*Article* **Flexible Carbon Nanotubes Confined Yolk-Shelled Silicon-Based Anode with Superior Conductivity for Lithium Storage**

**Na Han 1,†, Jianjiang Li 1,†, Xuechen Wang <sup>1</sup> , Chuanlong Zhang <sup>1</sup> , Gang Liu <sup>1</sup> , Xiaohua Li <sup>1</sup> , Jing Qu <sup>1</sup> , Zhi Peng <sup>1</sup> , Xiaoyi Zhu 1,\* and Lei Zhang 2,\***


**Abstract:** The further deployment of silicon-based anode materials is hindered by their poor rate and cycling abilities due to the inferior electrical conductivity and large volumetric changes. Herein, we report a silicon/carbon nanotube (Si/CNT) composite made of an externally grown flexible carbon nanotube (CNT) network to confine inner multiple Silicon (Si) nanoparticles (Si NPs). The in situ generated outer CNTs networks, not only accommodate the large volume changes of inside Si NPs but also to provide fast electronic/ionic diffusion pathways, resulting in a significantly improved cycling stability and rate performance. This Si/CNT composite demonstrated outstanding cycling performance, with 912.8 mAh g−<sup>1</sup> maintained after 100 cycles at 100 mA g−<sup>1</sup> , and excellent rate ability of 650 mAh g−<sup>1</sup> at 1 A g−<sup>1</sup> after 1000 cycles. Furthermore, the facial and scalable preparation method created in this work will make this new Si-based anode material promising for practical application in the next generation Li-ion batteries.

**Keywords:** silicon; yolk−shell structure; anode; lithium-ion batteries

### **1. Introduction**

Silicon (Si) is the most promising anode candidate in lithium-ion batteries (LIBs) due to its high theoretical specific capacity (~4200 mAh g−<sup>1</sup> ) and cut-price [1–4]. However, the large volume changes (over 400% expansion after full lithiation) induced poor structural stability and continuous breaking and regenerating of the solid-electrolyte interphase (SEI) cause's short working life for Si-based anodes [5–7]. Moreover, the low electrical conductivity of the Si limits its rate performance under high current densities [8–10]. Up till now, introducing a reserved void space and conductive framework into silicon-based materials has been regarded as the most effective strategy to fundamentally improve the electrochemical behavior of Si-based anodes [11–13]. The introduced reserved space can buffer the huge changes in volume of Si during cycling, leading to the enhanced structural integrity and cycling stability [14–16]. Additionally, the conductive framework within the composites increases the overall conductivity of the electrodes, resulting in the high-rate capacities under high current densities [17–20].

Among various Si-based composites, the yolk-shelled Si/carbon (Si/C) composites are the most promising candidate because of their distinctive advantages over the existing Si-based composites in terms of cycling stability and rate behavior [21–24]. Many previous reports confirmed the effective structure [25–27]. For these yolk-shelled Si/C composites,

**Citation:** Han, N.; Li, J.; Wang, X.; Zhang, C.; Liu, G.; Li, X.; Qu, J.; Peng, Z.; Zhu, X.; Zhang, L. Flexible Carbon Nanotubes Confined Yolk-Shelled Silicon-Based Anode with Superior Conductivity for Lithium Storage. *Nanomaterials* **2021**, *11*, 699. https://doi.org/10.3390/ nano11030699

Academic Editors: Henry Radamson and Guilei Wang

Received: 14 January 2021 Accepted: 2 March 2021 Published: 11 March 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the Si-yolk was encapsulated within a hollow C-shell with reserved space between the Siyolk and the C-shell. Therefore, the volume changes of inner Si-yolk can be accommodated by the void space and confined within the hollow C-shell, leading to increased structural stability and limited formation of the outer generated SEI film [25–27]. However, the introduced void space limits the conductive contact between Si-yolk and C-shell and further decreases the tap density of the composite [28–30]. Carbon nanotubes (CNTs) with excellent mechanical properties and high electrical conductivity are regarded as another hopeful carbon matrix to increase the overall behavior of Si-based materials [31–36]. Currently, most of the reported Si/CNT anodes are synthesized by directly using expensive commercialized CNTs to mix with Si nanoparticles (Si NPs), causing increased production cost [37–39]. Moreover, it is difficult to achieve the uniform distribution between CNTs and Si NPs due to their large surface area [40,41]. Currently, new Si/CNT anodes composites have been developed via a chemical vapor deposition (CVD) process, which provides distinguished structural stability and electrochemical performance, enhances the overall conductivity of the electrode, and increases the safety of the battery [32,42]. Moreover, it is remaining a great challenge to prepare promising Si/CNT composites with low-cost methods while preserving the unique volume change containment functionality of Si/C yolk–shell structures.

Herein, we overcome these obstacles by developing new Si/CNTs anodes (Scheme 1). Si NPs were successively double-coated with rigid carbon and silica layers (Si@C@SiO2) to better encapsulate the incorporated multiple Si NPs to realize good safety levels. Furthermore, the SiO<sup>2</sup> coating layer on the outer surface of Si@C@SiO<sup>2</sup> further provided active position for in situ CNTs grown via a CVD method, resulting in a new Si/CNT composite. For this new Si/CNT, the flexible CNT networks were grown on the surface of Si@C@SiO<sup>2</sup> particles. Therefore, the aggregation for both the CNTs and Si NPs can be significantly suppressed due to the external in-situ grown CNTs networks. Additionally, compared with the traditional yolk-shell structure, the CNT networks and the carbon coating shell effectively increase the conductive contact, not only between the inner Si-yolks and CNT networks but also among different Si/CNT microparticles, leading to increased electronic conductivity and rate capacities. Moreover, the overall structural stability and integrity of this new Si/CNT can also be enhanced by flexible porous CNT networks and rigid carbon coating [42].

**Scheme 1.** Schematic illustration of the preparation of the Si@C@v@CNTs.

### **2. Materials and Methods**

### *2.1. Synthesis of Si NPs*

All reagents in this paper were purchased from Sinopharm Co (Shanghai, China). The nano-sized silica (SiO2) spheres were firstly synthesized by the well-established Stöber method. In the following magnesiothermic reduction (MR) process, Mg powders (99%) and the obtained SiO<sup>2</sup> spheres were uniformly mixed and placed in one side of a crucible boat. After that, a certain amount of NaCl (AR) (SiO2:NaCl = 1:10) was placed in the other side of the crucible boat. The crucible boat was then placed in the center of the tube furnace (OTF-1200X, Shenzhen kejing-zhida Co, Shenzhen, China) and increased to 700 ◦C

under an Ar/H<sup>2</sup> (95:5 vol. %) flow and retained for 6 h. After cooling down to normal condition, the obtained sample was dispersed in 1 M HCl for several hours to remove NaCl and byproduct MgO. The final porous Si NPs powders were obtained after a wash and vacuum dry.
