Novel Ag@Nitrogen-doped Porous Carbon Composite with High Electrochemical Performance as Anode Materials for Lithium-ion Batteries
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KeywordsNitrogen-doped porous carbon Ag nanoparticles Synergistic effects Lithium-ion batteries
A novel Ag@nitrogen-doped porous carbon (Ag-NPC) composite was applied to lithium-ion batteries. The encapsulation of Ag nanoparticles (Ag NPs) into NPC boosts reversible capacity from 501.6 to 852 mAh g−1.
Ag-NPC shows a much better cycling performance than NPC due to the synergistic effect of NPC and Ag NPs.
In recent years, lithium-ion batteries (LIBs) have not only been widely used for consumer electronics, but have also proved promising for electric vehicles, owing to their unique advantages, such as high energy and power density, no memory effect and environmental friendliness [1, 2, 3]. Currently, graphitic materials are the most commonly used commercial anode materials for LIBs by virtue of their superior cycling stability and high coulombic efficiency . However, due to quite a low theoretical capacity of 372 mAh g−1, it would be hard for graphite to meet increasingly high energy requirements in electric vehicles . A variety of materials have been exploited as anode materials for LIBs in the past decades, such as transition-metal oxides [6, 7, 8], and silicon-based [9, 10, 11] or tin-based [12, 13, 14] materials, which have ultra-high theoretical capacity. Unfortunately, these materials react with lithium and are more inclined to form Li2O than Li-M alloys. Due to the fact that it is an electrochemically irreversible reaction, it results in a large irreversible capacity . Silver is an appealing option for anode materials, owing to its relatively high specific capacity, which is attributed to the formation of several Ag-Li alloys (up to AgLi12) within a very low voltage range (0.25–0 V) . Moreover, silver has the best electrical conductivity among all metals and excellent lithium diffusivity, which can efficiently boost the electrochemical performance . However, silver also suffers from undesirable volumetric expansion during lithium insertion. To alleviate this harmful effect, different strategies have been developed, such as downsizing the particle dimensions to the nanoscale, introducing a porous structure to the solid particles and designing silver-containing composites [18, 19, 20]. Carbon is a common matrix for silver. Shilpa et al. used hollow carbon nanofibers as a buffer matrix and embedded silver nanoparticles in them through the coaxial electrospinning method . Hsieh et al. dispersed silver nanorods onto graphene nanosheets by the hydrothermal method .
Metal organic frameworks (MOFs) have been attracting increasing attention as carbon sources for anode materials because various types of MOF precursors can result in derived carbon with a uniform, controllable, porous structure and enable innate doping of heteroatoms [22, 23, 24]. On the basis of previous research, the nanopores can facilitate rapid electrolyte transfer . In addition, the heteroatom-doped carbon always performs at a higher specific capacity and outstanding cycling stability compared to the non-doped carbon [26, 27, 28, 29]. Song et al. prepared a cage-like carbon/nano-Si composite as anode materials by the template method to embed Si nanoparticles into ZIF-8. The resulting nano-Si/C composite showed a higher reversible capacity than many Si/C composites previously reported . Xie et al. fabricated a sandwich-like, graphene-based, porous nitrogen-doped carbon (PNCs@Gr) through the pyrolysis of zeolitic imidazolate framework nanoparticles grown in situ on GO (ZIF-8@GO), which exhibited outstanding electrochemical performance among carbonaceous materials used as anode materials .
We used ZIF-8-derived carbon as a matrix for silver nanoparticles (Ag NPs), which can provide not only rigid matrices with nanopores, but also a relatively high nitrogen content. We designed a strategy to incorporate Ag NPs into N-doped porous carbon uniformly via a facile hydrothermal method without any reduction agent. When applied as the anode material for the Li-ion battery, the Ag-NPC showed excellent electrochemical performance over bare NPC, which was attributed to the synergistic effect of Ag NPs and the carbon matrix.
Methanol (CH3OH, Sinopharm Chemical Reagent Co. Ltd, >99.5%), 2-methylimidazole (C4H6N2, Sinopharm Chemical Reagent Co. Ltd., 99%), zinc nitrate (Zn(NO3)2·6H2O, Shanghai Titanchem Co. Ltd., >99.8%), 1-methylimidazole (C4H6N2, Sinopharm Chemical Reagent Co. Ltd., 99%) and silver nitrate (AgNO3, Sinopharm Chemical Reagent Co. Ltd., >99.8%) were used. All reagents were used without further purification.
