratefully acknowledges financial support through the Marie Curie Actions People Program of the EU's Seventh Frame work Program (FP7/2007-2013), under the grant agreement n.317127, the 'pNMR project'. K.J.G. gratefully acknowledges funding from The Winston Churchill Foundation of the United States and the Herchel Smith Scholarship. This work made use of the facilities of the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. ; This is the final version of the article. It first appeared from American Chemical Society via http://dx.doi.org/10.1021/acs.chemmater.6b01091

Na-ion batteries are promising alternatives to Li-ion systems for electrochemical energy storage because of the higher natural abundance and widespread distribution of Na compared to Li. High capacity anode materials, such as phosphorus, have been explored to realize Na-ion battery technologies that offer comparable performances to their Li-ion counterparts. While P anodes provide unparalleled capacities, the mechanism of sodiation and desodiation is not well-understood, limiting further optimization. Here, we use a combined experimental and theoretical approach to provide molecular-level insight into the (de)sodiation pathways in black P anodes for sodium-ion batteries. A determination of the P binding in these materials was achieved by comparing to structure models created via species swapping, ab initio random structure searching, and a genetic algorithm. During sodiation, analysis of 31P chemical shift anisotropies in NMR data reveals P helices and P at the end of chains as the primary structural components in amorphous Na xP phases. X-ray diffraction data in conjunction with variable field 23Na magic-angle spinning NMR support the formation of a new Na3P crystal structure (predicted using density-functional theory) on sodiation. During desodiation, P helices are re-formed in the amorphous intermediates, albeit with increased disorder, yet emphasizing the pervasive nature of this motif. The pristine material is not re-formed at the end of desodiation and may be linked to the irreversibility observed in the Na-P system. ; L.E.M. acknowledges funding from the European Union's Horizon 2020 – European Union research and innovation program under the Marie Skłodowska-Curie grant agreement No. 750294, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, of the U.S. DOE under Contract no. DE-AC02-05CH11231, under the Batteries for Advanced Transportation Technologies (BATT) Program subcontract no. 7057154, and the Charles and Katharine Darwin Research Fellowship for support. K.J.G. gratefully acknowledges funding from the Winston Churchill Foundation of the United States and the Herchel Smith Schol-arship. M.F.G. is grateful to the Engineering and Physical Sci-ences Research Council (EPSRC Grant No: EP/P003532/1). M.E. would like to acknowledge the EPSRC Centre for Doc-toral Training in Computational Methods for Materials Science for funding under grant number EP/L015552/1. A.J.M. and J.N. acknowledge the Winton Programme for the Physics of Sustainability. J.N. also acknowledges support from the Isaac Newton Fund. L.E.M. thanks Dr. Derrick Kaseman for provid-ing the Matlab script used to process 2D PASS data. We acknowledge Josh Stratford, Dr. Elizabeth Castillo-Martínez, Dr. Michael Gaultois, Dr. Pieter Magusin, and Prof. Michael Ruck (TU Dresden) for helpful discussions. NMR calculations were performed using the Cambridge Service for Data Driven Discovery (CSD3) operated by the University of Cambridge Research Computing Service (http://www.csd3.cam.ac.uk/), provided by Dell EMC and Intel using Tier-2 funding from the Engineering and Physical Sciences Research Council, and Di-RAC funding from the Science and Technology Facilities Council (www.dirac.ac.uk). Structure prediction calculations were performed using the resources of the Center for Function-al Nanomaterials, which is a U.S. DOE Office of Science Facil-ity, at Brookhaven National Laboratory under Contract No. DE-SC0012704 and the Thomas Tier 2 facility of the UK na-tional high performance computing service, for which access was obtained via the UKCP consortium and funded by EPSRC grant no. EP/K014560/1. Charles and Katharine Darwin Research Fellowship Winston Churchill Foundation of the United States Herchel Smith Scholarship (University of Cambridge) Winton Programme for the Physics of Sustainability Isaac Newton Fund

