Rigid Polyimide Buffering Layer Enabling Silicon Nanoparticles

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Rigid Polyimide Buffering Layer Enabling Silicon Nanoparticles Prolonged Cycling Life for Lithium Storage Ruirui Fu, Ping Nie, Minyuan Shi, Jiang Wang, Jiangmin Jiang, Yadi Zhang, Yuting Wu, Shan Fang, Hui Dou, and Xiaogang Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00344 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Applied Energy Materials

Rigid Polyimide Buffering Layer Enabling Silicon Nanoparticles Prolonged Cycling Life for Lithium Storage Ruirui Fu, Ping Nie, Minyuan Shi, Jiang Wang, Jiangmin Jiang, Yadi Zhang, Yuting Wu, Shan Fang, Hui Dou and Xiaogang Zhang* College of Material Science and Engineering , Jiangsu Key Laboratory of Materials and Technology for Energy Conversion & Jiangsu Key Laboratory of Electrochemical Energy Storage Technologies, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China. *Correspondence and requests for materials should be addressed to [email protected] or [email protected]

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ABSTRACT: Structural design is an effective avenue to alleviate the volume expansion of silicon anode. Surface coating and/or encapsulation shows a significant advantage. However, most coating layer shows a poor mechanical strength, resulting in fast fading electrochemical performance. In this article, commercial silicon nanoparticles coated with polyimide (PI) film have been synthesized by a simple mechanical stirring process. The polyimide polymer film endows silicon with structural characteristics of intrinsic high ionic conductivity, stable SEI layer, and structure rigidness. The resulting composites display excellent capacity retention with about 1144 mAh g-1 after 430 cycles at 1 A g-1. The LiCoO2/Si@PI-15% full-cell displays a high initial capacity about ~126 mAh g-1 based on the cathode mass. After 50 cycles, it delivers 55 mAh g−1 reversible capacity with an average CE of ∼98% at 140 mA g−1.

KEYWORDS: Silicon anode; Polyimide; Rigid buffering layer; Lithium-ion batteries; Long cycle

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INTRODUCTION Negative electrode materials play a pivotal role for the electrochemical performance in lithium-ion batteries (LIBs).1-4 Commercial graphite anode is no longer suitable for the new development goals of high energy batteries. Silicon is an ideal anode material due to its outstanding specific capacity (4204 mAh g-1, based on Li4.4Si),5-7 low working potential (~0.3 V versus Li/Li+),8 low cost and environmentally friendly.9 Therefore, silicon anode becomes a strong contender for the next generation of high energy lithium-ion batteries.10-11 Unfortunately, the application of silicon anode was hampered by the massive volume expansion during the repeated charge-discharge process, which caused a series of issues. On the one hand, the aeolotropism of silicon makes it difficult to expand uniformly in all directions during lithium insertion.12 This is bound to make the structural distortion of electrode. Notedly, the electrode will separate from the current collector after few cycles.13 On the other hand, volume expansion brings about more fresh silicon surface exposed to the electrolyte and the continuous formation of solid electrolyte interphase (SEI) film.14-16 The lithium ion will be consumed continuously and leading to the capacity attenuates quickly. Furthermore, as the SEI film became more and more thick, the increased internal resistance makes against the diffusion of lithium ion.17 Therefore, silicon anode exhibits a tendency of significant decline about cyclic stability and rate capability. In order to improve the electrochemical performance, a lot of work has focused on solving the volume expansion and structuring stable interface, such as the electrolyte solution,18-20 binders,2123

structure design.24-26 Wakabayashi et al.27 successfully fabricated hollow core-shell type Si/C nanocomposites using

a simple sugar-blowing technique . The decomposition of the ammonium chloride produces voids between the silicon and carbon layers to accommodate volume expansion. Hollow core-


