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Mesoporous Silicon Anodes by Using Polybenzimidazole Derived Pyrrolic N-enriched Carbon Towards High-Energy Li-ion Batteries Ping Nie, Xiaoyan Liu, Ruirui Fu, Yuting Wu, Jiangmin Jiang, Hui Dou, and Xiaogang Zhang ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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ACS Energy Letters

Mesoporous Silicon Anodes by Using Polybenzimidazole Derived Pyrrolic N-enriched Carbon Towards High-Energy Li-ion Batteries Ping Nie †, Xiaoyan Liu ‡, Ruirui Fu †, Yuting Wu†, Jiangmin Jiang†, Hui Dou†, and Xiaogang Zhang*, † †

College of Material Science and Engineering & Jiangsu Key Laboratory of Materials and

Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P.R. China. ‡

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, P.R.

China. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

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ABSTRACT. Silicon anode holds great potential for next generation lithium ion batteries in view of its high gravimetric capacity and natural abundance. The main challenges associated with silicon are the structural degradation and instability caused by huge volume change upon cycling. We report herein polybenzimidazole (PBI) derived pyrrolic N-enriched carbon as an ideal encapsulation onto micro-sized silicon spheres, which is achieved by an aerosol-assisted assembly combined with a simple physisorption process. The new polymer derived carbon endows silicon with the structural and compositional characteristics of intrinsic high electronic conductivity, abundant pyrrolic nitrogen, and structure robustness. The resulting mesoporous Si-PBI carbon composite exhibits excellent lithium storage performance in terms of high reversible specific capacity of 2172 mAh g-1, superior rate capability (1186 mAh g-1 at 5 A g-1), and prolonged cycling life. As a result, a fabricated Si/LiCoO2 full battery demonstrates high energy density of 367 Wh kg-1 as well as good cycling stability for 100 cycles.

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Recent years have witnessed an ever-increasing demand for advanced lithium ion batteries (LIBs) to achieve lower cost, higher energy density as well as better safety.1-4 Silicon that couples earth abundance and unparalleled specific capacity of >4000 mAh g-1 (10 times larger than that of commercial graphite, i.e., 372 mAh g-1), has been considered among the most promising anode candidates for next generation LIBs application. Silicon shows attractive operating potential as low as ~0.3 V vs. Li/Li+, which guarantees a higher working voltage and energy density for a full cell.5-9 Despite the appealing properties, silicon suffers from severe capacity fade upon cycling due to its alloy reaction mechanism, which generally induce huge volume change nearly ~300% during the lithiation/delithiation process.10-12 The degradation of battery capacity can be attributed to pulverization of active materials, delamination from current collector, electrical isolation of fractured particles, and repeated chemical side reactions with organic electrolytes to form unstable thicker solid electrolyte interphase (SEI) layer.13-15 Therefore, of core importance to realize silicon’s success commercially is the cycling lifetime and mass production. Tremendous efforts have been made to address the above critical challenges. Strategies including through 1) nanostructuring of silicon,16-17 2) continuous electrically conductive network,18-19 3) porosity between crystals and particles,20-21 4) new binder concepts22-24 have been successful in extending the cycle life of silicon with suppressing volume expansion, mechanical robustness, and reduced ion/electron transportation path. Choi et al. presents a new strategy to enhance the electrochemical performance of micro-sized silicon by designing porous structure, construction of small primary nanoparticles and carbon nanotubes wedging.25 The desired Si composites were synthesized by a disproportion reaction, coupled with chemical etching and a ball-milling treatment using commercially available SiO as precursors. They further investigated the influences of various CNTs contents and ball milling time on the electrochemical properties of the obtained products. The porous characteristic, high conductive networks as well as micro-size morphology endows the resultant electrode 3 ACS Paragon Plus Environment

