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Surface-bound Silicon Nanoparticles with a Planar-oriented N-type Polymer for Cycle-Stable Li-ion Battery Anode Jingmin Zhang, Sijia Fan, Hui Wang, Jiangfeng Qian, Hanxi Yang, Xinping Ai, and Jincheng Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 15, 2019

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ACS Applied Materials & Interfaces

Surface-bound Silicon Nanoparticles with a Planar-oriented N-type Polymer for Cycle-Stable Li-ion Battery Anode Jingmin Zhang a, Sijia Fan a, Hui Wang a, Jiangfeng Qian a, Hanxi Yang a, Xinping Aia *，Jincheng Liu b* a

Hubei Key Lab of Electrochemical Power Sources, College of Chemistry &

Molecule,Science, Wuhan University, Wuhan, 430072, China. b

Research institute, EVE battery corporation limited, Huizhou, 516006，China

KEYWORDS: Silicon nanoparticles; Lithium-ion batteries; Mechanochemical synthesis; Si/polyphenylene composite; N-type conductive polymer. ABSTRACT: Silicon is now well-recognized to be a promising alternative anode for advanced lithium-ion batteries (LIBs) because of its highest capacity available today; however, its insufficiently high coulombic efficiency upon cycling remains a major challenge for practical application. To overcome this challenge, we have developed a facile

mechanochemical

method

to

synthesize

a

core-shell

structured

Si/polyphenylene composite (Si/PPP) with n-type conductive PPP layer tightly bonded in planar-orientation to the surfaces of Si nanocores. Because of its compactness and flexibility, the outer PPP layer can protect the Si core from contact with electrolyte and maintain the structural stability of electrode/electrolyte interface 1

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during cycles. As a result, the Si/PPP anode demonstrated a high reversible capacity of ~2387 mAh g-1, a stable cycleability with 88.5% capacity retention over 500 cycles and particularly, a high coulombic efficiency of 99.7% upon extended cycling, offering a new insight for future development of high capacity and cycle-stable Si anode.

Introduction Silicon has been extensively investigated in the past decade as an ideal alternative to carbonaceous anode materials for new-generation lithium-ion batteries (LIBs) because of its extremely high capacity (4200 mAh g−1) and appropriate lithiation potential (~ 0.2 V, vs. Li+/Li).

1-3

However, unlike the carbonaceous anodes with only 10%

expansion during Li+ insertion process, Si anode undergoes a drastic volume change (300%) upon lithiation/delitiation process, which unavoidably causes cracking, pulverization and eventually deactivation of Si particles.

4-6

To address this problem,

many efforts have been devoted to maintain the mechanical integrity of the cycled Si anodes by using downsized,

7, 8nanostructured 9, 10

and alloyed Si particles 11, 12, thus

preventing the pulverization of Si particles and enhancing the cyclability of Si anodes. Nevertheless, despite these materials show considerably improved cycling performance in lithium-half cells as compared with pristine Si particles, their commercial application in practical LIBs is still less successful, mainly due to their insufficiently high coulombic efficiency upon cycling, which is caused by the structure instability of the surface solid electrolyte interphase (SEI) films during 2


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charge/discharge cycles 13. As is well known, the huge volume change of bulk Si phase during lithiation/delithiation processes would cause fierce expansion/shrinkage of the Si surface, leading to destruction/reconstruction of the SEI film. This process is repeated at each cycle and causes a ceaseless consumption of electrolyte and Li+ ions, leading to a low coulombic efficiency. Since in a full battery, the cathode cannot provide a sufficient amount of Li+ ions like the metal Li counter electrode in Li-half cells to compensate for such a consumption, the practical battery with a Si anode frequently exhibits continuous capacity decay with increased cycles. Therefore, building a stable SEI film is a prerequisite for achieving a cycle-stable Si anode. To construct a stable SEI film on Si anodes, various Si/C nanocomposites were designed and fabricated by coating a carbon layer on Si particles to form a core-shell structure 14-16, where carbon coating acts not only as a buffer matrix to accommodate the mechanical stress, but also as a protective barrier to prevent electrolyte into the inner Si core, thus protecting the Si surface from direct contact with the electrolyte and stabilizing the electrode/electrolyte interface. Nevertheless, the coated carbon was usually made by a high temperature pyrolysis of hydrocarbon precursors and appeared as a porous layer that is easy to be infiltrated by electrolyte and fractured by the large volume change during charge/discharge cycles. To overcome this problem, a number of conductive polymers such as polybithiophene (PBT) 17, polyparaphenylene (PPP) 18, polypyrrole (PPy)

