Nanostructured Na2Ti9O19 for Hybrid Sodium-Ion Capacitors with Excellent Rate Capability

Coupling ultrafine TiO2 within pyridinic-N enriched porous carbon towards high-rate and long-life sodium ion capacitors

Coupling TiO2 within N-doped porous carbon (NPC) is essential for enhancing its Na+ storage performance. However, the role of different N configurations in NPC in improving the electrochemical performance of TiO2 is currently unknown. In this study, melamine is deliberately incorporated as a pore-forming agent in the self-assembly process of metal organic framework precursors (NH2-MIL-125(Ti)). This intentional inclusion of melamine leads to the one-pot and in-situ formation of highly active edge-N, which is vital for the development of TiO2/NPC with exceptional reactivity. Electrochemical performance characterization and density functional theory (DFT) calculation indicate that the interaction between TiO2 and pyridinic-N enriched NPC can effectively narrow the bandgap of TiO2/NPC, thereby significantly improving electron/ion transfer. Additionally, the abundant mesoporous channels, high N content and oxygen vacancies also contribute to the fast reaction kinetics of TiO2/NPC. As a result, the optimized TiO2/NPC-M, with high proportion of pyridinic-N (44.1 %) and abundant mesoporous channels (97.8 %), delivers high specific capacity of 282.1 mA h-1 at 0.05 A g-1, superior rate capability of 177.3 mA h-1 at 10 A g-1, and prominent capacity retention of 89.3 % over 5000 cycles even under ultrahigh 10 A g-1. Furthermore, the TiO2/NPC-M//AC sodium ion capacitors (SIC) device achieves a high energy density of 136.7 Wh kg-1 at 200 W kg-1. This research not only offers fresh perspectives on the production of high-performance TiO2-based anodes, but also paves the way for customizing other active materials for energy storage and beyond.

Anisotropic Amorphization and Phase Transition in Na2 Ti3 O7 Anode Caused by Electron Beam Irradiation

Na2 Ti3 O7 is considered one of the most promising anode materials for sodium ion batteries due to its superior safety, environmental friendliness, and low manufacturing cost. However, its structural stability and reaction mechanism still have not been fully explored. As the electron beam irradiation introduces a similar impact on the Na2 Ti3 O7 anode as the extraction of Na+ ions during the battery discharge process, the microstructure evolution of the materials is investigated by advanced electron microscopy techniques at the atomic scale. Anisotropic amorphization is successfully observed. Through the integrated differential phase contrast-scanning transmission electron microscopy technique and density functional theory calculation, a phase transition pathway involving a new phase, Na2 Ti24 O49 , is proposed with the reduction of Na atoms. Additionally, it is found that the amorphization is dominated by the surface energy and electron dose rate. These findings will deepen the understanding of structural stability and deintercalation mechanism of the Na2 Ti3 O7 anode, providing new insight into exploring the failure mechanism of electrode materials.

Altering the Alkaline Metal Ions in Lepidocrocite-Type Layered Titanate for Sodium-Ion Batteries

The relatively large ionic radius of the Na ion is one of the primary reasons for the slow diffusion of Na ions compared to that of Li ions in de/intercalation processes in sodium-ion batteries (SIBs). Interlayer expansion of intercalation hosts is one of the effective techniques for facilitating Na-ion diffusion. For most ionic layered compounds, interlayer expansion relies on intercalation of guest ions. It is important to investigate the role of these ions for material development of SIBs. In this study, alkali-metal ions (Li⁺, Na⁺, K⁺, and Cs⁺) with different sizes were intercalated into lepidocrocite-type layered titanate by a simple ion-exchange technique to achieve interlayer modulation and those were then evaluated as anode materials for SIBs. By controlling the intercalated alkaline ion species, basal spacings of layered titanates (LTs) in the range of 0.68 to 0.85 nm were obtained. Interestingly, the largest interlayer spacing induced by the large size of Cs did not yield the best performance, while the Na intercalated layered titanate (Na-ILT) demonstrated a superior performance with a specific capacity of 153 mAh g^-1 at a current density of 0.1 A g^-1. We found that the phenomena can be explained by the high alkaline metal ion concentration and the efficient utilization of the active sites in Na-ILT. The detailed analysis indicates that large intercalating ions like Cs can hamper sodium-ion diffusion although the interlayer spacing is large. Our work suggests that adopting an appropriate interlayer ion species is key to developing highly efficient layered electrode materials for SIBs.

