2F3 Cathode Research Articles

High-entropy strategy to suppress volumetric strain and enhance diffusion rate of Na3V2(PO4)2F3 cathode for durable and high-areal-capacity zinc-ion battery pouch cells

Aqueous zinc-ion batteries are pivotal contributors to the global energy transformation. Cathode materials with NASICON-type structures are promising candidates for zinc-ion batteries but are hindered by their low electrical conductivity, sluggish ionic diffusion, and structural instability. This work introduces a high-entropy, carbon-coated NASICON-type Na3V2(PO4)2F3 (HE-NVPF@C) cathode by incorporating five metal elements (Al, Zn, Mn, Cr, and Nb) mainly into the V sites of the VO4F2 octahedral structure. Systematic experimental and simulation studies of the Zn2+ storage mechanism in high-entropy NASICON-type cathode are presented for the first time. The high-entropy doping strategy contributes to significantly enhanced cycling stability by suppressing Jahn-Teller distortion, reducing lattice change during Zn2+ extraction and insertion, and decreasing the Zn2+ migration energy barrier. As a result, the HE-NVPF@C cathode demonstrates exceptional cycling stability over 6000 cycles at 20 C with a capacity loss of a mere 0.0031 % per cycle and a high areal capacity retention of 2.17 mAh cm−2. In addition, the pouch cell provides a long cycling lifespan with 90.8 % capacity retention at 5 C after 200 cycles. This feasible high-entropy approach broadens the perspective for developing practical zinc-ion batteries with a long cycle lifespan and high areal capacity.

Fluorinated ionic liquid mediated nanostructure with enhanced conductivity and fluorine retention in Na3V2(PO4)2F3 cathode toward high-performance sodium-ion batteries

Na3V2(PO4)2F3 (NVPF) is considering as a promising cathode material for sodium-ion batteries (SIBs), due to its framework robustness and high working voltage. For NVPF cathodes, the main obstacle of poor conductivity could be well-addressed by carbon hybridization or nano-crystallization strategies. However, carbon compositing process would bring up fluorine-loss issue and the designation of tailor-made nanocomposite remains challenging. Here, we realize the optimal regulation on crystal growth by virtue of a fluorinated ionic liquid usage, which could compensate the fluorine-loss during high-temperature annealing process. As a structural-directing agent, ionic liquid with low surface tension also helps to reduce surficial Gibbs free energy of NVPF crystal, facilitating to reduce the particle size. Meanwhile, the π-cation interaction between reduced graphene oxide (rGO) and cations of ionic liquid is beneficial to fabricate ultrathin rGO-wrapped NVPF nanoplates (I-NVPF@rGO). The tailored I-NVPF@rGO exhibits enhanced conductivity and optimized crystal planes, resulting in excellent rate capability and improved energy density. When coupling with MoS2/SnS/rGO anode, the MoS2/SnS/rGO||I-NVPF@rGO full battery delivers a capacity of 116 mA h g−1 at 0.2 C. This strategy offers valuable insights into the development of sodium-ion batteries.

High-Entropy Strategy to Suppress Volumetric Strain and Enhance Diffusion Rate of Na3V2(PO4)2F3 Cathode for Durable and High-Areal-Capacity Zinc-Ion Battery Pouch Cells

