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.