A Novel Hybrid Point Defect of Oxygen Vacancy and Phosphorus Doping in TiO2 Anode for High-Performance Sodium Ion Capacitor

Although sodium ion capacitors (SICs) are considered as one of the most promising electrochemical energy storage devices (organic electrolyte batteries, aqueous batteries and supercapacitor, etc.) due to the combined merits of battery and capacitor, the slow reaction kinetics and low specific capacity of anode materials are the main challenges. Point defects including vacancies and heteroatoms doping have been widely used to improve the kinetics behavior and capacity of anode materials. However, the interaction between vacancies and heteroatoms doping have been seldomly investigated. In this study, a hybrid point defects (HPD) engineering has been proposed to synthesize TiO₂ with both oxygen vacancies (OVs) and P-dopants (TiO₂/C-HPD). In comparison with sole OVs or P-doping treatments, the synergistic effects of HPD on its electrical conductivity and sodium storage performance have been clarified through the density functional theory calculation and sodium storage characterization. As expected, the kinetics and electronic conductivity of TiO₂/C-HPD3 are significantly improved, resulting in excellent rate performance and outstanding cycle stability. Moreover, the SICs assembled from TiO₂/C-HPD3 anode and nitrogen-doped porous carbon cathode show outstanding power/energy density, ultra-long life with good capacity retention. This work provides a novel point defect engineering perspective for the development of high-performance SICs electrode materials.

Mass Balancing of Hybrid Ion Capacitor Electrodes: A Simple and Generalized Semiempirical Approach

Hybrid ion capacitors (HICs) are emerging as promising energy-storage devices exhibiting the advantages of both batteries and supercapacitors. However, the difference in the electrodes' specific capacities and rate capabilities makes it extremely challenging to achieve optimum mass balancing for a full-cell HIC device. Here, we demonstrate a method to predict well-performing mass ratios of electrodes for a Na-HIC by analyzing the capacities of anodes and cathodes as a function of the actual current densities experienced by the individual electrodes. We employ a simple design tool, a "Ragone Plot Simulator", to predict specific energy and specific power on Ragone plots and study the performance trend of devices with varying electrode mass ratios. The validation of the proposed method is done based on the experimental data obtained from several hybrid ion capacitor devices reported in the literature, which closely matches with the simulated Ragone plots. Further, we exemplify the validity of our calculations by comparing the simulated Ragone plot with that of a Na-HIC fabricated using in-house-made carbon. This unique approach presents a simple, generalized, yet never reported, method, which could be employed as a design tool to guide the selection of optimized HIC devices for the intended applications.

New Diglyme-based Gel Polymer Electrolytes for Na-based Energy Storage Devices

This work presents for the first time a new diglyme-based gel polymer (DOBn-GPE) suitable for Na-based energy storage devices. The DOBn-GPE, which contains a methacrylate-based polymer, exhibited an excellent high ionic conductivity (2.3 mS cm-1 at 20 °C), broad electrochemical stability (>5.0 V), and high mechanical stability. DOBn-GPE could be successfully used for the realization of Na-ion capacitors, sodium-metal batteries, and sodium-ion batteries, displaying performance comparable with those of systems containing liquid electrolytes at room temperature and at 60 °C. The results of these investigation indicated that the development of diglyme-based gel polymer electrolytes represents a promising strategy for the realization of advanced Na-based energy storage devices.