The burgeoning global environmental awareness and the escalating demand for renewable energy storage have intensified the critical need for next-generation battery technologies.1,2 In recent decades, there has been a sustained pursuit of grid-scale energy storage systems that balance practicality and efficiency. In this context, aqueous zinc-ion batteries (AZIBs) have emerged as a promising contender, driven by the distinct advantages of the 2.8-fold higher volumetric capacity (5849 mAh cm-3) of zinc metal over lithium metal (2062 mAh cm-3), and the use of non-toxic, cost-effective aqueous electrolyte. To date, substantial efforts have been dedicated to investigating suitable AZIB cathode materials, including Prussian blue analogs (PBAs), transition-metal (TM) oxides (e.g., MnO2 and V2O5), organic compounds, and polyanion-type compounds.Recently, there has been a notable surge of interest in polyanion-type compounds. Na3V2(PO4)2F3 (NVPF), demonstrates an operating potential of exceeding 1.60 V and an enhanced energy density of 97.5 Wh kg-1. Regrettably, these advantages are often counterbalanced by undesirable side reactions and sluggish ion kinetics, culminating in incompetent rate capabilities and suboptimal cycling stability.3 High-entropy (HE) materials have emerged as promising candidates for facilitating ion transport and fortifying cathode stability across various energy storage systems due to their exceptional design flexibility and compositional tunability.4,5 Inspired by conventional HE stabilization strategies, this work introduces a carbon-coated Na3V1.9(AlZnMnCrNb)0.1(PO4)2F3 cathode, denoted as HE-NVPF@C. Employing this cathode, coupled with a carbon-film-functionalized Zn anode, referred to as Zn@CF, and acetonitrile(AN)-water hybrid electrolyte, we configured an AZIB with substantial enhancements in long-term cycling stability and high areal capacity. AZIBs based on HE-NVPF@C cathodes presented a high reversible capacity of 77.45 mAh g−1 at 2C and remarkable cycling stability with a capacity loss of a mere 0.0031% per cycle over 6,000 cycles at 20C. In particular, the areal capacity of the HE-NVPF@C cathode reached up to 2.17 mAh cm-2, and HE-NVPF@C also attained a high capacity density of 78.2 mAh g-1 and 61.6 mAh g-1 when working under 50 and -20°C, making it more competitive among the reported cathodes in AZIBs. In addition, the pouch cell provides a long cycling lifespan with 90.8% capacity retention at 5C after 200 cycles. More importantly, the Zn2+ storage mechanism of the high-entropy NASICON-type cathode was systematically studied for the first time. In-situ X-ray diffraction (XRD) and density functional theory (DFT) calculations revealed that NVPF@C with high-entropy doping improved cycling stability due to the mitigated Jahn-Teller distortion, enhanced Zn2+ diffusion rate, and reduced lattice volumetric strain upon Zn2+ extraction and insertion. This work not only lays the door for developing stable AZIBs but also enriches the mechanistic discussion about the improved cycling or structural stability of high-entropy NASICON structures.Keywords: Zinc-ion battery; High-entropy material; NASICON; Na3V2(PO4)2F3; Pouch cellReference1 Cao, Y. et al. Bridging the academic and industrial metrics for next-generation practical batteries. Nat. nanotechnol. 14, 200-207, (2019).2 Xue, M. et al. Carbon-assisted anodes and cathodes for zinc ion batteries: From basic science to specific applications, opportunities and challenges. Energy Storage Mater. 62, 102940, (2023).3 Zhu, L. et al. A comprehensive review on the fabrication, modification and applications of Na3V2(PO4)2F3 cathodes. J. Mater. Chem. A 8, 21387-21407, (2020).4 Zeng, Y. et al. High-entropy mechanism to boost ionic conductivity. Science 378, 1320-1324, (2022).5 Yang, C. et al. All-temperature zinc batteries with high-entropy aqueous electrolyte. Nat. Sustain. 6, 325-335, (2023).

Polyaniline-Coated Na3V2(PO4)2F3 Cathode Enables Fast Sodium Ion Diffusion and Structural Stability in Rechargeable Batteries.

Na3V2(PO4)2F3 (NVPF), a typical sodium superionic conductor (NASICON) type structure, has attracted much interest as a potential positive electrode in sodium-ion battery. However, the inherently poor electronic conductivity of phosphates compromises the electrochemical properties of this material. Here, we develop a general strategy to improve the electrochemical performance by preparing a new composite material "polyaniline (PANI)@NVPF" using a Pickering emulsion method. The X-ray diffraction and Raman results indicated a successful PANI coating without affecting the NASICON-type structure of NVPF, and they enhanced the interfacial bonding between the two components. Also, thermogravimetric analysis and scanning electron microscopy analyses revealed that the PANI content influenced the thermal stability and morphology of the nanocomposites. As a result, the sodium test cells exhibited multielectron reactions and a better rate performance for PANI@NVPF nanocomposites as compared to NVPF. Specifically, 2%PANI@NVPF maintained 70% of its initial capacity at 5C. Ex-situ electron paramagnetic resonance revealed the existence of mixed valence states of vanadium (V4+/V3+) in both discharge and charge processes. Consequently, the successful PANI coating into the sodium superionic conductor framework improved the sodium diffusion channels with a measurable increase of diffusion coefficients with cycling (ca. 3.25 × 10-11 cm2 s-1). Therefore, PANI@NVPF nanocomposites are promising cathode candidates for high-rate sodium-ion battery applications.