We have developed and explored an external automatic tuning/matching (eATM) robot that can be attached to commercial and/or home-built magic angle spinning (MAS) or static nuclear magnetic resonance (NMR) probeheads. Complete synchronization and automation with Bruker and Tecmag spectrometers is ensured via transistor-transistor-logic (TTL) signals. The eATM robot enables an automated "on-the-fly" re-calibration of the radio frequency (rf) carrier frequency, which is beneficial whenever tuning/matching of the resonance circuit is required, e.g. variable temperature (VT) NMR, spin-echo mapping (variable offset cumulative spectroscopy, VOCS) and/or in situ NMR experiments of batteries. This allows a significant increase in efficiency for NMR experiments outside regular working hours (e.g. overnight) and, furthermore, enables measurements of quadrupolar nuclei which would not be possible in reasonable timeframes due to excessively large spectral widths. Additionally, different tuning/matching capacitor (and/or coil) settings for desired frequencies (e.g. $^{7}$Li and $^{31}$P at 117 and 122MHz, respectively, at 7.05 T) can be saved and made directly accessible before automatic tuning/matching, thus enabling automated measurements of multiple nuclei for one sample with no manual adjustment required by the user. We have applied this new eATM approach in static and MAS spin-echo mapping NMR experiments in different magnetic fields on four energy storage materials, namely: (1) paramagnetic $^{7}$Li and $^{31}$P MAS NMR (without manual recalibration) of the Li-ion battery cathode material LiFePO$_{4}$; (2) paramagnetic $^{17}$O VT-NMR of the solid oxide fuel cell cathode material La$_{2}$NiO$_{4+δ}$; (3) broadband $^{93}$Nb static NMR of the Li-ion battery material BNb$_{2}$O$_{5}$; and (4) broadband static $^{127}$I NMR of a potential Li-air battery product LiIO$_{3}$. In each case, insight into local atomic structure and dynamics arises primarily from the highly broadened (1-25MHz) NMR lineshapes that the eATM robot is uniquely suited to collect. These new developments in automation of NMR experiments are likely to advance the application of in and ex situ NMR investigations to an ever-increasing range of energy storage materials and systems. ; This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 655444 (O.P.). D.M.H. acknowledges funding from the Cambridge Commonwealth Trusts. J.L. gratefully acknowledges Trinity College, Cambridge (UK) for funding. K.J.G. gratefully acknowledges funding from the Winston Churchill Foundation of the United States and the Herchel Smith Scholarship. M.B. is the CEO of NMR Service GmbH (Erfurt, Germany), which manufactures the eATM device; M.B. acknowledges funding of the Central Innovation Programme for small and medium-sized enterprises (SMEs; Zentrales Innovationsprogramm Mittelstand, ZIM) of the German Federal Ministry of Economic Affairs and Energy (Bundesministerium für Wirtschaft und Energie, BMWi) under the Grant No. KF 2845501UWF. DFT calculations were performed on (1) the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council and (2) the Center for Functional Nanomaterials cluster, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

Quantum-confined CsPbBr3 nanoplatelets (NPLs) are extremely promising for use in low-cost blue light-emitting diodes, but their tendency to coalesce in both solution and film form, particularly under operating device conditions with injected charge-carriers, is hindering their adoption. We show that employing a short hexyl-phosphonate ligand (C6H15O3P) in a heat-up colloidal approach for pure, blue-emitting quantum-confined CsPbBr3 NPLs significantly suppresses these coalescence phenomena compared to particles capped with the typical oleyammonium ligands. The phosphonate-passivated NPL thin films exhibit photoluminescence quantum yields of ∼40% at 450 nm with exceptional ambient and thermal stability. The color purity is preserved even under continuous photoexcitation of carriers equivalent to LED current densities of ∼3.5 A/cm2. 13C, 133Cs, and 31P solid-state MAS NMR reveal the presence of phosphonate on the surface. Density functional theory calculations suggest that the enhanced stability is due to the stronger binding affinity of the phosphonate ligand compared to the ammonium ligand. ; J. S. and S.D.S. acknowledge the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (HYPERION, grant agreement number 756962). S.D.S acknowledges funding from the Royal Society and Tata Group (UF150033). R.H.F. and Y.L. acknowledge sup-port from the Simons Foundation (grant 601946). M.A. and D.K. acknowledges funding from the European Union's Hori-zon 2020 research and innovation programme under the Ma-rie Skłodowska-Curie (grant agreement number 841386 and 841136, respectively). K.J. acknowledges funding from the Royal Society (RGFR1180002). K.F. acknowledges a George and Lilian Schiff Studentship, Winton Studentship, the Engineer-ing and Physical Sciences Research Council (EPSRC) student-ship, Cambridge Trust Scholarship, and Robert Gardiner Scholarship. C. P. G. acknowledges the European Research Council (ERC) under the European Union's Horizon 2020 re-search and innovation program (835073) and the Royal Society for a Research Professorship (RP\R1\180147). The authors acknowledge the EPSRC for funding (EP/R023980/1).

We report the exfoliation of layered Na2Ti3O7, a promising anode material for Na-ion batteries, and restacking using HNO3and NaOH to form H-[Ti3O7] and Na(x)-[Ti3O7] compositions, respectively. The materials were characterized by a range of techniques (SEM, TEM, solid-state NMR, XRD, PDF). Although the formation of aggregated nanoparticles is favored under acidic restacking conditions, the use of basic conditions can lead to control over the adherence between the exfoliated layers. Pair distribution function (PDF) analysis confirms that the local TiO6connectivity of the pristine material is maintained. The lowest sodium-containing phase Na(1)-[Ti3O7], which is the stable product upon Na+leaching after consecutive washing steps, displays the best performance among the compositions studied, affording a stable reversible capacity of about 200 mAh·g-1for 20 cycles at a C/20 rate. Washing removes the excess of "free/reactive" Na+, which otherwise forms inactive Na2CO3in the insufficiently washed compositions. ; M.A.T, A.J.P, J.M.S, and R.N.K. acknowledge funding from the United States Department of Energy (DOE, funder reference: 7057154). P.K.A. acknowledges a Junior Research Fellowship from Gonville and Caius College and an Oppenheimer Fellowship from the University of Cambridge. European Union's Horizon 2020 research and innovation programme under grant agreement No. 696656–GrapheneCore1 (G.K.)

