Development of electrode materials for lithium-ion batteries and sodium-ion batteries

University of Technology Sydney. Faculty of Science.Electrode materials are vital to the performance of lithium-ion batteries and sodium-ion batteries. A rational design of electrode materials depends critically on understanding of their electrochemical processes, which is highly desirable for the development of high performance electroactive materials towards different applications. The composition, morphology, structure and preparation method can affect the electrochemical performance. In this doctoral work, a series of electrode materials were designed and fabricated and their electrochemical properties for lithium-ion batteries and sodium-ion batteries were investigated. Single crystalline spinel LiMn₂O₄ nanorods were prepared as stable cathode materials for lithium-ion batteries. The preparation involves infiltration of LiOH into porous Mn₃O₄ nanorods by a vacuum-assisted impregnation route, which facilitates the homogeneous reaction to prepare LiMn₂O₄. The reaction parameters were optimized and Li-rich single crystalline LiMn₂O₄ nanorods were prepared, which retained 95.6 % of its initial capacity after 1000 cycles at 3C rate as cathode material for lithium-ion batteries. Considering the concerns of the increasing cost of lithium salts, the development of low-cost sodium-ion batteries is becoming a hot topic. In this doctoral work, a series of anode materials were explored for sodium-ion storage. The electrochemical performances of SnO₂/nitrogen-doped graphene and SnO₂/graphene were compared to investigate the effects of nitrogen-doping into graphene on enhancing the electrochemical performance for sodium-ion batteries. In contrast to the previous reports which often ascribe the enhanced electro-activity of nitrogen-doped graphene based composites to two nitrogen-doping effects (improving the electron transfer efficiency and increasing electro-active sites within the graphene network) in one single declaration, it was demonstrated that the improved electron transfer efficiency of SnO₂/nitrogen-doped graphene due to nitrogen-doping plays a more important role than the increased electro-active sites within graphene network in enhancing the electro-activity of SnO₂/nitrogen-doped graphene nanohybrids compared to the SnO₂/graphene counterpart. MoS₂/reduced graphene oxide (RGO) nanocomposites with intimate two-dimensional heterointerfaces were prepared by a facile one-pot hydrothermal method. The synergistic effect between MoS₂ and graphene contributing to the enhanced reversible capacity of MoS₂/RGO nanocomposites was investigated by experimental and computational studies. It was revealed that Na prefers to be adsorbed on MoS₂ in the MoS₂/RGO heterostructure rather than intercalate into the MoS₂/RGO heterointerface. Interestingly, the MoS₂/RGO heterointerfaces can significantly increase the electronic conductivity of MoS₂, and store more Na ions, while maintaining the high diffusion mobility of Na atoms on MoS₂ surface and high electron transfer efficiency from Na to MoS₂. SnS₂ nanoplatelet@graphene nanocomposites were prepared by using a morphology-controlled hydrothermal method. The as-prepared materials achieved a high reversible specific sodium-ion storage capacity of 725 mA h g⁻¹, stable cyclability, and an enhanced high-rate capability as anode materials for sodium-ion batteries. Three dimensional interconnected SnO₂/graphene aerogels with a hierarchically porous structure were constructed by a facile in situ process. Such a functional architecture not only facilitates the electrode–electrolyte interaction but also provides an efficient electron pathway within the graphene networks. The as-prepared SnO₂/graphene aerogels exhibited an initial reversible capacity of 451 mA h g⁻¹ with a stable cycling performance at a current density of 20 mA g⁻¹. Even at a high current density of 1000 mA g⁻¹, the electrode achieved a capacity of 168 mA h g⁻¹ after 500 cycles. A series of freestanding electrodes with distinct architectures and promising electrochemical performance for sodium-ion storage were prepared, including: 1) Three dimensional freestanding electrodes consisting of Sn@CNT nanopillar arrays grown on carbon paper, which achieved a reversible capacity of 887 μA h cm⁻² in the first cycle and good cyclability extending to 100 cycles. 2) Vertically aligned MoS₂ nanosheets/carbon paper electrodes as highly reversible anode materials. Coating with carboxy methyl cellulose sodium salt improved the cycling performance and a high reversible capacity of 286 mA h g⁻¹ was achieved after 100 cycles at a current density of 80 mA g⁻¹. The as-prepared electrodes delivered a high initial Coulombic efficiency of 79.5% and promising rate capability. Even at a high current density of 1000 mA g⁻¹, a reversible capacity of 205 mA h g⁻¹ was maintained. 3) Heterostructured Ti₃C₂ MXene/CNTs porous films with high volumetric capacity for sodium-ion storage. The open structure facilitates electrolyte transport and access of ions to the electrode and produces functional MXene-based electrodes for sodium-ion storage. When applied as freestanding electrodes for sodium-ion storage, the built-to-order Ti₃C₂ MXene/CNTs porous films showed a volumetric capacity of 421 mA h cm⁻³ at 20 mA g⁻¹, good rate performances, and excellent cycling stability

