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Lithium sulfur batteries are expected to be one of the most promising energy storage systems due to their high energy density, low cost and environmental friendliness. However, the shuttle effect of lithium polysulfides severely hampers their practical application. The design of the sulfur cathode is one of the most important approaches to overcome the problem. In this work, MoS2 nanosheets have been successfully grown on the surface of hollow carbon spheres (HCS) to obtain MoS2@HCS nanocomposites with uniform morphology. The growth behavior of MoS2 nanosheets was also proved by adjusting the pore structure of HCS. With a sulfur loading of 74%, the MoS2@HCS/S cathode exhibits a high initial reversible capacity of 1419 mA h g-1 at a current density of 0.1 C and remains at 1010 mA h g-1 after 100 cycles. Even at 0.5 C, a capacity of 795 mA h g-1 can be retained after 600 cycles, corresponding to a capacity retention rate of 63.1%. By adjusting the concentration of the sulfur source, the relationship between different growth quantities of MoS2 and the cycling performance of the battery was also investigated. The excellent electrochemical performance of the MoS2@HCS/S cathode can be fully attributed to its physical and chemical double adsorption effect on lithium polysulfides, which has been confirmed through the visible adsorption and X-ray Photoelectron Spectroscopy (XPS) experiments. This work provides a simple design concept and method to synthesize a nanocomposite-based sulfur host for high performance lithium sulfur batteries.

The carbothermal reduction reaction (CRR) is the most commonly used method for synthesizing Magneli-phase reduced titanium oxide nanocrystals. Though the reaction process has been investigated from thermodynamics and kinetics perspectives, research focused on the detailed formation mechanism of the interfaces from reactant to product is still limited. In this work, the structures and formation mechanisms of the interfaces between different TinO2n−1 phases were investigated using comprehensive XRD, Raman spectroscopy, and TEM characterization. Both XRD and Raman spectroscopy results indicated that the formation sequence of the TinO2n-1 phases during the CRR was TiO2 → TinO2n−1 (n = 4–9) → Ti3O5 → Ti2O3 → TiO, regardless of the method used to introduce the carbon source. Furthermore, the surface encapsulation of polydopamine (PDA) on TiO2 precursor effectively reduced the reaction temperature compared with samples prepared by ball-mill mixing of TiO2 and carbon. The interfaces of Ti4O7/Ti5O9 and Ti3O5/Ti4O7 were obtained from the CRR using different carbon source introduction methods. TEM results suggested the different coherency of these interfaces, which might be due to different formation mechanisms. By calculating the Gibbs free energies in the temperature range of 298–1598 K for some involved reactions, it was noted that the reaction between TiO2 and carbon proceeds spontaneously to form TinO2n−1 phases (n = 2–10) over the whole temperature range. In contrast, the reaction between TinO2n−1 phases with different n values was not spontaneous when n ≤ 3 from 298 K to 1598 K, which further validated the oriented attachment growth mechanism of the Ti3O5/Ti4O7 interface rather than direct CRR. Finally, the growth mechanisms of the two types of interfaces were summarized at both atomic level and nanoscales.

As the next generation of electrochemical energy storage devices, lithium sulfur (Li–S) batteries have many advantages such as high theoretical specific capacity (1675 mAh g−1) and energy density (2600 kw h kg−1), non-toxicity and low cost. However, the poor electrical conductivity of sulfur cathodes and the shuttling effect of lithium polysulfides during cycling strongly hinder the commercial application of Li–S batteries. In this study, Magneli phase Ti4O7 nanoparticles were successfully prepared on the surface of hollow carbon spheres (HCS) through a carbothermal reduction reaction. The synthesized HCS@Ti4O7 with a mesoporous structure exhibited a uniform spherical morphology with a large specific surface area (512 m2 g−1) and the pore volume of 0.58 cm3 g−1. The introduction of a polydopamine (PDA) layer during the preparation was confirmed to effectively inhibit the grain coarsening of Ti4O7. Moreover, the high content of sulfur loading (70%) did not affect the morphology of HCS@Ti4O7, which suggested its stable architecture. The assembled coin cells exhibited the superior reversible capacity of 1427 mAh g−1 at 0.1C and favorable cycling stability from 1168 mAh g−1 to 601 mAh g−1 after 800 cycles at 0.5C with the capacity decay rate of only 0.06% per cycle. This excellent performance and stability were attributed to the strong chemical binding effects of Ti4O7 on lithium polysulfides and the stable morphology maintenance during cycling. This study provides a novel conception to design and synthesize nanocomposites between reduced metal oxides and carbon with uniform and stable mesoporous structures for high-performance Li–S batteries.

