Gas free oxidation of NaCN for presodiating and stabilizing the anodic host of sodium-ion capacitors

Abstract

Sodium-ion capacitors (NICs) trigger considerable attention due to their higher specific energy than electrical double-layer capacitors (EDLCs) at comparable specific power. However, the presodiation process of the anodic host is extremely crucial for the construction of high-performance NICs. Herein, a positive EDL electrode containing activated carbon (AC) mixed with sodium cyanide (NaCN) as a sacrificial material was electrochemically oxidized to presodiate a Sn4P3 anodic host buffered by hard carbon (HC). The oxidation of CN− occurred ca. 2.9 V vs. Na/Na+ and finished by a short region of linearly increasing potential with a total capacity close to the theoretical value of 547 mAh g−1. The operando electrochemical mass spectrometry (EMS) analysis of the atmosphere in the cell together with the internal pressure measurements realized during the galvanostatic oxidation of a YP80F-NaCN electrode demonstrate that the process occurs without any gas evolution. A precursor cell of an NIC was constructed in a pouch with YP80F-NaCN and HC/Sn4P3 electrodes. After the oxidative sodium transfer from NaCN to HC/Sn4P3, the realized YP80F//Nax(HC/Sn4P3) NIC demonstrated a discharge capacitance retention higher than 80% for 8900 cycles in the voltage range from 2.0 to 3.8 V. The infrared analysis of the anode obtained by the herein described transfer process detected polycyanogen, which stabilizes the electrode structure during cycling, and thereof is at the origin of the enhanced life span of the NIC.