The development of aqueous rechargeable sodium batteries (ARSBs) demands high-performance electrode materials, especially anode materials with low operating potential and competent electrochemical properties. The lithium/sodium vanadium phosphate family with good structural stability and abundant vanadium chemistry versatility is a promising series for energy storage applications. Herein, a new member in the sodium vanadium phosphate family, i.e. NaV3(PO4)3, is introduced as a novel anode candidate for ARSBs. For the first time, its sodium intercalation mechanism in an aqueous electrolyte is explored, and moreover, the well-aligned NaV3(PO4)3@porous carbon nanofiber is constructed to fulfil its full potential. Based on the reversible phase transformation and 3D open framework, the NaV3(PO4)3 is demonstrated to be reliable in the aqueous electrolyte. Favored by the well-aligned, highly porous and hierarchical 1D nanoarchitecture, the freestanding aligned NaV3(PO4)3@porous carbon hybrid film achieves fast electron/ion transport capability and good mechanical flexibility, resulting in its superior high-rate properties and excellent cycling durability. Moreover, a full cell is fabricated using the aligned NaV3(PO4)3@C nanofiber as the anode and Na0.44MnO2 as the cathode. The cell is capable of high-rate long-term cycling, which retains 84% of the capacity after five hundred cycles at alternate 20 and 5C. Therefore, this work not only demonstrates a novel high-performance anode material for ARSBs, but also introduces a general applicable and highly efficient architecture of aligned 1D nanofibers for energy storage applications.

Significant interest has been devoted to the design and fabrication of flexible electronic devices owing to their immense potential in modern society. Constructing freestanding electrodes with superior electrochemical performance and excellent mechanical durability is the key to the development of high-performance energy storage devices. In this study, we introduce a new cross-linked Na2VTi(PO4)3/porous carbon nanofiber, which is employed as a self-supported bi-functional electrode in aqueous sodium ion batteries. The sodium intercalation mechanism of the Na2VTi(PO4)3/C electrode in an aqueous electrolyte is investigated. The reversible phase transformations and the valence state change of Na2VTi(PO4)3 in both high- and low-potential ranges (i.e. 0–0.6 V vs. Ag/AgCl; −1–0 V vs. Ag/AgCl) demonstrate its reliability as both the anode and the cathode in the aqueous electrolyte. Favored by the highly porous architecture and aligned cross-linked arrangement, the Na2VTi(PO4)3/C nanofiber achieves fast kinetics and good mechanical characteristics. Both result in its superior high-rate properties, good cycling durability and high structural stability. Moreover, a symmetric aqueous rechargeable sodium battery is prepared by assembling two cross-arranged Na2VTi(PO4)3/C nanofiber electrodes. The cell exhibits a flat potential plateau of ∼1.2 V and is capable of high-rate long-term cycling. It retains 83% of the capacity after six hundred cycles at alternate 40 and 4C. Therefore, this work not only introduces a novel symmetric aqueous sodium ion system, but also provides a highly efficient architecture for flexible high-performance electrodes. More importantly, it provides a new clue to construct energy storage devices with a simple configuration, low-cost and superior properties.

Rechargeable sodium–iodine and lithium–iodine batteries have been demonstrated to be promising and scalable energy-storage devices, but their development has been seriously limited by challenges such as their inferior stability and the poor kinetics of iodine. Anchoring iodine to 3D porous carbon is an effective strategy to overcome these defects; however, both the external architecture and internal microstructure of the 3D porous carbon host can greatly affect the ion intercalation of iodine/C electrodes. To realize the full potential of iodine electrodes, a biochemistry-enabled route was developed to enable the controllable design of different 3D porous architectures, from hollow microspheres to 3D foam, for use in iodine/C cathodes. Two types of spores with spherical cells, i.e. Cibotium Barometz (C. Barometz) and Oetes Sinesis (O. Sinesis), are employed as bio-precursors. By carefully controlling the degree of damage on the bio-precursors, different targeted carbon hosts were fabricated. Systematic studies were carried out to clarify the structural effects on modifying the ion-intercalation capabilities of the iodine/C cathodes in lithium–iodine and sodium–iodine batteries. Our results demonstrate the profound performance improvements of both 3D bio-foam and hollow sphere because their hierarchically porous structures can strongly immobilize iodine. Notably, the 3D bio-foam based iodine composites achieve faster ion kinetics and enhanced rate capability than their hollow sphere based counterparts. This was attributed to their higher micro/mesopore volume, larger surface area and improved packing density, which result in the highly efficient adsorption of iodine species. By virtue of the thinnest slices, the iodine/bio-foam derived from C. Barometz spores achieves the best high-rate long-term cycling capability, which retains 94% and 91% of their capacities in lithium–iodine and sodium–iodine batteries after 500 cycles, respectively. With the help of the biochemistry-assisted technique, our study provides a much-needed fundamental insight for the rational design of 3D porous iodine/C composites, which will promote a significant research direction for the practical application of lithium/sodium–iodine batteries.

High potential sodium hosts have attracted enormous attention recently in view of the requirement for improving energy density for sodium ion batteries. Na7V4(P2O7)4(PO4) and Na7V3(P2O7)4 with operating potentials near 4.0 V versus Na+/Na are promising cathode candidates. But the low conductivity, limited ion intercalation kinetics and inferior stability remain critical drawbacks for their practical applications. In this paper, the design of freestanding three-dimensional (3D) hybrid foams of high potential sodium hosts@biomass-derived porous carbon is reported. The biological fungus realizes the formation of highly porous graphene-like carbon, which constructs the 3D framework for the high potential polyanions. The polyanion nanocrystals are closely enwrapped by the biomass derived framework and build the hybrid architecture. Both the highly conductive skeleton and the hierarchically porous architecture of hybrid foams are favourable for highly efficient electron and ion transport. Furthermore, the depressed structural deterioration rate and improved contact between the active material and conductive substrate are achieved for the 3D hybrid foams in comparison with the conventional electrodes on the basis of the dynamic studies. Without additional additive or binders, the freestanding hybrid foams achieve desirable characteristics of high operating potentials, fast sodium intercalation and excellent cycling stability. The Na7V4(P2O7)4(PO4)- and Na7V3(P2O7)4-based hybrid foams retain 94% and 91%, respectively, of the capacities after 800 cycles at alternative 20C and 3C, demonstrating their superior high rate ultra long-term cycling capability. Therefore, this research provides a low-cost, highly efficient and widely applicable architecture to construct high potential and ultra long-life cathodes for sodium batteries.