Achieving High-Performance Na3V2(PO4)2F3 Cathode Material through a Bifunctional N-Doped Carbon Network.

Na3V2(PO4)2F3 (NVPF) is emerging as a popular cathode for sodium-ion batteries owing to its stable structure, high operating voltage, and large energy density. However, its practical application is hindered by its low conductivity. In addition, due to the loss of fluorine during synthesis, Na3V2(PO4)3 (NVP) impurity is often easily generated, resulting in a decrease in actual operating voltage. Herein, a bifunctional carbon network composed of an N-doped carbon layer and carbon bridge is constructed around NVPF particles. Through pyrolysis of polydopamine (PDA), the NVPF particles are covered in situ by an N-doped carbon layer, and the carbon bridge generated by polytetrafluoroethylene (PTFE) is also coated with N-doped carbon. Besides, PTFE also serves as a fluorine supplement to ensure that pure NVPF is obtained. As a result, the bifunctional N-doped carbon network-modified NVPF delivers a high reversible capacity (125.7 mA h g-1 at 0.2 C) and appreciable cycle stability (92.7% at 1 C over 300 cycles, and 89.8% at 10 C over 1500 cycles). When assembled into a full cell with a commercial hard carbon anode, it displays a discharge median voltage of up to 3.62 V at 0.2 C. Furthermore, it achieves a high energy density of 373.7 W h kg-1 at a power density of 461.2 W kg-1, with an excellent specific energy retention of 78.2% after 200 cycles. Therefore, this modification method is expected to be extended to other fluorine-containing materials with poor electrical conductivity.

Boosting sodium-ion battery performance with binary metal-doped Na3V2(PO4)2F3 cathodes

Na3V2(PO4)2F3 (NVPF), recognized for its Na superionic conductor architecture, emerges as a promising candidate among polyanion-type cathodes for sodium ion batteries (SIBs). However, its adoption in practical applications faces obstacles due to its inherently low electronic conductivity. To address this challenge, we employ a binary co-doped strategy to design Na3.3K0.2V1.5Mg0.5(PO4)2F3 cathode with nitrogen-doped carbon (NC) coating layer. This configuration enhances electronic conductivity and reduces diffusion barriers for sodium ion (Na+). The strategy of incorporating nitrogen-doped carbon coating not only facilitates the formation of a porous structure but also introduces additional defects and active sites. Such modifications accelerate the reaction kinetics and augment electrolyte interaction through an expanded specific surface area, thus streamlining the electrochemical process. Concurrently, strategic heteroatom substitution leads to a more efficient engagement of Na+ in the electrochemical activities, thereby bolstering the cathode's structural integrity. The vanadium fluorophosphate Na3.3K0.2V1.5Mg0.5(PO4)2F3@NC exhibits an electrochemical performance, including a high discharge specific capacity of 124.3 mA h g−1 at 0.1C, a long lifespan of 1000 cycles with a capacity retention of 93.1 % at 10C, and a rate property of 73.2 mA h g−1 at 20C. This research provides a method for preparing binary doped NVPF for energy storage electrochemistry.

Vanadium substituted Fe, Cr co-doped high performance C/Na3V2(PO4)2F3 cathode for sodium-ion batteries

In recent years, Na3V2(PO4)2F3 (NVPF) has been considered as one of the most promising candidates for cathodes in sodium ion batteries (SIBs). However, presence of vanadium makes the material costly. Besides, its electrochemical properties are hampered due to its low electronic conductivity. Herein, carbon coated Fe, Cr co-doped Na3V2(PO4)2F3 has been synthesized using rota-tumbler assisted sol–gel method. The effects of co-doping on structure, morphology and electrochemical performances are systematically studied. It is confirmed that co-doping of Fe and Cr can enhance the electrochemical performances of NVPF. Among all, Na3V1.9Fe0.095Cr0.005(PO4)2F3 shows a high initial capacity of 119.85 mAhg−1 at 0.1C and maintains a capacity retention of 88.58 % after 500 cycles at 1C. Increase in the size of the saddle point, planes of monocapped trigonal prisms and the hexagonal cavities help in decreasing the ion diffusion activation energy, thus improving the diffusivity of the doped material. The improved electrochemical performances can be attributed to the increase in the electronic conductivity of the material. Besides, vanadium reduction in the cathode aids in manufacturing cost reduction. Our work demonstrates that carbon coated Na3V1.9Fe0.095Cr0.005(PO4)2F3 can be considered as a potential cathode candidate for practical applications in SIBs.

