Nanoscale engineered electrochemically active silicon–CNT heterostructures-novel anodes for Li-ion application
Graphical abstract
Highlights
► Generation of vertically aligned Si/CNT heterostructures with novel Si morphologies. ► Nanoscale Si droplets show better cyclability than Si films on CNT. ► Stable capacity in excess of 2300 mAh/g for loading density of 2–3 mg/cm2. ► Low first cycle irreversible loss of only 10% seen for both Si morphologies.
Introduction
Lithium ion batteries have emerged as the flagship power sources for portable consumer electronic devices such as laptops, cell phones, camcorders as well as transportation systems due to their flexible design and high energy density [1]. Lithium ion batteries widely used in commercial devices today employ graphite anodes exhibiting a theoretical capacity of 372 mAh/g [2] and cathodes comprising primarily LiCoO2 and LiMn2O4. Early work of Besenhard and Huggins in the mid 1970s and early 1980s identified silicon and tin with a theoretical capacity of 4200 mAh/g and 990 mAh/g, respectively as promising anodes to replace graphite for the next generation of lithium ion batteries [3], [4], [5]. However, silicon is known to undergo colossal volume expansion (>300%) upon lithiation leading to pulverization of the active material and consequent loss of inter-particle contact resulting in poor capacity retention and cyclability [6], [7], [8]. Since early 2000, attempts have been made to address the resultant significant damage occurring due to the catastrophic volume expansion related stresses in silicon which include developing amorphous and nanostructured forms of silicon such as silicon nanoparticles, nanowires and nanotubes [7], [9], [10], [11], [12], [13], [14]. The concept of ex situ active–inactive nano-composites utilizing the generation of amorphous silicon and electrochemically inactive matrices comprising transition metal non-oxides such as TiN, TiB2, SiC and C was initially explored by our group earlier on [15], [16], [17], [18], [19]. The molar free volume and consequent voids available in the amorphous silicon structures render them more favorable for withstanding the ensuing large volume expansion thus alleviating the pulverization related loss of particle contact during cycling [11], [20], [21]. Unfortunately, the nanostructured and amorphous forms of silicon have lower electronic conductivity leading to poor charge transport, coulombic efficiencies, and rate capability characteristics. On the other hand one dimensional (1D) structures such as carbon nanotubes (CNTs) are known for their excellent electronic [22] and mechanical properties [23], [24] which have been exploited by us in our earlier work [13], [14] wherein we have shown that the nanoscale droplets of defined spacing deposited on vertically aligned CNTs on quartz substrates, subsequently stripped to form electrodes by slurry coating on Cu substrates as well as generation of vertically aligned Si–CNT (VASCNT) directly on Inconel substrates both demonstrate high capacity of 2000 mAh/g, low first cycle irreversible loss of ∼9% and capacity retention up to 100 cycles.
Following up on this earlier work further work was conducted in this system and herein we report the generation of heterostructures of CNTs and silicon wherein the CNTs are continued to be employed as the conducting substrates onto which silicon is deposited using (CVD). The CNT substrates provide good electronic conducting pathway for silicon to the current collector providing good charge and Li-ion transport during cycling. Moreover, the morphology of silicon deposited has been varied by changing the flow conditions inside the CVD reactor and its effect on the cycling performance has been investigated.
Section snippets
Experiment
Microscopic quartz slides, 3 in. × 1 in. were employed to grow the heterostructures of vertically aligned carbon nanotubes (CNT) and silicon employing a simple 2 step liquid injection CVD approach. The first step involved the growth of carbon nanotubes using liquid injection technique, wherein, m-xylene acted as the carbon source while ferrocene served as the catalyst. Specifically, 0.1 g of ferrocene, dissolved in 10 ml of m-xylene was loaded onto a syringe pump set to flow at 0.11 ml/min. A
Results and discussion
The heterostructures of CNT/Si were synthesized by a simple 2-step liquid injection approach in a CVD reactor. In the first step, growth of carbon nanotubes on the quartz slides was achieved by the decomposition of m-xylene used as the carbon source on the iron nanoparticle catalysts formed as a result of the decomposition of ferrocene in an hydrogen atmosphere at atmospheric pressure conditions. Fig. 1(a) and (b) represents the dense vertically aligned carbon nanotube forests generated on the
Conclusions
Droplet and thin film heterostructures of CNT/Si were synthesized on quartz substrates by a 2 step CVD approach. Uniform deposition of silicon in the form of droplets and thin films were obtained on the CNTs by varying the silane partial pressure inside the CVD reactor. The obtained silicon was found to be nanocrystalline as seen from the XRD pattern and some amount of amorphous silicon was also present as evidenced from the Raman spectra. The differential capacity plot showed significant
Acknowledgments
The authors would like to acknowledge the financial support from the US Department of Energy's Office of Vehicle Technologies BATT program (Contract DE-AC02-05CHI1231), the Ford Motor Company, the National Science Foundation (CBET-0933141) and the Edward R. Weidlein Chair Professorship Funds. PNK also acknowledges partial financial support from the Center for Complex Engineered Multifunctional Materials (CCEMM) at the University of Pittsburgh.
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2017, Journal of Power SourcesCitation Excerpt :Fig. 2c shows the Si 2p XPS spectra of porous Si, C-Si, and CNT-Si ranging from 106 to 96 eV. All of the samples display similar spectra, showing peaks centered at ca. 103 eV that are related to the silicon oxide on the surface of Si, which may originate from the natural oxidation of Si in air and the transformation caused by the local heating effect during the test process [2,13,38,41,45]. Additionally, the peaks located at ca. 99 eV can be assigned to Si (0) [2,8,38], which is the dominant species for all of the samples.