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© 2020 Wiley-VCH GmbH Sodium-ion batteries have attracted ever-increasing attention in view of the natural abundance of sodium resources. Sluggish sodiation kinetics, nevertheless, remain a tough challenge, in terms of achieving high rate capability and high energy density. Herein, a sheet-in-sphere nanoconfiguration of 2D titania–carbon superlattices vertically aligned inside of mesoporous TiO2@C hollow nanospheres is constructed. In such a design, the ultrathin 2D superlattices consist of ordered alternating monolayers of titania and carbon, enabling interpenetrating pathways for rapid transport of electrons and Na+ ions as well as a 2D heterointerface for Na+ storage. Kinetics analysis discloses that the combination of 2D heterointerface and mesoporosity results an intercalation pseudocapacitive charge storage mechanism, which triggers ultrafast sodiation kinetics. In situ transmission electron microscope imaging and in situ synchrotron X-ray diffraction techniques elucidate that the sheet-in-sphere architecture can maintain robust mechanical and crystallographic structural stability, resulting an extraordinary high rate capability, remarkable stable cycling with a low capacity fading ratio of 0.04% per cycle over 500 cycles at 0.2 C, and exceptionally long-term cyclability up to 20 000 cycles at 50 C. This study offers a method for the realization of a high power density and long-term cyclability battery by designing of a hierarchical nanoarchitecture.

© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Lithium-ion batteries (LIBs) have been widely applied and studied as an effective energy supplement for a variety of electronic devices. Titanium dioxide (TiO2), with a high theoretical capacity (335 mAh g−1) and low volume expansion ratio upon lithiation, has been considered as one of the most promising anode materials for LIBs. However, the application of TiO2 is hindered by its low electrical conductivity and slow ionic diffusion rate. Herein, a 2D ultrathin mesoporous TiO2/reduced graphene (rGO) heterostructure is fabricated via a layer-by-layer assembly process. The synergistic effect of ultrathin mesoporous TiO2 and the rGO nanosheets significantly enhances the ionic diffusion and electron conductivity of the composite. The introduced 2D mesoporous heterostructure delivers a significantly improved capacity of 350 mAh g−1 at a current density of 200 mA g−1 and excellent cycling stability, with a capacity of 245 mAh g−1 maintained over 1000 cycles at a high current density of 1 A g−1. The in situ transmission electron microscopy analysis indicates that the volume of the as-prepared 2D heterostructures changes slightly upon the insertion and extraction of Li+, thus contributing to the enhanced long-cycle performance.

2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Applications of room-temperature-sodium sulfur (RT-Na/S) batteries are currently impeded by the insulating nature of sulfur, the slow redox kinetics of sulfur with sodium, and the dissolution and migration of sodium polysulfides. Herein, a novel micrometer-sized hierarchical S cathode supported by FeS2 electrocatalyst, which is grown in situ in well-confined carbon nanocage assemblies, is presented. The hierarchical carbon matrix can provide multiple physical entrapment to polysulfides, and the FeS2 nanograins exhibit a low Na-ion diffusion barrier, strong binding energy, and high affinity for sodium polysulfides. Their combination makes it an ideal sulfur host to immobilize the polysulfides and achieve reversible conversion of polysulfides toward Na2S. Importantly, the hierarchical S cathode is suitable for large-scale production via the inexpensive and green spray-drying method. The porous hierarchical S cathode offers a high sulfur content of 65.5 wt%, and can deliver high reversible capacity (524 mAh g−1 over 300 cycles at 0.1 A g−1) and outstanding rate capability (395 mAh g−1 at 1 A g−1 for 850 cycles), holding great promise for both scientific research and real application.

© 2020 The Royal Society of Chemistry. Pseudocapacitive energy storage via Li+ storage at the surface/interface of the electrode is promising for achieving both high energy density and high power density in lithium-ion batteries (LIBs). Thus, we created holey graphene (HG) via an etching method, and then in situ grew atomically thin mesoporous NiCo2O4 nanosheets on the HG surface, resulting in a NiCo2O4-HG heterostructure. Since both NiCo2O4 and HG possess atomic thickness and porous structures, the as-prepared nanocomposite enables efficient electrolyte diffusion and mass transfer, providing abundant accessible surface atoms for enhanced redox pseudocapacitance. Moreover, the strong coupling effect between NiCo2O4 and graphene produces an ultra-large interfacial area and enhanced electrical conductivity, and subsequently promotes the intercalation pseudocapacitance. Consequently, the NiCo2O4@HG exhibits a high specific capacity of 1103.4 mA h g-1 at 0.2C, ∼88.9% contribution from pseudocapacitance at 1 mV s-1, excellent rate capability, and ultra-long life up to 450 cycles with 931.2 mA h g-1 retention, significantly outperforming previously reported electrodes. This work suggests that the maximum exposure and utilization of the surface/interfacial active sites is vital for the construction of high-performance pseudocapacitive energy storage devices.

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Mn-based hexacyanoferrate NaxMnFe(CN)6 (NMHFC) has been attracting more attention as a promising cathode material for sodium ion storage owing to its low cost, environmental friendliness, and its high voltage plateau of 3.6 V, which comes from the Mn2+/Mn3+ redox couple. In particular, the Na-rich NMHFC (x > 1.40) with trigonal phase is considered an attractive candidate due to its large capacity of ≈130 mAh g−1, delivering high energy density. Its unstable cycle life, however, is holding back its practical application due to the dissolution of Mn2+ and the trigonal-cubic phase transition during the charge–discharge process. Here, a novel hexacyanoferrate (Na1.60Mn0.833Fe0.167[Fe(CN)6], NMFHFC-1) with Na-rich cubic structure and dual-metal active redox couples is developed for the first time. Through multiple structural modulation, the stress distortion is minimized by restraining Mn2+ dissolution and the trigonal-cubic phase transition, which are common issues in manganese-based hexacyanoferrate. Moreover, NMFHFC-1 simultaneously retains an abundance of Na ions in the framework. As a result, Na1.60Mn0.833Fe0.167[Fe(CN)6] electrode delivers high energy density (436 Wh kg−1) and excellent cycle life (80.2% capacity retention over 300 cycles), paving the way for the development of novel commercial cathode materials for sodium ion storage.

