Stress-Relieving Carboxylated Polythiophene/Single-Walled Carbon Nanotube Conductive Layer for Stable Silicon Microparticle Lithium-Ion Battery Anodes

Abstract

Silicon is one of the most promising anode materials for next generation Lithium ion batteries (LIBs) on account of its ultrahigh specific capacity (4200 mAh g-1) and relatively low working potential of 0.3 V (vs. Li+/Li). However, practical commercial implementation of silicon poses a great challenge due to technical difficulties such as low conductivity (10-5-10-3 S cm-1), low initial Coulombic efficiency (50-80%) and stress-induced fracture, along with SEI accumulation due to severe volume changes (~400%) inherent in silicon-based anodes.To address these challenges and advance the next generation of LIBs, we present poly[3-(potassium-4-butanoate)thiophene] (PPBT) and single-walled carbon nanotube (SWNT) coated silicon microparticles (Si MPs) (PPBT/SWNT@Si MPs) as active materials for highly stable Si-based anodes for LIB systems. The water-soluble conjugated polymer, PPBT, was selected for its ability to serve as an ion and electron conducting physical/chemical linker (10-5 S cm-1 vs. PVDF; ~ 10-8 S cm-1) to the surface of various high capacity anode active materials systems. In consideration of current practical requirements for Si-based anode materials, Si MPs were selected as starting materials due to their lower surface area than the nanoscale analogs, a characteristic that can further improve LIB bulk capacity. In addition, Si MPs are known to have high initial Coulombic efficiency and industrial compatibility. With the help of PPBT, which facilitates unraveling of entangled single-walled carbon nanotubes, SWNTs were successfully attached to the surface of the active materials. PPBT carboxylate substituents were found to be chemically bound to the Si MP surface, providing an electrochemically conductive environment in intimate contact with the 1D SWNTs and Si MPs.Due to facilitated ion/electron transport kinetics, PPBT/SWNT@SiMP anodes exhibited superior capacity retention and higher reversible capacities, even at a high current density of 8 A g-1, compared to bare Si MP anodes. Improved initial Coulombic efficiency and stable cycling performance over 300 cycles were achieved. In situ Raman spectroscopy was employed to investigate the evolution of stress on the SWNTs during lithiation/delithiation, which elucidated the mechanism underlying the enhancement in PPBT/SWNT@Si MP anode cycling performance. The single-walled carbon nanotubes within the PPBT/SWNT@Si MP anodes consistently exhibited reversible stress recovery and 45 % less tensile stress variation after the 10th cycle than SWNT@Si MP control anodes fabricated by direct mixing of the microparticles with SWNTs to create SWNT@Si MPs.Combined, the results suggest that a well-developed conductive interface can enhance the rate performance, and improve electrode stability and Coulombic efficiency. Furthermore, the PPBT/SWNT coating directly bound to the Si MP surface provides for efficient stress relaxation of Si-based electrodes, resulting in reversible lithiation/delithiation, low electrode resistance, and reduced SEI layer formation.