Cobalt Doping of NaVTi(PO) Enables a High-Energy NASICON-Type Cathode Material for Sodium-Ion Batteries.

Natrium superionic conductor (NASICON) compounds have emerged as a rising star in the field of sodium-ion batteries (SIBs) owing to their stable framework structure and high Na ionic conductivity. The NASICON-structured NaVTi(PO) manifests significant potential as Na storage material, characterized by decent rate capability and cyclability. However, the low redox potential of Ti/Ti and undesirable energy density limit its practical applications. We developed a NASICON-structured NaCoVTi(PO) (NCTVP) cathode material by doping an appropriate amount of cobalt into NaVTi(PO). Cobalt doping introduces a Co/Co redox couple at ~4.1 V and activates the V/V redox at ~3.9 V, resulting in significantly increased medium discharge voltage and capacity. NCTVP demonstrates a high capacity of over 160 mAh g at 20 mA g. With a medium discharge voltage of ~2.7 V, the energy density of NCTVP reaches 432.0 Wh kg. NCTVP also demonstrates desirable cycling stability (87.4% retention for 100 cycles at 50 mA g). In situ X-ray diffraction discloses a solid solution reaction mechanism for NCTVP, while the galvanostatic intermittent titration technique demonstrates fast Na diffusion kinetics. NCTVP also demonstrates high capacity and good cyclability in full cells. This contribution demonstrates an effective approach for the construction of NASICON materials for SIBs.