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The lithium ions storage behavior of heteroatom-mediated echinus-like porous carbon spheres: From co-doping to multi-atom doping
Journal Pre-proofs The lithium ions storage behavior of heteroatom-mediated echinus-like porous carbon spheres: from co-doping to multi-atom doping Zhuo Chen, Haibo Li PII: DOI: Reference:
S0021-9797(20)30121-1 https://doi.org/10.1016/j.jcis.2020.01.107 YJCIS 25980
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Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
4 December 2019 23 January 2020 28 January 2020
Please cite this article as: Z. Chen, H. Li, The lithium ions storage behavior of heteroatom-mediated echinus-like porous carbon spheres: from co-doping to multi-atom doping, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.01.107
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The lithium ions storage behavior of heteroatom-mediated echinus-like porous carbon spheres: from co-doping to multi-atom doping Zhuo Chen, Haibo Li* Ningxia Key Laboratory of Photovoltaic Materials, Ningxia University, Yinchuan, Ningxia 750021, P. R. China *Corresponding author at the E-mail address: [email protected] (H. Li).
Abstract This study proposed a facile method to prepare echinus-like porous carbon spheres (PCS) with different heteroatom doping for lithium ions battery (LIBs). A metalorganophosphine framework (MOPF) was synthesized by employing riboflavin sodium phosphate as an organic ligand to conjugate with metal ions and then carbonized at mild temperature, leading to the formation of heteroatom doped PCS (H-PCS). As a result, (N, P) co-, (N, P, Ni) tri-, (N, P, Co) tri- and (N, Ni, Co, P) tetra-doped PCS were obtained to examine the insight into lithium-ion storage behavior of H-PCS. It was found that the specific surface area, pore texture and structural defects of H-PCS were dependent on doping of heteroatoms as well as the charge transfer resistance and Liion diffusion coefficient. Significantly, the redox reaction potential during the charge/discharge could be mediated upon the doping. Thus, when evaluated as anode for LIBs, the (N, Ni, Co, P) tetra-doped PCS exhibited highly reversible capacity of 680 mAh∙g-1 at 0.1 A∙g-1, excellent rate capability (115.9 mAh∙g-1 at 1.0 A∙g-1) and superior cycling performance (399.6 mAh∙g-1 at 0.1 A∙g-1). Moreover, the cyclic voltammogram measurements demonstrated that the doping of metal atoms was favorable for improving the capacitive contribution of surface limited diffusion. Thus, this work highlighted the importance of HCP with defined doping which could be considered as one of the prominent candidates for high-performance LIBs’ anode. Keywords: Porous carbon spheres, Heteroatom doping, Energy storage, Sol-gel method.
2
1.
Introduction With the tremendous usage of traditional fossil fuel and global environmental
concern, it is very urgent to explore green energy as the main energy source substitute. To continuously utilize renewable energy effectively, advanced energy storage technologies are needed. Lithium-ion batteries (LIBs) as one of the representative renewable energy sources that have been widely commercialized and applied in many fields since 1991, such as laptops, portable electronic devices and electric vehicles [13]. As a typical anode for LIBs, graphite offers a reversible capacity of ~372 mAh·g-1. However, it cannot catch up with the development requirements for high capacity [4, 5]. To satisfy the increasing demands of these energy storage applications, it is necessary to develop LIBs with high capacities based on advanced functional materials. Among all candidates, alloying types of materials can often deliver highly enhanced specific capacities [6, 7], nevertheless, they suffer from poor structural-stability, low irreversible capacity and complicated strategies of preparation [8, 9]. Owing to the advantages involving abundance, low-cost [10, 11], excellent cycling stability and electrochemical kinetics, various carbonaceous materials [12, 13], such as carbon nanotube [14], graphene [15, 16], carbon fiber, [17-19] mesoporous carbon and carbon hollow spheres [20, 21], have the greatest potential as anode for LIBs. Among these carbon anodes, porous carbon (PC) has attracted enormous attention due to its high porosity, specific surface area, mediated electrochemical activity and relatively low cost [22]. It is proposed that lithium storage in carbonaceous anode is performed by adsorption which is analog to electrochemical capacitors. As a result, high power 3
density is achieved. However, the energy density is unsatisfactory [23, 24]. To overcome this issue, heteroatom doped porous carbon (HPC) is developed aiming to achieve both high power and high density by combing adsorption and redox reactions. In application to LIBs, HPC and its derivative have rational pore texture. It promises to shorten the diffusion distance of Li+, alleviate the volume expansion during the charge/discharge and thereby improve the capacitance retention. In this aspect, many efforts have been devoted to design different strategies and optimize the heteroatom doped sites to promote the capacity. Zheng et al. proposed that the lithium-ion storage capacity of N-doped graphene depend on nitrogen-doping content. It is supposed that nitrogen atoms adopted on a 2D honeycomb lattice is closely related to structuralstability. As a result, nitrogen-doped graphene particle with nitrogen content of 17.72 wt% delivered a high capacity of 2132 mAh·g-1 after 50 cycles at the current density of 100 mA·g-1 and 785 mAh·g-1 after 1000 cycles at the current density of 5 A∙g-1 [25]. Pan et al. prepared N-doped porous carbon (NPC) as advanced anode for LIBs. It suggested that large interlayer spacing (~0.4 nm) and disordered structure are favorable for Li+ insertion/extraction. Moreover, N-doped NPC can enhance conductivity and provide more active sites for Li+ storage [26]. Regarding the fabrication of PC, it is accompanied by high-cost, high complexity and low yield. In most cases, it is basically necessary to remove the template or introduce further purification process. On the other hand, additional doping sources are required and involved into the preparation of HPC which to some extent leads to a large waste of doping sources and increasing expenses. In terms of P doping, the inevitable 4
release of the poisonous and flammable gas should be considered seriously. Therefore, it is still a challenge to prepare HPC using facile, economical and effective approach at the molecular level for LIBs with enhanced anode performance [27]. Metal-organic frameworks (MOFs) with abundant active site and well-defined pores have been viewed as one of the ideal choices to prepare nanostructured carbons by proper carbonization. To this end, it is considered that heteroatom doped carbons can be obtained by either introducing a doping source in the carbonating or selecting particular organic ligand with target doping atom. This study reports a facile strategy to prepare heteroatom doped porous carbon spheres (H-PCS) with co-, tri- and tetra-atom doping by carbonating a metalorganophosphine framework (MOPF) using riboflavin sodium phosphate. The MOPF can be deemed as organic ligand, nitrogen and phosphorus source as well (Fig. 1). In contrast to previous doping method, the H-PCS were directly obtained by introducing doping atoms at the molecular level. Significantly, this method does not introduce any additional doping source which is thought to be feasible and economical. On the other hand, the doping can be precisely controlled by selecting proper metal salts. It is explored that the H-PCS shows a typical echinus-like structure with high doping content. Such structure is very beneficial to increase the conductivity and create as much active sites for Li+ diffusion [28, 29]. Owing to the highly porous characteristic, high heteroatom doping content and moderate specific surface area, the (N, Ni, Co, P) tetra-doped PCS exhibited high reversible capacity, good rate capability and long cycling performance. The cyclic voltammograms (CV) measurements verified the high 5
ratio of capacitive capacities contribution for all H-PCS samples. It illustrated the improved Li+ storage capacity of H-PCS is ascribed to Li adsorption on active sites. Further, it is also found that the metal ions doping is favored for improving the capacitive contribution of surface limited diffusion behavior. Thus, this work examines the role of heteroatom doping in PCS on the electrochemical performance and provides a universal approach to design and mediate the heteroatom doping in carbon anode.
Figure 1 The schematic illustration of the synthesis of (N, P) co-doped PCS, (N, Co, P) tri-doped PCS, (N, Ni, P) tri-doped PCS) and (N, Ni, Co, P) tetra-doped PCS.
2.
Experimental section
2.1 Synthesis of (N, Ni, Co, P) tetra-doped PCS The synthesis of (N, Ni, Co, P) tetra-doped PCS is as follows. 0.5 g Riboflavin sodium phosphate (C17H20N4NaO9P) was dissolved in 45 ml deionized water under vigorous magnetic stirring for 10 minutes to form solution A. Similarly, 0.58206 g 6
Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 1.16316 g Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) was added into 40 ml ethanol solution (99.7%) to obtain solution B and C, respectively. Then, solution B was transferred to solution A under stirring at room temperature. Immediately, solution C was added to the mixed solution and was stirred continuously. The mixture was kept at room temperature for 24 h. After that, the products were washed with ethanol solution (99.7%) at least three times and dried for 12 h in a drying oven at 60 ℃. Subsequently, the dried precursor was annealed at 450 ℃ for 1 h under a nitrogen atmosphere at a heating rate of 10 ℃/min to obtain (N, Ni, Co, P) tetra-doped PCS. 2.2 Synthesis of (N, P) co-doped, (N, Ni, P) and (N, Co, P) tri-doped PCS To synthesize (N, Ni, P) tri-doped PCS, 0.5 g C17H20N4NaO9P was initially dissolved in 45 ml deionized water with vigorous magnetic stirring for 10 minutes to form solution A. Then, 0.58158 g Ni (NO3)2·6H2O was added into 40 ml ethanol solution (99.7%) to form the solution B. Subsequently, solution B was transferred to solution A under stirring at room temperature. After that, the process is the same as the synthesis of (N, Ni, Co, P) tetra-doped PCS. The synthesis of (N, Co, P) tri-doped PCS follows the method described as above. In addition to (N, Ni, P) and (N, Co, P) tri-doped PCS, the (N, P) co-doped PCS was directly obtained by carbonating C17H20N4NaO9P at 450 ℃ for 1 h under a nitrogen atmosphere at a heating rate of 10 ℃/min. The specifications of reagents used in the experiment is shown in Table 1.
