Yolk-Shell TiO2@C Nanocomposite as High-Performance Anode Material for Sodium-Ion Batteries

Morphology design and electronic configuration of MoSe2 anchored on TiO2 nanospheres for high energy density sodium-ion half/full batteries

Molybdenum selenide (MoSe2) has shown potential sodium storage properties due to its large layer spacing (0.646 nm) and high theoretical capacity and narrow band gap. However, as the anode material of sodium ion batteries (SIBs), the MoSe2's performance is not ideal, especially due to the layer agglomeration and stacking caused by volume expansion and low intrinsic conductivity. Hence, morphology design and electronic configuration of MoSe2 is proposed via building MoSe2 nanosheets and auxiliary sulfur doping on the surface of the TiO2 hollow nanosphere (S-MoSe2@TiO2). The hierarchical shaped S-MoSe2@TiO2 effectively overcomes the shortcomings of high surface energy and weak interlayer van der Waals force of MoSe2. As anode for SIBs, S-MoSe2@TiO2 delivers enhanced cycling life and rate capability (308 mAh/g at 10 A/g after 1000 cycles) with the comparison of MoSe2@TiO2 or pure MoSe2 and TiO2. Such excellent sodium storage performance is due to the fast diffusion kinetics of Na+. When it is applied in sodium ion full batteries, the S-MoSe2@TiO2 anode based cell can reach a high energy density of 187.8 W h kg-1 at 148.3 W kg-1. The design of the new MoSe2-based hybrid provides a novel scheme for the preparation of advanced anode in SIBs.

Hard-Carbon Negative Electrodes from Biomasses for Sodium-Ion Batteries

With the development of high-performance electrode materials, sodium-ion batteries have been extensively studied and could potentially be applied in various fields to replace the lithium-ion cells, owing to the low cost and natural abundance. As the key anode materials of sodium-ion batteries, hard carbons still face problems, such as poor cycling performance and low initial Coulombic efficiency. Owning to the low synthesis cost and the natural presence of heteroatoms of biomasses, biomasses have positive implications for synthesizing the hard carbons for sodium-ion batteries. This minireview mainly explains the research progress of biomasses used as the precursors to prepare the hard-carbon materials. The storage mechanism of hard carbons, comparisons of the structural properties of hard carbons prepared from different biomasses, and the influence of the preparation conditions on the electrochemical properties of hard carbons are introduced. In addition, the effect of doping atoms is also summarized to provide an in-depth understanding and guidance for the design of high-performance hard carbons for sodium-ion batteries.

Highly compacted TiO2/C micospheres via in-situ surface-confined intergrowth with ultra-long life for reversible Na-ion storage

TiO₂ as the promising anode material candidate of sodium-ion battery suffers from poor conductivity and slow ion diffusion rate, which severely hampers its development. Highly compacted TiO₂/C microspheres without inner pores/tunnels are synthesized by a very facile one-pot rapid processing method based on novel in-situ surface-confined inter-growth mechanism. This highly compacted TiO₂/C microspheres exhibit an excellent electrochemical performance of reversible Na⁺ storage despite with relatively large particle/aggregation size from submicrometer to micrometer. An outstanding cycling stability extending to 10,000 cycles is gained with a high retention capacity of 140.5 mAh g^-1 at a current rate of 2 A g^-1. An ultra-high reversible capacity of 362 mAh g^-1 close to its theoretic specific capacity is obtained at a current rate of 0.05 A g^-1. The successful combination of highly compacted structure with large particle size, excellent electrochemical performance as well as rapid cost-effective preparing process might provide a potential industrial approach for efficiently synthesizing electrode materials for Na ion batteries.

TiO2 quantum dots confined in 3D carbon framework for outstanding surface lithium storage with improved kinetics

Pseudocapacitive lithium storage is an effective way to promote the improvement of electrochemical performance for lithium ion batteries. However, the intrinsically sluggish lithium ionic diffusion and the low electronic conductivity of TiO₂ limit its capability of pseudocapacitive behavior with fast surface redox reaction. In this work, TiO₂ quantum dots confined in 3-dimensional carbon framework have been synthesized by a facile process of reverse microemulsion method combined with heat treatment. The obtained composites effectively combine electrochemical redox with surface pseudocapacitive, showing excellent electrochemical properties. An ultra-high discharge capacity of 370.5 mAh/g can be retained after 200 cycles at a current density of 0.1 A/g. Ultra-long life extends to 10,000 cycles with an average capacity loss of as low as 0.00314% per cycle can be obtained at a high current density of 5.0 A/g, due to the high pesudocapacitance contribution of fast surface redox reaction. Furthermore, the practice application of the obtained electrode is also investigated in a full cell with LiCoO₂ as the cathode and a high capacity retention of 93.5% is maintained after 100 cycles at the current density of 0.1 A/g.