The tailoring of materials into bio-inspired structures is triggering unprecedented innovations. Muscle tissue is composed of myofibrils and densely wired blood vessels; it is a perfect model for designing high-performance electrode materials that have the advantage of fast mass transport and superior durability. We design a top-down strategy as a facile approach to tailor the alluaudite Na2+2xFe2−x(SO4)3 into a muscle-like spindle. A precipitation process is employed to prepare the hydrated “top” precursor, which is subjected to dehydration and phase transformation to obtain the “down” product. The alluaudite sulfate nanoparticles closely anchor on the single-wall carbon nanotubes (SWNT), and they together aggregate into microscale particles in the shape of spindles. The Na2+2xFe2−x(SO4)3/SWNT composite as a whole copies the morphology and function of muscle tissue. Taking advantage of its 3D conductive framework and porous structure, the composite achieves fast electron/ion transport and sodium intercalation. Moreover, the single-phase reaction mechanism during sodium intercalation is beneficial to its cycling property. It exhibits such desirable electrochemical performance as an operating potential as high as ∼3.8 V and a high-rate capability, which achieves a capacity retention of 92% after 100 cycles at 5C. The muscle-inspired architecture makes electrode materials favorable for superior electrochemical performance.

The low-cost sodium iron pyrophosphate Na3.12Fe2.44(P2O7)2, which enables facile ion transport, presents a promising alternative for use as the cathode of sodium ion batteries. However, high moisture sensitivity and severe surface oxidation lead to a large irreversible capacity and inferior electrochemical kinetics. Herein, for the first time, we report the design of an ultrafast “flash-combustion synthesis” strategy to prepare coral-like Na3.12Fe2.44(P2O7)2/C with good surface properties and fast sodium intercalation. The single-phase Na3.12Fe2.44(P2O7)2 is successfully prepared in as short a time as three minutes. Each ultrafine Na3.12Fe2.44(P2O7)2 particle with a nanoscale carbon coating is enwrapped in a microscale carbon matrix, forming a coral-like architecture. Benefiting from the hierarchical carbon decoration, the coral-like composite attains good surface properties that enable low moisture sensitivity and fast sodium intercalation. Moreover, dynamic analysis reveals that surface oxidation proceeds through a “shell–core” mechanism and demonstrates the crucial role of the surface properties on sodium intercalation. Taking advantage of the good surface properties and the hierarchical porous architecture, the coral-like composite is capable of fast sodium intercalation and stable prolonged cycling. It retains 95% of the initial capacity after 200 cycles alternating between 20 and 5C rates. The clarification of the correlation between the surface properties and the sodium intercalation chemistry provides clues to the design and construction of high-performance Na3.12Fe2.44(P2O7)2 for large-scale applications.

Both high safety and low cost give aqueous rechargeable sodium-ion batteries (ARSB) the opportunity for application in stationary energy storage, but the low operating potential of the existing cathode materials limits its energy density. Here, we introduce a hydrothermal-assisted strategy to prepare the Na7V4(P2O7)4(PO4)/C nanorod and employ it as a novel high-property cathode material for ARSB. The hierarchical structure is formed by direct in situ carbonization of the surfactants (CTAB and oxalic acid) along with the crystallization of Na7V4(P2O7)4(PO4). The prepared Na7V4(P2O7)4(PO4) with a well-defined 1D nanostructure and uniform particle size is wrapped with a thin carbon layer. For the first time, its sodium intercalation chemistry in an aqueous electrolyte was investigated. Based on the reversible phase transformation and high sodium diffusion coefficient, it is demonstrated to be reliable in an aqueous electrolyte with the rapid ion transport capability. A pair of redox plateaus is observed in the charge and discharge curves at 0.961 and 0.944 V (vs. SCE) respectively with the capacity of 51.2 mA h g−1 at 80 mA g−1. Favored by the open ion channel and 1D morphology, the composite exhibits superior high rate capability and 72% of the capacity remains at 1000 mA g−1. The results not only demonstrate a high-property cathode material for ARSB, but also are helpful for design and synthesis of mixed-polyanion electrode materials with tailored architecture.

The three-dimensional (3D) hierarchical porous structure is ideal for constructing high-performance electrode materials and offers advantages such as large surface area, stable structural integrity and efficient ionic transport. In this report, we prepared a novel wafer-like 3D porous structured NaTi2(PO4)3/C by a facile self-assembled strategy. The NaTi2(PO4)3 crystal was not only coated by a nanoscale carbon layer but was also embedded in a microscale carbon network, which self-assembled into a secondary particle in a plate-like shape. The hierarchical carbon in the plate-like particle constitutes a 3D porous framework with a bicontinuous electronic conductive skeleton, showing a wafer-like structure. When used as an anode in an aqueous system, the wafer-like composite exhibited better sodium intercalation kinetics and enhanced high-rate capability than nonporous samples. Moreover, a full aqueous rechargeable sodium battery was fabricated using the wafer-like NaTi2(PO4)3 as the anode and Na0.44MnO2 as the cathode. The cell exhibited superior high rate property and an ultralong-life performance, which delivered 64% capacity at 30 C and retained 67% capacity after 400 cycles at alternate 50 and 5 C. In view of the highly efficient electron/ion transport pathways and robust structure stability, the wafer-like structure is put forward as a new strategy for nanoarchitecture tailoring to achieve high-performance electrodes.