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with high tap density of 1.103 g cm-3, and appealing lithium storage properties including higher initial Coulombic efficiency (83.4%), superior rate capability and excellent cycling stability with high specific capacity of 2028.6 mAh g-1 after 100 cycles, which is compared with those of micro Si electrodes reported in the literature. Among these structure design, stable coating layer on Si surface is considered as one of the most promising methods to enhance the cycling stability.26 Despite impressive advances achieved, challenge still remains in both manufacturing and fundamental understanding of the technology. For most cases, limited and incomplete coating present issues for widespread application of Si/carbon composites. Structural degradation and unstable SEI layer with electrolyte will lead to additional SEI formation during lithium insertion owing to continuous occurrence of side reactions, severely decreasing the cycle life of silicon anodes. Another concern is the lithium ion transfer behavior/electron transfer across the network structure of silicon and coating layer. Moreover, its robust property of coating layer when subjected to cycling for perfect volume change accommodation. Importantly, the established synthesis usually involves chemical vapour deposition or atomic layer deposition or requires rather complicated process. Therefore, there is an urgent need to develop an efficient approach to obtain robust coating material on silicon surface for high energy LIBs application. Among various polymers, polybenzimidazole (PBI) is generally synthesized by a condensation reaction between diphenyl isophthalate and 3, 3′, 4, 4′-tetraaminodiphenyl (inset in Figure 1) has been broadly used in high-performance protective applications including astronaut space suits, firefighter implements, aircraft wall fabrics, and chemical machinery owing to its superior thermal stability (above 500 ℃) and exceptional mechanical strength at elevated temperature.27 In the field of energy storage and conversion, PBI has attracted intense attention for use as promising electrocatalysts,28-30 proton conductive membranes for fuel cells31 and separators for all vanadium redox flow batteries.32 Very recently, Li et al. reported a modified polybenzimidazole (mPBI) in taming intermediate polysulfides 4 ACS Paragon Plus Environment

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dissolution and the shuttling effect of lithium-sulfur battery due to its excellent mechanical features and chemical interactions with polysulfides.27 The mPBI was used as dual functional agents of binder and separator, which rendered the Li-S battery delivering a stable capacity of 750 mAh g-1 after 500 cycles at 0.2C with a low capacity fading rate of 0.08% per cycle. However, such fascinating polymer pyrolyzed carbon and its properties have not been demonstrated in lithium ion batteries. In this work, polybenzimidazole was used as novel carbon sources for mesoporous silicon microspheres, which was achieved by an aerosol-assisted assembly combined with polymer solution physisorption process. The new polymer derived carbon coating is expected to bring some unique advantageous features: firstly, the surface carbon coating layers from polybenzimidazole avoid direct contact between silicon and electrolyte, and is expected to significantly prevent uncontrolled SEI film formation. Second, the nitrogen containing functional groups could act as novel nitrogen source for carbon doping, where pyrrolic Nenriched carbon significantly creates numerous extrinsic defects and active sites for extra lithium storage, thus enhancing the specific capacity.30 Lastly, the polymer pyrolyzed carbon exhibits high electrical conductivity due to lack of oxygen containing functional groups. To the best of our knowledge, we develop for the first time the promising Si-PBI derived carbon material and its attractive battery performance. The results show that the aerosol-assisted silicon microspheres are encapsulated completely by a conductive pyrrolic nitrogen rich carbon layer. The composite exhibits a high reversible specific capacity up to 2172 mAh g-1 at a current density of 0.2 A g-1 when test as an anode for LIBs. Charge-discharge cycling at different rates shows the electrode to be very stable, delivering high discharge capacity of 1186 mAh g-1 at 5 A g-1 and 65.85% capacity retention for 200 cycles with a CE greater than ~100% at 1 A g-1, suggesting a favorable anode for high-performance LIBs. The schematic illustration for the synthesis of silicon-PBI derived nitrogen doped carbon composite is shown in Figure 1. Mesoporous silica spheres were simply fabricated through an 5 ACS Paragon Plus Environment