Ultrafast Synthesis of Large‐Sized and Conductive Na3V2(PO4)2F3 Simultaneously Approaches High Tap Density, Rate and Cycling Capability

AbstractThe Na3V2(PO4)2F3 (NVPF) cathode material is usually nano‐sized particles exhibiting low tap density, high specific surface area, correspondingly low volume energy density, and cycle stability of the sodium‐ion batteries (SIBs). Herein, a high‐temperature shock (HTS) strategy is proposed to synthesize NVPF (HTS‐NVPF) with uniform conducting network and high tap density. During a typical HTS process (heating rate of 1100 °C s−1 for 10 s), the precursors rapidly crystallize and form large‐sized and dense particles. The tight connection between particles not only enhances their contact with carbon layers, but also reduces the specific surface area that inhibits side reactions between the interfaces and the electrolyte. Besides, ultrafast synthesis of NVPF reduces the F loss and amount of Na3V2(PO4)3 impurities, which improve cycling capability. The HTS‐NVPF demonstrates a high energy density of 413.4 Wh kg−1 and an ultra‐high specific capacity of 103.4 mAh g−1 at 10 C as well as 84.2% capacity retention after 1000 cycles. In addition, the excellent temperature adaptability of HTS‐NVPF (−45–55 °C) and remarkable electrochemical properties of NVPF||HC full cell demonstrate extreme competitiveness in commercial SIBs. Therefore, the HTS technique is considered to be a high‐efficiency strategy to synthetize NVPF and is expected to prepare other cathode materials.

A surface-modified Na3V2(PO4)2F3 cathode with high rate capability and cycling stability for sodium ion batteries.

High voltage, high rate, and cycling-stable cathodes are urgently needed for development of commercially viable sodium ion batteries (SIBs). Herein, we report a facial ball-milling to synthesize a carbon-coated Na3V2(PO4)2F3 composite (C-NVPF). Benefiting from the highly conductive carbon layer, the C-NVPF material exhibits a high reversible capacity (110.6 mA h g-1 at 0.1C), long-term cycle life (54% of capacity retention up to 2000 cycles at 5C), and excellent rate performance (35.1 mA h g-1 at 30C). The present results suggest promising applications of the C-NVPF material as a high-performance cathode for sodium ion batteries.

High power NVPF/HC-based sodium-ion batteries

The Na-ion battery consisting of Na3V2(PO4)2F3 (NVPF) cathode and hard carbon (HC) anode has been developed and commercialized by Tiamat. This study focuses on the assessments of NVPF/HC-based prototype cells in a cylindrical cell format. The specific cell for this investigation has a capacity of 0.61 Ah, a specific energy of 68 Wh/kg, and an energy density of 135 Wh/L at 1C. The cycle life of this specific cell generation (2020) shows 1600 cycles at 5C charge and 5C discharge, but more recent cell generations show improved cycle stability: a trend of enhancement in cell cycle life is presented in the development timeline, and the latest version cell achieves a cycle life of 3200 cycles at 2C charge and 5C discharge. The cell shows excellent power rate capabilities up to 20C discharge and 10C charge. It is highlighted that >90 % of capacity and >80 % of energy (compared to 1C) can still be accessed at 20C discharge. Safety tests with different abuse conditions (overcharge, external heating, short circuit, and nail penetration) were performed. For all these abuse conditions, the specific cell version showed no thermal runaway. Thus, the NVPF/HC-based Na-ion technology seems to develop towards a well-suited candidate for high-power energy storage applications.

A Novel Na4MnCr(PO4)2F3 Cathode with High Energy Density for Sodium-Ion Batteries.

