Freestanding Na3V2O2(PO4)2F/Graphene Aerogels as High-Performance Cathodes of Sodium-Ion Full Batteries

Microwave-Assisted Hydrothermal Synthesis of Na3V2(PO4)2F3 Nanocuboid@Reduced Graphene Oxide as an Ultrahigh-Rate and Superlong-Lifespan Cathode for Fast-Charging Sodium-Ion Batteries

Na3V2(PO4)2F3 (NVPF) has been regarded as a favorable cathode for sodium-ion batteries (SIBs) due to its high voltage and stable structure. However, the limited electronic conductivity restricts its rate performance. NVPF@reduced graphene oxide (rGO) was synthesized by a facile microwave-assisted hydrothermal approach with subsequent calcination to shorten the hydrothermal time. NVPF nanocuboids with sizes of 50-150 nm distributed on rGO can be obtained, delivering excellent electrochemical performance such as a longevity life (a high capacity retention of 85.6% after 7000 cycles at 10 C) and distinguished rate capability (116 mAh g-1 at 50 C with a short discharging/charging time of 1.2 min). The full battery with a Cu2Se anode represents a capacity of 116 mAh g-1 at 0.2 A g-1. The introduction of rGO can augment the electronic conductivity and advance the Na+ diffusion speed, boosting the cycling and rate capability. Besides, the small lattice change (3.3%) and high structural reversibility during the phase transition process between Na3V2(PO4)2F3 and NaV2(PO4)2F3 testified by in situ X-ray diffraction are also advantageous for Na storage behavior. This work furnishes a simple method to synthesize polyanionic cathodes with ultrahigh rate and ultralong lifespan for fast-charging SIBs.

Highly Aligned Graphene Aerogels for Multifunctional Composites

Stemming from the unique in-plane honeycomb lattice structure and the sp2 hybridized carbon atoms bonded by exceptionally strong carbon-carbon bonds, graphene exhibits remarkable anisotropic electrical, mechanical, and thermal properties. To maximize the utilization of graphene's in-plane properties, pre-constructed and aligned structures, such as oriented aerogels, films, and fibers, have been designed. The unique combination of aligned structure, high surface area, excellent electrical conductivity, mechanical stability, thermal conductivity, and porous nature of highly aligned graphene aerogels allows for tailored and enhanced performance in specific directions, enabling advancements in diverse fields. This review provides a comprehensive overview of recent advances in highly aligned graphene aerogels and their composites. It highlights the fabrication methods of aligned graphene aerogels and the optimization of alignment which can be estimated both qualitatively and quantitatively. The oriented scaffolds endow graphene aerogels and their composites with anisotropic properties, showing enhanced electrical, mechanical, and thermal properties along the alignment at the sacrifice of the perpendicular direction. This review showcases remarkable properties and applications of aligned graphene aerogels and their composites, such as their suitability for electronics, environmental applications, thermal management, and energy storage. Challenges and potential opportunities are proposed to offer new insights into prospects of this material.

Structural Evolution of Lithium-Exchanged Na3(VO)2(PO4)2F Cathode under Operation in Sodium Ion Batteries

Na superionic conductor (NASICON)-type Na3(VO)2(PO4)2F (NVOPF) exhibits excellent cycling stability for high-voltage sodium ion batteries. Various strategies have been developed to form ion-exchanged NVOPF which can enhance the ionic and electronic conductivity. However, the underlying ion transport mechanism and complex structural transitions during battery operation remained uninvestigated. In this work, we prepared lithium-exchanged NVOPF (namely NLVOPF) which shows improved ionic conductivity and increased capacity at high discharging rates. Solid-state nuclear magnetic resonance (SSNMR) revealed the distinctive presence of two kinds of Li-exchanged sites in the NLVOPF, which are attributed to the occupied lithium ions at the Na1 and Na2 sites (namely Li1 and Li2, respectively). The Li1 site was metastably replaced in the first cycle, yet the Li2 site participated in ion insertion/extraction in the subsequent cycles. Our characterizations show that the dynamic doping of lithium in NLVOPF could contribute to the improved cycling stability and capacity retention.