7
Table 1 Number of moles of reagents. Reagents
Mass (g)
Relative molecular mass
Number of moles (mol)
C17H20N4NaO9P
0.5
478.33
0.001
Co (NO3)2·6H2O
0.58206
291.03
0.002
Ni (NO3)2·6H2O
1.16316
290.79
0.004
Ni (NO3)2·6H2O
0.58158
290.79
0.002
2.5 Characterization The structure and morphology were tested by scanning electron microscope (SEM, Zeiss Supra 40, German), transmission electron microscopy (TEM, Hitachi HT7700, Japan), and high-resolution transmission electron microscope (HRTEM, FEI talos F200s, USA). The crystal structures were identified by X-ray diffractometer (DX-2700, CHN) using Cu-Ka radiation (40 KV/30mA) coverage 2 theta angles 10-85°. The specific surface area was measured by surface area analyzer with Brunauer-EmmettTeller (BET, JW-BK200C, China), which is relying on a single point. Raman spectra were recorded with the spectrometer (DXR, USA) at room temperature. X-ray photoelectron spectroscopy (XPS) was performed on Thermo ESCALAB 250Xi (America) with an Al Ka source gun (1361 eV). The FT-IR spectra were recorded by KBr
pellets
with
a
WQF-520A
FT-IR
spectrometer
(4000-400
cm-1).
Thermogravimetry (TG) spectra of as-prepared H-PCS were tested on a thermogravimetric analyzer (Pyris 1, USA) from room temperature to 1000 ℃ with a ramp rate of 10 ℃/min under N2 atmosphere. 2.6 Battery performance To prepare the PCS anode, a homogeneous mixture containing the active material, acetylene black and polyvinylidene fluoride (PVDF) with a mass ratio of 8:1:1 was 8
made first. Then, the N-methyl-2-pyrrolidone (NMP) was dropped into the mixture to make it moisture. After that, the anode was obtained by coating the slurry uniformly on a collector utilized disk-shaped foamed nickel. Finally, the as-prepared electrode was dried in the oven at 80 ℃ for 6 hours. It should be noted that the assembling of coin cells LIB battery was carried out in an argon-filled glove box (O2<5 ppm, H2O<1 ppm). The electrochemical performance was tested by using lithium-foil as the counter electrode and Celgard membrane as a cell separator, respectively. A mixture of EC, EMC and DEC with a ratio of 1: 1: 1 Vol% was used as the solvent. One mol/L LiPF6 solution with FEC as an additive was used to assemble a LIB battery (CR2032). The land battery measurement system (CT2001A, LANHE) was used to evaluate the constant current charge and discharge (GCD) test. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) measurements were performed on the electrochemical workstation (PARSTAT 3000A-DX, AMETEK). 2.7 PITT test The potentiostatic intermittent titration technique (PITT) experiments were performed with a potential step of 0.02 V and an interval of 15 minutes. The current with respect to time was measured for the initial charging process. Moreover, the voltage window was set from 2.1 to 3.0 V. 3.
Result and discussion Fig. 2 (a) to (d) display the SEM image of MOPF, Ni-MOPF, Co-MOPF and (Ni,
Co)-MOPF while (e) to (h) show the SEM image of (N, P) co-, (N, Ni, P) tri-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS, respectively. The figure clearly shows that 9
all M-MOPF (M=metal or bi-metal) revealed typical echinus-like structure with plenty of open pores present on the surface except for MOPF. After carbonization at mild temperature, the microstructure of (N, Ni, P) tri-, (N, Co, P) tri- and (N, Ni, Co, P) tetradoped PCS remained. However, the surface of H-PCS is more obvious and porous [30]. It is worthy of note that the H-PCS was obtained via a self-doping strategy without the introduction of any additional doping source, realizing the advances. Significantly, such structure is supposed to provide a large number of tunnels for penetration of Li+, improving the specific capacity. On the other hand, the electrochemical reaction was probably confined within the inner PCS. In this case, the volume expansion during the charge/discharge process could be restricted. In contrast to (N, Ni, P) tri-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS, the (N, P) co-doped PCS presented a ball-like structure with quite smooth surface, suggesting low specific surface area (SSA) and low LIBs performance. Moreover, (N, Ni, P) tri-doped PCS and (N, Co, P) tri-doped PCS showed similar structure with that of (N, Ni, Co, P) tetra-doped PCS. However, it is obtained from the magnified image that the porous structure of both samples is not as good as (N, Ni, Co, P) tetra-doped PCS. Besides, they both show compact surface which may decrease the SSA and unfavorable to facilitate the transportation of electrolytes. Table 2 shows the elemental ratio of various H-PCS (the data were recorded via Energy Dispersive Spectrometer). It was found that heteroatom occupied a high percentage, indicating the high heteroatom doping content. Interestingly, as compared to Ni, Co is more facilely conjugated with organic C17H20N4NaO9P since the high Co doping ratio. Further, as the doping atom increases, the carbon content is 10
increased as well which is 73.33, corresponding to (N, Ni, Co, P) tetra-doped PCS.
Figure 2 SEM images of (a) MOPF, (b) Ni-MOPF, (c) Co-MOPF, (d) (Ni, Co)-MOPF, (e) (N, P) co-doped PCS, (f) (N, Ni, P) tri-doped PCS), (g) (N, Co, P) tri-doped PCS), (h) (N, Ni, Co, P) tetra-doped PCS, inset is the magnified SEM image from the selected area.
Table 2. Elemental ratio of various H-PCS Samples
Ni (%)
Co (%)
P (%)
C (%)
N (%)
O (%)
(N, P) co-doped PCS
-
-
7.87
56.90
5.49
22.51
(N, Ni, P) tri-doped PCS
1.82
-
2.56
69.22
1.30
24.10
(N, Co, P) tri-doped PCS
-
5.22
6.85
54.21
6.05
26.81
(N, Ni, Co, P) tetra-doped PCS
0.99
0.77
2.06
73.33
5.87
16.98
The fine structure of (N, Ni, Co, P) tetra-doped PCS was examined by employing HRTEM. Fig. 3(a) to (c) shows the HRTEM image of (N, Ni, Co, P) tetra-doped PCS with different magnifications. As can be seen that the (N, Ni, Co, P) tetra-doped PCS are quite uniform and highly porous. The diameter of the individual (N, Ni, Co, P) tetradoped PCS is around ~5.0 μm. Fig. 3(d) shows the HAADF image of (N, Ni, Co, P) tetra-doped PCS with mapping of Ni, Co, P, C, N and O, verifying the even elemental distribution. 11
Figure 3 HRTEM image of (N, Ni, Co, P) tetra-doped PCS in low (a) and high magnifications (b), (c) HAADF image of (N, Ni, Co, P) tetra-doped PCS with mapping of Ni, Co, P, C, N, and O.
The XRD patterns of all samples display a broad diffraction peak (Fig. 4(a)), illustrating the presence of amorphous carbon. The peak for all samples is almost locating at ~26°, corresponding to (002) plane of graphite. Besides, none obvious peaks were found, indicating the doping is dominant in PCS. Fig. 4(b) draws the FTIR spectrums for all H-PCS. Basically, they all show the same absorption band except for the intensity due to different doping levels. The main peaks are locating at 3316, 1635, 1628 and 1097 cm-1 which correspond to N-H, C=C, N=N and C-O stretch vibration, respectively. Besides, the peak focused at 914, 828, 870 and 747 are ascribed to the existence of C=C-H. The Raman spectra of H-PCS are shown in Fig. 4(c). For all HPCS samples, two distinct peaks locating at ~1350 and 1580 cm-1 are found which can be assigned to D and G bond, respectively. The D and G bonds separately represent the defected sp2 hybridized carbon and graphitic mode of carbon materials. Moreover, the 12
intensity ratio of D peak to G peak indicates the ordering degree of the carbonaceous materials. As calculated from Fig. 4(c) that the ID/IG is 0.998, 0.995, 0.863 and 0.841, corresponding to (N, Ni, Co, P) tetra-doped PCS, (N, Ni, P) tri-doped PCS, (N, Co, P) tri-doped PCS and (N, P) co-doped PCS, respectively. This is implied that doping may introduce a large number of defects into the carbon skeleton due to the substitution of carbon atoms. Fig. 4(d) shows the TG curve of H-PCS samples. All curves revealed a similar behavior. Specifically, the mass decay below 300 °C was ascribed to the evaporation of solvents and the release of crystal water. When the temperature increased from 300 to 450 °C, the carbon-containing gas would be generated, resulting in a large fraction of mass loss.
