Polyvinylpyrrolidone-assisted synthesis of ultrathin multi-nanolayered Cu₂Nb₃₄O_87-x for advanced Li⁺ storage

The ultrathin multi-nanolayered structure with ultrathin monolayer thickness (<10 nm) and certain interlayer spacing can significantly shorten Li⁺ paths and alleviate the volume effect for Li⁺-storage materials. However, unlike layered materials such as MXene and MoS₂, shear ReO₃-type niobates have difficulty forming ultrathin multi-nanolayered structures due to their crystal structures, which still remains a challenge. Herein, by a polyvinylpyrrolidone (PVP)-assisted solvothermal method, we first synthesize ultrathin multi-nanolayered Cu₂Nb₃₄O_87-x with oxygen vacancies composed of ultrathin nanolayers (2-10 nm in thickness) and interlayer spacing (1-5 nm). Oxygen vacancies can radically enhance the inherent electronic/ionic conductivity and Li⁺ diffusion coefficient of this material. The PVP-induced formation mechanism of this material is expounded in detail. The well-preserved ultrathin multi-nanolayered structure and excellent multi-electron electrochemical reversibility (Nb⁵⁺ ↔ Nb⁴⁺ ↔N b³⁺ and Cu²⁺ ↔ Cu⁺) of this material during cycling are fully verified. Based on an ultrathin multi-nanolayered structure and oxygen vacancies, this material as the anode of lithium-ion batteries is highly competitive among reported shear ReO₃-type Cu-Nb-O anodes, displaying a high reversible capacity (315.3 mAh g^-1 after 300 cycles at 1 C), durable cycling stability (85.7 % capacity retention after 1000 cycles at 10 C), and outstanding rate performance. Moreover, the application of this material to lithium-ion capacitors generates a large energy density (97.9 Wh kg^-1 at 87.5 W kg^-1) and a high power density (17,500 W kg^-1 at 12.6 Wh kg^-1), thus further indicating its fast faradaic pseudocapacitive behavior for practical applications. The results of this work indicate a breakthrough in synthesizing ultrathin multi-nanolayered shear ReO₃-type niobates.