Progress on Fe-Based Polyanionic Oxide Cathodes Materials toward Grid-Scale Energy Storage for Sodium-Ion Batteries

The development of large-scale energy storage systems (EESs) is pivotal for applying intermittent renewable energy sources such as solar energy and wind energy. Lithium-ion batteries with LiFePO₄ cathode have been explored in the integrated wind and solar power EESs, due to their long cycle life, safety, and low cost of Fe. Considering the penurious reserve and regional distribution of lithium resources, the Fe-based sodium-ion battery cathodes with earth-abundant elements, environmental friendliness, and safety appear to be the better substitutes in impending grid-scale energy storage. Compared to the transition metal oxide and Prussian blue analogs, the Fe-based polyanionic oxide cathodes possess high thermal stability, ultra-long cycle life, and adjustable voltage, which is more commercially viable in the future. This review summarizes the research progress of single Fe-based polyanionic and mixed polyanionic oxide cathodes for the potential sodium-ion batteries EESs candidates. In detail, the synthesized method, crystal structure, electrochemical properties, bottlenecks, and optimization method of Fe-based polyanionic oxide cathodes are discussed systematically. The insights presented in this review may serve as a guideline for designing and optimizing Fe-based polyanionic oxide cathodes for coming commercial sodium-ion batteries EESs.

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.

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).

Recent Progress in Rechargeable Sodium-Ion Batteries: toward High-Power Applications

The increasing demands for renewable energy to substitute traditional fossil fuels and related large-scale energy storage systems (EES) drive developments in battery technology and applications today. The lithium-ion battery (LIB), the trendsetter of rechargeable batteries, has dominated the market for portable electronics and electric vehicles and is seeking a participant opportunity in the grid-scale battery market. However, there has been a growing concern regarding the cost and resource availability of lithium. The sodium-ion battery (SIB) is regarded as an ideal battery choice for grid-scale EES owing to its similar electrochemistry to the LIB and the crust abundance of Na resources. Because of the participation in frequency regulation, high pulse-power capability is essential for the implanted SIBs in EES. Herein, a comprehensive overview of the recent advances in the exploration of high-power cathode and anode materials for SIB is presented, and deep understanding of the inherent host structure, sodium storage mechanism, Na⁺ diffusion kinetics, together with promising strategies to promote the rate performance is provided. This work may shed light on the classification and screening of alternative high rate electrode materials and provide guidance for the design and application of high power SIBs in the future.

Control of SEI Formation for Stable Potassium-Ion Battery Anodes by Bi-MOF-Derived Nanocomposites

Bismuth (Bi)-based electrodes are highly attractive for potassium-ion batteries (PIBs) while suffering from a short cycle life due to the larger diameter of K ion, leading to unstable solid electrolyte interface (SEI) films during continuous potassiation/depotassiation. Herein, we developed novel ultrathin carbon film@carbon nanorods@Bi nanoparticle (UCF@CNs@BiN) materials for the long cycle life anode of PIBs. Bi nanoparticles are uniformly distributed in carbon nanorods, which not only provides a high-speed channel for ion transport but also accommodates the volume change of Bi nanoparticles during continuous potassiation/depotassiation processes. The UCF@CN matrix can direct most SEI film formation on the surface of the carbon film, not on the surface of individual Bi nanoparticles, avoiding the fracture of the matrix. Benefiting from their unique structure, the UCF@CNs@BiN anodes exhibit an outstanding capacity of ∼425 mAh g^-1 at 100 mA g^-1 and a capacity decay of 0.038% per cycle over 600 cycles. Even at a higher current density of 1000 mA g^-1, there is a capacity decay as low as 0.036% per cycle during 700 cycles. Meanwhile, this work provides a new way of utilizing the metal-organic framework structure and reveals a highly promising PIB anode.