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aerosol spray process by using colloidal silica solution as precursors (Scheme S1).33 The SiO2 microspheres were next subjected to magnesiothermic reduction after considering stoichiometric ratio of reactants (weight ratio of silica: Mg = 1:1) following the reaction of SiO2(s)+2Mg(g) = Si(s)+2MgO(s). The mixture of silica and Mg powders was loaded to a stainless-steel reactor and heated at 650 ℃ for 5 h under Ar gas flow. After etching of the MgO byproducts by HCl solution, large amounts of mesopores appeared to form porous silicon microsphere. Meanwhile, PBI solution was firstly obtained by a facile solvothermal method using commercially available PBI in NMP solvent (Figure S1). A PBI polymer is directly coated on mesoporous silicon microsphere through a liquid infiltration-evaporation method. Lastly, Si-PBI derived N-doped carbon composites are obtained after carbonization at 700 ℃ under nitrogen atmosphere. Both theoretical and experimental investigation has disclosed that the chemical doping of carbon using nitrogen could improve the lithium storage performance.34-35 The structural and morphological information of the products was checked by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Highly spherical SiO2 particles with a size ranging from 100 to 2 µm (Figure 2a) were successfully prepared by the continuous aerosol spraying process followed by removal the organic species in an air flow. TEM of SiO2 microspheres (Figure 2a1) confirms its typical porous structure with small primary nanocrystals. The resulting silicon materials have more substantial porosity after Mg reduction (Figure 2b, b1). The numerous pores generated by the removal the MgO byproducts facilitate the infiltration of PBI solution, easy deposition of during coating and accommodating volume changes upon electrochemical cycling. After PBI derived carbon coating, the surface of the particles roughened due to the accumulation of a conformal layer of carbon (Figure 2c). However, its original spherical morphology was well maintained, with negligible size change even annealing at high temperature, which is most likely due to the protection of PBI polymer coating on the surface of the Si spheres, demonstrating its 6 ACS Paragon Plus Environment

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excellent structural stability. Evident core-shell structure can be observed from the lowmagnification TEM image (Figure 2c1), where the carbon shell appeared to be continuous and the thickness was measured to be several tens of nanometers. Furthermore, it appears that highly interconnected sheet-like carbon was also formed to fully cover the whole surface of mesoporous Si particles (Figure 1, 2c1 and Figure S2). To gain a further understanding of the porosity of materials, nitrogen adsorption and desorption measurements were performed (Figure S3). The Brunauer-Emmett-Teller (BET) surface areas of SiO2 and Si-carbon composites were calculated to be 187.5, and 145.8 m2 g-1 with a total pore volume of 0.19 and 0.48 cm3 g-1, respectively. The nearly two times enlargement of pore volume mainly originates from the PBI carbon. X-ray diffraction (XRD) was used to understand the compositional evolution from the SiO2 precursors to resulting products. XRD pattern of silica precursors can be indexed to an amorphous phase (Figure S4, black curve). When the silica reacted with magnesium powder at 650 °C, the intermediate shows clear peaks related to Si (JPCDS 27-1402), MgO (JPCDS No.75-1525), Mg2Si (JPCDS No. 65-0690), and unreacted Mg metal (red curve in Figure S4). By removing the byproducts using acid etching, pure silicon is obtained (green curve). Si-PBI carbon composite exhibits actually identical XRD pattern of pristine Si, suggesting that the main structure of Si remained unchanged and successful conversion from PBI to carbon during the annealing process. After magnesiothermic reduction, the products were soaked in 1 M HCl solution for 12 h to remove the unreacted Mg powder, MgO and Mg2Si byproducts. The unreacted or newly formed SiO2 on silicon surface was not removed by hydrofluoric acid etching. The broad peak in the range of 15-26° (2θ value) observed on the XRD pattern could be contributed to amorphous SiO2 phase, and the peak at around 26° is indexed to the (002) plane of PBI carbon. To gain further insight into the structure and compositional features of nitrogen-enriched carbon coated silicon composites, high-resolution TEM, X-ray photoelectron spectroscopy (XPS), and Raman spectrum are carried out. As shown in Figure 3a, b, the surface of the Si 7 ACS Paragon Plus Environment

