Nanostructuring Strategies for Silicon‐based Anodes in Lithium‐ion Batteries: Tuning Areal Silicon Loading, SEI Formation/Irreversible Capacity Loss, Rate Capability Retention and Electrode Durability

Abstract

Silicon is one of the most promising anode materials for Lithium-ion batteries. Silicon endures volume changes upon cycling, which leads to subsequent pulverization and capacity fading. These drawbacks lead to a poor lifespan and hamper the commercialization of silicon anodes. In this work, a hybrid nanostructured anode based on silicon nanoparticles (SiNPs) anchored on vertically aligned carbon nanotubes (VACNTs) with defined spacing to accommodate volumetric changes is synthesized on commercial macroscopic current collector. Achieving electrodes with good stability and excellent electrochemical properties remain a challenge. Therefore, we herein tune the active silicon areal loading either through the modulation of the SiNPs volume by changing the silicon deposition time at a fixed VACNTs carpet length or through the variation of the VACNT length at a fixed SiNPs volume. The low areal loading of SiNPs improves capacity stability during cycling but triggers large irreversible capacity losses due to the formation of the solid electrolyte interphase (SEI) layer. By contrast, higher areal loading electrode reduces the quantity of the SEI formed, but negatively impacts the capacity stability of the electrode during the subsequent cycles. A higher gravimetric capacity and higher areal loading mass of silicon is achieved via an increase of VACNTs carpet length without compromising cycling stability. This hybrid nanostructured electrode shows an excellent stability with reversible capacity of 1330 mAh g -1 after 2000 cycles.

Thick Architected Silicon Composite Battery Electrodes Using Honeycomb Patterned Carbon Nanotube Forests

“…Furthermore, conformal Si coatings have been achieved on a number of high aspect ratio structures using CVD deposition methods from conventional semiconductor processing. 5,7,22,34,38,[44][45][46][47] Figure 1d shows a schematic of a honeycomb VA-CNT electrode with a conformal Si coating to enable high capacity.…”

“…Despite having a high specific capacity, Si electrodes suffer from significant capacity fading as Si undergoes a ∼300% volume change during cycling. 38,48 This expansion and contraction process, as experienced in the work of Ezzedine et al, causes active material loss through delamination or pulverization, and repeatedly destroys the protective SEI layer that forms during cycling. 38 This continual SEI reformation not only depletes active material but also consumes electrolyte and leads to increased cell impedance.…”

“…36,37 CNTs can be grown directly on Cu foil by using thicker Al 2 O 3 layers and carefully controlling annealing time. 37,38 Ezzedine et al fabricated a 50 nm thick Al 2 O 3 film beneath their Fe catalyst layer to grow CNTs with heights up to 145 μm and then utilized 30 to 60 μm tall Si coated forests as electrodes. 38 These non-patterned forests demonstrated respectable areal capacities up to 8.5 mAh cm −2 but suffered poor capacity retention at higher Si loadings, as ∼40% of the starting capacity had been lost after 40 cycles.…”

“…37,38 Ezzedine et al fabricated a 50 nm thick Al 2 O 3 film beneath their Fe catalyst layer to grow CNTs with heights up to 145 μm and then utilized 30 to 60 μm tall Si coated forests as electrodes. 38 These non-patterned forests demonstrated respectable areal capacities up to 8.5 mAh cm −2 but suffered poor capacity retention at higher Si loadings, as ∼40% of the starting capacity had been lost after 40 cycles. The authors attributed these losses to Si and solid electrolyte interphase (SEI) cracking and pulverization during cycling.…”

“…The authors attributed these losses to Si and solid electrolyte interphase (SEI) cracking and pulverization during cycling. 38 More flexibility in the growth process and taller CNTs can be obtained by utilizing a thin, electrically conductive diffusion barrier between the metallic substrate and catalyst layer. 36,39 Recently, Lettiere et al demonstrated that a thin W layer, deposited in the middle of the Al 2 O 3 layer between the Cu and Fe, prevented Fe catalyst poisoning and, by tailoring of the CVD recipe, grew tall (∼270 μm) CNTs directly on Cu foils.…”