Towards enhanced sodium storage of anatase TiO2via a dual-modification approach of Mo doping combined with AlF3 coating

Recent studies on anatase TiO₂ have demonstrated its capability of performing as an anode material for sodium-ion batteries (SIBs) even though, due to poor conductivity, realistic applications have not yet been foreseen. In order to try to address this issue, herein, we shall introduce a cost effective and facile route based on the co-precipitation method for the synthesis of Mo-doped anatase TiO₂ nanoparticles with AlF₃ surface coating. The electrochemical measurements demonstrate that the Mo-doped anatase TiO₂ nanoparticles deliver an ∼40% enhanced reversible capacity compared to pristine TiO₂ (139.8 vs. 100.7 mA h g^-1 at 0.1 C after 50 cycles) due to an improved electronic/ionic conductivity. Furthermore, upon AlF₃ coating, the overall system can deliver a much higher reversible capacity of 178.9 mA h g^-1 (∼80% increase with respect to pristine TiO₂) with good cycling stability and excellent rate capabilities of up to 10 C. The experimental results indicate that the AlF₃ surface coating could indeed effectively reduce the solid electrolyte interfacial resistance, enhance the electrochemical reactivity at the surface/interface region, and lower the polarization during cycling. The improved performance achieved using a cost-effective fabrication approach makes the dually modified anatase TiO₂ a promising anode material for high-performance SIBs.

Sulfur-Doped TiO2 Anchored on a Large-Area Carbon Sheet as a High-Performance Anode for Sodium-Ion Battery

Well-tailored sulfur-doped anatase titanium dioxide nanoparticles anchored on a large-area carbon sheet are designed, where the in situ sulfur-doped titanium dioxide directly comes from titanium oxysulfate and the large-area carbon sheet is derived from glucose. When applied as an anode material for sodium-ion batteries, it exhibits an excellent electrochemical performance including a high capacity [256.4 mA h g^-1 at 2 C (1 C = 335 mA h g^-1) after 500 cycles] and a remarkable rate of cycling stability (100.5 mA h g^-1 at 30 C after 500 cycles). These outstanding sodium storage behaviors are ascribed to the nanosized particles (about 8-12 nm), good electronic conductivity promoted by the incorporation of carbon sheet and sulfur, as well as the unique chemical bond based on the electrostatic interaction.

The Application of Hollow Structured Anodes for Sodium-Ion Batteries: From Simple to Complex Systems

Hollow structures exhibit fascinating and important properties for energy-related applications, such as lithium-ion batteries, supercapacitors, and electrocatalysts. Sodium-ion batteries, as analogs of lithium-ion batteries, are considered as promising devices for large-scale electrical energy storage. Inspired by applications of hollow structures as anodes for lithium-ion batteries, the application of these structures in sodium-ion batteries has attracted great attention in recent years. However, due to the difference in lithium and sodium-ion batteries, there are several issues that need to be addressed toward rational design of hollow structured sodium anodes. Herein, this research news article presents the recent developments in the synthesis of hollow structured anodes for sodium-ion batteries. The main strategies for rational design of materials for sodium-ion batteries are presented to provide an overview and perspectives for the future developments of this research area.

Designing Hierarchical Assembly of Carbon-Coated TiO2 Nanocrystals and Unraveling the Role of TiO2/Carbon Interface in Lithium-Ion Storage in TiO2

Despite the many benefits of hierarchical nanostructures of oxide-based electrode materials for lithium-ion batteries, it remains a challenging task to fully exploit the advantages of such materials partly because of their intrinsically poor electrical conductivities. The resulting limited electron supply to primary particles inside secondary microparticles gives rise to significant variation in the lithium-ion (Li⁺) storage capability within the nanostructured particles. To address this, facile annealing, where in situ generated carbon-coated primary particles were assembled into porous microagglomerates, is demonstrated to prepare nanostructured titanium dioxide (TiO₂). A systematic study on the effect of the carbon coating reveals that it is exclusively governed by the characteristics of the TiO₂/carbon interface rather than by the nature of the carbon coating. Depending on their number, oxygen vacancies created by carbothermal reduction on the TiO₂ surface are detrimental to Li⁺ diffusion in the TiO₂ lattice, and structural distortion at the interface profoundly influences the Li⁺ (de)intercalation mechanism. This new insight serves as a stepping stone toward understanding an important yet often overlooked effect of the oxide/carbon interface on Li⁺ storage kinetics, thereby demanding more investigations to establish a new design principle for carbon-coated oxide electrode materials.

Evolution of the electrochemical interface in sodium ion batteries with ether electrolytes

Ether based electrolytes have surfaced as alternatives to conventional carbonates allowing for enhanced electrochemical performance of sodium-ion batteries; however, the primary source of the improvement remains poorly understood. Here we show that coupling titanium dioxide and other anode materials with diglyme does enable higher efficiency and reversible capacity than those for the combination involving ester electrolytes. Importantly, the electrolyte dependent performance is revealed to be the result of the different structural evolution induced by a varied sodiation depth. A suit of characterizations show that the energy barrier to charge transfer at the interface between electrolyte and electrode is the factor that dominates the interfacial electrochemical characteristics and therefore the energy storage properties. Our study proposes a reliable parameter to assess the intricate sodiation dynamics in sodium-ion batteries and could guide the design of aprotic electrolytes for next generation rechargeable batteries.

