Self-assembled ping-pong chrysanthemum spherality NASICON cathode with intercalated structure enables ultralong lifespan and superior thermal safety

Nowadays, morphological regulation and heteroatom substitution are dominated modified methods to optimize Na₃V₂(PO₄)₃ (NVP). Nevertheless, few studies could effectively combine these two routes. Herein, a facile sol-gel method is explored to successfully synthesize ping-pong chrysanthemum-like and Eu-substituted NVP system under the electrostatic self-assembly effects. Notably, the formation mechanism of this unique morphology is completely investigated by ex-situ SEM/Raman/XRD/FTIR. The highly innovative morphology possesses intercalated structure, which elevates the specific surface area and numerous active sites, conducive to reducing the Na⁺ diffusion distance and augmenting the interface area with the electrolyte. The introduced Eu³⁺ plays pillar effects by generating solid Eu-O bond and inducing the lengthen of V-O bond (from 1.97 Å to 2.03 Å) to expand the crystal framework and thus facilitating Na⁺ migration, which has been demonstrated by XAFS. Moreover, in-situ XRD reveals the charge compensation mechanism of reversible phase transition reaction in the Eu-doped NVP composite. The slighter volume shrinkage from 0.688 % to 0.159 % after Eu replacing V indicates the enhanced structural stability of Eu0.07@CNTs sample. Furthermore, DFT calculations deeply uncover the beneficial effects on electronic structure of Eu-substitution for NVP, containing the reduced DOS band energy and Na⁺ migration barrier, as well as lower binding energy between Na⁺ and NVP framework. Consequently, Eu0.07@CNTs achieves superior electrochemical performance in half and three type full cells. Moreover, ARC also verifies it has optimized thermal stability.