Electrochemical reaction of lithium with orthorhombic bismuth tungstate thin films fabricated by radio-frequency sputtering

Abstract

Bi₂WO₆ thin films with fast deposition rate have been fabricated by radio-frequency (R.F.) sputtering deposition, and are used as positive electrodes in rechargeable thin film lithium batteries. An initial discharge capacity of 113 μAh/cm²-μm is obtainable for Bi₂WO₆ film electrode with good capacity reversibility. A multiple-center reactive mechanism associated with both Bi³⁺/Bi⁰ and W⁶⁺/W^x+(x < 6) is investigated by ex situ X-ray diffraction patterns, transmission electron microscopy and selected area electron diffraction techniques, apart from the direct comparison of Bi₂WO₆ electrochemical performance with those of Bi₂O₃ and WO₃ thin films. A possible explanation about smooth capacity loss of Bi₂WO₆ after long-term cycling is suggested from the incomplete reaction of Bi component. The advantages of Bi₂WO₆ thin films over the singer-center Bi₂O₃ or WO₃ thin films are shown in both the aspects of volumetric capacity and cycling life.

Introduction

For the last two decades, thin film rechargeable micro-batteries have been played enough attention due to their potential applications on many micro-electronic or micro-mechanical devices as power sources, such as radio-frequency identification (RFID), silicon complementary metal-oxide semiconductor (CMOS), supraconductor devices, implantable medical devices and microelectromechanical system (MEMS) [1], [2], [3], [4]. On the purpose of enhancing the volumetric capacity, which should be the most important performance index in the fields of thin film electrodes and batteries, the conception of multiple reactive centers in electrodes for lithium batteries is being concerned about. There is a considerable interest in positive electrode materials based on both deoxidable metal ions and reactive polyanion frameworks, such as W–O, Mo–O and V–O polyanions [5], [6], [7], [8], [9], which indicate the characteristic of more than one electrochemical active center. Very high lithium insertion number (more than 5 Li) can be achieved for some positive electrodes of this kind during the first discharge process [5], [6], [9].

In our previous works, both the active components of transitional metal ions and polyanion frameworks have been chosen for the fabrication of thin film positive electrodes with multiple reactive couples by R.F. sputtering [10], [11]. Despite the heavy weight, these compact thin films have been expected to enhance the volumetric rate capacity of thin film batteries. For examples, nano-sized CuWO₄ thin films of anorthic structure showed the capacity as high as 145 μAh/cm²-μm during the first discharge, based on the electrochemistry of both Cu²⁺/Cu⁰ and W⁶⁺/W⁴⁺ [10]. Amorphous LiFe(WO₄)₂ thin films displayed a large initial discharge capacity of 104 μAh/cm²-μm associated with the reaction of both Fe³⁺/Fe²⁺ and W⁶⁺/W^x+ (x < 6) [11]. Even under very large current density of 50 μAh/cm², the capacity of LiFe(WO₄)₂ thin films was still kept at about 60 μAh/cm²-μm after hundreds of cycles [12].

Recently, the materials with an opened Aurivillius structure have attractively been regarded as electrodes with multiple redox couples for lithium batteries [13], [14], [15], [16], [17], [18]. For the maintenance of Aurivillius framework, the presence of bismuth or antimony is considered to be essential. Interestingly, this kind of electrode materials could reach very high lithium insertion number (more than 10 Li) from the first discharge process, so a large volumetric capacity could be expected for thin film lithium batteries based on thin film electrode with Aurivillius framework. Bismuth tungstate (Bi₂WO₆), as the most typical material with Aurivillius framework, has been widely used as photocatalyst [18], [19], oxygen ionic conductor [20], ferroelectrics [21], apart from as promising electrode materials. But the electrochemical reactivity of bulk Bi₂WO₆ was characterized by large initial irreversibility and poor cycling stability in previous reports [13], [14], [18]. Moreover, the reactions of Bi³⁺ and W⁶⁺ at different voltage regions were not detailedly explained. Further, the effective fabrication of Bi₂WO₆ thin films will extend the potential application in these above-mentioned fields. In this work, Bi₂WO₆ thin film materials as positive electrodes are deposited by R.F. sputtering for the first time, and systematically investigated in rechargeable thin film lithium batteries. Meanwhile, a reaction mechanism associated with dual redox couples and a negative role of Bi component on electrochemical behavior are reasonably suggested.