Synthesis and electrochemical performance of bismuth–vanadium oxyfluoride

Abstract

Bismuth–vanadium oxyfluoride (Bi₂VO₅F) has been synthesized using a simple, solid-state reaction process at different sintering temperatures. The structure and performance of the samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge experiments. The results show that bismuth–vanadium oxyfluoride belongs to a tetragonal crystal system with space group I4mm. The sample that was synthesized at 550 °C (P550) exhibits relatively good electrochemical properties. Sample P550 shows a high, initial discharge capacity of 222 mAh g⁻¹ at a rate of 100 mA g⁻¹ between 1.4 and 3.5 V. Sample P550 also shows acceptable electrochemical cycling properties. After the first cycle, the discharge specific capacity remains between 106 and 155 mAh g⁻¹, which plateaus between 2.1 and 1.9 V during the first 15 cycles.

Introduction

Almost all of the cathode electrodes for lithium ion batteries are based on intercalation reactions [1], [2]. In the age of portable technologies, the need for small electricity supplies continues to grow steadily, and the energy densities of these cathode materials will not be sufficient to meet society's demands. Recently, new cathode materials have attracted considerable attention.

Li [3] and Badway [4] have reported on the electrochemical reactions of a variety of metal fluorides (TiF₃, VF₃, MnF₂, FeF₂, CoF₂, NiF₂, CuF₂, CaF₂, BaF₂), metal oxides (TiO, TiO₂, VO₂, V₂O₃, V₂O₅, NbO, NbO₂, Cr₂O₃, RuO₂, MoO₃), TiS₂, and TiN with lithium in nonaqueous lithium cells. In most cases, deep Li uptake occurs via heterogeneous reversible conversion reactions that results in the transformation of MX_m (M = transition mental; X = F or S or N) into a nanocrystalline or amorphous Li_m/nX/M composite from which Li can be extracted to restore the MX_m phase [5]. It has been demonstrated that large capacities can be obtained through these reversible conversion reactions. In reversible conversion reactions, all of the oxidation states of the active material participate. The overall reaction for the conversion can be summarized as follows:

$$m{\text{Li}}^{+}+m{\text{e}}^{-}+{\text{MX}}_n\iff {\text{nLi}}_{m/n}\text{X}+\text{M}$$

where M stands for a cation and X stands for an anion [6].

The primary lithium battery, which uses BiF₃ as the cathode material, gives practical energy densities in excess of 200 mAh g⁻¹ at a rate of 0.5 mA cm⁻² at 1.5 V at room temperature based on the conversion reactions [7]. Due to the large band-gaps and poor ionic and electronic conductivities, the conversion reaction between BiF₃ and Li is not reversible; thus, it is not suitable for use as the cathode electrode of a rechargeable lithium battery [6]. Recently, a BiF₃/C nanocomposite with electrochemical activity was prepared by high-energy milling and was used as the cathode material for rechargeable lithium batteries. It exhibited a capacity of 230 mAh g⁻¹ (for the composite), corresponding to the reaction of 2.6 Li per BiF₃ [6], [8]. Unfortunately, it could not maintain good cycling efficiency and showed a large, irreversible capacity loss during each cycle due to the large band-gaps. In order to decrease band-gaps between the bismuth cation and fluoride, the effect of oxygen anion substitution on the reversibility of electrochemical activity of metal fluorides was investigated by studying the properties of bismuth oxyfluorides. It was shown that relatively low oxygen content was sufficient to drastically enhance the electrochemical activity of the electronically insulating fluoride [8]. Although cycling was poor, it has been demonstrated that oxyfluorides could be an attractive alternative to fluorides, combining the high voltage of the fluorides with the high electrochemical activity of the oxides [9]. In our previous work, the BiO_0.1F_2.8 was synthesized via a liquid phase precipitation method. It was found that doping BiO_0.1F_2.8 with activated carbon improved its electrochemical performance, and the energy density of the BiO_0.1F_2.8/C composite was as high as 613 Wh kg⁻¹ at a rate of 16.5 mA g⁻¹ [10].

Bi₂VO_5.5 is the one of most promising ferroelectric materials with a single bismuth layer structure, a relatively low crystallization temperature, and a low dielectric constant. Thus, Bi₂VO_5.5 thin films have much potential, which has been demonstrated in devices such as integrated ferroelectrics [11], [12]. This compound has attracted the attention of many researchers for its dielectric and ferroelectric properties, and its high ionic conductivity (at elevated temperatures) in the bulk and in thin film configurations [13]. It has been reported that a new, anion-conductive oxyfluoride Bi₂VO₅F_x (0 < x < 1) can be prepared from BiF₃, V₂O₅ and B₂O₃ [14]. To the best of our knowledge, studies on the electrochemical behavior of bismuth–vanadium oxyfluoride as a cathode material for secondary lithium batteries has not been reported. In this paper, bismuth–vanadium oxyfluoride was synthesized via a simple, solid-state reaction. The effects of temperature on the structure and electrochemical properties of bismuth–vanadium oxyfluoride have been examined in detail.