A structural investigation of organic battery anode materials by NMR crystallography.

Conjugated alkali metal dicarboxylates have recently received attention for applications as organic anode materials in lithium- and sodium-ion batteries. In order to understand and optimise these materials, it is important to be able to characterise both the long-range and local aspects of the crystal structure, which may change during battery cycling. Furthermore, some materials can display polymorphism or hydration behaviour. NMR crystallography, which combines long-range crystallographic information from diffraction with local information from solid-state NMR via interpretation aided by DFT calculations, is one such approach, but this has not yet been widely applied to conjugated dicarboxylates. In this work, we evaluate the application of NMR crystallography for a set of model lithium and sodium dicarboxylate salts. We investigate the effect of different DFT geometry optimisation strategies and find that the calculated NMR parameters are not systematically affected by the choice of optimisation method, although the inclusion of dispersion correction schemes is important to accurately reproduce the experimental unit cell parameters. We also observe hydration behaviour for two of the sodium salts and provide insight into the structure of an as-yet uncharacterised structure of sodium naphthalenedicarboxylate. This highlights the importance of sample preparation and characterisation for organic sodium-ion battery anode materials in particular.

Solvothermal Synthesis of a Novel Calcium Metal-Organic Framework: High Temperature and Electrochemical Behaviour.

The rapid growth in the field of metal-organic frameworks (MOFs) over recent years has highlighted their high potential in a variety of applications. For biological and environmental applications MOFs with low toxicity are vitally important to avoid any harmful effects. For this reason, Ca-based MOFs are highly desirable owing to their low cost and high biocompatibility. Useful Ca MOFs are still rare owing to the ionic character and large size of the Ca2+ ion tending to produce dense phases. Presented here is a novel Ca-based MOF containing 2,3-dihyrdoxyterephthalate (2,3-dhtp) linkers Ca(2,3-dhtp)(H2O) (SIMOF-4). The material undergoes a phase transformation on heating, which can be followed by variable temperature powder X-ray diffraction. The structure of the high temperature form was obtained using single-crystal X-ray diffraction. The electrochemical properties of SIMOF-4 were also investigated for use in a Na ion battery.

Synthesis, Structure, and Electrochemical Properties of Some Cobalt Oxalates.

Transition-metal oxalates have wide applications in magnetics, photoemission, electrochemistry, etc. Herein, using hydrothermal reactions, five cobalt(II) oxalates, Na2Co2(C2O4)3·2H2O (I), Na2Co(C2O4)2·8H2O (II), KLi3Co(C2O4)3 (III), Li4Co(C2O4)3 (IV), and (NH4)2Co2(C2O4)F4 (V) have been synthesized, and their structures are determined from single-crystal X-ray diffraction or Rietveld refinement of powder X-ray diffraction data. Notably, IV and V are identified for the first time. The structures of these cobalt oxalates are versatile, covering 0D, 1D, 2D, and 3D frameworks, while the coordination environments of Co2+ centers are uniquely distorted octahedra. As representative examples, I and III are investigated as cathode materials for secondary batteries. Both exhibited electrochemical activity despite large cell polarization. The present study enriches the transition-metal oxalate family and provides new options for energy storage materials.

NaFe3(HPO3)2((H,F)PO2OH)6: A Potential Cathode Material and a Novel Ferrimagnet.

A novel iron fluorophosphite, NaFe3(HPO3)2((H,F)PO2OH)6, was synthesized by a dry low-temperature synthesis route. The phase was shown to be electrochemically active for reversible insertion of Na(+) ions, with an average discharge voltage of 2.5 V and an experimental capacity at low rates of up to 90 mAhg(-1). Simple synthesis, low-cost materials, excellent capacity retention, and efficiency suggest this class of material is competitive with similar oxyanion-based compounds as a cathode material for Na batteries. The characterization of physical properties by means of magnetization, specific heat, and electron spin resonance measurements confirms the presence of two magnetically nonequivalent Fe(3+) sites. The compound orders magnetically at TC ≈ 9.4 K into a state with spontaneous magnetization.

ß-NaMnO2: a high-performance cathode for sodium-ion batteries.

