Insight on the Failure Mechanism of Sn Electrodes for Sodium-Ion Batteries: Evidence of Pore Formation during Sodiation and Crack Formation during Desodiation

The development of Sn based anode materials for sodium ion batteries is mainly hindered by the limited understanding of sodiation/desodiation mechanisms inside the active material, which typically results in electrode damage. Herein, we report a post-mortem ex-situ scanning electron microscopic analysis of Sn thin film motivated by the intention to elucidate these structural mechanisms. Our results reveal for the first time that the surface of Sn electrode film becomes highly porous during sodiation with no presence of obvious cracks, a surprising result when compared to previous reports performed on Sn particles. Even more surprisingly, sequential ex-situ SEM observations demonstrate that, once the desodiation starts and reaches the second desodiation plateau (0.28 V), obvious cracks in the Sn film are instead observed along with porous islands of active material. These islands appear as aggregated particles which further split into smaller islands when the desodiation potential reaches its maximum value (2.0 V). Finally, for the first time, the experimental value of the sodium diffusion coefficient inside Sn was measured (3.9 × 10–14 cm2 s–1) using electrochemical impedance spectroscopy

Direct Synthesis of Carbon-Doped TiO₂–Bronze Nanowires as Anode Materials for High Performance Lithium-Ion Batteries

Carbon-doped TiO₂–bronze nanowires were synthesized via a facile doping mechanism and were exploited as active material for Li-ion batteries. We demonstrate that both the wire geometry and the presence of carbon doping contribute to the high electrochemical performance of these materials. Direct carbon doping for example reduces the Li-ion diffusion length and improves the electrical conductivity of the wires, as demonstrated by cycling experiments, which evidenced remarkably higher capacities and superior rate capability over the undoped nanowires. The as-prepared carbon-doped nanowires, evaluated in lithium half-cells, exhibited lithium storage capacity of ∼306 mA h g^–1 (91% of the theoretical capacity) at the current rate of 0.1<i>C</i> as well as excellent discharge capacity of ∼160 mAh g^–1 even at the current rate of 10 C after 1000 charge/discharge cycles