High-Rate and Long-Life Sodium-Ion Batteries Based on Sponge-like Three-Dimensional Porous Na-Rich Ferric Pyrophosphate Cathode Material

Sponge-like three-dimensional porous carbon-encapsulated Na_3.32Fe_2.34(P₂O₇)₂ nanoparticles (labeled to NFPO@SC) were manufactured by a sol-gel method followed by multistage calcinations and utilized as the cathode material for sodium-ion batteries. The excellent electrochemical performance of the NFPO@SC cathode can be attributed to its unique porous structure, which facilitates electrolyte penetration, reduces the diffusion path of sodium ions, and increases electronic conductivity. In addition, the full battery is assembled by NFPO@SC and hard carbon, which are employed as cathode and anode electrodes, respectively. The full battery delivers a high discharge capacity (112.2 mA h g^-1 at 0.5 C) and maintains 93.9% stable capacity over 1000 cycles at 5 C.

Super long-life potassium-ion batteries based on an antimony@carbon composite anode

A highly stable Sb based anode material of well-confined Sb@graphene@carbon (Sb@G@C) was developed for high performance PIBs. The Sb@G@C electrode exhibits a reversible capacity of 474 mA h g-1 at 100 mA g-1 (second charge), an outstanding long cycle stability over 800 cycles with a capacity retention as high as 72.3% and an excellent rate performance.

Triclinic Off-Stoichiometric Na3.12Mn2.44(P2O7)2/C Cathode Materials for High-Energy/Power Sodium-Ion Batteries

The application of sodium-ion batteries (SIBs) requires a suitable cathode material with low cost, nontoxic, high safety, and high energy density, which is still a big challenge; thus, a basic research on exploring new types of materials is imperative. In this work, a manganic pyrophosphate and carbon compound Na_3.12Mn_2.44(P₂O₇)₂/C has been synthesized through a feasible sol-gel method. Rietveld refinement reveals that Na_3.12Mn_2.44(P₂O₇)₂ adopts a triclinic structure ( P1̅ space group), which possesses spacious ion diffusion channels for facile sodium migration. The off-stoichiometric phase is able to offer more reversible Na⁺, delivering an enhanced reversible capacity of 114 mA h g^-1 at 0.1 C, and because of the strong "inductive effect" that (P₂O₇)^4- groups imposing on the Mn³⁺/Mn²⁺ redox couple, Na_3.12Mn_2.44(P₂O₇)₂/C presents high platforms above 3.6 V, contributing a remarkable energy density of 376 W h kg^-1, which is among the highest Fe-/Mn-based polyanion-type cathode materials. Furthermore, the off-stoichiometric compound also presents satisfactory rate capability and long-cycle stability, with a capacity retention of 75% after 500 cycles at 5 C. Ex situ X-ray diffraction demonstrates a single-phase reaction mechanism, and the density functional theory calculations display two one-dimensional sodium migration paths with low energy barriers in Na_3.12Mn_2.44(P₂O₇)₂, which is vital for the facile sodium storage. We believe that this compound will be a competitive cathode material for large-scale SIBs.

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.

Phosphate Framework Electrode Materials for Sodium Ion Batteries

Sodium ion batteries (SIBs) have been considered as a promising alternative for the next generation of electric storage systems due to their similar electrochemistry to Li-ion batteries and the low cost of sodium resources. Exploring appropriate electrode materials with decent electrochemical performance is the key issue for development of sodium ion batteries. Due to the high structural stability, facile reaction mechanism and rich structural diversity, phosphate framework materials have attracted increasing attention as promising electrode materials for sodium ion batteries. Herein, we review the latest advances and progresses in the exploration of phosphate framework materials especially related to single-phosphates, pyrophosphates and mixed-phosphates. We provide the detailed and comprehensive understanding of structure-composition-performance relationship of materials and try to show the advantages and disadvantages of the materials for use in SIBs. In addition, some new perspectives about phosphate framework materials for SIBs are also discussed. Phosphate framework materials will be a competitive and attractive choice for use as electrodes in the next-generation of energy storage devices.

Superior sodium storage performance of reduced graphene oxide-supported Na3.12Fe2.44(P2O7)2/C nanocomposites

We prepared a rGO-supported Na_3.12Fe_2.44(P₂O₇)₂/C nanocomposite showing superior rate capability and a long lifespan as a NIB cathode material.

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.

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.

Layered SnS2 cross-linked by carbon nanotubes as a high performance anode for sodium ion batteries

Facile synthesized SnS₂@CNT hybrid nanocomposite exhibits high capacity and good cyclability as anode for sodium ion batteries.