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.

Interfacial Engineering of Na3V2(PO4)2O2F Cathode for Low-Temperature (-40 °C) Sodium-Ion Batteries

Na3V2(PO4)2O2F (NVPOF) is considered a promising cathode material for sodium-ion batteries (SIBs) on account of its attractive electrochemical properties such as high theoretical capacity, stable structure, and high working platform. Nevertheless, the inevitable interface problems like sluggish interfacial electrochemical reaction kinetics and poor interfacial ion storage capacity seriously hinder its application. Construction of chemical bonding is a highly effective way to solve interface problems. Herein, NVPOF with interfacial V-F-C bonding (CB-NVPOF) is developed. The CB-NVPOF cathode exhibits high rate capability (65 mA h g-1 at 40C) and long-term cycling stability (a capacity retention of 77% after 2000 cycles at 20C). Furthermore, it shows impressive electrochemical performance at temperatures as low as -40 °C, delivering a capacity of 56 mA h g-1 at 10C and a capacity retention of ∼80% after 500 cycles at 2C. The interfacial V-F-C bond engineering significantly advances the electronic conductivity, Na+ diffusion, as well as interface compatibility at -40 °C. This study provides a novel idea for improving the electrochemical performance of NVPOF-based cathodes for SIBs aiming for low-temperature applications.

Multi-component surface engineering of Na3V2(PO4)2O2F for low-temperature (-40 °C) sodium-ion batteries

A functional Na₃V₂(PO₄)₂O₂F (NVPOF) cathode with a multi-component (Na₃V(PO₄)₂, V₂O₃, and reduced graphene oxide) surface coating is developed via a facile hydrothermal reaction followed by calcination, and exhibits high reversible capability, and long-term cycling stability even at a low temperature of -40 °C. It is demonstrated that the multi-component-coating layer can significantly accelerate the e^-/Na⁺ transport and reduce the interfacial resistance at low temperature. This work provides a novel strategy to boost the kinetics and stability of electrode materials for low-temperature sodium ion batteries.

Phosphorus–carbon covalent bond induced kinetics modulation of vanadium diphosphide for room- and high-temperature sodium-ion batteries

The electrochemical kinetics of vanadium diphosphide is regulated by the phosphorus–carbon covalent bond to boost the sodium storage performance.

3D Carbon Networks: Design and Applications in Sodium Ion Batteries

As the key component of a new generation for low-cost energy storage systems, sodium-ion batteries (SIBs) have attracted enormous attention and research due to its promising potentiality in large-scale electrochemical energy storage. For practical application of SIBs, carbonaceous materials have been considered to be one of the best choices for electrodes in virtue of their abundant reserves, low cost, easy availability, and environmental friendliness. 3D carbon network (3D-carbon) is of particular interests, which has displayed outstanding features, including abundant active sites, interconnected multi-level pore structures, high electronic conductivity, and excellent mechanical stability. Herein, we review the structural advantages of 3D-carbon and its preparation methods, and then discuss recent progress in 3D carbon materials and their composites for SIBs. The superior functionalities of 3D-carbon are emphasized as support templates or encapsulation shell membranes. Finally, we summarize and outline the challenges and future prospects of 3D-carbon in SIBs.

Current Collector-Free Reduced Graphene Oxide Aerogel Cathode for High Energy Density Dual-Ion Batteries

As an important indicator to evaluate battery performance, the energy density of commercial lithium ion batteries is close to the limit. Herein, for the first time, we reported that the reduced oxide graphene (rGO) aerogel can be directly used for the cathode of a dual-ion battery to achieve a superior specific energy density by reducing the dead weight of the battery. The porous structure of an rGO aerogel facilitates the penetration of the electrolyte and provides more active sites for energy storage. At the same time, the continuous graphene network benefits the electron transport. The lithium-rGO battery shows a discharge specific capacity of 94 mA h g^-1 at 1 A g^-1 and a superior specific energy density of 213 W h kg^-1 at 2141 W kg^-1. It is worth noting that this current collector-free electrode design would promote the development and application of graphene-based batteries with a high energy density.