Self-Assembled Alluaudite Na2Fe3−xMnx(PO4)3Micro/Nanocompounds for Sodium-Ion Battery Electrodes: A New Insight into Their Electronic and Geometric Structure

State-of-the-Art Electrode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) were investigated as recently as in the seventies. However, they have been overshadowed for decades, due to the success of lithium-ion batteries that demonstrated higher energy densities and longer cycle lives. Since then, the witness a re-emergence of the SIBs and renewed interest evidenced by an exponential increase of the publications devoted to them (about 9000 publications in 2019, more than 6000 in the first six months this year). This huge effort in research has led and is leading to an important and constant progress in the performance of the SIBs, which have conquered an industrial market and are now commercialized. This progress concerns all the elements of the batteries. We have already recently reviewed the salts and electrolytes, including solid electrolytes to build all-solid-state SIBs. The present review is then devoted to the electrode materials. For anodes, they include carbons, metal chalcogenide-based materials, intercalation-based and conversion reaction compounds (transition metal oxides and sulfides), intermetallic compounds serving as functional alloying elements. For cathodes, layered oxide materials, polyionic compounds, sulfates, pyrophosphates and Prussian blue analogs are reviewed. The electrode structuring is also discussed, as it impacts, importantly, the electrochemical performance. Attention is focused on the progress made in the last five years to report the state-of-the-art in the performance of the SIBs and justify the efforts of research.

HPO32- as a building unit for sodium-ion battery cathodes: 3.1 V operation of Na2-xFe(HPO3)2 (0 < x < 1)

We report Na₂Fe(HPO₃)₂ as the first HPO₃^2--based iron(ii) cathode material for sodium-ion batteries, which delivers a reversible capacity of approximately 100 mA h g^-1 at an average reaction voltage of 3.1 V. In situ X-ray diffraction and ex situ⁵⁷Fe Mössbauer spectroscopy clarify reversible (de)sodiation associated with the Fe³⁺/Fe²⁺ redox reaction.

Exploring the Role of Manganese on Structural, Transport, and Electrochemical Properties of NASICON-Na3Fe2-yMny(PO4)3-Cathode Materials for Na-Ion Batteries

Given the extensive efforts focused on protecting the environment, eco-friendly cathode materials are a prerequisite for the development of Na-ion battery technology. Such materials should contain abundant and inexpensive elements. In the paper, we present NASICON-Na₃Fe_2-yMn_y(PO₄)₃ (y = 0, 0.1, 0.2, 0.3, and 0.4) cathode materials, which meet these requirements. Na₃Fe_2-yMn_y(PO₄)₃ compounds were prepared via a solid-state reaction at 600 °C, which allowed to obtain powders with submicron particles. The presence of manganese in the iron sub-lattice inhibits phase transitions, which occurs at ∼95 and ∼145 °C in Na₃Fe₂(PO₄)₃, changing the monoclinic structure to rhombohedral and affecting the structural and transport properties. The chemical stability of Na₃Fe_2-yMn_y(PO₄)₃ was thus higher than that of Na₃Fe₂(PO₄)₃, and it also exhibited enhanced structural, transport, and electrochemical properties. The observed correlation between the chemical composition and electrochemical properties proved the ability to precisely tune the crystal structure of NASICONs, allowing cathode materials with more desirable properties to be designed.

Structural properties of the alluaudite-type materials Ag2– x Na x Mn3(VO4)3 (x=0.62 and 1.85)

The new members of the Ag2− x Na x Mn3(VO4)3 (0 ≤ x ≤ 2) solid solution were synthesized by a solid-state reaction route. The crystal structures of Ag1.38Na0.62Mn3(VO4)3 (x = 0.62) and Ag0.15Na1.85Mn3(VO4)3 (x = 1.85) were solved using single crystal X-ray diffraction. These phases crystallize with a monoclinic symmetry (space group C2/c), and their structures are new members of the well-known alluaudite family. In both compounds, the Ag+/Na+, Mn2+/Mn3+ and V5+ cations are eight-, six-, and four-coordinated to oxygen atoms, respectively. All the atoms are perfectly ordered except for the Ag and Na atoms which are statistically disordered over a 4b and a 4e atomic position. This single-crystal structural study confirms the existence of a full solid solution Ag2− x Na x Mn3(VO4)3 (0 ≤ x ≤ 1.85).

