Short communicationPartially sulfurized MoO2 film for durable lithium storage
Introduction
Electrode materials play a key role in designing advanced battery systems [1], [2], [3], [4], [5], [6], [7]. Molybdenum oxide is a structurally unique material and has been widely exploited as electrode material in a broad spectrum of battery applications [8]. Depending on the arrangement and the relative ratio of Mo and O atoms, numerous molybdenum oxides such as MoO3, Mo17O47, and MoO2 have been identified, which are all attractive electrode materials for rechargeable lithium batteries [9], [10], [11], [12], [13], [14]. In particular, rutile MoO2 has captured most imagination owing to the large capacity of 838 mAh g−1, high conductivity (∼104 S cm−1), and rapid ion transport property. Previous works have shown that Li storage in MoO2 generally consists of two sequential processes with distinct mechanisms, i.e. insertion or conversion. The latter process that reduces MoO2 to metallic Mo enables a large capacity but also results in substantial volume expansion and breaking off of conducting pathways, leading to a poor cycling and rate capability [15], [16]. This is a fundamental issue in conventional two-dimensional (2D) thin film cells.
Three-dimensional (3D) ordered nanostructures have attracted intensive interest due to their fundamental importance and potential wide-ranging applications. 3D configuration electrodes can maximize power and energy density yet maintain short ion transport distances. This offers an efficient way that battery design does not trade off between available energy per unit area and the ability to release this energy [17], [18], [19], [20], [21], [22], [23]. Previously, our group has designed and fabricated 3D nanoarray through an electrochemical anodization method, and achieved ultrastable and robust battery performance in several metal oxides [24], [25], [26], [27]. Generally, preparation of array structure needs complex procedure or elaborate facility, while practical application calls for some innovative routes that are more affordable and producible. However, significant gaps in our knowledge still remain.
In this work, we propose a facile route to prepare 3D hybrid array structure based on thermal oxidation of Mo foil and subsequent mild sulfurization. The mild treatment is well controlled so that only partial conversion of molybdenum oxides to sulfide, leading to a hybrid film of MoO2 and MoS2 (designated as S-MoO2). With respect to other structures, the hybrid structure possesses a large reaction area, a short ion diffusion path, and intrinsic advantages of heterostructure [28]. When used as anode for Li storage, the S-MoO2 hybrid exhibits superior electrochemical performance, affording a reversible capacity of 899 mAh g−1. At an extremely high rate of 10C, it still affords a capacity of 630 mAh g−1 with favourable stability, outstripping most reported molybdenum dioxide materials.
Section snippets
Material preparation
The self-supported S-MoO2 hybrid film was fabricated using a simple two-step approach. Firstly, Mo foil (99%, 50 μm in thickness) was ultrasonically cleaned in hydrochloric acid, acetone, deionized water, respectively, and dried in vacuum. The Mo foil was then thermally oxidized in air at 450 °C for 6 h with a ramping rate of 1 °C min−1. The oxidized Mo foil was transferred to a tube furnace, in which S powder (99.9%, Sinopharm) was placed on the upstream side of the furnace at carefully adjusted
Results and discussion
As schematically present in Fig. 1a, 3D ordered S-MoO2 hybrid film can be produced on Mo substrate via a simple two-step approach. During the first step, an ordered MoO3 film grows on the surface of Mo foil when the substrate is subject to thermal oxidization at 450 °C in air. The MoO3 film is then heat treated in S vapor under Ar flow, resulting in the generation of MoO2 and its further conversion to MoS2. The photographs of Mo substrate, MO3 film, and S-MoO2 film are presented in Figure b-d,
Conclusion
In conclusion, we described a facile route to fabricate partially sulfurized MoO2 film directly grown on Mo substrate for highly active Li storage. which enhanced lithium storage performance as anode materials for LIBs. The S-MoO2 hybrid film affords a high reversible capacity of 899 mAh g−1 and a superior rate capability of 630 mAh g−1 at 10C with good stability. The approach is simple and producible in terms of cost and efficiency, and will find more applications in the preparation of electrode
Acknowledgments
We are grateful for the financial support of the National Natural Science Foundation of China (51672182, 51422206, 51302181, 51372159), 333 High-Level Talents Project in Jiangsu Province, the Thousand Young Talents Plan, the Jiangsu Natural Science Foundation (BK20151219, BK20140009), Six Talent Peaks Project in Jiangsu Province, and of the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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