Self-Supporting Negative Electrode of Silicon Monoxide Held by Carbon Nanotubes for Stable and High-Capacity Secondary Batteries

Silicon monoxide (SiO) is a promising anode material having high theoretical capacity (&gt;1710 mA h/g_SiO) with superior stability. However, SiO is used with polymeric binder, conductive filler and heavy current collector of metal [1,2], resulting in low delithiation capacity per electrode mass. Herein, we propose self-supporting SiO-carbon nanotube (CNT) electrode without metal foils nor polymeric binders, being expected to have high delithiation capacity based on electrode mass because CNT is light weight.<br/>Self-supporting electrode was prepared by simple co-dispersion and filtration process [3] using ~1 μm-sized SiO particles coated with carbon (SiO/C) and submillimeter-long few-wall CNTs synthesized by our fluidized bed [4] having large specific surface area of ~300 m²/g. The SiO/C-CNT (~1.9 mg_SiO/cm² and ~3.2 mA h/cm²) was evaluated by half-cell test with a Li foil counter electrode, a polypropylene separator, and an electrolyte (1 M LiTFSI + 0.6 M LiNO₃, DOL:DME=1:1, v:v). As a condition of electrochemical measurement, the SiO/C-CNT electrode was fully lithiated, fully delithiated, fully lithiated, and then the following cycles were performed with SiO utilization ratios of 42%, 67%, or 100% (1.5, 2, or 3.2 mA h/cm²). The 1st delithiation capacity per electrode mass achieved &gt;1200 mAh/g_electrode at the optimum SiO/C content of 85-90 mass%. The delithiation capacity faded rapidly with the high SiO utilization of 100% and 67% while remained high and stable at 709 mA h/g_SiO and 584 mA h/g_electrode for 236 cycles with the moderate SiO utilization of 42%. This is because the moderate SiO utilization keeps the volume change of SiO particles moderate, preventing damage to and regeneration of the solid-electrolyte interphase. And the flexible CNT sponge enabled reversible volume/thickness change of the electrode and retained the electrode structure even after 51st lithiation as evidenced by electron micrographs.<br/><br/>[1] T. Chen, et al., J. Power Sources 363, 126 (2017).<br/>[2] Q. Meng, et al, ACS Appl. Mater. Interfaces 11, 32062 (2019).<br/>[3] K. Hori, et al., J. Phys. Chem. C 123, 3951 (2019).<br/>[4] M. Li, et al., Carbon 167, 256 (2020).