Full paperBottom–up assembly of strongly–coupled Na3V2(PO4)3/C into hierarchically porous hollow nanospheres for high–rate and –stable Na–ion storage
Graphical abstract
Introduction
Sodium vanadium phosphate (Na3V2(PO4)3, NVP), as a typical sodium super–ionic conductor (NASICON) type compound, has attracted intensive attentions in sodium ion batteries owing to its open framework which can accommodate reversible sodium ions insertion/extraction, the relatively high and stable operating voltage at 3.4 V that is derived from the redox couples of V4+/V3+, as well as its good thermal stability [1], [2], [3], [4]. But it is also known that the low electrical conductivity of NVP is the most important barrier that affecting its practical applications [4], [5]. To date, many strategies, primarily including preparing nanostructural material to shorten the ion diffusion paths [6], doping with alien ions to enhance the mixed conductivity [7], [8], coating with conductive agents (typically carbons) [9], [10], [11], and optimizing the morphologies [12], [13], have been devoted to address this issue. And indeed all the above approaches have been proven to promote the battery performances, but few improvements can be achieved as adopting each of them independently. Innovative researchers learn quickly that the combination of these methods may make full use of integrated advantages [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Afterwards promising results are reported constantly as the successful developments of NVP–carbons nanocomposites with various architectures, such as reaching the maximal theoretical capacity (118 mA h g–1) for NVP/acetylene carbon [14], ultrafast–rate performances high to 40 C for porous NVP/C [15], 100 C for carbon coated NVP in a porous graphene network [16], and 200 C for NVP 3D forms [17], which also possess long lifespan. Despite these inspiring achievements, the realization of an advanced NVP–based cathode for satisfying commercial applications that can keep an acceptable capacity retention of ≥85% over 10,000 cycles and simultaneously provide extremely high–rate capability (i.e. capacity retention obtained at 400 C remain ≥80% of the 1 C capacity), is still a great challenge. To achieve this, it requires not only barrier–free sodium ions diffusion but also immediate electrons transfer.
In fact, previously–studied brilliant researches have offered lots of valuable experiences and abundant literature materials for us to use for reference [17], [18], [20], [23], [26], [27], [28], [29], [30]. For example, Fang et al. reported a carbon–coated NVP with the highest–rate performance up to now (the maximum rate of 500 C), in which the electrodes were consist of NVP particles embedded in highly conductive and interconnected carbon frameworks that showed effective in the ultra–fast transport of electrons [28]. However, its high–rate capability (34.2% retention at 500 C of the 1 C capacity) was unsatisfactory that might be attributed to the low ion diffusion rate in the big NVP particle and especially the reducing active areas since the dense accumulation of electrode materials. By constructing three–dimensional (3D) hierarchically porous conducting networks with large ion–accessible sites that can increase the interfacial area of electrolyte/electrode, lower the electronic conductivity and strengthen the structural stability is proven a useful way to achieve a full utilization of the active materials and already widely producing great enhancements in the high–rate performances of supercapacitors [31]. Capacity retentions obtained at extremely high rate can easily meet and even exceed 80% of that collected at low rate in various reported supercapacitors [31], [32], [33], [34]. Taking this concept into account, hierarchically porous NVP–C nanocomposites may be a visible solution to overcome the limitations of packed non–porous NVP–C materials.
Herein, we propose a rational design of 3D hierarchically porous (co–existence of micro–, meso– and macro– pores) carbon–coating NVP hollow nanospheres (denoted as 3DHP–NVP@C) via a facile soft–chemistry pre–treatment with a post calcination. Detailed characterizations reveal that the novel nanocomposite is assembled from the bottom–up growth of strongly–coupled NVP nanoparticles/sp2–bonded carbons. The highly conductive carbon network ensures the rapid electrons transfer, while the hierarchization of hollow nanospheres in porosities attaching with the possible homoepitaxial growth of NVP nanoparticles together eliminate the sodium ions diffusion barrier. Benefiting from these merits, the as–prepared 3DHP–NVP@C electrodes exhibit extremely high–rate (84 mA h g–1 at 400 C (46.8 A g–1), which is equivalent to 84.2% of that obtained at 1 C) and –stable (90.9% capacity retention over 10,000 cycles at 1 C rate) sodium–storage properties, both exceeding the performances of the reported NVP–based cathode materials.
Section snippets
Results and discussion
Fig. 1a depicts a panoramic view of the as–synthesized products, in which one can see that the samples present near–spherical shapes with the relatively uniform diameters between 100 – 200 nm. Through careful observations, there are many openings and lots of coupled nanoparticles existing in the products. A locally–enlarged SEM image in Fig. 1b attaching with a low–magnification TEM image in Fig. 1c demonstrate the novel channel–connected structure and also, to our best knowledge, for the first
Conclusion
In summary, we have developed a novel NVP–C nanocomposite that is consisting of bottom–up assembly of strongly coupled NVP/sp2–bonded carbon into hierarchically porous hollow nanospheres. This is the first report on the successful preparation of NVP–based hollow nanomaterial. In particular, the inner–connected NVP nanoparticles are embedded in 3D highly–conductive carbon framework. Benefits from these unique features, the 3DHP–NVP@C demonstrates the ultrafast and extremely–stable sodium–storage
Synthesis
Graphene oxide (GO) was prepared according to the famous Hummers method from natural graphite powders [41]. For the synthesis of 3DHP–NVP@C, 50 mg GO powders were severely stirred in 30 mL deionized water to form a homogenous dispersion. Then, 2.34 g sodium dihydrogen phosphate (1.5 mmoL NaH2PO4) and 0.5 g glucose (C6H12O6) were dissolved in GO/water dispersions to obtain a brown black colloidal suspension. Meanwhile, 2.65 g vanadium(IV)oxy acetylacetonate (1 mmoL VO(C5H7O2)2) is added into 30 mL
Supporting information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (5112508, 11274392, U1401241).
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