Caging Na3V2(PO4)2F3 Microcubes in Cross-Linked Graphene Enabling Ultrafast Sodium Storage and Long-Term Cycling

Intercalation Pseudocapacitive Zn2+ Storage with Hydrated Vanadium Dioxide toward Ultrahigh Rate Performance

The weak van der Waals interactions enable ion-intercalation-type hosts to be ideal pseudocapacitive materials for energy storage. Here, a methodology for the preparation of hydrated vanadium dioxide nanoribbon (HVO) with moderate transport pathways is proposed. Out of the ordinary, the intercalation pseudocapacitive reaction mechanism is discovered for HVO, which powers high-rate capacitive charge storage compared with the battery-type intercalation reaction. The main factor is that the defective crystalline structure provides suitable ambient spacing for rapidly accommodating and transporting cations. As a result, the HVO delivers a fast Zn²⁺ ion diffusion coefficient and a low Zn²⁺ diffusion barrier. The electrochemical results with intercalation pseudocapacitance demonstrate a high reversible capacity of 396 mAh g^-1 at 0.05 A g^-1 , and even maintain 88 mAh g^-1 at a high current density of 50 A g^-1 .

Facilitating Phase Evolution for a High-Energy-Efficiency, Low-Cost O3-Type NaxCu0.18Fe0.3Mn0.52O2 Sodium Ion Battery Cathode

The phase transition and lattice parameter evolution of O3 structure commonly occurs in O3-type sodium ion battery (SIB) cathodes, which might enlarge the voltage hysteresis and lower the energy efficiency. Given that the cost is one of the issues discouraging the application of SIBs in large-scale energy storage, here we focus on Co/Ni-free Na_xCu_0.18Fe_0.3Mn_0.52O₂ (x = 0.8, 0.85, 0.9) and propose a convenient strategy to reduce the voltage hysteresis. It is found that when the Na content is 0.8, the highest energy efficiency of 95.4% after activation is achieved (2.5-4.0 V, 50 mA g^-1, the 50th cycle), in addition to a satisfactory capacity retention (about 0.056% decay per cycle). The further characterizations reveal that Na_0.8Cu_0.18Fe_0.3Mn_0.52O₂ owns a gentle O3-P3 phase transition process and does not undergo O3 phase lattice parameter evolution. The key point lies in the attainability of the O3/P3 composite of the material. This work will provide a simple strategy for the rational design of O3-type cathodes with a high energy efficiency and might offer inspiration to search for layered oxides with a higher O3/P3 critical Na content through element adjustments.

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

Although sodium vanadium fluorophosphate, Na₃(VO_1-xPO₄)₂F_1+2x (0 ≤ x ≤ 1), is a highly promising cathode candidate for sodium-ion batteries because of its stable structure and high working voltage, the low charge diffusion dynamics and the inactive materials used in traditional coating electrodes reduce the energy density of a sodium-ion full battery. Hence, Na₃V₂O₂(PO₄)₂F/graphene aerogels (NVPF/GAs) with a three-dimensional continuous porous network are first prepared by coassembly and freeze-drying. The three-dimensional porous network helps to obtain a high NVPF content of 81 wt %, relieve the volume change for improving the cyclability, and enhance the wettability of the electrode with the electrolyte for accelerating the diffusion dynamics of sodium ions and electrons. As a directly used freestanding cathode without the use of any binder/collector, an optimized freestanding NVPF/GA electrode exhibits excellent cycling and rate performances compared to traditional coating electrodes. The average capacities at current densities of 0.2, 0.5, 1.0, 2.0, and 5.0 C are 135.4, 128.0, 125.1, 121.9, and 115.1 mA h g^-1, respectively. Especially, it maintains a capacity retention of 100% after 1000 cycles at an ultrahigh current of 40 C. A sodium-ion full battery with the NVPF/GA cathode and the Sb/graphene/carbon anode attains a of 82.1 mA h g^-1 without an obvious decline after 100 cycles.