3.2 Preparation of N-doped Porous Carbon (NPC)
ZIF-8 was synthesized according to method reported in the literature . Specifically, a methanolic solution (400 mL) of 2-methylimidazole (6.48 g) and 1-methylimidazole (6.28 mL) was quickly poured into a methanolic solution (400 mL) of Zn(NO3)2·6H2O (5.88 g) and stirred for 2 min and then kept still for 16 h. After that, the solution was centrifuged, washed by methanol and dried at 60 °C for 3 h to produce a white solid (ZIF-8). Then, the solid was ground into powder, followed by heat treatment at 800 °C for 5 h under an argon atmosphere. After letting it cooldown to the room temperature, the obtained product was dispersed into an HCl solution (100 mL, 20 wt% in water) and stirred for 24 h to remove residual metallic Zn and/or ZnO. The mixture was then washed thoroughly with distilled water several times until all the zinc ions were removed. Finally, the resultant product was dried at 60 °C in vacuum oven for 6 h to obtain an N-doped hierarchically porous carbon.
3.3 Synthesis of Ag-NPC Composite
The Ag-NPC composite was synthesized by a facile hydrothermal process without a reduction agent . The NPC (100 mg) was quickly dispersed into an aqueous solution of AgNO3 (25 mL, 20 mmol L−1), and the mixture was subsequently homogenized by intensely stirring it for 1 h in the dark. Then, the aqueous suspension was heated at 100 °C while gently stirring it under argon flow for 10 min, followed by quickly cooling it with running water. Next, the mixture was centrifuged and washed with distilled water several times and then dried at 60 °C in a vacuum oven for 5 h.
Powder X-ray diffraction (XRD) analysis was performed using a Bruker-Axs D8 Advance X-ray diffractometer with Cu Ka radiation (λ=0.15406 nm). The morphology of the sample was studied using a Hitachi SU70 field emission scanning electron microscope (SEM) at 10 kV. The high-resolution transmission electron microscopy (HRTEM) characterization was performed on a Tecnai F30 microscope at an accelerating voltage of 300 kV. The specific surface area and pore size distribution were analyzed by using a TriStar II 3020. The thermogravimetric analysis (TGA) was carried out on a SDTQ600 thermoanalyzer in air. The elemental analysis was performed using a Vario ELIII. The X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific ESCALAB 250Xi with Al Kα radiation (hν = 1486.8 eV).
3.5 Electrochemical Measurements
The active materials, NPC or Ag-NPC composite (70 wt%), acetylene black (20 wt%) and poly(vinyl difluoride) (PVDF, 10 wt%) were diffused in 1-methyl-2-pyrrolidinone (NMP) and stirred intensely to form a homogeneous slurry. Then, a copper foil was coated with the slurry and dried in a vacuum oven at 80 °C for 12 h. The lithium foil was used as the counter electrode as well as the reference electrode. A polypropylene membrane and LiPF6 were used as the separator and electrolyte, respectively. The galvanostatic discharge/charge experiments were performed on a Neware battery tester. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out on an Autolab electrochemical workstation (NOVA 1.9).
4 Results and Discussion
To determine the content of Ag NPs, thermogravimetric analysis in air was carried out for the Ag-NPC. As shown in Fig. 3c, a slight weight loss was observed at temperatures lower than 150 °C, which was probably caused by the evaporation of absorbed water. Then, the curve was flat until 500 °C, at which the nitrogen-doped carbon began to decompose. When the temperature increased over 900 °C, the curve became flat again, indicating complete decomposition of the carbon matrix. The resulting product of TGA proved to be metallic silver, confirmed by means of XRD. That is, the weight percentage of Ag NPs in Ag-NPC was 18.79%.
We found that the third cycle almost overlapped with the second cycle in the CV profile of Ag-NPC in contrast to NPC, proving that the Ag-NPC displayed a better cycling stability than the NPC did.
Figure 8b displays the galvanostatic charge–discharge profile of NPC and Ag-NPC for first, second and third cycles at a current density of 100 mA g−1. In the initial discharge curve of NPC, a broad plateau appeared near 0.75 V, which was usually related to the formation of the SEI films. The plateau for the Ag-NPC around 0.75 V was much narrower than that for the NPC, signaling that smaller areas of SEI films were formed, resulting in less capacity loss. However, no plateau could be detected in the charge curves of the NPC, implying continuous Li+ extraction from the NPC. It was noteworthy that for the charge–discharge profile of the Ag-NPC, several small plateaus can be observed apart from the one at 0.75 V. In the discharge curve, plateaus were at 0.076, 0.10, and 0.2 V, respectively. The charge curve presented several small plateaus within the range of 0.1–0.3 V. The step-like curves correlated with the complex alloying and dealloying processes of Ag NPs with Li+.
