Zinc niobate materials: crystal structures, energy-storage capabilities and working mechanisms

Abstract

W₅Nb₁₆O₅₅ is a very promising negative electrode compound for lithium-ion storage owing to its good safety, durable cyclability and high rate performance. However, its commercialization is hindered by its limited capacity and high tungsten cost. Here, we designed a tungsten-free and niobium-rich zinc niobate (Zn₂Nb₃₄O₈₇) with a large capacity (theoretically 389 mA h g⁻¹) as a new insertion negative electrode compound. Micron-sized Zn₂Nb₃₄O₈₇ blocks (Zn₂Nb₃₄O₈₇-B) and one-dimensional Zn₂Nb₃₄O₈₇ nanofibers (Zn₂Nb₃₄O₈₇-N) were prepared via a solid-state reaction and electrospinning methods, respectively. Zn₂Nb₃₄O₈₇-B and Zn₂Nb₃₄O₈₇-N have 3 × 4 × ∞ shear ReO₃ crystal structures of orthorhombic and monoclinic types, respectively. Both the Zn₂Nb₃₄O₈₇ materials exhibited better electrochemical performance than W₅Nb₁₆O₅₅ in terms of the capacity, lithium-ion diffusion coefficient, intercalation pseudocapacitive contribution, rate performance, and cyclability. In situ X-ray diffraction characterization demonstrated the excellent structural stability and electrochemical reversibility of Zn₂Nb₃₄O₈₇, and revealed that lithium ions were stored in the (010) crystallographic planes. Furthermore, a LiNi_0.5Mn_1.5O₄‖Zn₂Nb₃₄O₈₇-N full cell exhibited outstanding electrochemical performance, including a large capacity, prominent rate performance, and especially ultra-long cyclability (96.5% capacity retention even after 1000 cycles at 5C). Therefore, Zn₂Nb₃₄O₈₇ holds great promise as a practical negative electrode compound for large-capacity, cost-effective, rapid, durable and safe lithium-ion storage.