Sodium-ion storage technologies are promising candidates for large-scale grid systems due to the abundance and low cost of sodium. However, compared to well-understood lithium-ion storage mechanisms, sodium-ion storage remains relatively unexplored. Herein, we systematically determine the sodium-ion storage properties of anatase titanium dioxide (TiO 2 (A)). During the initial sodiation process, a thin surface layer (~3 to 5 nm) of crystalline TiO 2 (A) becomes amorphous but still undergoes Ti 4+ /Ti 3+ redox reactions. A model explaining the role of the amorphous layer and the dependence of the specific capacity on the size of TiO 2 (A) nanoparticles is proposed. Amorphous nanoparticles of ~10 nm seem to be optimum in terms of achieving high specific capacity, on the order of 200 mAh g −1 , at high charge/discharge rates. Kinetic studies of TiO 2 (A) nanoparticles indicate that sodium-ion storage is due to a surface-redox mechanism that is not dependent on nanoparticle size in contrast to the lithiation of TiO 2 (A) which is a diffusion-limited intercalation process. The surface-redox properties of TiO 2 (A) result in excellent rate capability, cycling stability and low overpotentials. Moreover, tailoring the surface-redox mechanism enables thick electrodes of TiO 2 (A) to retain high rate properties, and represents a promising direction for high-power sodium-ion storage.

Pseudocapacitive materials with surface‐redox reactions are capable of realizing high capacities at ultrahigh rates; however, it remains a challenge in the synthesis of active components with high surface area to boost surface‐redox sodiation but restrain side reactions. Herein, a two‐step, topochemical synthesis of 2D mesoporous TiN (2D‐meso‐TiN) with high surface area and rich mesoporosities is presented. It is demonstrated that the sodium‐ion storage mechanism of TiN anode is based on the existence of surficial titanium oxides via redox reactions between Ti4+ and Ti3+. The interconnected, highly conductive 2D‐meso‐TiN with high surface area largely increases the pseudocapacitive capacities, leading to a high capacity of 160/93 mAh g−1 at 0.1/10 A g−1, which is much higher than 2D‐TiN (120/72 mAh g−1) and commercial TiN nanoparticles (57/30 mAh g−1). The surface‐redox (de)sodiation undergoes no destruction of crystalline TiN, which enables high initial coulombic efficiency and long‐term cycles. Furthermore, a novel hybrid sodium‐ion capacitor consisting of 2D‐meso‐TiN anode and Na3V2(PO4)3 cathode is assembled without any presodiation treatments. The hybrid capacitor delivers both high energy density (94 Wh kg−1 at 64 W kg−1) and high power density (38 Wh kg−1 at 4.4 kW kg−1), as well as long cycling stability.

Electrochemical capacitors have faced the limitations of low energy density for decades, owing to the low capacity of electric double-layer capacitance (EDLC)-type positive electrodes. In this work, we reveal the functions of interlayer confined water in iron vanadate (FeV3O8.7nH(2)O) for sodium-ion storage in nonaqueous electrolyte. Using an electrochemical quartz crystal microbalance, in situ Raman, and ex situ X-ray diffraction and X-ray photoelectron spectroscopy, we demonstrate that both nonfaradaic (surficial EDLC) and faradaic (pseudocapacitance-dominated Na+ intercalation) processes are involved in the charge storages. The interlayer confined water is able to accelerate the fast Na+ intercalations and is highly stable (without the removal of water or co-intercalation of [Na-diglyme](+)) in the nonaqueous environment. Furthermore, coupling the pseudocapacitive FeV3O8.7nH(2)O with EDLC-type activated carbon (FeVO-AC) as the positive electrode brings comprehensive enhancements, displaying the enlarged compaction density of similar to 2 times, specific capacity of similar to 1.5 times, and volumetric capacity of similar to 3 times compared to the AC electrode. Furthermore, the as-assembled hybrid sodium-ion capacitor, consisting of an FeVO-AC positive electrode and a mesocarbon microbeads negative electrode, shows a high energy density of 108 Wh kg(-1) at 108 W kg(-1) and 15.3 Wh kg(-1) at 8.3 kW kg(-1). Our results offer an emerging route for improving both specific and volumetric energy densities of electrochemical capacitors.

Sodium-ion batteries (SIBs) are the promising candidate in grid systems owing to the wide distribution and abundance of sodium resources. However, the charge storage mechanism and size-effect of the active materials for SIBs remain largely unexplored. Herein, we synchronously investigate the electrochemical properties and structural evolutions of Li4Ti5O12 (LTO) anode for Li+ and Na+ storage, along with the nano-size effect on their charge storage behaviors. Through detailed kinetic analysis, it is found the Li+ storage in LTO is diffusion-controlled in bulk and shows increased surface-controlled contributions when the size is reduced to 18 nm. In comparison, for the Na+ storage, the domination step is surface-controlled in all the sizes of LTO from 260 to 18 nm, with over 55% contribution even at the low sweep rate of 0.1 mV s(-1). The limited near-surface reaction region and low diffusion kinetics determine the surface-controlled Na+ storage behaviors and the specific ca-pacities. The suitable size of LTO-18 nm anode displays a high capacity of-140 mAh g(-1) (at 0.05 A g(-1),-0.29C), high-rate capability (42 mAh g(-1) at similar to 57C), and long-term cycling stability (over 1200 cycles). Our finding provides insights into a precise design of nanomaterials for high-power sodium-ion storage devices.

