Gurpreet Singh

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%.

Silicon oxycarbide glass-graphene composite paper electrode for long-cycle lithium-ion batteries.

Silicon and graphene are promising anode materials for lithium-ion batteries because of their high theoretical capacity; however, low volumetric energy density, poor efficiency and instability in high loading electrodes limit their practical application. Here we report a large area (approximately 15 cm × 2.5 cm) self-standing anode material consisting of molecular precursor-derived silicon oxycarbide glass particles embedded in a chemically-modified reduced graphene oxide matrix. The porous reduced graphene oxide matrix serves as an effective electron conductor and current collector with a stable mechanical structure, and the amorphous silicon oxycarbide particles cycle lithium-ions with high Coulombic efficiency. The paper electrode (mass loading of 2 mg cm−2) delivers a charge capacity of ∼588 mAh g−1electrode (∼393 mAh cm−3electrode) at 1,020th cycle and shows no evidence of mechanical failure. Elimination of inactive ingredients such as metal current collector and polymeric binder reduces the total electrode weight and may provide the means to produce efficient lightweight batteries.

Sn– and SnO2–graphene flexible foams suitable as binder-free anodes for lithium ion batteries

With the objective of developing new advanced composite materials that can be used as anodes for lithium ion batteries (LIBs), herein we describe the synthesis of novel three dimensional (3D) macroporous foams formed by reduced graphene oxide (rGO) and submicron tin-based particles. The aerogels were obtained by freeze/freeze-drying a suspension of graphene oxide (GO) in the presence of a tin precursor and its subsequent thermal reduction under an argon atmosphere. The materials exhibited a 3D-macroporous structure formed by the walls of rGO decorated with Sn or SnO2 particles depending on the temperature of calcination. Self-standing compressed foams were directly assembled into coin cells without using any metallic support to be evaluated as binder-free anodes for LIBs. The homogeneous dispersion and intimate contact between the Sn-based particles and graphene walls were confirmed by scanning electron microscopy (SEM). The performance of SnO2–rGO composite materials as anodes for LIBs showed higher specific capacity compared with rGO and metallic Sn-containing samples, reaching a reversible capacity of 1010 mA h g−1 per mass of the electrode at 0.05 A g−1 and good capacity retention (470 mA h g−1) even at 2 A g−1 (∼2 C), among the highest reported for similar systems. The SEM images of selected electrodes after 50 charge–discharge cycles showed that even though SnO2 submicron particles were pulverized into small nanoparticles they remain intact upon cycling.

Polymer-Derived Ceramic Functionalized MoS2 Composite Paper as a Stable Lithium-Ion Battery Electrode.

A facile process is demonstrated for the synthesis of layered SiCN-MoS2 structure via pyrolysis of polysilazane functionalized MoS2 flakes. The layered morphology and polymer to ceramic transformation on MoS2 surfaces was confirmed by use of electron microscopy and spectroscopic techniques. Tested as thick film electrode in a Li-ion battery half-cell, SiCN-MoS2 showed the classical three-stage reaction with improved cycling stability and capacity retention than neat MoS2. Contribution of conversion reaction of Li/MoS2 system on overall capacity was marginally affected by the presence of SiCN while Li-irreversibility arising from electrolyte decomposition was greatly suppressed. This is understood as one of the reasons for decreased first cycle loss and increased capacity retention. SiCN-MoS2 in the form of self-supporting paper electrode (at 6 mg·cm−2) exhibited even better performance, regaining initial charge capacity of approximately 530 mAh·g−1 when the current density returned to 100 mA·g−1 after continuous cycling at 2400 mA·g−1 (192 mAh·g−1). MoS2 cycled electrode showed mud-cracks and film delamination whereas SiCN-MoS2 electrodes were intact and covered with a uniform solid electrolyte interphase coating. Taken together, our results suggest that molecular level interfacing with precursor–derived SiCN is an effective strategy for suppressing the metal-sulfide/electrolyte degradation reaction at low discharge potentials.

Very high laser-damage threshold of polymer-derived Si(B)CN-carbon nanotube composite coatings.

We study the laser irradiance behavior and resulting structural evolution of polymer-derived silicon-boron-carbonitride (Si(B)CN) functionalized multiwall carbon nanotube (MWCNT) composite spray coatings on copper substrate. We report a damage threshold value of 15 kWcm(-2) and an optical absorbance of 0.97 after irradiation. This is an order of magnitude improvement over MWCNT (1.4 kWcm(-2), 0.76), SWCNT (0.8 kWcm(-2), 0.65) and carbon paint (0.1 kWcm(-2), 0.87) coatings previously tested at 10.6 μm (2.5 kW CO2 laser) exposure. Electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy suggests partial oxidation of Si(B)CN forming a stable protective SiO2 phase upon irradiation.

In Vitro and in Vivo Studies of Single-Walled Carbon Nanohorns with Encapsulated Metallofullerenes and Exohedrally Functionalized Quantum Dots

Single-walled carbon nanohorns (SWNHs) are new carbonaceous materials. In this paper, we report the first successful preparation of SWNHs encapsulating trimetallic nitride template endohedral metallofullerenes (TNT-EMFs). The resultant materials were functionalized by a high-speed vibration milling method and conjugated with CdSe/ZnS quantum dots (QDs). The successful encapsulation of TNT-EMFs and external functionalization with QDs provide a dual diagnostic platform for in vitro and in vivo biomedical applications of these new carbonaceous materials.