19

and polyaniline (PANI)

20

were employed to build a flexible

surface coating that can not only tolerate the volume change but also prevent the 3



penetration of electrolyte into Si surface. Unfortunately, these polymer coatings often consist of disorderly stacked polymer clews and agglomerates, so that the electrolyte can still pass through the polymer layer, resulting in an incomplete shielding of the Si surfaces. Liquid metal (LM) coatings can provide a complete shielding for Si surface and spontaneously repair the structure destruction of Si cores upon cycling due to their good fluidity and self-healing ability, thus significantly enhancing the cycleability of Si anodes.21-23 However, the LM coatings is unfavorable for building a stable electrode/electrolyte interface, due to their constantly changed surface at the impact of Si volume change upon cycling. If a conductive polymer is composed of fully conjugated aromatic rings and can be bonded in planar orientation on the surface of Si nanoparticles, such a core-shell structure of Si/polymer nanoparticles would enable a structural stability and therefore a long term cycling stability of the Si anode, because the impenetrability of aromatic rings can enable the planar-oriented polymer coating to act as a defending armor for preventing the electrolyte from into and contact with Si nanocores. To realize this idea, we developed a facile mechanochemical method to synthesize a core-shell structured Si/polymer nanocomposite with n-type polyphenylene (PPP) planar-oriented on the Si surface (Si/PPP). Benefiting the large plane-conjugated structure and considerable mechanical flexibility, the PPP surface layer can protect the Si core from direct contact with electrolyte and also maintain its integrity during cycles. As a result, the Si/PPP anode demonstrated a high reversible capacity (~2387 mAh g-1 at 1/10C), a stable cycleability (500 cycles with 88.5% capacity retention) 4


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and particularly, a high coulombic efficiency (99.7%) upon cycling, offering a new structural design for future development of high capacity and cycle-stable Si anode.

Experimental Section Materials synthesis：PPP was synthesized in a similar way to Kovacic’s method 24, 25. A detailed synthetic procedure was as follows: 2g anhydrous AlCl3 (Alfa Aesar) and 2.01g CuCl2 (Alfa Aesar) were mixed in a round-bottom flask, then 4.67g benzene was added dropwise into the flask under N2 atmosphere. The reaction mixture was stirred at 0

oC

for 1 h and then at room temperature for 24 h. The

resulting precipitate was filtered and washed several times with 18% hydrochloric acid solution and finally dried at 80 oC under vacuum. The dark red powder product was further heat-treated at 400oC in a muffle furnace for 36h to remove the organic residues and the redundant catalyst. Si/PPP composite was synthesized by sand-milling a mixture of 25.5g silicon powders (325 mesh, Sigma-Aldrich), 4.5g as-prepared PPP polymers and 270g n-hexane (NT-1L, LONGLY Machinery Limited Company). The dispersion was milled for 1.5h at 2000 rpm to crush microsized Si into nanosized Si with fresh surfaces, meanwhile the PPP polymer was exfoliated into thin sheets to wrap the Si cores. The thus-formed Si/PPP nanocomposite were collected by centrifugation, followed by heat treatment at 80 oC under vacuum. For comparison, pristine Si nanoparticles without the PPP coating were also prepared in the same way. Structural Characterization ： X-ray diffraction (XRD) was performed on a Bruker 5



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D8 Advance X-ray diffraction instrument using Cu Kr radiation. The morphologies and structural features were analyzed by scanning-electron microscope (SEM, Zeiss, MERLIN