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sphere is uniformly covered with a carbon layer with thickness of ~25 nanometers. Thermal gravity analysis profile reveals an obvious mass loss due to the evaporation of moisture and burning out of the carbon in the temperature range of 150 ~ 600 °C, respectively, corresponding a carbon content of 23.1 wt.% in the composite (Figure S5). A closer examination reveals the Si microspheres are highly porous and composed of small primary nanocrystals with sizes of around 10 nm. HRTEM analysis in Figure 3c reveals obvious lattice fringes with an interplanar distance of 0.31 nm, which can be well assigned to the Si (111) plane.36 The selected area diffraction (SAED) pattern further indicates the polycrystalline characteristics of the resultant silicon. The carbon layer show partial graphitic properties as shown in Figure 3c, indicating the high crystalline of PBI derived carbon, which can be attributed to PBI polymer intrinsic properties and high temperature annealing at 700 °C for long duration. The graphitic carbon will provide some unique merits during electrochemical operation of the silicon batteries, e.g., high electric conductivity, stability for accommodating volume change, and combining with strong synergetic effect with silicon mesoporous silicon for fast electronic transport, ultrafast charge and discharge rates.37 X-ray photoelectron spectroscopy (XPS) survey spectra reveal the existence of Si2p peak at 99.5 eV, SiO2 peak at ~103.5 eV and N1s (Figure 3d, e). The SiO2 mainly originate from the surface silica layer and/or unreacted SiO2. The high resolution N1s spectra of the composite can be deconvoluted into three individual peaks at binding energies of 398.5, 400.7, 401.8 eV, corresponding to pyridinic N, pyrrolic N, and graphitic N, respectively. And the entire nitrogen content is measured to be 6.79 atom%. Raman spectroscopy (Figure 3f) of the thusproduced powders exhibits two characteristic peaks of carbon at 1366.4 cm-1 and 1591.1 cm-1, corresponding to a disorder-induced D band and graphitic G band (the ratio of ID/IG = 0.87), respectively. Two peaks centered at 512 cm-1 and 940 cm-1 assign to characteristic peaks of silicon. It is should be noted that the value of 512 cm-1 is lower than that of bulk silicon at 520 cm-1, implying its nano-sized morphology.36 The chemical composition of the composite is 8 ACS Paragon Plus Environment

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further examined by scanning transmission electron microscopy (STEM) image and EDX element mapping. Figure 3g shows that silicon, oxygen, nitrogen, and carbon elements are homogeneously distributed throughout the Si-PBI carbon composites, further demonstrating the homogeneous coverage of silicon by carbon with nitrogen doping in the structure. When most widely used glucose as carbon sources for comparison under similar condition, Si/carbon composite can still be prepared, as the XRD pattern and SEM shown in Figure S6a, b. However, the TEM images (Figure S6c, d) exhibit incomplete and limited carbon coating on silicon surface, demonstrating the benefits arising from the polymer pyrolyzed carbon coating. Limited and unstable carbon shell can cause loose mechanical stiffness and inferior electrochemical stability, which present challenges for widespread application of Si/carbon composites. Intrigued by the unique structure characteristics of the silicon-PBI carbon composites, we investigated its electrochemical properties in CR 2032-coin cells at room temperature. Figure 4a shows the cyclic voltammograms (CV) for the initial ten cycles in the voltage range of 0.01-1.5 V at a scan rate of 0.1 mV s-1. A broad irreversible peak at about 0.4-1.0 V in the first lithiation scan, which disappears in subsequent cycles, could be attributed to the decomposition of electrolyte and the formation of the SEI film. The reduction of ethylene carbonate (EC) and diethyl carbonate (DEC) occurs at around ~0.5 V and ~1.5 V vs. Li/Li+, respectively.38 The sharp peak below 0.1 V during the first lithiation scan corresponds to the phase transition from crystalline Si to amorphous lithium silicide phase (LixSi). In the following delithiation scan, two peaks around 0.34 and 0.51 V are ascribed to the delithiation of LixSi to amorphous silicon. The subsequent CV curves show two anodic peaks at 0.19 and ∼0.02 V, and two cathodic peaks at 0.33 and 0.49 V, which is typical electrochemical behavior of phase transition in amorphous silicon. The peak positions retained consistently in the following scans, indicating good concurrency and reversibility. Furthermore, the