Na3V2(PO4)2F3 (NVPF) is a representative cathode material of sodium-ion batteries (SIBs) due to its high working voltage and high structural stability. However, its specific capacity is limited to the reversible inserting and extracting of two sodium ions per formula unit, which hampers the improvement of its energy density. In this study, we propose a new NASICON-type Na4MnCr(PO4)2F3 (NMCPF) cathode and systematically investigate its key properties using first-principles calculations. NMCPF exhibits the ability to extract/insert three sodium ions per formula unit, resulting in a high specific capacity of 180.34 mAh/g, and demonstrates three-electron redox reactions involving three redox couples of Mn2+/3+ (3.05 V), Mn3+/4+ (4.11 V), and Cr3+/4+ (4.64 V). Consequently, its energy density can reach 709.33 Wh/kg. In addition, NMCPF exhibits a small volume change of 8.2% during the charging/discharging process and sodium ion diffusion properties comparable to those of NVPF. This study demonstrates the potential of NMCPF as a promising cathode material with high energy density for SIBs.

Construction of superior performance Na3V2-xCrx(PO4)2F3/C cathode by homovalent doping strategy toward enhanced sodium ion storage

Na3V2(PO4)2F3, with robust 3D structural framework and high operating potential, is regarded as a promising cathode candidate for sodium-ion batteries. However, the low electrical conductivity caused by poly-anionic polyhedron leads to slow kinetics and poor rate performances. In this work, Cr3+ doped Na3V2(PO4)2F3/C is prepared by using chromium as homovalent dopant ion. The synergistic effect of Cr3+ doping and carbon coating promotes the rapid transportation of electrons in Na3V2(PO4)2F3/C particles. The Rietveld refinements show that Cr3+ doping can adjust the crystal structure and accelerate the diffusion kinetics of Na+. The Cr3+ doping improves the electrical conductivity of Na3V2-xCrx(PO4)2F3/C. In-situ electrochemical impedance spectroscopy measurements show that Cr3+ doping effectively reduces the charge transfer impedance and plays an important role in improving the charge transfer kinetics. Consequently, the optimized Na3V1.98Cr0.02(PO4)2F3/C exhibits excellent rate capability of 91.2 mAh·g−1 at 30 C and long cyclic stability, in which the capacity decay per cycle is 0.0029% over 1000 cycles at 10 C. The results provide a facile strategy and beneficial way to design advanced Na3V2(PO4)2F3 cathode toward sodium-ion batteries.

High-performance Na3V2(PO4)2F3 cathode obtained by a three-in-one strategy for self-sodium compensation, interface modification, and crosslinked carbon coatings

Sodium-ion batteries (SIBs) are encouraging substitute to lithium-ion batteries. However, the application of SIBs is limited by their sluggish kinetics and unstable cyclic stability. Here, a crosslinked carbon-covered porous Na3V2(PO4)2F3 (NVPF) composite was prepared via a carboxymethyl cellulose (CMC)-assisted sol–gel strategy. The influences of the CMC on the interface and electrochemical properties were investigated. CMC supplemented additional Na+ that compensated for the sodium loss during the generation of the solid electrolyte interface. Besides, the crosslinked carbon formed by CMC strengthened the structure. Meanwhile, the porous coating layer with a large pore provided a fast transfer path for Na+. Thus, the optimized NVPF@C@CMC delivered an outstanding discharge capacity of 125.8 mAh g-1 at 0.2C, superb rate capability of 70.4 mAh g-1 at 20C, and good high-temperature adaptability (122.8 mAh g-1 at 0.2C and 45 °C). Moreover, in the full battery composed of the NVPF@C@CMC and a hard carbon, a high discharge capacity of 123.3 mAh g-1 was acquired at 0.2C. These superior results can be attributed to the simultaneous realization of self-sodium compensation, interface modification, and crosslinked carbon coating. Therefore, the proposed modified sol–gel method is a multi-effect synergistic approach for the advance of high-performance energy-storage materials.

Crumpling Carbon‐Pillared Atomic‐Thin Dichalcogenides and CNTs into Elastic Balls as Superior Anodes for Sodium/Potassium‐Ion Batteries