Compact)

and

transmission

electron

microscopy

(TEM,

JEOL,

JEM-2010-FEF). Thermal gravimetric analyses (TG) of the Si/PPP composite was tested using a TGA Q500 under air. The Si/PPP composite and pristine Si particles were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher, ESCALAB 250Xi). Electron Paramagnetic Resonance (EPR, bruker A200) was used to characterize the unpaired electrons in the composite materials. Electrochemical characterization ： The electrochemical performance of the as-prepared Si/PPP nanocomposite was tested in 2016-type coin-type cells with a glass microfiber membrane as a separator and a lithium disk as a counter electrode assembled in an argon filled glovebox. The working electrodes were made by blade-coating a slurry of 70% wt Si/PPP composite, 20 % wt acetylene carbon and 10% wt PAA binder onto a 9-μm thick copper foil and then drying at 100 oC under vacuum for overnight. The mass loading of Si/PPP composite was 1.7 mg cm2. The electrolyte was 1 M LiPF6 dissolving in a mixture solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (1:1:1, by volume, Shinestar Battery Materials Co., Ltd., China) with 10 vol% Fluoroethylene carbonate (FEC). The PAA binder used in this study was purchased from Alfa Asear with an average molecule weight of 240 000. All the cells were cycled in a voltage interval of 0.01 to 1.5 V versus Li/Li+ on Land Battery Testing System (Wuhan Kingnuo Electronics Co., Ltd., China) at ambient temperature. Except as specifically noted, the 6


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electrodes were first discharged and charged at 100 mA g−1 (0.1C) for the first cycle and then cycled at 500 mA g−1 (0.5C) for the following cycles. Cyclic voltammetry (CV) was performed on an electrochemical workstation (CHI660c, Shanghai, China). The electrochemical impedance measurement was measured on an Impedance Measuring Unit (IM 6e, Zahner).

Results and Discussion The selection of PPP as a surface protection layer on Si particles was based on several considerations: firstly, PPP is n-type redox-active polymer that allows reversible doping/de-doping of Li+ ions into/from PPP chains, [22] thus enabling Li+ ions to pass through. Secondly, PPP has a plane-conjugated structure with a large delocalized π-electron system and therefore has a tendency to lie down in plane-orientation on electron-rich Si surface, leading to a strong bonding interaction between PPP molecules and Si surface. In addition, PPP is a flexible molecule with strong elasticity to accommodate huge mechanical stress during charge/discharge cycles. Despite of these reasonable considerations, it is not certain whether or how a PPP polymer can be chemically bonded to a Si surface. Density functional theory calculations (DFT) provide a theoretical insight into the orientation and bonding of PPP on Si surface (see the Supporting Information for computation details). In the DFT calculation, PPP with two, three and five degrees of polymerization were used as model molecules for their geometry optimization (Figure S1). The DFT result revealed that PPP molecules are preferentially adsorbed in a 7



planar-orientation on the Si surface through their π-electron bonding with the dangling-bond electrons of surface Si atoms, meanwhile producing a surface lattice distortion. Such a chemical bonding enables the PPP polymer to be tightly bounded to the Si surface. As a result, the distance between the surface-bonded Si atoms enlarges 0.249 Å than those between bulk Si atoms (see the sideview in Fig. S1), apparently leading to an enlarged bond length between the surface and the inner Si atoms. This phenomenon suggests a strong bonding between the polymer and the surface Si atoms. With increasing the number of aromatic rings from 2 to 3 and 5, the adsorption energy (Eads) rises from 0.93 eV to 0.97 eV and 1.486 eV (Fig.S1), respectively, indicating that the bonding of PPP molecules to the Si surface becomes stronger with the higher degree of polymerization. In other words, the larger-conjugated PPP molecules would become more stable on the Si surface. In light of the DFT calculations, we tried to realize a core-shell structure of Si/PPP nanoparticles by mechanochemical reaction of PPP molecules with freshly generated surfaces of Si particles. Figure 1 illustrates the fabrication process of the Si/PPP composite. During the sand-milling process, Micro-sized Si particles were continuously crushed into nanoparticles with large amount of fresh surfaces, while the ductile PPP polymer was exfoliated into thin sheets under the collision of grinding balls. Due to the high reactivity of dangling-bond electrons on the surface defects of Si and the strong electron affinity of unsaturated aromatic rings of PPP, a chemical bonding is expected to occur through an electron backdonation from Si surface to the PPP molecules, thus tightly anchoring PPP molecules on the Si surface. This process 8


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proceeds repeatedly to produce PPP-coated Si nanoparticles. To optimize the PPP content, we prepared a series of Si/PPP composites with different Si to PPP mass ratios and compared their electrochemical performance in Li-half cells. As revealed by Figure S4, the Si/PPP composite with a PPP content of 15% exhibits the most stable cycling performance, while too high or too low PPP contents will deteriorate the cycling stability of Si/PPP composites. The possible reason is that the low PPP content can only produce a thin surface protective layer, which is insufficient to prevent the outer electrolyte from penetrating into the inner Si cores, while too thick surface layer produced by the high PPP content will bring about a huge kinetic obstacle for the transporting of Li+ ions into and out of Si cores. For this reason, we selected a mass ratio of Si : PPP = 51 : 9 to synthesize the Si/PPP composite in this work.

