Effects of the chemical etching of single-walled carbon nanotubes on their lithium storage properties

Research highlights: → The SWCNTs are effectively purified by acidic treatment and gas-phase oxidation. → The etched SWCNTs have excellent Li storage properties. → The etched SWCNTs have the opened ends and the shortened length. → The etched SWCNTs have various surface functional groups. → The etched SWCNTs are consistent with a nongraphitic-carbon-insertion mechanism. - Abstract: The effects of chemical etching on lithium (Li) storage properties of single-walled carbon nanotubes (SWCNTs) were investigated. The SWCNTs were synthesized on supported catalysts by thermal chemical-vapor deposition method, purified, and chemically etched in an acid solution. The purified SWCNTs and the etched SWCNTs were electrochemically inserted and extracted with Li. The structural and chemical modifications in the etched SWCNTs change the Li storage properties of the etched SWCNTs. The reversible capacity (Crev) increases with the etching time, from 616 mAh g-1 (Li1.7C6) for the purified SWCNTs to 878 mAh g-1 (Li2.4C6) for the etched SWCNTs after etching for 10 h, and decreases slightly after further etching. The irreversible capacity (Cirr) also increases continuously with the etching time, from 1573 mAh g-1 (Li4.2C6) for the purified SWCNTs to 1772 mAh g-1 (Li4.8C6) for the etched SWCNTs after etching for 20 h. The insertion of Li ions into the etched SWCNTs is facilitated by various Li insertion sites formed during the chemical etching process. The Li ions inserted into various insertion sites enhance the Crev in the etched SWCNTs with the large voltage hysteresis by hindrance of the extraction of Li ions from the etched SWCNTs. During the charge/discharge cycling, the Li ions inserted into the etched SWCNTs are extracted easily at the initial stage of cycling, but an amount of the extracted Li ions from the etched SWCNTs decreases due to the accumulation of Li ions in the etched SWCNTs.