N-Doped C@Zn 3 B 2 O 6 as a Low Cost and Environmentally Friendly Anode Material for Na-Ion Batteries: High Performance and New Reaction Mechanism.

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.