There is much interest in Na-ion batteries for grid storage because of the lower projected cost compared with Li-ion. Identifying Earth-abundant, low-cost, and safe materials that can function as intercalation cathodes in Na-ion batteries is an important challenge facing the field. Here we investigate such a material, ß-NaMnO2, with a different structure from that of NaMnO2 polymorphs and other compounds studied extensively in the past. It exhibits a high capacity (of ca. 190 mA h g(-1) at a rate of C/20), along with a good rate capability (142 mA h g(-1) at a rate of 2C) and a good capacity retention (100 mA h g(-1)after 100 Na extraction/insertion cycles at a rate of 2C). Powder XRD, HRTEM, and (23)Na NMR studies revealed that this compound exhibits a complex structure consisting of intergrown regions of α-NaMnO2 and ß-NaMnO2 domains. The collapse of the long-range structure at low Na content is expected to compromise the reversibility of the Na extraction and insertion processes occurring upon charge and discharge of the cathode material, respectively. Yet stable, reproducible, and reversible Na intercalation is observed.

Lithium-ion diffusion mechanisms in the battery anode material Li(1+x)V(1-x)O2.

Layered Li(1+x)V(1-x)O2 has attracted recent interest as a potential low voltage and high energy density anode material for lithium-ion batteries. A greater understanding of the lithium-ion transport mechanisms is important in optimising such oxide anodes. Here, stoichiometric LiVO2 and Li-rich Li1.07V0.93O2 are investigated using atomistic modelling techniques. Lithium-ion migration is not found in LiVO2, which has also previously shown to be resistant to lithium intercalation. Molecular dynamics simulations of lithiated non-stoichiometric Li(1.07+y)V0.93O2 suggest cooperative interstitial Li(+) diffusion with favourable migration barriers and diffusion coefficients (D(Li)), which are facilitated by the presence of lithium in the transition metal layers; such transport behaviour is important for high rate performance as a battery anode.

Rapid preparation of binary mixtures of sodium carboxylates as anodes in sodium-ion batteries.

Sodium-ion batteries are emerging as a sustainable solution to tackle the growing global energy demands. In this context, organic electrode materials complement such technologies as they are composed of earth-abundant elements. As organic anodes, sodium carboxylates exhibit promising applicability in a wide range of molecules. To harness the advantages of individual systems and to minimise their limitations, in this work, an approach to form binary mixtures of sodium carboxylates using one-pot, microwave-assisted synthesis is presented. The target mixtures were synthesised in 30 min with disodium naphthalene-2,6-dicarboxylate (Na-NDC) as a common constituent in all. Both components in all mixtures were shown to participate in the charge storage and had a considerable effect on the performance characteristics, such as specific capacity and working voltage, in half and full cell formats. This approach opens a new avenue for enabling organic materials to be considered as more competitive candidates in sodium-ion batteries and promote their use in other material classes to overcome their limitations.

K2Fe(C2O4)2: An Oxalate Cathode for Li/Na-Ion Batteries Exhibiting a Combination of Multielectron Cation and Anion Redox.

The development of multielectron redox-active cathode materials is a top priority for achieving high energy density with long cycle life in the next-generation secondary battery applications. Triggering anion redox activity is regarded as a promising strategy to enhance the energy density of polyanionic cathodes for Li/Na-ion batteries. Herein, K2Fe(C2O4)2 is shown to be a promising new cathode material that combines metal redox activity with oxalate anion (C2O4 2-) redox. This compound reveals specific discharge capacities of 116 and 60 mAh g-1 for sodium-ion batterie (NIB) and lithium-ion batterie (LIB) cathode applications, respectively, at a rate of 10 mA g-1, with excellent cycling stability. The experimental results are complemented by density functional theory (DFT) calculations of the average atomic charges.

Azo-functionalised metal-organic framework for charge storage in sodium-ion batteries.

Sodium-ion batteries (NIBs) are emerging as promising devices for energy storage applications. Porous solids, such as metal-organic frameworks (MOFs), are well suited as electrode materials for technologies involving bulkier charge carriers. However, only limited progress has been made using pristine MOFs, primarily due to lack of redox-active organic groups in the materials. In this work a azo-functional MOF, namely UiO-abdc, is presented as an electrode compound for sodium-ion insertion. The MOF delivers a stable capacity (∼100 mA h g-1) over 150 cycles, and post-cycling characterisation validates the stability of the MOF and participation of the azo-group in charge storage. This study can accelerate the realisation of pristine solids, such as MOFs and other porous organic compounds, as battery materials.

Surface-Functionalized Metal-Organic Frameworks for Binding Coronavirus Proteins.

