Van der Waals sheets for rechargeable metal-ion batteries

Doctor of PhilosophyDepartment of Mechanical and Nuclear EngineeringGurpreet SinghThe inevitable depletion of fossil fuels and related environmental issues has led to exploration of alternative energy sources and storage technologies. Among various energy storage technologies, rechargeable metal-ion batteries (MIB) are at the forefront. One dominant factor affecting the performance of MIB is the choice of electrode material. This thesis reports synthesis of paper like electrodes composed for three representative layered materials (van der Waals sheets) namely reduced graphene oxide (rGO), molybdenum disulfide (MoS₂) and hexagonal boron nitride (BN) and their use as a flexible negative electrode for Li and Na-ion batteries. Additionally, layered or sandwiched structures of vdW sheets with precursor-derived ceramics (PDCs) were explored as high C-rate electrode materials. Electrochemical performance of rGO paper electrodes depended upon its reduction temperature, with maximum Li charge capacity of 325 mAh.g⁻¹ observed for specimen annealed at 900°C. However, a sharp decline in Na charge capacity was noted for rGO annealed above 500 °C. More importantly, annealing of GO in NH₃ at 500 °C showed negligible cyclability for Na-ions while there was improvement in electrode's Li-ion cycling performance. This is due to increased level of ordering in graphene sheets and decreased interlayer spacing with increasing annealing temperatures in Ar or reduction at moderate temperatures in NH₃. Further enhancement in rGO electrodes was achieved by interfacing exfoliated MoS₂ with rGO in 8:2 wt. ratios. Such papers showed good Na cycling ability with charge capacity of approx. 225.mAh.g⁻¹ and coulombic efficiency reaching 99%. Composite paper electrode of rGO and silicon oxycarbide SiOC (a type of PDC) was tested as high power-high energy anode material. Owing to this unique structure, the SiOC/rGO composite electrode exhibited stable Li-ion charge capacity of 543.mAh.g⁻¹ at 2400 mA.g⁻¹ with nearly 100% average cycling efficiency. Further, mechanical characterization of composite papers revealed difference in fracture mechanism between rGO and 60SiOC composite freestanding paper. This work demonstrates the first high power density silicon based PDC/rGO composite with high cyclic stability. Composite paper electrodes of exfoliated MoS₂ sheets and silicon carbonitride (another type of PDC material) were prepared by chemical interfacing of MoS₂ with polysilazane followed by pyrolysis . Microscopic and spectroscopic techniques confirmed ceramization of polymer to ceramic phase on surfaces on MoS₂. The electrode showed classical three-phase behavior characteristics of a conversion reaction. Excellent C-rate performance and Li capacity of 530 mAh.g⁻¹ which is approximately 3 times higher than bulk MoS₂ was observed. Composite papers of BN sheets with SiCN (SiCN/BN) showed improved electrical conductivity, high-temperature oxidation resistance (at 1000 °C), and high electrochemical activity (~517 mAh g⁻¹ at 100 mA g⁻¹) toward Li-ions generally not observed in SiCN or B-doped SiCN. Chemical characterization of the composite suggests increased free-carbon content in the SiCN phase, which may have exceeded the percolation limit, leading to the improved conductivity and Li-reversible capacity. The novel approach to synthesis of van der Waals sheets and its PDC composites along with battery cyclic performance testing offers a starting point to further explore the cyclic performance of other van der Waals sheets functionalized with various other PDC chemistries

MoS2/graphene composite paper for sodium-ion battery electrodes.

We study the synthesis and electrochemical and mechanical performance of layered freestanding papers composed of acid-exfoliated few-layer molybdenum disulfide (MoS₂) and reduced graphene oxide (rGO) flakes for use as a self-standing flexible electrode in sodium-ion batteries. Synthesis was achieved through vacuum filtration of homogeneous dispersions consisting of varying weight percent of acid-treated MoS₂ flakes in GO in DI water, followed by thermal reduction at elevated temperatures. The electrochemical performance of the crumpled composite paper (at 4 mg cm⁻²) was evaluated as counter electrode against pure Na foil in a half-cell configuration. The electrode showed good Na cycling ability with a stable charge capacity of approximately 230 mAh g⁻¹ with respect to total weight of the electrode with Coulombic efficiency reaching approximately 99%. In addition, static uniaxial tensile tests performed on crumpled composite papers showed high average strain to failure reaching approximately 2%

