Lithium-Ion Battery Anodes Based on Silicon Nanoparticles and Vertically Aligned Carbon Nanotubes

Abstract

A new concept for silicon-based anodes with potentially high rate capabilities is investigated. Vertically-aligned carbon nanotubes (CNT-forests/VACNTs) grown via chemical vapor deposition act as electrically conductive scaffolding with the aim to expand the two-dimensional surface of Cu current collectors (fig. 1 a) & b)). Silicon nanoparticles (Si-NP), integrated into this CNT-structure, are used as active material due to their high Li storage capability and their high tolerance against pulverization. Compared to conventional anodes, this concept offers a higher proportion of active material in direct electrical contact with the current collector and hence, possibly better performance at higher C-rates. Figure 1 b) illustrates the basic anode concept pursued by this contribution.As grown VACNTs are densely packed and therefore, leave no space for a coating with Si-NPs beyond the topmost surface (Fig. 1 a) & c)). In order to be able to benefit from the CNT’s advantages, such as an enlarged electrode surface area, two methods for structuring VACNTs were developed: UV-Laser and solvent treatment. These process steps induce a CNT density dependent “CNT-bunching” which yields a compacted structure with gaps that can be used for the infiltration of Si-NPs (Fig. 1 b) & d)). The Si-NP coating is investigated in dependence of different slurry coating methods. The experiments are evaluated using a SEM study in the interest of finding the most suitable approach for a homogeneous coating of the entire three-dimensional CNT-surface. In order to enhance the CNT/Si-NP adhesion, as well as to increase the active material’s electrical conductivity, UV-laser experiments were performed and evaluated with the aim to sinter the Si-NPs and thereby reduce the number of NP grain boundaries, which typically negatively influence the electronic transport and thereby the C-rate.These anodes are evaluated for their battery suitability, using electrochemical cycling tests in a half cell setup with Lithium as counter electrode in a potential range between 10 mV and 1 V. The VACNT anodes have allowed for a capacity of 1030 mAh/g after 300 cycles with an initial discharge capacity of 2638 mAh/g and an average Coulombic efficiency of 99.59 % (cycle 4 to 300) at a rate of C/2. First results on tests with high C-rates of up to 10C will be discussed for the purpose of proving the superiority of our anode concept in terms of rate capability. Figure 1