Sn4P3 nanoparticles confined in multilayer graphene sheets as a high-performance anode material for potassium-ion batteries
Graphical abstract
Sn4P3 ultrasmall nanoparticles embedded in multilayer graphene sheets (Sn4P3/MGS) were synthesized via a ball-milling method and used as an advanced anode material for potassium-ion batteries.
Introduction
The rapid expansion of commercial electronic vehicles and grid energy storage calls for high-energy–density and inexpensive rechargeable batteries [1], [2], [3], [4]. Potassium, with wide availability of mineral reserves and low standard electrode potential (–2.93 V vs. SHE), close to lithium (–3.04 V vs. SHE), enables potassium-ion batteries (PIBs) an attractive alternative to lithium-ion batteries (LIBs) in stationary energy storage systems [5], [6], [7], [8], [9]. Besides, it has been reported that K ion-based electrolytes can display superior ionic conductivity than those of Li+ and Na+ caused by the weaker interactions between potassium ions and organic electrolyte solvents, which is favorable for achieving the ideal rate performance of the cell device [10], [11], [12], [13], [14]. Nonetheless, it is a great challenge to overcome the difficulty concerning the huge volumetric changes upon cycling arising from the larger size of K+ (1.38 Å) [15], [16], [17], [18], [19], [20].
Tin phosphides have captured considerable attention as a class of materials relying on their unique physical characteristics, chemical properties, and crystal phases. Among these tin phosphides, metal-rich rhombohedral Sn4P3 has a great development prospect as an anode material for alkali-ion batteries because it has the advantages of high theoretical capacity, layered crystal structure, and metallic nature [21], [22], [23], [24], [25]. The theoretical capacity of the Sn4P3 anode is 614 mAh g−1, which is achieved by the generation of K3P and KSn phases via sequential conversion and alloying reactions in the potassiation process [26]. However, such reactions are always accompanied by severe volumetric expansion/contraction, giving rise to the pulverization of the anode and poor cycling stability [27], [28]. Guo’s group firstly prepared a Sn4P3/C anode of PIBs through a ball-milling method. The obtained electrode showed a high capacity of about 384.8 mAh g−1 at 0.05 A g−1, exceeding most of the reported anodes for PIBs [29]. Unfortunately, though the combination of carbon could enhance the structure stability to some degree, the anode experienced serious capacity decay after only 35 cycles, which resulted from the large particle diameter and self-agglomeration of Sn4P3 nanoparticles [30], [31].
In this work, ultrasmall Sn4P3 nanoparticles uniformly confined in multilayer graphene sheets (Sn4P3/MGS) were fabricated by using a high-energy ball-milling approach and used as an anode material for PIBs. The ultrafine and evenly distributed Sn4P3 nanoparticles and highly conductive MGS endue Sn4P3/MGS with prominent transporting dynamics for both electrons/ions. Furthermore, MGS can efficiently buffer the expansion of Sn and P upon cycling processes, which maintains the structural integrity of the hybrid. By optimizing the proportion of Sn4P3 to MGS, a Sn4P3/MGS hybrid with optimal electrochemical properties was obtained, which delivered a reversible capacity of 393.3 mAh g−1 at 0.1 A g−1 after 100 cycles, and good rate performance of 215.7 mAh g−1 at a high current density of 2 A g−1.
Section snippets
Material synthesis
Sn4P3 nanoparticles were initially fabricated via a high-energy ball-milling strategy using stannum (Aladdin, 99.5%) and red phosphorus powders (Alfa Aesar, 98.9%) as the raw materials with a molar proportion of 4:3. Then by milling 800 mg Sn4P3 and 200 mg expanded graphite (XFNANO) (in a weight ratio of 8:2), the final product Sn4P3/MGS-80 can be achieved. All the above ball milling processes were conducted under an Ar atmosphere for 100 min. For comparison, Sn4P3/MGS-70 and Sn4P3/MGS-90 were
Results and discussion
Fig. 1 shows the detailed synthesis procedures for the Sn4P3/MGS-80 nanocomposite. In the first step, the Sn4P3 particles were facilely synthesized from the reaction between the tin and phosphorus powder in a high-energy ball-milling tank. Subsequently, these Sn4P3 particles were milled with expanded graphite, during which expanded graphite was transformed into multilayer graphene sheets and Sn4P3 particles were uniformly confined in these graphene sheets, resulting in the Sn4P3/MGS-80 hybrid.
Conclusions
In conclusion, we developed a facile high-energy ball-milling route to homogeneously load Sn4P3 nanoparticles between multilayered graphene sheets as new anode materials for PIBs. Benefiting from the large surface area-to-volume ratio, superior structural stability upon the potassiation/depotassiation process, and high overall electronic conductivity, the Sn4P3/MGS-80 electrode delivers a large charge capacity (378.2 mAh g−1 at 0.1 A g−1), remarkable rate property (260.2 mAh g−1 at 1 A 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
This work was supported by the National Natural Science Foundation of China (22075147) and the Natural Science Foundation of Jiangsu Province of China (BK20180086).
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These authors contributed equally to this work.