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increasing CV peak intensity is observed during first few cycles, which is a common phenomenon for silicon based anode materials reported previously. Figure 4b exhibits galvanostatic charge-discharge curves of the silicon-PBI carbon composites during the initial five cycles at a current density of 0.2 A g-1 in the potential range of 0.01-1.5 V. The voltage profiles gave the typical electrochemical features of silicon anode. During the first discharge, the curve display a sloping voltage plateau between 1.5 and 0.1 V, and a relatively flat plateau below 0.1 V, which is associated with the formation of SEI layer and the lithiation of crystal silicon, consistent with the CV analysis. The first cycle discharge and charge capacities were 3540 and 2133 mAh g-1, respectively, corresponding an initial Coulombic efficiency of 60.27%. After the first cycle, the Coulombic efficiency value increases to 93.45% at the 2nd cycle, and achieves 97.53% at the 5th cycle. Additionally, the voltage profiles in the following cycles are well maintained, suggesting stable electrochemical reaction of the silicon electrode. The rate capability of the Si-PBI derived carbon composite was evaluated at various current density from 0.1 to 5 A g-1. As presented in Figure 4c, the silicon electrode exhibits reversible capacity of 2300 mAh g-1 at 0.1 A g-1. With increasing current density from 0.2 A g-1 to 2 A g-1, high specific capacity of 1956, 1462 mAh g-1 can be obtained. Even at a high current rate of 5 A g-1, the electrode still delivered a high capacity of 1186 mAh g-1. Importantly, the discharge capacity can be recovered as the rate goes back to 0.2 A g-1, indicating its excellent stability at high rate cycling. In comparison, for the Siglucose derived carbon composite, it exhibits a high discharge capacity of 2055 mAh g−1 at 0.2 A g−1 (Figure 4c). However, the rate capability is inferior, the specific capacity of material decreases dramatically to 722 mAh g-1 as the current rate reaches 5 A g−1. Such enhancement demonstrates that the polybenzimidazole derived pyrrolic N-enriched carbon is favorable for enhancing the electrochemical properties of silicon anode by significantly increasing the electronic conductivity and creating numerous extrinsic defects and active sites (Table 1).30, 39 10 ACS Paragon Plus Environment

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Figure 4d represented the cycling performance of the composite electrode at a current rate of 1 A g-1.

The cell delivers an initial charge/discharge capacity of 1713 mAh g-1,

respectively. A high reversible capacity of 1128 mAh g-1 could be maintained after 200 cycles, corresponding a capacity retention of 65.85% with a Coulombic efficiency of ~100 % during this cycling, which is comparable to many previous reports of Si anode. Furthermore, the long-term cycling of the electrode at higher rate is conducted. As shown is Figure 4e, a reversible capacity remains at 674 mAh g-1 after 500 cycles, suggesting a capacity fading rate of 0.12 % per cycle. Even at 2 A g-1, the material exhibited excellent capacity retention of 35.75% after 500 cycles. The presented electrochemical performance clearly suggests that synergetic effect of nitrogen-doping carbon coating and porous spherical structure leads to significantly improved rate capability. In principle, porous structure with high specific surface facilitates the accessibility of electrolytes, higher electrode/electrolyte contact area.40-41 Moreover, the carbon layer significantly enhances the electronic transport. In addition, the nanometre-sized domains could reduce the Li+ diffusion length.13,

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Bruce et al. has

demonstrated that the diffusion time constant t for electrode reactions in battery increases with the square of particle size: t=L2/D, where L is the diffusion length and D the diffusion constant.44-45 Last, pyrrolic N-enriched carbon could maintain high electrical connectivity for silicon anode on both the particle and electrode level. In contrast, when using glucose as carbon sources, the control sample delivers a moderate initial capacity of 1787 mAh g-1 but with fast capacity decay. The electrode presents only a capacity retention of 29.22% after 190 cycles with a poor capacity of 522 mAh g-1 (Figure 4d). We further offer detailed experimental results and insights into the origin for improved performance and the structural evolution of material after cycling. The cells were disassembled and the electrode was rinsed with dimethyl carbonate solvent in a glovebox. From the SEM images in Figure 5a, the electrode shows compact and flat surface, no cracks and distinct fracture surface are found, indicating that there is good cohesion between the 11 ACS Paragon Plus Environment

