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

Abstract

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 electrodeactive 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 mechanicalmilling 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 "alloyingreaction"-based anode materials for Na-ion batteries and beyond, sans the need for ultrafine/nanoscaled electrode particles or "inactive" nanoscaled-carbon-based "buffer" materials.

Understanding the Electrolyte Chemistry Induced Enhanced Stability of Si Anodes in Li-Ion Batteries based on Physico-Chemical Changes, Impedance, and Stress Evolution during SEI Formation

“…In-plane equi-biaxial stress was then estimated from the substrate curvature data using Stoney's equation [σ = Eh s 2 /6(1-ν)h f R; where, E, h s, and ʋ are Young's modulus, thickness, and Poisson's ratio of the substrate, respectively, and h f is the thickness of the active film (Si)], which is also mentioned, along with further details of the setup and terminologies used, in our previously published works. 34,[52][53][54] Since the as-measured substrate curvature has been normalized by the initial thickness of the Si film and not the instantaneous thickness with progress in lithiation/ delithiation, the as-reported stress has been referred to as "nominal stress." In addition to the stress measurements during galvanostatic cycling, the stress development associated just with the SEI formation was monitored by holding the cells at a constant potential of 0.8 V (vs. Li/Li + ) for 20 h (as per pre-established protocols) in the presence of the two electrolyte types (viz., with and without FEC).…”