Synthesis of large-area few layer graphene films by rapid heating and cooling in a modified apcvd furnace

Master of ScienceDepartment of Mechanical and Nuclear EngineeringGurpreet SinghGraphene because of its unique electrical (electron mobility = 2 x 10[superscript]5 cm[superscript]2 V[superscript]-1 s[superscript]-1), mechanical (E = 1 TPa), optical, thermal and chemical properties has generated a lot of interest among the research community in recent years. One of the most notable methods of synthesizing large area pristine graphene sheets, which are several 100 micrometers wide, is through thermal chemical vapor deposition (CVD). But very little has been known about the effects of heating and cooling rate of the substrate on the quality of graphene produced. Hence we varied various growth parameters to understand the process of graphene growth on Cu and Ni substrates when subjected to fast heating and quenching. This allowed optimization of the CVD process to achieve large-area graphene films consistently and repeatedly. This work provides new insights on synthesis of graphene at atmospheric pressures and the effect of (a) fast heating and fast cooling of substrates, (b) catalyst type and (c) gas flow rates on quality of the graphene produced. A carbon nanotube CVD furnace was restored and modified to accommodate graphene synthesis. We started with synthesis of graphene on Cu substrate following procedures already available in the literature (heating rate ~ 15 °C/min and cooling rate ~ 5 °C/min; total processing time 7 hours). This provided a good reference point for the particular furnace and the test setup. The best results were obtained for 15 minutes of growth at a CH4:H2 ratio of 1:30 at 950 °C. SEM images showed full coverage of the substrate by few layer graphene (FLG), which was indicated by the relatively high I[subscript]2D/I[subscript]G ratio of 0.44. The furnace was further modified to facilitate fast cooling (~4 °C/sec) of substrate while still being in inert atmosphere (Argon). The effect of growth time and concentration of CH[subscript]4 was studied for this modified procedure (at H[subscript]2 flow rate of 300 SCCM). SEM images showed full coverage for a CH[subscript]4 flow rate of 10 SCCM in as little as 6 minutes of growth time. This coupled with the fast cooling cycle effectively reduced the overall time of graphene synthesis by 7 times. The I[subscript]2D/I[subscript]G ratio in Raman spectrum was 0.4 indicating that the quality of graphene synthesized was similar to that obtained in conventional CVD. This modification also facilitated introduction of catalyst substrate after the furnace has reached growth temperature (fast heating ~8 °C/sec). Hence, the overall time required for graphene synthesis was reduced to ~6 % (30 minutes) when compared to the traditional procedure. SEM images showed formation of high concentration few layer graphene islands. This was attributed to the impurities on the catalyst surface, which in the traditional procedure would have been etched away during the long heating period. The optimum process parameters were 30 minutes of growth with 20 SCCM of CH[subscript]4 and 300 SCCM of H[subscript]2 at 950 °C. The Raman spectrum for this condition showed a relatively high I[subscript]2D/I[subscript]G ratio of 0.66. We also studied the effect of Ni as a catalyst. Similar to Cu, for Ni also, traditional procedure found in the literature was used to optimize the graphene growth for this particular furnace. Best results were obtained for 10 minutes of growth time with 120 SCCM of CH[subscript]4 in 300 SCCM of H[subscript]2 at 950 °C. SEM images showed large grain growth (~50 μm) with full coverage. The Raman spectrum showed formation of bi-layer graphene with a I[subscript]2D/I[subscript]G ratio of 1.03. Later the effect of growth time and concentration of the hydrocarbon precursor for Ni substrate subjected to fast heating (~ 8 °C/sec) was studied. It was found that because the process of graphene synthesis on Ni is by segregation, growth period or gas flow rate had little effect on the quality and size of the graphene sheets because of the presence of impurities on the substrate. This procedure yielded multilayer graphite instead of graphene under all conditions. Future work will involve study of changing several other parameters like type of hydrocarbon precursor and pressure in the chamber for graphene synthesis. Also various other substrates like Cu or Ni based alloys will be studied to identify the behavior of graphene growth using this novel procedure

Reduced Graphene Oxide Paper Electrode: Opposing Effect of Thermal Annealing on Li and Na Cyclability