Tailoring materials into a hierarchical porous micro/nanostructure offers unprecedented opportunities in the utilization of their functional properties. Particularly, it is crucial for the electrode materials to realize high-performance because of the advantages such as large surface area, superior structure stability and short ion transport pathway. Here we report the design of a new architecture, named “honeycomb-type hierarchical porous microball”, for Na3V2(PO4)3 by a facile one-pot synthesis. The network between nanovoids is formed by in situ carbonization of surfactants (CTAB) along with the crystallization of Na3V2(PO4)3, which results in the hierarchical porous Na3V2(PO4)3 skeleton with a surface conductive layer. The prepared Na3V2(PO4)3/C composite consists of spherical particles filled with hierarchical pores and interconnective nanochannels, resulting in the honeycomb-type architecture. It not only enables easier electrolyte penetration, but also provides a high-efficiency electron/ion transport pathway for fast sodium intercalation. Both the GITT and EIS results demonstrate the improved sodium diffusion capability and decreased electrochemical resistance for the honeycomb-structured microball in comparison to the microsized nonporous reference samples. Moreover, it also delivers superior high rate capability and cycling stability, which retains 93.6% of the initial capacity after 200 cycles at the 1 C rate. Even at 20 C, it still delivers a high capacity of 80.2 mA h g−1 corresponding to 71% of the capacity. Given the superior ion intercalation kinetics and excellent structure stability, the honeycomb-type structure puts forward a new strategy to develop high-performance polyanion-based materials for low-cost and high-power “rocking-chair” batteries.

Sulfate-based (SO42−) polyanionic materials with low cost and ionic-conduction break fresh ground for “rocking-chair” systems. But the high moisture sensitivity and limited conductivity led to their poor crystal stability and inferior alkaline-ion intercalation chemistry. Here we report the design of graphene-based sandwich-type nanoarchitecture for sulfate. The three-dimensional graphene-based network not only provides continuous electron/ion pathways for fast intercalation kinetics, but also effectively protects the crystal structure from deterioration to depress moisture sensitivity. As a case study, the hydrated sulfate (Na2Fe(SO4)2·2H2O)–graphene composite with a sandwich-type structure is prepared by a facile low-temperature synthesis. The depressed moisture sensitivity of hierarchical graphene–Na2Fe(SO4)2·2H2O compared to the pristine one is demonstrated by comparing their hydration process, and moreover, a shell–core hydration mechanism is disclosed. The hierarchical composite exhibits improved electronic conductivity and better sodium–lithium insertion capability than the pristine one. It delivers a reversible capacity of 72 and 69 mA h g−1 with redox potentials of 3.415/3.234 V (vs. Na+/Na) and 3.579/3.483 V (vs. Li+/Li) in sodium and lithium intercalation systems, respectively. Moreover, it also exhibits superior high rate capabilities and good cycling stability, which delivers 81% (for sodium) and 70% (for lithium) of the capacity at a 5 C rate. Therefore, the hierarchical sandwich-type architecture is favorable to realizing superior electrochemical performance for the sulfate, which presents a significant step forward in the development of low-cost large-scale batteries.

•Li0.85Na0.15V3O8 nanosheet with superionic conductive layer was constructed.•LixV2O5 surface layer provides facile pathways for lithium migration.•LixV2O5-Li0.85Na0.15V3O8 composite displays good high rate capability.Poor ion transport and rate capability are the main challenges for LiV3O8 as cathode material for lithium ion batteries. Here we report a novel strategy for enhancing lithium ion transport by building superionic pathways on the surface of Li0.85Na0.15V3O8 nanosheet. The two-dimensional Li0.85Na0.15V3O8 nanoparticle with an ion conductive layer of LixV2O5 on its surface is constructed by a modified sol–gel strategy with carefully controlled sodium incorporation and elements stoichiometry. Ultrathin LixV2O5 surface layer not only provides facile pathways for lithium migration, but also increases the structure stability during cycling. The LixV2O5-Li0.85Na0.15V3O8 composite displays good high rate capability of 172.3 mAh g−1 at 5C and excellent cycling stability of 98.9% over fifty cycles. This superior electrochemical property is attributed to the occupation of lithium site by Na+ in LiV3O8 host crystals and the surface superionic pathways of LixV2O5 phase. Therefore, the advantages of both high ion transport and the structure stabilization in present study put forward a new strategy for achieving high-performance LiV3O8 electrode material with tailored nanoarchitecture.

An iron-based mixed-polyanion compound, Li9Fe3(P2O7)3(PO4)2, is introduced as a possible cathode material for Li-ion batteries. Phase-pure Li9Fe3(P2O7)3(PO4)2 is successfully prepared by a sol–gel method, and its physicochemical properties are investigated in detail. Special attention is paid on making clear the variation of the phase composition with the annealing temperature and the effect of carbon coating on the electrochemical performance. Apparently phase-pure Li9Fe3(P2O7)3(PO4)2 can only be obtained in a narrow temperature range, either higher or lower annealing temperature outside this temperature range always leads to the impurity phase. The pristine Li9Fe3(P2O7)3(PO4)2 is suffering from its low electronic conductivity (10−9 S cm−1) and theoretical capacity (85 mA h g−1), it has a first discharge capacity of only 36 mA h g−1. Carbon coating is employed to improve the electrochemical performance. When the carbon content is 10 wt%, the discharge capacity of Li9Fe3(P2O7)3(PO4)2/C reaches the maximum value of 60 mA h g−1. The electronic conductivity of the composite, the exact discharge capacity of Li9Fe3(P2O7)3(PO4)2 in the composite and the capacity retention of the composite after 30 cycles vary in the same fashion with an increase in carbon content, i.e. first quickly increase and then stabilize.

Mixed polyanion materials with a 3D framework for battery electrodes have been attracting significant attention recently in view of the requirements to further improve energy storage and power densities. Herein, we present a design of a hierarchical Na7V4(P2O7)4(PO4)/C nanorod–graphene composite as sodium- and lithium-storage cathode materials. The hierarchical structure is composed of a 1D rectangular Na7V4(P2O7)4(PO4)/C nanorod, which is coated by in situ residual carbon and wrapped by a reduced graphene-oxide sheet. The open network of graphene and the surface carbon coating of the Na7V4(P2O7)4(PO4)/C nanorod provide bicontinuous electron and ion pathways, providing a three-dimensional conductive network for efficient electron and ion transfer. The flexible electrode built from the hierarchical composite free of binder or conductive additive exhibits improved electron conductivity and higher sodium/lithium ion migration coefficients than the pristine Na7V4(P2O7)4(PO4)/C nanorod. It approaches the initial reversible electrochemical capacities of 91.4 and 91.8 mA h g−1 with high discharge potentials over 3.8 V (vs. Na/Na+ or Li/Li+) and good cycling properties with capacity retentions of 95% and 83% after 200 cycles at a 1 C rate in sodium and lithium intercalation systems, respectively. Even at 10 C, it still delivers 87.4% (for sodium) and 78.2% (for lithium) of the capacity and high cycling stability. Taking into consideration the compatibilities of both sodium/lithium ions and their superior electrochemical characteristics, the bicontinuous hierarchical composite is considered to be a promising high-rate capability electrode material for advanced energy storage applications.

