Sodium is globally available, which makes a sodium-ion rechargeable battery preferable to a lithium-ion battery for large-scale storage of electrical energy, provided a host cathode for Na can be found that provides the necessary capacity, voltage, and cycle life at the prescribed charge/discharge rate. Low-cost hexacyanometallates are promising cathodes because of their ease of synthesis and rigid open framework that enables fast Na(+) insertion and extraction. Here we report an intriguing effect of interstitial H2O on the structure and electrochemical properties of sodium manganese(II) hexacyanoferrates(II) with the nominal composition Na2MnFe(CN)6·zH2O (Na2-δMnHFC). The newly discovered dehydrated Na2-δMnHFC phase exhibits superior electrochemical performance compared to other reported Na-ion cathode materials; it delivers at 3.5 V a reversible capacity of 150 mAh g(-1) in a sodium half cell and 140 mAh g(-1) in a full cell with a hard-carbon anode. At a charge/discharge rate of 20 C, the half-cell capacity is 120 mAh g(-1), and at 0.7 C, the cell exhibits 75% capacity retention after 500 cycles.

A generalized solid-state nudged elastic band (G-SSNEB) method is presented for determining reaction pathways of solid-solid transformations involving both atomic and unit-cell degrees of freedom. We combine atomic and cell degrees of freedom into a unified description of the crystal structure so that calculated reaction paths are insensitive to the choice of periodic cell. For the rock-salt to wurtzite transition in CdSe, we demonstrate that the method is robust for mechanisms dominated either by atomic motion or by unit-cell deformation; notably, the lowest-energy transition mechanism found by our G-SSNEB changes with cell size from a concerted transformation of the cell coordinates in small cells to a nucleation event in large cells. The method is efficient and can be applied to systems in which the force and stress tensor are calculated using density functional theory.

Reversible high voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen anion redox has garnered intense interest for such applications, particularly lithium ion batteries, as it offers substantial redox capacity at > 4 V vs. Li/Li + in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis, and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li 2-x Ir 1-y Sn y O 3 model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure-redox coupling arises from the local stabilization of short ~ 1.8 Å metal-oxygen π bonds and ~ 1.4 Å O-O dimers during oxygen 42 redox, which occurs in Li 2-x Ir 1-y Sn y O 3 through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighboring cation sites, driving cation disorder. These insights establish a point defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry. 3 Main Text: Reversible redox chemistry in solids under highly oxidizing conditions (e.g. vs H 2 /H + , Li/Li + , or 52 O) is a powerful tool in (electro)chemical systems, increasing the catalytic activity of oxygenevolution and methane-functionalization (electro)catalysts as well as the energy and power densities of lithium-ion batteries (LIBs). 1 In LIBs in particular, employing high-voltage redox has been identified as a promising avenue to meeting the energy density demands of nextgeneration technologies such as plug-in electric vehicles. Recently, anionic oxygen redox has been shown to offer access to substantial high-voltage (de)intercalation capacity in a range of electrode materials, 2-7 spurring an intense research effort to understand this phenomenon. While many oxygen-redox-active materials have been developed, they almost universally exhibit a host of irreversible electrochemical behaviors such as voltage hysteresis and voltage fade. 8 This is most notable in the anion-redox-active Li-rich 62 layered oxides, Li 1+x M 1-x O 2 (M = a transition metal (TM) or non-transition metal such as Al, Sn, Mg, etc.), which exhibit capacities approaching 300 mAh g-1 but have yet to achieve commercial success due to such electrochemical behaviors. 5, 9 It has been shown both experimentally 10-12 and 65 from first-principles thermodynamics 13 that the migration of M into empty Li sites 9-creating structural disorder in the form of M Li /V M antisite/cation vacancy point defect pairs-is at the root of voltage profile...

In the synthesis of inorganic materials, reactions often yield non-equilibrium kinetic byproducts instead of the thermodynamic equilibrium phase. Understanding the competition between thermodynamics and kinetics is fundamental towards the rational synthesis of target materials. Here, we use in situ synchrotron X-ray diffraction to investigate the multistage crystallization pathways of the important two-layer (P2) sodium oxides Na 0.67 MO 2 (M = Co, Mn). We observe a series of fast non-equilibrium phase transformations through metastable three-layer O3, O3' and P3 phases before formation of the equilibrium two-layer P2 polymorph. We present a theoretical framework to rationalize the observed phase progression, demonstrating that even though P2 is the equilibrium phase, compositionally-unconstrained reactions between powder precursors favor the formation of non-equilibrium three-layered intermediates. These insights can guide the choice of precursors and parameters employed in the solidstate synthesis of ceramic materials, and constitutes a step forward in unraveling the complex interplay between thermodynamics and kinetics during materials synthesis.