Superior Na-Storage Properties of Nickel-Substituted Na2FeSiO4@C Microspheres Encapsulated with the In Situ-Synthesized Alveolation-like Carbon Matrix

The poor electronic conductivity of Na₂FeSiO₄ always limits its electrochemical reactivities and no effective solution has been found to date. Herein, the novel Ni-substituted Na₂Fe_1-xNi_xSiO₄@C nanospheres (50-100 nm) encapsulated with a 3D hierarchical porous skeleton (named as alveolation-like configuration) constructed using in situ carbon are first synthesized via a facile sol-gel method, and the effects of Ni substitution combined with the design of a unique carbon network on Na-storage properties are assessed systematically, focusing on alleviating the inherent defects of the Na₂FeSiO₄ cathode material. A series of characterization technologies such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and so forth, coupled with the electrochemical measurements and first-principles calculations, are used to explore the structure, morphology and electrochemical behaviors of the as-prepared materials. The results show that the synergism between Ni substitution and the special alveolation-like configuration enables fast Na ions mobility (from 10^-14 to 10^-12 cm² s^-1), reduces band gap energy (from 2.82 to 1.79 eV) and lowers Na-ion diffusion barriers, finally reciprocating the vigorous electrochemical kinetics of the electrode. Especially, the elaborately designed material-Na₂Fe_0.97Ni_0.03SiO₄@C-displays superior Na-storage properties of around 197.51 mA h g^-1 (corresponding to 1.43 Na⁺ intercalation) at 0.1 C within 1.5-4.5 V along with desirable capacity retention (84.44% after 100 cycles), and the rate capability is also markedly enhanced (a capacity of 133.62 mA h g^-1 at 2 C). Such the effective methodology employed in this work opens a potential pathway to synthesize the silicate cathode material with excellent electrochemical properties.

Three-Dimensional Topotactic Host Structure-Secured Ultrastable VP-CNO Composite Anodes for Long Lifespan Lithium- and Sodium-Ion Capacitors

Performance degradation of lithium/sodium-ion capacitors (LICs/SICs) mainly originates from anode pulverization, particularly the alloying and conversion types, and has spurred research for alternatives with an insertion mechanism. Three-dimensional (3D) topotactic host materials remain much unexplored compared to two-dimensional (2D) ones (graphite, etc.). Herein, vanadium monophosphide (VP) is designed as a 3D topotactic host anode. Ex situ electrochemical characterizations reveal that there are no phase changes during (de)intercalation, which follows the topotactic intercalation mechanism. Computational simulations also confirm the metallic feature and topotactic structure of VP with a spacious interstitial position for the accommodation of guest species. To boost the electrochemical performance, carbon nano-onions (CNOs) are coupled with 3D VP. Superior specific capacity and rate capability of VP-CNOs vs lithium/sodium can be delivered due to the fast ion diffusion nature. An exceptional capacity retention of above 86% is maintained after 20 000 cycles, benefitting from the topotactic intercalation process. The optimized LICs/SICs exhibit high energy/power densities and an ultrastable lifespan of 20 000 cycles, which outperform most of the state-of-the-art LICs and SICs, demonstrating the potential of VP-CNOs as insertion anodes. This exploration would draw attention with regard to insertion anodes with 3D topotactic host topology and provide new insights into anode selection for LICs/SICs.

Carbon-coating-increased working voltage and energy density towards an advanced Na3V2(PO4)2F3@C cathode in sodium-ion batteries