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active material and conductive carbon networks. As shown in Figure 5b-d, the silicon-PBI derived carbon composites almost retained their original sphere morphology after 80 cycles at 1 A g-1, indicating that the unique porous structure and polymer pyrolyzed carbon coating could remarkably suppress pulverization of the active material and maintain its structural integrity, confirming its robust properties of carbon layer. Furthermore, the cross-sectional SEM images (Figure S8) results show that the PBI derived carbon effectively accommodate large volume expansion after cycling (only 64.7% change in electrode thickness), reflecting a great consistency in above TEM observation. Electrochemical impedance spectroscopy (EIS) was recorded from 100 kHz to 0.1 Hz at open circuit voltage. The Nyquist plots exhibit a single semicircle in the high-frequency region and a straight line in the low-frequency region associated with the charge transfer resistance (Rct), and mass transfer process, respectively (Figure 6a). The Si-PBI carbon anode has a smaller Rct value than that of Si-glucose carbon composite, indicating improved electrical conductivity of Si-PBI derived carbon. The Nyquist plots are further modelled and fitted. The calculated Rct extracted from the high-frequency range was 270 and 644 Ω for the two electrodes as shown in Figure 6b. Inspired by the attractive lithium storage properties of the silicon anode, from a practical point of view, the full battery performance of the Si-PBI derived carbon composite is further evaluated in a coin-cell configuration. The full cell is established based on as prepared Si/carbon composite anode and commercially available LiCoO2 positive electrode. The LiCoO2 electrode delivers an initial charge-dischage capacity of 140 and 129 mA g-1, respectivley, corresponding to a Coulombic efficiency of ~92 % (Figure S9). A stable reversible capacity of 118 mAh g-1 can be remained after 100 cycles, indicating a capacity retention of 91.4%. As analysis above, the anode exhibits an initial irreversible capacity loss of 39.73%. The initial lithium loss is a critical issue, which significantly reduce the specific capacity, initial Coulombic efficiency and energy density in a practical battery. A simple and effective method is prelithiation, e.g. electrochemical prelithiation or using prelithiation 12 ACS Paragon Plus Environment

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additives to work as a secondary lithium source to compensate the initial lithium loss.46 Herein, the prelithiation is achieved by using the silicon electrode in direct contact with a lithium foil with electrolyte additive. Before assembling the full cells, the silicon carbon composite anodes were firstly electrochemically prelithiated by direct contact with a lithium chip (15.6 mm diameter × 0.25 mm thickness) wetted with electrolyte solution for ca.45 min to eliminate the first irreversible capacity loss. External pressure was applied using two glass slides to induce electrical shorting of the two electrodes.9, 47 The prelithium is simlilar to battery shorting mechanism. As soon as the Si electrode touches Li metal, electrons will pass through to silicon electrode and the silicon begins to be electrochemically lithiated. The full cell was designed to cathode limited (excessive silicon could prevent lithium deposition during the charging),48 and assembled with anode/cathode capacity ratio of ca. 1.2~1.5: 1 using the same electrolyte components and separator as that in half cell test. Figure 6c shows the charge/discharge profiles of the Si-LiCoO2 full battery at a current density of 1C rate with a cutoff potential window of 2.5-4.0 V. The battery shows a practical working voltage in the range of 3.1-3.8 V, and delivers high initial charge/discharge capacities of 114 and 103 mAh g-1 based on the positive material mass, corresponding a Coulombic efficiency of about 90.3%. A discharge capacity of 55 mAh g-1 can still be delivered after 100 cycles with an average CE of ~99% (Figure 6d). The highest discharge capacity is 105 mAh g-1, the Si-LiCoO2 full cell demonstrate a maximum energy density of 367 Wh kg-1 when calculated based on the positive active material if the average working voltage is assumed to be 3.5 V, which is considerably higher than that offered by the state-of-the-art Li-ion batteries (~170 Wh kg-1). In conclusion, we have explored the use of polybenzimidazole derived carbon for silicon anodes, which exhibits the merits of intrinsic high electronic conductivity, rich in pyrrolic nitrogen, flexible in morphology and good mechanical property. Our results show that the aerosol-assisted assembly of silicon microspheres are encapsulated uniformly by a conductive pyrrolic nitrogen rich carbon layer. With these extraordinary characteristics, thus-prepared 13 ACS Paragon Plus Environment

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silicon carbon composites exhibit outstanding lithium storage properties and great potential, including higher capacity, superior rate capability and better cycling stability. Paired with a LiCoO2 cathode, the Si/LiCoO2 full cell delivered high energy density of 367 Wh kg-1 as well as moderate cycling stability for 100 cycles with an extremely high Coulombic efficiency. It is demonstrated that the inherent conductivity, high activity of pyrrolic nitrogen with Li ions, as well as the porous characteristic facilitate high electron/ion transport, volume expansion suppressing, and accelerated interfacial electrochemical reaction are critical in contributing to the remarkably improved battery performance. The rational design and attractive electrochemical properties provided will rapidly accelerate the development of stable silicon anodes with long cycle life for high energy lithium ion batteries.