Figure 4 (a) XRD pattern (b) FTIR spectra (c) Raman spectra and (d) TG curve of all as-prepared samples. 13
Fig. 5(a) presents the nitrogen adsorption/desorption isotherm of all H-PCS. Interestingly, the isotherms of (N, Ni, P) co-, (N, Co, P) co- and (N, Ni, Co, P) tetradoped PCS are classified as type-IV isotherms where there is an obvious hysteresis loop which appeared within the relative pressure of 0.43-1.0 V [31, 32]. By contrast, the loop of isotherm vanished for (N, P) co-doped PCS. This hysteresis loop actually has manifested the existence of a large number of mesopores. It is favorable to shorten the ion/electron transport distance and enhance the active reaction sites. As a result, the kinetics can be promoted in the charge/discharge process [33]. Fig. 5(b) draws the pore size distribution of various H-PCS. As analyzed, there is only one prominent peak locating at 2.5 nm associating with (N, P) co-doped PCS. However, it was found that an intensive broaden peak emerges at 12-15 nm once the metal ions were doped, suggesting the mesoporosity of (N, Ni, P) co-, (N, Co, P) co- and (N, Ni, Co, P) tetradoped PCS. This is also consistent with the observation from the SEM image of H-PCS (Fig. 2). Further, the pore textures of H-PCS are summarized in Table 3. Among all HPCS, the largest BET surface area and pore volume are associated with (N, Ni, Co, P) tetra-doped PCS. It is 112.137 m2·g-1 and 0.604 cm3·g-1, respectively, indicating the good potential for LIBs. Without metal ions doping, the SSA of (N, P) co-doped PCS only obtained as 7.987 m2·g-1 with mesopore volume of 0.05 cm3·g-1, predicting the low capacitance and poor electrochemical kinetics. Thus, it is confirmed that doping has a significant impact on the SSA and pore structure of PCS due to the redistribution of atoms in the carbon skeleton [34].
14
Figure 5 (a) N2 adsorption-desorption isotherms and (b) pore size distribution of H-PCS.
Table 3. Porous parameters of H-PCS Sample
SBET (m2·g-1)
Smeso (m2·g-1)
Vt (cm3·g-1)
Vmeso (cm3·g-1)
Dm (m2·g-1)
(N, P) co-doped PCS
7.987
27.587
0.042
0.05
9.686
(N, Ni, P) tri-doped PCS
89.201
101.079
0.503
0.512
22.002
(N, Co, P) tri-doped PCS
81.047
95.343
0.473
0.480
22.432
(N, Ni, Co, P) tetra-doped
112.137
116.912
0.604
0.608
20.882
PCS
SBET: BET surface area, Vmeso: Mesopore volume, Vt : total pore volume, Dm: average pore size.
High-resolution XPS spectra were employed to analyze the elementary chemistry of H-PCS, equipped with the corresponding fitted data. Theoretically, the chemical shift is related to the total charge of the atom while the bonding energy is affected by the adjacent atom. As compared to carbon atom, the doping atoms including Ni, Co, N and P have shown relatively high electronegativity. Therefore, when the carbon atom was substituted by these heteroatoms from the skeleton, the surface charge may be changed. As a result, it is expected that the binding energy would shift to a high level with the increase of doping level. Fig. 6 draws the XPS spectra of N 1s, P 2p, Ni 2p and Co 2p. 15
Basically, all characteristic peaks are shifted to the right as the doping atom increases. In terms of N 1s spectrum, it can be decomposed into three peaks which are assigned to pyridine N at 398.5 eV, pyrrole N at 399.2 eV and graphite N at 401.1 eV. Among three N species, Pyridine N is proposed as an electrochemically active site to increase the capacitance of H-PCS for batteries. On the other hand, it is obtained from Fig. 6(a) that the Pyridine N is only examined from (N, Ni, Co, P) tetra-doped PCS, implying the high electrochemical activity. Fig. 6(b) shows the spectra of P 2p, confirming the presence of a prominent peak relating to P-C and P-O-C. Fig. 6(c) draws the Ni 2p spectra of (N, Ni, P) tri- and (N, Ni, Co, P) tetra-doped PCS. Both spectrums are quite similar and illustrate four obvious peaks. Regarding (N, Ni, P) tri-doped PCS, the peak located at 855.8 eV, 860.4 eV, 873.4 eV and 880.2 eV are attributed to the Ni 2p3/2 (Ni-P band) [35], the satellite speak (Ni-O band), Ni 2p1/2 (Ni-P band) and the satellite peaks (Ni-O band), respectively. All corresponding peak position has a little shift in terms of (N, Ni, Co, P) tetra-doped PCS due to the doping. Fig. 6(d) displays the Co 2p spectra of (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS. They both confirm the presence of Co 2p3/2 (Co-O, 781.3 eV) and Co 2p1/2 eV (Co-P, 797.57 eV) with satellite peak centering at 786.7 eV and 804.1 eV, respectively.
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Figure 6 (a) N 1s (b) P 2p (c) Ni 2p, (d) Co 2p and XPS spectra of all H-PCS.
The lithium ions storage performance of all H-PCS was evaluated within the voltage window of 0-3 V at a current density of 0.1 A·g-1. The GCD curves of various H-PCS anode in the 1st, 50th, 10th, 20th, 50th and 100th at a current density of 0.1 A·g-1 are showed in Fig. S2. As exhibited in Fig. 7(a), the highest initial capacity among all H-PCS electrodes was obtained as 900 mAh·g-1, corresponding to (N, Ni, P) tri-doped PCS. However, the stable reversible capacity of (N, Ni, P) tri-doped PCS after 100 cycles was 189.7 mAh·g-1, illustrating only 21.1% capacitance retaining which implies poor cycling performance. As compared to (N, Ni, P) tri-doped PCS, the reversible capacity, as well as capacitance retaining (Fig. 7(a)) of (N, Co, P) tri-doped PCS, were improved greatly due to high Co doping level and electrochemical activity. In contrast 17
to (N, Ni, P) and (N, Co, P) tri-doped PCS, the (N, Ni, Co, P) tetra-doped PCS electrode demonstrated the reversible capacity of 399.6 mAh·g-1 and capacitance retaining of 59.2% after 100 cycles, which are the highest value among all samples. Actually, after 10 cycles, the capacity of (N, Ni, Co, P) tetra-doped PCS was almost stabilized at ～ 400 mAh·g-1 with Coulombic efficiency of ~98% till 100 cycles, suggesting the highly durable characteristics. Further, the long cycling performance of (N, Ni, Co, P) tetradoped PCS was explored at 0.2 A·g-1. As exhibited in Fig. S1, the specific capacity was varied from 456.8 mAh·g-1 to 220.9 mAh·g-1 after 200 cycles, revealing 48.4% capacitance retaining. Different from the above-mentioned H-PCS, the (N, P) co-doped PCS presented the lowest reversible capacity although its capacitance retaining is impressive (~90%), suggesting that the porous structure of PCS is beneficial to provide the stable capacitance during the charge/discharge process. On the other hand, it is proposed that metal ions doping is favored improving the specific capacity by increasing the SSA with rational pore texture and introducing the redox reaction. Thus, the (N, Ni, Co, P) tetra-doped PCS is expected to have the best LIB performance among all samples. Besides the cycling performance, Fig. 7(b) shows the rate performance of various H-PCS anodes at current densities ranging from 0.05 to 1.0 A·g-1. Obviously, the tri- and tetra-doped PCS electrode exhibited the superior rate capability. Specifically, (N, Ni, Co, P) tetra-doped PCS displays the highly reversible discharge capacity decreased from 760.8 to 115.9 mAh·g-1 when the current density increased from 0.05 to 1.0 A·g-1. Once the current density recovered to 0.05 A·g-1, the specific capacity still remains at 698 mAh·g-1, demonstrating the good rate capability. 18
Fig. 7(c) draws the CV curve of various H-PCS tested at a scan rate of 0.1 mV·s-1 after three cycles. Overall, all curves are almost symmetrical which indicates the good electrochemical reversibility. In terms of oxidation, there are two obvious peaks locating at 0.9354 to 1.3258 V associating with tri- and tetra-doped PCS excepting for (N, P) co-doped PCS, which are attributed to the formation of the electrolyte interface (SEI) layer and Li2O [36]. It is worth mentioning that the oxidation is more significant regarding (N, Ni, Co, P) tetra-doped PCS. Further, the reduction peak location is closely related to heteroatom doping. For instance, two broad cathodic peaks can be observed at 0.4959 and 1.6901 V for (N, Ni, P) tri-doped PCS (Fig. 7(c)) while the corresponding peak shifted to 0.4707 and 1.6163 V for (N, Ni, Co, P) tetra-doped PCS. The arises of reduction peak are due to the intercalation of Li+ into PCS with sequential phase transitions. In contrast to tri- and tetra-doped PCS, the (N, P) co-doped PCS only presented one pair of oxidation-reduction peak, implying low specific capacity. Fig. S3 shows the CV curves of various PCS electrode tested at 1st, 2nd and 3rd cycles, exhibiting that CV curves are overlapped with each other after the 1st cycle. It suggests good electrochemical stability. Especially for (N, P) co-doped PCS, CV curves are almost coincidence, confirming the excellent reversibility [37-39]. EIS was utilized to investigate the properties of kinetics. Nyquist plot of all HPCS electrodes is illustrated in Fig. 7(d). All curves consist of a depressed semicircle from the high to medium frequency which implies the presence of charge transfer resistance (Rct) at the interface of electrode and electrolyte. Meanwhile, the Warburg tail emerged in the low-frequency region is associated with the Li-ion diffusion 19
resistance (Rs) in the electrode [40, 41]. Based on the equivalent circuit (Fig. 7d), the fitted value is presented in Table 4. The Rct of (N, Ni, Co, P) tetra-doped PCS is the lowest among all samples, which is good to improve the charging efficiency of the LIBs. However, the Rs is variable, depending on metal atom doping.
