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Though the intercalation mechanism of hard carbon anodes for Na-ion batteries (NIBs) gain decent electrochemical performances, the sluggish diffusion kinetics of Na-ions at low potential present huge challenge to high power applications. To improve the rate capability and cycling stability, especially the performance at high rates, herein, a novel N, S co-doping strategy based on the pseudocapacitive mechanism is developed via one-step synthesis, which endows the hard carbon with excellent rate performances and long-term cycling stability. Moreover, the electrochemical behaviors of N, S co-doping carbon materials outperform those of single heteroatom (N) doping materials, owing to the additional effective covalent S bonds and more defects. The optimized NSC2 (derived from 2:1 mass ratio of thiourea and sodium citric) delivers a high reversible capacity of 280 ​mAh g−1 at 0.05 ​A ​g−1 for 200 cycles and 223 ​mAh g−1 at 1 ​A ​g−1 for 2000 cycles, respectively, and preserves 102 ​mAh g−1 at 10 ​A ​g−1. This work provides a universal co-doping approach on modification of hard carbon materials for high power battery applications.

A simple method was applied to control the morphology of LiMn2O4 and Li1.05Mn1.97Nb0.03O4 in the sintering process by premixing a suitable proportion of acetylene black in the raw material. Both specific discharge capacity and cycling stability of the samples were improved. The results demonstrated that the doped samples showed excellent electrochemical performance at both 25 °C and 55 °C.

Since the exfoliation of graphene in 2005, two-dimensional (2D) materials have become the subject of exploiting interest. In particular, graphene is regarded as a serious alternative to many conventional materials in various applications. The rapid and prosperous development of graphene stimulates numerous research interests on other 2D materials, such as transition metal dichalcogenides (TMDCs). Although the 2D materials have been continuously refreshing and enriching their family, a pure material may not meet the demands for versatile applications. Therefore, this thesis focuses on the modulation of the properties of 2D materials by means of different methods. In this thesis, three approaches are presented to control the properties of 2D systems involving stacking, covalent functionalization and defect engineering in combination with molecule adsorption. To fully understand the effect of these methods on the 2D materials, we explore the structural, electronic and optical properties of the monolayered graphene and MoS2 (a traditional representative of TMDCs) by state-of-the-art ab-initio computational methods. Specifically, we study (a) the intercalation of surface and subsurface Co-Ir alloy between graphene/Ir(111); (b) the covalent functionalization of 2H-MoS2, 1T'-MoS2 and graphene by various chemical groups such as -F, -CH3, -C6H5 and so on; (c) the physisorption and chemisorption of small molecules on the pristine and defective 2H-MoS2 monolayer. Our results reveal that stacking is an effective method to tune the moire superstructure and electronic properties of graphene and the interaction strength between graphene and substrate is strongly influenced by the composition and nature of an alloy; covalent functionalization results in dramatic changes to the electronic and optical properties of MoS2 and graphene, achieving semiconductor-to-metal or metal-to-semiconductor transition; the band gap and optical absorption display a strong dependence on the covalent functionalization coverage, suggesting that the ability to accurately select the coverage of groups attached to the monolayer surfaces, may be an effective way to engineer the optoelectronic properties of graphene and MoS2 for selected device applications; defects in the MoS2 are active centers for the molecule adsorption and chemical functionalization; the chemisorption and dissociation of O2 on the defective surface tend to passivate S defect states, while the physisorption of O2 and NO molecules on the defective and pristine MoS2 could enhance the optical absorption peak and the excitonic binding energy. Our work confirms the tunability of properties of the considered systems and further indicates the possibility of artificially controlling the properties of 2D materials. The deep insights into the functionalized graphene and MoS2 from this thesis are expected to provide a useful guide for the design of 2D-based devices., Seit der Exfoliation von Graphen im Jahr 2005 sind zweidimensionale (2D) Materialien Gegenstand von wissenschaftlichem Interesse geworden. Insbesondere Graphen wird in verschiedenen Anwendungen als ernsthafte Alternative zu vielen herk��mmlichen Materialien diskutiert. Die schnelle und florierende Entwicklung von Graphen stimuliert das gro�� Forschungsinteresse an anderen 2D-Materialien, wie z.B. den ��bergangsmetalldichalkogeniden (TMDCs). Obwohl die Familie der 2D-Materialien kontinuierlich w��chst, erf��llt ein pure Material m��glicherweise nicht die Anforderungen f��r vielseitige Anwendungen. Daher konzentriert sich diese Arbeit auf die Modulation der Eigenschaften von 2D-Materialien mit Hilfe verschiedener Methoden. In dieser Dissertation werden drei Ans��tze vorgestellt um die Eigenschaften von 2D-Systemen zu steuern: Stapelung, kovalente Funktionalisierung und Defekt-Engineering in Kombination mit Molek��ladsorption. Um die Auswirkungen dieser Methoden auf die 2D-Materialien vollst��ndig zu verstehen, untersuchen wir die strukturellen, elektronischen und optischen Eigenschaften des einschichtigen Graphens und MoS2 (eines traditionellen Vertreters von TMDCs) mit modernsten ab-initio Berechnungsmethoden. Insbesondere untersuchen wir (a) die Interkalation von Oberfl��chen- und Suboberfl��chen von Co-Ir-Legierungen zwischen Graphen/Ir(111); (b) die kovalente Funktionalisierung von 2H-MoS2, 1T'-MoS2 und Graphen durch verschiedene chemische Gruppen wie -F, -CH3, -C6H5 und ��hnliche; (c) die Physisorption und Chemisorption kleiner Molek��le auf der idealen und defekten 2H-MoS2-Monoschicht. Unsere Ergebnisse zeigen, dass das Stapeln eine effektive Methode ist, um die Moir��-��berstruktur und die elektronischen Eigenschaften von Graphen abzustimmen, und dass die Wechselwirkungsst��rke zwischen Graphen und Substrat stark von der Zusammensetzung und Art einer Legierung beeinflusst wird; kovalente Funktionalisierung f��hrt zu dramatischen ��nderungen der elektronischen und optischen Eigenschaften von MoS2 und Graphen, wodurch ein Halbleiter-zu-Metall- oder Metall-zu-Halbleiter-��bergang erreicht wird; die Bandl��cke und die optische Absorption zeigen eine starke Abh��ngigkeit von dem ausma�� an kovalenter Funktionalisierung; dies deutet darauf hin, dass die F��higkeit die Konzentration der Funktionalisierung auf den Monoschichtoberfl��chen genau zu bestimmen ein effektiver Weg sein k��nnte, um die optoelektronischen Eigenschaften von Graphen und MoS2 f��r gezielt Anwendungen zu optimieren; Defekte in MoS2 sind aktive Zentren f��r Molek��ladsorption und chemische Funktionalisierung; die Chemisorption und Dissoziation von O2 auf der defekten Oberfl��che tendiert dazu, S-Defektzust��nde zu passivieren, w��hrend die Physisorption von O2 und NO-Molek��len auf dem defekten und idealen MoS2 den optischen Absorptionspeak und die exzitonische Bindung verst��rken k��nnen. Unsere Arbeit best��tigt die Einstellbarkeit der Eigenschaften der betrachteten Systeme und weist ferner auf M��glichkeiten hin die Eigenschaften von 2D-Materialien k��nstlich zu steuern. Die tiefen Einblicke in funktionalisiertes Graphen und MoS2 aus dieser Dissertation sollen einen n��tzlichen Leitfaden f��r das Design von 2D-Material-basierten Bauelementen darstellen.

