Three-dimensional carbon nanotube networks enhanced sodium trimesic: a new anode material for sodium ion batteries and Na-storage mechanism revealed by ex situ studies

Recently, room temperature sodium ion batteries (SIBs) have attracted considerable attention as one of the promising candidates to replace lithium ion batteries. Nevertheless, achieving high capacity and cycling stability remains a great challenge for the electrode materials of SIBs. Compared to the traditional inorganic electrode materials, organic ones should be more attractive because of their easier sodium (Na)-transport accessibility as well as their diversities of organic skeleton and functional groups. In this work, a new carboxyl-based organic, sodium trimesic (Na3TM), is proposed for the first time as an anode material for SIBs, and its Na-storage properties are significantly enhanced by constructing three-dimensional conductive networks of carbon nanotubes (CNT-NWs) in the Na3TM microparticles. In comparison to the pure Na3TM exhibiting almost inactive Na storage, the prepared CNT-NWs@Na3TM composite delivers a reversible capacity of 214.6 mA h g−1 at 0.1 A g−1, and exhibits excellent rate performance with the specific capacities of 149 and 87.5 mA h g−1 at 1 and 10 A g−1, respectively. The CNT-NWs@Na3TM also exhibit good cycling performance. More importantly, the Na-storage mechanism of CNT-NWs@Na3TM was ascertained using several ex situ technologies of Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and 23Na solid-state nuclear magnetic resonance spectroscopy. It is discovered that the two Na uptake/release processes were reversible during cycling and contributed to the Na-storage capacity except for the 1st sodiation process with a three Na uptake.