Crystal structure of disilver(I) dizinc(II) iron(III) tris-(orthovanadate) with an alluaudite-type structure

The title compound, Ag2Zn2Fe(VO4)3, has been synthesized by solid-state reactions and belongs to the alluaudite structure family. In the crystal structure, four sites are positioned at special positions. One silver site is located on an inversion centre (Wyckoff position 4b), and an additional silver site, as well as one zinc and one vanadium site, on twofold rotation axes (4e). One site on a general position is statistically occupied by FeIII and ZnII cations that are octa-hedrally surrounded by O atoms. The three-dimensional framework structure of the title vanadate results from [(Zn,Fe)2O10] units of edge-sharing [(Zn,Fe)O6] octa-hedra that alternate with [ZnO6] octa-hedra so as to form infinite chains parallel to [10]. These chains are linked through VO4 tetra-hedra by sharing vertices, giving rise to layers extending parallel to (010). Such layers are shared by common vanadate tetra-hedra. The resulting three-dimensional framework delimits two types of channels parallel to [001] in which the silver sites are located with four- and sixfold coordination by oxygen.

Recent Progress in Iron-Based Electrode Materials for Grid-Scale Sodium-Ion Batteries

Grid-scale energy storage batteries with electrode materials made from low-cost, earth-abundant elements are needed to meet the requirements of sustainable energy systems. Sodium-ion batteries (SIBs) with iron-based electrodes offer an attractive combination of low cost, plentiful structural diversity and high stability, making them ideal candidates for grid-scale energy storage systems. Although various iron-based cathode and anode materials have been synthesized and evaluated for sodium storage, further improvements are still required in terms of energy/power density and long cyclic stability for commercialization. In this Review, progress in iron-based electrode materials for SIBs, including oxides, polyanions, ferrocyanides, and sulfides, is briefly summarized. In addition, the reaction mechanisms, electrochemical performance enhancements, structure-composition-performance relationships, merits and drawbacks of iron-based electrode materials for SIBs are discussed. Such iron-based electrode materials will be competitive and attractive electrodes for next-generation energy storage devices.

Carbon-Coated Na3.32 Fe2.34 (P2 O7 )2 Cathode Material for High-Rate and Long-Life Sodium-Ion Batteries

Rechargeable sodium-ion batteries are proposed as the most appropriate alternative to lithium batteries due to the fast consumption of the limited lithium resources. Due to their improved safety, polyanion framework compounds have recently gained attention as potential candidates. With the earth-abundant element Fe being the redox center, the uniform carbon-coated Na_3.32 Fe_2.34 (P₂ O₇ )₂ /C composite represents a promising alternative for sodium-ion batteries. The electrochemical results show that the as-prepared Na_3.32 Fe_2.34 (P₂ O₇ )₂ /C composite can deliver capacity of ≈100 mA h g^-1 at 0.1 C (1 C = 120 mA g^-1 ), with capacity retention of 92.3% at 0.5 C after 300 cycles. After adding fluoroethylene carbonate additive to the electrolyte, 89.6% of the initial capacity is maintained, even after 1100 cycles at 5 C. The electrochemical mechanism is systematically investigated via both in situ synchrotron X-ray diffraction and density functional theory calculations. The results show that the sodiation and desodiation are single-phase-transition processes with two 1D sodium paths, which facilitates fast ionic diffusion. A small volume change, nearly 100% first-cycle Coulombic efficiency, and a pseudocapacitance contribution are also demonstrated. This research indicates that this new compound could be a potential competitor for other iron-based cathode electrodes for application in large-scale Na rechargeable batteries.

Crystal structure of calcium dinickel(II) iron(III) tris-(orthophosphate): CaNi2Fe(PO4)3

The title compound, CaNi2Fe(PO4)3, was synthesized by solid-state reactions. Its structure is closely related to that of α-CrPO4 in the space group Imma. Except for two O atoms in general positions, all atoms are located in special positions. The three-dimensional framework is built up from two types of sheets extending parallel to (100). The first sheet is made up from two edge-sharing [NiO6] octa-hedra, leading to the formation of [Ni2O10] double octa-hedra that are connected to two PO4 tetra-hedra through a common edge and corners. The second sheet results from rows of corner-sharing [FeO6] octa-hedra and PO4 tetra-hedra forming an infinite linear chain. These layers are linked together through common corners of PO4 tetra-hedra and [FeO6] octa-hedra, resulting in an open three-dimensional framework that delimits two types of channels parallel to [100] and [010] in which the eightfold-coordinated CaII cations are located.

