CONTENTS 6524 4.2. Bond-Length Variations 6524 4.3. Bond-Angle Variations 6524 4.4. Variations of the A•••A Distances 6524 5. Concluding Remarks 6524 Author Information 6525 Corresponding Author 6525 Notes 6525 Biographies 6525 Acknowledgments 6526 References 6526

The crystal structure of the NASICON Na3V2(PO4)3 phase (NVP) has been investigated as a function of temperature. Combining laboratory and synchrotron X-ray powder diffraction with single crystal X-ray diffraction, we demonstrate the presence of four polymorphs of NVP from 30 to 225 °C. While the high temperature γ-NVP crystallizes in the classical rhombohedral cell (S.G. R3̅c, a = 8.73382(4) Å, c = 21.91438(17) Å), the low temperature α-NVP undergoes a monoclinic distortion (S.G. C2/c, a = 15.1244(6) Å, b = 8.7287(3) Å, c = 21.6143(8) Å, β = 90.163(2)°) together with an ordering of the Na atoms. In the middle temperature range, two incommensurate modulated structures (β- and β′-NVP) are also reported for the first time.

Currently, batteries are being both considered and utilized in a variety of large-scale applications. Materials sustainability stands as a key issue for future generations of batteries. One alternative to the use of a finite supply of mined materials is the use of renewable organic materials. However, before addressing issues regarding the sustainability of a given organic electrode, fundamental questions relating to the structure-function relationships between organic components and battery performance must first be explored. Herein we report the synthesis, characterization, and device performance of an organic salt, lithium 2,6-bis(ethoxycarbonyl)-3,7-dioxo-3,7-dihydro-s-indacene-1,5-bis(olate), capable of reversibly intercalating with minimal polarization 1.8 Li per unit formula over two main voltage plateaus located at approximately 1.96 and approximately 1.67 V (vs. Li/Li(+)), leading to an overall capacity of 125 mAh/g. Proton NMR and in situ XRD analyses of battery cycling versus Li at room temperature reveal that the insertion-deinsertion process is fully reversible with the dips in the voltage-composition traces, which are associated with changes in the 3D structural packing of the electrochemically active molecules.

Among them, Na 3 V 2 (PO 4 ) 3 (NVP) has been identified so far as the most interesting one as it possesses satisfactory energy density and high power for extended cycle life. Its crystal structure can be described as a 3D framework of VO 6 octahedra and PO 4 tetrahedra connected to each other by common corners forming so-called "lantern units" along the c direction of the commonly used hexagonal cell. Sodium cations were described as randomly disordered over two sodium sites (Na(1), 6b and Na(2), 18e) [18] until Chotard et al. discovered that below 280 K, the so called α-NVP form crystallized in a monoclinic superstructure due to a fully ordered distribution of Na +[19] similar to the ordering previously reported in α-Na 3 Ti 2 (PO 4 ) 3 . [51] The synthesis of Na 3 V 2 (PO 4 ) 3 was first reported by Delmas. [20] Gopalakrishnan [21] later on reported on the possible extraction of three Na + toward the novel sodium-free V IV V V (PO 4 ) 3 composition. Afterward, the electrochemical extraction of Na + from Na 3 V 2 (PO 4 ) 3 to NaV 2 (PO 4 ) 3 (with a theoretical capacity of 117.6 mAh g −1 at 3.4 V vs Na/Na) was extensively investigated. [22][23][24][25][26] A large number of special treatments (e.g., carbon coating, particle shape controlling) was also proposed to improve battery performances. [27][28][29][30][31] It is important to note that only 2Na formula unit −1 have been completely removed from the structure during charging up to now. A possible activation of the V 4+/5+ redox couple at higher voltages may also contribute to the increasing of the energy density of NVP-based materials, as demonstrated in a series of works by using a metal substitution of a part of V 3+ in the structure of NVP. In recent years, several elements have been chosen for the partial substitution of V into the crystal structure of this promising material (such as Ni, [32][33][34] Al, [35,36] Fe 3+ , [34,37] Zr 4+ , [38] Mn 3+ , [39] Mn 2+ , [34,40] Cr 3+ , [41][42][43] Ti 4+ , [44][45][46] Mo 6+ , [47] and Mg 2+[48] ).In this work, Mn 2+ was used as a substituting ion to enhance the capacity of the Na 3 V 2 (PO 4 ) 3 cathode material. Inspired by the recent work of Zhou et al., [34] nearly single-phase Na 4 MnV(PO 4 ) 3 (98.5 wt%) powders were synthesized and studied structurally and electrochemically in details. In operando X-ray diffraction (XRD) studies during electrochemical operation show for the first time that Na 4 MnV(PO 4 ) 3 can deliver 156 mAh g −1 toward the new composition NaMnV(PO 4 ) 3 . Results and DiscussionThe crystal structure of Na 4 MnV(PO 4 ) 3 has been fully determined using high-resolution synchrotron powder XRD (SXRD) dataThe mixed Mn 2+ /V 3+ Na-super-ionic-conductor (NASICON) cathode material Na 4 MnV(PO 4 ) 3 is prepared by solid-state reaction at 800 °C under argon. When used as a positive electrode in Na batteries, this material can exchange three electrons for two transition metals, that is, yielding a high gravimetric capacity of 156 mAh g −1 on charge when the upper cutoff voltage is set to 4.3 V...