2D Titanium Carbide (MXene) Based Films: Expanding the Frontier of Functional Film Materials

2D titanium carbide (Ti3C2Tx) MXene films, with their well-defined microstructures and chemical functionality, provide a macroscale use of nano-sized Ti3C2Tx flakes. Ti3C2Tx films have attractive physicochemical properties favorable for device design, such as high electrical conductivity (up to 20 000 S cm–1), impressive volumetric capacitance (1500 F cm–3), strong in-plane mechanical strength (up to 570 MPa), and a high degree of flexibility. Here, the appealing features of Ti3C2Tx-based films enabled by the layer-to-layer arrangement of nanosheets are reviewed. We devote attention to the key strategies for actualizing desirable characteristics in Ti3C2Tx-based functional films, such as high and tunable electrical conductivity, outstanding mechanical properties, enhanced oxidation-resistance and shelf life, hydrophilicity/hydrophobicity, adjustable porosity, and convenient processability. This review further discusses fundamental aspects and advances in the applications of Ti3C2Tx-based films with a focus on illuminating the relationship between the structural features and the resulting performances for target applications. Finally, the challenges and opportunities in terms of future research, development, and applications of Ti3C2Tx-based films are suggested. A comprehensive understanding of these competitive features and challenges shall provide guidelines and inspiration for the further development of Ti3C2Tx-based functional films, and contribute to the advances in MXene technology

SnS2 nanoplatelet@graphene nanocomposites as high-capacity anode materials for sodium-ion batteries

Na-ion batteries have been attracting intensive investigations as a possible alternative to Li-ion batteries. Herein, we report the synthesis of SnS2 nanoplatelet@graphene nanocomposites by using a morphology-controlled hydrothermal method. The as-prepared SnS2/graphene nanocomposites present a unique two-dimensional platelet-on-sheet nanoarchitecture, which has been identified by scanning and transmission electron microscopy. When applied as the anode material for Na-ion batteries, the SnS2/graphene nanosheets achieved a high reversible specific sodium-ion storage capacity of 725 mA h g−1, stable cyclability, and an enhanced high-rate capability. The improved electrochemical performance for reversible sodium-ion storage could be ascribed to the synergistic effects of the SnS2 nanoplatelet/graphene nanosheets as an integrated hybrid nanoarchitecture, in which the graphene nanosheets provide electronic conductivity and cushion for the active SnS2 nanoplatelets during Na-ion insertion and extraction processes

Changes in Fat Oxidation and Body Composition after Combined Exercise Intervention in Sedentary Obese Chinese Adults

(1) Background: Evidence suggests that aerobic exercise and high-intensity interval training (HIIT) might increase fat oxidation and reduce fat. However, limited research has examined the effects of combining progressive aerobic exercise and HIIT interventions in sedentary adults with overweight and obesity, and differences in its effects between men and women remain unclear. The purpose of this study was to investigate the effects of combined progressive aerobic exercise and HIIT (CAEH) on fat oxidation and fat reduction in sedentary Chinese adults and compare sex differences in sedentary adults after seven weeks. (2) Methods: Eighty-four sedentary obese adults were enrolled and allocated to two groups in baseline (experimental (EXP) group:42; control (CON) group:42), and fifty-six subjects (EXP:31; CON:25) completed the experiments and were included in the final analysis. Subjects in the EXP group performed CAEH three times per week for seven weeks. Subjects in the CON group were advised to continue with their normal daily activities. Anthropometric, lipid profile, cardiorespiratory fitness, and fat oxidation outcomes were assessed before and after the intervention. (3) Results: After seven weeks of the CAEH intervention, compared with the CON group, the EXP group showed significant increases in fat oxidation at rest (FO_rest) (+0.03 g/min, p < 0.01), maximal fat oxidation (MFO) (+0.05 g/min, p < 0.01), and maximal oxygen intake (VO2max) (+3.2 mL/kg/min, p < 0.01). The changes in the percentages of the FO_rest (+57%) and the VO2max (+16%) were significantly greater (+20%, +6%) in males than in females (p < 0.05, p < 0.05). The body mass index (BMI) (−1.2 kg/m2, p < 0.01), body fat percentage (−3.2%, p < 0.001), visceral fat area (−12.8 cm2, p < 0.001), and total cholesterol (TC) levels (−0.4 mmol/L, p < 0.05) were significantly decreased in the EXP group. (4) Conclusions: Seven weeks of the CAEH intervention effectively improved FO_rest, MFO, and VO2max in sedentary obese adults, and the improvements in FO_rest and VO2max were more pronounced in males than in females. CAEH also improved body composition and TC levels in sedentary obese adultspeerReviewe