Tailoring materials into nanostructure offers unprecedented opportunities in the utilization of their functional properties. High-purity Na7V4(P2O7)4(PO4) with 1D nanostructure is prepared as a cathode material for rechargeable Na-ion batteries. An efficient synthetic approach is developed by carefully controlling the crystal growth in the molten sodium phosphate. Based on the XRD, XPS, TG, and morphological characterization, a molten-salt assisted mechanism for nanoarchitecture formation is revealed. The prepared Na7V4(P2O7)4(PO4) nanorod has rectangle sides and preferential [001] growth orientation. GITT evaluation indicates that the sodium de/intercalation of Na7V4(P2O7)4(PO4) nanorod involves V3+/V4+ redox reaction and Na5V3.5+4(P2O7)4(PO4) as intermediate phase, which results in two pairs of potential plateaus at the equilibrium potentials of 3.8713 V (V3+/V3.5+) and 3.8879 V (V3.5+/V4+), respectively. The unique nanoarchitecture of the phase-pure Na7V4(P2O7)4(PO4) facilitates its reversible sodium de/intercalation, which is beneficial to the high-rate capability and the cycling stability. The Na7V4(P2O7)4(PO4) cathode delivers 80% of the capacity (obtained at C/20) at the 10 C rate and 95% of the initial capacity after 200 cycles. Therefore, it is feasible to design and fabricate an advanced rechargeable sodium-ion battery by employment of 1D nanostructured Na7V4(P2O7)4(PO4) as the cathode material.Keywords: 1D nanostructure; intermediate phase; mixed-polyanion material; Na7V4(P2O7)4(PO4); sodium ion battery;

•1D nanostructured Na2V6O16·nH2O was introduced as a novel anode material for aqueous sodium ion batteries.•The solid-state diffusion coefficient of sodium ion in the bulk of Na2V6O16·nH2O is in the order of magnitude of 10−14 cm S−1.•A full aqueous sodium ion battery built by Na2V6O16·nH2O/Na0.44MnO2 exhibits stable cycling property after the initial cycle.1D nanostructured sodium vanadium oxide, i.e. Na2V6O16·nH2O, was introduced as a novel anode material for aqueous sodium ion batteries. A simple hydrothermal method is employed to prepare bundles of straight nanobelts whose crystals grow along the (010) direction. In each bundle, most nanobelts are aligned along the same direction. Sodium vanadium oxide hydrate has a layered structure, and that sodium ions are located at the interstices between layers. The solid-state diffusion coefficient of sodium ion in the bulk of Na2V6O16·nH2O is in the order of magnitude of 10–14 cm S−1. The discharge/charge capacity fades quickly in the initial few cycles upon galvanostatic cycling. It is revealed by ex-situ XRD analyses that this fast capacity fading can be attributed to the irreversible phase transition which mainly occurs in the first discharge. A full aqueous sodium ion battery was built using Na2V6O16·nH2O as anode and Na0.44MnO2 as cathode. Although its charge capacity fades quickly in the initial few cycles and stabilizes in the following cycles, its discharge capacity is comparatively stable upon cycling.

Carbon coating is an effect way to improve the electronic conductivity of polyanion materials for lithium ion batteries. A citric acid assisted sol–gel method is employed in this study to prepare Li2MnSiO4/C composite, where carbon is in situ coated on Li2MnSiO4 during the synthetic process. The effect of in situ carbon coating on the structural, morphological and electrochemical characteristics is studied in detail. The particle size decreases whereas the amount of impurities increases with increasing amount of carbon. The Li2MnSiO4/C sample with 2.1 wt% carbon shows the best electrochemical performance, and the particle of it is uniformly coated by a thin layer of carbon. The carbon coating is incomplete if the amount of carbon is lower than 2.1 wt%. However, uneven thick layer and/or agglomerates of carbon are formed if the amount of carbon is higher than 2.1 wt%. Li+ transfer on the surface of particles will be hindered if the carbon layer is too thick. Although the smaller particles of the samples with 8.5 wt% and 10.5 wt% carbon have a positive effect on their electrochemical performance, the combined negative effect of higher amount of impurities and thicker carbon layer with carbon agglomerates finally lead to poorer electrochemical performance.Highlights► The content of in situ carbon is crucial to the electrochemistry of Li2MnSiO4. ► Higher content of carbon leads to better conductivity and higher impurities. ► The Li2MnSiO4 with 2.1 wt% carbon shows the best electrochemical performance.

Ti–Mn and Ti–Fe codoped Li3V2(PO4)3 samples, i.e. Li3V2−2xTixMnx(PO4)3 and Li3V2−2xTixFex(PO4)3 (x = 0, 0.05, 0.1, 0.15, 0.2 and 0.25), are prepared by a sol–gel method. Li3V2−2xTixMnx(PO4)3 and Li3V2−2xTixFex(PO4)3 are phase-pure when x is not higher than 0.05. LiMnPO4 and LiFePO4 begin to form as impurity phases in Li3V2−2xTixMnx(PO4)3 and Li3V2−2xTixFex(PO4)3, respectively, when x is equal to 0.1. And another impurity of Mn2P2O7 appears in Li3V2−2xTixMnx(PO4)3 when x is equal to 0.2. All these impurities increase with increasing x. XPS analyses indicate that the oxidation states of Ti, Mn and Fe are +4, +2 and +2 respectively. The first charge/discharge capacities of both Li3V2−2xTixMnx(PO4)3 and Li3V2−2xTixFex(PO4)3 at 0.2 C decline with an increase of x. Both the high-rate discharge capability and long term cycling performance of Li3V1.9Ti0.05Fe0.05(PO4)3 are much better than those of Li3V2(PO4)3, which can be attributed to the smaller particle size, larger lattice parameters and better structural stability induced by Ti and Fe codoping. However, the electrochemical performance of Li3V1.9Ti0.05Mn0.05(PO4)3 is worse than that of Li3V2(PO4)3, which is due to the structural instability induced by the incorporation of Mn.Highlights► Ti–Mn and Ti–Fe codoped Li3V2(PO4)3 samples were prepared by a sol-gel method. ► Single phase Li3V2−2xTixMnx(PO4)3 and Li3V2−2xTixFex(PO4)3 were detected as x ≤ 0.05. ► Li3V1.9Ti0.05Fe0.05(PO4)3 has improved electrochemical performance than undoped one.

