CoMoP2 nanoparticles anchored on N, P doped carbon nanosheets for high-performance lithium-oxygen batteries
Graphical abstract
CoMoP2@NPC with high electrical conductivity could not only expose abundant active sites of CoMoP2 nanoparticles, but also effectively relive volume changes during cycling. CoMoP2@NPC electrodes thus delivered superior electrochemical activities for both Li-O2 batteries and Li-ion storage.
Introduction
Nowadays, the over-consumption and shortage of fossil energy promote the development of green and renewable energy. Since the commercial production of Li-ion batteries (LIBs), they have quickly occupied the market of electronic equipment due to their portable size, high operating voltage, good safety and long cycle life [1], [2], [3]. Their low energy densities, however, largely hinder the applications of emerging power devices. Li-O2 batteries (LOBs) are attracted increasing research attention [4], [5] due to their extraordinarily high theoretical capacity and energy density based on the reversible reaction of O2 + 2Li+ + 2e− ↔ Li2O2 [6], [7], [8], [9]. Currently, several problems such as high overpotentials, poor cycling stability and low rate performance have to be addressed for promising applications [10], [11], [12], [13].
Employing a highly efficient catalyst in the cathode is an effective way to solve the above problems of LOBs. The ideal catalyst can optimize the electrochemical performance of the LOBs by catalyzing the two reaction processes of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during cycling. Carbon materials are evidently proven to be the satisfying catalysts for ORR [14], [15], [16], [17], while other active materials are still needed to be composited to realize the bi-functional catalytic properties for improving the battery performance. A large number of catalysts as active materials were previously investigated, such as Pt [18], [19], Ru [20], [21], Pd [22], RuO2 [23], [24]. Although the catalytic activities of noble metals are excellent, the low reserves and high prices limit their commercial application. In recent years, transition metal oxides (TMOs, such as MnO2 [25], [26], Co3O4 [27], [28], TiO2 [29], [30]) and transition metal phosphides (TMPs, such as Co2P [31], [32], [33], [34], Ni2P [35], [36], [37], MoP [38], [39]) have attracted more and more research attention owing to their potential to replace noble-metal-based catalysts. Compared with TMOs, TMPs feature excellent electrical conductivity close to those of metals, which is of importance in improving the rate performance and catalytic activity of the electrodes for batteries [31], [40], [41], and TMPs are thus considered as superior alternatives to the noble metal. Besides, the electrochemical properties of materials are closely related to their structure. TMPs normally show a triangular prism structure, in which metal atoms occupy its apex position and phosphorus atoms occupy its internal space [42]. This special crystal structure endows the TMPs more electrochemical active sites. It has been reported that the electron transfer in TMPs mainly results from metal atoms with cationic state to P atoms with anionic state, and P atoms with charge redistribution could act as active sites for boosting electrochemical reactions [43], [44], [45]. Notably, single TMPs could only provide one kind of anionic active sites, while binary TMPs could offer more electron-donating active sites [46], which thus effectively improving their electrochemical performance. On the other hand, high-efficient electrode materials are also of vital importance in LIBs. Traditional commercial batteries used graphite as the anode material, and the theoretical specific capacity is only 372 mAh g−1, which is difficult to meet the increasing demand for electric vehicles. Based on the various benefits of the above TMPs, lots of TMPs have been used as anode materials in LIBs, such as CoP [47], [48], [49], NiP [50], [51], FeP [52], [53], etc., also show good electrochemical performance.
Among binary TMPs, CoMoP2 has already been extensively studied as electrode materials in water-splitting devices based on the hydrogen evolution reaction (HER)/OER [43], [54], [55]. To the best of our knowledge, however, the application of CoMoP2-based materials as electrode materials for lithium storage was rarely attempted. Therefore, the construction of the CoMoP2-based materials with high performance for Li-O2 catalysis is highly desired.
Herein, this work demonstrated the preparation of CoMoP2 nanoparticles anchored on N, P doped carbon nanosheets (CoMoP2@NPC) as cathode catalyst, the LOBs exhibited excellent specific capacities, higher round-trip efficiency and stable cycling property of more than 100 cycles. Furthermore, they delivered large specific capacities, favourable rate capability and superior cycling stability for lithium ion storage. The remarkable improvements in the electrochemical performance are attributed to the synergistic effect between the CoMoP2 nanoparticles and NPC in the composite.
Section snippets
Synthesis of CoMoP2@NPC and NPC
In a typical synthesis process, 200 mg of glucose, 4 g of urea, 185 mg of H3PO4, 274 mg of Co(NO3)2·6H2O and 167 mg of (NH4)6Mo7O24 were dispersed into 40 mL deionized water. After stirring for 10 min, the solution was dried at 80 °C. The resulting product was then annealed in the porcelain boat at 900 °C for 2 h with the heating rate of 3 °C min−1 under the N2 atmosphere. When cooling down to room temperature, CoMoP2@NPC was finally gained (Scheme 1). NPC was also fabricated using the same
Results and discussion
The precursors of CoMoP2@NPC and NPC are shown in Fig. S1, XRD and FTIR results show that the main component of the precursor is urea. To investigate the morphology and structure of the CoMoP2@NPC and NPC samples, SEM and TEM were conducted. As revealed in Fig. 1a and b, CoMoP2@NPC exhibits the typical lamellar structure. In contrast with the thin nanosheets of NPC in Fig. S2, the thickness increased obviously after thermal treatment. The element mapping images in Fig. 1c–h revealed that Co,
Conclusions
In summary, the CoMoP2@NPC was prepared and intensively investigated as cathode catalyst for LOBs. The CoMoP2@NPC cathode delivers satisfying electrochemical performance, including large discharge/charge specific capacity of 15095/14810 mAh g−1, high round-trip efficiency of 64.7% at the current density of 100 mA g−1, more than 100 cycles at a current density of 100 mA g−1 and 80 cycles at a current density of 500 mA g−1 at a fixed capacity of 1000 mAh g−1, without obvious terminal voltage
CRediT authorship contribution statement
Haoran Xu: Data curation, Writing - original draft. Lanling Zhao: Software, Visualization. Xiaomeng Liu: Data curation, Validation. Deyuan Li: Data curation, Validation. Qing Xia: Data curation, Validation. Xueying Cao: Data curation, Validation. Jun Wang: Supervision, Conceptualization, Methodology. Weibin Zhang: Writing - review & editing. Huaisheng Wang: Writing - review & editing. Jintao Zhang: Supervision, Conceptualization, Methodology.
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.
Acknowledgements
Funding support is gratefully acknowledged from the Taishan Scholars Programme of Shandong Province (No. tsqn20161004), Project for Scientific Research Innovation Team of Young Scholar in Colleges and Universities of Shandong Province (2019KJC025), Young Scholars Program of Shandong University (2019WLJH21) and China Postdoctoral Science Foundation (2020M672054).
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