Figure 7 (a) Cycling performance at 0.1 A·g-1, (b) rate capabilities, (c) CV curves at 3rd cycles, (d) EIS spectra of various H-PCS anodes.
20
Table 4 Simulated values of Rs and Rct from the equivalent circuit Sample
Rs (Ω)
Rct (Ω)
(N, P) co-doped PCS
8.849
60.66
(N, Ni, P) tri-doped PCS
3.549
84.4
(N, Co, P) tri-doped PCS
16.17
72.5
(N, Ni, Co, P) tetra-doped PCS
12.46
57.99
To further explore the electrochemical behavior of heteroatom doped PCS, the CV curves were carried out at various scan rates from 0.2 to 0.8 mV·s-1 (Fig. 8(a), Fig. S4). As exhibited, there are obvious oxidation-reduction peaks presented on each curve regardless of the scan rate for all samples. However, the intensity is strengthened with the scan rate increased. Furthermore, the logarithm peak current (i) as a function of logarithm sweep rates (v) can be expressed according to Eq. (1) [42, 43] (1)
lg (𝑖) = lg (𝑎) +𝑏lg(ν)
Where a and b as variable value which can be obtained from the fitted line. Fig. 8(b) draws the relationship between the lg(i) vs lg(v) of (N, Ni, Co, P) tetra-doped PCS. Typically, b-value is related to the contribution of the capacitance in the reaction. When the b-value is approaching ~0.5, the capacitance is mainly contributed from the lithium insertion/extraction diffusion behavior. Once the b-value is close to 1, it indicates that the capacitance is derived from the surface-limited capacitive behavior. As shown in Fig. 8(b) that the b-value of (N, Ni, Co, P) tetra-doped PCS is ranged from ~0.74141 to ~0.90068, suggesting the capacitive capacitance is dominant. On the other hand, the bvalue of co-atom and tri-atom doped PCS is close to 1 as well (Fig. S5), also manifesting 21
similar behavior. Moreover, the respective contribution from capacitive and diffusion behavior can be confirmed according to Eq. (2) [44] 𝑖 = k1v + k2v1/2
(2)
where k1 and k2 are adjustable constants at a certain sweep rate. k1ν and k2ν1/2 are often dominated to separate surface-limited capacitive capacitance and the diffusioncontrolled capacitance, respectively. Fig. 8(c) exhibits the CV curve of (N, Ni, Co, P) tetra-doped PCS with respective capacitance contribution at the scan rate of 0.8 mV·s1,
illustrating the contribution of the capacitive process is approximately 68.70%. Fig.
8(d) states the contributions ratios of the diffusion-controlled charge and capacitive charge at different scan rates for all H-PCS. Regardless of doping, the ratio of capacitive contribution was gradually increased with the increase of scan rate. Nevertheless, the ratio was exceeded 80% for (N, P) co-doped PCS at any scan rate, indicating the complete capacitive behavior. Once the metal atom doped, the corresponding ratio was dramatically decreased to ~40% to ~60%, revealing the diffusion-controlled capacitance was reinforced due to the involvement of the Faradaic reaction. Moreover, it is necessary to mention that such an effect is more distinct in (N, Co, P) tri-doped PCS due to the high level of Co doping as well as its electrochemical activity.
22
Figure 8 (a) CV curves of (N, Ni, Co, P) tetra-doped PCS at various scan rates, (b) lg(i) vs lg(v), (c) The capacitive charge storage contributions at the scan rate of 0.8 mV·s-1, (d) Contributions ratios of capacitive charge at different scan rates for all H-PCS anodes.
The Li+ diffusion in (N, Ni, Co, P) tetra-doped PCS electrode was examined for a potential step of 2.1 to 3.0 V by implementing PITT test. Fig. 9(a) and (b) show the I vs t and ln(I) vs t at a voltage platform of 2.191 V. During the PITT test, the cells were charged to 3 V with a voltage step of 20 mV. Since Li-ion transport in the (N, Ni, Co, P) tetra-doped PCS electrode follows Fick’s second law, thus the DLi+ can be calculated according to Eq. (3) [45, 46] 𝐷𝐿𝑖 + = ―
4𝐿2d𝑙𝑛(𝐼)
(3)
π2 𝑑(𝑡)
where L indicates the thickness of the active material on the electrode, I refer to 23
the step current, and t is related to the step time during the test process [47]. Fig. 9(c) exhibits the calculated logarithm Li-ion diffusion coefficient plots of (N, P) co-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS. From the figure, DLi+ of (N, P) co-doped PCS anode was varied from 8.99×10-10 to 1.01×10-9 cm2·s-1 when the cell potential was changed from 2.1 to 2.65 V while it was changed from 3.96×10-10 to 1.45×10-9 cm2·s-1 and 8.18×10-10 to 9.24×10-9 cm2·s-1 when the cell potential was changed from 2.1 to 3.0 V, corresponding to (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS. Obviously, the highest DLi+ corresponds to (N, Ni, Co, P) tetra-doped PCS, which determines the fast reaction and superior LIBs performance. This indicates the heteroatom doping are beneficial to improving DLi+ value and therefore enhance the electrochemical performance of H-PCS. Fig. 9(d) shows the EIS spectra of (N, P) co-, (N, Co, P) triand (N, Ni, Co, P) tetra-doped PCS anode after 100 cycles. As compared to Fig. 7(d), both the Rct and Rs value are greatly increased for all samples, implying the formation of electrolyte interphase substance. Table S1 presented the fitted value of Rct and Rs for (N, P) co-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS. When compared with the corresponding data shown in Table 4, it was found that bi-metal atom doping is favored for restraining the increase of Rct. On the other hand, another small arc emerged on the EIS curve of (N, P) co-doped PCS after cycling which may have resulted from the structural deterioration.
24
Figure 9. (a) Time dependence of the current in the (N, Ni, Co, P) tetra-doped PCS at a voltage platform of 2.191 V vs Li/Li+, (b) ln(I) vs t, (c) calculated Li-ion diffusion coefficient of (N, P) co-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS, (d) EIS spectra of (N, P) co-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS anode after 100 cycles at 100 mA·g-1.
To examine the structural durability of H-PCS, the morphology of anode after cycling was characterized. Fig. 10(a), (b) and (c) exhibit the SEM image of (N, P) co-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS anode after 100 cycles, respectively. Obviously, we can obtain from the image that the spherical morphology of (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS were well retained while severe structural damage was found in terms of (N, P) co-doped PCS, suggesting the volume change was restricted during the charge/discharge process in H-PCS with doping of metal atom [48]. 25
Fig. 10(d) shows the HRTEM of (N, Ni, Co, P) tetra-doped PCS after 100 cycles with SAED pattern from the selected area (b). As compared to observation from Fig. 3(b), some dark substitutes were found which can be classified as the by-products from the electrochemical process. In Fig. 10(e), three diffraction rings were clearly captured which are associated with (214), (123) and (311) of Li3PO4.
Figure 10. SEM images of (a) (N, P) co-doped PCS, (b) (N, Co, P) tri-doped PCS and (c) (N, Ni, Co, P) tetra-doped PCS anode after 100 cycles (inset is the magnified SEM image), (d) HRTEM of (N, Ni, Co, P) tetra-doped PCS after 100 cycles and (e) SAED pattern.
Conclusion In summary, we have proposed a universal approach to prepare echinus-like porous carbon spheres with controlled heteroatom doping (H-PCS). The morphology, structure, electrochemical behavior and lithium ions storage performance of (N, P) co-, 26
(N, Ni, P) tri-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS have been comprehensively investigated. The results suggested that the (N, Ni, Co, P) tetra-doped PCS exhibited highly reversible capacity, excellent rate capability and superior cycling performance. This is benefited from the highly porous characteristic, high heteroatom doping content and moderate specific surface which improved the Li+ diffusion and adsorption on active sites. Thus, this work provides an insight into the electrochemical behavior of H-PCS and strengthens our understanding of carbon nanomaterials with confined heteroatom doping.
Acknowledgment This work was supported by project of Ningxia key R&D plan (2018BEE03013).
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Zhuo Chen: Investigation, Writing - Original Draft. Haibo Li: Conceptualization, Methodology, Supervision, Writing Review & Editing.