Designing Uniformly Layered FeTiO3 Assemblies Consisting of Fine Nanoparticles Enabling High-Performance Quasi-Solid-State Sodium-Ion Capacitors

Sodium-ion capacitors (NICs) that have integrated the dual advantages of the high output of supercapacitors and the high energy density of batteries have stimulated growing attention for the next generation of practical electrochemical energy storage devices. The last years have seen the unprecedentedly rapid emergence of ilmenite materials, which present great promise in the realm of energy storage. However, NICs based on ilmenite materials have been scarcely researched so far. Instead, most of the current devices explored applied flammable liquid electrolytes, leading to a concern about unexpected leakage and potential safety problems. Herein, a quasi-solid-state NIC is constructed by employing the prepared uniformly layered FeTiO₃ assemblies consisting of fine nanoparticles as anode and sodium ion conducting gel polymer as electrolyte. The resulting device delivers a high-energy-high-power density (79.8 Wh kg⁻¹, 6,750 W kg⁻¹), putting it among the state-of-the-art NICs. Furthermore, the assembled quasi-solid-state device also manifests long-term cycling stability over 2,000 cycles with a capacity retention ~80%. The uniformly layered FeTiO₃ has great potential in developing low-cost and high-performance electrodes for the next generation of sodium and other metal ions-based energy storage devices.

Sodium-ion capacitors: Materials, Mechanism, and Challenges

Sodium-ion capacitors (SICs), designed to attain high energy density, rapid energy delivery, and long lifespan, have attracted much attention because of their comparable performance to lithium-ion capacitors (LICs), alongside abundant sodium resources. Conventional SIC design is based on battery-like anodes and capacitive cathodes, in which the battery-like anode materials involve various reactions, such as insertion, alloying, and conversion reactions, and the capacitive cathode materials usually depend on activated carbon (AC). However, researchers have attempted to construct SICs based on battery-like cathodes and capacitive anodes or a combination of both in recent years. In this Minireview, charge storage mechanisms and material design strategies for SICs are summarized, with a focus on the battery-like anode materials from both inorganic and organic sources. Additionally, the challenges in the fabrication of SICs and future research directions are discussed.

Advanced Materials for Sodium-Ion Capacitors with Superior Energy-Power Properties: Progress and Perspectives

Developing electrochemical energy storage devices with high energy-power densities, long cycling life, as well as low cost is of great significance. Sodium-ion capacitors (NICs), with Na⁺ as carriers, are composed of a high capacity battery-type electrode and a high rate capacitive electrode. However, unlike their lithium-ion analogues, the research on NICs is still in its infancy. Rational material designs still need to be developed to meet the increasing requirements for NICs with superior energy-power performance and low cost. In the past few years, various materials have been explored to develop NICs with the merits of superior electrochemical performance, low cost, good stability, and environmental friendliness. Here, the material design strategies for sodium-ion capacitors are summarized, with focus on cathode materials, anode materials, and electrolytes. The challenges and opportunities ahead for the future research on materials for NICs are also proposed.

Electrospun Nb2O5 nanorods/microporous multichannel carbon nanofiber film anode for Na+ ion capacitors with good performance

For the disadvantages of both the slow reaction kinetics and the poor conductivity for Nb₂O₅ electrode materials as sodium-ion capacitors (SICs), Nb₂O₅ NRs/NMMCNF film electrode with good flexibility and high electrochemical property has been fabricated by electrospinning PAN/PMMA/H₂Nb₂O₆·H₂O homogeneous viscous suspension and followed by an annealing treatment, in which the precursor H₂Nb₂O₆·H₂O nanorods are obtained by grinding H₂Nb₂O₆·H₂O nanowires, and Nb₂O₅ nanorods are uniformly embedded in nitrogen doped microporous multichannel carbon nanofiber. Benefiting from the multichannel network structure, Nb₂O₅ NRs/NMMCNF film electrode delivers the fast kinetics of Na⁺-storage and the superior Na-ion storage performance, it delivers outstanding rate capability (101 mAh g^-1 at 4 A g^-1) and ultralong lifespan (91% capacity retention after 10,000 cycles at 2 A g^-1). A Nb₂O₅ NRs/NMMCNF//AC SIC based on the Nb₂O₅ NRs@NMMCNF fiber film anode and the AC cathode is assembled. The energy density of the as-assembled device is as high as 91 Wh kg^-1 and its maximum power density is 7499 W kg^-1. This work offers a new structure design strategy toward intercalation-type metal oxide electrodes for application in SICs.

