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Single wall carbon nanotubes (SWCNTs) were incorporated into lithium ion battery anodes as conductive additives in mesocarbon microbead (MCMB) composites and as a free-standing support for silicon active materials. In the traditional MCMB composite, 0.5% w/w SWCNTs were used to replace 0.5% w/w SuperP conductive additives. The composite with 0.5% SWCNTs had nearly three times the conductivity which leads to improved electrochemical performance at higher discharge rates with a 20% increase in capacity at greater than a C/2 rate. The thermal stability and safety was measured using differential scanning calorimetry (DSC), and a 35% reduction in exothermic energy released was measured using the highly thermally conductive SWCNTs as an additive. Alternatively, free-standing SWCNT papers were coated with increasing amounts of silicon using a low pressure chemical vapor deposition technique and a silane precursor. Increasing the amount of silicon deposited led to a significant increase in specific capacity (>2000 mAh/g) and coulombic efficiency (>90%). At the highest silicon loading, the surface area of the electrode was reduced by over an order of magnitude which leads to lower solid electrolyte interface formation and improved safety as measured by DSC.
High-capacity thin-film germanium was coupled with free-standing single-wall carbon nanotube (SWCNT) current collectors as a novel lithium ion battery anode. A series of Ge–SWCNT compositions were fabricated and characterized by scanning electron microscopy and Raman spectroscopy. The lithium ion storage capacities of the anodes were measured to be proportional to the Ge weight loading, with a 40 wt% Ge–SWCNT electrode measuring 800 mAh/g. Full batteries comprising a Ge–SWCNT anode in concert with a LiCoO2 cathode have demonstrated a nominal voltage of 3.35 V and anode energy densities 3× the conventional graphite-based value. The higher observed energy density for Ge–SWCNT anodes has been used to calculate the relative improvement in full battery performance when capacity matched with conventional cathodes (e.g., LiCoO2, LiNiCoAlO2, and LiFePO4). The results show a >50% increase in both specific and volumetric energy densities, with values approaching 275 Wh/kg and 700 Wh/L.
The electrochemical cycling performance of high purity single wall carbon nanotube (SWCNT) paper electrodes has been measured for a series of electrolyte solvent compositions. The effects of varying the galvanostatic charge rate and cycling temperature on lithium ion capacity have been evaluated between 25-100 °C. The measured reversible lithium ion capacities for SWCNT anodes range from 600-1000 mAh/g for a 1M LiPF6 electrolyte, depending on solvent composition and cycling temperature. The solid-electrolyte-interface (SEI) formation and first cycle charge loss are also shown to vary dramatically with carbonate solvent selection and illustrate the importance of solvent alkyl chain length and polarity on SWCNT capacity. SWCNT anodes have also been incorporated into full battery designs using LiCoO2 cathode composites. An electrochemical pre-lithiation sequence, prior to battery assembly, has been developed to mitigate the first cycle charge loss of SWCNT anodes. The pre-lithiated SWCNT anodes show reversible cycling at varying charge rates and depths of discharge with the cathode system. The summary of data shows that the structural integrity of individual SWCNTs is preserved after cycling, and that free-standing SWCNT paper electrodes represent an attractive material for lithium ion batteries.
Carbonaceous purity assessment methods are being sought after for all types of carbon nanotubes as a means to standardize the material metrology. Our most recent work has evaluated chemical vapor synthesized multi-walled carbon nanotubes (MWNTs). This effort included a protocol for assessment involving qualitative information from scanning electron microscopy (SEM) images and quantitative information from thermogravimetric analysis (TGA) and Raman spectroscopy. Presently, the analysis using Raman spectroscopy on a constructed sample set has been extended to a second excitation energy (HeNe laser at 1.96 eV) and the similar trends in the relative Raman peak ratios have been measured. In contrast to the G-band, the D and G' peaks demonstrate a Raman shift that is excitation energy-dependent, consistent with the double resonance theory. However, the Raman ratio of IG'/ID is independent of excitation energy and is observed to be the most sensitive to MWNT carbonaceous purity. Application of this approach to MWNT arrays grown on SiO2 is compared to conventional bulk powders synthesized under similar conditions. The MWNT arrays show a high degree of vertical alignment based upon SEM and a measured carbonaceous purity using the IG'/ID ratio of 75% w/w.
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