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 1500 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 a very promising anode material for future use in high-performance Li⁺ storage.