AbstractWith abundant electroactive sites and rapid ion diffusion paths, ultrathin dichalcogenides such as MoS2 demonstrate enormous potential as anodes for sodium/potassium‐ion batteries (SIBs/PIBs). However, ultrahigh‐aspect‐ratio nanosheets are very easy to aggregate and re‐stack, drastically weakening their intrinsic merits. Here a sustainable dichalcogenide anode is designed via crumpling carbon‐pillared atomic‐thin MoS2 nanosheets with CNTs into an elastic ball structure (C‐p‐MoS2/CNTs). In this architecture, the glucose‐derived carbon pillars atomic‐thin MoS2 nanosheets and broadens interlayer spacing, ensuring fast Na+/K+ diffusion; CNTs act as 3D scaffolds to impede re‐stacking of MoS2 while providing high‐speed pathways for electrons; the integration of flexible atomic‐thin sheets and high‐toughness CNTs further endows the balls with great elasticity to release the cycling stress. Consequently, the C‐p‐MoS2/CNTs material delivers high reversible capacities, outstanding cycling stability, and superior rate performance as anodes for both SIBs and PIBs. Pairing with Na3V2(PO4)2F3 cathode, the sodium‐ion coin‐cell could operate at a rate up to 50 C at high mass loading of 9.4 mg cm−2 and manifest ultrastable cycling stability at 40 C over 600 cycles. Impressively, the assembled pouch cell can be cycled stably with a high energy density of 188 Wh kg−1. This study is anticipated to provide inspiration for designing innovative metal dichalcogenides as battery anodes.

Constructing Stable Anion‐Tuned Electrode/Electrolyte Interphase on High‐Voltage Na 3 V 2 (PO 4 ) 2 F 3 Cathode for Thermally‐Modulated Fast‐Charging Batteries

Constructing stable electrode/electrolyte interphase with fast interfacial kinetics is vital for fast-charging batteries. Herein, we investigate the interphase that forms between a high-voltage Na3V2(PO4)2F3 cathode and the electrolytes consisting of 3.0, 1.0, or 0.3 M NaClO4 in an organic carbonate solvent (47.5:47.5:5 mixture of EC: PC: FEC) during charging up to 4.5 V at 55oC. It is found that a higher anion/solvent ratio in electrolyte solvation structure induces anion-dominated interphase containing more inorganic species and more anion derivatives (CxClOy), which leads to a larger interfacial Na+ transport resistance and more unfavorable gas evolution. In comparison, a low anion/solvent ratio derives stable anion-tuned interphase that enables better interfacial kinetics and cycle ability. Importantly, the performance of a failed cathode is restored by triggering the decomposition of CxClOy species. This work elucidates the role of tuning interphase in fast-charging batteries.

Repurposing Face Masks after Use: From Wastes to Anode Materials for Na-Ion Batteries

Nowadays, face masks play an essential role in limiting coronavirus diffusion. However, their disposable nature represents a relevant environmental issue. In this work, we propose the utilization of two types of disposed (waste) face masks to prepare hard carbons (biochar) by pyrolytic conversion in mild conditions. Moreover, we evaluated the application of the produced hard carbons as anode materials in Na-ion batteries. Pristine face masks were firstly analyzed through infrared spectroscopy and thermogravimetric analysis. The pyrolysis of both mask types resulted in highly disordered carbons, as revealed by field-emission scanning electron microscopy and Raman spectroscopy, with a very low specific surface area. Anodes prepared with these carbons were tested in laboratory-scale Na-metal cells through electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic cycling, displaying an acceptable specific capacity along a wide range of current regimes, with a good coulombic efficiency (>98% over at least 750 cycles). As a proof of concept, the anodes were also used to assemble a Na-ion cell in combination with a Na3V2(PO4)2F3 (NVPF) cathode and tested towards galvanostatic cycling, with an initial capacity of almost 120 mAhg−1 (decreasing at about 47 mAhg−1 after 50 cycles). Even though further optimization is required for a real application, the achieved electrochemical performances represent a preliminary confirmation of the possibility of repurposing disposable face masks into higher-value materials for Na-ion batteries.

High-Voltage and Intrinsically Safe Sodium Metal Batteries Enabled by Nonflammable Fluorinated Phosphate Electrolytes.

Sodium metal is a promising anode for high-energy-density sodium rechargeable batteries (RSBs). However, the low Coulombic efficiency (CE) of the Na plating/stripping process and the problem of safety hinder their practical application. Herein, we report a facile strategy for employing the fluorinated phosphate solvents to realize highly reversible Na plating/stripping and improve the safety performance. The fluorinated phosphate molecules reduce the polarity of the solvent and lower the coordination number to Na+, which makes it possible to form the anion-induced ion-solvent-coordinated (AI-ISC) structures with high reduction tolerance. Moreover, the fluorination treatment enhances the oxidation resistance of the phosphate solvent, enabling compatibility with the high-voltage Na3V2(PO4)2F3 (NVPF) cathode. As expected, the Na@Al//NVPF full cell with the as-prepared 0.9 M NaFSI/tris(2,2,2-trifluoroethyl) phosphate (TFEP) demonstrates a capacity retention of 83.4% after 200 cycles with an average CE of 99.6%. This work opens a new avenue for designing high-energy-density RSBs with improved safety performance.

