An Abnormal 3.7 Volt O3‐Type Sodium‐Ion Battery Cathode

Layered O3‐type sodium oxides (NaMO₂, M=transition metal) commonly exhibit an O3–P3 phase transition, which occurs at a low redox voltage of about 3 V (vs. Na⁺/Na) during sodium extraction and insertion, with the result that almost 50 % of their total capacity lies at this low voltage region, and they possess insufficient energy density as cathode materials for sodium‐ion batteries (NIBs). Therefore, development of high‐voltage O3‐type cathodes remains challenging because it is difficult to raise the phase‐transition voltage by reasonable structure modulation. A new example of O3‐type sodium insertion materials is presented for use in NIBs. The designed O3‐type Na_0.7Ni_0.35Sn_0.65O₂ material displays a highest redox potential of 3.7 V (vs. Na⁺/Na) among the reported O3‐type materials based on the Ni²⁺/Ni³⁺ couple, by virtue of its increased Ni?O bond ionicity through reduced orbital overlap between transition metals and oxygen within the MO₂ slabs. This study provides an orbital‐level understanding of the operating potentials of the nominal redox couples for O3‐NaMO₂ cathodes. The strategy described could be used to tailor electrodes for improved performance.     

Superionic Conduction in Co‐Vacant P2‐NaxCoO2 Created by Hydrogen Reductive Elimination

The layered P2‐Na_xMO₂ (M: transition metal) system has been widely recognized as electronic or mixed conductor. Here, we demonstrate that Co vacancies in P2‐Na_xCoO₂ created by hydrogen reductive elimination lead to an ionic conductivity of 0.045 S cm^?1 at 25 °C. Using in situ synchrotron X‐ray powder diffraction and Raman spectroscopy, the composition of the superionic conduction phase is evaluated to be Na_0.61(H₃O)_0.18Co_0.93O₂. Electromotive force measurements as well as molecular dynamics simulations indicate that the ion conducting species is proton rather than hydroxide ion. The fact that the Co‐stoichiometric compound Na_x(H₃O)_yCoO₂ does not exhibit any significant ionic conductivity proves that Co vacancies are essential for the occurrence of superionic conductivity.     

A Low‐Cost and Environmentally Friendly Mixed Polyanionic Cathode for Sodium‐Ion Storage

Sodium‐ion batteries (NIBs) are the most promising alternatives to lithium‐ion batteries in the development of renewable energy sources. The advancement of NIBs depends on the exploration of new electrode materials and fundamental understanding of working mechanisms. Herein, via experimental and simulation methods, we develop a mixed polyanionic compound, Na₂Fe(C₂O₄)SO₄?H₂O, as a cathode for NIBs. Thanks to its rigid three dimensional framework and the combined inductive effects from oxalate and sulfate, it delivered reversible Na insertion/desertion at average discharging voltages of 3.5 and 3.1 V for 500 cycles with Coulombic efficiencies of ca. 99 %. In situ synchrotron X‐ray measurements and DFT calculations demonstrate the Fe²⁺/Fe³⁺ redox reactions contribute to electron compensation during Na⁺ desertion/insertion. The study suggests mixed polyanionic frameworks may provide promising materials for Na ion storage with the merits of low cost and environmental friendliness.     

High‐Entropy Layered Oxide Cathodes for Sodium‐Ion Batteries

Material innovation on high‐performance Na‐ion cathodes and the corresponding understanding of structural chemistry still remain a challenge. Herein, we report a new concept of high‐entropy strategy to design layered oxide cathodes for Na‐ion batteries. An example of layered O3‐type NaNi_0.12Cu_0.12Mg_0.12Fe_0.15Co_0.15Mn_0.1Ti_0.1Sn_0.1Sb_0.04O₂ has been demonstrated, which exhibits the longer cycling stability (ca. 83 % of capacity retention after 500 cycles) and the outstanding rate capability (ca. 80 % of capacity retention at the rate of 5.0 C). A highly reversible phase‐transition behavior between O3 and P3 structures occurs during the charge‐discharge process, and importantly, this behavior is delayed with more than 60 % of the total capacity being stored in O3‐type region. Possible mechanism can be attributed to the multiple transition‐metal components in this high‐entropy material which can accommodate the changes of local interactions during Na⁺ (de)intercalation. This strategy opens new insights into the development of advanced cathode materials.     