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shell type Si/C nanocomposites have effectively release capacity attenuation. Yang et al.28 synthesized core-shell nanostructure consisting of pure silicon nanoparticles coated amorphous TiO2 (Si@a-TiO2) through a facile sol–gel approach. The amorphous TiO2 shows elastic characteristic, which can relieved the volume expansion of silicon during lithium insertion and extraction.29 Nie et al.30 used aerosol spray pyrolysis24, 31 and in situ chemical vapor deposition (CVD) technology to produce core-shell Si@graphene cage composites. The graphene cage was deposited uniformly on the surface of silicon. In addition, there are many studies related to the synthesis of core-shell or encapsulation structure32 to oppose the issue of volume deformation, which has been considered as one of the most effective solutions. The design of the structure can build an interface between silicon and electrolyte except relieving volume expansion. However, many preparation methods perhaps required special equipment or complex process flow. In addition, the weak mechanical properties of carbon layer are difficult to maintain the deformation of silicon. Polyimide (PI) polymer has been widely used in lithium-ion batteries owing to the excellent heat stability33-34 and good wettability for electrolyte, such as separators.35 Furthermore, PI binders are mightily researched because of favorable adhesive characteristics even at high temperatures (>150℃).36 Even in the field of organic cathode material for LIBs, we can still find the silhouette of PI.37 However, such fascinating polymer and its properties have not been demonstrated as a coating layer for silicon in lithium-ion batteries. We herein used a simple mechanical stirring process to synthetize commercial silicon nanoparticles coated with PI film (signed as Si@PI) generated from polyamic acid (PAA) precursor. The effect of PI coating on electrochemical performance of silicon has been investigated in detail. The PI film plays a significant role during discharge-charge process due to excellent mechanical properties. It can resist the volume expansion of silicon anode. The PI,


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meanwhile, has established steady SEI film and decreased direct contact between the silicon and the electrolyte. And the polymer film provides more efficient lithium-ion transport because of its ionic conductivity. Moreover, the Si@PI composites can still maintain the electronic connection due to the heterogeneity of traditional mechanical mixing method even low electrical conductivity of PI. Consequently, the Si@PI electrode delivers a long cycle life with about 1144 mAh g-1 capacity retention after 430 cycles. And a LiCoO2/Si@PI-15% full-cell displays a high initial capacity about ~126 mAh g-1 based on the cathode mass. After 50 cycles, it delivers 55 mAh g−1 reversible capacity with an average CE of ∼98% at 140 mA g−1. RESULTS AND DISCUSSION The PAA precursor was mixed with commercial silicon nanoparticles in DMA solvent. The obtained Si@PAA was successfully converted to Si@PI composite via a thermally cured process (Figure 1). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in Figure 2a, d show the morphology and size of commercial silicon nanoparticles. The particles distribution is in the range of nanometer with average size of about ~100 nm. Obviously, the morphology of the Si@PI is similar to pure silicon from Figure 2b, e. Further observation proved that the resultant Si@PI nanoparticles showing rough surfaces, suggesting the deposition of PI. Further, X-ray diffraction (XRD) pattern (Figure 2c) suggests that Si@PI composites exhibit clear five peaks related to (111), (220), (311), (400), and (331) planes of cubic silicon (JCPDS 27-1402), which means that the structure of silicon remains the same even after the thermally cured via a stepwise imidization process. The high-resolution transmission electron microscopy (HRTEM) image of Si@PI in Figure 3a indicates evident lattice fringe, corresponding to the (111) plane of Si, in agreement with the


