Significant improvement in reversibility of MWCNT-Sn compound composite electrode: Nanostructure effect of MWCNT-Sn compound composite on high initial reversible capacity

Abstract

Using a simple wet method, a composite anode in which Sn, SnO, and SnO2 were inserted into a multiwall carbon nanotube (MWCNT), was fabricated. The synthesized composites had Sn, SnO, and SnO2 compounds in the MWCNT only, with these compounds not present outside the MWCNT. This is because the precursors penetrate the MWCNT through channel-like defects (CLDs) formed on the MWCNT walls by acid treatment; the Sn-compounds are formed from these precursors by post-annealing. The synthesized MWCNT-Sn compound composite as an anode showed high capacity retention efficiency during 85 charge and discharge cycles. The initial average discharge capacity after 5 cycles, and the average discharge capacity in the 75th - 85th cycles were 743 mAh/g and 660 mAh/g, respectively. The initial Coulombic efficiency was particularly high, reaching similar to 96.3%. The dramatic improvement in the initial capacity retention characteristics, as well as the capacity retention during these subsequent cycles is attributed to four effects: (1) reduction of irreversible Li2O phase formation during charging due to low oxidation states of Sn such as zero and monovalent states; (2) easy reduction of the Li2O phase during discharge by a large Sn-Li2O grain-boundary area formed by small Sn compound particles; (3) suppression of the Sn compounds' particle destruction due to the wrapping effect of the MWCNT; and (4) suppression of the increase in interfacial resistance between the current collector and the negative electrode slurry. These results provide a meaningful insight into the rational design of composites in which Sn, SnO, and SnO2 nanoparticles exist in the MWCNT only. This is significant because MWCNT is one of the anode materials for power supplies that can be used in various environments in which the application of metal Li is difficult. (C) 2018 Elsevier B.V. All rights reserved.