One main challenge for phosphate cathodes in sodium-ion batteries (SIBs) is to increase the working voltage and energy density to promote its practicability. Herein, an advanced Na₃V₂(PO₄)₂F₃@C cathode is prepared successfully for sodium-ion full cells. It is revealed that, carbon coating can not only enhance the electronic conductivity and electrode kinetics of Na₃V₂(PO₄)₂F₃@C and inhibit the growth of particles (i.e., shorten the Na⁺-migration path), but also unexpectedly for the first time adjust the dis-/charging plateaux at different voltage ranges to increase the mean voltage (from 3.59 to 3.71 V) and energy density (from 336.0 to 428.5 Wh kg^-1) of phosphate cathode material. As a result, when used as cathode for SIBs, the prepared Na₃V₂(PO₄)₂F₃@C delivers much improved electrochemical properties in terms of larger specifc capacity (115.9 vs. 93.5 mAh g^-1), more outstanding high-rate capability (e.g., 87.3 vs. 60.5 mAh g^-1 at 10 C), higher energy density, and better cycling performance, compared to pristine Na₃V₂(PO₄)₂F₃. Reasons for the enhanced electrochemical properties include ionicity enhancement of lattice induced by carbon coating, improved electrode kinetics and electronic conductivity, and high stability of lattice, which is elucidated clearly through the contrastive characterization and electrochemical studies. Moreover, excellent energy-storage performance in sodium-ion full cells further demonstrate the extremely high possibility of Na₃V₂(PO₄)₂F₃@C cathode for practical applications.

Exploration of Energy Storage Materials for Water Desalination via Next-Generation Capacitive Deionization

Clean energy and environmental protection are critical to the sustainable development of human society. The numerous emerged electrode materials for energy storage devices offer opportunities for the development of capacitive deionization (CDI), which is considered as a promising water treatment technology with advantages of low cost, high energy efficiency, and wide application. Conventional CDI based on porous carbon electrode has low salt removal capacity which limits its application in high salinity brine. Recently, the faradaic electrode materials inspired by the researches of sodium-batteries appear to be attractive candidates for next-generation CDI which capture ions by the intercalation or redox reactions in the bulk of electrode. In this mini review, we summarize the recent advances in the development of various faradaic materials as CDI electrodes with the discussion of possible strategies to address the problems present.

One-Step Electrochemically Prepared Graphene/Polyaniline Conductive Filter Membrane for Permeation Enhancement by Fouling Mitigation

In the electrofiltration process, membrane conductivity plays a decisive role in improving the antifouling performance of the membrane. In this paper, combining the preparation of graphene (Gr) with the fabrication of the Gr layer on the surface of a polyaniline (PANI) membrane, a graphene/PANI (Gr/PANI) conductive membrane was prepared creatively by the one-step electrochemical method. The properties of the as-prepared Gr/PANI membrane were studied systematically. By the tests of Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and atomic force microscopy, it was confirmed that Gr was successfully produced and was combined with the PANI membrane well. Field scanning electron microscopy with energy-dispersive X-ray analysis further confirmed that the top surface and the upper layer pore walls of the membrane were randomly covered by Gr. The antifouling performance of the prepared membrane was evaluated by studying the permeation flux of the yeast suspension, compared with the ones with no electric field: the total permeation flux at 1 V direct current (dc) increased by 109%; besides, under 1 V dc, the average flux of the Gr/PANI membrane was approximately 1.4 times that of the PANI membrane. This approach may provide a promising strategy for the combination of Gr with conductive polymers to produce separation membranes.

Hydrogel-derived VPO4/porous carbon framework for enhanced lithium and sodium storage

Vanadium phosphate (VPO₄) is attracting extensive attention because of its advantages of low cost, stable structure and high theoretical capacity. However, similar to other phosphates, VPO₄ suffers from low electrical conductivity and large volume expansion, adversely influencing its electrochemical performance and thus limiting its application as an anode in lithium and sodium ion batteries. Herein, we propose a novel, facile strategy based on the organic-inorganic network of a nanostructured hybrid hydrogel for immobilizing VPO₄ in a hierarchically porous carbon framework (3DHP-VPO₄@C). VPO₄ chemically interacts with the carbon framework via a P-C bond, functioning as a buffer layer to maintain structural stability during charge/discharge cycles. The carbon framework offers an efficient pathway for electron and Li⁺/Na⁺ transport to ensure high electronic conductivity of the electrode. The 3DHP-VPO₄@C anode exhibits excellent lithium and sodium storage performances, and notably high capacities of 957 mA h g^-1 at 0.1 A g^-1 and 345.3 mA h g^-1 at 5 A g^-1 for lithium ion batteries. Full cells consisting of a LiFePO₄ cathode and the 3DHP-VPO₄@C anode also prove to have superior cycling stability and rate performance for LIBs.