Attempts to dope Zn2+, Cu2+ or Ni2+ are made for Li2FeSiO4. The effects of dopant on the physical and electrochemical characteristics of Li2FeSiO4 were investigated. Zn2+ successfully entered into the lattice of Li2FeSiO4 and induced the change of lattice parameters. Compared with the undoped Li2FeSiO4, Li2Fe0.97Zn0.03SiO4 has higher discharge capacity, better electrochemical reversibility and lower electrode polarization. The improved electrochemical performance of Li2Fe0.97Zn0.03SiO4 can be attributed to the improved structural stability and the enhanced lithium ion diffusivity brought about by Zn2+ doping. However, Ni2+ and Cu2+ cannot be doped into the lattice of Li2FeSiO4. Cu and NiO are formed as impurities in the Cu- and Ni-containing samples, respectively. Compared with the undoped Li2FeSiO4, the Cu- and Ni-containing samples have lower capacities and higher electrochemical polarization.

Three precipitators, i.e. Na2CO3, (NH4)2CO3 and NH4HCO3, are employed to prepare Li[Ni1/3Co1/3Mn1/3]O2 via the carbonate coprecipitation method. The effects of precipitator on the morphological, structural and electrochemical characteristics of the prepared samples are studied. The sample prepared by using Na2CO3 as precipitator has irregular particle shape and nonuniform particle size, while the sample prepared by using (NH4)2CO3 as precipitator has spherical particle shape and uniform particle size. Among all the samples, the one prepared with (NH4)2CO3 exhibits the best hexagonal layered structure, which results in its highest discharge capacity and best cycling performance. Therefore, precipitator plays an important role in the coprecipitation reaction and makes a great impact on the characteristics of Li[Ni1/3Co1/3Mn1/3]O2.Research highlights▶ The precipitator plays an important role in the carbonate coprecipitation reaction. ▶ The kind of precipitator greatly influences the electrochemical characteristics of Li[Ni1/3Co1/3Mn1/3]O2.

Ti and Mg codoped Li3V2–2xTixMgx(PO4)3 (x = 0, 0.05, 0.10, 0.20, and 0.25) samples were prepared by a sol–gel method. The effects of Ti and Mg codoping on the physical and electrochemical characteristics of Li3V2(PO4)3 were investigated. Compared with the XRD pattern of the undoped sample, those of the Ti and Mg codoped samples have no extra reflections, which indicates that Ti and Mg enter the structure of Li3V2(PO4)3. According to the results of charge–discharge measurements, the initial capacity of Li3V2–2xTixMgx(PO4)3 at a low current density (0.2 C) decreases with increasing x. However, the discharge capacities at higher current densities (1 and 2 C) and the cycling stability are improved by a low amount of Ti and Mg codoping (x = 0.05), and moreover, EIS measurements indicate the lower charge transfer resistance of Li3V1.9Ti0.05Mg0.05(PO4)3. The improved electrochemical performance of Li3V1.9Ti0.05Mg0.05(PO4)3 can be attributed to its higher structural stability and smaller particle size. When x is higher than 0.05, the charge transfer resistance increases with increasing x, which leads to their poor electrochemical performance.

Cr-doped Li2FeSiO4 was prepared by a sol–gel method. The effects of Cr doping on the characteristics of Li2FeSiO4 were carefully investigated. Compared with the XRD pattern of the Li2FeSiO4 sample, the XRD patterns of the Cr-doped samples have no extra reflections. This indicates that Cr enters the structure of Li2FeSiO4 rather than forming impurities. As indicated by the charge–discharge measurements, the highest capacity is obtained by 3% Cr doping. The particle size of the Li2Fe0.97Cr0.03SiO4 sample was smaller than that of the Li2FeSiO4 sample, and the BET surface area of the Li2Fe0.97Cr0.03SiO4 sample was more than twice as high as that of the Li2FeSiO4 sample. Compared with the Li2FeSiO4 sample, the Li2Fe0.97Cr0.03SiO4 sample shows faster activation, higher reversible capacity, and better rate capability, which can be attributed to the smaller particle size and larger surface area as well as the crystal defects induced by Cr doping.

Li2Fe0.97Mg0.03SiO4 and Li2FeSiO4 have been synthesized via a sol–gel method. The effects of Mg doping on the characteristics of Li2FeSiO4 are investigated. Both Li2Fe0.97Mg0.03SiO4 and Li2FeSiO4 have a monoclinic structure (space group: P21) and their lattice parameters are similar, which indicates that Mg2+ has been doped into the structure of Li2FeSiO4 without destroying its lattice structure. Mg doping improves the discharge capacity and cycle stability of Li2FeSiO4. Electrochemical impedance analysis shows that Mg doping decreases charge transfer resistance of Li2FeSiO4, and moreover, Mg doping increases the lithium ion diffusion coefficient of Li2FeSiO4 by one order of magnitude. Compared with Li2FeSiO4, the higher lithium ion diffusion capability of Li2Fe0.97Mg0.03SiO4 results in its higher reversible capacity, especially at high rates. Furthermore, Mg2+ is unchangeable during cycling, which stabilizes the crystal structure and results in higher cycle stability of Li2Fe0.97Mg0.03SiO4.

