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OperandoNa 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.

Operando pair distribution function (PDF) analysis and ex situ 23Na 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 23Na ssNMR in reference to relevant model compounds, identifies two previously uncharacterized intermediate species formed electrochemically; a-Na3–xSb (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-Na1.7Sb, a highly amorphous structure featuring some Sb–Sb bonding. The first sodiation breaks down the crystalline antimony to form first a-Na3–xSb 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-Na1.7Sb, then a-Na3–xSb, whereas lower potentials are required for the sodiation of crystalline antimony, which reacts to form a-Na3–xSb without the formation of a-Na1.7Sb. a-Na3–xSb is converted to crystalline Na3Sb at the end of the second discharge. We find no evidence of formation of NaSb. Variable temperature 23Na NMR experiments reveal significant sodium mobility within c-Na3Sb; this is a possible contributing factor to the excellent rate performance of Sb anodes.

Metallic germanium is a promising anode material in secondary lithium-ion batteries (LIBs) due to its high theoretical capacity (1623 mAh/g) and low operating voltage, coupled with the high lithium-ion diffusivity and electronic conductivity of lithiated Ge. Here, the lithiation mechanism of micron-sized Ge anodes has been investigated with X-ray diffraction (XRD), pair distribution function (PDF) analysis, and in-/ex-situ high-resolution 7Li solid-state nuclear magnetic resonance (NMR), utilizing the structural information and spectroscopic fingerprints obtained by characterizing a series of relevant Li x Ge y model compounds. In contrast to previous work, which postulated the formation of Li9Ge4 upon initial lithiation, we show that crystalline Ge first reacts to form a mixture of amorphous and crystalline Li7Ge3 (space group P3212). Although Li7Ge3 was proposed to be stable in a recent theoretical study of the Li–Ge phase diagram (MorrisA. J.GreyC. P.PickardC. J. Morris, A. J. Grey, C. P. Pickard, C. J. 054111Phys. Rev. B: Condens. Matter Mater. Phys.201490), it had not been identified in prior experimental studies. Further lithiation results in the transformation of Li7Ge3, via a series of disordered phases with related structural motifs, to form a phase that locally resembles Li7Ge2, a process that involves the gradual breakage of the Ge–Ge bonds in the Ge–Ge dimers (dumbbells) on lithiation. Crystalline Li15Ge4 then grows, with an overlithiated phase, Li15+δGe4, being formed at the end of discharge. This study provides comprehensive experimental evidence, by using techniques that probe short-, medium-, and long-range order, for the structural transformations that occur on electrochemical lithiation of Ge; the results are consistent with corresponding theoretical studies regarding stable lithiated Li x Ge y phases.

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 Na solid-state nuclear magnetic resonance (ssNMR), along with ex situSn 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 NaSn). Following this, NaSn, which is predicted to be thermodynamically stable by AIRSS, forms; this contains hexagonal layers closely related to NaSn, but has no tin atoms between the layers. NaSn is broken down into an amorphous phase of approximate composition NaSn. 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 NaSn, with increasing occupancy of one of its sodium sites commensurate with the amount of sodium added. ssNMR results indicate that the final product, NaSn, can store additional sodium atoms as an off-stoichiometry compound (NaSn) in a manner similar to LiSi.

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.

The structural characterization of nanoporous carbons is a challenging task as they generally lack long-range order and can exhibit diverse local structures. Such characterization represents an important step toward understanding and improving the properties and functionality of porous carbons, yet few experimental techniques have been developed for this purpose. Here we demonstrate the application of nuclear magnetic resonance (NMR) spectroscopy and pair distribution function (PDF) analysis as new tools to probe the local structures of porous carbons, alongside more conventional Raman spectroscopy. Together, the PDFs and the Raman spectra allow the local chemical bonding to be probed, with the bonding becoming more ordered for carbide-derived carbons (CDCs) synthesized at higher temperatures. The ring currents induced in the NMR experiment (and thus the observed NMR chemical shifts for adsorbed species) are strongly dependent on the size of the aromatic carbon domains. We exploit this property and use computer simulations to show that the carbon domain size increases with the temperature used in the carbon synthesis. The techniques developed here are applicable to a wide range of porous carbons and offer new insights into the structures of CDCs (conventional and vacuum-annealed) and coconut shell-derived activated carbons. ■ INTRODUCTIONNanoporous carbons are an important class of materials used in a range of applications including capacitive energy storage, gas storage, water treatment, and catalysis. 1−3 In each case, the nanoporosity and high specific surface areas (typically >1500 m 2 g −1 ), achieved by activating carbonaceous precursors, are exploited to store molecules or ions. In principle, carbon structures can be engineered for a given application, though characterization of the highly disordered structures poses a significant challenge. The challenges in determining local-and long-range structure make it extremely difficult to establish structure−function correlations beyond those simply derived from surface area and pore-size distributions.The structures of carbon materials 4,5 have been actively researched since the pioneering X-ray diffraction studies of Franklin. 6,7 She distinguished between graphitizing and nongraphitizing carbons, the former transforming into graphite upon heating to high temperature, and the latter showing no such transformation at temperatures as high as 3000°C. 7 For nanoporous carbons, analysis of the broad Bragg peaks is generally of limited use due to the long-range disordered structures of these materials. However, inclusion of the diffuse scattering in the analysis allows the extraction of a pair distribution function (PDF), which is a weighted histogram of atom-to-atom distances showing the likelihood of finding an atom pair separated by a certain distance. 8 PDF studies show that porous carbons often exhibit a high degree of local ordering, with a propensity for hexagonal carbon rings in which the carbon atoms are sp 2 hybridized. 9−11 Correlations in the PDFs typical...

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 23 Na 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.