The alloying mechanism of high-capacity tin anodes for sodium-ion batteries is investigated using a combined theoretical and experimental approach. Ab initio random structure searching (AIRSS) and high-throughput screening using a species-swap method provide insights into a range of possible sodium-tin structures. These structures are linked to experiments using both average and local structure probes in the form of operando pair distribution function analysis, X-ray diffraction, and 23Na solid-state nuclear magnetic resonance (ssNMR), and ex situ 119Sn ssNMR. Through this approach, we propose structures for the previously unidentified crystalline and amorphous intermediates. The first electrochemical process of sodium insertion into tin results in the conversion of crystalline tin into a layered structure consisting of mixed Na/Sn occupancy sites intercalated between planar hexagonal layers of Sn atoms (approximate stoichiometry NaSn3). Following this, NaSn2, which is predicted to be thermodynamically stable by AIRSS, forms; this contains hexagonal layers closely related to NaSn3, but has no tin atoms between the layers. NaSn2 is broken down into an amorphous phase of approximate composition Na1.2Sn. Reverse Monte Carlo refinements of an ab initio molecular dynamics model of this phase show that the predominant tin connectivity is chains. Further reaction with sodium results in the formation of structures containing Sn-Sn dumbbells, which interconvert through a solid-solution mechanism. These structures are based upon Na5-xSn2, with increasing occupancy of one of its sodium sites commensurate with the amount of sodium added. ssNMR results indicate that the final product, Na15Sn4, can store additional sodium atoms as an off-stoichiometry compound (Na15+xSn4) in a manner similar to Li15Si4. ; This work was supported by STFCBatteries.org through the STFC Futures Early Career Award (J.M.S.). J.M.S. acknowledges funding from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, of the U.S. DOE under Contract no. DE-AC02-05CH11231, under the Batteries for Advanced Transportation Technologies (BATT) Program subcontract no. 7057154, and the European Commission under grant agreement no. 696656 (Graphene Flagship). P.K.A. acknowledges the School of the Physical Sciences of the University of Cambridge for funding through an Oppenheimer Research Fellowship and a Junior Research Fellowship from Gonville and Caius College, Cambridge. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 655444 (O.P.). M.M. and A.J.M. acknowledge the support from the Winton Programme for the Physics of Sustainability. A.J.M. and C.J.P. were supported by Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom (Grant no. EP/G007489/2). C.J.P. is also supported by the Royal Society through a Royal Society Wolfson Research Merit award. Calculations were performed using the Archer facility of the UK national high performance computing service, for which access was obtained via the UKCP consortium and funded by EPSRC grant no. EP/K014560/1.

Hybrid perovskite-based optoelectronic devices are demonstrating unprecedented growth in performance, and defect passivation approaches are highly promising routes to further improve properties. Here, the effect of the molecular ion BF4-, introduced via methylammonium tetrafluoroborate (MABF4) in a surface treatment for MAPbI3 perovskite is reported. The optical spectroscopic characterisations shows that the introduction of tetrafluoroborate leads to reduced non-radiative charge carrier recombination with a reduction in first order recombination rate from 6.5 × 106 to 2.5 × 105 s-1 in BF4--treated samples, and a consequent increase in photoluminescence quantum yield by an order of magnitude (from 0.5% to 10.4%). 19F, 11B and 14N solid-state NMR is used to elucidate the atomic-level mechanism of the BF4- additive-induced improvements, revealing that the BF4- acts as a scavenger of excess MAI by forming MAI–MABF4 cocrystals. This shifts the equilibrium of iodide concentration in the perovskite phase is presumably due to the formation of MAI-MABF4 cocrystal, thereby reducing the concentration of interstitial iodide defects that act as deep traps and non-radiative recombination centers. These collective results allow us, for the first time, to elucidate the microscopic mechanism of action of BF4-. ; S.N. would like to acknowledge Royal Society-SERB Newton International Fellowship for funding. S.D.S. acknowledges the Royal Society and Tata Group (UF150033) and the EPSRC (EP/R023980/1). This work has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 841136. M.A.H. acknowledges support from the Royal Society (RP/R1/180147). S.M. thanks the EPRSC for funding. J.L.M-D. and W.-W. L. thank the UK Royal Academy of Engineering, grant CiET1819_24, EPSRC grants EP/N004272/1, EP/P007767/1, the Winton Programme for the Physics of Sustainability, and Bill Welland.