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information from XRD pattern. The polycrystalline properties of the silicon nanoparticles can be revealed by the selected area electron diffraction (SAED) pattern (Figure 3b). Synthetic route of the polyimide was given in Figure 2f. To make sure that the PAA was successfully transformed into the PI, we measured FT-IR spectra to analyze the chemical bonds of silicon nanoparticles, PAA solutions and Si@PI composites. As shown in Figure 3c, the polyamic acid exhibits the peak of the C=O bond at 1635 cm-1, the C=O peak of the PI transfers to 1725 cm-1.36, 38 This change is attributed to the formation of the amide bond from dehydration reaction, demonstrating the PI has been successfully deposited on the surface of silicon nanoparticles. To further gain the distribution situation of the PI, we measured the dark-field scanning transmission electron microscopy (STEM) and corresponding elemental mapping of Si, O, N and C. In Figure 3d, uniform distribution of N, O and C elements can be observed on the outside of the silicon. X-ray photoelectron spectroscopy (XPS) was used to view the surface composition of the Si@PI-15% hybrid. In Figure 4a, the XPS spectra of O 1s not only reveal C=O (532.2 eV), C-O (532 eV) bond of PI film, but also exhibit the existence of SiO2 (532.9 eV). The spectra of Si 2p was also show a SiO2 peak at 103.3 eV (Figure 4b).39 We attribute the existence of SiO2 to the surface oxidation of silicon in air.40 The XPS spectra also exhibit the existence of N 1s (Figure 4c) and C 1s (Figure 4d), further demonstrating successful PI coating on silicon particles. We synthesized Si@PI with different PI content (5 wt.%, 10 wt.%, 15 wt.%) to investigate the influence on the cycling stability. The result was plotted on the curves about the variations of the specific capacity of Si@PI of different PI content with cycling performance at 1 A g-1 in Figure 5a. Clearly, with the increase of PI content, the cycle stability of Si@PI anode was improved. In particular, the Si@PI-15% anode exhibits outstanding cycle performance. Although further


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studies may find a better proportion for cycling stability, it will be at the cost of the decrease of specific capacity. Therefore, we did not do any further investigation. We selected the Si@PI-15% composites based on its stability to study the electrochemical response. The cyclic voltammetry (CV) curves of the Si@PI-15% of the first five cycles were obtained in the voltage range of 0.01-1.5 V (vs. Li/Li+) at a scanning rate of 0.1 mV s−1.41 As can be seen from Figure 5b, the Si@PI-15% exhibits typical Li–Si alloying/de-alloying behaviors. During the first lithiation process, it appeared three peaks at around ~1.2, ~0.8 and 0.1-0.2 V (vs. Li/Li+) which corresponds to the formation of SEI film, the decomposition of electrolyte and the presence of Li–Si compounds.28, 42-43 Among them, the two peaks related to the electrolyte and the SEI film disappeared in the subsequent scan. In the cathodic scan, two anodic peaks observed in ~0.33 V and ~0.5 V reflected the delithiation process of Li−Si alloy. With the increase of the number of cycles, the peak intensity was enhanced. We can find that the Si@PI-15% electrode material was activated. After several cycles, the CV curves almost coincide. We attribute this phenomenon to the excellent mechanical properties of PI films. This film effectively inhibited the expansion and deformation of silicon nanoparticles and promoted the formation of stable SEI films. This phenomenon of no other redox peak in CV curves led us conclude that the PI films has excellent electrochemical stability in the test voltage range. This is supported by the previous reports about the PI materials used as binder for lithium-ion battery anode. Similarly, a small voltage platform (Figure 5d) can be observed from the first cycle of chargedischarge profiles at ~0.8 and ~1.2 V. This is consistent with the observation from cyclic voltammetry curves. The initial Coulombic Efficiency (CE) is only 77%. It shows no obvious change in initial CE for Si@PI. The formation of the SEI film was the decisive factor for the irreversible capacity loss.


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It must also be mentioned that the Si@PI-15% composites still displayed a higher reversible capacity about ~2860 mAh g-1. In the second cycle, the Coulombic efficiency was rapidly increased to ~94%. Furthermore, the commercial silicon nanoparticles were selected as the control sample to test the cycling performance and CE with the same preparation procedure of half-cell. The results was shown in the Figure 5e. The half-cell was activated at a low current density of 0.2 A g-1 for the first three cycles, and then cycled at 1 A g-1. The Si@PI-15% half-cell displays an excellent reversible capacity retention about 1144 mAh g-1 after 430 cycles, while the commercial silicon nanoparticles exhibit rapid capacity decay with a capacity of only 236 mAh g-1 after 430 cycles. The Coulombic efficiency of Si@PI electrode is a little higher than pristine silicon anode. Moreover, the CE of Si@PI electrode increases more rapidly. And the CE of Si@PI electrode can maintain a higher level, while the pristine silicon electrode has an obvious reduction during subsequent cycling. Figure 5c showed the rate capability of the Si@PI-15% composite at the current density of 0.1, 0.2, 0.5, 1, 2, 5 A g-1 respectively. The Si@PI-15% composite exhibits general electrochemical performance because the PI has ionic conductivity and no electrical conductivity. The electrode maintained a reversible capacity of 750 mAh g−1 at a high current density of 5 A g−1. Figure 6a, b show the variation of silicon electrode thickness before and after 100 cycles, respectively. Obviously, the Si@PI-15% electrode after 100 cycles (Figure 6d) exhibits a smaller thickness change (3.4%) than that of the Si@PI-15% electrode before cycling. For the pristine silicon electrode, 23.9% change in electrode thickness was observed, demonstrating rigid polyimide buffering layer weakens the deformation caused by the stress. To explore the influence of PI film on the internal resistance of silicon anode, we tested the electrochemical impedance spectroscopy of Si@PI-15% (Figure 6e) and Si (Figure 6f) under delithiation state of


