Aluminum substitution for vanadium in the Na3V2(PO4)2F3 and Na3V2(PO4)2FO2 type materials

Electrolyte Design with Dual -C≡N Groups Containing Additives to Enable High-Voltage Na3V2(PO4)2F3-Based Sodium-Ion Batteries

Na3V2(PO4)2F3 is recognized as a promising cathode for high energy density sodium-ion batteries due to its high average potential of ∼3.95 V (vs Na/Na+). A high-voltage-resistant electrolyte is of high importance due to the long duration of 4.2 V (vs Na/Na+) when improving cyclability. Herein, a targeted electrolyte containing additives with two -C≡N groups like succinonitrile has been designed. In this design, one -C≡N group is accessible to the solvation sheath and enables the other -C≡N in dinitrile being exposed and subsequently squeezed into the electric double layer. Then, the squeezed -C≡N group is prone to a preferential adsorption on the electrode surface prior to the exposed -CH2/-CH3 in Na+-solvent and oxidized to construct a stable and electrically insulating interface enriched CN-/NCO-/Na3N. The Na3V2(PO4)2F3-based sodium-ion batteries within a high-voltage of 2-4.3 V (vs Na/Na+) can accordingly achieve an excellent cycling stability (e.g., 95.07% reversible capacity at 1 C for 1,5-dicyanopentane and 98.4% at 2 C and 93.0% reversible capacity at 5 C for succinonitrile after 1000 cycles). This work proposes a new way to design high-voltage electrolytes for high energy density sodium-ion batteries.

Recent Advances and Perspectives on the Promising High-Voltage Cathode Material of Na3 (VO)2 (PO4 )2 F

Na3 (VO)2 (PO4 )2 F (NVOPF) has emerged as one of the most promising cathode materials for sodium-ion batteries (SIBs) attributed to its high specific capacity (130 mAh g-1 ), high operation voltage (>3.9 V vs Na+ /Na), and excellent structural stability (<2% volume change). However, the comparatively low intrinsic electronic conductivity (≈10-7 S cm-1 ) of NVOPF leads to unsatisfactory electrochemical performance, especially at high rates, limiting its practical applications. To improve the conductivity and enhance Na storage performance, many efforts have been devoted to designing NVOPF, including morphology optimization, hybridization with conductive materials, metal-ion doping, Na-site regulation, and F/O ratio adjustment. These attempts have shown some encouraging achievements and shed light on the practical application of NVOPF cathodes. This work aims to provide a general introduction, synthetic methods, and rational design of NVOPF to give a deeper understanding of the recent progress. Additionally, the unique microstructure of NVOPF and its relationship with Na storage properties are also described in detail. The current status, as well as the advances and limitations of such SIB cathode material, are reported. Finally, future perspectives and guidance for advancing high-performance NVOPF cathodes toward practical applications are presented.

Vanadium fluorophosphates: advanced cathode materials for next-generation secondary batteries

Next-generation secondary batteries including sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) are considered the most promising candidates for application to large-scale energy storage systems due to their abundant, evenly distributed and cost-effective sodium/potassium raw materials. The electrochemical performance of SIBs (PIBs) significantly depends on the inherent characteristics of the cathode material. Among the wide variety of cathode materials, sodium/potassium vanadium fluorophosphate (denoted as MVPF, M = Na and K) composites are widely investigated due to their fast ion transportation and robust structure. However, their poor electron conductivity leads to low specific capacity and poor rate capacity, limiting the further application of MVPF cathodes in large-scale energy storage. Accordingly, several modification strategies have been proposed to improve the performance of MVPF such as conductive coating, morphological regulation, and heteroatomic doping, which boost the electronic conductivity of these cathodes and enhance Na (K) ion transportation. Furthermore, the development and application of MVPF cathodes in SIBs at low temperatures are also outlined. Finally, we present a brief summary of the remaining challenges and corresponding strategies for the future development of MVPF cathodes.

Hierarchical Na3V2(PO4)2F3 Microsphere Cathodes for High-Temperature Li-Ion Battery Application

Sodium superionic conductor (NASICON)-structured Na₃V₂(PO₄)₂F₃ cathode materials have received vast attention in the high-temperature storage performance due to their structural and thermal stability. Herein, hierarchical Na₃V₂(PO₄)₂F₃ microspheres (NVPF-HMSs) consisting of nanocubes were designed by a one-pot facial solvothermal method. The hierarchical Na₃V₂(PO₄)₂F₃ microsphere size is 2-3 μm, which is corroborated by FE-SEM and HR-TEM analyses. The NVPF-HMSs have been demonstrated as a cathode in Li-ion batteries at both low and elevated temperatures (25 and 55 °C, respectively). The NVPF-HMS cathode in a Li-ion cell exhibits reversible capacities of 119 mA h g^-1 at 0.1 C and 85 mA h g^-1 at 1 C with an 82% retention after 250 cycles at 25 °C. At elevated temperatures, the NVPF-HMS cathode exhibits a superior capacity of 110 mA h g^-1 at 1 C along with a retention of 90% after 150 cycles at 55 °C. Excellent capacity and cyclability were achieved at 55 °C due to its hierarchical morphology with a robust crystal structure, low charge-transfer resistance, and improved ionic diffusivity. The Li-ion storage performance of the NVPF-HMS cathode material at elevated temperatures was analyzed for the first time to understand the high-temperature storage property of the material, and it was found to be a promising candidate for elevated-temperature energy storage applications.