P2‐Na0.67AlxMn1−xO2: Cost‐Effective,Stable and High‐Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility

Sodium layered P2‐stacking Na_0.67MnO₂ materials have shown great promise for sodium‐ion batteries. However, the undesired Jahn–Teller effect of the Mn⁴⁺/Mn³⁺ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition‐metal layers to decrease the number of Mn³⁺, we obtain the low cost pure P2‐type Na_0.67Al_xMn_1?xO₂ (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al‐doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid‐state NMR techniques. Our results reveal that Al‐doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na⁺ layer thus producing a remarkable rate capability of 96 mAh g^‐1 at 1200 mA g^‐1.     

P2‐Na0.67AlxMn1−xO2: Cost‐Effective,Stable and High‐Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility

Sodium layered P2‐stacking Na_0.67MnO₂ materials have shown great promise for sodium‐ion batteries. However, the undesired Jahn–Teller effect of the Mn⁴⁺/Mn³⁺ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition‐metal layers to decrease the number of Mn³⁺, we obtain the low cost pure P2‐type Na_0.67Al_xMn_1?xO₂ (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al‐doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid‐state NMR techniques. Our results reveal that Al‐doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na⁺ layer thus producing a remarkable rate capability of 96 mAh g^‐1 at 1200 mA g^‐1.     

Realizing Complete Solid‐Solution Reaction in High Sodium Content P2‐Type Cathode for High‐Performance Sodium‐Ion Batteries

P2‐type layered oxides suffer from an ordered Na⁺/vacancy arrangement and P2→O2/OP4 phase transitions, leading them to exhibit multiple voltage plateaus upon Na⁺ extraction/insertion. The deficient sodium in the P2‐type cathode easily induces the bad structural stability at deep desodiation states and limited reversible capacity during Na⁺ de/insertion. These drawbacks cause poor rate capability and fast capacity decay in most P2‐type layered oxides. To address these challenges, a novel high sodium content (0.85) and plateau‐free P2‐type cathode‐Na_0.85Li_0.12Ni_0.22Mn_0.66O₂ (P2‐NLNMO) was developed. The complete solid‐solution reaction over a wide voltage range ensures both fast Na⁺ mobility (10^?11 to 10^?10 cm² s^?1) and small volume variation (1.7 %). The high sodium content P2‐NLNMO exhibits a higher reversible capacity of 123.4 mA h g^?1, superior rate capability of 79.3 mA h g^?1 at 20 C, and 85.4 % capacity retention after 500 cycles at 5 C. The sufficient Na and complete solid‐solution reaction are critical to realizing high‐performance P2‐type cathodes for sodium‐ion batteries.     

Sodium‐ and Potassium‐Hydrate Melts Containing Asymmetric Imide Anions for High‐Voltage Aqueous Batteries

Aqueous Na‐ or K‐ion batteries could virtually eliminate the safety and cost concerns raised from Li‐ion batteries, but their widespread applications have generally suffered from narrow electrochemical potential window (ca. 1.23 V) of aqueous electrolytes that leads to low energy density. Herein, by exploring optimized eutectic systems of Na and K salts with asymmetric imide anions, we discovered, for the first time, room‐temperature hydrate melts for Na and K systems, which are the second and third alkali metal hydrate melts reported since the first discovery of Li hydrate melt by our group in 2016. The newly discovered Na‐ and K‐ hydrate melts could significantly extend the potential window up to 2.7 and 2.5 V (at Pt electrode), respectively, owing to the merit that almost all water molecules participate in the Na⁺ or K⁺ hydration shells. As a proof‐of‐concept, a prototype Na₃V₂(PO₄)₂F₃|NaTi₂(PO₄)₃ aqueous Na‐ion full‐cell with the Na‐hydrate‐melt electrolyte delivers an average discharge voltage of 1.75 V, that is among the highest value ever reported for all aqueous Na‐ion batteries.     