A carbonate co-precipitation method was employed to prepare spherical Li[Ni1/3Co1/3Mn1/3]O2 cathode material. The precursor, [Ni1/3Co1/3Mn1/3]CO3, was prepared using ammonia as chelating agent under CO2 atmosphere. The spherical Li[Ni1/3Co1/3Mn1/3]O2 was prepared by mixing the precalcined [Ni1/3Co1/3Mn1/3]CO3 with LiOH followed by high temperature calcination. The preparation conditions such as ammonia concentration, co-precipitation temperature, calcination temperature and Li/[Ni1/3Co1/3Mn1/3] ratio were varied to optimize the physical and electrochemical properties of the prepared Li[Ni1/3Co1/3Mn1/3]O2. The structural, morphological, and electrochemical properties of the prepared LiNi1/3Co1/3Mn1/3O2 were characterized by XRD, SEM, and galvanostatic charge–discharge cycling. The optimized material has a spherical particle shape and a well ordered layered structure, and it also has an initial discharge capacity of 162.7 mAh g− 1 in a voltage range of 2.8–4.3 V and a capacity retention of 94.8% after a hundred cycles. The optimized ammonia concentration, co-precipitation temperature, calcination temperature, and Li/[Ni1/3Co1/3Mn1/3] ratio are 0.3 mol L− 1, 60 °C, 850 °C, and 1.10, respectively.Spherical Li[Ni1/3Co1/3Mn1/3]O2 cathode material was prepared via a carbonate co-precipitation method. The effects of synthetic conditions including ammonia concentration, co-precipitation temperature, calcination temperature, Li/[Ni1/3Co1/3Mn1/3] ratio on the structural, morphological, and electrochemical properties of the prepared materials were carefully investigated.

Two members of the family of orthosilicate, Li2FeSiO4 and Li2MnSiO4, are prepared by a citric acid assisted sol–gel method. As cathode materials for lithium-ion batteries, their structural, morphological and electrochemical characteristics are investigated and compared. Both cathode materials have nanoparticles with similar lattice parameters. Li2FeSiO4 has a maximum discharge capacity of 152.8 mAh g−1, and 98.3% of its maximum discharge capacity is retained after fifty cycles. However, the discharge capacity of Li2MnSiO4 fades rapidly and stabilized at about 70 mAh g−1 after twenty cycles. The electrochemical impedance and differential capacity analysis indicate that Li2MnSiO4 has larger charge transfer impedance and higher electrochemical irreversibility than Li2FeSiO4, which makes its electrochemical behaviors seriously deteriorate and leads to difference between two silicate materials.

Special synthetic conditions at 0 °C were used to prepare nanostructured Li[Ni1/3Co1/3Mn1/3]O2 via chemical coprecipitation synthesis. The precursor preparation shows platelet shape with thickness of 10 nm and width of 100 nm. After calcination, the particles change to spherical or rectangle shape with a size of 100~200 nm. The nanostructured Li[Ni1/3Co1/3Mn1/3]O2 shows a well-ordered layered hexagonal lattice with low cation mixing. Galvanostatic testing showed good electrochemical properties and high rate capability, which may be due to its unique morphological and structural characteristics. Synthesis at 0 °C effectively prevented growth of the precursor particles and produced nanosize Li[Ni1/3Co1/3Mn1/3]O2, which gave improvement in high rate performance and favoring the future use of this cathode material for high power applications.

Li2(Fe1−xMnx)SiO4 (x = 0, 0.3, 0.5, 0.7, 1) cathode materials have been synthesized via citric acid assisted sol–gel method. The effects of Mn substitution on the structural, morphological and electrochemical behaviors of Li2(Fe1−xMnx)SiO4 are investigated. The prepared Li2(Fe1−xMnx)SiO4 samples can all be indexed on the basis of the orthorhombic unit cell in space group Pmn21, and their lattice parameters are similar. Nanoparticles can be observed in the samples. The discharge capacity first increases and then decreases with increasing x. The maximum discharge capacity is obtained when x is equal to 0.5. However, the cycling performance of the Li2(Fe1−xMnx)SiO4 samples decreases with increasing x. At the same time, Mn substitution lowers the electrochemical reversibility of the Li2(Fe1−xMnx)SiO4 samples.

An improved carbonate co-precipitation method was employed to prepare spherical Li[Ni1/3Co1/3Mn1/3]O2 cathode material. The precursor, [Ni1/3Co1/3Mn1/3]CO3, was prepared using ammonia as chelating agent under CO2 atmosphere. The spherical Li[Ni1/3Co1/3Mn1/3]O2 was prepared by mixing the precalcined [Ni1/3Co1/3Mn1/3]CO3 with 7% excess LiOH followed by high-temperature calcination. Compared with the Li[Ni1/3Co1/3Mn1/3]O2 prepared by the conventional carbonate co-precipitation method, the Li[Ni1/3Co1/3Mn1/3]O2 prepared by the improved carbonate co-precipitation method showed larger specific surface area and better electrochemical performance. The initial discharge capacity of the Li[Ni1/3Co1/3Mn1/3]O2 prepared by the improved carbonate co-precipitation method was 162 mAh g−1 and 96.6% of the initial discharge capacity was retained after 50 charge–discharge cycling. However, the tap density of the Li[Ni1/3Co1/3Mn1/3]O2 prepared by the improved carbonate co-precipitation method is lower than that of the Li[Ni1/3Co1/3Mn1/3]O2 prepared by the conventional carbonate co-precipitation method, and the tap densities of both Li[Ni1/3Co1/3Mn1/3]O2 samples are lower than that of the Li[Ni1/3Co1/3Mn1/3]O2 prepared by the hydroxide co-precipitation method.

The development of sodium ion battery demands high-performance materials, especially the polyanion compounds with rich crystal chemistry and competent electrochemical properties. Herein, for the first time, we report the sodium vanadium pyrophosphate of Na7V3(P2O7)4 as a new high-potential cathode material for sodium ion battery. A facile synthetic strategy assisted by the molten-salt mechanism is developed to prepare high-pure single-phase Na7V3(P2O7)4. The prepared material has ultrafine nanoparticles and surface carbon decoration, which facilitates its fast electron/ion transport and results in superior kinetics. The quasi-2D framework with well-defined ion-conducting planes enables it to have good electrochemical activities, which delivers a high average potential of 4.0 V on the basis of one-electron reaction. The high efficiency in sodium-intercalation capability is demonstrated by the superior high-rate capability and long-term cycling property. As a new electroactive sodium vanadium pyrophosphate for sodium ion battery, the high operating voltage and superior high-rate capability of Na7V3(P2O7)4 are among the best of state-of-art polyanion-based sodium hosts. Therefore, the discovery of Na7V3(P2O7)4 opens a new opportunity for the development of the cathode materials for sodium ion batteries.The new Na7V3(P2O7)4 with high-voltage and superior-performance provides a new cathode material for sodium ion battery.Download high-res image (209KB)Download full-size image

