Decorated Carbon Nanotubes for Electrochemical Energy Storage ( EES ) systems

Abstract

Electrochemical Energy Storage (EES) systems have various applications ranging from large‐scale generation and transmission‐related systems, to distribution networks, customer/end‐user sites or electrical mobility and transportation. [1] Still higher performance and more environmentally acceptable component materials are continuously required. This can only be overcome by major advances in materials with nanomaterials playing a central role. Lithium‐Ion batteries, by far the most common form of EES system, share three common features: two electrodes and an ionically conducting electrolyte which provides means of transporting ions to and from the component electrodes. The electrodes comprise a current collector and an active material. Improvement in the design of one or more of these features may lead to EES systems with improved performance, delivering higher energy densities and characteristics such as increased capacity, charge/discharge rates and cycle lifetime. Along this line the key‐issue addressed by the present work relates to the assessment of the batteries performances when nanostructured electrode materials are considered for both the anode and the cathode. Grown directly on the current collector (copper for anode and respectively aluminum for cathode) substrate, thin walled carbon nanotubes in the form of very dense vertically aligned (VACNT) carpets are considered as ideal support matrix (current collector) for the insertion active materials due to their efficient electron transport pathways and stable mechanical support. [3] The present work focuses in presenting preliminary 2D analysis based on advanced electron microscopy techniques (TEM, HR‐TEM, STEM) coupled with the spectroscopy ones (EELS and EDX), dedicated to the characterization of both anode and cathode decorated carbon nanotubes based electrodes. The different analyses allowed the validation of the CNTs carpets fabrication, which becomes nanostructured current collector upon their coupling (decoration) with Si nanoparticles for anodes and metallic lithiated nanoparticles (LiMO 2 ) for cathode. The electrochemical experiences performed on the nanostructured system confirmed the devices performance. Regarding the anode fabrication, the synthesis of CNTs directly on the copper substrate is visible from figure 1a) that using a dHF‐CVD technique one can obtain a homogenous and very dense carpet of VACNT having a 60µm length. The HR‐TEM analysis allowed identifying a double walled structure for the nano‐tube with a diameter size of 3‐4nm presenting surface defects which act as anchorage sites for the eletrodeposition of the nanoparticles. Concerning the nano‐tubes decoration with silicon nanoparticles the process has been carried following an innovative approach using ionic liquid with SiCl 4 as precursor for Si. These hybrid nanostructures are expected to enhance the electrode capacity since the VACNT acts as both mechanical support and electrical conductor while silicon nanoparticles as high capacity insertion anode material. Figure 2b) illustrates silicon decorated CNTs. The HR‐TEM analyses on several areas on the current collector allowed evidencing that the deposition is obtained homogenously on all nanotubes with nanoparticles sizes ranging from 5 to 30nm. Regarding the cathode fabrication, similar TEM analyses have been performed to validate the synthesis of CNTs carpets on aluminum foil substrate as well as their subsequent decoration with LiMO 2 nanoparticles.