A High‐Voltage and Ultralong‐Life Sodium Full Cell for Stationary Energy Storage

Recently, there has been great interest in developing advanced sodium‐ion batteries for large‐scale application. Most efforts have concentrated on the search for high‐performance electrode materials only in sodium half‐cells. Research on sodium full cells for practical application has encountered many problems, such as insufficient cycles with rapid capacity decay, low safety, and low operating voltage. Herein, we present a layered P2‐Na_0.66Ni_0.17Co_0.17Ti_0.66O₂, as both an anode (ca. 0.69 V versus Na⁺/Na) and as a high‐voltage cathode (ca. 3.74 V versus Na⁺/Na). The full cell based on this bipolar electrode exhibits well‐defined voltage plateaus near 3.10 V, which is the highest average voltage in the symmetric cells. It also shows the longest cycle life (75.9 % capacity retention after 1000 cycles) in all sodium full cells, a usable capacity of 92 mAh g^?1, and superior rate capability (65 mAh g^?1 at a high rate of 2C).     

Suppressing the P2–O2 Phase Transition of Na0.67Mn0.67Ni0.33O2 by Magnesium Substitution for Improved Sodium‐Ion Batteries

Room‐temperature sodium‐ion batteries (SIBs) have shown great promise in grid‐scale energy storage, portable electronics, and electric vehicles because of the abundance of low‐cost sodium. Sodium‐based layered oxides with a P2‐type layered framework have been considered as one of the most promising cathode materials for SIBs. However, they suffer from the undesired P2–O2 phase transition, which leads to rapid capacity decay and limited reversible capacities. Herein, we show that this problem can be significantly mitigated by substituting some of the nickel ions with magnesium to obtain Na_0.67Mn_0.67Ni_0.33?xMg_xO₂ (0≤x≤0.33). Both the reversible capacity and the capacity retention of the P2‐type cathode material were remarkably improved as the P2–O2 phase transition was thus suppressed during cycling. This strategy might also be applicable to the modulation of the physical and chemical properties of layered oxides and provides new insight into the rational design of high‐capacity and highly stable cathode materials for SIBs.     

Processable and Moldable Sodium‐Metal Anodes

Sodium‐ion batteries are similar in concept and function to lithium‐ion batteries, but their development and commercialization lag far behind. One obstacle is the lack of a standard reference electrode. Unlike Li foil reference electrodes, sodium is not easily processable or moldable and it deforms easily. Herein we fabricate a processable and moldable composite Na metal anode made from Na and reduced graphene oxide (r‐GO). With only 4.5 % percent r‐GO, the composite anodes had improved hardness, strength, and stability to corrosion compared to Na metal, and can be engineered to various shapes and sizes. The plating/stripping cycling of the composite anode was significantly extended in both ether and carbonate electrolytes giving less dendrite formation. We used the composite anode in both Na‐O₂ and Na‐Na₃V₂(PO₄)₃ full cells.     