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Figures and captions

Figure 1. Schematic illustration for the fabrication of silicon composite anodes by using polybenzimidazole (PBI) polymer derived carbon.

Figure 2. SEM and TEM images of as-prepared products at different steps: (a, a1) mesoporous SiO2 precursors synthesized by an aerosol spray method, (b, b1) silicon produced via magnesiothermic reduction, (c, c1) Si-PBI derived carbon composites.


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Si

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Figure 3. Characterization of the PBI derived carbon coating on silicon: TEM images and the corresponding HRTEM images of Si carbon composites (a-c), (d) XPS spectra of Si 2p peaks, (e) XPS analysis of N 1s, (f) Raman spectrum, and (g) STEM image of the composite and corresponding elemental mapping of Si, O, N and C.


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20 500

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Figure 4. Electrochemical performance of Si carbon composites. (a) CV curves at a scan rate of 0.1 mV s-1, (b) first five galvanostatic discharge-charge curves at 200 mA g-1, (c, d) rate capabilities at different current densities from 100 mA g-1 to 5 A g-1 and cycling stability for Si-PBI carbon composite and Si/C composite prepared by using glucose as carbon sources, (e) cycling performance and corresponding Coulombic efficiency for Si-PBI carbon composite at the current densities of 1 A g-1 and 2 A g-1.


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Figure 5. SEM characterization of Si-PBI carbon electrodes after cycling for 100 cycles at 1 A g-1.


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a 1800

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Specific capacity / mAh g

Figure 6. Electrochemical impedance spectra for the Si-carbon composite electrodes prepared by using PBI and glucose as carbon sources, respectively, (a) Nyquist plots in the frequency range of 0.1 Hz-100 kHz with a voltage perturbation of 5 mV before cycling (Inset: schematic of the equivalent circuit), (b) comparison of the charge transfer resistance among the two samples (values obtained from the fitted data), (c) Charge/discharge profiles for a LiCoO2/silicon composite cell at 1C rate with LiCoO2 capacity limited cycled between 2.5 and 4.0 V, (d) Cycling performance of battery at a current density of 1 C.

Table 1. The ratio of various types of nitrogen in silicon-PBI derived carbon composite. Silicon-PBI derived carbon composite Content (at. %) Pyrrolic 59.33 Nitrogen types Pyridinic 27.78 Graphitic 12.89


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ASSOCIATED CONTENT Supporting Information. Supporting Information Available: < Experimental section; Schematic of aerosol spray; photograph of PBI solution, TEM image; N2 adsorption-desorption isotherms for Si-PBI derived carbon composites and SiO2 precursors; XRD patterns; TGA curve; XRD and corresponding electron microscope images of the Si/C composite prepared by using glucose as carbon sources; cross-sectional SEM images of silicon-PBI derived carbon composite electrodes before and after cycling; electrochemical performance of PBI carbon and LiCoO2 cathode.> AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Program on Key Basic Research Project of China (973 Program, no. 2014CB239701), Natural Science Foundation of China (no. 51372116, 51672128), Prospective Joint Research Project of Cooperative Innovation Fund of Jiangsu Province(BY2015003-7), Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). P. Nie acknowledges Funding for Outstanding Doctoral Dissertation in NUAA (no. BCXJ14-12), Funding of Jiangsu Innovation Program for Graduate Education (no. KYLX_0254), and China Scholarship Council (no. 201406830023). Y Wu and R. Fu acknowledges Founding of Graduate Innovation Center in NUAA (no. kfjj20160601, kfjj20170607). P. Nie thanks Prof. Yunfeng Lu, Dr. Haobin Wu, 20 ACS Paragon Plus Environment

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