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Declaration of interests ☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Graphical abstract Text: A universal strategy to prepare heteroatom doped echinus-like porous carbon spheres as advanced anode for lithium ions battery has been developed.
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S0021-9797(20)30121-1 https://doi.org/10.1016/j.jcis.2020.01.107 YJCIS 25980
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4 December 2019 23 January 2020 28 January 2020
Please cite this article as: Z. Chen, H. Li, The lithium ions storage behavior of heteroatom-mediated echinus-like porous carbon spheres: from co-doping to multi-atom doping, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.01.107
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The lithium ions storage behavior of heteroatom-mediated echinus-like porous carbon spheres: from co-doping to multi-atom doping Zhuo Chen, Haibo Li* Ningxia Key Laboratory of Photovoltaic Materials, Ningxia University, Yinchuan, Ningxia 750021, P. R. China *Corresponding author at the E-mail address: [email protected] (H. Li).
Abstract This study proposed a facile method to prepare echinus-like porous carbon spheres (PCS) with different heteroatom doping for lithium ions battery (LIBs). A metalorganophosphine framework (MOPF) was synthesized by employing riboflavin sodium phosphate as an organic ligand to conjugate with metal ions and then carbonized at mild temperature, leading to the formation of heteroatom doped PCS (H-PCS). As a result, (N, P) co-, (N, P, Ni) tri-, (N, P, Co) tri- and (N, Ni, Co, P) tetra-doped PCS were obtained to examine the insight into lithium-ion storage behavior of H-PCS. It was found that the specific surface area, pore texture and structural defects of H-PCS were dependent on doping of heteroatoms as well as the charge transfer resistance and Liion diffusion coefficient. Significantly, the redox reaction potential during the charge/discharge could be mediated upon the doping. Thus, when evaluated as anode for LIBs, the (N, Ni, Co, P) tetra-doped PCS exhibited highly reversible capacity of 680 mAh∙g-1 at 0.1 A∙g-1, excellent rate capability (115.9 mAh∙g-1 at 1.0 A∙g-1) and superior cycling performance (399.6 mAh∙g-1 at 0.1 A∙g-1). Moreover, the cyclic voltammogram measurements demonstrated that the doping of metal atoms was favorable for improving the capacitive contribution of surface limited diffusion. Thus, this work highlighted the importance of HCP with defined doping which could be considered as one of the prominent candidates for high-performance LIBs’ anode. Keywords: Porous carbon spheres, Heteroatom doping, Energy storage, Sol-gel method.
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1.
Introduction With the tremendous usage of traditional fossil fuel and global environmental
concern, it is very urgent to explore green energy as the main energy source substitute. To continuously utilize renewable energy effectively, advanced energy storage technologies are needed. Lithium-ion batteries (LIBs) as one of the representative renewable energy sources that have been widely commercialized and applied in many fields since 1991, such as laptops, portable electronic devices and electric vehicles [13]. As a typical anode for LIBs, graphite offers a reversible capacity of ~372 mAh·g-1. However, it cannot catch up with the development requirements for high capacity [4, 5]. To satisfy the increasing demands of these energy storage applications, it is necessary to develop LIBs with high capacities based on advanced functional materials. Among all candidates, alloying types of materials can often deliver highly enhanced specific capacities [6, 7], nevertheless, they suffer from poor structural-stability, low irreversible capacity and complicated strategies of preparation [8, 9]. Owing to the advantages involving abundance, low-cost [10, 11], excellent cycling stability and electrochemical kinetics, various carbonaceous materials [12, 13], such as carbon nanotube [14], graphene [15, 16], carbon fiber, [17-19] mesoporous carbon and carbon hollow spheres [20, 21], have the greatest potential as anode for LIBs. Among these carbon anodes, porous carbon (PC) has attracted enormous attention due to its high porosity, specific surface area, mediated electrochemical activity and relatively low cost [22]. It is proposed that lithium storage in carbonaceous anode is performed by adsorption which is analog to electrochemical capacitors. As a result, high power 3
density is achieved. However, the energy density is unsatisfactory [23, 24]. To overcome this issue, heteroatom doped porous carbon (HPC) is developed aiming to achieve both high power and high density by combing adsorption and redox reactions. In application to LIBs, HPC and its derivative have rational pore texture. It promises to shorten the diffusion distance of Li+, alleviate the volume expansion during the charge/discharge and thereby improve the capacitance retention. In this aspect, many efforts have been devoted to design different strategies and optimize the heteroatom doped sites to promote the capacity. Zheng et al. proposed that the lithium-ion storage capacity of N-doped graphene depend on nitrogen-doping content. It is supposed that nitrogen atoms adopted on a 2D honeycomb lattice is closely related to structuralstability. As a result, nitrogen-doped graphene particle with nitrogen content of 17.72 wt% delivered a high capacity of 2132 mAh·g-1 after 50 cycles at the current density of 100 mA·g-1 and 785 mAh·g-1 after 1000 cycles at the current density of 5 A∙g-1 [25]. Pan et al. prepared N-doped porous carbon (NPC) as advanced anode for LIBs. It suggested that large interlayer spacing (~0.4 nm) and disordered structure are favorable for Li+ insertion/extraction. Moreover, N-doped NPC can enhance conductivity and provide more active sites for Li+ storage [26]. Regarding the fabrication of PC, it is accompanied by high-cost, high complexity and low yield. In most cases, it is basically necessary to remove the template or introduce further purification process. On the other hand, additional doping sources are required and involved into the preparation of HPC which to some extent leads to a large waste of doping sources and increasing expenses. In terms of P doping, the inevitable 4
release of the poisonous and flammable gas should be considered seriously. Therefore, it is still a challenge to prepare HPC using facile, economical and effective approach at the molecular level for LIBs with enhanced anode performance [27]. Metal-organic frameworks (MOFs) with abundant active site and well-defined pores have been viewed as one of the ideal choices to prepare nanostructured carbons by proper carbonization. To this end, it is considered that heteroatom doped carbons can be obtained by either introducing a doping source in the carbonating or selecting particular organic ligand with target doping atom. This study reports a facile strategy to prepare heteroatom doped porous carbon spheres (H-PCS) with co-, tri- and tetra-atom doping by carbonating a metalorganophosphine framework (MOPF) using riboflavin sodium phosphate. The MOPF can be deemed as organic ligand, nitrogen and phosphorus source as well (Fig. 1). In contrast to previous doping method, the H-PCS were directly obtained by introducing doping atoms at the molecular level. Significantly, this method does not introduce any additional doping source which is thought to be feasible and economical. On the other hand, the doping can be precisely controlled by selecting proper metal salts. It is explored that the H-PCS shows a typical echinus-like structure with high doping content. Such structure is very beneficial to increase the conductivity and create as much active sites for Li+ diffusion [28, 29]. Owing to the highly porous characteristic, high heteroatom doping content and moderate specific surface area, the (N, Ni, Co, P) tetra-doped PCS exhibited high reversible capacity, good rate capability and long cycling performance. The cyclic voltammograms (CV) measurements verified the high 5
ratio of capacitive capacities contribution for all H-PCS samples. It illustrated the improved Li+ storage capacity of H-PCS is ascribed to Li adsorption on active sites. Further, it is also found that the metal ions doping is favored for improving the capacitive contribution of surface limited diffusion behavior. Thus, this work examines the role of heteroatom doping in PCS on the electrochemical performance and provides a universal approach to design and mediate the heteroatom doping in carbon anode.
Figure 1 The schematic illustration of the synthesis of (N, P) co-doped PCS, (N, Co, P) tri-doped PCS, (N, Ni, P) tri-doped PCS) and (N, Ni, Co, P) tetra-doped PCS.
2.
Experimental section
2.1 Synthesis of (N, Ni, Co, P) tetra-doped PCS The synthesis of (N, Ni, Co, P) tetra-doped PCS is as follows. 0.5 g Riboflavin sodium phosphate (C17H20N4NaO9P) was dissolved in 45 ml deionized water under vigorous magnetic stirring for 10 minutes to form solution A. Similarly, 0.58206 g 6
Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 1.16316 g Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) was added into 40 ml ethanol solution (99.7%) to obtain solution B and C, respectively. Then, solution B was transferred to solution A under stirring at room temperature. Immediately, solution C was added to the mixed solution and was stirred continuously. The mixture was kept at room temperature for 24 h. After that, the products were washed with ethanol solution (99.7%) at least three times and dried for 12 h in a drying oven at 60 ℃. Subsequently, the dried precursor was annealed at 450 ℃ for 1 h under a nitrogen atmosphere at a heating rate of 10 ℃/min to obtain (N, Ni, Co, P) tetra-doped PCS. 2.2 Synthesis of (N, P) co-doped, (N, Ni, P) and (N, Co, P) tri-doped PCS To synthesize (N, Ni, P) tri-doped PCS, 0.5 g C17H20N4NaO9P was initially dissolved in 45 ml deionized water with vigorous magnetic stirring for 10 minutes to form solution A. Then, 0.58158 g Ni (NO3)2·6H2O was added into 40 ml ethanol solution (99.7%) to form the solution B. Subsequently, solution B was transferred to solution A under stirring at room temperature. After that, the process is the same as the synthesis of (N, Ni, Co, P) tetra-doped PCS. The synthesis of (N, Co, P) tri-doped PCS follows the method described as above. In addition to (N, Ni, P) and (N, Co, P) tri-doped PCS, the (N, P) co-doped PCS was directly obtained by carbonating C17H20N4NaO9P at 450 ℃ for 1 h under a nitrogen atmosphere at a heating rate of 10 ℃/min. The specifications of reagents used in the experiment is shown in Table 1.
