High Energy Density CNT/NaI Composite Cathodes for Sodium‐Ion Batteries

Abstract The Na‐iodine system has potential as a positive electrode in sodium‐ion batteries due to its fast reaction kinetics and high energy density. Here, the fabrication of a NaI‐loaded carbon nanotube (CNT) mat cathode is presented and the use of NaI as a Na‐ion battery cathode is reported for the first time. Freestanding electrodes show a capacity retention of 92% after 100 cycles at 100 mA h g −1 and a high specific energy density (≈473 W h kg −1 , second discharge basis vs Na metal) and small hysteresis (0.03 V). Adding a CNT mat interlayer between the NaI composite cathode and the separator, and fluoroethylene carbonate to the electrolyte significantly suppresses shuttling of active materials during cycling. Using ex situ X‐ray photoelectron spectroscopy and Raman data, an understanding of the relevant processes occurring during cycling of the CNT/NaI cathode is developed.