High-Energy-Density Sodium-Ion Hybrid Capacitors Enabled by Interface-Engineered Hierarchical TiO2 Nanosheet Anodes

Sodium-ion hybrid capacitors are known for their high power densities and superior cycle life compared to Na-ion batteries. However, low energy densities (<100 Wh kg^-1) due to the lack of high-capacity (>150 mAh g^-1) anodes capable of fast charging are delaying their practical implementation. Herein, we report a high-performance Na-ion hybrid capacitor based on an interface-engineered hierarchical TiO₂ nanosheet anode consisting of bronze (∼15%) and anatase (∼85%) crystallites (∼10 nm). This pseudocapacitive dual-phase anode demonstrated exceptional specific capacity of 289 mAh g^-1 at 0.025 A g^-1 and excellent rate capability (110 mAh g^-1 at 1.0 A g^-1). The Na-ion hybrid capacitor integrating a dual-phase hierarchical TiO₂ nanosheet anode and an activated carbon cathode exhibited a high energy density of 200 Wh kg^-1 (based on the total mass of active materials in both electrodes) and power density of 6191 W kg^-1. These values are in the energy and power density range of Li-ion batteries (100-300 Wh kg^-1) and supercapacitors (5000-15 000 W kg^-1), respectively. Furthermore, exceptional capacity retention of 80% is observed after 5000 charge-discharge cycles. Outstanding electrochemical performance of the demonstrated Na-ion hybrid capacitor is credited to the enhanced pseudocapacitive Na-ion intercalation of the two-dimensional TiO₂ anode resulting from nanointerfaces between bronze and anatase crystallites. Mechanistic investigations evidenced Na-ion storage through intercalation pseudocapacitance with minimal structural changes. This approach of nanointerface-induced pseudocapacitance presents great opportunities toward developing advanced electrode materials for next-generation Na-ion hybrid capacitors.

The Roles of the Structure and Basic Sites of Sodium Titanates on Transesterification Reactions to Obtain Biodiesel

Sodium titanates were evaluated as heterogeneous catalysts for biodiesel production. Materials were prepared using an experimental design considering NaOH and TiO2 concentrations and hydrothermal and calcination temperatures as input variables. Materials characterization was carried out by DRX-Rietveld refinement, CO2-TPD, and XPS. Statistical analysis of the experimental results indicates that the calcination temperature is the most influential factor in the formation of sodium titanates with high catalytic performance in transesterification reactions. Further analysis of the oil-to-biodiesel conversion revealed that the catalytic activity of sodium titanates is directly correlated to the catalyst associated species and to the density of medium-strong basic sites on the surface of the material, obtaining up to 95% conversion to biodiesel at 60 °C using 3.6% weight catalyst with respect to oil.

