A MoS2 coating strategy to improve the comprehensive electrochemical performance of LiVPO4F
Graphical abstract
Introduction
With the increasing demands for energy conversion and storage systems, Li-ion batteries (LIBs) continue to attract a tremendous amount of interest around the world owing to their high energy densities and good design flexibilities [1], [2], [3], [4], [5]. As a crucial component, cathode material determines the performance of LIBs [6], [7], [8]. Framework materials based on the phosphate polyanion have been extensively investigated as cathode materials for LIBs in recent years [9], [10], [11], [12], [13]. Typically, lithium vanadium fluorophosphates, LiVPO4F, is proposed as one of alternatives to lithiated transition metal oxides because of its high operating voltage (about 4.2 V vs. Li+/Li) and excellent structure stability [14], [15], [16], [17], [18], [19], [20]. However, like many other phosphate-based cathodes, LiVPO4F suffers a major drawback of poor cyclic performance because of the relatively poor electronic conductivity and slow lithium ion diffusivity, and it is an obstacle for its further practical application [21], [22]. To date, tremendous efforts have been made to improve the electrochemical performance of LiVPO4F. Elemental substitutions (Na [23], Al [24], [25], Mn [19], Ti [26], etc.) have been proved to be a possible approach to expand the Li+ diffusion channel of LiVPO4F cathode material. As a result, the Li+ mobility is improved. The other efficient route is coating with the conducting materials (graphene [21], [27], carbon nanotube [28], polyaniline [29], etc.) to enhance the electronic transport ability of LiVPO4F. However, all previous studies focus on the structural modification and electronic conductivity improvement, and to the best of our knowledge, no literature pays attention to the LiVPO4F/electrolyte interfacial chemistry and stability. Nevertheless, it is a fact that LiVPO4F suffers very poor cycle performance at elevated temperature [19], which may be caused by unstable interface at high voltage. Therefore, it is of great significance to pay attention to the interfacial stability of LiVPO4F and develop an advanced route to improve its performance especially at high-temperature.
As a typical layered transition metal sulfide, molybdenum disulfide (MoS2) with unique physical and chemical properties including high surface area, good structural flexibility and acceptable conductivity, has received greatly increased attention in recent areas [30], [31], [32], [33]. Herein, for the first time, we report the usage of layered MoS2 to coat LiVPO4F/C composite. Due to the high surface area and structural flexibility, MoS2 naonsheets can be effectively coated on the surface of the LiVPO4F nanoparticles through a skillful solution method. Moreover, MoS2 owns the analogous structure of graphene, composed of three stacked atom layers (S–Mo–S) held together by van der Waals force, which enables the convenient intercalation and de-intercalation of Li+ ions [34], [35]. In addition, MoS2 owns improved electronic conductivity compared to traditional metal oxides/fluorides coatings, which is in favor of electronic transfer at the interface [36]. Most importantly, because of the chemical stability of MoS2 at high voltage, the interfacial stability of MoS2-coated LiVPO4F is expected to be improved. Therefore, the MoS2 coating layer is designed to act as a charge transport media and most importantly a protecting barrier against side reactions at the electrode/electrolyte interface. As a result, the electrochemical performance especially at elevated temperature can be enhanced significantly.
Section snippets
Experimental section
LiVPO4F/C nanoparticles (LVPF) with the carbon content of 1.56 wt% were prepared by the method as introduced in our previous work [14]. The MoS2 nanosheets were synthesized by a modified hydrothermal method [31]. Details about the experimental processes are described in Supporting Information (ESI-1, ESI-2). To prepare the MoS2 ethanol dispersion, the obtained MoS2 powders were dispersed into 200 mL ethanol by ultrasonic agitation. After that, the suspension was aged for 6 h and the supernatant
Results and discussion
Fig. 2 shows the crystal structures and morphologies of LiVPO4F materials before and after MoS2 modification. After coated by MoS2, the 2M-LVPF sample presents high-intensity diffraction peaks of crystalline LiVPO4F without any decrease in peaks' intensities, indicating that the solution coating of MoS2 has no obvious effect on the crystal structure of LiVPO4F. In addition, a small peak is found at the diffraction angle range from 13.1° to 15.3° which can be indexed with MoS2 [37], and becomes
Conclusions
For the first time, MoS2 nanosheets-modified LiVPO4F/C composites were synthesized via a solution method followed by low-temperature annealing. In the designed architecture, LiVPO4F/C nanoparticles were wrapped by amorphous carbon as interlayer and MoS2 coating layer as outer layer. Such MoS2-modified LiVPO4F cathodes delivered superior electrochemical performance including high reversible capacity, excellent rate capacity as well as remarkable elevated-temperature cycling stability. It was
Acknowledgments
This work is supported byNational Natural Science Foundation of China (Grant no. 51574287, 21501015). We also thank the Advanced Research Center of CSU for performing HRTEM examination and EDS elemental mapping. Dr. Jiexi Wang appreciates the supporting from Bao Steel Education Foundation.
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