Figure 1. Schematic illustration of the fabrication process of the Si/PPP composite

SEM and TEM images provide a clear picture for the core-shelled structure of Si/PPP particles. As shown in Figure S2, the pristine Si is composed of irregularly shaped dense particles. After sand-milling with PPP polymer, these particles were 9



pulverized and became loose. As can be visualized from Figure 2, the Si/PPP composite appear as irregular microparticles aggregated with PPP-coated Si nanoparticles with a diameter of 50 nm to 200 nm. It is worth noting that all the Si nanoparticles are tightly embedded in the PPP matrix, which forms compact, voidless and flexible coating layers. The TEM and HRTEM images in Figure 2c and d clearly show a single Si particle surrounded by a polymer shell of a few nanometers thick. The EDX mapping in Figure S3 suggests the uniform distribution of PPP polymer on the Si nanoparticle surface. In the Si cores, a well-defined lattice fringe of 0.31 nm emerges, corresponding to the d-spacing value of the (111) plane of Si phase.

10


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Figure 2. a, b, Low- and high magnification SEM images of the Si/PPP composite; c, d, TEM and High-resolution TEM images of Si/PPP composite

The surface protection effect of PPP shell on the Si core can be evidenced from the TG behavior of the Si/PPP composite. Derived from the TG data in Figure 3a, the Si/PPP composite contains ~ 15 wt% PPP and ~ 85 wt% Si. As the temperature was scanned to 800oC, the weight increase of pristine Si nanoparticles was only 1%, while the Si/PPP composite gained 5% of its weight, indicating that the PPP polymer can protect the fresh surface of silicon from oxidation. Once the polymer layer was pyrolyzed, the Si surface had to be exposed to the air and then oxidized rapidly at a high temperature of over 400 C. X-ray diffraction (XRD) (Fig. 2b) and X-ray photoelectron spectroscopy (XPS) (Fig. 2c) results further confirm the complete coverage of Si/PPP composite. The pristine Si nanoparticles show sharp and strong diffraction lines in the XRD pattern, while the Si/PPP composite gives greatly decreased XRD signals, suggesting that the Si cores were tightly covered by a compact polymer layer. Except for a broad and weak peak at 2θ=20o, there was no obvious peak in the XRD pattern of PPP, indicating an amorphous state of PPP on the Si surface. XPS analysis provides a further evidence for the core-shelled structure of Si/PPP particles. As shown in Figure 3c, the pristine Si nanoparticles demonstrated a binding energy of electrons at 98.85 eV and a weaker peak at 102.80 eV, corresponding to the elemental Si and its slightly oxidized surface, respectively. Nevertheless, these two features were indiscernible in the XPS spectra of Si/PPP 11



composite, indicating an effective protective layer to shield the surface of Si. Instead, a very weak XPS peak appeared at ~99.30 eV in the Si/PPP composite, which is about 0.45 eV higher than Si 2p binding energy of the pristine Si particles, possibly arising from the PPP-adsorbed Si atoms. Electron paramagnetic resonance (EPR) results in Figure 3d reveals a bonding interaction between the PPP polymers and Si nanoparticles. Since both the silicon nanoparticles and the PPP molecules have no EPR signal, appearance of a strong signal in the Si/PPP composite indicates the presence of unpaired electrons, which are brought about from the dangling bonds on the surface defects of Si due to the lattice distortion. Overall, the morphological and structural characterizations point out a core-shelled structure of the Si/PPP particles with their nanosized Si cores completely and tightly encapsulated in conductive PPP polymer matrix. A key question that arises is how the outer PPP layer can prevent the penetration of electrolyte but selectively allow Li+ ions to pass through itself and arrive at the Si cores. As is well known, PPP is an n-type redox-active polymer that can undergo its redox reaction through a reversible Li+-doping/de-doping mechanism.26,27 To ensure this mechanism, we tested the charge-discharge behaviors of a PPP electrode in 1 M LiPF6/EC-DMC-EMC-10%FEC electrolyte. As given in Figure S6, after a few of initial cycles, the PPP electrode shows a stable reversible capacity of ~ 220 mAh g-1 at low potential region of  0.5 V with a high coulombic efficiency of nearly 99.9%, demonstrating a reversible n-doping behavior of Li+ ions. Driven by the n-doping reaction, Li+ ions have to enter into and move between the PPP chains for charge 12