Hierarchical hollow microspheres Na3V2(PO4)2F3C@rGO as high-performance cathode materials for sodium ion batteries

Nanosheet-assembled Na₃V₂(PO₄)₂F₃ microspheres were synthesized by a one-step polyol-assisted hydrothermal method and combined with graphene as the cathode material for SIBs.

Enhancing Na-ion storage in Na3V2(PO4)3/C cathodes for sodium ion batteries through Br and N co-doping

Br and N co-doping can enhance the electronic/ionic conductivities of carbon-coating layers and lower the activation energy of V⁴⁺/V⁵⁺ redox reactions.

Facile One-Step Hydrothermal Synthesis of Na3V2(PO4)2F3@C/CNTs Tetragonal Micro-Particles as High Performance Cathode Material for Na-Ion Batteries

In this paper, we report a facile one-step hydrothermal method to synthesize tetragonal Na₃V₂(PO₄)₂F₃@C particles which are connected by carbon nanotubes (CNTs) networks, using water as hydrothermal solvents. In this strategy, the reduction and crystallization of materials are carried out in the hydrothermal process (180°C, 12 h), no additional heat treatment is required. The well-crystallized Na₃V₂(PO₄)₂F₃ tetragonal grains (5–10 μm) are coated with amorphous nano-carbon and connected by highly conductive CNTs. The addition of CNTs can not only improve the conductivity of materials but also effectively inhibit the Na₃V₂(PO₄)₂F₃ grains over growth. The Na₃V₂(PO₄)₂F₃@C/CNTs composite possesses very flat charge/discharge platforms of 3.6 and 4.1 V. The sample exhibits an initial discharge specific capacity of 120.2 and 74.3 mAh g⁻¹ at 0.1 and 10 C rate, respectively, and shows excellent cyclical stability. The composite owns excellent electrochemical performances owing to the three-dimensional highly conductive network which is co-constructed by the CNTs and nano-carbon coating layer.

Large-Area, Uniform, Aligned Arrays of Na3 (VO)2 (PO4 )2 F on Carbon Nanofiber for Quasi-Solid-State Sodium-Ion Hybrid Capacitors

Sodium-vanadium fluorophosphate (Na₃ V₂ O_{2 x} (PO₄ )₂ F_{3-2 x} , NVPF, 0 ≤ x ≤ 1) is considered to be a promising Na-storage cathode material due to its high operation potentials (3.6-4 V) and minor volume variation (1.8%) during Na⁺ -intercalation. Research about NVPF is mainly focused on powder-type samples, while its ordered array architecture is rarely reported. In this work, large-area and uniform Na₃ (VO)₂ (PO₄ )₂ F cuboid arrays are vertically grown on carbon nanofiber (CNF) substrates for the first time. Owing to faster electron/ion transport and larger electrolyte-electrode contact area, the as-prepared NVPF array electrode exhibits much improved Na-storage properties compared to its powder counterpart. Importantly, a quasi-solid-state sodium-ion hybrid capacitor (SIHC) is constructed based on the NVPF array as an intercalative battery cathode and porous CNF as a capacitive supercapacitor anode together with the P(VDF-HFP)-based polymer electrolyte. This novel hybrid system delivers an attractive energy density of ≈227 W h kg^-1 (based on total mass of two electrodes), and still remains as high as 107 Wh kg^-1 at a high specific power of 4936 W kg^-1 , which pushes the energy output of sodium hybrid capacitors toward a new limit. In addition, the growth mechanism of NVPF arrays is investigated in detail.