on the cathode and anode side of the battery. [5][6][7][8][9][10] The positive electrode is the main limiting factor for the gravimetric capacity of the battery [11] and no material has yet shown capacities equal to those of Li-ion battery cathodes (typically, stable specific energies of ≈750 Wh kg −1 for LiCoO 2 , [12] ≈610 Wh kg −1 for LiMn 2 O 4 , [13] or ≈580 Wh kg −1 for LiFePO 4 ). [14] The three main classes of materials investigated as cathodes in Na-ion are: P2 or O3 transition metal oxides NaTMO 2 , [15][16][17][18] where TM can be one or often several transition metals, and polyanionic compounds. The layered oxides have a large diversity of compositions due to the low tendency for TM migration into the slab space upon desodiation, [19] and multiple binary, [20] ternary, [21,22] and quaternary [23] mixtures of transition metals have been shown to work. The P2 structure, in particular, is promising and was shown to outperform O3 in terms of high mobility of sodium ions. [24] However, despite their rather high gravimetric energy density, often approaching or exceeding 600 Wh kg −1 , most layered materials tend to lose capacity upon cycling to high voltages. [9] Polyanionic materials are structurally diverse compounds in which the arrangement of sodium, transition metals, and polyanions give rise to numerous structural frameworks. [25] At present, the most studied candidate materials have been vanadium-containing phosphates and fluorophosphates of compositions Na 3 V 2 (PO 4 ) 3 [26][27][28] or Na 3 V 2 (PO 4 ) 2 F 3 . [29] This second compound, in particular, offers great promises. With an average operating potential of 3.9 V and a capacity of 128 mAh g −1 , its specific energy reaches 500 Wh kg −1 , an important milestone Na-ion technology is increasingly studied as a low-cost solution for grid storage applications. Many positive electrode materials have been reported, mainly among layered oxides and polyanionic compounds. The vanadium oxy/flurophosphate solid solution Na 3 V 2 (PO 4 ) 2 F 3-y O 2y (0 ≤ y ≤ 1), in particular, has proven the ability to deliver ≈500 Wh kg −1 , operating on the V 3+ /V 4+ (y = 0) or V 4+ /V 5+ redox couples (y = 1). This paper reports here on a significant increase in specific energy by enabling sodium insertion into Na 3 V 2 (PO 4 ) 2 FO 2 to reach Na 4 V 2 (PO 4 ) 2 FO 2 upon discharge. This occurs at ≈1.6 V and increases the theoretical specific energy to 600 Wh kg −1 , rivaling that of several Li-ion battery cathodes. This improvement is achieved by the judicious modification of the composition either as O for F substitution, or Al for V substitution, both of which disrupt Na-ion ordering and thereby enable insertion of the 4th Na. This paper furthermore shows from operando X-Ray Diffraction (XRD) that this energy is obtained in the cycling range

Amorphous transition metal oxides are recognized as leading candidates for electrochromic window coatings that can dynamically modulate solar irradiation and improve building energy efficiency. However, their thin films are normally prepared by energy-intensive sputtering techniques or high-temperature solution methods, which increase manufacturing cost and complexity. Here, we report on a room-temperature solution process to fabricate electrochromic films of niobium oxide glass (NbO) and 'nanocrystal-in-glass' composites (that is, tin-doped indium oxide (ITO) nanocrystals embedded in NbO glass) via acid-catalysed condensation of polyniobate clusters. A combination of X-ray scattering and spectroscopic characterization with complementary simulations reveals that this strategy leads to a unique one-dimensional chain-like NbO structure, which significantly enhances the electrochromic performance, compared to a typical three-dimensional NbO network obtained from conventional high-temperature thermal processing. In addition, we show how self-assembled ITO-in-NbO composite films can be successfully integrated into high-performance flexible electrochromic devices.

Oxygen loss can lead to high-capacity Li 2 MnO 3 -based lithiumrich layered cathodes. Substitution of Mn with other transition metals (Ti and Co) significantly affects the amount of oxygen loss and capacity during the first charge/discharge cycle. An explanation of these results is provided with density functional theory (DFT+U) electronic structure calculations. Oxygen is found to bind more strongly to Ti and more weakly to Co. The influence of the substitution is attributed to changes of the band gap. Ti lifts the nonbonding band and increases the band gap of the compound, thus raising the energy required to redistribute the electrons released upon oxygen loss. Co lowers the nonbonding band and facilitates oxygen loss.

The Prussian Blue analog, Na x FeMn(CN) 6 , is a potential new cathode material for Na-ion batteries. During Na intercalation, the dehydrated material exhibits a monoclinic to rhombohedral phase transition, while the hydrated material remains in the monoclinic phase. With density functional theory calculations, the phase transition is explained in terms of a competition between Coulomb attraction, Pauli repulsion, and d−π covalent bonding. The interstitial Na cations have a strong Coulomb attraction to the N anions in the host material, which tend to bend the Mn−N bonds and reduce the volume of the structure. The presence of lattice H 2 O enhances the Pauli repulsion so that the volume reduction is suppressed. The calculated volume change, as it depends upon the presence of lattice H 2 O, is consistent with experimental measurements. Additionally, a new LiFeMn(CN) 6 phase is predicted where MnN 6 octahedra decompose into LiN 4 and MnN 4 edge-sharing tetrahedra.