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balance. From the SEM images in Fig.S7, it can be seen that the PPP polymer has a smooth and dense surface, making it capable of acting as protective layer to prevent the penetration of electrolyte.

Figure 3. (a) TG curves of the pristine nano-Si and the Si/PPP composite under the air atmosphere; (b) XRD patterns of the pristine Si nanoparticles, Si/PPP composite and PPP polymers; (c) Si 2p XPS spectra of the pristine Si and Si/PPP composite; (d) The EPR response of the pristine Si, PPP and Si/PPP composite, respectively

Li+ insertion properties of the Si/PPP anode were investigated by cyclic voltammetry (CV) and galvanostatic charge/discharge. As displayed in Figure 4a, a weak and broad reduction current arises from 1.0 to 0.5V at first cathodic scan and 13



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disappears at the following scans, corresponding to the formation of a SEI film on the Si/PPP composite. At lower potential range of 0.5 V, there appeared two pairs of reduction/oxidation

peaks,

which

are

characteristic

of

stepwise

Li+

insertion/extraction into/from the Si phase. Compared with the CV patterns from pristine nano-Si anode, the initial irreversible capacity from the Si/PPP electrode is much smaller, suggesting that the electrochemical decomposition of electrolyte for the SEI formation is greatly suppressed, due to an effective shielding of the Si surface by the outer PPP layer. The XPS spectra in Figure S5 reveal that the SEI film formed on the surface of Si/PPP composite anode mainly consists of Li2CO3, lithium alkyl oxide (ROLi) and LiF. In

consistence

with

the

CV

features,

the

initial

charge/discharge

(lithiation/delithiation) capacities of the Si/PPP electrode are 2862.0 mAh g-1 and 2429.0 mAh g-1, respectively, corresponding to a high initial coulombic efficiency of 85% (Figure 4 b). This depressed irreversible capacity is exclusively benefited from the protection of the surface PPP layer for the Si cores from direct contact with electrolyte. From the 2nd cycle, the charge/discharge curves remain almost the same during the followed 10 cycles, demonstrating a structural and electrochemical stability of the Si/PPP electrode. Figure 4c shows the rate capability of the Si/PPP electrode at changing current densities. With increasing the current rate to 1000, 2000, and 5000 mA g−1, the Si/PPP electrode can deliver considerably high capacities of 1653, 1286 and 1095 mAh g-1, respectively. Even at an extremely high rate of 10 A g−1, this electrode can still release a reversible capacity of 727 mA h g−1. 14


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When the current density returned to 100 mA g−1, this electrode can almost fully recover its initial reversible capacity ( 2336 mAh g-1)，exhibiting a strong tolerance to the impact of high rates. This excellent rate capability is most likely resulted from the high electronic and Li+ conductivities of the surface PPP layer. The long-term cycle performances of the Si/PPP and pristine nano-Si anodes are tested by cycling the electrodes at 100 mA g−1 for the initial ten cycles and then at 500 mA g−1 for the later cycles. As displayed in Figure 4d, though the pristine nano-Si anode can deliver a high capacity of 2564 mAh g-1 at initial cycles at 500 mA g−1, its reversible capacity decreases rapidly with only a few tens of milliamper-hours available after 200 cycles. In contrast, the Si/PPP electrode deliver a smaller reversible capacity of 1628 mAh g-1 at initial cycles at a current density of 500 mA g−1, but shows a greatly improved cyclability. Even over 500 cycles, the Si/PPP electrode can still remain a reversible capacity of 1438 mAh g-1, corresponding to capacity retention of 88.3%. Particularly, the average coulombic efficiency keeps steadily at 99.7% upon extended cycling, implying a structural integrity of the Si/PPP nanocomposite during prolonged cycles. This structural stability is also revealed by the SEM and TEM images of the Si/PPP particles after fully lithiation (Figure S8). Compared to the unlithiated Si particle, the diameters of lithiated Si particles are significantly enlarged from 50 -200 nm to 100 – 500 nm. The HR-TEM image clearly revealed that, even after undergoing a drastic volume expansion, the Si/PPP composites can still maintain their structural integrity with a PPP layer tightly bonded on the nano-Si surface. This structure stability apparently arises from the excellent 15