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before cycling, after 10 cycles and 30 cycles. Clearly, the slope of the Si@PI-15% is larger than the slope of silicon nanoparticle at low frequencies, which means the existence of PI film on the silicon increased the solid diffusion for lithium ion. Because the PI film has no electrical conductivity, The Si@PI-15% composite has a larger charge-transfer resistance compared to the commercial silicon nanoparticles in pristine condition. With the increase of cycle number, the electrolyte resistance and the interfacial resistance of the Si@PI-15% was decreased but the pure silicon was increased.44-46 Owing to the stable SEI formation for the Si@PI-15%, there was no fresh silicon surface exposed on the electrolyte to continue form thicker SEI film. That is why the Si@PI-15% exhibits a better cycling stability compared with pure silicon. Consequently, we investigated the mechanical properties of PI film. As can be seen from the stress-strain curve of Figure 6g, the PI film has larger tear strength. When the PI film was subjected to greater force, it only exhibited a small strain. Accordingly, we have reason to believe that the PI film can promote the formation of stable SEI film and strengthening of cyclic stability. This is consistent with the observation from EIS. In addition, ex-situ electron microscopy was used to observe the morphology of the Si@PI-15% electrode during cycling (Figure 6 h-j).47 It prove that the Si@PI15% maintained the spherical morphology well even after 430 cycles. It is important to highlight that rigid polyimide buffering layer possess enough strength to undergo the large volume expansion. We have achieved the practical application of the Si@PI-15% materials. The lithium-ion full batteries were assembled using the Si@PI-15% as an anode and commercial available LiCoO2 as a cathode.48 We measured the cycle stability of Si@PI-15% and LiCoO2 half-cells at the same electrolyte (1 M LiPF6 in EC:DMC=1:1, 5 vol.% FEC)49-51 and current densities (1C-rate, 140 mA g-1) before assembling full batteries, respectively. To determine the operating voltage range


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of the full cell, the charge capacity of the cathode was normalized to 1.0 mAh and the discharge capacity of the anode was normalized to 1.2 mAh. That means the negative capacity was in excess of 20% for positive capacity. The details of capacity matchups for the Si@PI-15% anode and LiCoO2 cathode was revealed in the Figure 7a. Because LiCoO2 cathode was used as limited electrode, voltage range was determined the cutoff voltage of full cells. When the cathode was fully charged (point A, VA=4.2 V vs. Li+/Li), the corresponding negative voltage was point B (VB=0.07 V vs. Li+/Li). While the cathode was fully discharged (point C, VC=3.0 V vs. Li+/Li), the anode rise to point D (VD=0.55 V vs. Li+/Li). Finally, the operating voltage ranges from 2.5 to 4.1 V in the LiCoO2/Si@PI-15% full-cell. The electrode preparation method has been mentioned in the Experimental. For a change, the negative electrode was cut into a wafer with a bigger diameter of 13 mm, which can make the positive electrode react adequately. And the negative electrode was directly contacted with the lithium metal in the electrolyte under certain pressure.52 The purpose of the prelithiation was to complement the lithium loss caused by the formation of SEI film. The cycle performance of the LiCoO2/Si@PI-15% full-cell was shown in Figure 7b. The full-cell displayed a high initial capacity of ~126 mAh g-1 based on the cathode mass. The corresponding initial Coulombic efficiency is about 91.3%. The LiCoO2-limited LiCoO2/Si@PI-15% full cell delivers a high reversible capacity of 55 mAh g−1 with an average CE of ∼98% at 140 mA g−1 after 50 cycles. CONCLUSIONS In conclusion, we have developed polyimide film coated silicon composites with outstanding cyclability. Owing to the excellent ionic conductivity and wettability of PI film, Si@PI composites exhibit much lower Li+ diffusion resistances compared to pure silicon. And the breaking resistance of PI film enables the Si@PI composites with stable interface between


