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Improved electrochemical performance of SiO-based anode by N, P binary doped carbon coating
Applied Surface Science 507 (2020) 145060
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Full Length Article
Improved electrochemical performance of SiO-based anode by N, P binary doped carbon coating Manxin Peng, Yechao Qiu, Meixia Zhang, Yabin Xu, Li Yi, Kui Liang
T
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College of Materials Science and Engineering, Hunan University, Changsha 410082, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen-doping Phosphate-doping Carbon coating Silicon monoxide Anode
Silicon monoxide (SiO), a prospective anode candidate for lithium-ion batteries (LIBs), has drawn the extensive attention of many researchers. In this work, we coated nitrogen, phosphorus (N, P) binary doped carbon onto the surface of SiO by homogeneous mixing gelatin, melamine, phosphate and SiO along with subsequent pyrolysis. Higher electronic conductivity and lithium-ion diffusion coefficient obtained by the synergistic effect of the N, P binary doping, make the modified SiO-based anode exhibit preferable electrochemical performance. Specifically, the as-prepared anode material delivered a reversible capacity of 1223 mAh g−1 at 100 mA g−1, and had capacity retention of 98% after 200 cycles at 1000 mA g−1, while the raw SiO only retained 14%. This work will be committed to providing a possible method of preparing a practical anode material for high-performance lithium-ion batteries.
1. Introduction Lithium-ion batteries (LIBs) have a broad prospect in the field of electric vehicles and grid storage, however, the limited reversible capacity of 372 mAh g−1 and poor rate capability of the current commercial graphite anodes cannot satisfy the demand for the development of this industry, which stimulates numerous researches to study alternative anode materials [1–5]. As one of the prospective candidates of the next-generation anode material, SiO with a high theoretical capacity of 2600 mAh g−1 is cost-effective and rich in reserves. However, the large volume expansion (approximately 200%), poor coulomb efficiency and low conductivity (6.7 × 10−4 S cm−1) limit the industrialization of SiO [6–8]. To solve the problems mentioned above and obtain a high reversible capacity of SiO anode, some research has been carried out [9–11]. Carbon coating is an impact means to meliorate the electrochemical properties of SiO by improving electronic conductivity of the electrode, promoting the formation of a stable SEI layer [12,13]. Moreover, it is accepted that heteroatom doping (N, S, F, P, B) can modulate electronic and physicochemical properties, and heteroatom doped materials with good electrochemistry performances have been reported [14–19]. Most of the related research has focused on the doping of single element such as nitrogen for meliorating the electrochemical performance, only a tiny proportion of the studies focus on binary doping associated with the field and its synergistic effect on electrochemical
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properties [20–22]. Nitrogen doping can improve the electronic properties of carbon materials, in various types of N (pyridine N, pyrrole N and graphite N), the more active pyridine N cannot only improve the electronic conductivity of the materials and enhance the interaction between Li+ and electrode surface but also induce more defect sites and facilitate the adsorption of lithium ions [23–25]. Meanwhile, replacing the carbon atoms with the larger size phosphorus atoms will enlarge the plane spacing of the carbon layer, induce more defects and edge sites, it also can be promoting electrochemical performance by enhancing the interaction of Li+ and accelerating Li+ diffusion [25–28]. Some research suggests that the content of defects and edge positions in the carbon coating is a key factor affecting the proportion of pyridine N, so the introduction of additional phosphorus atoms may be a feasible method to obtain a high proportion of pyridine N [25,29]. Based on these considerations, it is believed that constructing a carbon layer coating with a high proportion of active pyridine N and sufficient defect sites by N, P binary doping is a promising way to improve the performance of silicon monoxide anode. Herein, we choose melamine as nitrogen doping source and phosphate as phosphorus doping sources, and a facile mixing and sintering approach is proposed to synthesize the SiO/N, P binary doped carbon composite. The synergistic action of phosphorus atoms and nitrogen atoms can enhance the conductivity of the electrode, facilitate the reactivity between the electrode and the lithium ions and increase the lithium-ion diffusion coefficient. As we had expected, the modified
Corresponding author. E-mail address: [email protected] (K. Liang).
https://doi.org/10.1016/j.apsusc.2019.145060 Received 2 September 2019; Received in revised form 25 November 2019; Accepted 11 December 2019 Available online 13 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Schematic chart of the fabrication for SiO/NPC.
ray diffraction (XRD, Brooke, Advance D8) were used to investigate the morphology and crystalline structure of the samples, respectively. The microstructure and elemental distribution were visualized by transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN TMP) and corresponding energy-dispersive X-ray spectroscopy (EDS). The surface chemical characteristics and the configuration proportional of doping elements were observed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, K-Alpha 1063). The carbon content of the modified sample was determined by thermo-gravimetric analysis (TGA, Japan, TG/DTA 7300). The conductivity of all samples was determined by the ST-2722 Semiconductor resistivity of the powder tester.
anode material exhibits high capacity, long cycle life and excellent rate performance. 2. Experimental 2.1. Chemicals The average particle size of raw SiO powder was D50 = 5 μm (Sichuan Chuangneng New Energy Materials Co. Ltd.). gelatin (CP), melamine (CP) and phosphoric acid (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Materials synthesis
2.4. Electrochemical characterization
Scheme 1 provides the flow chart of the synthesis process for SiO/N, P binary doped carbon composite. After dissolving 1.25 g of gelatin with 80 mL of deionized water, 2.0 g of melamine, 0.67 mL of phosphoric acid and 2.0 g of SiO were added to the gelatin aqueous solution in order and magnetic stirring at a constant temperature of 70 °C until completely evaporated to obtain the precursor, transfer the precursor to a tube furnace and heat-treated at 850 °C for 4 h under nitrogen atmosphere, the as-obtained sample was expressed as SiO/NPC. For comparison, the sample modified only with melamine denoted as SiO/ NC, the heat-treated SiO without carbon coating denoted as H-SiO.