Understanding of the Mechanism and Kinetics of the Fast Solid-State Reaction between NaF and VPO4 to Form Na3V2(PO4)2F3

The solid-state reaction between NaF and VPO₄ is widely used to produce Na₃V₂(PO₄)₂F₃, a promising cathode material for sodium-ion batteries. In the present work, the mechanism and kinetics of the reaction between NaF and VPO₄ were investigated, and the effect of preliminary high-energy ball milling (HEBM) was studied using in situ time-resolved synchrotron powder X-ray diffraction, in situ transmission electron microscopy, differential scanning calorimetry, etc. The reaction was attributed to a "dimensional reduction" formalism; it proceeds quickly with the unilateral diffusion of Na⁺ and F^- ions into VPO₄ particles as a limiting stage. The use of HEBM leads to the mechanism corresponding to the third-order reaction model and accelerates the interaction. The rate constant k increases from 3.5 × 10^-5 to 3.4 × 10^-3 s^-1, and diffusion coefficient D increases from 2 × 10^-14 to 4 × 10^-13 cm² s^-1 when HEBM is used. The calculated apparent activation energy is ∼290 kJ mol^-1. The electrochemical properties of the as-prepared Na₃V₂(PO₄)₂F₃ are not inferior to the properties of the materials prepared by conventional solid-state synthesis.

Particle nanosizing and coating with an ionic liquid: two routes to improve the transport properties of Na3V2(PO4)2FO2

Na₃V₂(PO₄)₂FO₂ is a promising candidate for practical use as a positive electrode material in Na-ion batteries thanks to its high voltage and excellent structural stability upon cycling. However, its limited intrinsic transport properties limit its performance at fast charge/discharge rates. In this work, two efficient approaches are presented to optimize the electrical conductivity of the electrode material: particle nanosizing and particle coating with an ionic liquid (IL). The former reveals that particle downsizing from micrometer to nanometer range improves the electronic conductivity by more than two orders of magnitude, which greatly improves the rate capability without affecting the capacity retention. The second approch dealing with an original surface modification by applying an IL coating strongly enhances the ionic mobility and offers new perspectives to improve the energy storage performance by designing the electrode materials' surface composition.

Manipulating the Phase Compositions of Na3(VO1-xPO4)2F1+2x (0 ≤ x ≤ 1) and Their Synergistic Effects with Reduced Graphene Oxide toward High-Rate Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have aroused intense research and academic interest due to the natural abundance and cost-effectiveness of sodium resources. Presently, cathode materials based on the Na₃(VO_1-xPO₄)₂F_1+2x (0 ≤ x ≤ 1, NVPF_1+2x) polyanionic framework show intriguing electrochemical performances toward practical and advanced SIBs due to its high operating voltage (>3.9 V) and high energy density (>500 Wh kg^-1). Different from conventional approaches focusing on delicate morphology design, metal ion substitution, and the conductive matrix's incorporation to overcome the low intrinsic electrical conductivity, here we adopt a one-step microwave-assisted hydrothermal approach to optimize the electrochemical performances of NVPF_1+2x via manipulating its phase compositions with different vanadium sources and distinguishing the tetragonal (I4/mmm) symmetry of the Na₃(VOPO₄)₂F phase from the orthorhombic symmetry (Amam) of the Na₃V₂(PO₄)₂F₃ phase. The introduction of the conductive reduced graphene oxide (rGO) framework and its impacts on the phase compositions were systematically investigated. The rGO framework with different calcination temperature can alter the phase composition and the electrical conductivity of NVPF_1+2x cathodes significantly, thus having a great impact on their electrochemical performances. Galvanostatic charge/discharge, cyclic voltammetry, electrochemical impedance spectroscopy, and the galvanostatic intermittent titration technique are adopted to compare their electrode polarization and kinetics difference and show that NVPF@rGO-600 °C possesses a high rate, small polarization, and fast kinetics electrochemical properties. This work provides new insights into manipulating phase compositions of the NVPF_1+2x cathode by modulating the synthesis conditions and revealing their synergistic effect with a rGO conductive framework toward a superior rate capability and more realistic practical applications for SIBs.

Monitoring the Crystal Structure and the Electrochemical Properties of Na3(VO)2(PO4)2F through Fe3+ Substitution

We here present the synthesis of a new material, Na₃(VO)Fe(PO₄)₂F₂, by the sol-gel method. Its atomic and electronic structural descriptions are determined by a combination of several diffraction and spectroscopy techniques such as synchrotron X-ray powder diffraction and synchrotron X-ray absorption spectroscopy at V and Fe K edges, ⁵⁷Fe Mössbauer, and ³¹P solid-state nuclear magnetic resonance spectroscopy. The crystal structure of this newly obtained phase is similar to that of Na₃(VO)₂(PO₄)₂F, with a random distribution of Fe³⁺ ions over vanadium sites. Even though Fe³⁺ and V⁴⁺ ions situate on the same crystallographic position, their local environment can be studied separately using ⁵⁷Fe Mössbauer and X-ray absorption spectroscopy at Fe and V K edges, respectively. The Fe³⁺ ion resides in a symmetric octahedral environment, while the octahedral site of V⁴⁺ is greatly distorted due to the presence of the vanadyl bond. No electrochemical activity of the Fe⁴⁺/Fe³⁺ redox couple is detected, at least up to 5 V, whereas the reduction of Fe³⁺ to Fe²⁺ has been observed at ∼1.5 V versus Na⁺/Na through the insertion of 0.5 Na⁺ into Na₃(VO)Fe(PO₄)₂F₂. Comparing to Na₃(VO)₂(PO₄)₂F, the electrochemical profile of Na₃(VO)Fe(PO₄)₂F₂ in the same cycling condition shows a smaller polarization which could be due to a slight improvement in Na⁺ diffusion process thanks to the presence of Fe³⁺ in the framework. Furthermore, the desodiation mechanism occurring upon charging is investigated by operando synchrotron X-ray diffraction and operando synchrotron X-ray absorption at V K edge.