SnS Nanosheets Confined Growth by S and N Codoped Graphene with Enhanced Pseudocapacitance for Sodium-Ion Capacitors

Layered tin monosulfide (SnS) is a promising anode material for sodium-ion batteries because of its high theoretical capacity of 1020 mA h g^-1. Its large interlayer spacing permits fast sodium-ion transport, making it a viable candidate for sodium-ion capacitors (SICs). In this work, we designed and synthesized oriented SnS nanosheets confined in graphene in the presence of poly(diallyl dimethyl ammonium chloride) by electrostatic self-assembly during hydrothermal growth. SnS nanosheets growing along (l00) and (0l0) directions are suppressed because of the confinement by graphene, which exhibit smaller thickness and particle size. These nanostructures expose abundant open edges because of the presence of Sn⁴⁺-O, which offers rich active sites and Na⁺ easy transport pathways. Vacancies formed at these edges along with S and N codopants in the graphitic structure synergistically promoted Na⁺ surface adsorption/desorption. Such nanocomposites with SnS nanosheets confined by N,S codoped graphene demonstrated significantly enhanced pseudocapacitance. The SICs delivered excellent energy densities of 113 and 54 W h kg^-1 at power densities of 101 and 11 100 W kg^-1, respectively, with 76% capacity retention after 2000 cycles at 1 A g^-1.

MoS2-Coupled Carbon Nanosheets Encapsulated on Sodium Titanate Nanowires as Super-Durable Anode Material for Sodium-Ion Batteries

There is an ever-increasing demand for rechargeable batteries with fast charging, long cycling, high safety, and low cost in new energy storage systems. Herein, a heterogeneous architecture composed of MoS2-coupled carbon nanosheets encapsulated on sodium titanate nanowires is developed and demonstrated as an advanced anode for sodium-ion batteries (SIBs). Owing to the synergistic effects of ultrastable substrate of 1D sodium titanate (NTO) nanowires, high-capacity promoter of 2D MoS2 nanosheets as well as the 2D conductive carbon matrix, the resulting 1D/2D-2D hybrid demonstrates excellent high-rate capacity and super-durable cyclability, delivering a stable capacity of up to 425.5 mAh g-1 at 200 mA g-1. Even at an ultrafast charging/discharging process within 80 s, the capacity can be maintained at 201 mAh g-1 after 16 000 cycles with only 0.0012% capacity loss per cycle, one of the best high-rate capacities and cyclabilities for NTO-based hybrid composites. The present work highlights the designing protocol of hierarchical nanoarchitectures with stable substrate and high-capacity electrodes for next-generation energy storage applications.

Mesopore-Induced Ultrafast Na+ -Storage in T-Nb2 O5 /Carbon Nanofiber Films toward Flexible High-Power Na-Ion Capacitors

Hybrid Na-ion capacitors (NICs) are receiving considerable interest because they combine the merits of both batteries and supercapacitors and because of the low-cost of sodium resources. However, further large-scale deployment of NICs is impeded by the sluggish diffusion of Na⁺ in the anode. To achieve rapid redox kinetics, herein the controlled fabrication of mesoporous orthorhombic-Nb₂ O₅ (T-Nb₂ O₅ )/carbon nanofiber (CNF) networks is demonstrated via in situ SiO₂ -etching. The as-obtained mesoporous T-Nb₂ O₅ (m-Nb₂ O₅ )/CNF membranes are mechanically flexible without using any additives, binders, or current collectors. The in situ formed mesopores can efficiently increase Na⁺ -storage performances of the m-Nb₂ O₅ /CNF electrode, such as excellent rate capability (up to 150 C) and outstanding cyclability (94% retention after 10 000 cycles at 100 C). A flexible NIC device based on the m-Nb₂ O₅ /CNF anode and the graphene framework (GF)/mesoporous carbon nanofiber (mCNF) cathode, is further constructed, and delivers an ultrahigh power density of 60 kW kg^-1 at 55 Wh kg^-1 (based on the total weight of m-Nb₂ O₅ /CNF and GF/mCNF). More importantly, owing to the free-standing flexible electrode configuration, the m-Nb₂ O₅ /CNF//GF/mCNF NIC exhibits high volumetric energy and power densities (11.2 mWh cm^-3 , 5.4 W cm^-3 ) based on the full device, which holds great promise in a wide variety of flexible electronics.