Porous Nb4W7O31 microspheres with a mixed crystal structure for high-performance Li+ storage

Niobium-tungsten oxides with tungsten bronze and confined ReO3 crystal structures are prospective anode candidates for lithium-ion batteries since the multi-electron transfer per niobium/tungsten offers large specific capacities. To combine the merits of the two structures, porous Nb4W7O31 microspheres constructed by nanorods are synthesized based on a facile solvothermal method. This new material contains different tungsten bronze structures and 4 × 4 ReO3-type blocks confined by tungsten bronze matrices, generating plenty of pentagonal and quadrangular tunnels for Li+ storage, as confirmed by spherical-aberration-corrected scanning transmission electron microscopy. Such structural mixing enables three-dimensionally uniform and small lattice expansion/shrinkage during lithiation/delithiation, leading to good structural and cyclic stability (95.2% capacity retention over 1,500 cycles at 10C). The large interlayer spacing (~3.95 Å), coupled with the abundant pentagonal/quadrangular tunnels, results in ultra-high Li+ diffusion coefficients (1.24 × 10-11 cm2 s-1 during lithiation and 1.09 × 10-10 cm2 s-1 during delithiation) and high rate capability (10C vs. 0.1C capacity retention percentage of 47.6%). Nb4W7O31 further exhibits a large reversible capacity (252 mAh g-1 at 0.1C), high first-cycle Coulombic efficiency (88.4% at 0.1C), and safe operating potential (~1.66 V vs. Li/Li+). This comprehensive study demonstrates that the porous Nb4W7O31 microspheres are very promising anode materials for future use in high-performance Li+ storage.