Superior Na‐Storage Performance of Low‐Temperature‐Synthesized Na3(VO1−xPO4)2F1+2x (0≤x≤1) Nanoparticles for Na‐Ion Batteries

Na‐ion batteries are becoming comparable to Li‐ion batteries because of their similar chemical characteristics and abundant sources of sodium. However, the materials production should be cost‐effective in order to meet the demand for large‐scale application. Here, a series of nanosized high‐performance cathode materials, Na₃(VO_1?xPO₄)₂F_1+2x (0≤x≤1), has been synthesized by a solvothermal low‐temperature (60–120 °C) strategy without the use of organic ligands or surfactants. The as‐synthesized Na₃(VOPO₄)₂F nanoparticles show the best Na‐storage performance reported so far in terms of both high rate capability (up to 10 C rate) and long cycle stability over 1200 cycles. To the best of our knowledge, the current developed synthetic strategy for Na₃(VO_1?xPO₄)₂F_1+2x is by far one of the least expensive and energy‐consuming methods, much superior to the conventional high‐temperature solid‐state method.     

Hydrothermal Synthesis and X‐Ray Structural Characterization of Manganese,Nickel, and Cadmium Coordination Polymers Containing 2,3‐Pyridinedicarboxylate as Multidentate Ligands

Three divalent transiton‐metal complexes of 2,3‐pyridinedicarboxylate (2,3‐pda²), [Mn(2,3‐pda)‐(H₂O)3]_∞ ( 7 ), [Ni(2,3‐pda)(H₂O)3]_∞ ( 8 ), and [Cd2(2,3‐pda)₂(H₂O)]_∞ ( 9 ) have been hydrothermally synthesized and structurally characterized. X‐ray diffraction analyses reveal that compounds 7 and 8 are zigzag‐ and linear‐type one‐dimension (1D) coordination polymers, respectively, whereas compound 9 is a three‐dimension (3D) coordination polymer. A simple comparison of the coordination geometries with the available neutral analogues {M_x(2,3‐pda)_x(H₂O)_y}_∞ containing one or two divalent transition‐metal atoms and equal ligands in the presence or absence of coordinated water molecules is also presented.     

Poly[tetrasodium(I)‐tetra‐μ2‐bis(butane‐1,4‐diyldioxy)borato‐μ2‐1,4‐butane­diol]

In the title compound, [Na₄(C₈H₁₆BO₄)₄(C₄H₁₀O₂)]_n, there are two coordination types for the four independent Na⁺ cations: two Na⁺ cations bond to six diolate O atoms [Na—O = 2.305 (2)–2.609 (2) Å], while the other two are five‐coordinate via one 1,4‐butane­diol [2.289 (2) and 2.349 (3) Å] and four diolate O atoms [2.295 (2)–2.408 (2) Å]. Corresponding to this, there are three‐ and four‐coordinate diolate O atoms, the latter bridging Na atoms. The 1,4‐butane­diol mol­ecules lie on inversion centres. The boron stereochemistry shows minor local perturbations from its usual tetrahedral state [B—O = 1.457 (4)–1.503 (4) Å]. The resulting polymer packs as sheets parallel to the (10) plane crosslinked by the butane­diol mol­ecules. The structure was solved using data from a multiple crystal.     

A Low-Cost and Environmentally Friendly Mixed Polyanionic Cathode for Sodium-Ion Storage

Sodium-ion batteries (NIBs) are the most promising alternatives to lithium-ion batteries in the development of renewable energy sources. The advancement of NIBs depends on the exploration of new electrode materials and fundamental understanding of working mechanisms. Herein, via experimental and simulation methods, we develop a mixed polyanionic compound, Na₂Fe(C₂O₄)SO₄⋅H₂O, as a cathode for NIBs. Thanks to its rigid three dimensional framework and the combined inductive effects from oxalate and sulfate, it delivered reversible Na insertion/desertion at average discharging voltages of 3.5 and 3.1 V for 500 cycles with Coulombic efficiencies of ca. 99 %. In situ synchrotron X-ray measurements and DFT calculations demonstrate the Fe²⁺/Fe³⁺ redox reactions contribute to electron compensation during Na⁺ desertion/insertion. The study suggests mixed polyanionic frameworks may provide promising materials for Na ion storage with the merits of low cost and environmental friendliness.     