The structure of the fully ordered α-Na(3)Ti(2)(PO(4))(3) NASICON compound was elucidated using high-quality single-crystal data. The cation/vacancy distribution forms a homogeneous 3D arrangement and could represent the absolute cationic ordering available in the full Na(3)M(2)(PO(4))(3) series, as verified for M = Fe. For M = Ti, the reversible α → γ transition was observed at 85 °C, leading to the standard disordered R ̅3c γ model. Through JPDF analysis, the most probable Na(+) zigzag M(2)-M(1) diffusion scheme was directly deduced using our accurate crystallographic data.

Efficient organic Li-ion batteries require air-stable lithiated organic structures that can reversibly deintercalate Li at sufficiently high potentials. To date, most of the cathode materials reported in the literature are typically synthesized in their fully oxidized form, which restricts the operating potential of such materials and requires use of an anode material in its lithiated state. Reduced forms of quinonic structures could represent examples of lithiated organic-based cathodes that can deintercalate Li(+) at potentials higher than 3 V thanks to substituent effects. Having previously recognized the unique electrochemical properties of the C(6)O(6)-type ring, we have now designed and then elaborated, through a simple three-step method, lithiated 3,6-dihydroxy-2,5-dimethoxy-p-benzoquinone, a new redox amphoteric system derived from the tetralithium salt of tetrahydroxy-p-benzoquinone. Electrochemical investigations revealed that such an air-stable salt can reversibly deintercalate one Li(+) ion on charging with a practical capacity of about 100 mAh g(-1) at about 3 V, albeit with a polarization effect. Better capacity retention was obtained by simply adding an adsorbing additive. A tetrahydrated form of the studied salt was also characterized by XRD and first-principles calculations. Various levels of theory were probed, including DFT with classical functionals (LDA, GGA, PBEsol, revPBE) and models for dispersion corrections to DFT. One of the modified dispersion-corrected DFT schemes, related to a rescaling of both van der Waals radii and s(6) parameter, provides significant improvements to the description of this kind of crystal over other treatments. We then applied this optimized approach to the screening of hypothetical frameworks for the delithiated phases and to search for the anhydrous structure.

Inorganic compounds made up of low-dimensional ferromagnetic (FM) units display fascinating properties and provide a rich opportunity to investigate FM ground states, fieldinduced transitions, [1] and magnetization steps. [2][3][4] Even the spin-valve effect, realized in multilayer thin films, is found in the magnetic metal Ca 3 Ru 2 O 7[5] and its Cr-doped analogue [6] in which FM double-perovskite layers are antiferromagnetically coupled. [7] The inorganic compound Cr 2 Si 2 Te 6 , also consisting of FM layers, [8a,b] turns out to be a unique example of a bulk 2D FM Ising system. [8c] It is a great synthetic challenge to discover new magnetic transition-metal oxides made up of FM layers. In searching for such materials, a rational approach rather than by a blind exploration of chemical systems should be used. In general, a transition-metal cation at a coordinate site with three-fold or higher rotational symmetry can lead to uniaxial magnetism if its d electron count and spin state are such that there occurs an unevenly-filled degenerate level, as found for high-spin Fe 2+ ions at linear-coordinate sites [8a] and high-spin Co 3+ and Co 2+ ions at trigonal-prismatic sites. [8b,c] In principle, high-spin Fe 2+ O 6 octahedra can support uniaxial magnetism as long as they possess three-fold rotational symmetry. It can be imagined that isolated FM layers form from such FeO 6 octahedra by edge-sharing because the Fe-O-Fe angle will be close to 908 so that the nearest-neighbor spin exchange would be FM.Our guided search for such a magnetic system led to the synthesis of BaFe 2 (PO 4 ) 2 that turns out to be the first oxide 2D Ising ferromagnet. It consists of FM honeycomb layers of edge-sharing FeO 6 octahedra containing high-spin Fe 2+ ions. Such FeO 6 octahedra showing uniaxial magnetism are expected to be susceptible to Jahn-Teller (JT) instability. [9] Indeed, on cooling, BaFe 2 (PO 4 ) 2 undergoes a rare re-entrant structural transition owing to the competition between uniaxial magnetism and the JT distortion.The three main 2D triangular lattices consisting of edgesharing MO 6 octahedra are shown in Figure 1. Starting from the [MO 2 ] triangular lattice, the ordering of one quarter of the M vacancies leads to the [M 3/4 O 2 ] KagomØ lattice, and that of one third of the M vacancies to the [M 2/3 O 2 ] honeycomb lattice. They all possess similar intralayer spin exchange paths in different ratios (Figure 1). The superexchange (SE) path J 1 consists of two M-O-M bridges with a M-O-M angle of about 908, while the super-superexchange (SSE) paths J 2 and J 3 involve M À O···O À M bridges. For M = Fe, it is expected that the 908 SE J 1 path is ferromagnetic (FM) for high-spin (HS) Fe 2+ (d 6 , S = 2) but antiferromagnetic (AFM) for HS Fe 3+ (d 5 , S = 5/2) cations, according to dominant direct t 2g -t 2g overlaps. [10] Results of a literature search for such magnetic iron oxides with pertinent 2D or pseudo 2D lattices are summarized in Table 1, [11][12][13] which indicates that the Fe 2+ ions is the key cation lea...

The magnetic properties of BiCu 2 PO 6 have been analyzed by means of magnetic-susceptibility and inelastic neutron-scattering measurements on powder samples by evaluating the spin-exchange interactions on the basis of density-functional calculations and by simulating the inelastic neutron scattering in terms of spin-exchange parameters. BiCu 2 PO 6 exhibits magnetic properties described by the two-leg spin ladder with strong spin frustration along each leg chain and has a gapped quantum singlet ground state with excited magnetic states, showing an incommensurate dispersion arising from frustration.