Correlating Local Structure and Sodium Storage in Hard Carbon Anodes: Insights from Pair Distribution Function Analysis and Solid-State NMR.

Hard carbons are the leading candidate anode materials for sodium-ion batteries. However, the sodium-insertion mechanisms remain under debate. Here, employing a novel analysis of operando and ex situ pair distribution function (PDF) analysis of total scattering data, supplemented by information on the local electronic structure provided by operando 23Na solid-state NMR, we identify the local atomic environments of sodium stored within hard carbon and provide a revised mechanism for sodium storage. The local structure of carbons is well-described by bilayers of curved graphene fragments, with fragment size increasing, and curvature decreasing with increasing pyrolysis temperature. A correlation is observed between the higher-voltage (slope) capacity and the defect concentration inferred from the size and curvature of the fragments. Meanwhile, a larger lower-voltage (plateau) capacity is observed in samples modeled by larger fragment sizes. Operando PDF data on two commercially relevant hard carbons reveal changes at higher-voltages consistent with sodium ions stored close to defective areas of the carbon, with electrons localized in the antibonding π*-orbitals of the carbon. Metallic sodium clusters approximately 13-15 Å in diameter are formed in both carbons at lower voltages, implying that, for these carbons, the lower-voltage capacity is determined by the number of regions suitable for sodium cluster formation, rather than by having microstructures that allow larger clusters to form. Our results reveal that local atomic structure has a definitive role in determining storage capacity, and therefore the effect of synthetic conditions on both the local atomic structure and the microstructure should be considered when engineering hard carbons.

Fast operando X-ray pair distribution function using the DRIX electrochemical cell.

In situ electrochemical cycling combined with total scattering measurements can provide valuable structural information on crystalline, semi-crystalline and amorphous phases present during (dis)charging of batteries. In situ measurements are particularly challenging for total scattering experiments due to the requirement for low, constant and reproducible backgrounds. Poor cell design can introduce artefacts into the total scattering data or cause inhomogeneous electrochemical cycling, leading to poor data quality or misleading results. This work presents a new cell design optimized to provide good electrochemical performance while performing bulk multi-scale characterizations based on total scattering and pair distribution function methods, and with potential for techniques such as X-ray Raman spectroscopy. As an example, the structural changes of a nanostructured high-capacity cathode with a disordered rock-salt structure and composition Li4Mn2O5 are demonstrated. The results show that there is no contribution to the recorded signal from other cell components, and a very low and consistent contribution from the cell background.

Investigating Sodium Storage Mechanisms in Tin Anodes: A Combined Pair Distribution Function Analysis, Density Functional Theory, and Solid-State NMR Approach.

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), along with 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.

Tracking Sodium-Antimonide Phase Transformations in Sodium-Ion Anodes: Insights from Operando Pair Distribution Function Analysis and Solid-State NMR Spectroscopy.

Operando pair distribution function (PDF) analysis and ex situ (23)Na magic-angle spinning solid-state nuclear magnetic resonance (MAS ssNMR) spectroscopy are used to gain insight into the alloying mechanism of high-capacity antimony anodes for sodium-ion batteries. Subtraction of the PDF of crystalline NaxSb phases from the total PDF, an approach constrained by chemical phase information gained from (23)Na ssNMR in reference to relevant model compounds, identifies two previously uncharacterized intermediate species formed electrochemically; a-Na(3-x)Sb (x ≈ 0.4-0.5), a structure locally similar to crystalline Na3Sb (c-Na3Sb) but with significant numbers of sodium vacancies and a limited correlation length, and a-Na(1.7)Sb, a highly amorphous structure featuring some Sb-Sb bonding. The first sodiation breaks down the crystalline antimony to form first a-Na(3-x)Sb and, finally, crystalline Na3Sb. Desodiation results in the formation of an electrode formed of a composite of crystalline and amorphous antimony networks. We link the different reactivity of these networks to a series of sequential sodiation reactions manifesting as a cascade of processes observed in the electrochemical profile of subsequent cycles. The amorphous network reacts at higher voltages reforming a-Na(1.7)Sb, then a-Na(3-x)Sb, whereas lower potentials are required for the sodiation of crystalline antimony, which reacts to form a-Na(3-x)Sb without the formation of a-Na(1.7)Sb. a-Na(3-x)Sb is converted to crystalline Na3Sb at the end of the second discharge. We find no evidence of formation of NaSb. Variable temperature (23)Na NMR experiments reveal significant sodium mobility within c-Na3Sb; this is a possible contributing factor to the excellent rate performance of Sb anodes.

In situ studies of materials for high temperature CO2 capture and storage.

