Silicon sp2-Hybridized Multiwall Nanotubes for Lithium-Ion Battery Application
Authors: Kolla Praveen , McGraw Mike , Cook Rob , Lvovich Vadim , Smirnova Alevtina 
Published: February 2020

Development of Li-on battery technology is crucial for sustainable energy growth as Li-ion batteries have potential in many applications, e.g. potable electronics, automotive and spacecraft power sources. Recent research efforts have been focused in finding alternative electrode and electrolyte materials in order to improve their energy and power density, cycle-life, operating voltage, and safety of operation. One of the strategies is to develop silicon as active anode material in Li-ion batteries due to its highest theoretical capacity of ~4200 mAh/g which is ten times higher than that of conventional carbon anodes. However, mechanical stress due to 300-400% volume change during lithiation-delithiation cycles causes pulverization of Si and degradation of Solid Electrolyte Interface (SEI). In the past few years, Si-nanostructures having critical dimensions (~50-100 nm), such as nanowires, nanoparticles, and nanospheres, have been successfully used to alleviate the effects of volume expansion. However, SEI-degradation in these Si-nanostructures greatly affects their cycle-life and irreversible specific capacity losses. Developing hollow Si-nanostructures with an outer mechanically clamping layer to selectively allow the internal volume change into hallow-space during charge-discharge cycles and to control SEI-layer degradation have significantly improved their cycle-life and irreversible-capacity losses (1). The objective of this research is to synthesize sp2-hybridized via a scalable hydrothermal process and investigate the advantages of Si-Nanotubes (SiNTs) with SiO2mechanical clamping layer for the application in Li-ion battery anode materials.
The self-assembled multiwall SiNTs with ~2-3 nm thick amorphous SiO2 were synthesized from SiO as reported earlier (2) using hydrothermal supercritical conditions of ~6-7 MPa at ~470oC (Fig. 1A). The SiO2 is removed using HF-treatment and two samples having SiO2/SiNTs (Fig.1A, on left) and SiNTs (Fig.1A, right) are comparatively investigated. The mechanical clamping layer allows internal expansion of SiNTs during lithiation and avoids the direct contact of electrolyte with SiNT surface (as in Fig. 1B). This process further prevents SEI-degradation upon cycling and improves their cycle-life and irreversible specific-capacity losses. The advantages and the novelty of the proposed multiwall SiNTs as Li-ion anode materials compared to other one and two-dimensional nanostructures, e.g. silicon nanoparticles and silicon aerogel will be presented and discussed in terms of SiNT morphology (FESEM and HRTEM), and electrochemical properties (CV data, AC impedance analysis, and cell performance in a half-cell configuration).
Acknowledgements: The authors gratefully acknowledge the financial support from NASA Initiation Grant at SDSMT and NASA EPSCoR (Grant No.: NNX14AN22A).
References:
1. H. Wu, G. Chan, J. W. Choi, I. Ryu, Y. Yao, M. T. McDowell, S. W. Lee, A. Jackson, Y. Yang, L. Hu and Y. Cui, Nat Nano, 7, 310 (2012).
2. Y. H. Tang, L. Z. Pei, Y. W. Chen and C. Guo, Phys. Rev. Lett., 95, 116102 (2005).

Figure 1