Since the outbreak of SARS-CoV-2, a multitude of strategies have been explored for the means of protection and shielding against virus particles: filtration equipment (PPE) has been widely used in daily life. In this work, we explore another approach in the form of deactivating coronavirus particles through selective binding onto the surface of metal-organic frameworks (MOFs) to further the fight against the transmission of respiratory viruses. MOFs are attractive materials in this regard, as their rich pore and surface chemistry can easily be modified on demand. The surfaces of three MOFs, UiO-66(Zr), UiO-66-NH2(Zr), and UiO-66-NO2(Zr), have been functionalized with repurposed antiviral agents, namely, folic acid, nystatin, and tenofovir, to enable specific interactions with the external spike protein of the SARS virus. Protein binding studies revealed that this surface modification significantly improved the binding affinity toward glycosylated and non-glycosylated proteins for all three MOFs. Additionally, the pores for the surface-functionalized MOFs can adsorb water, making them suitable for locally dehydrating microbial aerosols. Our findings highlight the immense potential of MOFs in deactivating respiratory coronaviruses to be better equipped to fight future pandemics.

The lithium intercalation process in the low-voltage lithium battery anode Li(1+x)V(1-x)O2.

Lithium can be reversibly intercalated into layered Li(1+x)V(1-x)O(2) (LiCoO(2) structure) at ~0.1 V, but only if x>0. The low voltage combined with a higher density than graphite results in a higher theoretical volumetric energy density; important for future applications in portable electronics and electric vehicles. Here we investigate the crucial question, why Li cannot intercalate into LiVO(2) but Li-rich compositions switch on intercalation at an unprecedented low voltage for an oxide? We show that Li(+) intercalated into tetrahedral sites are energetically more stable for Li-rich compositions, as they share a face with Li(+) on the V site in the transition metal layers. Li incorporation triggers shearing of the oxide layers from cubic to hexagonal packing because the Li(2)VO(2) structure can accommodate two Li per formula unit in tetrahedral sites without face sharing. Such understanding is important for the future design and optimization of low-voltage intercalation anodes for lithium batteries.

Exploiting anion and cation redox chemistry in lithium-rich perovskite oxalate: a novel next-generation Li/Na-ion battery electrode.

The fundamental understanding of electrochemical reaction kinetics for lithium/sodium-ion batteries (LIBs & NIBs) is a significant criterion for advancing new-generation electrode materials. Herein, we demonstrate a novel lithium-rich perovskite oxalate KLi3Fe(C2O4)3 (KLFC) cathode with the combination of cation and anion redox delivering discharge capacities of 86 and 99 mA h g-1 after 100 cycles for a LIB and NIB, respectively, with good cyclability. Experimental Raman spectroscopy analysis combined with DFT calculations of charged/discharged samples illustrate the oxalate anion redox activity. Further, first-principles calculations of the partial density of states and Bader charges analysis have also characterised the redox behaviour and charge transfer during the potassium extraction processes.

Structure and lithium transport pathways in Li2FeSiO4 cathodes for lithium batteries.

The importance of exploring new low-cost and safe cathodes for large-scale lithium batteries has led to increasing interest in Li(2)FeSiO(4). The structure of Li(2)FeSiO(4) undergoes significant change on cycling, from the as-prepared γ(s) form to an inverse ß(II) polymorph; therefore it is important to establish the structure of the cycled material. In γ(s) half the LiO(4), FeO(4), and SiO(4) tetrahedra point in opposite directions in an ordered manner and exhibit extensive edge sharing. Transformation to the inverse ß(II) polymorph on cycling involves inversion of half the SiO(4), FeO(4), and LiO(4) tetrahedra, such that they all now point in the same direction, eliminating edge sharing between cation sites and flattening the oxygen layers. As a result of the structural changes, Li(+) transport paths and corresponding Li-Li separations in the cycled structure are quite different from the as-prepared material, as revealed here by computer modeling, and involve distinct zigzag paths between both Li sites and through intervening unoccupied octahedral sites that share faces with the LiO(4) tetrahedra.

Dependence of Li2FeSiO4 electrochemistry on structure.

Small differences in the FeO(4) arrangements (orientation, size, and distortion) do influence the equilibrium potential measured during the first oxidation of Fe(2+) to Fe(3+) in all polymorphs of Li(2)FeSiO(4).

Influence of size on the rate of mesoporous electrodes for lithium batteries.

