Zn 3 B 2 O 6 as a Low Cost and Environmentally Friendly Anode Material for NaIon Batteries : High Performance and New Reaction Mechanism

Content Provider	Semantic Scholar
Author	Wang, Sai Zhang, Xin‐bo
Copyright Year	2018
Abstract	DOI: 10.1002/adma.201805432 transitional metal (Mn, Fe, Cu) oxides. But some drawbacks of these materials are noticeable; for instance, the main capacity of hard carbon is exhibited at potentials that close to Na metal plating which would result in Na dendrite formation and causing safe hidden trouble.[6,7] And the drawback of MnO2 lies in the low capacity which is even lower than 200 mAh g−1 at 50 mA g−1[8]; the application of Fe2O3 as anodes in NIBs calls for necessary integrations with carbon materials which involves the troublesome methods or expensive instruments[9,10]; and about half the charge capacity of CuO originates from the anodic reaction at 2.3 V, indicating it is not suitable for full cells when considering available capacity and average voltage.[11,12] Therefore, efforts in exploring and synthesizing suitable anode materials with convenient technologies and low-cost raw materials are very necessary and significant. Inorganic borates have been widely studied in the past decades due to their practical values as phosphor host materials,[13] microwave dielectric materials,[14] piezoelectricity materials,[15] flame-resistant materials, and so on.[16,17] The borates were also identified as possible candidates of anode materials in Li-ion batteries (LIBs) since 1997 and some follow-up studies researches were focused on FeBO3, Cu3B2O6, Ni3B2O6, Co3B2O6, but they showed much inferior electrochemical performance when compared with the corresponding metal oxides; and the lithium-ion storage mechanisms of these transition metal borates were understood to be similar to those of the corresponding metal oxides that the generated boracic phases had no electrochemistry activity.[18–20] These relatively poor lithium-ion storage abilities might be reasons that the borates were not studied in hot spot of NIBs research field in recent years when considering that the anodes of LIBs with satisfactory performance could be usually directly proved to be practicable in NIBs. Fortunately, in 2017, Fe3BO6@C was synthesized by calcination with the assistance of preball milling and proved to be as anode material for NIBs with reversible capacity of 514.3 mAh·g−1; the sodiumion storage mechanism was studied by ex situ X-ray diffraction (XRD) method and deduced to be the transformation reaction between Na ion and iron oxides after the initial irreversible structural evolution. Namely, the generated boracic phases were Na-ion batteries (NIBs) are ideal candidates for solving the problem of largescale 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@Zn3B2O6 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@Zn3B2O6 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: Zn3B2O6 + 6Na+ + 6e− ⇋ 3Zn + B2O3 ∙ 3Na2O, which indicates that the generated boracic phase is electrochemically active and participates in the later discharge/charge progress. Sodium-Ion Storage
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