7
Table 1 Number of moles of reagents. Reagents
Mass (g)
Relative molecular mass
Number of moles (mol)
C17H20N4NaO9P
0.5
478.33
0.001
Co (NO3)2·6H2O
0.58206
291.03
0.002
Ni (NO3)2·6H2O
1.16316
290.79
0.004
Ni (NO3)2·6H2O
0.58158
290.79
0.002
2.5 Characterization The structure and morphology were tested by scanning electron microscope (SEM, Zeiss Supra 40, German), transmission electron microscopy (TEM, Hitachi HT7700, Japan), and high-resolution transmission electron microscope (HRTEM, FEI talos F200s, USA). The crystal structures were identified by X-ray diffractometer (DX-2700, CHN) using Cu-Ka radiation (40 KV/30mA) coverage 2 theta angles 10-85°. The specific surface area was measured by surface area analyzer with Brunauer-EmmettTeller (BET, JW-BK200C, China), which is relying on a single point. Raman spectra were recorded with the spectrometer (DXR, USA) at room temperature. X-ray photoelectron spectroscopy (XPS) was performed on Thermo ESCALAB 250Xi (America) with an Al Ka source gun (1361 eV). The FT-IR spectra were recorded by KBr
pellets
with
a
WQF-520A
FT-IR
spectrometer
(4000-400
cm-1).
Thermogravimetry (TG) spectra of as-prepared H-PCS were tested on a thermogravimetric analyzer (Pyris 1, USA) from room temperature to 1000 ℃ with a ramp rate of 10 ℃/min under N2 atmosphere. 2.6 Battery performance To prepare the PCS anode, a homogeneous mixture containing the active material, acetylene black and polyvinylidene fluoride (PVDF) with a mass ratio of 8:1:1 was 8
made first. Then, the N-methyl-2-pyrrolidone (NMP) was dropped into the mixture to make it moisture. After that, the anode was obtained by coating the slurry uniformly on a collector utilized disk-shaped foamed nickel. Finally, the as-prepared electrode was dried in the oven at 80 ℃ for 6 hours. It should be noted that the assembling of coin cells LIB battery was carried out in an argon-filled glove box (O2<5 ppm, H2O<1 ppm). The electrochemical performance was tested by using lithium-foil as the counter electrode and Celgard membrane as a cell separator, respectively. A mixture of EC, EMC and DEC with a ratio of 1: 1: 1 Vol% was used as the solvent. One mol/L LiPF6 solution with FEC as an additive was used to assemble a LIB battery (CR2032). The land battery measurement system (CT2001A, LANHE) was used to evaluate the constant current charge and discharge (GCD) test. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) measurements were performed on the electrochemical workstation (PARSTAT 3000A-DX, AMETEK). 2.7 PITT test The potentiostatic intermittent titration technique (PITT) experiments were performed with a potential step of 0.02 V and an interval of 15 minutes. The current with respect to time was measured for the initial charging process. Moreover, the voltage window was set from 2.1 to 3.0 V. 3.
Result and discussion Fig. 2 (a) to (d) display the SEM image of MOPF, Ni-MOPF, Co-MOPF and (Ni,
Co)-MOPF while (e) to (h) show the SEM image of (N, P) co-, (N, Ni, P) tri-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS, respectively. The figure clearly shows that 9
all M-MOPF (M=metal or bi-metal) revealed typical echinus-like structure with plenty of open pores present on the surface except for MOPF. After carbonization at mild temperature, the microstructure of (N, Ni, P) tri-, (N, Co, P) tri- and (N, Ni, Co, P) tetradoped PCS remained. However, the surface of H-PCS is more obvious and porous [30]. It is worthy of note that the H-PCS was obtained via a self-doping strategy without the introduction of any additional doping source, realizing the advances. Significantly, such structure is supposed to provide a large number of tunnels for penetration of Li+, improving the specific capacity. On the other hand, the electrochemical reaction was probably confined within the inner PCS. In this case, the volume expansion during the charge/discharge process could be restricted. In contrast to (N, Ni, P) tri-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS, the (N, P) co-doped PCS presented a ball-like structure with quite smooth surface, suggesting low specific surface area (SSA) and low LIBs performance. Moreover, (N, Ni, P) tri-doped PCS and (N, Co, P) tri-doped PCS showed similar structure with that of (N, Ni, Co, P) tetra-doped PCS. However, it is obtained from the magnified image that the porous structure of both samples is not as good as (N, Ni, Co, P) tetra-doped PCS. Besides, they both show compact surface which may decrease the SSA and unfavorable to facilitate the transportation of electrolytes. Table 2 shows the elemental ratio of various H-PCS (the data were recorded via Energy Dispersive Spectrometer). It was found that heteroatom occupied a high percentage, indicating the high heteroatom doping content. Interestingly, as compared to Ni, Co is more facilely conjugated with organic C17H20N4NaO9P since the high Co doping ratio. Further, as the doping atom increases, the carbon content is 10
increased as well which is 73.33, corresponding to (N, Ni, Co, P) tetra-doped PCS.
Figure 2 SEM images of (a) MOPF, (b) Ni-MOPF, (c) Co-MOPF, (d) (Ni, Co)-MOPF, (e) (N, P) co-doped PCS, (f) (N, Ni, P) tri-doped PCS), (g) (N, Co, P) tri-doped PCS), (h) (N, Ni, Co, P) tetra-doped PCS, inset is the magnified SEM image from the selected area.
Table 2. Elemental ratio of various H-PCS Samples
Ni (%)
Co (%)
P (%)
C (%)
N (%)
O (%)
(N, P) co-doped PCS
-
-
7.87
56.90
5.49
22.51
(N, Ni, P) tri-doped PCS
1.82
-
2.56
69.22
1.30
24.10
(N, Co, P) tri-doped PCS
-
5.22
6.85
54.21
6.05
26.81
(N, Ni, Co, P) tetra-doped PCS
0.99
0.77
2.06
73.33
5.87
16.98
The fine structure of (N, Ni, Co, P) tetra-doped PCS was examined by employing HRTEM. Fig. 3(a) to (c) shows the HRTEM image of (N, Ni, Co, P) tetra-doped PCS with different magnifications. As can be seen that the (N, Ni, Co, P) tetra-doped PCS are quite uniform and highly porous. The diameter of the individual (N, Ni, Co, P) tetradoped PCS is around ~5.0 μm. Fig. 3(d) shows the HAADF image of (N, Ni, Co, P) tetra-doped PCS with mapping of Ni, Co, P, C, N and O, verifying the even elemental distribution. 11
Figure 3 HRTEM image of (N, Ni, Co, P) tetra-doped PCS in low (a) and high magnifications (b), (c) HAADF image of (N, Ni, Co, P) tetra-doped PCS with mapping of Ni, Co, P, C, N, and O.
The XRD patterns of all samples display a broad diffraction peak (Fig. 4(a)), illustrating the presence of amorphous carbon. The peak for all samples is almost locating at ~26°, corresponding to (002) plane of graphite. Besides, none obvious peaks were found, indicating the doping is dominant in PCS. Fig. 4(b) draws the FTIR spectrums for all H-PCS. Basically, they all show the same absorption band except for the intensity due to different doping levels. The main peaks are locating at 3316, 1635, 1628 and 1097 cm-1 which correspond to N-H, C=C, N=N and C-O stretch vibration, respectively. Besides, the peak focused at 914, 828, 870 and 747 are ascribed to the existence of C=C-H. The Raman spectra of H-PCS are shown in Fig. 4(c). For all HPCS samples, two distinct peaks locating at ~1350 and 1580 cm-1 are found which can be assigned to D and G bond, respectively. The D and G bonds separately represent the defected sp2 hybridized carbon and graphitic mode of carbon materials. Moreover, the 12
intensity ratio of D peak to G peak indicates the ordering degree of the carbonaceous materials. As calculated from Fig. 4(c) that the ID/IG is 0.998, 0.995, 0.863 and 0.841, corresponding to (N, Ni, Co, P) tetra-doped PCS, (N, Ni, P) tri-doped PCS, (N, Co, P) tri-doped PCS and (N, P) co-doped PCS, respectively. This is implied that doping may introduce a large number of defects into the carbon skeleton due to the substitution of carbon atoms. Fig. 4(d) shows the TG curve of H-PCS samples. All curves revealed a similar behavior. Specifically, the mass decay below 300 °C was ascribed to the evaporation of solvents and the release of crystal water. When the temperature increased from 300 to 450 °C, the carbon-containing gas would be generated, resulting in a large fraction of mass loss.