We study long-term electrochemical sodium and lithium cycling, and tensile testing behavior of thermally reduced graphene oxide (rGO) paper electrodes. We find strong dependence of annealing temperature and gas environment on the electrical conductivity, electrochemical capacity, and rate capability of the electrodes. The effect, however, was opposing for the two cell types. Lithium charge capacity increased with increasing annealing temperatures reaching a stable value of ∼325 mAh·g_anode^–1 (∼100 mAh·cm^–3_anode at ∼48 μA·cm^–2 with respect to total volume of the electrode) for specimen annealed at 900 °C, while a sharp decline in Na charge capacity was noted for rGO annealed above 500 °C. Maximum sodium charge capacity of ∼140 mAh·g^–1_anode at 100 mA·g^–1_anode (∼98 mAh·cm_anode^–3 at ∼70 μA·cm^–2) was realized for specimen reduced at 500 °C. These values are the highest reported for GO paper electrodes. More important, annealing of GO in NH₃ environment resulted in a complete shutdown of its Na-ion cyclability showing near-zero charge capacity. On the contrary, NH₃ annealing only improved the electrode’s Li-ion cycling efficiency and rate capability. This behavior is attributed to the increased level of ordering in graphene sheets and decreased interlayer spacing with increasing annealing temperatures in Ar or reduction at moderate temperatures in NH₃ atmosphere. Further, uniaxial tensile tests and videography highlighted the superior elasticity and high strain to failure of crumpled paper electrodes. The present work provides new insights toward the optimization and design of Li and other larger metal-ion battery electrodes where graphene is utilized as an active material, conductive agent, or a flexible mechanical support

Reduced Graphene Oxide Paper Electrode: Opposing Effect of Thermal Annealing on Li and Na Cyclability

We study long-term electrochemical sodium and lithium cycling, and tensile testing behavior of thermally reduced graphene oxide (rGO) paper electrodes. We find strong dependence of annealing temperature and gas environment on the electrical conductivity, electrochemical capacity, and rate capability of the electrodes. The effect, however, was opposing for the two cell types. Lithium charge capacity increased with increasing annealing temperatures reaching a stable value of ∼325 mAh·g_anode^–1 (∼100 mAh·cm^–3_anode at ∼48 μA·cm^–2 with respect to total volume of the electrode) for specimen annealed at 900 °C, while a sharp decline in Na charge capacity was noted for rGO annealed above 500 °C. Maximum sodium charge capacity of ∼140 mAh·g^–1_anode at 100 mA·g^–1_anode (∼98 mAh·cm_anode^–3 at ∼70 μA·cm^–2) was realized for specimen reduced at 500 °C. These values are the highest reported for GO paper electrodes. More important, annealing of GO in NH₃ environment resulted in a complete shutdown of its Na-ion cyclability showing near-zero charge capacity. On the contrary, NH₃ annealing only improved the electrode’s Li-ion cycling efficiency and rate capability. This behavior is attributed to the increased level of ordering in graphene sheets and decreased interlayer spacing with increasing annealing temperatures in Ar or reduction at moderate temperatures in NH₃ atmosphere. Further, uniaxial tensile tests and videography highlighted the superior elasticity and high strain to failure of crumpled paper electrodes. The present work provides new insights toward the optimization and design of Li and other larger metal-ion battery electrodes where graphene is utilized as an active material, conductive agent, or a flexible mechanical support

MoS₂/Graphene Composite Paper for Sodium-Ion Battery Electrodes

We study the synthesis and electrochemical and mechanical performance of layered free-standing papers composed of acid-exfoliated few-layer molybdenum disulfide (MoS₂) and reduced graphene oxide (rGO) flakes for use as a self-standing flexible electrode in sodium-ion batteries. Synthesis was achieved through vacuum filtration of homogeneous dispersions consisting of varying weight percent of acid-treated MoS₂ flakes in GO in DI water, followed by thermal reduction at elevated temperatures. The electrochemical performance of the crumpled composite paper (at 4 mg cm^–2) was evaluated as counter electrode against pure Na foil in a half-cell configuration. The electrode showed good Na cycling ability with a stable charge capacity of approximately 230 mAh g^–1 with respect to total weight of the electrode with Coulombic efficiency reaching approximately 99%. In addition, static uniaxial tensile tests performed on crumpled composite papers showed high average strain to failure reaching approximately 2%

Galvanostatic charge discharge

First cycle galvanostatic charge discharge for the TMD samples

TEM and EDX

TEM images and EDX spectra for the various TMD sample