Univ. of Waterloo and GM team develop simple flash heat treatment to boost performance of Si-based anodes for Li-ion batteries
17 December 2013
|Flash heat treatment significantly boosted the performance of silicon-based electrode materials. Credit: ACS, Hassan et al. Click to enlarge.|
Researchers at the University of Waterloo (Canada) and the General Motors Global Research and Development Center have developed a novel, economical flash heat treatment (FHT) for fabricated silicon-based Li-ion electrodes to boost the performance and cycle capability of Li-ion batteries.
In a paper published in the ACS journal Nano Letters, they report that the flash-heat-enhanced electrodes achieve a first cycle efficiency of 84% and a maximum charge capacity of 3,525 mAh g–1—almost 84% of silicon’s theoretical maximum. Further, a stable reversible charge capacity of 1,150 mAh g–1 at 1.2 A g–1 can be achieved over 500 cycles.
Silicon (Si) is a promising candidate to replace graphite as the anode material in commercial Li-ion batteries (LIBs) due to its high theoretical storage capacity of ∼4,200 mAh g−1, natural abundance and low cost, and low working potential 0−0.4 V vs Li/Li+. However, extreme volume change (∼400%) experienced during lithiation/delithiation results in pulverization of the silicon and loss of the electrical connection, leading to rapid capacity loss. Additionally, the solid electrolyte interphase (SEI) layer on the Si surface—which has to bear the same volume expansion and contraction—will also crack, fracture, or delaminate from the Si, leading to low Columbic efficiency.
Numerous strategies have been pursued to address these limitations, including tailoring the nanostructure and buffering the mechanical strain by preparing Si/C composites, dispersing silicon particles within or coating them with a porous carbon; including commercial Si nanoparticles mixed into a functional polymer binding matrix; and utilizing sophisticated methodologies to produce Si and Si@carbon core−shell nanowires, as well as porous composite structures coated with Si nanoparticles.
All of these, the authors note, have limitations either (or both) of fabrication complexity and cost.
In this present work, we present a safe, economical, and short duration strategy for post-treatment of Si electrodes made with commercially available Si nanoparticles (Si-NP) to achieve high-performance. The single-step flash heat treatment (FHT) process, which simultaneously engineered the electrode matrix and the surface architecture of Si, is compatible with continuous roll-to-roll electrode processing.
To our knowledge, this is the first time that the direct engineering of the electrode structure by the rapid post-treatment of commercial Si particles has been reported to be successful. Specifically, the benefits of the FHT postprocess include: (1) the creation of a SiO2/C shell around the Si-NP which restricts volume expansion and stabilizes the SEI layer; (2) the carbonized binder generates a dense cellular network throughout the entire electrode, interconnecting the Si particles, boosting the electrical conductivity, and attributing to the enhanced electrode integrity; and (3) manipulation of the copper current collector to catalyze graphene growth, resulting in a strong interfacial contact mechanically and electrically.
In addition, all of the binders were converted into graphitic carbon, which should give the electrode much better durability in the electrochemical environment. Also, the dispersion of Si powder in the binder before the FHT treatment would be much better since no conductive additive is needed. All of those synergetic functions contribute to the significantly improved performance in terms of cycle stability and rate capability (500 cycles, retaining capacity of 1150 mAh g−1 at a high discharge rate of 1.2 A g−1).—Hassan et al.
The researchers took Si-NP/PVDF (60/40 wt %) electrodes on copper foil and moved them through a high temperature furnace (900 °C) purged with a mixture of Ar(g) and H2(g), at a constant rate. The high ramp results in carbonization of the polymer matrix starting at ∼450 °C. After FHT, the Si content within the electrodes increased to 87.2 wt %, and no deflagration of the electrode occurred.
The resulting electrode material features good adherence to the current collector and excellent flexibility. The researchers bent one of the FHT electrodes—no rupture or microcrack formation arising from the tensile stress was observed. They attributed this flexibility to the polymer carbonization leading to an interconnected carbon network with a coherent, intact structure.
For testing, fabricated coin-type half cells with the working electrode and a Li metal counter electrode. A polypropylene separator was employed to separate the two electrodes and the electrolyte composed of 1 M LiPF6 in 30 wt % ethylene carbonate (EC), 60 wt % dimethyl carbonate (DMC), and 10 wt % fluorinated ethylene carbonate (FEC). For a reference coin cell, an electrode was prepared with the ratio of 60 wt % Si-NP, 20% Super-P as a the conductive material, and 20% polyvinylidene fluoride (PVdF) as a binder. These electrodes were used without FHT treatment.
…the success of this strategy reveals an elegant approach to solving an old problem, providing a scalable methodology for treating commercial Si particles. The controlled heat treatment provides an effective approach to engineer, in a single step: the interfacial contact with copper, the binding matrix, and the creation of a synergistic SiO2/C coating. The treated electrodes possess built-in void space for rapid ion transport and successfully retain strong contact between the SiO2/C shell and the Si-NP, promoting efficient electron transport even after long-term cycling. As a result the enhanced electrodes allow for the controlled expansion of Si and achieve higher reversible capacity (2240 mAh g-1 @120 mA g−1), as well as good rate capability and durability (1150 mAh g−1 @ 1200 mA g−1 over 500 cycles).
Further, elimination of binder facilitates high temperature operation in industrial applications which limit the current electrode design standard. The emphasis of a simplified process represents a promising avenue for the production of industrially viable high-performance Si-based electrodes, which could be extended for roll-to-roll manufacturing of next-generation Li-Ion batterie.—Hassan et al.
Fathy M Hassan, Victor Chabot, Abdel Rahman Elsayed, Xingcheng Xiao, and Zhongwei Chen (2013) “Engineered Si Electrode Nanoarchitecture: A Scalable Postfabrication Treatment for the Production of Next-Generation Li-Ion Batteries.” Nano Letters doi: 10.1021/nl403943g
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