Carbon capture and storage (CCS) offers a possible solution to curb the CO2 emissions from stationary sources in the coming decades, considering the delays in shifting energy generation to carbon neutral sources such as wind, solar and biomass. The most mature technology for post-combustion capture uses a liquid sorbent, amine scrubbing. However, with the existing technology, a large amount of heat is required for the regeneration of the liquid sorbent, which introduces a substantial energy penalty. The use of alternative sorbents for CO2 capture, such as the CaO-CaCO3 system, has been investigated extensively in recent years. However there are significant problems associated with the use of CaO based sorbents, the most challenging one being the deactivation of the sorbent material. When sorbents such as natural limestone are used, the capture capacity of the solid sorbent can fall by as much as 90 mol% after the first 20 carbonation-regeneration cycles. In this study a variety of techniques were employed to understand better the cause of this deterioration from both a structural and morphological standpoint. X-ray and neutron PDF studies were employed to understand better the local surface and interfacial structures formed upon reaction, finding that after carbonation the surface roughness is decreased for CaO. In situ synchrotron X-ray diffraction studies showed that carbonation with added steam leads to a faster and more complete conversion of CaO than under conditions without steam, as evidenced by the phases seen at different depths within the sample. Finally, in situ X-ray tomography experiments were employed to track the morphological changes in the sorbents during carbonation, observing directly the reduction in porosity and increase in tortuosity of the pore network over multiple calcination reactions.

Mitigating Strain Accumulation in Li2RuO3 via Fluorine Doping.

Lithium ruthenium oxide (Li2RuO3) is an archetypal lithium rich cathode material (LRCM) with both cation and anion redox reactions (ARRs). Commonly, the instability of oxygen redox activities has been regarded as the root cause of its performance degradation in long-term operation. However, we find that not triggering ARRs does not improve and even worsens its cyclability due to the detrimental strain accumulation induced by Ru redox activities. To solve this problem, we demonstrate that F-doping in Li2RuO3 can alter its preferential orientation and buffer interlayer repulsion upon Ru redox, both of which can mitigate the strain accumulation along the c-axis and improve its structural stability. This work highlights the importance of optimizing cation redox reactions in LRCMs and provides a new perspective for their rational design.

Non-equilibrium metal oxides via reconversion chemistry in lithium-ion batteries.

Binary metal oxides are attractive anode materials for lithium-ion batteries. Despite sustained effort into nanomaterials synthesis and understanding the initial discharge mechanism, the fundamental chemistry underpinning the charge and subsequent cycles-thus the reversible capacity-remains poorly understood. Here, we use in operando X-ray pair distribution function analysis combining with our recently developed analytical approach employing Metropolis Monte Carlo simulations and non-negative matrix factorisation to study the charge reaction thermodynamics of a series of Fe- and Mn-oxides. As opposed to the commonly believed conversion chemistry forming rocksalt FeO and MnO, we reveal the two oxide series topotactically transform into non-native body-centred cubic FeO and zincblende MnO via displacement-like reactions whose kinetics are governed by the mobility differences between displaced species. These renewed mechanistic insights suggest avenues for the future design of metal oxide materials as well as new material synthesis routes using electrochemically-assisted methods.

In situ single-crystal diffraction studies of the structural transition of metal-organic framework copper 5-sulfoisophthalate, Cu-SIP-3.

The flexibility of the metal-organic framework Cu(2)(OH)(C(8)H(3)O(7)S)(H(2)O) x 2 H(2)O (Cu-SIP-3) toward reversible single-crystal to single-crystal transformations is demonstrated using in situ diffraction methods at variable temperature. At temperatures below a dehydration-induced phase transition (T < 370 K) the structure is confirmed as being hydrated. In the temperature range where the transition takes place (370 K < T < 405 K) no discrete, sharp Bragg peaks can be seen in the single-crystal X-ray diffraction pattern, indicating significant loss of long-range order. At temperatures higher than 405 K, the Bragg peaks return and the structure can be refined as dehydrated Cu-SIP-3. The loss of guest water molecules can be followed at temperatures below the phase transition giving insight into the mechanism of the dehydration. Addition of nitric oxide gas to the material above the gating opening pressure of 275 mbar also leads to loss of Bragg scattering in the diffraction pattern.

An operando spatially resolved study of alkaline battery discharge using a novel hyperspectral detector and X-ray tomography.

An experimental technique is described for the collection of time-resolved X-ray diffraction information from a complete commercial battery cell during discharging or charging cycles. The technique uses an 80 × 80 pixel 2D energy-discriminating detector in a pinhole camera geometry which can be used with a polychromatic X-ray source. The concept was proved in a synchrotron X-ray study of commercial alkaline Zn-MnO2 AA size cells. Importantly, no modification of the cell was required. The technique enabled spatial and temporal changes to be observed with a time resolution of 20 min (5 min of data collection with a 15 min wait between scans). Chemical changes in the cell determined from diffraction information were correlated with complementary X-ray tomography scans performed on similar cells from the same batch. The clearest results were for the spatial and temporal changes in the Zn anode. Spatially, there was a sequential transformation of Zn to ZnO in the direction from the separator towards the current collector. Temporally, it was possible to track the transformation of Zn to ZnO during the discharge and follow the corresponding changes in the cathode.