Polyanion-Type Electrode Materials for Sodium-Ion Batteries

Sodium-ion batteries, representative members of the post-lithium-battery club, are very attractive and promising for large-scale energy storage applications. The increasing technological improvements in sodium-ion batteries (Na-ion batteries) are being driven by the demand for Na-based electrode materials that are resource-abundant, cost-effective, and long lasting. Polyanion-type compounds are among the most promising electrode materials for Na-ion batteries due to their stability, safety, and suitable operating voltages. The most representative polyanion-type electrode materials are Na3V2(PO4)3 and NaTi2(PO4)3 for Na-based cathode and anode materials, respectively. Both show superior electrochemical properties and attractive prospects in terms of their development and application in Na-ion batteries. Carbonophosphate Na3MnCO3PO4 and amorphous FePO4 have also recently emerged and are contributing to further developing the research scope of polyanion-type Na-ion batteries. However, the typical low conductivity and relatively low capacity performance of such materials still restrict their development. This paper presents a brief review of the research progress of polyanion-type electrode materials for Na-ion batteries, summarizing recent accomplishments, highlighting emerging strategies, and discussing the remaining challenges of such systems.

Electrospun graphene-wrapped Na6.24Fe4.88(P2O7)4 nanofibers as a high-performance cathode for sodium-ion batteries

Na_6.24Fe_4.88(P₂O₇)₄ is one of the intensively investigated polyanionic compounds and has shown high rate discharge capacity, but its relatively low electronic conductivity hampers the high performance of the batteries.

New alluaudite-related triple molybdates Na25Cs8R5(MoO4)24(R = Sc, In): synthesis, crystal structures and properties

New triple molybdates Na₂₅Cs₈R₅(MoO₄)₂₄(R = Sc, In) were synthesized and structurally characterized, and their physicochemical properties were studied.

Crystal structure of strontium dicobalt iron(III) tris-(orthophosphate): SrCo2Fe(PO4)3

The title compound, SrCo2Fe(PO4)3, has been synthesized by a solid-state reaction. It crystallizes with the α-CrPO4 type structure. In this structure, all atoms are on special positions of the Imma space group, except for two O atoms which are located on general positions. The three-dimensional network in the crystal structure is made up of two types of layers stacked normal to (100). The first layer is built from two edge-sharing CoO6 octa-hedra, leading to the formation of Co2O10 dimers that are connected to two PO4 tetra-hedra by a common edge and corners. The second layer results from apex-sharing FeO6 octa-hedra and PO4 tetra-hedra, which form linear chains alternating with a zigzag chain of Sr(II) cations. These layers are linked together by common vertices of PO4 tetra-hedra and FeO6 octa-hedra to form an open three-dimensional framework that delimits two types of channels parallel to [100] and [010] where the Sr(II) cations are located. Each Sr(II) cation is surrounded by eight O atoms.

Crystal structure of (Na0.70)(Na0.70,Mn0.30)(Fe(3+),Fe(2+))2Fe(2+)(VO4)3, a sodium-, iron- and manganese-based vanadate with the alluaudite-type structure

The title compound, sodium (sodium,manganese) triiron(II,III) tris[vana-date(V)], (Na0.70)(Na0.70,Mn0.30)(Fe(3+),Fe(2+))2Fe(2+)(VO4)3, was prepared by solid-state reactions. It crystallizes in an alluaudite-like structure, characterized by a partial cationic disorder. In the structure, four of the 12 sites in the asymmetric unit are located on special positions, three on a twofold rotation axis (Wyckoff position 4e) and one on an inversion centre (4b). Two sites on the twofold rotation axis are entirely filled by Fe(2+) and V(5+), whereas the third site has a partial occupancy of 70% by Na(+). The site on the inversion centre is occupied by Na(+) and Mn(2+) cations in a 0.7:0.3 ratio. The remaining Fe(2+) and Fe(3+) atoms are statistically distributed on a general position. The three-dimensional framework of this structure is made up of kinked chains of edge-sharing [FeO6] octa-hedra stacked parallel to [10-1]. These chains are held together by VO4 tetra-hedral groups, forming polyhedral sheets perpendicular to [010]. Within this framework, two types of channels extending along [001] are present. One is occupied by (Na(+)/Mn(2+)) while the second is partially occupied by Na(+). The mixed site containing (Na(+)/Mn(2+)) has an octa-hedral coordination sphere, while the Na(+) cations in the second channel are coordinated by eight O atoms.