The low-cost sodium iron pyrophosphate Na3.12Fe2.44(P2O7)2, which enables facile ion transport, presents a promising alternative for use as the cathode of sodium ion batteries. However, high moisture sensitivity and severe surface oxidation lead to a large irreversible capacity and inferior electrochemical kinetics. Herein, for the first time, we report the design of an ultrafast “flash-combustion synthesis” strategy to prepare coral-like Na3.12Fe2.44(P2O7)2/C with good surface properties and fast sodium intercalation. The single-phase Na3.12Fe2.44(P2O7)2 is successfully prepared in as short a time as three minutes. Each ultrafine Na3.12Fe2.44(P2O7)2 particle with a nanoscale carbon coating is enwrapped in a microscale carbon matrix, forming a coral-like architecture. Benefiting from the hierarchical carbon decoration, the coral-like composite attains good surface properties that enable low moisture sensitivity and fast sodium intercalation. Moreover, dynamic analysis reveals that surface oxidation proceeds through a “shell–core” mechanism and demonstrates the crucial role of the surface properties on sodium intercalation. Taking advantage of the good surface properties and the hierarchical porous architecture, the coral-like composite is capable of fast sodium intercalation and stable prolonged cycling. It retains 95% of the initial capacity after 200 cycles alternating between 20 and 5C rates. The clarification of the correlation between the surface properties and the sodium intercalation chemistry provides clues to the design and construction of high-performance Na3.12Fe2.44(P2O7)2 for large-scale applications.

Mixed polyanion materials with a 3D framework for battery electrodes have been attracting significant attention recently in view of the requirements to further improve energy storage and power densities. Herein, we present a design of a hierarchical Na7V4(P2O7)4(PO4)/C nanorod–graphene composite as sodium- and lithium-storage cathode materials. The hierarchical structure is composed of a 1D rectangular Na7V4(P2O7)4(PO4)/C nanorod, which is coated by in situ residual carbon and wrapped by a reduced graphene-oxide sheet. The open network of graphene and the surface carbon coating of the Na7V4(P2O7)4(PO4)/C nanorod provide bicontinuous electron and ion pathways, providing a three-dimensional conductive network for efficient electron and ion transfer. The flexible electrode built from the hierarchical composite free of binder or conductive additive exhibits improved electron conductivity and higher sodium/lithium ion migration coefficients than the pristine Na7V4(P2O7)4(PO4)/C nanorod. It approaches the initial reversible electrochemical capacities of 91.4 and 91.8 mA h g−1 with high discharge potentials over 3.8 V (vs. Na/Na+ or Li/Li+) and good cycling properties with capacity retentions of 95% and 83% after 200 cycles at a 1 C rate in sodium and lithium intercalation systems, respectively. Even at 10 C, it still delivers 87.4% (for sodium) and 78.2% (for lithium) of the capacity and high cycling stability. Taking into consideration the compatibilities of both sodium/lithium ions and their superior electrochemical characteristics, the bicontinuous hierarchical composite is considered to be a promising high-rate capability electrode material for advanced energy storage applications.

The three-dimensional (3D) hierarchical porous structure is ideal for constructing high-performance electrode materials and offers advantages such as large surface area, stable structural integrity and efficient ionic transport. In this report, we prepared a novel wafer-like 3D porous structured NaTi2(PO4)3/C by a facile self-assembled strategy. The NaTi2(PO4)3 crystal was not only coated by a nanoscale carbon layer but was also embedded in a microscale carbon network, which self-assembled into a secondary particle in a plate-like shape. The hierarchical carbon in the plate-like particle constitutes a 3D porous framework with a bicontinuous electronic conductive skeleton, showing a wafer-like structure. When used as an anode in an aqueous system, the wafer-like composite exhibited better sodium intercalation kinetics and enhanced high-rate capability than nonporous samples. Moreover, a full aqueous rechargeable sodium battery was fabricated using the wafer-like NaTi2(PO4)3 as the anode and Na0.44MnO2 as the cathode. The cell exhibited superior high rate property and an ultralong-life performance, which delivered 64% capacity at 30 C and retained 67% capacity after 400 cycles at alternate 50 and 5 C. In view of the highly efficient electron/ion transport pathways and robust structure stability, the wafer-like structure is put forward as a new strategy for nanoarchitecture tailoring to achieve high-performance electrodes.

Tailoring materials into a hierarchical porous micro/nanostructure offers unprecedented opportunities in the utilization of their functional properties. Particularly, it is crucial for the electrode materials to realize high-performance because of the advantages such as large surface area, superior structure stability and short ion transport pathway. Here we report the design of a new architecture, named “honeycomb-type hierarchical porous microball”, for Na3V2(PO4)3 by a facile one-pot synthesis. The network between nanovoids is formed by in situ carbonization of surfactants (CTAB) along with the crystallization of Na3V2(PO4)3, which results in the hierarchical porous Na3V2(PO4)3 skeleton with a surface conductive layer. The prepared Na3V2(PO4)3/C composite consists of spherical particles filled with hierarchical pores and interconnective nanochannels, resulting in the honeycomb-type architecture. It not only enables easier electrolyte penetration, but also provides a high-efficiency electron/ion transport pathway for fast sodium intercalation. Both the GITT and EIS results demonstrate the improved sodium diffusion capability and decreased electrochemical resistance for the honeycomb-structured microball in comparison to the microsized nonporous reference samples. Moreover, it also delivers superior high rate capability and cycling stability, which retains 93.6% of the initial capacity after 200 cycles at the 1 C rate. Even at 20 C, it still delivers a high capacity of 80.2 mA h g−1 corresponding to 71% of the capacity. Given the superior ion intercalation kinetics and excellent structure stability, the honeycomb-type structure puts forward a new strategy to develop high-performance polyanion-based materials for low-cost and high-power “rocking-chair” batteries.

