Lamuel David

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.

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.

Synthesis and extreme rate capability of Si-Al-C-N functionalized carbon nanotube spray-on coatings as Li-ion battery electrode.

Silicon-based precursor derived glass-ceramics or PDCs have proven to be an attractive alternative anode material for Li ion batteries. Main challenges associated with PDC anodes are their low electrical conductivity, first cycle loss, and meager C-rate performance. Here, we show that thermal conversion of single source aluminum-modified polysilazane on the surfaces of carbon nanotubes (CNTs) results in a robust Si-Al-C-N/CNT shell/core composite that offers extreme C-rate capability as battery electrode. Addition of Al to the molecular network of Si-C-N improved electrical conductivity of Si-C-N by 4 orders of magnitude, while interfacing with CNTs showed 7-fold enhancement. Further, we present a convenient spray-coating technique for PDC composite electrode preparation that eliminates polymeric binder and conductive agent there-by reducing processing steps and eradicating foreign material in the electrode. The Si-Al-C-N/CNT electrode showed stable charge capacity of 577 mAh g(-1) at 100 mA g(-1) and a remarkable 400 mAh g(-1) at 10,000 mA g(-1), which is the highest reported value for a silazane derived glass-ceramic or nanocomposite electrode. Under symmetric cycling conditions, a high charge capacity of ∼350 mA g(-1) at 1600 mA g(-1) was continuously observed for over 1000 cycles.

Synthesis of Graphene Films by Rapid Heating and Quenching at Ambient Pressures and Their Electrochemical Characterization

We study the process of graphene growth on Cu and Ni substrates subjected to rapid heating (approximately 8 °C/s) and cooling cycles (approximately 10 °C/s) in a modified atmospheric pressure chemical vapor deposition furnace. Electron microscopy followed by Raman spectroscopy demonstrated successful synthesis of large-area few-layer graphene (FLG) films on both Cu and Ni substrates. The overall synthesis time was less than 30 min. Further, the as-synthesized films were directly utilized as anode material and their electrochemical behavior was studied in a lithium half-cell configuration. FLG on Cu (Cu-G) showed reduced lithium-intercalation capacity when compared with SLG, BLG and Bare-Cu suggesting its substrate protective nature (barrier to Li-ions). Although graphene films on Ni (Ni-G) showed better Li-cycling ability similar to that of other carbons suggesting that the presence of graphene edge planes (typical of Ni-G) is important in effective uptake and release of Li-ions in these materials.

Three-dimensional polymer-derived ceramic/graphene paper as a Li-ion battery and supercapacitor electrode

We study the synthesis and electrochemical performance of molecular precursor-derived ceramic (PDC)/carbon nanotube-embedded graphene self-supporting composite papers as Li-ion battery and supercapacitor electrodes. The composite papers are prepared using vacuum filtration of PDC-graphene oxide (GO) dispersion, followed by thermal reduction at 500 °C. Tested as a Li-ion battery electrode, the composite papers deliver a reversible capacity as high as 300 mA h g−1 (normalized with respect to total mass of the electrode) with negligible capacity loss after 1000 charge/discharge cycles. Boron-doped silicon carbon nitride (Si(B)CN) outperforms its undoped counterpart (SiCN) in terms of rate capability, cyclic stability, and coulombic efficiency. Among the PDC materials analyzed, Si(B)CN–CNT–rGO demonstrates the lowest ohmic resistance and highest specific capacitance of approximately 269.52 F g−1 at a current density of 5 A g−1, making it a promising electrode material for electrochemical energy storage applications.

Evaluating the thermal damage resistance of graphene/carbon nanotube hybrid composite coatings.

We study laser irradiation behavior of multiwalled carbon nanotubes (MWCNT) and chemically modified graphene (rGO)-composite spray coatings for use as a thermal absorber material for high-power laser calorimeters. Spray coatings on aluminum test coupon were exposed to increasing laser irradiance for extended exposure times to quantify their damage threshold and optical absorbance. The coatings, prepared at varying mass % of MWCNTs in rGO, demonstrated significantly higher damage threshold values at 2.5 kW laser power at 10.6 μm wavelength than carbon paint or MWCNTs alone. Electron microscopy and Raman spectroscopy of irradiated specimens show that the coating prepared at 50% CNT loading endure at least 2 kW.cm−2 for 10 seconds without significant damage. The improved damage resistance is attributed to the unique structure of the composite in which the MWCNTs act as an efficient absorber of laser light while the much larger rGO sheets surrounding them, dissipate the heat over a wider area.

Polysiloxane-functionalized graphene oxide paper: pyrolysis and performance as a Li-ion battery and supercapacitor electrode

Exfoliated graphene oxide (GO) and polysiloxane were blended and pyrolyzed to synthesize free-standing SiOC–graphene composite papers. Characterization techniques reveal a layer-by-layer stacking of GO sheets and an increase in interlayer spacing due to the functionalization of SiOC with GO. This unique structure of the SiOC–graphene composite paper makes it suitable for energy storage applications in batteries and supercapacitors. A reversible electrochemical capacity ∼750 mA h g−1 which is stabilized to ∼400 mA h g−1 after 5 cycles was recorded when tested as a battery electrode. Also, a maximum specific capacitance of 75.72 F g−1 at a current density of 6.7 A g−1 was observed while studying its electrochemical performance as a supercapacitor.