Probing the Critical Role of Sn Content in SnSb@C Nanofiber Anode on Li Storage Mechanism and Battery Performance

The minimization of the detrimental effects as a result of the drastic volume changes (few hundred times) occurring during repeated alloying-dealloying of lithium with group IV elements, e.g., tin (Sn), is a major challenge. An important design strategy is to have Sn as a component in a binary compound. SnSb is an important example where the antimony (Sb) itself is redox active at a potential higher than that of Sn. The ability of Sb to alloy with Li reduces the Li uptake amount of Sn in SnSb compared to that in bare Sn. Thus, the volume changes of Sn in SnSb will expectedly be much lower compared to that in bare Sn, leading to greater mechanical stability and cyclability. As revealed recently, the complete reformation of SnSb (for a molar ratio of Sn/Sb = 1:1) during charging is not achieved due to the loss of some fraction of Sn. Thus, the molar concentration of Sn and Sb in SnSb is also absolutely important for the optimization of battery performance. We discuss here SnSb with varying compositions of Sn encapsulated inside an electrospun carbon nanofiber (abbreviated as CF). The carbon-nanofiber matrix not only provides electron transport pathways for the redox process but also provides ample space to accommodate the drastic volume changes occurring during successive charge and discharge cycles. The systematic changes in the chemical composition of SnSb minimize the instabilities in SnSb structure as well as replenish any loss in Sn during repeated cycling. The composition plays a very crucial role, as magnitude of specific capacities and cyclability of SnSb are observed to depend on the variable percentage of Sn. SnSb-75-25-CF, which contains excess Sn, exhibits the highest specific capacity of 550 mAh g^-1 after 100 cycles in comparison with pure SnSb (1:1) anode material at a current density of 0.2 A g^-1 and shows excellent rate capability over widely varying current densities (0.2-5 A g^-1).