Electrochemical performances were evaluated by assembling a CR2032 coin-type cells, the electrolyte adopted was 1M LiPF6 in a mixture of EC, EMC and DMC (volume ratio of 1:1:1), and containing 10 wt% FEC additive. The composition of the working electrodes were active material (60 wt%), sodium alginate (20 wt%) and Super P (20 wt %). Galvanostatic discharge and charge profiles in 0.05–2.0 V was recorded at constant temperature (28 °C) by applying a battery test system (NEWARE CT-3008) after assembling the battery. An electrochemical workstation (CHI604E) was adopted to perform the measure of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).
2.3. Structural characterization Scanning electron microscopy (SEM, FEI Quanta FEG 250) and X-
Fig. 1. (a) XRD patterns, (b) Raman spectrum, (c) TG curves of the samples. 2
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3. Results and discussion
homogeneously distributed in the SiO/NPC. The bonding configurations of the doping elements were characterized by XPS. As shown in Fig. 4a, five main peaks standing for O 1s, N 1s, C 1s, Si 2s, Si 2p, respectively. As described in Fig. 4b–c, the C 1s spectrum confirms the existence of CeC (284.43 eV), CeN (285.68 eV) and OeC]O (287.28 eV), the content of N is 4.92 at.% [37]. The peak at ~133 eV in SiO/NPC verified the existence of P and the content of P is 1.43 at.%. The P 2p spectrum (Fig. 4d) showed three fitted peaks at 131.1, 132.6, and 134.1 eV, conforming with the bonds of P-C, P-N, and P-O, respectively [25]. As displayed in Fig. 4e–f, three peaks of pyridinic N (~298 eV), pyrrolic N (~400 eV) and graphitic N (~402 eV) can be revealed, and illustrated in the schematic of Fig. 4g [4,38]. Interestingly, the proportion of pyridine N is 32.8% when the melamine was introduced as nitrogen doping source, and it was increased from 32.8% to 60.2% when melamine and phosphoric acid were simultaneously introduced for N, P binary doping, indicating that the proportion of pyridine N is effectively raised by phosphorus doping. The introduction of high content of pyridine N is identified as an efficient method to improve the electrochemical performance. On one aspect, it can enhance the electronic conductivity, the conductivity of SiO/NPC increased from 5.540 × 10−2 S/cm to 2.732 × 10−1 S/cm compared to SiO/NC (Table 1), allowing lithium ions to react with a larger portion of Si domains. On the other aspect, it can promote the diffusion of Li+ by inducing the formation of more defects and has higher reactivity to favor the lithium dealloying reaction [23,25,27,28].
3.1. Analysis of structures The crystal structures of the samples were characterized by XRD and illustrated in Fig. 1a. The broad peak between 20° and 30° indicates that bare SiO maintains the structure of the Si surrounded by the amorphous SiOx matrix [30,31]. The graphite peaks were not discovered in the SiO/NC and SiO/NPC, meaning that the coated carbon layer is amorphous [32]. After the sintering treatment, the XRD patterns of the modified samples show three week slight peaks appeared at 28.4°, 47.3° and 56.1°, corresponding to the (1 1 1), (2 2 0) and (3 1 1) planes of Si [22]. These results illustrate that a few crystalline nano-Si domains were formed and embedded in the amorphous SiOx matrix during the rearrangement of high-temperature sintering, which is beneficial to optimize the cycle life of SiO [33]. To further confirm the structure of the samples, Raman spectroscopy is applied to SiO/NC and SiO/NPC composite (Fig. 1b). Two characteristic peaks at 1360 and 1580 cm−1 of the all samples correspond to disordered carbon (D band) and graphitic carbon (G band), respectively [21]. The peak area ratio of D band and G band is obtained by fitting the peak area, the values of the ID/IG were 2.486 for SiO/NC and 2.904 for SiO/NPC, indicating that an amorphous feature of the carbon layer, which was in agreement with the XRD analysis [17]. Besides, SiO/NPC showed a higher intensity ratio (ID: IG) than SiO/NC, which also showed that the sample produced more defects after phosphorus doping [25]. TG curves (Fig. 1c) were measured under an air atmosphere and temperature range of 30 ℃ to 700 ℃, obtaining the carbon content of SiO/NC and SiO/NPC were 11.4% and 12.0%, respectively. The start temperature of the thermal decomposition of SiO/NPC is 475 °C, while for the SiO/NC is 350 °C. The higher weight loss temperature of SiO/ NPC is related to the mixture generated in the pyrolysis reaction of melamine with phosphoric acid. When heated above 600 °C, phosphoric acid interacts with the triazine ring in the melamine molecule to form a (PON)x crosslinked inorganic polymer of high thermal stability, this is also why SiO/NPC has a wide range of thermal decomposition temperatures [34,35]. Fig. 2 depicts the SEM images of the SiO and SiO/NPC, revealing similar morphologies. They are all composed of irregularly shaped blocks that exhibit a unique microstructure of small particles attached to micron-sized large particles, indicating that the doped carbon layer is a conformal coating. The amorphous carbon coating of the SiO/NPC particles in TEM images shown in Fig. 3a, presenting that the thickness of carbon layer ranging from 25 to 30 nm. A small quantity of nanocrystals with d-spacing of 0.317 nm is dispersed in an amorphous matrix as shown in Fig. 3b, which matches with the (1 1 1) planes of Si and it conforms to the analysis of XRD [32,36]. The elemental mapping images are shown in Fig. 3c–h, indicate that Si, O, C, N and P are
3.2. Electrochemical performances The first three cycle voltammetry curves of all samples are carried out at a rate of 0.1 mV s−1 over a voltage range of 0.05–2.0 V (vs. Li/ Li+) and presented in Fig. 5a–c. There were two broad peaks about 0.7 V and 1.2 V exists in the first cathode scan curve of all samples, which attributes to the formation of the SEI layer and the decomposition of the electrolyte FEC additive, respectively [17,32]. It can be clearly observed that the peak intensity of SiO, SiO/NC and SiO/NPC at 0.7 V is sequentially slightly decreased, suggesting that formed SEI layer is thinner in turn, because the carbon coating effectively reduces the contact area with the electrolyte to reduce the formation of the SEI layer, which helps to reduce the SEI impedance and improve the initial coulombic efficiency. Two main peaks are observable during the scanning, the cathode scan peak at less than 0.2 V is relevant in he alloying of LixSi and the formation of irreversible Li2O, LixSiOy, the anode peak at 0.53 V is attributed to the de-alloying process of LixSi [39]. It is noteworthy that peak amplitude of the first three cycles is gradually increased, which can ascribe to the formation of a preferable conductive route with the activation of electrode material and can be demonstrated by the capacity of SiO/NPC increasing from 1223 to 1320 mAh g−1 after three cycles of activation (Fig. 5d). The major irreversible reaction can be identified in the differential
Fig. 2. SEM images of (a) SiO, (b) SiO/NPC. 3
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Fig. 3. TEM images of (a, b) SiO/NPC. (c) STEM images of SiO/NPC and corresponding EDS element distribution images of (d) Si, (e) O, (f) C, (g) N, (h) P.
modified samples has an increasing tendency at about the twentieth cycle. A possible explanation is that the composite material cannot be completely lithiated in the initial cycle, and then the active particles are pulverized during the alloying/dealloying process to provide a higher specific surface area and a shorter lithium-ion diffusion distance, allowing the material to be fully activated and obtain a higher capacity [32]. As presented in Fig. 7b, the rate capability was tested at current densities of 100–4000 mA g−1. It provides reversible capacity of 1254, 1096, 976 and 817 mAh g−1 for SiO/NPC, obtained at 100, 500, 1000, and 2000 mA g−1, respectively. Even at a higher current density of 4000 mA g−1, a high reversible capacity of 650 mAh g−1 can be obtained, while the raw SiO electrode is only 155 mAh g−1. More strikingly, the reversible capacity can be restored to 1210 mAh g−1 when the current density is recovered to 100 mA g−1, and electrochemical reversibility and structural integrity of SiO/NPC have been demonstrated. Rapid charge transfer and lithium-ion diffusion are the root causes for excellent rate performance, which is consistent with the subsequent EIS analysis. To further analyze the effect of phosphorus doping on charge transfer and lithium-ion diffusion kinetics of the SiO/NPC, the AC impedance test was performed and recorded in Fig. 8a. The Nyquist plot includes two semi-circular and an oblique line, corresponding to the solid electrolyte membrane resistance (Rs), charge transfer resistance (Rct) and Warburg’s resistance (Zw), respectively, as well as the Re represents electrolyte resistance. The fitting data shown in Table 2 is obtained by using the equivalent circuit model as shown in Fig. 8c. It can be seen intuitively that the values of the charge transfer resistance were 339 Ω for SiO, 305 Ω for H-SiO, 88 Ω for SiO/NC and 33 Ω for SiO/NPC, so electrochemical activity is sequentially increased corresponding to the decrease of charge transfer resistance in turn. The oblique line in low frequency can be used to compare the lithium diffusion coefficient (DLi+ ) and following formulas are employed [28,42]: 1 Z' = Re + Rs + Rct + σω ω− 2 2 2 2 2 4 4 2 DLi + =R T /2A n F C σ where σ ω represents the Warburg impedance factor, and R, T, A, n, F, and C are representing gas constant, room temperature, electrode surface area, number of transferred electrons, faraday constant and lithium-ion concentration, respectively. The lithium-ion diffusion coefficient can be compared by the Warburg impedance factor, which can be calculated from the plot slopes of Z' vs. ω−1/2 (Fig. 8b). It is clearly seen that the values of the σ ω were 442 for SiO, 383 for H-SiO, 163 for SiO/NC and 74 for SiO/NPC, therefore, the lithium diffusion coefficients were calculated to be 2.21 × 10−14,
capacity plot (Fig. 6). An intense peak appearing at less than 0.25 V in the first discharge, which is not found in the subsequent cycle, and is related to the formation of LixSiOy and Li2O, in a certain extent, the LixSiOy and Li2O with electrochemically inert can be acted as buffer components to enhance the cycle stability, but would significantly increase the irreversible capacity, suggesting the occurrence of irreversible reactions, it can be seen that the irreversibility of the electrode is reduced after carbon-coating [40]. The other peaks centered at 0.18 and 0.08 V upon discharging and 0.28 and 0.45 V upon charging in the subsequent cycle are similar in both shape and position to those of amorphous Si electrode, can be assigned to the reversible reaction [9]. The discharge/charge curves of the SiO, H-SiO, SiO/NC and SiO/ NPC during the first cycle were evaluated at 100 mA g−1, and the results are illustrated in Fig. 7a. The discharge voltage platform of raw SiO is 0.1–0.2 V higher than that of three heat-treated samples, the possible reason may be the disproportion of SiO during the heat-treated process and leads to structural changes. On the one hand, this structural change affect the charge-discharge curve by affecting the SEI layer structure. On the other hand, the discharge-charge curve between nanoscale Si and micro-scale SiO is different [41]. The initial reversible capacities are 998, 1037, 1126 and 1223 mAh g−1, corresponding to initial coulomb efficiencies approximately 63, 65, 71 and 72%, respectively. All samples have two short discharge platforms at about 0.75 V and 1.2 V, which can be attributed to the formation of SEI layer and the decomposition of the FEC additive, which is consistent with the CV analysis. The initial reversible capacities of all samples at 1000 mA g−1 are exhibited as 562, 602, 777 and 864 mAh g−1, respectively (Fig. 7c). Due to inherent volume expansion, SiO shows a quite low capacity of 80 mAh g−1 after 200 cycles and a capacity retention of 14%, while the cycle performance of the sample after heat treatment was improved, the capacity retention was 27.8%. When coated with a nitrogen-doped carbon layer, the obtained capacity retention of the SiO/NC (77%) have a significant improvement in comparison with the SiO. The increase in capacity and cycle life should be attributed to that the conductivity is increased by nitrogen doping and volume expansion is mitigated by carbon coating [4,22]. The SiO/NPC composite shows a quite high capacity of 847 mAh g−1 after 200 cycles, with the capacity retention as high as 98%. This increase in reversible capacity and cycle stability may be due to the fact that phosphorus-doped improves the electronic conductivity and promotes electrochemical reactions, as well as accommodates volume changes of the SiO during charge/discharge under the cooperation of the carbon layer. Besides, the capacity of the
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Fig. 4. (a) XPS survey spectra of modified samples, C 1s XPS spectrum for (b) SiO/NC and (c) SiO/NPC, (d) P 2p XPS spectrum for SiO/NPC, N 1s XPS spectrum for (e) SiO/NC and (f) SiO/NPC, (g) Schematic of nitrogen atoms and phosphorus atoms replacing carbon atoms.
Table 1 The conductivity data from SiO, H-SiO, SiO/NC and SiO/NPC electrodes. Sample
Resistivity (Ω cm−1) Conductivity (S cm−1)
SiO
H-SiO
SiO/NC
SiO/NPC
11,660 8.576 × 10−5
11,190 8.937 × 10−5
18.05 5.540 × 10−2
3.66 2.732 × 10−1
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Fig. 5. (a–c) Cyclic voltammograms of SiO, SiO/NC and SiO/NPC. (d) Charge-discharge curve at 100 mA g−1 of the SiO/NPC.
Fig. 6. Differential-capacity plots (dQ/dV vs V) of (a) SiO, (b) SiO/NPC in the initial three cycles.
2.53 × 10−14, 1.63 × 10−13 and 8.77 × 10−13 cm2 s−1 for SiO, HSiO, SiO/NC and SiO/NPC, which implies that the phosphorus doping can meliorate the Li+ diffusion coefficient. Fig. 9 presents the SEM images of different electrodes before and after 200 cycles to comfirm the structural stability properties. After 200 cycles, the cross-sectional images of the SiO composite anode reveal a serious volume expansion, since the thickness of the SiO anode with a 5.92 μm increases to 11.89 μm and a expansion ratio is 101% (Fig. 9a and d). Comparatively, the thickness of the SiO/NPC composite anode slightly expanded from 18.20 μm to 24.08 μm, corresponding to 32% (Fig. 9c and f), which was much lower than that of the raw SiO anode, it is known that the electrode maintains good structural integrity. Thus, it is confirmed that the SiO/NPC composite fabricated in this study shows
excellent structural stability. 4. Conclusion N, P binary doped carbon has been introduced into silicon monoxide anode material, and the synergistic effect of binary doping effectively increases the content of active pyridine N, not only improves the electron conductivity of the material, but also induces more defects and edge sites to provide more lithium storage sites and facilitate the diffusion of Li+, thereby effectively optimized the electrochemical performance, reversible capacity, cycle life and rate performance of the electrode are significantly improved. And the as-prepared SiO/NPC material has an initial reversible capacity of 1223 mAh g−1 at 6
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Fig. 7. (a) Initial charge-discharge curve at 100 mA g−1, (b) Cycling performance profiles at 1000 mA g−1 and (c) Rate capability of the SiO, H-SiO, SiO/NC and SiO/ NPC.
Fig. 8. (a) Resistance comparison of the SiO, H-SiO, SiO/NC, and SiO/NPC after the first cycle at 100 mA g−1. (b) The relationship of real impedance versus low frequencies of the samples. (c) Equivalent circuit model.
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Table 2 The fitted EIS data from equivalent circuit and lithium ion diffusion coeffcients DLi+ of SiO, H-SiO, SiO/NC and SiO/NPC electrodes. Sample
Rs (Ω) Rct (Ω) σ ω (Ω cm2·s−1/2) DLi+ (cm2·s−1)
SiO
H-SiO
SiO/NC
SiO/NPC
42.36 339 442 2.21 × 10−14
40.87 305 383 2.53 × 10−14
13.76 99 163 1.63 × 10−13
10.58 33 74 8.77 × 10−13
Fig. 9. (a–c) Cross-sectional SEM images of the fresh electrode. (a) SiO (b) SiO/NC (c) SiO/NPC. (e–h) Cross-sectional of the electrode after 200 cycles. (d) SiO (e) SiO/NC (f) SiO/NPC.
100 mA g−1 and capacity retention of 98% after 200 cycles at 1000 mA g−1, indicating the powerful potential of high-performance SiO/NPC anode material for LIBs.
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nitrogen-doped carbon hollow spheres as high-performance anodes for lithium-ion batteries, J. Power Sources 324 (2016) 233–238. Y.S. Yun, V.D. Le, H. Kim, S.J. Chang, S.J. Baek, S. Park, B.H. Kim, Y.H. Kim, K. Kang, H.J. Jin, Effects of sulfur doping on graphene-based nanosheets for use as anode materials in lithium-ion batteries, J. Power Sources 262 (2014) 79–85. L. Guo, H. He, Y. Ren, C. Wang, M. Li, Core-shell SiO@F-doped C composites with interspaces and voids as anodes for high-performance lithium-ion batteries, Chem. Eng. J. (2017) S1385894717318594. H.J. Denisa, A.M. Puziy, O.I. Poddubnaya, S.G. Fabian, J.M.D. Tascón, L.G. Qing, Highly stable performance of supercapacitors from phosphorus-enriched carbons, J. Am. Chem. Soc. 131 (2009) 5026–5027. J. Woo, S.H. Baek, J.S. Park, Y.M. Jeong, J.H. Kim, Improved electrochemical performance of boron-doped SiO negative electrode materials in lithium-ion batteries, J. Power Sources 299 (2015) 25–31. S. Lu, W. Wang, A. Wang, K. Yuan, Z. Jin, Y. Yang, Si nanoparticles adhering to the nitrogen-rich porous carbon framework and their application as lithium-ion battery anode materials, J. Mater. Chem. A 3 (2015), https://doi.org/10.1039/ C5TA03974F. J. Zhu, J. Yang, Z. Xu, J. Wang, Y. Nuli, X. Zhuang, X. Feng, Silicon anodes protected by a nitrogen-doped porous carbon shell for high-performance lithium-ion batteries, Nanoscale 9 (2017). D.J. Lee, M.H. Ryou, J.N. Lee, B.G. Kim, Y.M. Lee, H.W. Kim, B.S. Kong, J.K. Park, J.W. Choi, Nitrogen-doped carbon coating for a high-performance SiO anode in lithium-ion batteries, Electrochem. Commun. 34 (2013) 98–101. Y. Xie, Y. Chen, L. Liu, P. Tao, M. Fan, N. Xu, X. Shen, C. Yan, Ultra-high pyridinic N-doped porous carbon monolith enabling high-capacity K-ion battery anodes for both half-cell and full-cell applications, Adv. Mater. 29 (2017) 1702268. N. Ding, Z.H. Huang, R. Lv, Y. Lu, F. Kang, Nitrogen-enriched electrospun porous carbon nanofiber networks as high-performance free-standing electrode materials, J. Mater. Chem. A 2 (2014) 19678–19684. H. He, D. Huang, Y. Tang, Q. Wang, X. Ji, H. Wang, Z. Guo, Tuning nitrogen species in three-dimensional porous carbon via phosphorus doping for ultrafast potassium storage, Nano Energy 57 (2019) 728–736. X. Ma, G. Ning, C. Qi, C. Xu, J. Gao, Phosphorus and nitrogen dual-doped fewlayered porous graphene: a high-performance anode material for lithium-ion batteries, ACS Appl. Mater. Interfaces 6 (2014) 14415–14422. J. Gu, Z. Du, C. Zhang, S. Yang, Pyridinic nitrogen-enriched carbon nanogears with thin teeth for superior lithium storage, Adv Energy Mater. (2016). D. Li, D. Wang, K. Rui, Z. Ma, X. Ling, J. Liu, Z. Yu, R. Chen, Y. Yan, H. Lin, Flexible phosphorus doped carbon nanosheets/nanofibers: electrospun preparation and enhanced Li-storage properties as free-standing anodes for lithium ion batteries, J. Power Sources 384 (2018) 27–33.
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Full Length Article
Improved electrochemical performance of SiO-based anode by N, P binary doped carbon coating Manxin Peng, Yechao Qiu, Meixia Zhang, Yabin Xu, Li Yi, Kui Liang
T
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College of Materials Science and Engineering, Hunan University, Changsha 410082, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen-doping Phosphate-doping Carbon coating Silicon monoxide Anode
Silicon monoxide (SiO), a prospective anode candidate for lithium-ion batteries (LIBs), has drawn the extensive attention of many researchers. In this work, we coated nitrogen, phosphorus (N, P) binary doped carbon onto the surface of SiO by homogeneous mixing gelatin, melamine, phosphate and SiO along with subsequent pyrolysis. Higher electronic conductivity and lithium-ion diffusion coefficient obtained by the synergistic effect of the N, P binary doping, make the modified SiO-based anode exhibit preferable electrochemical performance. Specifically, the as-prepared anode material delivered a reversible capacity of 1223 mAh g−1 at 100 mA g−1, and had capacity retention of 98% after 200 cycles at 1000 mA g−1, while the raw SiO only retained 14%. This work will be committed to providing a possible method of preparing a practical anode material for high-performance lithium-ion batteries.
1. Introduction Lithium-ion batteries (LIBs) have a broad prospect in the field of electric vehicles and grid storage, however, the limited reversible capacity of 372 mAh g−1 and poor rate capability of the current commercial graphite anodes cannot satisfy the demand for the development of this industry, which stimulates numerous researches to study alternative anode materials [1–5]. As one of the prospective candidates of the next-generation anode material, SiO with a high theoretical capacity of 2600 mAh g−1 is cost-effective and rich in reserves. However, the large volume expansion (approximately 200%), poor coulomb efficiency and low conductivity (6.7 × 10−4 S cm−1) limit the industrialization of SiO [6–8]. To solve the problems mentioned above and obtain a high reversible capacity of SiO anode, some research has been carried out [9–11]. Carbon coating is an impact means to meliorate the electrochemical properties of SiO by improving electronic conductivity of the electrode, promoting the formation of a stable SEI layer [12,13]. Moreover, it is accepted that heteroatom doping (N, S, F, P, B) can modulate electronic and physicochemical properties, and heteroatom doped materials with good electrochemistry performances have been reported [14–19]. Most of the related research has focused on the doping of single element such as nitrogen for meliorating the electrochemical performance, only a tiny proportion of the studies focus on binary doping associated with the field and its synergistic effect on electrochemical
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properties [20–22]. Nitrogen doping can improve the electronic properties of carbon materials, in various types of N (pyridine N, pyrrole N and graphite N), the more active pyridine N cannot only improve the electronic conductivity of the materials and enhance the interaction between Li+ and electrode surface but also induce more defect sites and facilitate the adsorption of lithium ions [23–25]. Meanwhile, replacing the carbon atoms with the larger size phosphorus atoms will enlarge the plane spacing of the carbon layer, induce more defects and edge sites, it also can be promoting electrochemical performance by enhancing the interaction of Li+ and accelerating Li+ diffusion [25–28]. Some research suggests that the content of defects and edge positions in the carbon coating is a key factor affecting the proportion of pyridine N, so the introduction of additional phosphorus atoms may be a feasible method to obtain a high proportion of pyridine N [25,29]. Based on these considerations, it is believed that constructing a carbon layer coating with a high proportion of active pyridine N and sufficient defect sites by N, P binary doping is a promising way to improve the performance of silicon monoxide anode. Herein, we choose melamine as nitrogen doping source and phosphate as phosphorus doping sources, and a facile mixing and sintering approach is proposed to synthesize the SiO/N, P binary doped carbon composite. The synergistic action of phosphorus atoms and nitrogen atoms can enhance the conductivity of the electrode, facilitate the reactivity between the electrode and the lithium ions and increase the lithium-ion diffusion coefficient. As we had expected, the modified
Corresponding author. E-mail address: [email protected] (K. Liang).
https://doi.org/10.1016/j.apsusc.2019.145060 Received 2 September 2019; Received in revised form 25 November 2019; Accepted 11 December 2019 Available online 13 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Schematic chart of the fabrication for SiO/NPC.
ray diffraction (XRD, Brooke, Advance D8) were used to investigate the morphology and crystalline structure of the samples, respectively. The microstructure and elemental distribution were visualized by transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN TMP) and corresponding energy-dispersive X-ray spectroscopy (EDS). The surface chemical characteristics and the configuration proportional of doping elements were observed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, K-Alpha 1063). The carbon content of the modified sample was determined by thermo-gravimetric analysis (TGA, Japan, TG/DTA 7300). The conductivity of all samples was determined by the ST-2722 Semiconductor resistivity of the powder tester.
anode material exhibits high capacity, long cycle life and excellent rate performance. 2. Experimental 2.1. Chemicals The average particle size of raw SiO powder was D50 = 5 μm (Sichuan Chuangneng New Energy Materials Co. Ltd.). gelatin (CP), melamine (CP) and phosphoric acid (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Materials synthesis
2.4. Electrochemical characterization
Scheme 1 provides the flow chart of the synthesis process for SiO/N, P binary doped carbon composite. After dissolving 1.25 g of gelatin with 80 mL of deionized water, 2.0 g of melamine, 0.67 mL of phosphoric acid and 2.0 g of SiO were added to the gelatin aqueous solution in order and magnetic stirring at a constant temperature of 70 °C until completely evaporated to obtain the precursor, transfer the precursor to a tube furnace and heat-treated at 850 °C for 4 h under nitrogen atmosphere, the as-obtained sample was expressed as SiO/NPC. For comparison, the sample modified only with melamine denoted as SiO/ NC, the heat-treated SiO without carbon coating denoted as H-SiO.
Electrochemical performances were evaluated by assembling a CR2032 coin-type cells, the electrolyte adopted was 1M LiPF6 in a mixture of EC, EMC and DMC (volume ratio of 1:1:1), and containing 10 wt% FEC additive. The composition of the working electrodes were active material (60 wt%), sodium alginate (20 wt%) and Super P (20 wt %). Galvanostatic discharge and charge profiles in 0.05–2.0 V was recorded at constant temperature (28 °C) by applying a battery test system (NEWARE CT-3008) after assembling the battery. An electrochemical workstation (CHI604E) was adopted to perform the measure of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).
2.3. Structural characterization Scanning electron microscopy (SEM, FEI Quanta FEG 250) and X-
Fig. 1. (a) XRD patterns, (b) Raman spectrum, (c) TG curves of the samples. 2
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3. Results and discussion
homogeneously distributed in the SiO/NPC. The bonding configurations of the doping elements were characterized by XPS. As shown in Fig. 4a, five main peaks standing for O 1s, N 1s, C 1s, Si 2s, Si 2p, respectively. As described in Fig. 4b–c, the C 1s spectrum confirms the existence of CeC (284.43 eV), CeN (285.68 eV) and OeC]O (287.28 eV), the content of N is 4.92 at.% [37]. The peak at ~133 eV in SiO/NPC verified the existence of P and the content of P is 1.43 at.%. The P 2p spectrum (Fig. 4d) showed three fitted peaks at 131.1, 132.6, and 134.1 eV, conforming with the bonds of P-C, P-N, and P-O, respectively [25]. As displayed in Fig. 4e–f, three peaks of pyridinic N (~298 eV), pyrrolic N (~400 eV) and graphitic N (~402 eV) can be revealed, and illustrated in the schematic of Fig. 4g [4,38]. Interestingly, the proportion of pyridine N is 32.8% when the melamine was introduced as nitrogen doping source, and it was increased from 32.8% to 60.2% when melamine and phosphoric acid were simultaneously introduced for N, P binary doping, indicating that the proportion of pyridine N is effectively raised by phosphorus doping. The introduction of high content of pyridine N is identified as an efficient method to improve the electrochemical performance. On one aspect, it can enhance the electronic conductivity, the conductivity of SiO/NPC increased from 5.540 × 10−2 S/cm to 2.732 × 10−1 S/cm compared to SiO/NC (Table 1), allowing lithium ions to react with a larger portion of Si domains. On the other aspect, it can promote the diffusion of Li+ by inducing the formation of more defects and has higher reactivity to favor the lithium dealloying reaction [23,25,27,28].
3.1. Analysis of structures The crystal structures of the samples were characterized by XRD and illustrated in Fig. 1a. The broad peak between 20° and 30° indicates that bare SiO maintains the structure of the Si surrounded by the amorphous SiOx matrix [30,31]. The graphite peaks were not discovered in the SiO/NC and SiO/NPC, meaning that the coated carbon layer is amorphous [32]. After the sintering treatment, the XRD patterns of the modified samples show three week slight peaks appeared at 28.4°, 47.3° and 56.1°, corresponding to the (1 1 1), (2 2 0) and (3 1 1) planes of Si [22]. These results illustrate that a few crystalline nano-Si domains were formed and embedded in the amorphous SiOx matrix during the rearrangement of high-temperature sintering, which is beneficial to optimize the cycle life of SiO [33]. To further confirm the structure of the samples, Raman spectroscopy is applied to SiO/NC and SiO/NPC composite (Fig. 1b). Two characteristic peaks at 1360 and 1580 cm−1 of the all samples correspond to disordered carbon (D band) and graphitic carbon (G band), respectively [21]. The peak area ratio of D band and G band is obtained by fitting the peak area, the values of the ID/IG were 2.486 for SiO/NC and 2.904 for SiO/NPC, indicating that an amorphous feature of the carbon layer, which was in agreement with the XRD analysis [17]. Besides, SiO/NPC showed a higher intensity ratio (ID: IG) than SiO/NC, which also showed that the sample produced more defects after phosphorus doping [25]. TG curves (Fig. 1c) were measured under an air atmosphere and temperature range of 30 ℃ to 700 ℃, obtaining the carbon content of SiO/NC and SiO/NPC were 11.4% and 12.0%, respectively. The start temperature of the thermal decomposition of SiO/NPC is 475 °C, while for the SiO/NC is 350 °C. The higher weight loss temperature of SiO/ NPC is related to the mixture generated in the pyrolysis reaction of melamine with phosphoric acid. When heated above 600 °C, phosphoric acid interacts with the triazine ring in the melamine molecule to form a (PON)x crosslinked inorganic polymer of high thermal stability, this is also why SiO/NPC has a wide range of thermal decomposition temperatures [34,35]. Fig. 2 depicts the SEM images of the SiO and SiO/NPC, revealing similar morphologies. They are all composed of irregularly shaped blocks that exhibit a unique microstructure of small particles attached to micron-sized large particles, indicating that the doped carbon layer is a conformal coating. The amorphous carbon coating of the SiO/NPC particles in TEM images shown in Fig. 3a, presenting that the thickness of carbon layer ranging from 25 to 30 nm. A small quantity of nanocrystals with d-spacing of 0.317 nm is dispersed in an amorphous matrix as shown in Fig. 3b, which matches with the (1 1 1) planes of Si and it conforms to the analysis of XRD [32,36]. The elemental mapping images are shown in Fig. 3c–h, indicate that Si, O, C, N and P are
3.2. Electrochemical performances The first three cycle voltammetry curves of all samples are carried out at a rate of 0.1 mV s−1 over a voltage range of 0.05–2.0 V (vs. Li/ Li+) and presented in Fig. 5a–c. There were two broad peaks about 0.7 V and 1.2 V exists in the first cathode scan curve of all samples, which attributes to the formation of the SEI layer and the decomposition of the electrolyte FEC additive, respectively [17,32]. It can be clearly observed that the peak intensity of SiO, SiO/NC and SiO/NPC at 0.7 V is sequentially slightly decreased, suggesting that formed SEI layer is thinner in turn, because the carbon coating effectively reduces the contact area with the electrolyte to reduce the formation of the SEI layer, which helps to reduce the SEI impedance and improve the initial coulombic efficiency. Two main peaks are observable during the scanning, the cathode scan peak at less than 0.2 V is relevant in he alloying of LixSi and the formation of irreversible Li2O, LixSiOy, the anode peak at 0.53 V is attributed to the de-alloying process of LixSi [39]. It is noteworthy that peak amplitude of the first three cycles is gradually increased, which can ascribe to the formation of a preferable conductive route with the activation of electrode material and can be demonstrated by the capacity of SiO/NPC increasing from 1223 to 1320 mAh g−1 after three cycles of activation (Fig. 5d). The major irreversible reaction can be identified in the differential
Fig. 2. SEM images of (a) SiO, (b) SiO/NPC. 3
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Fig. 3. TEM images of (a, b) SiO/NPC. (c) STEM images of SiO/NPC and corresponding EDS element distribution images of (d) Si, (e) O, (f) C, (g) N, (h) P.
modified samples has an increasing tendency at about the twentieth cycle. A possible explanation is that the composite material cannot be completely lithiated in the initial cycle, and then the active particles are pulverized during the alloying/dealloying process to provide a higher specific surface area and a shorter lithium-ion diffusion distance, allowing the material to be fully activated and obtain a higher capacity [32]. As presented in Fig. 7b, the rate capability was tested at current densities of 100–4000 mA g−1. It provides reversible capacity of 1254, 1096, 976 and 817 mAh g−1 for SiO/NPC, obtained at 100, 500, 1000, and 2000 mA g−1, respectively. Even at a higher current density of 4000 mA g−1, a high reversible capacity of 650 mAh g−1 can be obtained, while the raw SiO electrode is only 155 mAh g−1. More strikingly, the reversible capacity can be restored to 1210 mAh g−1 when the current density is recovered to 100 mA g−1, and electrochemical reversibility and structural integrity of SiO/NPC have been demonstrated. Rapid charge transfer and lithium-ion diffusion are the root causes for excellent rate performance, which is consistent with the subsequent EIS analysis. To further analyze the effect of phosphorus doping on charge transfer and lithium-ion diffusion kinetics of the SiO/NPC, the AC impedance test was performed and recorded in Fig. 8a. The Nyquist plot includes two semi-circular and an oblique line, corresponding to the solid electrolyte membrane resistance (Rs), charge transfer resistance (Rct) and Warburg’s resistance (Zw), respectively, as well as the Re represents electrolyte resistance. The fitting data shown in Table 2 is obtained by using the equivalent circuit model as shown in Fig. 8c. It can be seen intuitively that the values of the charge transfer resistance were 339 Ω for SiO, 305 Ω for H-SiO, 88 Ω for SiO/NC and 33 Ω for SiO/NPC, so electrochemical activity is sequentially increased corresponding to the decrease of charge transfer resistance in turn. The oblique line in low frequency can be used to compare the lithium diffusion coefficient (DLi+ ) and following formulas are employed [28,42]: 1 Z' = Re + Rs + Rct + σω ω− 2 2 2 2 2 4 4 2 DLi + =R T /2A n F C σ where σ ω represents the Warburg impedance factor, and R, T, A, n, F, and C are representing gas constant, room temperature, electrode surface area, number of transferred electrons, faraday constant and lithium-ion concentration, respectively. The lithium-ion diffusion coefficient can be compared by the Warburg impedance factor, which can be calculated from the plot slopes of Z' vs. ω−1/2 (Fig. 8b). It is clearly seen that the values of the σ ω were 442 for SiO, 383 for H-SiO, 163 for SiO/NC and 74 for SiO/NPC, therefore, the lithium diffusion coefficients were calculated to be 2.21 × 10−14,
capacity plot (Fig. 6). An intense peak appearing at less than 0.25 V in the first discharge, which is not found in the subsequent cycle, and is related to the formation of LixSiOy and Li2O, in a certain extent, the LixSiOy and Li2O with electrochemically inert can be acted as buffer components to enhance the cycle stability, but would significantly increase the irreversible capacity, suggesting the occurrence of irreversible reactions, it can be seen that the irreversibility of the electrode is reduced after carbon-coating [40]. The other peaks centered at 0.18 and 0.08 V upon discharging and 0.28 and 0.45 V upon charging in the subsequent cycle are similar in both shape and position to those of amorphous Si electrode, can be assigned to the reversible reaction [9]. The discharge/charge curves of the SiO, H-SiO, SiO/NC and SiO/ NPC during the first cycle were evaluated at 100 mA g−1, and the results are illustrated in Fig. 7a. The discharge voltage platform of raw SiO is 0.1–0.2 V higher than that of three heat-treated samples, the possible reason may be the disproportion of SiO during the heat-treated process and leads to structural changes. On the one hand, this structural change affect the charge-discharge curve by affecting the SEI layer structure. On the other hand, the discharge-charge curve between nanoscale Si and micro-scale SiO is different [41]. The initial reversible capacities are 998, 1037, 1126 and 1223 mAh g−1, corresponding to initial coulomb efficiencies approximately 63, 65, 71 and 72%, respectively. All samples have two short discharge platforms at about 0.75 V and 1.2 V, which can be attributed to the formation of SEI layer and the decomposition of the FEC additive, which is consistent with the CV analysis. The initial reversible capacities of all samples at 1000 mA g−1 are exhibited as 562, 602, 777 and 864 mAh g−1, respectively (Fig. 7c). Due to inherent volume expansion, SiO shows a quite low capacity of 80 mAh g−1 after 200 cycles and a capacity retention of 14%, while the cycle performance of the sample after heat treatment was improved, the capacity retention was 27.8%. When coated with a nitrogen-doped carbon layer, the obtained capacity retention of the SiO/NC (77%) have a significant improvement in comparison with the SiO. The increase in capacity and cycle life should be attributed to that the conductivity is increased by nitrogen doping and volume expansion is mitigated by carbon coating [4,22]. The SiO/NPC composite shows a quite high capacity of 847 mAh g−1 after 200 cycles, with the capacity retention as high as 98%. This increase in reversible capacity and cycle stability may be due to the fact that phosphorus-doped improves the electronic conductivity and promotes electrochemical reactions, as well as accommodates volume changes of the SiO during charge/discharge under the cooperation of the carbon layer. Besides, the capacity of the
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Fig. 4. (a) XPS survey spectra of modified samples, C 1s XPS spectrum for (b) SiO/NC and (c) SiO/NPC, (d) P 2p XPS spectrum for SiO/NPC, N 1s XPS spectrum for (e) SiO/NC and (f) SiO/NPC, (g) Schematic of nitrogen atoms and phosphorus atoms replacing carbon atoms.
Table 1 The conductivity data from SiO, H-SiO, SiO/NC and SiO/NPC electrodes. Sample
Resistivity (Ω cm−1) Conductivity (S cm−1)
SiO
H-SiO
SiO/NC
SiO/NPC
11,660 8.576 × 10−5
11,190 8.937 × 10−5
18.05 5.540 × 10−2
3.66 2.732 × 10−1
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Fig. 5. (a–c) Cyclic voltammograms of SiO, SiO/NC and SiO/NPC. (d) Charge-discharge curve at 100 mA g−1 of the SiO/NPC.
Fig. 6. Differential-capacity plots (dQ/dV vs V) of (a) SiO, (b) SiO/NPC in the initial three cycles.
2.53 × 10−14, 1.63 × 10−13 and 8.77 × 10−13 cm2 s−1 for SiO, HSiO, SiO/NC and SiO/NPC, which implies that the phosphorus doping can meliorate the Li+ diffusion coefficient. Fig. 9 presents the SEM images of different electrodes before and after 200 cycles to comfirm the structural stability properties. After 200 cycles, the cross-sectional images of the SiO composite anode reveal a serious volume expansion, since the thickness of the SiO anode with a 5.92 μm increases to 11.89 μm and a expansion ratio is 101% (Fig. 9a and d). Comparatively, the thickness of the SiO/NPC composite anode slightly expanded from 18.20 μm to 24.08 μm, corresponding to 32% (Fig. 9c and f), which was much lower than that of the raw SiO anode, it is known that the electrode maintains good structural integrity. Thus, it is confirmed that the SiO/NPC composite fabricated in this study shows
excellent structural stability. 4. Conclusion N, P binary doped carbon has been introduced into silicon monoxide anode material, and the synergistic effect of binary doping effectively increases the content of active pyridine N, not only improves the electron conductivity of the material, but also induces more defects and edge sites to provide more lithium storage sites and facilitate the diffusion of Li+, thereby effectively optimized the electrochemical performance, reversible capacity, cycle life and rate performance of the electrode are significantly improved. And the as-prepared SiO/NPC material has an initial reversible capacity of 1223 mAh g−1 at 6
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Fig. 7. (a) Initial charge-discharge curve at 100 mA g−1, (b) Cycling performance profiles at 1000 mA g−1 and (c) Rate capability of the SiO, H-SiO, SiO/NC and SiO/ NPC.
Fig. 8. (a) Resistance comparison of the SiO, H-SiO, SiO/NC, and SiO/NPC after the first cycle at 100 mA g−1. (b) The relationship of real impedance versus low frequencies of the samples. (c) Equivalent circuit model.
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Table 2 The fitted EIS data from equivalent circuit and lithium ion diffusion coeffcients DLi+ of SiO, H-SiO, SiO/NC and SiO/NPC electrodes. Sample
Rs (Ω) Rct (Ω) σ ω (Ω cm2·s−1/2) DLi+ (cm2·s−1)
SiO
H-SiO
SiO/NC
SiO/NPC
42.36 339 442 2.21 × 10−14
40.87 305 383 2.53 × 10−14
13.76 99 163 1.63 × 10−13
10.58 33 74 8.77 × 10−13
Fig. 9. (a–c) Cross-sectional SEM images of the fresh electrode. (a) SiO (b) SiO/NC (c) SiO/NPC. (e–h) Cross-sectional of the electrode after 200 cycles. (d) SiO (e) SiO/NC (f) SiO/NPC.
100 mA g−1 and capacity retention of 98% after 200 cycles at 1000 mA g−1, indicating the powerful potential of high-performance SiO/NPC anode material for LIBs.
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