Full Length ArticleUltrafine CoP nanoparticles encapsulated in N/P dual-doped carbon cubes derived from 7,7,8,8-tetracyanoquinodimethane for lithium-ion batteries
Graphical abstract
Introduction
In modern society, a rechargeable battery is considered to be good if it can achieve high energy density, incurs low cost, and has a long cycle life [1], [2], [3]. The demand for high energy density is continuously increasing for mobile devices with high power consumption. Batteries with a high energy density have also extended the driving range of electric vehicles. Therefore, more suitable active electrode materials with excellent electrochemical performance are required to improve the performance of lithium-ion batteries [4], [5], [6].
As active electrode materials, transition metal phosphides such as FeP, CoP, Ni2P, Cu2P, and MoP exhibit significant promise for lithium-ion batteries [7], [8], [9], [10], [11]. The high capacity of transition metal phosphides is related to the conversion reaction during lithiation and delithiation, which is expressed as follows:MPx + 3xLi+ + 3xe– →M + xLi3P (M = Fe, Co, Ni, Mo)
Moreover, the electronic conductivity of Li3P formed from transition metal phosphides is much higher (~1 × 10−4 S cm−1) than that of Li2O derived from transition metal oxides (~5 × 10−8 S cm−1), which improves its electrochemical reactivity [6], [12].
However, the application of transition metal phosphides as anodes has two main limitations. First, the intrinsic electronic conductivities of these phosphides are relatively low (leading to poor lithiation and delithiation kinetics), resulting in practical capacities lower than the theoretical capacities. Second, significant volume changes occur during the charge–discharge processes, which cause pulverization of the electrode materials and loss of contact between the electrode materials and current collector. This can result in a low cycling stability and rate capability [8], [13].
Several approaches have been proposed to achieve a high energy density and long life cycle, and thereby address the limitations of transition metal phosphides. One effective approach is to downsize transition metal phosphides to the nanoscale. Nanosized materials can decrease the solid diffusion distance between lithium ions and compensate the strain due to volume expansion [14], [15], [16]. However, inevitable issues lead to unsatisfactory cycling stability, such as a high interfacial charge-transfer resistance and insufficient electrolyte–electrode contact area due to high aggregation during lithiation–delithiation reactions. To mitigate the high aggregation of the nanoparticles, ultrafine nanoparticles have been immobilized on highly electronically conductive carbon matrices to achieve zero-dimensional encapsulation in nanoarchitectures [16], [17], [18]. The accelerated electron-transport pathways and the interactions between the nanoparticles and elastic carbon matrix can enhance the electrochemical performance of transition metal phosphides.
Considering the aforementioned advantages, N-doped carbon materials have been developed through post-treatment of carbon sources with nitrogen-containing chemicals and in situ doping with nitrogen-containing precursors [16], [19], [20], [21], [22], [23]. Recently, coordination polymers have been used as precursors to prepare nitrogen-doped carbon materials through direct carbonization. Among the coordination polymers, 7,7,8,8-tetracyanoquinodimethane (TCNQ) is a multi-redox active organic ligand with multisite properties. In addition, TCNQ can produce nitrogen-rich coordination polymers owing to its high nitrogen content (27.5 wt%) [24].
In this study, TCNQ was selected as the ligand in three-dimensional coordination polymers to immobilize ultrafine nanoparticles for obtaining a highly electronically conductive carbon nanoarchitecture, that served as a nitrogen source for the nitrogen-doped carbon matrix. TCNQ-derived cubic carbon structures composed of zero-dimensional CoP nanoparticles were prepared and then distributed in a nitrogen-doped carbon sheath considering an inter-assembled structure on three-dimensional carbon cubes (denoted as CoP@NP-CCs). CoP was selected owing to its theoretical capacity of 895 mA h g−1. When CoP@NP-CC-based anodes were tested in lithium-ion batteries, they exhibited excellent cyclability. In particular, the capacity retention of the CoP@NP-CCs after 200 cycles decreased at a rate of 0.024% per cycle over 1000 cycles. In addition, during the discharge–charge process, the CoP@NP-CC-based anodes retained their original cubic morphology, exhibiting a volume variation as low as 20%.
Section snippets
Synthesis of Co(TCNQ)2 cubes
Co(TCNQ)2 cubes were synthesized by co-precipitation using CoI2 (Alfa Aesar, purity 99.5%) and TCNQ (Sigma-Aldrich, purity 98%). CoI2 (0.52 g) and TCNQ (0.34 g) were added to 40 mL of acetonitrile (Samchun, purity 99.9%) at 60 °C for 3 min. The color of the solution turned dark blue, indicating the formation of Co(TCNQ)2 cubes. The reaction mixture was washed several times with acetonitrile and then dried at 70 °C for 4 h.
Synthesis of CoP@NP-CCs and post-annealed CoP@NP-CCs
CoP@NP-CCs were produced via phosphidation. First, 0.2 g of Co(TCNQ)2
Characterization of CoP@N/P dual-doped carbon cubes
Ultrafine CoP nanoparticles were successfully embedded in a nitrogen-doped carbon cube (NP-CC), which exhibited a structure that resembled cut dragon fruit. A schematic of the synthesis of CoP@NP-CCs is shown in Fig. 1. The first step was to prepare NP-CCs using TCNQ-derived carbon. Phase-pure Co(TCNQ)2 cubes were synthesized by co-precipitation using cobalt iodide (CoI2) and TCNQ (Fig. S1) [25]. The chemical composition of TCNQ is C12H4N4. TCNQ is easy to carbonize at a relatively low
Conclusions
Herein, we successfully developed ultrafine CoP nanoparticles embedded in nitrogen-doped carbon cubes. After a post-heating treatment under Ar, the nitrogen-doped carbon matrix (i.e., NP-CC) in CoP@NP-CCs provided an elastic buffer to mitigate volume variation and a conductive channel to improve the cycling stability and rate performance during the discharge–charge process. In addition, the ultrafine CoP nanoparticles of the PA-CoP@NP-CCs maintained the excellent capacity of the material
CRediT authorship contribution statement
Gwang-Hee Lee: Data curation, Formal analysis, Investigation, Writing - original draft. Yoon Seon Kim: Data curation, Formal analysis, Methodology, Writing - original draft. Myeong-Chang Sung: Formal analysis, Investigation, Methodology. Dong-Wan Kim: Conceptualization, Funding acquisition, Investigation, Supervision, Validation, Writing - review & editing.
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 a Korea University Grant and the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT and Future Planning [NRF-2017R1C1B2004869, 2019R1A2B5B02070203]. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2020R1A6A1A03045059).
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These authors contributed equally to this work.