Two-dimensional (2D) superlattices offer promising technological opportunities in tuning the intercalation chemistry of metal ions. Now, well-ordered 2D superlattices of monolayer titania and carbon with tunable interlayer-spacing are synthesized by a molecularly mediated thermally induced approach. The 2D superlattices are vertically encapsulated in hollow carbon nanospheres, which are embedded with TiO2 quantum dots, forming a 0D-2D-3D multi-dimensional architecture. The multi-dimensional architecture with the 2D superlattices encapsulated inside exhibits a near zero-strain characteristic and enriched electrochemical reactivity, achieving a highly efficient Na+ storage performance with exceptional rate capability and superior long-term cyclability.

Abrupt Li dendrite growth and the safety hazards caused by liquid electrolytes are generally acknowledged as major technical barriers for the practical application of Li metal batteries. Solid polymer electrolytes (SPEs) are promising to overcome these obstacles, but suffer from rigidity-conductivity inconsistency, ununiform ion distribution and inferior interfacial compatibility. Herein, an anion-immobilized SPE using vinylene carbonate as the rigid polymer backbone and flexible ether oxygen chains containing anion-trapping boron moieties is proposed, which facilitates the Li+ transport and adjusts the ion distribution. This ingenious design along with facile in situ preparation effectively integrates a favorable Young's modulus (2.41 GPa), high ionic conductivity (9.11 x 10-4 S cm-1 at 25 °C) and a high Li+ transference number (0.68), as well as achieving a stable solid electrolyte interface layer. As a result, these integrative properties enable dendrite-free LiFePO4/Li batteries with excellent rate capacity (8C, 98.3 mA h g-1) and superior long-term cyclability over 600 cycles at 30 °C, providing a new strategy for safe and high-energy all-solid-state energy storage systems.

Sodium-ion batteries (SIBs) have been deemed a promising low-cost battery technology for grid-scale energy storage in view of the abundance of sodium resources. The large ionic size (1.02 Å) and different electronegativity of Na+ nevertheless, make it difficult to reversibly insert/extract into/out of most host materials, resulting in inferior battery performance. Searching for viable anode materials with high efficiency that can accommodate the Na+ while achieving long-term stable electrochemical performance is a tough issue for SIBs. With the characteristics of high theoretical capacity (335 mAh g-1), reasonable insertion voltage (~0.75 V vs. Na/Na+), chemical stability, and abundant reserves, titanium oxides have shown promise as anodes for SIBs. The slow progress on titania-based SIBs, however, has been due mainly to their inferior conductivity and sluggish Na+ reaction kinetics. In this doctoral work, several strategies, including atomic-level P-doping, surface structural engineering, constructing two dimensional (2D) superlattices, and optimizing Na+ intercalation kinetics by tailoring-designed nanostructures, have been investigated to enhance the electrochemical reactivity of titania anodes. Structural modulation and surface engineering have remarkable advantages for fast and efficient charge storage. In Chapter 4, a phosphorus modulation strategy is presented that can induce surface structural-disorder while realizing interior phosphorus-doping to enhance the Na+ storage capability of TiO2. The P-modulated TiO2 nanocrystals present an improved structural stability and Na+ diffusion kinetics, achieving excellent high rate performance and long cycling life without dramatically capacity fading, even after 5000 cycles at a current density of 30 C. Meanwhile, the Na+ insertion mechanism was symmetrically studied using in situ synchrotron X-ray diffraction (XRD). The results show that the phosphorus modulation strategy can enable negligible volume expansion of onl

Carbon-coated SiO is the most promising alternative to the graphite anode for improving the energy density of currently commercialized lithium-ion batteries but exhibits poor cyclic stability that leaves an unclear mechanism. To address this issue, the surface properties of commercial carbon-coated silicon monoxide are investigated in a 1.0 M LiPF6-dimethyl carbonate electrolyte with and without ethylene carbonate (EC), with a comparison of graphite. Unlike graphite that can work well in the electrolytes with and without EC during initial 30 cycles, carbon-coated SiO suffers a serious capacity decay, especially in the electrolyte without EC. By identifying the samples after various cycles, it is found that a relatively stable interphase that makes graphite work well cannot be built on carbon-coated SiO. The chemical analyses demonstrate that there is a strong interaction between SiO with hydrofluoric acid in the electrolyte, which leads to destruction of the carbon-coating layer and prevents the formation of a protective interphase.

Structural modulation and surface engineering have remarkable advantages for fast and efficient charge storage. Herein, we present a phosphorus modulation strategy which simultaneously realizes surface structural disorder with interior atomic-level P-doping to boost the Na + storage kinetics of TiO 2 . It is found that the P-modulated TiO 2 nanocrystals exhibit a favourable electronic structure, and enhanced structural stability, Na + transfer kinetics, as well as surface electrochemical reactivity, resulting in a genuine zero-strain characteristic with only approximately 0.1 % volume variation during Na + insertion/extraction, and exceptional Na + storage performance including an ultrahigh rate capability of 210 mAh g −1 at 50 C and a strong long-term cycling stability without significant capacity decay up to 5000 cycles at 30 C.