Improved Electrochemical Capacity of Precursor-Derived Si(B)CN-Carbon Nanotube Composite as Li-Ion Battery Anode

We study the electrochemical behavior of precursor-derived siliconboron carbonitride (Si(B)CN) ceramic and Si(B)CN coated-multiwalled carbon nanotube (CNT) composite as a lithium-ion battery anode. Reversible capacity of Si(B)CN was observed to be 138 mA h/g after 30 cycles, which is four times that of SiCN (~25 mA h/g) processed under similar conditions, while the Si(B)CN-CNT composite showed further enhancement demonstrating 412 mA h/g after 30 cycles. Improved performance of Si(B)CN is attributed to the presence of boron that is known to modify SiCNs nanodomain structure resulting in improved chemical stability and electronic conductivity. Post-cycling microscopy and chemical analysis of the anode revealed formation of a stable passivating layer, which resulted in stable cycling.

Graphene-based technologies for energy applications, challenges and perspectives

Here we report on technology developments implemented into the Graphene Flagship European project for the integration of graphene and graphene-related materials (GRMs) into energy application devices. Many of the technologies investigated so far aim at producing composite materials associating graphene or GRMs with either metal or semiconducting nanocrystals or other carbon nanostructures (e.g., CNT, graphite). These composites can be used favourably as hydrogen storage materials or solar cell absorbers. They can also provide better performing electrodes for fuel cells, batteries, or supercapacitors. For photovoltaic (PV) electrodes, where thin layers and interface engineering are required, surface technologies are preferred. We are using conventional vacuum processes to integrate graphene as well as radically new approaches based on laser irradiation strategies. For each application, the potential of implemented technologies is then presented on the basis of selected experimental and modelling results. It is shown in particular how some of these technologies can maximize the benefit taken from GRM integration. The technical challenges still to be addressed are highlighted and perspectives derived from the running works emphasized.

Structural evolution during sodium deintercalation/intercalation in Na2/3[Fe1/2Mn1/2]O2

Among the various possible earth abundant electrode materials, P2-Na2/3[Fe1/2Mn1/2]O2 is one of the most promising cathode materials for sodium ion batteries. Most transition metal oxide materials undergo various structural transitions during the sodiation/desodiation process and it is crucial to understand such transitions for further development of these materials. In the present research, in situ X-ray diffraction (XRD), in situ Raman spectroscopy and ex situ solid-state Nuclear Magnetic Resonance (NMR) were used as tools to understand such transitions for the specific case of P2-Na2/3[Fe1/2Mn1/2]O2. Both in situ Raman spectroscopy and X-ray diffraction measurements, together with ex situ solid-state NMR studies revealed that the synthesized P2-Na2/3[Fe1/2Mn1/2]O2 hexagonal crystal structure undergoes P2–OP4 reversible phase transitions at the end of the charge process and at the end of the discharge process a biphasic mechanism follows. Moreover, ex situ solid state NMR measurements revealed the fast diffusion of Na+ ions in the 2D layers of P2-Na2/3[Fe1/2Mn1/2]O2, which led to an average 23Na NMR signal that reflects very accurately the average oxidation state of the metal centres. Solid state NMR data also shows that the diffusion of sodium ions is frozen at high levels of sodiation, at low voltages and that at the end of the charge process the prismatic coordinated sites are preferentially populated in the OP4 phase.

Investigating the dendritic growth during full cell cycling of garnet electrolyte in direct contact with Li metal.

All-solid-state batteries including a garnet ceramic as electrolyte are potential candidates to replace the currently used Li-ion technology, as they offer safer operation and higher energy storage performances. However, the development of ceramic electrolyte batteries faces several challenges at the electrode/electrolyte interfaces, which need to withstand high current densities to enable competing C-rates. In this work, we investigate the limits of the anode/electrolyte interface in a full cell that includes a Li-metal anode, LiFePO4 cathode, and garnet ceramic electrolyte. The addition of a liquid interfacial layer between the cathode and the ceramic electrolyte is found to be a prerequisite to achieve low interfacial resistance and to enable full use of the active material contained in the porous electrode. Reproducible and constant discharge capacities are extracted from the cathode active material during the first 20 cycles, revealing high efficiency of the garnet as electrolyte and the interfaces, but prolonged cycling leads to abrupt cell failure. By using a combination of structural and chemical characterization techniques, such as SEM and solid-state NMR, as well as electrochemical and impedance spectroscopy, it is demonstrated that a sudden impedance drop occurs in the cell due to the formation of metallic Li and its propagation within the ceramic electrolyte. This degradation process is originated at the interface between the Li-metal anode and the ceramic electrolyte layer and leads to electromechanical failure and cell short-circuit. Improvement of the performances is observed when cycling the full cell at 55 °C, as the Li-metal softening favors the interfacial contact. Various degradation mechanisms are proposed to explain this behavior.