High potential sodium hosts have attracted enormous attention recently in view of the requirement for improving energy density for sodium ion batteries. Na7V4(P2O7)4(PO4) and Na7V3(P2O7)4 with operating potentials near 4.0 V versus Na+/Na are promising cathode candidates. But the low conductivity, limited ion intercalation kinetics and inferior stability remain critical drawbacks for their practical applications. In this paper, the design of freestanding three-dimensional (3D) hybrid foams of high potential sodium hosts@biomass-derived porous carbon is reported. The biological fungus realizes the formation of highly porous graphene-like carbon, which constructs the 3D framework for the high potential polyanions. The polyanion nanocrystals are closely enwrapped by the biomass derived framework and build the hybrid architecture. Both the highly conductive skeleton and the hierarchically porous architecture of hybrid foams are favourable for highly efficient electron and ion transport. Furthermore, the depressed structural deterioration rate and improved contact between the active material and conductive substrate are achieved for the 3D hybrid foams in comparison with the conventional electrodes on the basis of the dynamic studies. Without additional additive or binders, the freestanding hybrid foams achieve desirable characteristics of high operating potentials, fast sodium intercalation and excellent cycling stability. The Na7V4(P2O7)4(PO4)- and Na7V3(P2O7)4-based hybrid foams retain 94% and 91%, respectively, of the capacities after 800 cycles at alternative 20C and 3C, demonstrating their superior high rate ultra long-term cycling capability. Therefore, this research provides a low-cost, highly efficient and widely applicable architecture to construct high potential and ultra long-life cathodes for sodium batteries.

Sulfate-based (SO42−) polyanionic materials with low cost and ionic-conduction break fresh ground for “rocking-chair” systems. But the high moisture sensitivity and limited conductivity led to their poor crystal stability and inferior alkaline-ion intercalation chemistry. Here we report the design of graphene-based sandwich-type nanoarchitecture for sulfate. The three-dimensional graphene-based network not only provides continuous electron/ion pathways for fast intercalation kinetics, but also effectively protects the crystal structure from deterioration to depress moisture sensitivity. As a case study, the hydrated sulfate (Na2Fe(SO4)2·2H2O)–graphene composite with a sandwich-type structure is prepared by a facile low-temperature synthesis. The depressed moisture sensitivity of hierarchical graphene–Na2Fe(SO4)2·2H2O compared to the pristine one is demonstrated by comparing their hydration process, and moreover, a shell–core hydration mechanism is disclosed. The hierarchical composite exhibits improved electronic conductivity and better sodium–lithium insertion capability than the pristine one. It delivers a reversible capacity of 72 and 69 mA h g−1 with redox potentials of 3.415/3.234 V (vs. Na+/Na) and 3.579/3.483 V (vs. Li+/Li) in sodium and lithium intercalation systems, respectively. Moreover, it also exhibits superior high rate capabilities and good cycling stability, which delivers 81% (for sodium) and 70% (for lithium) of the capacity at a 5 C rate. Therefore, the hierarchical sandwich-type architecture is favorable to realizing superior electrochemical performance for the sulfate, which presents a significant step forward in the development of low-cost large-scale batteries.

The tailoring of materials into bio-inspired structures is triggering unprecedented innovations. Muscle tissue is composed of myofibrils and densely wired blood vessels; it is a perfect model for designing high-performance electrode materials that have the advantage of fast mass transport and superior durability. We design a top-down strategy as a facile approach to tailor the alluaudite Na2+2xFe2−x(SO4)3 into a muscle-like spindle. A precipitation process is employed to prepare the hydrated “top” precursor, which is subjected to dehydration and phase transformation to obtain the “down” product. The alluaudite sulfate nanoparticles closely anchor on the single-wall carbon nanotubes (SWNT), and they together aggregate into microscale particles in the shape of spindles. The Na2+2xFe2−x(SO4)3/SWNT composite as a whole copies the morphology and function of muscle tissue. Taking advantage of its 3D conductive framework and porous structure, the composite achieves fast electron/ion transport and sodium intercalation. Moreover, the single-phase reaction mechanism during sodium intercalation is beneficial to its cycling property. It exhibits such desirable electrochemical performance as an operating potential as high as ∼3.8 V and a high-rate capability, which achieves a capacity retention of 92% after 100 cycles at 5C. The muscle-inspired architecture makes electrode materials favorable for superior electrochemical performance.

An iron-based mixed-polyanion compound, Li9Fe3(P2O7)3(PO4)2, is introduced as a possible cathode material for Li-ion batteries. Phase-pure Li9Fe3(P2O7)3(PO4)2 is successfully prepared by a sol–gel method, and its physicochemical properties are investigated in detail. Special attention is paid on making clear the variation of the phase composition with the annealing temperature and the effect of carbon coating on the electrochemical performance. Apparently phase-pure Li9Fe3(P2O7)3(PO4)2 can only be obtained in a narrow temperature range, either higher or lower annealing temperature outside this temperature range always leads to the impurity phase. The pristine Li9Fe3(P2O7)3(PO4)2 is suffering from its low electronic conductivity (10−9 S cm−1) and theoretical capacity (85 mA h g−1), it has a first discharge capacity of only 36 mA h g−1. Carbon coating is employed to improve the electrochemical performance. When the carbon content is 10 wt%, the discharge capacity of Li9Fe3(P2O7)3(PO4)2/C reaches the maximum value of 60 mA h g−1. The electronic conductivity of the composite, the exact discharge capacity of Li9Fe3(P2O7)3(PO4)2 in the composite and the capacity retention of the composite after 30 cycles vary in the same fashion with an increase in carbon content, i.e. first quickly increase and then stabilize.

The design of a freestanding electrode is the key to the development of energy storage devices with superior electrochemical performance and mechanical durability. Herein, we propose a highly-scalable strategy for the facile synthesis of a freestanding alluaudite Na2+2xFe2−x(SO4)3@porous carbon-nanofiber hybrid film, which is used as a self-supported and flexible electrode for sodium ion batteries. By the combined use of electrospinning and electrospraying, the freestanding hybrid film is constructed in the form of sulfate nanoparticles enwrapped in highly porous graphitic-like carbon-nanofibers. The multimodal porous architecture of the freestanding hybrid film ensures its superiority in mechanical flexibility and structural stability during repeated electrochemical processes, which meets the long-standing challenge of practical application. Moreover, both the highly conductive and porous framework and the nanoscale particles are favorable for promoting fast electron/ion transport capability. Compared with other carbon based supports such as graphene (GA), carbon nanotubes (CNTs) and active carbons (ACs), the flexible carbon nanofiber shows better interaction with electrochemical active materials and superior electrochemical properties. It retains over 95% of the capacity after five hundred cycles at alternate rates of 40C and 5C, which demonstrates the superior ultralong time and high-rate cycling capability. Therefore, the present work provides a facile and highly scalable strategy for the design and fabrication of high-performance freestanding sulfate cathodes for advanced sodium ion batteries.