High power rechargeable lithium batteries are a key target for transport and load leveling, in order to mitigate CO(2) emissions. It has already been demonstrated that mesoporous lithium intercalation compounds (composed of particles containing nanometer diameter pores separated by walls of similar size) can deliver high rate (power) and high stability on cycling. Here we investigate how the critical dimensions of pore size and wall thickness control the rate of intercalation (electrode reaction). By using mesoporous beta-MnO(2), the influence of these mesodimensions on lithium intercalation via single and two-phase intercalation processes has been studied in the same material enabling direct comparison. Pore size and wall thickness both influence the rate of single and two-phase intercalation mechanisms, but the latter is more sensitive than the former.

Crystal structure of a new polymorph of Li2FeSiO4.

We report on the crystal structure of a new polymorph of Li(2)FeSiO(4) (prepared by annealing under argon at 900 degrees C and quenching to 25 degrees C) characterized by electron microscopy and powder X-ray and neutron diffraction. The crystal structure of Li(2)FeSiO(4) quenched from 900 degrees C is isostructural with Li(2)CdSiO(4), described in the space group Pmnb with lattice parameters a = 6.2836(1) A, b = 10.6572(1) A, and c = 5.0386(1) A. A close comparison is made with the structure of Li(2)FeSiO(4) quenched from 700 degrees C, published recently by Nishimura et al. (J. Am. Chem. Soc. 2008, 130, 13212). The two polymorphs differ mainly on the respective orientations and alternate sequences of corner-sharing FeO(4) and SiO(4) tetrahedra.

Advances in Organic Anode Materials for Na-/K-Ion Rechargeable Batteries.

Electrochemical energy storage (EES) devices are gaining ever greater prominence in the quest for global energy security. With increasing applications and widening scope, rechargeable battery technology is gradually finding avenues for more abundant and sustainable systems such as Na-ion (NIB) and K-ion batteries (KIB). Development of suitable electrode materials lies at the core of this transition. Organic redox-active molecules are attractive candidates as negative electrode materials owing to their low redox potentials and the fact that they can be obtained from biomass. Also, the rich structural diversity allows integration into several solid-state polymeric materials. Research in this domain is increasingly focused on deploying molecular engineering to address specific electrochemical limitations that hamper competition with rival materials. This Minireview aims to summarize the advances in both the electrochemical properties and the materials development of organic anode materials.

Conversion of a microwave synthesized alkali-metal MOF to a carbonaceous anode for Li-ion batteries.

Hierarchical carbon-rich materials have shown immense potential for various electrochemical applications. Metal-organic frameworks (MOFs) are well suited precursors for obtaining such templated carbon matrices. Usually these conversions are carried out by energy intensive processes and lead to the presence of toxic transition metal residues. Herein, we demonstrate the green, scalable, microwave-assisted synthesis of a three-dimensional s-block metal based MOF and its efficient transformation into a carbonaceous material. The MOF-derived solid functions as a negative electrode for lithium-ion batteries having moderate low-rate capacities and cycling stability.

Sodium Naphthalene-2,6-dicarboxylate: An Anode for Sodium Batteries.

The conjugated dicarboxylate sodium naphthalene-2,6-dicarboxylate (Na2 NDC) was prepared by a low-energy-consumption reflux method, and its performance as a negative electrode for sodium-ion batteries was evaluated in electrochemical cells. The structure of Na2 NDC was solved for the first time (monoclinic P21 /c) from powder XRD data and consists of π-stacked naphthalene units separated by sodium-oxygen layers. Through an appropriate choice of binder and conducting carbon additive, Na2 NDC exhibited a reversible two electron sodium insertion at approximately 0.4 V (vs. Na+ /Na) with remarkably stable capacities of approximately 200 mAh g-1 at a rate of C/2 and good rate capability (≈133 mAh g-1 at 5 C). In parallel, the high thermal stability of the material was demonstrated by high-temperature XRD: the framework remained intact to above 500 °C.

An oxalate cathode for lithium ion batteries with combined cationic and polyanionic redox.

The growing demand for advanced lithium-ion batteries calls for the continued development of high-performance positive electrode materials. Polyoxyanion compounds are receiving considerable interest as alternative cathodes to conventional oxides due to their advantages in cost, safety and environmental friendliness. However, polyanionic cathodes reported so far rely heavily upon transition-metal redox reactions for lithium transfer. Here we show a polyanionic insertion material, Li2Fe(C2O4)2, in which in addition to iron redox activity, the oxalate group itself also shows redox behavior enabling reversible charge/discharge and high capacity without gas evolution. The current study gives oxalate a role as a family of cathode materials and suggests a direction for the identification and design of electrode materials with polyanionic frameworks.