Mechanistic insights into sodium storage in hard carbon anodes using local structure probes.

Operando23Na 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.

A Neutron Diffraction Study of the Electrochemical Double Layer Capacitor Electrolyte Tetrapropylammonium Bromide in Acetonitrile.

Neutron diffraction with isotopic substitution has been used to characterize the bulk liquid structure of the technologically relevant electrolyte solution, 1 M tetrapropylammonium bromide (TPA Br) in acetonitrile (acn), and of pure deuterated acetonitrile. Empirical potential structure refinement modeling procedures have been used to extract detailed structural information about solvent-solvent, solvent-ion, and ion-ion correlations. Analysis of the refined data shows the expected local dipolar conformation of acn in the pure solvent. This short-range dipolar ordering is also present within the solutions of TPA Br in acn, and it affects how the solvent orders itself around the ions. The solvation numbers of the TPA cations and the bromide anions are deduced, 8 and 5, respectively, as are the orientations of the solvent molecules that surround the ions. Evidence for ion association is also presented, with nearly two-thirds of the ions in the system being in associated pairs or clusters.

Ultrasound-driven preparation and pair distribution function-assisted structure solution of a copper-based layered coordination polymer.

Nanoparticles of a copper-based layered coordination polymer, STAM-2, have been prepared via an ultrasound mediated transformation from a layered metal-organic framework, STAM-1. The structure of the material was then solved using pair distribution function analysis to identify the structural units present and the final structural model refined against the pair distribution function data.

Synthesis and structural characterization of a single-crystal to single-crystal transformable coordination polymer.

A single-crystal to single-crystal transformable coordination polymer compound was hydrothermally synthesized. The structural rearrangement is induced by selecting a ligand that contains both strong and weaker coordinating groups. Both hydrated and dehydrated structures were determined by single crystal X-ray analysis.

Metal-organic frameworks for the storage and delivery of biologically active hydrogen sulfide.

Hydrogen sulfide is an extremely toxic gas that is also of great interest for biological applications when delivered in the correct amount and at the desired rate. Here we show that the highly porous metal-organic frameworks with the CPO-27 structure can bind the hydrogen sulfide relatively strongly, allowing the storage of the gas for at least several months. Delivered gas is biologically active in preliminary vasodilation studies of porcine arteries, and the structure of the hydrogen sulfide molecules inside the framework has been elucidated using a combination of powder X-ray diffraction and pair distribution function analysis.

Increasing the dimensionality of hybrid vanadium oxyfluorides using ionothermal synthesis.

A unique organically-templated vanadium(IV) oxyfluoride, [pyH][V(2)O(2)F(5)] has been prepared using an ionothermal approach. This compound has a novel layered structure which may be considered as arising via condensation of previously known 'ladder-like' structural building units. This suggests that ionothermal synthesis may provide a more effective route to extended network structures in these systems than the more widely explored hydrothermal-based media.

Metal-organophosphine and metal-organophosphonium frameworks with layered honeycomb-like structures.

Phosphanotriylbenzenecarboxylic acid (ptbcH(3); P(C(6)H(4)-p-CO(2)H)(3)) and its methyl phosphonium iodide derivative (mptbcH(3)I; {H(3)CP(C(6)H(4)-p-CO(2)H)(3)}I) have been used as organic building blocks in reaction with Zn(ii) salts to obtain a series of related two-dimensional coordination polymers with honeycomb-like networks. The variable coordination number and oxidation states available to phosphorus have been exploited to produce a family of related phosphine coordination materials (PCMs) using a single ligand precursor. The phosphine carboxylate trianion, ptbc(3-), reacted with Zn(ii) to form 3,3-connected undulating hexagonal sheets based on tetrahedral P and Zn nodes, where Zn-ptbc = 1 : 1. When hydroxide was used as an additional framework ligand, Zn(4)(OH)(2) clusters were obtained. The clusters support 6,3-connected bilayers that consist of pairs of fused hexagonal sheets (Zn-ptbc = 2 : 1) with intra-layer pore spaces. The Zn(4)(OH)(2) clusters are also coordinated by solvent, which was preferentially displaced when the bilayer material was synthesized in the presence of ethylene diamine. Treatment of ptbc(3-) with MeI resulted in methylation of the phosphine to give the P(v) phosphonium iodide salt derivative. The formally dianionic methylphosphonium tricarboxylate building block, mptbc(2-), has the same trigonal-pyramidal bridging geometry as the parent phosphine. However, mptbc(2-) reacted with Zn(ii) on a 1 : 1 stoichiometric ratio to give an unusual trilayer sheet polymer that is based exclusively on 3-connected nodes. Solid-state (31)P NMR studies confirmed that the phosphine ligands were resistant to oxidation upon solvothermal reaction under aerobic conditions.