Facile Pyrolyzed N-Doped Binder Network for Stable Si Anodes - ACS

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A Facile Pyrolyzed N-doped Binder Network for Stable Si anodes Zhenggang Zhang, Yang Jiang, Zhe Peng, Shanshan Yang, Huan Lin, Meng Liu, and Deyu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10314 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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ACS Applied Materials & Interfaces

A Facile Pyrolyzed N-doped Binder Network for Stable Si anodes Zhenggang Zhang,a,1 Yang Jiang,a,b,1 Zhe Peng,*,a Shanshan Yang,a Huan Lin,a Meng Liu,a Deyu Wang*,a

a. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. b. Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China. Keywords: Silicon, Polyacrylonitrile, Pyrolysis, Binder network, Lithium ion batteries.

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Abstract: Although nano-engineering provides improved stability of Si-based nanostructures, a facile and efficacious method to directly use raw Si practices is still absent. Herein, we report a pyrolyzed N-doped binder network to improve the cycling stability of raw Si particles. Such an N-doped binder network is formed at a conformal pyrolysis condition of the electrode binder using Polyacrylonitrile (PAN), and provides a tight encapsulation of Si particles with significantly improved cycling stability. In contrast to single Si particles that pulverize and loss the total capacity at the 20th cycle, the discharge capacity could be retained ~1700 mAh g-1 at the 100th cycle for the Si particles imbedded in the pyrolyzed N-doped binder network. Our results demonstrate that such a facile remedy could significantly improve the cycling stability of raw Si particles for high energy-density lithium ion batteries.


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1. Introduction To fulfill the applications of lithium ion batteries (LIBs) in electronic devices, electric vehicles and grid-scale storage system, designing stable and high-capacity anode materials is urgently demanded.1-3 As a low cost abundant material, silicon (Si) offers a remarkable theoretical capacity of 4200 mAh g-1 which is ten times higher than that of graphite. However, large volume expansion higher than 300% during cycling is still a major concern blocking the practical application of Si anodes.4-6 Such a drastic volume change leads to severe mechanical strains, and causes pulverization/disintegration of the active materials,7 accumulation of highly resistive solid electrolyte interphase (SEI) layer,8 and rapid capacity decay of the electrodes (Scheme 1a).

Nano-engineering has been devoted to address the structural instability of Si, including the fabrication of nano-porous Si particles,9-11 growth of core-shell Si nanowires,12-14 and confinement of Si particles in carbon-cages.15-19 Although improved cycling stability has been achieved for these Si-based nano-structures, the related methods are still too complex for scalable production. Therefore, exploring facile and versatile strategies to promote the stability of raw Si particles is highly desired.

In our previous study, we have assessed the mechanical properties of polyacrylonitrile (PAN) fiber as function of pyrolysis temperature, and found that the PAN fiber pyrolyzed at 400 ℃ could match an optimal compromise between its tensile strength and rigidity, thus offers a remarkable ability to hinder volume expansion of Li metal anode.20


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Herein, an N-doped binder network obtained by pyrolyzing the PAN binder is proposed to provide stable cycling of raw Si particles. Sub-micro Si particles are firstly dispersed in graphene sheets to improve the electronic conduction and avoid local agglomeration.21,22 The as-obtained composite is mixed with polymer binder of PAN and subjected to pyrolysis process. After a pyrolysis at 400 ℃, all the Si particles and graphene sheets are encapsulated in the pyrolyzed Ndoped binder network (Scheme 1b), which exhibits outstanding structural stability. The encapsulated Si particles could retain a discharge capacity ~ 1700 mAh g-1 at the 100th cycle, in sharp contrast to the total capacity loss of single Si particles at the 20th cycle. As this method does not require any complex preparation, it could open up a facile and efficacious route to improve the cycling stability of raw Si particles for high-energy density batteries.


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Scheme 1. Illustrations of structural variations during Li+ insertion/extraction processes in (a) single Si particles and (b) Si particles encapsulated in the pyrolyzed N-doped binder network.

2. Experimental section 2.1 Electrode preparations


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Three samples, denoted as Si, Si/G and Si/G/PAN400 are prepared and correspond to the electrodes of single Si particles, Si particles dispersed in graphene sheets, and Si particles dispersed in graphene sheets then encapsulated in pyrolyzed N-doped binder network, respectively.

The dispersion of Si particles in graphene sheets is realized through a hydrothermal method. Si particles and graphene (Suzhou Graphene Nanotechnology Co., Ltd.) are separately dispersed in methanol, with a weight ratio of Si:G of 85:15. After vigorous stirring, the Si suspension is slowly added into the graphene suspension, followed by another stirring for 2 h. The mixed Si:G suspension (50-60 mL) is then transferred to a Teflon-line stainless steel autoclave (100 mL) and reacted at 180 ℃ for 6 h. The obtained products are centrifuged and laved by deionized water and methanol.

To obtain the Si and Si/G electrodes, active materials (Si particles and Si/G composite) are mixed with Super-P and Polyacrylonitrile (PAN, Sigma-Aldrich Co. LLC.) at a weight ratio of 7:1:2 in N-methyl-2-pyrrolidone (NMP, Aladdin Industrial, Inc.) until fully homogeneous slurries are obtained. The slurries are casted onto Cu foils. After vacuum drying at 120 ℃ for 12 h, the electrodes are obtained and transferred to an argon-filled glovebox (MBRAUN, H2O ⩽ 0.1 ppm, O2 ⩽ 0.1 ppm) for further cell assembly. To obtain the Si/G/PAN400 electrodes, the aboveobtained Si/G electrodes are transferred to furnace and heated under Ar at 400 ℃ for 90 min, the temperature ramping rate is 5 ℃ min-1.

2.2 Electrochemical measurements


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All the electrochemical tests are performed using coin cells CR2032, with Celgard separator film (diameter: 18 mm; thickness: 20 µm) in which an electrolyte amount of 70 µL is deposed. The electrolyte consists of a commercial electrolyte (1 M LiPF6 in 1:1:1 ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), Guotai-Huarong New Chemical Materials Co., Ltd.). Si, Si/G and Si/G/PAN400 electrodes are used as working electrodes. Li foils are used as counter electrodes.

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests are performed using an electrochemical workstation 1470E equipped with a frequency response analyzer (FRA, 1455A from Solartron). The CV measurements are carried out between 0.01 and 1.5 V vs Li/Li+ at a scanning rate of 0.1 mV s-1. The EIS measurements are conducted in the frequency range between 10-1 and 105 Hz with a voltage perturbation of 5 mV. Cycling tests are performed using a battery testing system (LandCT2001 from LAND electronics Co., Ltd.). The fully discharge-charge mode is realized in a voltage window of 0.01-1.5 V vs. Li/Li+, at a current density of 800 mA g-1, and the fixed discharge capacity mode is realized with a fixed discharge capacity of 1000 mAh g-1 and a cut-off charge potential of 1.5 V vs. Li/Li+, at a current density of 800 mA g-1. The capacity rate tests are caught out with current densities of 400, 800, 2000, 4000 and 8000 mA g-1, within the voltage window of 0.01-1.5 V vs Li/Li+.

2.3 Electrode characterizations The microscopy analysis of the electrode surface morphology is performed using scanning electron microscopy (SEM, FEI, QUANTA 250 FEG) and transmission electron microscopy (TEM, FEI, TecnaiF20, 200 kV). Prior to the TEM tests, all the electrodes are scrapped off from


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the current collectors and immersed in ethanol ultrasonic bath realized by using a Qsonica Q700 sonicator with output power of 700 W. The crystalline phase of the prepared samples is characterized by X-ray diffraction (XRD) with a Bruker D8 advanced diffractometer using CuKα (λ = 1.5406 Å) radiation (Bruker axs, D8 Advance). Surface analysis is conducted with a PHI 3056 X-ray photoelectron spectrometer (XPS), which is excited by an MgKα radiation source at a constant power of 100 W (15 kV and 6.67 mA). Raman spectra are obtained with a Renishaw inVia Reflex Raman spectrometer at an excitation wavelength of 532 nm from an Nd:YAG (neodymium-doped yttrium aluminum garnet) laser operating at 120 mW.

3. Results and discussions In our previous work, the carbonization of PAN binder as function of pyrolysis temperatures has been assessed with Raman spectroscopy. The simultaneous appearances of D and G bands only occur at 400 ℃ and higher temperatures, justifying the conversion of PAN into carbon-like structures from these temperatures.20 Indeed, PAN has been widely used as the precursor to N-doped graphitic anode materials or carbon coating materials for application in lithium ion batteries. In these cases, PAN is often pyrolyzed at high temperatures beyond 700 ℃. Different from previous cases, in this work, the pyrolyzed PAN is rather used as a rigid binder material. Thus, the pyrolysis temperature has to be very carefully controlled, to avoid the overcarbonization of PAN, and to achieve a rigid state of “partially-carbonized” PAN. Intrinsically, the increase in rigidity of PAN is based on the sacrificed tensile property, as we showed in our previous work.20 In this work, as shown in Figure 1a, the PAN pyrolyzed at 500 ℃ already losses its tensile property leading to the disintegration of the composite electrode. Thus, we have fixed in this work the pyrolysis temperature of 400 ℃ for the conversion of polymeric PAN binder into


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the pyrolyzed N-doped binder network. Figure 1c shows a PAN film pyrolyzed at 400 ℃ with a dense and compact structure, totally different from the porous structure of its polymeric state (Figure 1b).

Figure 1. (a) Digital photos of the Si/G/PAN anodes pyrolyzed at 120, 400 and 500 ℃; SEM image of PAN films (b) heated in vacuum at 120 ℃ and (c) pyrolyzed at 400 ℃; (d) XRD patterns and (e) Raman spectra of Si, Si/G and Si/G/PAN400 anodes.

The electrodes of Si particles dispersed in graphene sheets then encapsulated in pyrolyzed binder network are denoted as Si/G/PAN400. As controls, single Si particles and Si particles dispersed in graphene sheets without encapsulation of pyrolyzed binder network are also compared, separately denoted as Si and Si/G. The success mixture of Si particles and graphene sheets is confirmed by the coexistence of diffraction peaks of Si (111) and C (002) in the XRD patterns (Figure 1d). Raman spectra have also been measured for the Si, Si/G and Si/G/PAN400 electrodes, as shown in Figure 1e. Significant D and G peaks are observed on the spectra of


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Si/G/PAN400, further indicating the conversion of polymeric PAN binder into a carbon-like state. It should be mentioned that the electronic conductivity has also been improved via the encapsulation of the pyrolyzed PAN binder. As shown in Figure S1, both the charge transfer impedances at lithiated and delithiated states of the Si/G/PAN400 anode are lower than that of the Si/G anode.

Amorphous state of the pyrolyzed binder on Si/G/PAN400 has been clearly observed via TEM, as shown in Figure 2a whereas a part of encapsulated graphene with a crystal lattice spacing of 0.344 nm has also been identified (magnified image from the surrounded zone). It should be mentioned that the amorphous layer derived from the pyrolyzed PAN is based on Ndoped structure. For the latter, C 1s and N 1s XPS spectra of Si/G and Si/G/PAN400 electrodes are separately compared (Figure 2b and c). On the surface of Si/G/PAN400, besides the large CC peak related to graphene, C=N and C-N could also be identified (Figure 2b), whereas only C=O and C-O of oxide impurities are observed on the surface of Si/G. Consistently, cyanic group (399.6 eV), pyridinic group (398.5 eV) and substitutional graphite group (400.1 eV) consisting the amorphous layer are separately identified on the N 1s spectrum of Si/G/PAN400 (Figure 2c).23,24 For the Si/G, only cyanic group is observed for the PAN polymer binder. A brief structural illustration of the PAN as function of pyrolysis temperatures is shown in Figure S2.


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Figure 2. (a) TEM image of the amorphous pyrolyzed binder on the Si/G/PAN400 anodes, magnified image of the surrounded zone shows a crystal lattice spacing of 0.344 nm of the encapsulated graphene; (b) C 1s and (c) N 1s XPS spectra of Si/G and Si/G/PAN400 anodes.

An important effect of the pyrolyzed N-doped binder network is to mechanically keep the structure of encapsulated Si particles. To prove this, the Si, Si/G and Si/G/PAN400 electrodes are separately scrapped off from the current collectors, and immersed in an aggressive ultrasonic bath for 0.5 h. As shown in Figure 3a and b, disintegrations of active materials are observed for the Si and Si/G electrodes. The released single Si particles have an individual size grain ~ 100 nm, lower than that ~ 500 ߤm measured by the granulometer data (Malvern Instruments Ltd.). In sharp contrast, the Si particles are tightly encapsulated with the graphene sheets in the pyrolyzed binder network (Figure 3c). Magnified images shown in Figure 3d clearly indicate the closely packed Si particles and graphene sheets by the pyrolyzed binder network, large amorphous zones (1 and 4) indicate that the pyrolyzed binder provides a conformal encapsulation whereas crystal phases of zone 2 and 3 show separately the underneath encapsulated graphene and Si.


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Figure 3. TEM images of (a) Si, (b) Si/G and (c) Si/G/PAN400 anodes after aggressive ultrasonic bath; (d) Magnified zone of the Si/G/PAN400 anode.

The multiple redox peaks on the Cyclic Voltammetry (CV) curves confirm the electrochemical activities for both Si and graphene in the Si/G and Si/G/PAN400 electrodes (Figure 4a). The oxidative peaks of Si are separately ~0.57, 0.55 and 0.53 V vs. Li/Li+ for the Si, Si/G and Si/G/PAN400 electrodes. The improved polarizations from Si to Si/G should be due to


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superior electronic properties of the graphene sheets, while further reduced polarization from Si/G to Si/G/PAN400 should originate from the pyrolyzed N-doped binder network with substitutional graphite groups.

Figure 4. (a) CV curves of the Si, Si/G and Si/G/PAN400 anodes; (b) Charge-discharge curves of the Si/G/PAN400 anodes; (c) Discharge capacities of the Si particles in the Si, Si/G and


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Si/G/PAN400 anodes at a current density of 800 mA g-1; In-situ EIS measurements for the (d) Si and (e) Si/G/PAN400 anodes.

The cycling in full discharge-charge mode has been performed within 0.01-1.5 V vs. Li/Li+ at a current density of 800 mA g-1. The discharge capacities depicted in Figure 4c correspond to capacity contributions of the Si particles in each electrode (the capacities of graphene sheets have been retracted, detailed method is shown in Figure S3). As shown in Figure 4c, the Si anode losses totally its capacity at the 20th cycle, meanwhile similar tendency is observed for the Si/G anode, with slightly improved capacity ~110 mAh g-1 at the 100th cycle. In sharp contrast, for the Si/G/PAN400 anode, the capacity retention is significantly improved. After 100 cycles, the capacity could still be maintained at ~ 1700 mAh g-1. Stable polarization is also observed for the Si/G/PAN400 anode (Figure 4b), while it rapidly increases for the discharge-charge curves of Si anode (Figure S4). These results indicate that the pulverization of active materials is obviously slowed down in the presence of pyrolyzed N-doped binder network. It should be mentioned that the pyrolyzed binder network was also applied for the singe Si particles, denoted as Si/PAN400 in Figure 4c. However, a sudden capacity drop was observed at the 20th cycle. One possible reason might be the local agglomeration of Si particles during electrode preparation, which hinders the formation of a well cross-linked pyrolyzed binder network, and thus seriously reduces its ability to encapsulate the Si particles, as shown in Figure S5.

Without notice, the Si/G/PAN400 anode investigated in this work has a Si:G weight ratio of 85:15. We have compared a Si/G/PAN400 electrode with a Si:G weight ratio of 50:50, as shown in Figure S6a. Obviously, lowering the content of Si in the Si/G/PAN400 anode leads to


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the decrease of the overall capacity. Besides, almost identical capacity retention was observed while we only compared the capacity contributions of the Si particles in these composite anodes (Figure S6b). These results reveal that introducing excessive graphene sheets in the Si/G/PAN400 anode is not useful since it reduces the overall capacity of the composite anode without conducting to any improvement of cycling stability.

The structural stability of the Si/G/PAN400 anode has also been verified via in-situ Electrochemical Impedance Spectroscopy, EIS (Figure 4d and e). According to the Nyquist plots of the Si/G/PAN400 anode, the depressed semicircle related to the interfacial charge transfer is small and stable ~ 5 ᅅ upon cycling, which indicate fast charge transfer kinetic in the encapsulated Si particles (Figure 4e). Only the increase of overall impedance is due to the electrolyte depletion caused by the side reactions at the counter-electrode side of Li foil. In sharp contrast, the charge transfer impedance of Si anode fast increases from 70 ᅅ after the 1st cycle to 170 ᅅ after the 100th cycle (Figure 4d). These large impedances indicate sluggish charge transfer of Li ions in the collapsed structure of Si particles.

The morphology of the electrodes after cycling has been observed with SEM (Figure 5). For the Si anode, the Si particles are dilated and agglomerated in large grains, with micro-scale cracks on the electrode, indicating the large volume change and pulverization of the Si particles during cycling (Figure 5a). For the Si/G anode, a rough surface likely covered by pulverized Si particles is observed on the graphene sheet, indicating that the presence of graphene sheets only provides the dispersion of Si particles without any protective effect (Figure 5b). For the Si/G/PAN400 electrode, neither micro-scale cracks nor electrode disintegration have been


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observed (Figure 5c). As shown on the magnified zone in Figure 5d, a clearly layered surface consisted of pyrolyzed binder out-layer and graphene sheet interlayer can be observed, confirming the high structural stability enabled by the pyrolyzed N-doped binder network.

Figure 5. SEM images of (a) Si, (b) Si/G and (c) Si/G/PAN400 anodes after cycling; (d) Magnified zone of the Si/G/PAN400 anode.


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The rate capability tests have been performed in the investigated electrodes (Figure S7). The best performances are observed for the Si/G/PAN400 electrode. For the encapsulated Si particles, it delivers capacities of 3826 mAh g-1 at 400 mA g-1, 3419 mAh g-1 at 800 mA g-1, 2724 mAh g-1 at 2000 mA g-1, 1951 mAh g-1 at 4000 mA g-1 and 1077 mAh g-1 at 8000 mA g-1. Thus, even at a high current density of 8000 mA g-1, the delivered capacity of the Si particles in Si/G/PAN400 anode is still higher than that of graphite (370 mAh g-1), indicating its exceptional rate capability.

Finally, in a galvanostatic mode at a current density of 800 mA g-1 for a fixed discharge capacity of 1000 mAh g-1, the discharge end-voltage of the Si anode quickly drops below 0 V vs. Li/Li+ at the 10th cycle (Figure 6a and d). It’s well known that the negative potential during lithiation corresponds to the deposition of Li metal. In other words, the structure collapse of the Si anode quickly causes the loss of its host capacity below 1000 mAh g-1 (~800 mAh g-1 at the 10th cycle), followed by further Li metal deposition at out-surfaces of the Si particles. The Si/G anode could maintain a cycling above 0 V vs. Li/Li+ up to ~50 cycles (Figure 6b and d). Only for the Si/G/PAN400 anode, the end-voltage in discharge is sustainably remained above 0 V vs. Li/Li+ for more than 260 cycles (Figure 6c and d). Therefore, the lifespan of the Si/G/PAN400 anode is ~25 and 5 times longer than that of the Si and Si/G anodes. These results prove the ability of the Si/G/PAN400 anode for its implementation in high-energy lithium batteries for long-term operation.


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Figure 6. Discharge curves of (a) Si, (b) Si/G and (c) Si/G/PAN400 anodes in a fixed discharge capacity mode; (d) End-voltages of discharge for these anodes upon cycling. The current density and discharge capacity are separately fixed at 800 mA g-1 and 1000 mAh g-1.

It should be noted that only the N-doped binder could achieve a cyclized state (detailed discussion has been provided in our previous work20) with improved mechanical strength that provides the conformal encapsulation of the Si particles. As a comparison, polyvinylidene fluoride, PVDF, has been studied through the same pyrolysis at 400 ℃ and the as-obtained Si/G/PVDF400 anode showed much poorer performances than that of Si/G/PAN400 (Figure S8). These results certify the pertinent choice of PAN binder investigated in this work.

4. Conclusion


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In summary, a facile and efficacious approach to enhance the structural stability of raw Si particles has been successively realized by the use of a pyrolyzed N-doped binder network. Such a pyrolyzed N-doped binder network is directly obtained by pyrolyzing the PAN polymer binder at a conformal temperature of 400 ℃, and could tightly enable the structural stability of the entire electrode. The Si particles encapsulated in the pyrolyzed N-doped binder network could maintain a discharge capacity of ~1700 mAh g-1 at the 100th cycle, in sharp contrast to the single Si particles which loss totally their capacity at the 20th cycle. Good rate capability and long-term cycling have also been achieved via this pyrolyzed binder network. Our results demonstrate the potential of this method for improving the cycling stability of raw Si particles in high energydensity batteries.

ASSOCIATED CONTENT Supporting Information SEM image of a PAN film heated at 120 ℃, molecular structural illustration of PAN as function of pyrolysis temperature, discharge-charge curves of Si anodes, SEM image of pyrolyzed Ndoped binder network on single Si particles, rate capability tests of Si, Si/G and Si/G/PAN400 anodes. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 1

These authors contribute equally to this work.

Corresponding Author *Zhe Peng, E-mail: [email protected]


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*Deyu Wang, E-mail: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Ningbo Natural Science Foundation (Grant No. 2016A610278), International S & T Cooperation Program of Ningbo (No. 2016D10011), Zhejiang Provincial Natural Science Foundation of China (Grant No. Q17E020023) and National Natural Science Foundation of China (NSFC) for the research fund for International Young Scientists (Grant No. 51650110490).

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Table of Contents Graphic


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Facile Pyrolyzed N-Doped Binder Network for Stable Si Anodes - ACS

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