A scalable strategy to synthesize TiO2-V2O5 nanorods as high performance cathode for lithium ion batteries from VOx quasi-aerogel and tetrabutyl titanate
Introduction
Lithium ion batteries (LIBs) have become an important energy storage device with a wide range of applications including portable electronics, electronic automobiles, large-scale power grid, etc. Among the multiple components in LIBs, the electrode is the key element which is directly correlated with the energy density, power density and cycle life of LIBs. Traditional cathode materials for LIBs, such as LiCoO2, LiNiO2, LiMn2O4, LiFePO4 and their modified counterparts, deliver relatively low specific capacity, generally reaching 140–170 mAh g−1 [1], [2], [3]. This capacity value is not comparable to that of new-type anode materials which own very high capacity such as nano-scaled Si, SiO2, SnO2 and many other metal oxides [4], [5], [6], [7]. Therefore, it is urgent and meaningful to develop high-performance cathode with large specific capacity, stable cycling property and good rate capability.
The orthorhombic V2O5, which is a promising cathode candidate for LIBs, had been extensively investigated by many scientists owing to its high theoretical lithium intercalation capacity [8], [9]. When three moles of Li+ are inserted into one mole of V2O5, its specific capacity could exceed 400 mAh g−1 [10]. Unfortunately, the bulky V2O5 suffers from poor structural stability, low Li+ diffusion coefficient (10–14 ~ 10–12 cm2 s−1) and moderate electric conductivity (10−7 ~ 10−6 S cm−1), resulting in poor cycling and rate capabilities [11], [12]. In order to resolve these drawbacks, nanotechnology was adopted by researchers to prepare a series of V2O5 nanostructures including 0 dimensional (D) V2O5 nanoparticles, 1 D V2O5 nanofibers, 2 D V2O5 nanosheets and 3 D hierarchical V2O5 nano-materials via diverse methods involving hydrothermal treatment, electro-deposition, vapor deposition, electro-spinning, spray pyrolysis, atomic layer deposition, etc [13], [14], [15], [16], [17], [18], [19]. All these V2O5 nanostructures exhibit higher capacity, better cyclic stability and rate capability compared to their bulky counterparts when used as cathodes for LIBs, which can be ascribed to the large active surface area, effective strain buffering function and faster Li+ diffusion rate enabled by nanostructure itself.
However, many approaches for the preparation of V2O5 nanostructures usually refer to high-energy consumption, low-yield rate, usage of a certain template or sophisticated equipment to some extent [17], [18], [19]. This limits the large-scaled and low-cost production of V2O5 nanostructures, hindering their practical application. Hence, it will be of great significance to develop a facile and scalable strategy to fabricate nano-structured V2O5 with good uniformity.
Moreover, some necessary modifications, which usually involve doping, surface coating and combination with other conductive networks, were performed so as to further improve the electrochemical performance of nano-structured V2O5 [20], [21], [22]. In these measures, the surface coating has aroused wide attention from materials scientists because the coating layer can effectively modify the interfacial properties between active electrode and organic electrolyte, reduce the dissolution of active material and protect the nanostructure of electrode against rapid degradation, leading to improved capacity retention and rate capability [23]. The most common three kinds of surface modifications to electrodes are carbon coating, metallic oxide coating and organic polymer coating, in which the metallic oxide (MOx) coating is often adopted owing to its simple operation and readily accessible condition, such as Al2O3-coated LiV3O8, Fe2O3-coated LiNi0.5Mn1.5O4, La2O3-coated Li1.2Mn0.54Ni0.13Co0.13O2, etc [24], [25], [26]. These surface modified cathode materials with various MOx coating layers display obviously promoting long-term cyclic stability and high-rate capability. Among the common MOx coatings, Al2O3 as a kind of successful coating layer for many cathodes has been reported a lot and its realization approaches are relatively mature [24], [27]. Anatase phase TiO2 layer can be generated on surface of active particle by hydrolysis of tetrabutyl titanate (C16H36O4Ti) and following direct sintering in air, whose preparation has the advantages of easy operation, facile controllability and large-scale output [28], [29]. Furthermore, the molecular weight of TiO2 is relatively smaller compared to some other MOx such as Fe2O3 and La2O3, which will be conducive to achieving high specific capacity of the whole cathode.
In this work, a simple sol-gel method was employed to prepare VOx hydrogel on a large scale by utilizing V2O5 commercial powder as precursor material. After the subsequent solvent exchange, vacuum drying, mechanical grinding and post-annealing to as-prepared VOx hydrogel, the orthorhombic V2O5 nanorods (NRs) with good crystallinity could be obtained in batches. In order to further improve the electrochemical performance of V2O5 NRs as high-capacity cathode material for LIBs, TiO2 coating layer, which has hardly any Li+ insertion/extraction capacity above 2 V (vs. Li/Li+) and will not undergo the volume expansion/shrinkage by itself [30], was introduced on the surface of V2O5 NRs through hydrolysis of C16H36O4Ti under wet-chemistry condition and followed by solvent elimination as well as post-sintering. The generated TiO2-V2O5 NRs demonstrated more stable cyclic property and better rate capability compared to bare V2O5 NRs during long-term charge/discharge tests. Here, TiO2 layer plays a role of protective coating, whose effect was analyzed and discussed. Our research provides a facile and scalable way to realize the fabrication of nano-sized V2O5 and its surface modification by metal oxide coating.
Section snippets
VOx quasi-aerogel (VOx nanofibers)
Firstly, the commercial V2O5 powder, benzyl alcohol and isopropyl alcohol were fully blended with a molar ratio of 1:8:80 in a heating reflux bottle under magnetic stirring. The mixture solution was kept at 90 °C by heating in water-bath for 4 h under reflux condensation. After filtration and concentrating, a light yellow VOx sol with functional VOx oligomers was acquired, in which the content of V2O5 was approximately 40 mg ml−1. Then, the deionized water and VOx sol with identical volume (20 ml)
Discussion
Fig. 1 described the synthetic process of VOx quasi-aerogel. First of all, the light-yellow VOx sol was prepared by heating reflux condensation and concentration using V2O5 powder, benzyl alcohol and isopropyl alcohol as reaction reagents. Rapid hydrolysis would occur as the functional VOx oligomers in VOx sol was mixed with H2O, leading to the generation of VOx hydrogel. After aging, the stable VOx gel was formed. The whole process involves complicated chemical reactions (seeing ESI†) [31],
Conclusion
In this research, we prepare VOx quasi-aerogel with nano-porous architecture on a large scale by sol-gel method, solvent exchange and following vacuum drying. VOx NFs are acquired through directly grinding the as-prepared VOx quasi-aerogel, whose electrochemical performance is unsatisfactory due to its low crystallinity, low vanadium oxidation state and high crystal water content. The well-crystallized V2O5 NRs are formed through directly annealing VOx NFs in air. V2O5 NRs display high specific
Acknowledgement
We appreciate the funding from the Youth Project of Applied Basic Research of Yunnan Science and Technology Department (grant No. 2015FD001) and the Young/Middle-aged Backbone Teacher Cultivating Scheme of Yunnan University (grant No. WX069051). This work is also financially supported by the Imported Project of Yunnan Province for High-end Scientific and Technological Talents (grant No. 2013HA019) and the Natural Science Foundation of China (grant No. 61366002, 61664009).
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