Cu3P nanoparticles confined in nitrogen/phosphorus dual-doped porous carbon nanosheets for efficient potassium storage
Graphical abstract
An integrated hybrid architecture composed of ultrathin Cu3P nanoparticles confined in porous carbon nanosheets (Cu3P ⊂ NPCSs) is synthesized by a self-designed self-templating strategy. Benefiting from the unique structural advantages, the two-dimensional hybrid electrode indicates excellent electrochemical performance as an anode material for potassium-ion batteries.
Introduction
Potassium-ion batteries (PIBs) have been regarded as a prospective alternative to lithium–ion batteries for next-generation economical and efficient energy storage devices in recent years due to the low cost, natural abundance of potassium resources, and the redox potential of K+/K (−2.93 V) close to that of Li+/Li (−3.04 V) [1], [2], [3], [4], [5], [6], [7], [8]. Among the potential anode material for PIBs, transition-metal phosphides (TMPs) have recently been attracting attention due to their relatively enhanced electrical conductivity, excellent thermal stability, and higher theoretical capacity compared with their oxide and sulfide counterparts [9], [10], [11], [12], [13], [14], [15], [16]. However, the large volume change during the repeated K+ insertion/extraction remains a great challenge that demands to integrate with other optimization strategies to address in order to further strengthen the potassium storage abilities of metal phosphide anodes [17], [18], [19], [20], [21].
The immobilization of primary electroactive nanomaterials on or in porous conductive carbon frameworks has been emerging as an effective approach for managing the above problems [22], [23], [24]. The conductive carbon frameworks can not only bolster the mechanical flexibility and stability of hybrid architectures and alleviate their volume expansion, but also prevent the aggregation of primary electroactive nanomaterials during cycling processes, as a direct consequence of enhanced rate capability and cycling stability for lithium/sodium/potassium storage [10], [23], [24], [25]. Currently, researchers have made some progress in the anode materials of PIBs, including carbon-based materials [9], transition metal hybrids (metal oxides, selenides, sufidies, phosphide [26], [27]), alloy-based materials and so on [28], [29], [30], [31]. Owning to non-toxicity, high theoretical capacity and cost-effectiveness, TMPs [28], have won wide-ranging attention from researchers. Thereinto, CoP-based and FeP-based materials have been reported one after another [31], [32]. Moreover, the engineering of hierarchical TMPs-based hybrid nanostructures with more active sites is regarded as an efficient strategy to improve electrochemical performance for PIBs [33]. Particularly, the highly-dispersed ultrafine (<20 nm) electroactive materials inherently bear rich active sites and shorten the transportation length of charge carriers, leading to higher specific capacity and higher rate capability [34]. As a result, the reasonable integration of two structural merits above to engineer and construct ultrafine TMPs nanocrystals uniformly immobilized on porous carbon framework is of great potential and prospect for advanced PIBs. For example, we have recently reported a self-templating and self-catalytic strategy to synthesize hierarchical microcables composed of CoP@C nanoparticles encapsulated in carbon matrix intertwined with carbon nanotubes with enhanced lithium storage [28]. In spite of the achievements, the construction of hybrid hierarchical architectures composed of ultrafine TMPs embedded uniformly in the porous carbon framework still requires earnest and conscientious endeavors because of the lack of applicable and effective synthetic strategies compared with their oxide and sulfide counterparts.
Herein, we demonstrate a facile self-template strategy for the elaborate regulation and synthesis of a two-dimensional (2D) hybrid architecture consisting of dispersive and uniform Cu3P nanoparticles (NPs) with controllable size embedded in nitrogen/phosphorus dual-doped porous carbon nanosheets (denoted as Cu3P⊂NPCSs) applied for potassium storage for the first time. This delicate hybrid architecture integrates the remarkable structural merits of two functional subunits. Specifically, ultrathin Cu3P NPs significantly facilitate K+ ions diffusion, and effectively relieve the volume change during the repeated K+ insertion/extraction. The assemblage furnishes ample active heterointerfacial sites and the intimate and stable electrical contact between Cu3P and conductive carbon matrix can guarantee the rapid migration of K+ ions and impede the aggregation of Cu3P NPs during the potassiation/depotassiation process. As a result, when evaluated as an anode material for PIBs, the obtained Cu3P⊂NPCSs manifest superior potassium storage properties with high specific capacity, excellent rate capability, and long cycle life. The theoretical calculations and the galvanostatic intermittent titration technique (GITT) results distinctly demonstrate the unique hybrid structures strengthen the fast diffusion of K-ions. Furthermore, Cu3P insertion mechanism for K+ storage has been unveiled, for the first time, on the basis of in-situ X-ray diffraction (XRD) and ex-situ transmission electron microscopy (TEM) results. This elaborate strategy with synchronous regulation in composition, size, and structure could illuminate the engineering of more efficient energy storage materials.
Section snippets
Synthesis of Cu3P⊂NPCSs
In the synthesis, first of one, 0.2 mL inositol-hexaphosphoric acid (IP6) was added in 10 mL distilled water and then stirred for 30 min to form a homogeneous solution. Secondly, 0.1 mol Cu(NO3)2·3H2O was dissolved in the above solution. After 30 min of stirring, the solution became light blue, followed by adding 1 g melamine (MA). After 8 h of stirring, the final mixture was dried to attain the precursor in the oven at 80 °C. In order to prepare S-Cu3P⊂NPCSs (NPs with the average size of 20 nm
Results and discussion
The fabrication process of Cu3P⊂NPCSs is schematically depicted in Fig. 1. To be specific, the layered precursor was fabricated by room-temperature liquid phase-based reaction. In a nutshell, IP6, MA and Cu(NO3)2·3H2O of different amounts were dissolved in water and stirred to polymerize and then maintained at 80 °C for 30 h to form a gray sheet-like sample (denoted as IP6-MA-Cu; Fig. S1, Supporting Information). Subsequently, it was collected and then annealed in argon at 900 °C for 2 h to
Conclusion
In summary, we have designed a novel 2D hybrid architecture composed of highly dispersed uniform Cu3P NPs with controllable in size confined in N,P co-doped Cu3P⊂NPCSs for efficient potassium storage for the first time. The 2D hybrid architectures with integrated merits of different components can promote the transport, diffusion, and storage of electrons/K+ ions to obtain enhanced potassium storage abilities. In particular, the architectures can deliver high reversible capacities of 149 mAh g−1
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors gratefully acknowledge the financial supports provided by the National Natural Science Foundation of China (Nos. 21971145, 21871164), the Taishan Scholar Project Foundation of Shandong Province (No. ts20190908), the Natural Science Foundation of Shandong Province (No. ZR2019MB024), and the Young Scholars Program of Shandong University (No. 2017WLJH15).
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