A safe and non-flammable sodium metal battery based on an ionic liquid electrolyte

Rechargeable sodium metal batteries with high energy density could be important to a wide range of energy applications in modern society. The pursuit of higher energy density should ideally come with high safety, a goal difficult for electrolytes based on organic solvents. Here we report a chloroaluminate ionic liquid electrolyte comprised of aluminium chloride/1-methyl-3-ethylimidazolium chloride/sodium chloride ionic liquid spiked with two important additives, ethylaluminum dichloride and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide. This leads to the first chloroaluminate based ionic liquid electrolyte for rechargeable sodium metal battery. The obtained batteries reached voltages up to ~ 4 V, high Coulombic efficiency up to 99.9%, and high energy and power density of ~ 420 Wh kg^-1 and ~ 1766 W kg^-1, respectively. The batteries retained over 90% of the original capacity after 700 cycles, suggesting an effective approach to sodium metal batteries with high energy/high power density, long cycle life and high safety.

Toward High Power-High Energy Sodium Cathodes: A Case Study of Bicontinuous Ordered Network of 3D Porous Na3 (VO)2 (PO4 )2 F/rGO with Pseudocapacitance Effect

Developing high power-high energy electrochemical energy storage systems is an ultimate goal in the energy storage field, which is even more difficult but significant for low-cost sodium ion batteries. Here, fluoride is successfully prepared by the electrostatic spray deposition (ESD) technique, which greatly expands the application scope of ESD. A two-step strategy (solvothermal plus ESD method) is proposed to construct a bicontinuous ordered network of 3D porous Na₃ (VO)₂ (PO₄ )₂ F/reduced graphene oxide (NVOPF/rGO). This two-step strategy makes sure that NVOPF can be prepared by ESD, since it avoids the loss of F element during synthesis. The obtained NVOPF particles are as small as 15 nm, and the carbon content is only 3.5% in the final nanocomposite. Such a bicontinuous ordered network and small size of electroactive particles lead to the significant contribution of the pseudocapacitance effect to sodium storage, resulting in real high power-high energy sodium cathodes. The cathode exhibits excellent rate capability and cycling stability, whose rate performance is one of the best ever reported in both half cells and full cells. Moreover, this work provides a general and promising strategy for developing high power-high energy electrode materials for various electrochemical energy storage systems.

Precisely controlled preparation of an advanced Na3V2(PO4)2O2F cathode material for sodium ion batteries: the optimization of electrochemical properties and electrode kinetics

Electrochemical properties of the Na₃V₂(PO₄)₂O₂F cathode for sodium-ion batteries are optimized by precisely controlling the preparation parameters.

The rapid microwave-assisted hydrothermal synthesis of NASICON-structured Na3V2O2x(PO4)2F3−2x (0 < x ≤ 1) cathode materials for Na-ion batteries

NASICON-structured Na₃V₂O_2x(PO₄)₂F_3−2x (0 < x ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique. Well-crystallized phases were obtained for x = 1 and 0.4 by reacting V₂O₅, NH₄H₂PO₄, and NaF precursors at temperatures as low as 180–200 °C for less than 15 min. Various available and inexpensive reducing agents were used to control the vanadium oxidation state and final product morphology. The vanadium oxidation state and O/F ratios were assessed using electron energy loss spectroscopy and infrared spectroscopy. According to electron diffraction and powder X-ray diffraction, the Na₃V₂O_2x(PO₄)₂F_3−2x solid solutions crystallized in a metastable disordered I4/mmm structure (a = 6.38643(4) Å, c = 10.62375(8) Å for Na₃V₂O₂(PO₄)₂F and a = 6.39455(5) Å, c = 10.6988(2) Å for Na₃V₂O_0.8(PO₄)₂F_2.2). With respect to electrochemical Na⁺ (de)insertion as positive electrodes (cathodes) for Na-ion batteries, the as-synthesized materials displayed two sloping plateaus upon charge and discharge, centered near 3.5–3.6 V and 4.0–4.1 V vs. Na⁺/Na, respectively, with a reversible capacity of ∼110 mA h g⁻¹. The application of a conducting carbon coating through the surface polymerization of dopamine with subsequent annealing at 500 °C improved both the rate capability (∼55 mA h g⁻¹ at a discharge rate of 10C) and capacity retention (∼93% after 50 cycles at a discharge rate of C/2).

NASICON-structured Na₃V₂O_2x(PO₄)₂F_3−2x (0 < x ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique.