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

Abstract

Long term galvanostatic charge/discharge cycling of oxygen deficient, carburized and self‐organized titanium dioxide (TiO2) nanotubes (NTs) in sodium ion (Na) batteries (SIBs) are subject to a significant self‐improving charge storage behavior. Surface reactions upon sodiation of carburized NTs form acicular surface films that can be reversibly cycled. We show that, alongside organic species from the decomposition of the electrolyte, mainly inorganic compounds, such as Na2O2 and Na2CO3, are the main constituents. These components possess a characteristic acicular morphology. Na2O2 is found to form upon sodiation and converted to NaO2 upon desodiation. This, in combination with its pseudo‐capacitive charge storage characteristics, explains the excellent rate capability measured for TiO2‐x‐C NTs. The observed high reversibility of this surface chemistry is also essential for the fast kinetics and the high capacity retention found in the system. Our findings point to a more general Na‐ion storage mechanism, that is potentially relevant to other transition metal oxides also.

Substantial Na-Ion Storage at High Current Rates: Redox-Pseudocapacitance through Sodium Oxide Formation

“…The analysis at different scan rates shows that the capacitive contribution progressively increases with increasing scan rate ( Figure 1 d) up to 94% capacitive current at a scan rate of 20 mV s −1 . The Na-ion charge storage is therefore governed by pseudo-capacitive characteristics, allowing for the excellent rate capabilities and storage capacities measured [ 21 , 24 ]. The prevailing Na-ion storage mechanism, as elucidated in a previous publication, shows that mainly inorganic compounds, such as NaO 2 , Na 2 O 2 , and NaCO 3 , are the main constituents formed at the electrodes surface with a characteristic acicular morphology ( Figure 2 ) [ 24 ].…”

“…The whole assembling process has been carried out in an Ar-filled glove box with H 2 O and O 2 levels below 0.1 ppm. The galvanostatic sodiation/desodiation was carried out between 3.0 and 0.1 V vs. Na/Na + until the current increase, described by a logistic growth function [ 24 ], levels off at 117, 59, 24, 12, 6 µA for the TiO 2−x flat surface films and 200 µA, 40 µA, 20 µA, and 10 µA for TiO 2−x NTs.…”

“…( c ) SEM top view image of TiO 2−x NTs after 230 galvanostatic cycles between 3.0 V and 0.1 V showing the surface coverage after sodiation by an acicular surface film. Reproduced and modified in part with permission from reference [ 24 ].…”

“…It was found that sodium superoxide (NaO 2 ) and sodium peroxide (Na 2 O 2 ), formed at the electrode surface in an acicular surface film, are the main constituents of the charge-storage products. The Na-ion storage is thereby dominated by pseudocapacitive charge storage at high sodiation rates [ 24 ]. Furthermore, in a very recent publication, it has been demonstrated that this surface chemistry is not unique to TiO 2 NTs, but can be seen as a more general characteristic for Na-ion storage at metal oxide surfaces.…”

Sodium‐Containing Surface Film Formation on Planar Metal–Oxide Electrodes with Potential Application for Sodium‐Ion and Sodium–Oxygen Batteries

“…[12] The prevailing Na-ion storage mechanism has been elucidated in a previous publication, showing that alongside organic species from the decomposition of the electrolyte, mainly inorganic compounds, such as Na 2 O 2 and NaCO 3 , with a characteristic acicular morphology, are the main constituents. [13] The formation of Na 2 O 2 upon sodiation and its conversion to sodium superoxide (NaO 2 ) upon desodiation are characterized by pseudocapacitive charge storage characteristics, allowing excellent rate capabilities and storage capacities measured for TiO 2Àx -C NTs. [12,13] This is particularly interesting since the initial discharge product in sodium-oxygen batteries is shown to be NaO 2 , which undergoes dissolution and then transforms to Na 2 O 2 and Na 2 O 2 dihydrate.…”