Figure 4 (a) XRD pattern (b) FTIR spectra (c) Raman spectra and (d) TG curve of all as-prepared samples. 13
Fig. 5(a) presents the nitrogen adsorption/desorption isotherm of all H-PCS. Interestingly, the isotherms of (N, Ni, P) co-, (N, Co, P) co- and (N, Ni, Co, P) tetradoped PCS are classified as type-IV isotherms where there is an obvious hysteresis loop which appeared within the relative pressure of 0.43-1.0 V [31, 32]. By contrast, the loop of isotherm vanished for (N, P) co-doped PCS. This hysteresis loop actually has manifested the existence of a large number of mesopores. It is favorable to shorten the ion/electron transport distance and enhance the active reaction sites. As a result, the kinetics can be promoted in the charge/discharge process [33]. Fig. 5(b) draws the pore size distribution of various H-PCS. As analyzed, there is only one prominent peak locating at 2.5 nm associating with (N, P) co-doped PCS. However, it was found that an intensive broaden peak emerges at 12-15 nm once the metal ions were doped, suggesting the mesoporosity of (N, Ni, P) co-, (N, Co, P) co- and (N, Ni, Co, P) tetradoped PCS. This is also consistent with the observation from the SEM image of H-PCS (Fig. 2). Further, the pore textures of H-PCS are summarized in Table 3. Among all HPCS, the largest BET surface area and pore volume are associated with (N, Ni, Co, P) tetra-doped PCS. It is 112.137 m2·g-1 and 0.604 cm3·g-1, respectively, indicating the good potential for LIBs. Without metal ions doping, the SSA of (N, P) co-doped PCS only obtained as 7.987 m2·g-1 with mesopore volume of 0.05 cm3·g-1, predicting the low capacitance and poor electrochemical kinetics. Thus, it is confirmed that doping has a significant impact on the SSA and pore structure of PCS due to the redistribution of atoms in the carbon skeleton [34].
14
Figure 5 (a) N2 adsorption-desorption isotherms and (b) pore size distribution of H-PCS.
Table 3. Porous parameters of H-PCS Sample
SBET (m2·g-1)
Smeso (m2·g-1)
Vt (cm3·g-1)
Vmeso (cm3·g-1)
Dm (m2·g-1)
(N, P) co-doped PCS
7.987
27.587
0.042
0.05
9.686
(N, Ni, P) tri-doped PCS
89.201
101.079
0.503
0.512
22.002
(N, Co, P) tri-doped PCS
81.047
95.343
0.473
0.480
22.432
(N, Ni, Co, P) tetra-doped
112.137
116.912
0.604
0.608
20.882
PCS
SBET: BET surface area, Vmeso: Mesopore volume, Vt : total pore volume, Dm: average pore size.
High-resolution XPS spectra were employed to analyze the elementary chemistry of H-PCS, equipped with the corresponding fitted data. Theoretically, the chemical shift is related to the total charge of the atom while the bonding energy is affected by the adjacent atom. As compared to carbon atom, the doping atoms including Ni, Co, N and P have shown relatively high electronegativity. Therefore, when the carbon atom was substituted by these heteroatoms from the skeleton, the surface charge may be changed. As a result, it is expected that the binding energy would shift to a high level with the increase of doping level. Fig. 6 draws the XPS spectra of N 1s, P 2p, Ni 2p and Co 2p. 15
Basically, all characteristic peaks are shifted to the right as the doping atom increases. In terms of N 1s spectrum, it can be decomposed into three peaks which are assigned to pyridine N at 398.5 eV, pyrrole N at 399.2 eV and graphite N at 401.1 eV. Among three N species, Pyridine N is proposed as an electrochemically active site to increase the capacitance of H-PCS for batteries. On the other hand, it is obtained from Fig. 6(a) that the Pyridine N is only examined from (N, Ni, Co, P) tetra-doped PCS, implying the high electrochemical activity. Fig. 6(b) shows the spectra of P 2p, confirming the presence of a prominent peak relating to P-C and P-O-C. Fig. 6(c) draws the Ni 2p spectra of (N, Ni, P) tri- and (N, Ni, Co, P) tetra-doped PCS. Both spectrums are quite similar and illustrate four obvious peaks. Regarding (N, Ni, P) tri-doped PCS, the peak located at 855.8 eV, 860.4 eV, 873.4 eV and 880.2 eV are attributed to the Ni 2p3/2 (Ni-P band) [35], the satellite speak (Ni-O band), Ni 2p1/2 (Ni-P band) and the satellite peaks (Ni-O band), respectively. All corresponding peak position has a little shift in terms of (N, Ni, Co, P) tetra-doped PCS due to the doping. Fig. 6(d) displays the Co 2p spectra of (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS. They both confirm the presence of Co 2p3/2 (Co-O, 781.3 eV) and Co 2p1/2 eV (Co-P, 797.57 eV) with satellite peak centering at 786.7 eV and 804.1 eV, respectively.
16
Figure 6 (a) N 1s (b) P 2p (c) Ni 2p, (d) Co 2p and XPS spectra of all H-PCS.
The lithium ions storage performance of all H-PCS was evaluated within the voltage window of 0-3 V at a current density of 0.1 A·g-1. The GCD curves of various H-PCS anode in the 1st, 50th, 10th, 20th, 50th and 100th at a current density of 0.1 A·g-1 are showed in Fig. S2. As exhibited in Fig. 7(a), the highest initial capacity among all H-PCS electrodes was obtained as 900 mAh·g-1, corresponding to (N, Ni, P) tri-doped PCS. However, the stable reversible capacity of (N, Ni, P) tri-doped PCS after 100 cycles was 189.7 mAh·g-1, illustrating only 21.1% capacitance retaining which implies poor cycling performance. As compared to (N, Ni, P) tri-doped PCS, the reversible capacity, as well as capacitance retaining (Fig. 7(a)) of (N, Co, P) tri-doped PCS, were improved greatly due to high Co doping level and electrochemical activity. In contrast 17
to (N, Ni, P) and (N, Co, P) tri-doped PCS, the (N, Ni, Co, P) tetra-doped PCS electrode demonstrated the reversible capacity of 399.6 mAh·g-1 and capacitance retaining of 59.2% after 100 cycles, which are the highest value among all samples. Actually, after 10 cycles, the capacity of (N, Ni, Co, P) tetra-doped PCS was almost stabilized at ～ 400 mAh·g-1 with Coulombic efficiency of ~98% till 100 cycles, suggesting the highly durable characteristics. Further, the long cycling performance of (N, Ni, Co, P) tetradoped PCS was explored at 0.2 A·g-1. As exhibited in Fig. S1, the specific capacity was varied from 456.8 mAh·g-1 to 220.9 mAh·g-1 after 200 cycles, revealing 48.4% capacitance retaining. Different from the above-mentioned H-PCS, the (N, P) co-doped PCS presented the lowest reversible capacity although its capacitance retaining is impressive (~90%), suggesting that the porous structure of PCS is beneficial to provide the stable capacitance during the charge/discharge process. On the other hand, it is proposed that metal ions doping is favored improving the specific capacity by increasing the SSA with rational pore texture and introducing the redox reaction. Thus, the (N, Ni, Co, P) tetra-doped PCS is expected to have the best LIB performance among all samples. Besides the cycling performance, Fig. 7(b) shows the rate performance of various H-PCS anodes at current densities ranging from 0.05 to 1.0 A·g-1. Obviously, the tri- and tetra-doped PCS electrode exhibited the superior rate capability. Specifically, (N, Ni, Co, P) tetra-doped PCS displays the highly reversible discharge capacity decreased from 760.8 to 115.9 mAh·g-1 when the current density increased from 0.05 to 1.0 A·g-1. Once the current density recovered to 0.05 A·g-1, the specific capacity still remains at 698 mAh·g-1, demonstrating the good rate capability. 18
Fig. 7(c) draws the CV curve of various H-PCS tested at a scan rate of 0.1 mV·s-1 after three cycles. Overall, all curves are almost symmetrical which indicates the good electrochemical reversibility. In terms of oxidation, there are two obvious peaks locating at 0.9354 to 1.3258 V associating with tri- and tetra-doped PCS excepting for (N, P) co-doped PCS, which are attributed to the formation of the electrolyte interface (SEI) layer and Li2O [36]. It is worth mentioning that the oxidation is more significant regarding (N, Ni, Co, P) tetra-doped PCS. Further, the reduction peak location is closely related to heteroatom doping. For instance, two broad cathodic peaks can be observed at 0.4959 and 1.6901 V for (N, Ni, P) tri-doped PCS (Fig. 7(c)) while the corresponding peak shifted to 0.4707 and 1.6163 V for (N, Ni, Co, P) tetra-doped PCS. The arises of reduction peak are due to the intercalation of Li+ into PCS with sequential phase transitions. In contrast to tri- and tetra-doped PCS, the (N, P) co-doped PCS only presented one pair of oxidation-reduction peak, implying low specific capacity. Fig. S3 shows the CV curves of various PCS electrode tested at 1st, 2nd and 3rd cycles, exhibiting that CV curves are overlapped with each other after the 1st cycle. It suggests good electrochemical stability. Especially for (N, P) co-doped PCS, CV curves are almost coincidence, confirming the excellent reversibility [37-39]. EIS was utilized to investigate the properties of kinetics. Nyquist plot of all HPCS electrodes is illustrated in Fig. 7(d). All curves consist of a depressed semicircle from the high to medium frequency which implies the presence of charge transfer resistance (Rct) at the interface of electrode and electrolyte. Meanwhile, the Warburg tail emerged in the low-frequency region is associated with the Li-ion diffusion 19
resistance (Rs) in the electrode [40, 41]. Based on the equivalent circuit (Fig. 7d), the fitted value is presented in Table 4. The Rct of (N, Ni, Co, P) tetra-doped PCS is the lowest among all samples, which is good to improve the charging efficiency of the LIBs. However, the Rs is variable, depending on metal atom doping.