Lithium sulfur batteries are expected to be one of the most promising energy storage systems due to their high energy density, low cost and environmental friendliness. However, the shuttle effect of lithium polysulfides severely hampers their practical application. The design of the sulfur cathode is one of the most important approaches to overcome the problem. In this work, MoS2 nanosheets have been successfully grown on the surface of hollow carbon spheres (HCS) to obtain MoS2@HCS nanocomposites with uniform morphology. The growth behavior of MoS2 nanosheets was also proved by adjusting the pore structure of HCS. With a sulfur loading of 74%, the MoS2@HCS/S cathode exhibits a high initial reversible capacity of 1419 mA h g-1 at a current density of 0.1 C and remains at 1010 mA h g-1 after 100 cycles. Even at 0.5 C, a capacity of 795 mA h g-1 can be retained after 600 cycles, corresponding to a capacity retention rate of 63.1%. By adjusting the concentration of the sulfur source, the relationship between different growth quantities of MoS2 and the cycling performance of the battery was also investigated. The excellent electrochemical performance of the MoS2@HCS/S cathode can be fully attributed to its physical and chemical double adsorption effect on lithium polysulfides, which has been confirmed through the visible adsorption and X-ray Photoelectron Spectroscopy (XPS) experiments. This work provides a simple design concept and method to synthesize a nanocomposite-based sulfur host for high performance lithium sulfur batteries.

Electrocatalysts play an important role in oxygen reduction reaction (ORR) in promoting the reaction process. Although commercial Pt/C exhibits excellent performance in ORR, the low duration, high cost, and poor methanol tolerance seriously restrict its sustainable development and application. TinO2n&minus, 1 (3 &le, n &le, 10) is a series of titanium sub-oxide materials with excellent electrical conductivity, electrochemical activity, and stability, which have been widely applied in the field of energy storage and catalysis. Herein, we design and synthesize Ti4O7/Ti3O5 (T4/T3) dual-phase nanofibers with excellent ORR catalytic performance through hydrothermal growth, which is followed by a precisely controlled calcination process. The H2Ti3O7 precursor with uniform size can be first obtained by optimizing the hydrothermal growth parameters. By precisely controlling the amount of reducing agent, calcination temperature, and holding time, the T4/T3 dual-phase nanofibers with uniform morphology and coherent interfaces can be obtained. The orientation relationships between T4 and T3 are confirmed to be [ 001 ] T 3 / / [ 031 ] T 4 , ( 100 ) T 3 / / ( 92 6 &macr, ) T 4 , and ( 010 ) T 3 / / ( 1 2 &macr, 6 ) T 4 , respectively, based on comprehensive transmission electron microscopy (TEM) investigations. Furthermore, such dual-phase nanofibers exhibit the onset potential and half-wave potential of 0.90 V and 0.75 V as the ORR electrocatalysts in alkaline media, respectively, which illustrates the excellent ORR catalytic performance. The rotating ring-disk electrode (RRDE) experiment confirmed the electron transfer number of 3.0 for such catalysts, which indicates a mixture of two electron and four electron transfer reaction pathways. Moreover, the methanol tolerance and cycling stability of the catalysts are also investigated accordingly.

As a green pollution-free reaction, oxygen reduction reaction (ORR) plays a significant role in many energy storage and conversion devices. Although commercial Pt/C possesses the highest ORR activity (especially in acidic medium), the high cost, low stability and poor methanol tolerance making the development of non-platinum electrocatalysts with high performance especially important. Herein, we design and synthesize a carbon nanofiber decorated by Ti4O7/Ti3O5 dual-phase nanoparticles (CNF/T4/T3) as ORR catalysts with excellent activity. With the employment of electrospinning followed by precisely controlled calcination process, the composite catalysts with uniform fibrous morphology can be obtained. Such CNF/T4/T3 composite catalysts exhibited an improved electrochemical performance with onset and half-wave potential of 0.91 V and 0.78 V respectively in alkaline solution. By comparing the catalytic performance with the pure T4 or T3 phase decorated on the same carbon nanofiber, it is found that the mixture of the two titanium sub-oxide phases can optimize the onset potential and limited current density at the same time. Density functional theory calculations suggest that different steps of ORR take place on T3 and T4 successively, which is the main reason of the excellent ORR performance for the dual-phase structure. This work provided a novel designing concept for the functional electrocatalysts using electrospinning technique for the application of energy storage and conversion.