By employing first-principles simulations, we present theoretical predictions regarding the modification of structural, electronic and optical properties of 2H- and 1T′-MoS2 monolayers by covalent mono- and bisfunctionalization. Specifically, non-aromatic groups (–F, –NH2, –CH3, –CH2CH2 CN and –CH2CH2 OH) and aromatic (–Ph, –PhNO2 and PhOH) groups are utilized for monofunctionalization, and –F/–NH2, –NH2/–CH3 and –CH3/–Ph for bisfunctionalization. The stability of functionalized 2H- and 1T′-MoS2 monolayers mainly depends on the bonded groups and their surface coverage. In particular, the mixed bisfunctionalization with –F/–CH3 and –NH2/–CH3 groups enhances the stability of 2H-MoS2 through the formation of intermolecular hydrogen bonds. Both 2H- and 1T′-MoS2 can serve not only as electron donors, but also as electron acceptors, subject to the charge transfer behavior of the attached groups. Furthermore, mono- and bisfunctionalization are predicted to be efficient approaches to control the electronic band gaps in 2H- and 1T′-MoS2, where the corresponding values can be tuned by varying the coverage of the absorbed groups. At the same time, the choice of the chemical groups and their coverage also effectively determines the optical adsorption range and intensity. Therefore, our work shows that chemical functionalization of 2D materials with varying coverage can be an important approach to extend the scope of 2D materials in specific electronic and optoelectronic applications.