Facile and Scalable Development of High-Performance Carbon-Free Tin-Based Anodes for Sodium-Ion Batteries

Tin (Sn)-based anodes for sodium (Na)-ion batteries possess higher Na-storage capacity and better safety aspects compared to hard carbon -based anodes but suffer from poor cyclic stability due to volume expansion/contraction and concomitant loss in mechanical integrity during sodiation/desodiation. To address this, the usage of nanoscaled electrode-active particles and nanoscaled-carbon-based buffers has been explored, but with compromises with the tap density, accrued irreversible surface reactions, overall capacity (for "inactive" carbon), and adoption of non-scalable/complex preparation routes. Against this backdrop, anode-active "layered" bismuth (Bi) has been incorporated with Sn via a facile-cum-scalable mechanical-milling approach, leading to individual electrode-active particles being composed of well-dispersed Sn and Bi phases. The optimized carbon-free Sn-Bi compositions, benefiting from the combined effects of "buffering" action and faster Na transport of Bi, to go with the greater Na-storage capacity and lower operating potential of Sn, exhibit excellent cyclic stability (viz., ∼83-92% capacity retention after 200 cycles at 1C) and rate capability (viz., no capacity drop from C/5 to 2C, with only ∼25% drop at 5C), despite having fairly coarse particles (∼5-10 μm). As proven by operando synchrotron X-ray diffraction and stress measurements, the sequential sodiation/desodiation of the components and, concomitantly, stress build-ups at different potentials provide "buffering" action even for such "active-active" Sn-Bi compositions. Furthermore, the overall stress development upon sodiation of Bi has been found to be significantly lower than that of Sn (by a factor of ∼3.8), which renders Bi promising as a "buffer" material, in general. Dissemination of such complex interplay between electrode-active components during electrochemical cycling also paves the way for the development of high-performance, safe, and scalable "alloying-reaction"-based anode materials for Na-ion batteries and beyond, sans the need for ultrafine/nanoscaled electrode particles or "inactive" nanoscaled-carbon-based "buffer" materials.