Figure 7 (a) Cycling performance at 0.1 A·g-1, (b) rate capabilities, (c) CV curves at 3rd cycles, (d) EIS spectra of various H-PCS anodes.
20
Table 4 Simulated values of Rs and Rct from the equivalent circuit Sample
Rs (Ω)
Rct (Ω)
(N, P) co-doped PCS
8.849
60.66
(N, Ni, P) tri-doped PCS
3.549
84.4
(N, Co, P) tri-doped PCS
16.17
72.5
(N, Ni, Co, P) tetra-doped PCS
12.46
57.99
To further explore the electrochemical behavior of heteroatom doped PCS, the CV curves were carried out at various scan rates from 0.2 to 0.8 mV·s-1 (Fig. 8(a), Fig. S4). As exhibited, there are obvious oxidation-reduction peaks presented on each curve regardless of the scan rate for all samples. However, the intensity is strengthened with the scan rate increased. Furthermore, the logarithm peak current (i) as a function of logarithm sweep rates (v) can be expressed according to Eq. (1) [42, 43] (1)
lg (𝑖) = lg (𝑎) +𝑏lg(ν)
Where a and b as variable value which can be obtained from the fitted line. Fig. 8(b) draws the relationship between the lg(i) vs lg(v) of (N, Ni, Co, P) tetra-doped PCS. Typically, b-value is related to the contribution of the capacitance in the reaction. When the b-value is approaching ~0.5, the capacitance is mainly contributed from the lithium insertion/extraction diffusion behavior. Once the b-value is close to 1, it indicates that the capacitance is derived from the surface-limited capacitive behavior. As shown in Fig. 8(b) that the b-value of (N, Ni, Co, P) tetra-doped PCS is ranged from ~0.74141 to ~0.90068, suggesting the capacitive capacitance is dominant. On the other hand, the bvalue of co-atom and tri-atom doped PCS is close to 1 as well (Fig. S5), also manifesting 21
similar behavior. Moreover, the respective contribution from capacitive and diffusion behavior can be confirmed according to Eq. (2) [44] 𝑖 = k1v + k2v1/2
(2)
where k1 and k2 are adjustable constants at a certain sweep rate. k1ν and k2ν1/2 are often dominated to separate surface-limited capacitive capacitance and the diffusioncontrolled capacitance, respectively. Fig. 8(c) exhibits the CV curve of (N, Ni, Co, P) tetra-doped PCS with respective capacitance contribution at the scan rate of 0.8 mV·s1,
illustrating the contribution of the capacitive process is approximately 68.70%. Fig.
8(d) states the contributions ratios of the diffusion-controlled charge and capacitive charge at different scan rates for all H-PCS. Regardless of doping, the ratio of capacitive contribution was gradually increased with the increase of scan rate. Nevertheless, the ratio was exceeded 80% for (N, P) co-doped PCS at any scan rate, indicating the complete capacitive behavior. Once the metal atom doped, the corresponding ratio was dramatically decreased to ~40% to ~60%, revealing the diffusion-controlled capacitance was reinforced due to the involvement of the Faradaic reaction. Moreover, it is necessary to mention that such an effect is more distinct in (N, Co, P) tri-doped PCS due to the high level of Co doping as well as its electrochemical activity.
22
Figure 8 (a) CV curves of (N, Ni, Co, P) tetra-doped PCS at various scan rates, (b) lg(i) vs lg(v), (c) The capacitive charge storage contributions at the scan rate of 0.8 mV·s-1, (d) Contributions ratios of capacitive charge at different scan rates for all H-PCS anodes.
The Li+ diffusion in (N, Ni, Co, P) tetra-doped PCS electrode was examined for a potential step of 2.1 to 3.0 V by implementing PITT test. Fig. 9(a) and (b) show the I vs t and ln(I) vs t at a voltage platform of 2.191 V. During the PITT test, the cells were charged to 3 V with a voltage step of 20 mV. Since Li-ion transport in the (N, Ni, Co, P) tetra-doped PCS electrode follows Fick’s second law, thus the DLi+ can be calculated according to Eq. (3) [45, 46] 𝐷𝐿𝑖 + = ―
4𝐿2d𝑙𝑛(𝐼)
(3)
π2 𝑑(𝑡)
where L indicates the thickness of the active material on the electrode, I refer to 23
the step current, and t is related to the step time during the test process [47]. Fig. 9(c) exhibits the calculated logarithm Li-ion diffusion coefficient plots of (N, P) co-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS. From the figure, DLi+ of (N, P) co-doped PCS anode was varied from 8.99×10-10 to 1.01×10-9 cm2·s-1 when the cell potential was changed from 2.1 to 2.65 V while it was changed from 3.96×10-10 to 1.45×10-9 cm2·s-1 and 8.18×10-10 to 9.24×10-9 cm2·s-1 when the cell potential was changed from 2.1 to 3.0 V, corresponding to (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS. Obviously, the highest DLi+ corresponds to (N, Ni, Co, P) tetra-doped PCS, which determines the fast reaction and superior LIBs performance. This indicates the heteroatom doping are beneficial to improving DLi+ value and therefore enhance the electrochemical performance of H-PCS. Fig. 9(d) shows the EIS spectra of (N, P) co-, (N, Co, P) triand (N, Ni, Co, P) tetra-doped PCS anode after 100 cycles. As compared to Fig. 7(d), both the Rct and Rs value are greatly increased for all samples, implying the formation of electrolyte interphase substance. Table S1 presented the fitted value of Rct and Rs for (N, P) co-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS. When compared with the corresponding data shown in Table 4, it was found that bi-metal atom doping is favored for restraining the increase of Rct. On the other hand, another small arc emerged on the EIS curve of (N, P) co-doped PCS after cycling which may have resulted from the structural deterioration.
24
Figure 9. (a) Time dependence of the current in the (N, Ni, Co, P) tetra-doped PCS at a voltage platform of 2.191 V vs Li/Li+, (b) ln(I) vs t, (c) calculated Li-ion diffusion coefficient of (N, P) co-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS, (d) EIS spectra of (N, P) co-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS anode after 100 cycles at 100 mA·g-1.
To examine the structural durability of H-PCS, the morphology of anode after cycling was characterized. Fig. 10(a), (b) and (c) exhibit the SEM image of (N, P) co-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS anode after 100 cycles, respectively. Obviously, we can obtain from the image that the spherical morphology of (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS were well retained while severe structural damage was found in terms of (N, P) co-doped PCS, suggesting the volume change was restricted during the charge/discharge process in H-PCS with doping of metal atom [48]. 25
Fig. 10(d) shows the HRTEM of (N, Ni, Co, P) tetra-doped PCS after 100 cycles with SAED pattern from the selected area (b). As compared to observation from Fig. 3(b), some dark substitutes were found which can be classified as the by-products from the electrochemical process. In Fig. 10(e), three diffraction rings were clearly captured which are associated with (214), (123) and (311) of Li3PO4.
Figure 10. SEM images of (a) (N, P) co-doped PCS, (b) (N, Co, P) tri-doped PCS and (c) (N, Ni, Co, P) tetra-doped PCS anode after 100 cycles (inset is the magnified SEM image), (d) HRTEM of (N, Ni, Co, P) tetra-doped PCS after 100 cycles and (e) SAED pattern.
Conclusion In summary, we have proposed a universal approach to prepare echinus-like porous carbon spheres with controlled heteroatom doping (H-PCS). The morphology, structure, electrochemical behavior and lithium ions storage performance of (N, P) co-, 26
(N, Ni, P) tri-, (N, Co, P) tri- and (N, Ni, Co, P) tetra-doped PCS have been comprehensively investigated. The results suggested that the (N, Ni, Co, P) tetra-doped PCS exhibited highly reversible capacity, excellent rate capability and superior cycling performance. This is benefited from the highly porous characteristic, high heteroatom doping content and moderate specific surface which improved the Li+ diffusion and adsorption on active sites. Thus, this work provides an insight into the electrochemical behavior of H-PCS and strengthens our understanding of carbon nanomaterials with confined heteroatom doping.
Acknowledgment This work was supported by project of Ningxia key R&D plan (2018BEE03013).
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Zhuo Chen: Investigation, Writing - Original Draft. Haibo Li: Conceptualization, Methodology, Supervision, Writing Review & Editing.
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Declaration of interests ☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Graphical abstract Text: A universal strategy to prepare heteroatom doped echinus-like porous carbon spheres as advanced anode for lithium ions battery has been developed.
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