PTFE-derived carbon-coated Na3V2(PO4)2F3 cathode material for high-performance sodium ion battery

Na3V2(PO4)2F3 (NVPF) is considered to be a promising cathode for sodium-ion batteries (SIBs) due to its large theoretical capacity and high average operating voltage. However, the capacity is limited by its low intrinsic electronic conductivity and the inevitable fluorine loss during synthesis. Herein, carbon-coated NVPF composites were successfully synthesized using polytetrafluoroethylene (PTFE) as a carbon source and fluorine supplement agent. The NVP impurity in these carbon-coated NVPF composites was significantly reduced or eliminated. As a cathode material for SIBs, the optimized PTFE-derived carbon-coated NVPF exhibits excellent reversible capacity, superior rate capability, and considerable long-term cycle stability. This simple and feasible modification strategy of using PTFE as a carbon source and fluorine supplement agent can also be extended to Na2FePO4F, LiVPO4F, K3V2(PO4)2F3, and other fluorine-containing polyanionic cathode materials.

Effects of nitrogen and sulfur atom regulation on electrochemical properties of Na3V2(PO4)2F3 cathode material for Na-ion batteries

The cathode material Na3V2(PO4)2F3 of sodium-ion battery is well-known for its large number of ion migration channels and high working voltage. However, the electrochemical performance of Na3V2(PO4)2F3 is not very outstanding. Thus, in the present study, Na3V2(PO4)2F3 cathode materials were successfully synthesized by using the sol-gel method and mechanical milling method to enhance the electrochemical performance. The physicochemical properties of synthesized Na3V2(PO4)2F3 were investigated by using X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transition electron microscopy. X-ray diffraction spectroscopy indicates that the doping of nitrogen and sulfur did not alter the crystal form of Na3V2(PO4)2F3. Transition electron microscopy image shows that Na3V2(PO4)2F3 has a thin carbon layer, and x-ray photoelectron spectroscopy illustrates the successful doping of nitrogen and sulfur into the carbon layer. The cyclic voltammetry curves show that the nitrogen and sulfur co-doped Na3V2(PO4)2F3 samples have good reversibility and low polarization. Materials with 15% thiourea has a high discharge specific capacity (126.9 mA h g−1 at 0.2 C) at the first cycle and excellent cycle stability (126.3 mA h g−1 after 100 cycles, a capacity retention of 99.5%) among the synthesized cathode materials. In the present study, the electrochemical performance of the Na3V2(PO4)2F3 cathode material was enhanced by regulation of co-doping of nitrogen and sulfur atoms.

Utilizing weakly-solvated diglyme-based electrolyte to achieve a 10,000-cycles durable Na3V2(PO4)2F3 cathode endured at − 20 ℃

The commercialization of sodium-ion batteries (SIBs) is tremendously limited as performance decline in extreme environments (such as below 0 ℃). This work proposes remarkable low-temperature (low-T) SIBs based on 0.5 M NaPF6 diglyme electrolyte. By a series of in-depth analysis, including XPS, temperature-dependent EIS, in- and ex-situ XRD, etc., finding that the weakly solvated structure of Na+-diglyme is beneficial to accelerating diffusivity of Na+ and reducing charge transfer energy (from 461.9 to 158.6 meV), which is effectively to ameliorate the low-T sodium storage mechanism. Moreover, The NaF-rich cathode electrolyte interphase (CEI) formed in diglyme is conducive to stabilize the interfacial structure and accelerate transport of Na+. All these merits collectively lead to high adaptability of low-concentration diglyme-based electrolyte to low-T. It has been verified that this system delivers 95.6% of room-temperature (RT) capacity, and full-cell delivers huge superiority in lifespan (working stably for 10,000 cycles) at − 20 ℃. Even at − 60 ℃, the battery still attains high capacity density (79.2 mAh g−1). These results provide new research ideas for improving the low-T performance of energy storage equipment.