A silicon-impregnated carbon nanotube mat as a lithium-ion cell anode

High Electrochemical Performance Silicon Thin-Film Free-Standing Electrodes Based on Buckypaper for Flexible Lithium-Ion Batteries

Recently, applications for lithium-ion batteries (LIBs) have expanded to include electric vehicles and electric energy storage systems, extending beyond power sources for portable electronic devices. The power sources of these flexible electronic devices require the creation of thin, light, and flexible power supply devices such as flexile electrolytes/insulators, electrode materials, current collectors, and batteries that play an important role in packaging. Demand will require the progress of modern electrode materials with high capacity, rate capability, cycle stability, electrical conductivity, and mechanical flexibility for the time to come. The integration of high electrical conductivity and flexible buckypaper (oxidized Multi-walled carbon nanotubes (MWCNTs) film) and high theoretical capacity silicon materials are effective for obtaining superior high-energy-density and flexible electrode materials. Therefore, this study focuses on improving the high-capacity, capability-cycling stability of the thin-film Si buckypaper free-standing electrodes for lightweight and flexible energy-supply devices. First, buckypaper (oxidized MWCNTs) was prepared by assembling a free stand-alone electrode, and electrical conductivity tests confirmed that the buckypaper has sufficient electrical conductivity (10-4(S m-1) in LIBs) to operate simultaneously with a current collector. Subsequently, silicon was deposited on the buckypaper via magnetron sputtering. Next, the thin-film Si buckypaper freestanding electrodes were heat-treated at 600 °C in a vacuum, which improved their electrochemical performance significantly. Electrochemical results demonstrated that the electrode capacity can be increased by 27/26 and 95/93 μAh in unheated and heated buckypaper current collectors, respectively. The measured discharge/charge capacities of the USi_HBP electrode were 108/106 μAh after 100 cycles, corresponding to a Coulombic efficiency of 98.1%, whereas the HSi_HBP electrode indicated a discharge/charge capacity of 193/192 μAh at the 100th cycle, corresponding to a capacity retention of 99.5%. In particular, the HSi_HBP electrode can decrease the capacity by less than 1.5% compared with the value of the first cycle after 100 cycles, demonstrating excellent electrochemical stability.

Minimizing two-dimensional Ti3C2Tx MXene nanosheet loading in carbon-free silicon anodes

Silicon anodes are promising for high energy batteries because of their excellent theoretical gravimetric capacity (3579 mA h g-1). However, silicon's large volume expansion and poor conductivity hinder its practical application; thus, binders and conductive additives are added, effectively diluting the active silicon material. To address this issue, reports of 2D MXene nanosheets have emerged as additives for silicon anodes, but many of these reports use high MXene compositions of 22-66 wt%, still presenting the issue of diluting the active silicon material. Herein, this report examines the question of what minimal amount of MXene nanosheets is required to act as an effective additive while maximizing total silicon anode capacity. A minimal amount of only 4 wt% MXenes (with 16 wt% sodium alginate and no carbon added) yielded silicon anodes with a capacity of 900 mA h gSi-1 or 720 mA h gtotal-1 at the 200th cycle at 0.5 C-rate. Further, this approach yielded the highest specific energy on a total electrode mass basis (3100 W h kgtotal-1) as comapared to other silicon-MXene constructs (∼115-2000 Wh kgtotal-1) at a corresponding specific power. The stable electrode performance even with a minimal MXene content is attributed to several factors: (1) highly uniform silicon electrodes due to the dispersibility of MXenes in water, (2) the high MXene aspect ratio that enables improved electrical connections, and (3) hydrogen bonding among MXenes, sodium alginate, and silicon particles. All together, a much higher silicon loading (80 wt%) is attained with a lower MXene loading, which then maximizes the capacity of the entire electrode.

Self-Catalyzed Vertically Aligned Carbon Nanotube-Silicon Core-Shell Array for Highly Stable, High-Capacity Lithium-Ion Batteries

Here, we report on the simple, catalyst-free preparation and application of carbon nanotube-silicon core-shell composite anodes on stainless steel. The stainless steel mesh structure acts as a self-catalyzing agent for the plasma-enhanced chemical vapor deposition (PECVD) growth of vertically aligned, dense, multiwalled carbon nanotube arrays. The carbon nanotube array then serves as a bed for silicon deposition by the decomposition of silane through chemical vapor deposition (CVD). This approach leads to the formation of highly conductive and stable composite anodes. Silicon deposition on the substrate is controlled in terms of the optimal silicon shell thickness, thus enhancing the performance of the cell. These extremely stable, binder-free composite electrodes were characterized as potential anodes in Li-ion batteries, exhibiting long cycle life (>700 cycles), high gravimetric capacity (>4000 mAh/gSi), low irreversible capacity (<10%), and high Coulombic efficiency (>99.5%). These composite anodes meet the requirements of Li-ion batteries for future portable electronics and electric vehicle applications.