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electrode and electrolyte. Furthermore, the Si@PI shows better safety due to excellent heat stability of PI polymer. The resulting composites display excellent cyclability with about 1144 mAh g-1 capacity retention over 430 cycles at a current density of 1 A g-1. As a further demonstration, a full cell coupled with LiCoO2 cathode shows a high initial capacity of ~126 mAh g−1 with a high CE and good cycling performance. This work provides a feasible and simple approach to deposit rigid polyimide protecting layer on nanosized silicon with unique merits, which would benefit the development of Si-based anode materials for next-generation high energy batteries. EXPERIMENTAL SECTION Material Synthesis We synthesized the rigid layer polyimide coated Si nanoparticles (Si@PI) composites using a simple method. As-purchased Si nanoparticles (Gexin nano, Shanghai, China) of about 50-100 nm in average diameter were mixed with dilute polyamic acid that has been reported in details in previous study (The details were shown in supporting information). Through a vigorous stirring process under a nitrogen atmosphere, the PAA was uniformly adsorbed on the surface of the silicon nanoparticles (Si@PAA). The Si@PAA mixture was thermally cured via a stepwise imidization process (60 °C for 30 min, 120 °C for 30 min, 200 °C for 60 min, 300 °C for 60 min, 400 °C for 10 min) under a stream of nitrogen gas, the rigid layer polyimide coated Si nanoparticles (Si@PI) composites were obtained. Weight ratio of the silicon nanoparticles and PI was varied from 1:1.05 to 1:1.15 (The composites were marked as Si@PI-5%, Si@PI-10%, Si@PI-15%). Characterizations


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The morphologies of the pure silicon and the composites were characterized by scanning electron microscopy (SEM, HITACHI S-4800) and transmission electron microscopy (TEM, JEOL JEM-2010, FEI TECNAI G2). The crystal structure was characterized by X-ray diffraction (XRD) (Bruker D8 advance) with Cu Kα radiation (the 2θ range from 10º to 80º). The X-ray photoelectron spectroscopy (XPS) analysis was measured on a Perkin-Elmer PHI 550 spectrometer with Al Kα (1486.6 eV) as the X-ray source. Fourier transform infrared spectra was obtained from a Shimadzu IR Prestige-21 FT-IR spectrometer using the KBr disk in the wave number range of 500 - 4000 cm-1. The stress−strain curves were performed on TA Instrument Q800 dynamic mechanical analyzer. The contact angle of electrolyte (1 M LiPF6 in EC:DMC=1:1, 5 vol.% FEC) on the composites was observed through a microscope (DSA1000 from Kruss Corporation, Germany) after spreading over substrate surface fully. Electrochemical Measurements The slurry of Si@PI electrodes is composed of Si@PI active material (60 wt.%), acetylene black (25 wt.%) and carboxymethyl cellulose (CMC) binder (15 wt.%), which was mixed with water. It was evenly coated on the copper collector by a doctor blade. After drying at 60 °C for 12 h under a vacuum oven, it was cut into a wafer with a diameter of 12 mm. The active material mass loading is about 0.56 mg based on the Si@PI active material. 1 M LiPF6 dissolved in the ethylene carbonate/dimethycarbonate (EC:DMC=1:1, by volume) with 5 vol.% fluoroethylene carbonate (FEC) additive was used as the electrolyte. The separator used is Celgard® 2400 polypropylene membrane. Coin-type half-cells (CR2032) were assembled using Li foil as counter and reference electrode under an argon-filled glovebox with water/oxygen content lower than 1 ppm. The cycling performance and the rate capability were tested on a Land Battery Tester (Wuhan LAND electronics Co., Ltd., China) and Neware Battery Testing System between


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0.01 V and 1.5 V vs. Li/Li+. The cyclic voltammetry (CV) curves were performed at a scan rate of 0.1 mV s-1 between 0.01 V and 1.5 V vs. Li/Li+ using CHI660C electrochemical workstation (Chenhua, Shanghai). Electrochemical impedance spectroscopy (EIS) was collected in a frequency range from 100 kHz to 0.1 Hz with an AC amplitude of 5 mV by ZIVE SP2 electrochemical workstation. For full cells, the commercial LiCoO2 (LCO) powders (EQ-Lib-LCO, MTI Corporation) were mixed with carbon black and PVDF (8:1:1 by weight) in N-methyl-2-pyrrolidone (NMP). The obtained slurry was coated on Al foil and dried at 110 °C overnight in a vacuum oven. The positive electrode was cut into a disk with a diameter of 13 mm. The electrolyte and separator were the same to half-cell.


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Figure 1. Schematic of the fabrication process for the rigid polyimide layer coated Si composites.

Figure 2. SEM and TEM images of the samples: (a, d) commercial silicon nanoparticles; (b, e) Si@PI composites; (c) XRD patterns of the Si@PI composites; (f) Synthetic route of the polyimide.


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Figure 3. (a) HRTEM images of Si@PI composites; (b) SAED image of Si@PI composites; (c) FT-IR spectra of Si, PAA and Si@PI composites; (d) STEM images of the Si@PI composites and corresponding elemental mapping of Si, O, N and C.


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Figure 4. XPS spectrum of O 1s (a), Si 2p (b), N 1s (c), C 1s (d) of the Si@PI composites.


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Figure 5. (a) Cycle stability of Si@PI-5%, Si@PI-10% and Si@PI-15% at the current densities of 0.2 A g-1 and 1 A g-1; (b) Cyclic voltammetry curves of Si@PI-15% composite from 0.01 to 1.5 V at a scan rate of 0.1 mV s−1; (c) Rate capabilities of Si@PI-15% composite and commercial silicon nanoparticles at different current densities; (d) The first three cycles chargedischarge profiles of the Si@PI-15% electrodes at 0.2 A g−1 current density; (e) Cycle stability and Coulombic efficiency of Si@PI-15%, commercial silicon nanoparticles at the current densities of 0.2 A g-1 and 1 A g-1.

Figure 6. Thickness changes of commercial Si (a, b) and Si@PI-15% (c, d) electrode before and after 100 cycle. Electrochemical impedance spectroscopy (EIS) of the Si@PI-15% (e) and silicon nanoparticles (f) at the delithiation state before cycle, after 10 cycle and 30 cycle at 1 A g-1. (g)


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Stress-strain curve of the PI film. The structural evolution of Si@PI-15% electrode before cycling (h), initial lithiation (i), and after 430 cycles (j) via ex situ TEM measurements.

Figure 7. (a) Details of capacity matchup in the LiCoO2-limited LiCoO2/Si@PI full cell; (b) Cycle performance for a LCO/Si@PI-15% cell at 1C between 2.5-4.1 V.


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ASSOCIATED CONTENT Supporting Information Supporting Information Available: < synthesis of polyamic acid; mechanical testing of polyimide; XRD pattern of commercial silicon nanoparticles; the contact angle of electrolyte (1 M LiPF6 in EC:DMC=1:1, 5 vol.% FEC) on Si@PI composites; electrochemical characterization of LiCoO2 cathode; the rate capability of different samples > This material is available online or from the authors. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program) (2014CB239701), National Natural Science Foundation of China (51372116, 51672128, 21773118), Natural Science Foundation of Jiangsu Province (BK20151468), Prospective Joint Research Project of Cooperative Innovation Fund of Jiangsu Province (BY-2015003-7), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). R. Fu acknowledges Founding of Graduate Innovation Center in NUAA (kfjj20170607).


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Rigid Polyimide Buffering Layer Enabling Silicon Nanoparticles

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