Detailed investigation of a NaTi2(PO4)3 anode prepared by pyro-synthesis for Na-ion batteries

NaTi₂(PO₄)₃ nanoparticles are synthesized by a facile polyol-assisted pyro-synthetic reaction.

Crystal structure of strontium dinickel iron orthophosphate

The title compound, SrNi2Fe(PO4)3, synthesized by solid-state reaction, crystallizes in an ordered variant of the α-CrPO4 structure. In the asymmetric unit, two O atoms are in general positions, whereas all others atoms are in special positions of the space group Imma: the Sr cation and one P atom occupy the Wyckoff position 4e (mm2), Fe is on 4b (2/m), Ni and the other P atom are on 8g (2), one O atom is on 8h (m) and the other on 8i (m). The three-dimensional framework of the crystal structure is built up by [PO4] tetra-hedra, [FeO6] octa-hedra and [Ni2O10] dimers of edge-sharing octa-hedra, linked through common corners or edges. This structure comprises two types of layers stacked alternately along the [100] direction. The first layer is formed by edge-sharing octa-hedra ([Ni2O10] dimer) linked to [PO4] tetra-hedra via common edges while the second layer is built up from a strontium row followed by infinite chains of alternating [PO4] tetra-hedra and FeO6 octa-hedra sharing apices. The layers are held together through vertices of [PO4] tetra-hedra and [FeO6] octa-hedra, leading to the appearance of two types of tunnels parallel to the a- and b-axis directions in which the Sr cations are located. Each Sr cation is surrounded by eight O atoms.

Crystal structure of alluaudite-type NaMg3(HPO4)2(PO4)

The title compound, sodium trimagnesium bis-(hydrogen phosphate) phosphate, was obtained under hydro-thermal conditions. In the crystal, two types of [MgO6] octa-hedra, one with point group symmetry 2, share edges to build chains extending parallel to [10-1]. These chains are linked together by two kinds of phosphate tetra-hedra, HPO4 and PO4, the latter with point group symmetry 2. The three-dimensional framework delimits two different types of channels extending along [001]. One channel hosts the Na(+) cations (site symmetry 2) surrounded by eight O atoms, with Na-O bond lengths varying between 2.2974 (13) and 2.922 (2) Å. The OH group of the HPO4 tetra-hedron points into the other type of channel and exhibits a strong hydrogen bond to an O atom of the PO4 tetra-hedron on the opposite side.

Crystal structure of a sodium, zinc and iron(III)-based non-stoichiometric phosphate with an alluaudite-like structure: Na1.67Zn1.67Fe1.33(PO4)3

The new title compound, disodium dizinc iron(III) tris-(phosphate), Na1.67Zn1.67Fe1.33(PO4)3, which belongs to the alluaudite family, has been synthesized by solid-state reactions. In this structure, all atoms are in general positions except for four, which are located on special positions of the C2/c space group. This structure is characterized by cation substitutional disorder at two sites, one situated on the special position 4e (2) and the other on the general position 8f. The 4e site is partially occupied by Na(+) [0.332 (3)], whereas the 8f site is entirely filled by a mixture of Fe and Zn. The full-occupancy sodium and zinc atoms are located at the Wyckoff positions on the inversion center 4a (-1) and on the twofold rotation axis 4e, respectively. Refinement of the occupancy ratios, bond-valence analysis and the electrical neutrality requirement of the structure lead to the given composition for the title compound. The three-dimensional framework of this structure consists of kinked chains of edge-sharing octa-hedra stacked parallel to [10-1]. The chains are formed by a succession of trimers based on [ZnO6] octa-hedra and the mixed-cation Fe(III)/Zn(II) [(Fe/Zn)O6] octa-hedra [Fe(III):Zn(III) ratio 0.668 (3)/0.332 (3)]. Continuous chains are held together by PO4 phosphate groups, forming polyhedral sheets perpendicular to [010]. The stacked sheets delimit two types of tunnels parallel to the c axis in which the sodium cations are located. Each Na(+) cation is coordinated by eight O atoms. The disorder of Na in the tunnel might presage ionic mobility for this material.

Porous graphene to encapsulate Na6.24Fe4.88(P2O7)4 as composite cathode materials for Na-ion batteries

Na_6.24Fe_4.88(P₂O₇)₄@porous graphene composite is fabricated as a cathode of Na-ion batteries for the first time by a hydrothermally assisted sol–gel process.