Sodium ion batteries are known to benefit from the use of ether electrolytes. Here the authors reveal the origin showing that the energy barrier of charge transfer at the electrolyte/electrode interface dominates the interfacial electrochemical characteristics and is favorably small.

Two-Dimensional Unilamellar Cation-Deficient Metal Oxide Nanosheet Superlattices for High-Rate Sodium Ion Energy Storage

Cation-deficient two-dimensional (2D) materials, especially atomically thin nanosheets, are highly promising electrode materials for electrochemical energy storage that undergo metal ion insertion reactions, yet they have rarely been achieved thus far. Here, we report a Ti-deficient 2D unilamellar lepidocrocite-type titanium oxide (Ti_0.87O₂) nanosheet superlattice for sodium storage. The superlattice composed of alternately restacked defective Ti_0.87O₂ and nitrogen-doped graphene monolayers exhibits an outstanding capacity of ∼490 mA h g^-1 at 0.1 A g^-1, an ultralong cycle life of more than 10000 cycles with ∼0.00058% capacity decay per cycle, and especially superior low-temperature performance (100 mA h g^-1 at 12.8 A g^-1 and -5 °C), presenting the best reported performance to date. A reversible Na⁺ ion intercalation mechanism without phase and structural change is verified by first-principles calculations and kinetics analysis. These results herald a promising strategy to utilize defective 2D materials for advanced energy storage applications.

Hierarchical Porous Nanosheets Constructed by Graphene-Coated, Interconnected TiO2 Nanoparticles for Ultrafast Sodium Storage

Sodium-ion batteries (SIBs) are considered promising next-generation energy storage devices. However, a lack of appropriate high-performance anode materials has prevented further improvements. Here, a hierarchical porous hybrid nanosheet composed of interconnected uniform TiO₂ nanoparticles and nitrogen-doped graphene layer networks (TiO₂ @NFG HPHNSs) that are synthesized using dual-functional C₃ N₄ nanosheets as both the self-sacrificing template and hybrid carbon source is reported. These HPHNSs deliver high reversible capacities of 146 mA h g^-1 at 5 C for 8000 cycles, 129 mA h g^-1 at 10 C for 20 000 cycles, and 116 mA h g^-1 at 20 C for 10 000 cycles, as well as an ultrahigh rate capability up to 60 C with a capacity of 101 mA h g^-1 . These results demonstrate the longest cyclabilities and best rate capability ever reported for TiO₂ -based anode materials for SIBs. The unprecedented sodium storage performance of the TiO₂ @NFG HPHNSs is due to their unique composition and hierarchical porous 2D structure.

A Review: Enhanced Anodes of Li/Na-Ion Batteries Based on Yolk-Shell Structured Nanomaterials

Lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) have received much attention in energy storage system. In particular, among the great efforts on enhancing the performance of LIBs and SIBs, yolk-shell (YS) structured materials have emerged as a promising strategy toward improving lithium and sodium storage. YS structures possess unique interior void space, large surface area and short diffusion distance, which can solve the problems of volume expansion and aggregation of anode materials, thus enhancing the performance of LIBs and SIBs. In this review, we present a brief overview of recent advances in the novel YS structures of spheres, polyhedrons and rods with controllable morphology and compositions. Enhanced electrochemical performance of LIBs and SIBs based on these novel YS structured anode materials was discussed in detail.

Synthesis of Uniquely Structured Yolk-Shell Metal Oxide Microspheres Filled with Nitrogen-Doped Graphitic Carbon with Excellent Li-Ion Storage Performance

Novel structured composite microspheres of metal oxide and nitrogen-doped graphitic carbon (NGC) have been developed as efficient anode materials for lithium-ion batteries. A new strategy is first applied to a one-pot preparation of composite (FeO_x -NGC/Y) microspheres via spray pyrolysis. The FeO_x -NGC/Y composite microspheres have a yolk-shell structure based on the iron oxide material. The void space of the yolk-shell microsphere is filled with NGC. Dicyandiamide additive plays a key role in the formation of the FeO_x -NGC/Y composite microspheres by inducing Ostwald ripening to form a yolk-shell structure based on the iron oxide material. The FeO_x -NGC/Y composite microspheres with the mixed crystal structure of rock salt FeO and spinel Fe₃ O₄ phases show highly superior lithium-ion storage performances compared to the dense-structured FeO_x microspheres with and without carbon material. The discharge capacities of the FeO_x -NGC/Y microspheres for the 1st and 1000th cycle at 1 A g^-1 are 1423 and 1071 mAh g^-1 , respectively. The microspheres have a reversible discharge capacity of 598 mAh g^-1 at an extremely high current density of 10 A g^-1 . Furthermore, the strategy described in this study is generally applied to multicomponent metal oxide-carbon composite microspheres with yolk-shell structures based on metal oxide materials.