The cycling performance of NPC and Ag-NPC was investigated at a current density of 0.1 A g−1 (Fig. 8c). The initial discharge capacity for NPC was very high (2226.64 mAh g−1), but a dramatic capacity loss occurred in the first 25 cycles, leading to a low initial coulombic efficiency (43.41%) as well as poor cycling performance. After the encapsulation of Ag NPs, the initial discharge capacity dropped to 1374.64 mAh g−1 though its initial coulombic efficiency improved to 64.33%. In the following cycles, the coulombic efficiency quickly increased above 95%, which meant a better cycling performance of Ag-NPC than NPC. The reason was that Ag-NPC had a much smaller specific surface area than NPC (270.174 vs 844.715 cm3 g−1), so smaller areas of SEI films were formed during cycles. Because the formation of SEI films contributed significantly to the initial capacity , the NPC with a much larger area showed a very high initial discharge capacity. However, the reaction of the formation of SEI was always irreversible, so NPC suffered a dramatic capacity loss in the second cycle. In addition, Ag NPs could improve the quality of SEI films, leading to a better cycling performance. Further, after 200 cycles, the reversible capacity for Ag-NPC remained at 852 mAh g−1, which was 96.4% of the discharge capacity in the second cycle, while that for NPC was only 501.6 mAh g−1. In other words, the Ag NPs can not only efficiently improve the reversible discharge capacity, but also enhance the cycling stability of the NPC. This remarkable improvement was attributed to the synergistic effect of Ag NPs and the carbon matrix. The theoretical capacity of metallic Ag was 993 mAh g−1 (for Li4Ag), although it suffered from severe volume expansion and agglomeration, leading to poor cycling performance. In the Ag-NPC, the NPC acted as a robust matrix for Ag NPs, hence, alleviating the volumetric expansion and preventing particles from aggregating. Moreover, the encapsulation of Ag NPs reduced the surface contact of the NPC with the electrolytes, preventing the formation of unstable SEI films and improving the quality of SEI films and thus enhancing the cycling performance . Further, the excellent electrical conductivity of Ag NPs accelerated electron transfer and lithium-ion transfer, thus boosting the electrochemical performance.
It is noteworthy that the calculated theoretical capacity for Ag-NPC was found to be 488 mAh g−1 based on AgLi4 and C6Li, which was much lower than the measured capacity of 852 mAh g−1. This phenomenon had also been reported by previous papers [41, 42, 43]. This was contributed to the highly disordered structure of carbon matrices, high nitrogen content, as well as the improved conductivity of Ag NPs. The carbon matrix was highly disordered according to the broad peak in the XRD pattern and the Raman spectroscopy results of Ag-NPC discussed above. The defects in the carbon matrix promoted Li+ diffusion and intercalation into carbon, thereby improving its reversible capacity . Besides, the high nitrogen content (17 wt%) can efficiently facilitate electron transfer, thus increasing the capacity. Furthermore, the superior conductivity of Ag NPs raised the electrical conductivity of the overall Ag-NPC composite and boosted the reversible capacity.
The rate performance for NPC and Ag-NPC was analyzed at different current rates as shown in Fig. 8d. The reversible capacity for both materials gradually decreased with the current rate. The Ag-NPC delivered a much higher discharge capacity than the NPC did at each current density level. The average discharge capacities of Ag-NPC were 777.69, 696.08, 557.30, 459.47, 397.98, and 344.40 mAh g−1 for the increasing current values 0.1, 0.2, 0.5, 1.0, 1.5, and 2.0 A g−1, respectively. When the current density was lowered back to 0.1 A g−1, its discharge capacity recovered to 851.02 mAh g−1 at the 100th cycle. The rate behavior demonstrated its superior rate capacity, which could be attributed to the rigid porous structure of the carbon matrix.
In summary, a novel Ag-NPC composite with uniformly embedded Ag NPs was prepared through the facile hydrothermal method. The Ag-NPC composite showed remarkable enhancement in the reversible capacity (852 mAh g−1 after 200 cycles) as well as cycling performance, compared to NPC without incorporated Ag NPs. This was attributed to the synergistic effect of Ag NPs and N-doped porous carbon. The N-doped porous carbon acted as a robust matrix for Ag NPs, which could alleviate the volumetric expansion and prevent particle aggregation. In return, the Ag NPs that exhibited a relatively high specific capacity as well as superior conductivity could efficiently raise its reversible capacity and enhance cycling performance by improving the quality of SEI films. This research demonstrated that the NPC derived from ZIF-8 was an excellent matrix for nanoparticles in anode materials for lithium-ion batteries. By providing a relatively rigid structure and avoiding particle aggregation, the encapsulated nanoparticles could maximize the reversible capacities.
This work was supported by the Scientific and Technological Innovation Platform of Fujian Province (2006L2003).
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