flexibility and strong elasticity of PPP polymer layers, enabling them to accommodate huge mechanical stress produced by the volume changes of Si cores during lithiation/delithiation cycles. However, it should be pointed out that the initial capacity of Si/PPP composite anode is lower than that of pristine nano-Si anode, even if we only consider the net weight of Si in composite. This is most likely due to the PPP layer, which imposes a kinetic barrier to the transporting of Li+ ions into and out of Si cores, thus decreasing the utilization of Si in some extent. The electrochemical impedance spectra (EIS) of the Si/PPP electrode at different cycles are shown in Figure S9. As can be seen, the EIS spectra are composed of three parts. The diameter of the two depressed semi-circles at the high frequency region represent the impedance of SEI film (RSEI) and the interface charge transfer impedance (RCT), respectively. The slopping line appearing at the low frequency region corresponds to the diffusion impedance of lithium within the composite particles (Rw). It can be found that both the RSEI and RCT decrease gradually with the increasing of cycle number at the first 50 cycles, reflecting an active process of Si/PPP composite anode. After then, the RSEI and RCT become almost stable, indicating that a complete and stable SEI film and electrode/electrolyte interface have been formed. Different from the RSEI and RCT, the Warburg impedance (Rw) almost keeps unchanged during cycling except for the first cycle, revealing the structural stability of Si cores and integrity of Si/PPP composite during cycling. This result demonstrates that the long-term cyclability and high coulombic efficiency of the Si/PPP anode mainly arise from their stable electrode/electrolyte interfaces. 16


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Figure

4.

(a)

CV

curves

of

the

Si/PPP

electrode

in

1

M

LiPF6/EC-DMC-EMC-10%FEC electrolyte; (b) charge/discharge curves of the Si/PPP electrode at first 10 cycles; (c) Cycling performance of the Si/PPP electrode at changing current rates; (d) a comparison of the cycling stabilities of and coulombic efficiency of pristine Si electrode and the Si/PPP electrodes. All the electrodes were cycled at 100 mA g−1 during the first ten cycles and at 500 mA g−1 during the later cycles

CONCLUSIONS In summary, we have synthesized a core-shell structured Si/polyphenylene composite (Si/PPP) by a facile mechanochemical process, during which the rigid Si microparticles were crushed into the nanoparticles with abundant active surface sites, meanwhile ductile PPP polymer was exfoliated into thin sheets to wrap the Si surface 17



due to chemical bonding between the largelyπ-conjugated aromatic rings of PPP and dangling-bond electrons on the Si surface. Such a core-shell structure can protect the Si cores from contact with electrolyte and also maintain its integrity during cycles, thus eliminating the repeated destruction/reconstruction processes of the SEI films usually observed from the Si/C composites. As a result, the Si/PPP electrode demonstrates a high reversible capacity (~2387 mAh g-1 at 1/10C), a stable cycleability (500 cycles with 88.5% capacity retention) and particularly, a high coulombic efficiency (99.7%), possibly providing a new structural design for development of high capacity and cycle-stable Si anode of Li-ion batteries.

Associated Content Supporting Information Available:: Computation details for the DFT calculations, the

adsorption of PPP with two and three degrees of polymerization, SEM images of the pure PPP polymers, cycling performance and the voltage–capacity curves of the pure PPP anode, and the electrochemical impedance spectra (EIS) of the Si/PPP composite electrode at different cycles. These materials are available free of charge via the Internet at http://pubs.acs.org.

Conflict of Interest The authors declare no competing financial interest

Corresponding Author 18


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* E-mail: [email protected]. Phone: +86-27-68754526.

ACKNOWLEDGMENT

The authors acknowledge the financial support from the National Key Research and Development Program for New Energy Vehicles (No. 2016YFB0100200), and National Science Foundation of China (Nos. 21373155 and 21333007).

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