Studies on the improved electro-chemical performance and the sodium ion migration mechanism of Na0·44MnO2-CNT electrodes for aqueous sodium batteries

Abstract

Here, A kind of carbon nanotubes (CNT) wrapping Na0·44MnO2 has been synthesized via a high-temperature solid state method and showed better electro-chemical performance in 1 M Na2SO4 aqueous solution as cathode material for aqueous sodium ion batteries. This rod-like nano-composite exhibited a first charge capacity of 70.1 mA h·g−1 with the reversible capacity retention of about 63.4% after 300 cycles at a current density of 50 mA g−1. It can also remain reversible specific capacity of 53.1, 44.7, 34.8, and 19.7 mA h·g−1 at the current density of 1 C, 2 C, 5 C and 10 C, respectively. The Na+ migration mechanism and kinetics of Na0·44MnO2 are fully investigated by molecular dynamics (MD) and the density functional theory (DFT). A SQS-8 quasi-random structure is used to model the structure of the material. The results show that Na+ is highly vigorous and propagates 1D along the c-axis. A minimum of 0.081 eV and 0.51 eV is required for Na+ to travel through the large S-shaped tunnel and the small tunnel, respectively.