Constructing dimensional gradient structure of Na3V2(PO4)3/C@CNTs-WC by wolfram substitution for superior sodium storage

Na3V2(PO4)(3) (NVP) has gained enormous attentions due to the high capacity and voltage platform. Nevertheless, poor intrinsic electronic conductivities severely restrict the further development. Herein, a feasible strategy to construct unique dimensional gradient structure of the wolfram doped NVP is proposed via a facile sol-gel method. The introduction of W6+ with the smaller ionic radius parallelized with local V3+ ions could stabilize the open NASICON structure of NVP and generate vast of beneficial holes to accelerate the electronic transportation. Notably, the wolfram can combine with the superfluous carbon layer and form the new conductive WC phase to further promote the electronic conductivity. Meanwhile, the coated carbon layers and enwrapped CNTs construct an effective conductive framework for accelerated electronic migration. The twining CNTs promote to inhibit the growth of the sintered grains, reducing the particle size to provide shortened pathway for the migration of Na+. Distinctively, the modified Na2.97V2.99W0.019(PO4)(3)/C@CNTs (W0.01-NVP) composite delivers a superior electrochemical performance. Significantly, it can submit a high reversible capability of 112.5 mAh g(-1) at 5C with retaining 86% initial capacity over 500 cycles. As for a super high rate of 50C, it reveals an incredible capacity of 92.6 mAh g(-1) and remains 84.5 mAh g(-1) over 400 cycles, corresponding to a high retention of 91.30%. The superior electrochemical performance is derived from the enhanced crystal structure resulted from wolfram substitution and the beneficial dimensional conductive system consisting of new WC phase, coated carbon layers and enwrapped CNTs.