Pseudocapacitance shows emerging properties to achieve both high capacity and high-rate performance simultaneously, including lithium ion and sodium ion storage systems. Herein, this work investigated the insightful mechanisms of pseudocapacitive lithium ion and sodium ion storage of anatase titanium dioxide anode. A transient two-dimensional continuum electrochemical modeling was performed for electrodes consisting of a single TiO2 nanoparticle immersed in the electrolyte. The process of intrinsic surface-redox pseudocapacitance showed a uniform ion concentration across the electrochemical-active amorphous layer without hysteresis. By contrast, typical hysteresis and concentration gradients in the electrode were observed in the process of extrinsic intercalation pseudocapacitance, which enlarged with the increase in nanoparticle size or scan rate. The results indicated that sodium ion storage in TiO2 was a surface-redox pseudocapacitive reaction. Design guidelines for sodium ion storage in TiO2 electrodes was proposed by dimensional analysis. The dimensionless capacity for sodium ion storage remained nearly constant when the dimensionless scan rate was less than a critical value. This provided the guidance for the design and synthesis of the TiO electrode to enhance sodium ion storage by balancing nanoparticle size, thickness of NaxTiO2 surface layer, and effective diffusion coefficient of the electrode. Furthermore, such surface-redox pseudocapacitive sodium ion storage model could be extended to analyze other electrode materials.

Abstract Sodium-ion storage technologies are promising candidates for large-scale grid systems owing to the abundance and low cost of sodium. However, compared to well-understood lithium-ion storage mechanisms, sodium-ion storage remains relatively unexplored. Herein, we systematically determine the sodium-ion storage properties of anatase titanium dioxide (TiO2(A)). During the initial sodiation process, a thin surface layer (~3 to 5 nm) of crystalline TiO2(A) becomes amorphous but still undergoes Ti4+/Ti3+ redox reactions. A model explaining the role of the amorphous layer and the dependence of the specific capacity on the size of TiO2(A) nanoparticles is proposed. Amorphous nanoparticles of ~10 nm seem to be optimum in terms of achieving high specific capacity, on the order of 200 mAh g-1, at high charge/discharge rates. Kinetic studies of TiO2(A) nanoparticles indicate that sodium-ion storage is due to a surface-redox, capacitor-like reaction mechanism that is not dependent on nanoparticle size in contrast to the lithiation of TiO2(A) which is a diffusion-limited intercalation process. The surface-redox properties of TiO2(A) result in excellent rate capability, cycling stability and low overpotentials. Moreover, the surface-redox mechanism is instrumental in enabling thick electrodes of TiO2(A) retain high rate properties, and represent a promising direction for high-power sodium-ion storage.

Sodium-ion storage technology holds great promise for large-scale, sustainable, and low-cost green energy storage systems. Overcoming the main limitations of sluggish Na+ diffusion kinetics and achieving high-power sodium-ion storage devices is critical but full of challenges. Recently, emerging pseudocapacitive sodium-ion storage materials are attracting attention due to their superior rate capability and other comprehensive electrochemical performance. Herein, this review covers the charge storage mechanism, features, and performance of pseudocapacitive sodium-ion storage anode materials and the assembled sodium-ion capacitors. The research processes of state-of-the-art pseudocapacitive sodium-ion storage anode materials, which display remarkable rate performance, high specific capacity and cycling stability, are introduced and discussed. Using the pseudocapacitive anode materials, advanced sodium-ion capacitors deliver high energy density at high power density. In the end, the outlooks of pseudocapacitive sodium-ion storage materials toward future high-power sodium-ion capacitors are briefly proposed.

Lightweight 3D-SiC(rGO, Gx) PDCs were fabricated from polycarbosilane-vinyltriethoxysilane-graphene oxide (PCS-VTES-GO) precursor added by different amounts of graphene fillers via direct cold molding and pyrolysis at 1400 degrees C in an easy manner. Results reveal that SiC(rGO, Gx) PDCs consist of beta-SiC nanocrystals homogeneously embedded within amorphous SiOxCy/C-free and graphene is well compatible with SiOxCy/C-free for void-free bonded interface, efficiently delaying decomposition of SiOxCy phase into beta-SiC. The nanocomposite structure provides an ingenious strategy for constructing complexes with good integrity, high ceramic yield, excellent thermal stability, high electrical and thermal conductivities. This improvement is primarily attributed to the presence of graphene with considerably increasing electric-charge carriers and wider phonon-channel. Such 3D-SiC(rGO, G(20%)) PDCs possess satisfying hardness (12.02 GPa), high electrical conductivity (23.82 S cm(-1)) and thermal conductivity (7.47 W m(-1)K(-1)), which make them attractive candidates for microelectromechanical systems (MEMS) devices, energy storage/conversion systems and high precision components, etc.