Mechanically Robust Sandwich-Structured C@Si@C Nanotube-Based Li-Ion Battery Anodes

Advanced secondary (rechargeable) batteries require high energy density and high structural stability anodes and cathodes. High volumetric energy density is particularly desirable for micro-electronic devices and portable applications, however it can impact almost any application. In current commercial lithium-ion batteries, carbon-based anodes are dominant; however, their capacity and structural stability are not ideal due to the low theoretical capacity (372 mAh g^‒1) and intrinsically fragile structure of carbon. Silicon is a very attractive anode material because of its huge theoretical capacity (~4,200 mAh g^‒1). Nevertheless, the solid electrolyte interphase (SEI) formation on Si surface is generally unstable due to the large volume change during lithiation-delithiation (400%), which leads to substantial capacity decay with cycling.

  Here we present an anode design consisting of closed-end Si nanotubes coated with carbon on both sides (C@Si@C). This sandwich-like structure with double-sided carbon coatings provides both good electrical conductivity and effective protection of electrochemically active Si, leading to a stable SEI. Because the C@Si@C nanotube array was fabricated onto a three-dimensional (3D) scaffold, the anode exhibits a considerable energy density.

  Our results show this nanotube array exhibits a capacity of about 2,200 mAh g^-1, and a nearly constant Coulombic efficiency of about 98% over 60 cycles. Correspondingly, the volumetric energy density (~750 mAh cm^–3) considerably exceeds the volumetric energy density of commercial graphite anodes (~300 mAh cm^–3). Through a series of control experiments, we find the C@Si@C nanotube array gives much better capacity and structure stability compared to the Si nanotubes without carbon coatings, the ZnO@C@Si@C nanorods, a Si thin film on Ni foam, and C@Si and Si@C nanotubes. In-situ SEM observations on lithiation-delithiation shows a stable structure evolution of the sandwich nanotube. Through stress and plastic strain modeling, while the plastic strain in the nanotubes increases gently, the one in the nanorods increases dramatically. Since the plastic strain amplitude in the nanorods is significantly higher than that in the nanotubes, in terms of a low-cycle fatigue mechanism of Si during lithiation-delithiation, it is expected that the nanorods would fail with significantly fewer lithiation-delithiation cycles compared to the nanotubes. Our findings show this design of sandwich-structured nanotube array is effective for fabricating anodes with both stable structure retention and high energy density. We believe this strategy is quite general can find application for a large variety of other anode and cathode designs.