W₇Nb₄O₃₁ Nanorods with a Mixed Crystal Structure: A Very Fast- and Stable-Charging Anode Material for Aqueous Lithium-Ion Batteries

Aqueous lithium-ion batteries (ALIBs) have attracted extensive interest since the safety problems of traditional lithium-ion batteries are waived. Although the energy density of ALIBs is improved, their rate capability and power density are still poor due to the slow Li+ diffusivity of the existing anode materials, and their cyclic stability is also poor. Here, W7Nb4O31 nanorods with very fast- and stable-charging capability are explored as a new anode material for ALIBs for the first time. This material owns different tetragonal tungsten bronze (TTB) structures together with 4 x 4 ReO3-type blocks confined by TTB matrices, allowing abundant pentagonal and quadrangular tunnels for Li+ transport. These large-sized tunnels combine with the large interlayer spacing (approximate to 3.95 & Aring;) not only lead to extremely fast Li+ diffusivity but also small unit-cell volume variations (maximum 2.1%) during lithiation/delithiation, thereby enabling the LiMn2O4//W7Nb4O31 full cell to possess excellent rate capability with a 50C versus 1C capacity ratio of 68.3%, ultrahigh power density of 9854 W kg(-1), and superior cyclic stability with capacity retention of 89.7/66.7/72.0% at 1C/5C/50C over 1000/10 000/10 000 cycles. This comprehensive study demonstrates that the W7Nb4O31 nanorods are highly promising for fast- and stable-charging ALIBs.