Polyoxometalate‐Based Entangled Coordination Networks Induced by an Extended Bis(triazole) Ligand

The introduction of an extended bridging bis(triazole) ligand, that is, 4,4′‐bis(1,2,4‐triazol‐1‐ ylmethyl)biphenyl (BBPTZ), into the hydrothermal reaction system containing transition metal ions and Keggin‐type polyoxometalates (POMs) led to the isolation of three new organic–inorganic hybrid entangled coordination networks, [Cu^I₂Cu^II(BBPTZ)₆][SiW₁₂O₄₀]?12 H₂O ( 1 ), [Ni(BBPTZ)₂(H₂O)][H₂SiW₁₂O₄₀]?11 H₂O ( 2 ), and [Ni₂(BBPTZ)₄(H₂O)₂][SiW₁₂O₄₀]?3 H₂O ( 3 ). All three compounds were characterized by elemental analysis, IR spectroscopy, TG analysis, powder X‐ray diffraction, and single‐crystal X‐ray diffraction. Compound 1 contains a 2‐D POM‐based metal–organic layer entangled with 1‐D ladder‐like metal–organic chains. The adjacent 2‐D networks are parallel to each other, further stacking into a 3‐D supramolecular framework with 1‐D channels. Compound 2 exhibits a 1‐D cantilever‐type loop‐containing chain. The Keggin‐type POMs act as the cantilever groups, leading to the adjacent catilever‐type chains interwaving together to form a 3‐D supramolecular open framework with two types of channels. Compound 3 possesses a 3‐D open framework based on 2‐D metal–organic undulated layer and Keggin‐type POM clusters. Three sets of such frameworks further interpenetrate with each other to form an interesting three‐fold interpenetrating framework. The photocatalytic activities of compounds 1–3 for the decomposition of methylene blue (MB) under UV light have been investigated.     

Copper(II) and Sodium(I) Complexes based on 3,7‐Diacetyl‐1,3,7‐triaza‐5‐phosphabicyclo[3.3.1]nonane‐5‐oxide: Synthesis,Characterization, and Catalytic Activity

The reaction of 3,7‐diacetyl‐1,3,7‐triaza‐5‐phosphabicyclo[3.3.1]nonane (DAPTA) with metal salts of Cu^II or Na^I/Ni^II under mild conditions led to the oxidized phosphane derivative 3,7‐diacetyl‐1,3,7‐triaza‐5‐phosphabicyclo[3.3.1]nonane‐5‐oxide (DAPTA=O) and to the first examples of metal complexes based on the DAPTA=O ligand, that is, [Cu^II(μ‐CH₃COO)₂(κO‐DAPTA=O)]₂ ( 1 ) and [Na(1κOO′;2κO‐DAPTA=O)(MeOH)]₂(BPh₄)₂ ( 2 ). The catalytic activity of 1 was tested in the Henry reaction and for the aerobic 2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO)‐mediated oxidation of benzyl alcohol. Compound 1 was also evaluated as a model system for the catechol oxidase enzyme by using 3,5‐di‐tert‐butylcatechol as the substrate. The kinetic data fitted the Michaelis–Menten equation and enabled the obtainment of a rate constant for the catalytic reaction; this rate constant is among the highest obtained for this substrate with the use of dinuclear Cu^II complexes. DFT calculations discarded a bridging mode binding type of the substrate and suggested a mixed‐valence Cu^II/Cu^I complex intermediate, in which the spin electron density is mostly concentrated at one of the Cu atoms and at the organic ligand.     

