Self-Improving Na Ion Storage in Oxygen Deficient, Carbon Coated Self-Organized TiO₂ Nanotubes

Abstract

The development of sodium ion batteries is largely motivated by the growing cost and limited resources of lithium. Titanium dioxide (TiO 2 ), in the form of selforganized and well-oriented nanotube arrays, are considered as a highly attractive anode material for sodium ion batteries, due to their inherent safety, low cost, and structural stability. This work reports on the sodiation and desodiation characteristics of anodically grown, self-organized TiO 2 nanotubes annealed in an Ar/C 2 H 2 atmosphere (TiO 2−x − C). It is found that anatase TiO 2−x −C nanotubes demonstrate substantial self-improving charge storage capacities as cycling proceeds, leading to a specific capacity of 202.2 ± 50.6 mAh g −1 at a current rate of 30 mA g −1 (C/20). Subsequent kinetic analysis reveals a pseudocapacitive contribution which dominates the Na storage process in TiO 2−x −C nanotubes at fast sodiation rates. This pseudocapacitance in TiO 2−x −C nanotubes is found to enable exceptional high-rate capabilities with a specific capacity of 58.4 ± 14.6 mAh g −1 at a current rate of 12 A g −1 (20C).

Recent Progress in Understanding Ion Storage in Self‐Organized Anodic TiO2 Nanotubes

“…At fast sodiation rates, the Na‐ion storage process in TiO 2− x –C NTs is mainly dominated by pseudocapacitive contributions, attributed to faradaic charge‐transfer processes at the surface. At very slow rates of 0.1 mV s −1 , unity between faradaic contributions, which arise from Na‐ion insertion into the NTs and which are accompanied by a reduction of Ti 4+ to Ti 3+ , and the capacitive charge was found . This study reveals that the insertion reaction into carbon‐coated anatase NTs is mainly dominated by two processes: an interfacial Na‐ion electrosorption at high scan rates and Na‐ion insertion into the bulk structure that proceeds at relatively slow rates .…”

“…At very slow rates of 0.1 mV s −1 , unity between faradaic contributions, which arise from Na‐ion insertion into the NTs and which are accompanied by a reduction of Ti 4+ to Ti 3+ , and the capacitive charge was found . This study reveals that the insertion reaction into carbon‐coated anatase NTs is mainly dominated by two processes: an interfacial Na‐ion electrosorption at high scan rates and Na‐ion insertion into the bulk structure that proceeds at relatively slow rates . Although the clearly visible peaks in the CVs indicate an insertion/extraction mechanism, detailed understanding of this insertion reaction, which only appears after prolonged cycling, still needs to be gained.…”

“…The specific capacity of the anatase TiO 2 NTs steadily increased during cycling, reaching 130 µAh cm −2 after 300 reversible charge/discharge cycles. A recent study by Portenkirchner et al demonstrated a self‐improving cycling behavior for oxygen‐deficient, carbon‐coated anatase TiO 2 NTs, which leads to a capacity of 202.2 mAh g −1 at slow rates (C/20) and 58.4 mAh g −1 at fast rates (20 C) . Remarkable Na‐ion storage capacity was found for sulfur (S)‐doped and phosphorylated TiO 2 NT arrays.…”

“…Analogous to the previously discussed carbon‐coated TiO 2− x –C for LIBs, Portenkirchner et al studied the Na storage mechanism of oxygen‐deficient, carbon‐coated, anatase TiO 2− x –C NT . A special focus was put on the determination of surface vs bulk contribution to the overall sodiation process.…”

Sodiation mechanism via reversible surface film formation on metal oxides for sodium‐ion batteries

“…Surprisingly though, little attention has been devoted to unraveling the reaction pathway and mechanism of oxide film sodiation. We believe that the understanding of surface film formation and removal, [ 32 ] during battery charging and discharging is key to understanding sodiation reactions of oxide based and oxide covered materials, in general. Previously, we were able to demonstrate that long term galvanostatic charge/discharge cycling of oxygen deficient, carbon coated self‐organized titanium dioxide (TiO 2‐x −C) NTs in SIBs is subject to a significant self‐improving charge storage behavior.…”