Porous Ti₃C₂T_<i>x</i> MXene for Ultrahigh-Rate Sodium-Ion Storage with Long Cycle Life

The development of anode materials remains a challenge to satisfy the requirements of sodium-ion storage for large-scale energy-storage applications, which is ascribed to the low kinetics of ionic/electron transfer of electrode materials. Here we show that the controlled anisotropic assembly of highly conductive Ti₃C₂T_<i>x</i> MXene nanosheets to form a porous structure can enhance the sodium-ion storage kinetics. At high current densities of 1 and 10 A g^–1, the porous Ti₃C₂T_<i>x</i> electrode delivered capacities of 166 and 124 mA h g^–1, respectively. Even at an extremely high current density of 100 A g^–1, a capacity of 24 mA h g^–1 could be achieved. The porous Ti₃C₂T_<i>x</i> electrode also exhibited a long cycle life that can be extended to 1000 cycles with no capacity decay at a current density of 1 A g^–1. This work demonstrates successful control of the Ti₃C₂T_<i>x</i> architecture to push electrochemical sodium-ion storage closer to large-scale applications and is expected to shed light on the rational utilization of the outstanding properties of MXenes by controlling their microscopic assembly

Nitrogen-Doped Graphene Nanosheets as Metal-Free Catalysts for Aerobic Selective Oxidation of Benzylic Alcohols

This work demonstrates the molecular engineering of active sites on a graphene scaffold. It was found that the N-doped graphene nanosheets prepared by a high-temperature nitridation procedure represent a novel chemical function of efficiently catalyzing aerobic alcohol oxidation. Among three types of nitrogen species doped into the graphene latticepyridinic N, pyrrolic N, and graphitic Nthe graphitic sp² N species were established to be catalytically active centers for the aerobic oxidation reaction based on good linear correlation with the activity results. Kinetic analysis showed that the N-doped graphene-catalyzed aerobic alcohol oxidation proceeds via a Langmuir–Hinshelwood pathway and has moderate activation energy (56.1 ± 3.5 kJ·mol^–1 for the benzyl alcohol oxidation) close to that (51.4 kJ·mol^–1) proceeding on the catalyst Ru/Al₂O₃ reported in literature. An adduct mechanism was proposed to be different remarkably from that occurring on the noble metal catalyst. The possible formation of a sp² N–O₂ adduct transition state, which can oxidize alcohols directly to aldehydes without any byproduct, including H₂O₂ and carboxylic acids, may be a key element step. Our results advance graphene chemistry and open a window to study the graphitic sp² nitrogen catalysis

Microwave-assisted Synthesis of Mesoporous Co₃O₄ Nanoflakes for Applications in Lithium Ion Batteries and Oxygen Evolution Reactions

Mesoporous Co₃O₄ nanoflakes with an interconnected architecture were successfully synthesized using a microwave-assisted hydrothermal and low-temperature conversion method, which exhibited excellent electrochemical performances as anode materials in lithium ion batteries and as catalysts in the oxygen evolution reaction (OER). Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) observations showed the unique interconnected and mesoporous structure. When employed as anode materials for lithium ion batteries, mesoporous Co₃O₄ nanoflakes delivered a high specific capacity of 883 mAh/g at 0.1C current rate and stable cycling performances even at higher current rates. Post-mortem analysis of <i>ex situ</i> FESEM images revealed that the mesoporous and interconnected structure had been well maintained after long-term cycling. The mesoporous Co₃O₄ nanoflakes also showed both OER active properties and good catalytic stability. This could be attributed to both the stability of unique mesoporous structure and highly reactive facets