A one‐dimensional heterometallic coordination polymer with a three‐dimensional supramolecular framework: poly[μ2‐aqua‐diaqua(2,2′‐bipyridyl)(μ5‐2‐sulfonatobutanedioato)copper(II)sodium(I)]

The title compound, [CuNa(C₄H₃O₇S)(C₁₀H₈N₂)(H₂O)₃]_n, consists of one Cu^II cation, one Na^I cation, one 2‐sulfonatobutanedioate trianion (SSC³⁻), one 2,2′‐bipyridyl (bpy) ligand and three coordinated water molecules as the building unit. The coordination of the Cu^II cation is composed of two pyridyl N atoms, one water O atom and two carboxylate O atoms in a distorted square‐pyramidal coordination geometry with an axial elongation. The Na^I cation is six‐coordinated by three water molecules and three carboxylate O atoms from three SSC³⁻ ligands in a distorted octahedral geometry. Two SSC³⁻ ligands link two Cu^II cations to form a Cu₂(SSC)₂(bpy)₂ macrocyclic unit lying across an inversion centre, which is further linked by Na^I cations via Na—O bonds to give a one‐dimensional chain. Interchain hydrogen bonds link these chains to form a two‐dimensional layer, which is further extended into a three‐dimensional supramolecular framework through π–π stacking interactions. The thermal stability of the title compound has also been investigated.     

Imidazole Coordinated Sandwich‐type Antimony Poly‐oxotungstates Na9[{Na(H2O)2}3{M(C3H4N2)}3(SbW9O33)2]·xH2O (M=NiII,CoII, ZnII,MnII)

The imidazole covalently coordinated sandwich‐type heteropolytungstates Na₉[{Na(H₂O)₂}₃{M(C₃H₄N₂)}₃‐ (SbW₉O₃₃)₂]·xH₂O (M=Ni^II, x=32; M=Co^II, x=32; M=Zn^II, x=33; M=Mn^II, x=34) were obtained by the reaction of Na₂WO₄·2H₂O, SbCl₃·6H₂O, NiCl₂·6H₂O [MnSO₄·H₂O, Co(NO₃)₂·6H₂O, ZnSO₄·7H₂O] and imidazole at pH≈7.5. The structure of Na₉[{Na(H₂O)₂}₃{Ni(C₃H₄N₂)}₃(SbW₉O₃₃)₂]·32H₂O was determined by single crystal X‐ray diffraction. Polyanion [{Na(H₂O)₂}₃{Ni(C₃H₄N₂)}₃(SbW₉O₃₃)₂}₃]^9? has approximate C_3v symmetry, imidazole coordinated six‐nuclear cluster [{Na(H₂O)₂}₃{Ni(C₃H₄N₂)}₃]⁹⁺ is encapsulated between two (α‐SbW₉O₃₃)^9?, the three rings of imidazole in the polyanion are perpendicular to the horizontal plane formed by six metals (Na‐Ni‐Na‐Ni‐Na‐Ni) in the central belt, and π‐stacking interactions exist between imidazoles of neighboring polyanions with dihedral angel of 60°. The compounds were also characterized by IR, UV‐Vis spectra, TG and DSC, and the thermal decomposition mechanism of the four compounds was suggested by TG curves.     

Development and Investigation of a NASICON‐Type High‐Voltage Cathode Material for High‐Power Sodium‐Ion Batteries

Herein, we introduce a 4.0 V class high‐voltage cathode material with a newly recognized sodium superionic conductor (NASICON)‐type structure with cubic symmetry (space group P2₁3), Na₃V(PO₃)₃N. We synthesize an N‐doped graphene oxide‐wrapped Na₃V(PO₃)₃N composite with a uniform carbon coating layer, which shows excellent rate performance and outstanding cycling stability. Its air/water stability and all‐climate performance were carefully investigated. A near‐zero volume change (ca. 0.40 %) was observed for the first time based on in situ synchrotron X‐ray diffraction, and the in situ X‐ray absorption spectra revealed the V^3.2+/V^4.2+ redox reaction with high reversibility. Its 3D sodium diffusion pathways were demonstrated with distinctive low energy barriers. Our results indicate that this high‐voltage NASICON‐type Na₃V(PO₃)₃N composite is a competitive cathode material for sodium‐ion batteries and will receive more attention and studies in the future.