N-Doped C@Zn₃ B₂ O₆ as a Low Cost and Environmentally Friendly Anode Material for Na-Ion Batteries: High Performance and New Reaction Mechanism.

Abstract

Na-ion batteries (NIBs) are ideal candidates for solving the problem of large-scale energy storage, due to the worldwide sodium resource, but the efforts in exploring and synthesizing low-cost and eco-friendly anode materials with convenient technologies and low-cost raw materials are still insufficient. Herein, with the assistance of a simple calcination method and common raw materials, the environmentally friendly and nontoxic N-doped C@Zn₃ B₂ O₆ composite is directly synthesized and proved to be a potential anode material for NIBs. The composite demonstrates a high reversible charge capacity of 446.2 mAh g^-1 and a safe and suitable average voltage of 0.69 V, together with application potential in full cells (discharge capacity of 98.4 mAh g^-1 and long cycle performance of 300 cycles at 1000 mA g^-1 ). In addition, the sodium-ion storage mechanism of N-doped C@Zn₃ B₂ O₆ is subsequently studied through air-insulated ex situ characterizations of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared (FT-IR) spectroscopy, and is found to be rather different from previous reports on borate anode materials for NIBs and lithium-ion batteries. The reaction mechanism is deduced and proposed as: Zn₃ B₂ O₆ + 6Na⁺ + 6e^- ⇋ 3Zn + B₂ O₃ ∙ 3Na₂ O, which indicates that the generated boracic phase is electrochemically active and participates in the later discharge/charge progress.