Surface Modification of Silicon Nanoparticles by an âInkâ Layer for

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Surface Modification of Silicon Nanoparticles by an “Ink” Layer for Advanced Lithium Ion Batteries Fang Wu, Hao Wang, Jiayuan Shi, Zongkai Yan, Shipai Song, Bangheng Peng, Xiaokun Zhang, and Yong Xiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03000 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Applied Materials & Interfaces

Surface Modification of Silicon Nanoparticles by an “Ink” Layer for Advanced Lithium Ion Batteries Fang Wu,a Hao Wang,b Jiayuan Shi,a Zongkai Yan,a Shipai Song,a Bangheng Peng,a Xiaokun Zhang,a Yong xiang*a a

School of Materials and Energy, University of Electronic Science and Technology of China,

Chengdu, 610054, China. E-mail: [email protected] b

University of Chinese Academy of Sciences, Beijing, 100049, China

KEYWORDS silicon, prussian blue analogues (PBAs), nanocomposite, lithium ion batteries (LIBs), anode

ABSTRACT

Due to its high specific capacity, silicon is considered as a promising anode materials for LIBs. While, the synthesis strategies for those previous silicon-based anode materials with delicate hierarchically-structure are complicated or hazardous. Here, prussian blue analogues (PBAs), as widely used in ink, is deposited on the silicon nanoparticles surface (PBAs@Si-450) to modify silicon nanoparticles with a transition metal atoms and N doped carbon layer. A facile and green synthesis procedure of PBAs@Si-450 nanocomposites are precipitated in a coprecipitation process combined with a thermal treatment process at 450 oC. As prepared,

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PBAs@Si-450 delivers a reversible charge capacity of 725.02 mAh g−1 at 0.42 A g−1 after 200 cycles. Moreover, this PBAs@Si-450 composite exhibits an exceptional rate performance of ~ 1203 and 263 mAh g−1 at the current densities of 0.42 and 14 A g−1, and fully recovered to 1136 mAh g-1 with the current density returning to 0.42 A g-1. Such a novel architecture of PBAs@Si450 via a facile fabrication process represents a promising candidate with a high-performance silicon-based anode for LIBs.

1 Introduction Lithium ion batteries (LIBs) have been dominating the energy storage market owing to their high capacity and high energy density with extended cycle life for future electric vehicles and portable electronics.1,

2

However, their performances must be further promoted to satisfy the

increasing demand for high energy density.3, 4 Among the various anode materials, silicon (Si) delivers the highest specific capacity (~4200 mAh g-1 in a form of Li4.4Si and 3600 mAh g-1 in a form of Li3.75Si), which is ten times larger than that of commercial graphite anode.5 Furthermore, Si is abundance in nature and nontoxic, along with a relatively low working potential (~ 0.4 V versus Li/Li+).6 However, it is still in infant stage for commercialization and its development is impeded by three critical issues:7,

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(a) the severe volumetric change (~300%) resulting in

structural fracture and pulverization of electrodes during the insertion and extraction processes of Li+. (b) the incessant decomposition of electrolytes through the aforementioned cracks leading to the formation of a thicker and electrically insulating solid-electrolyte-interphase (SEI) layer. (c) a low electronic conductivity of Si leading to a high potential polarization and poor rate performance. Therefore, the key challenges in Si anode are confining the volumetric change and enhancing the electronic conductivity.


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Designing nanostructures and incorporating active or inactive component are the common strategies to confine the volumetric change and enhance the electronic conductivity of Si. A variety of nanostructured Si or Si-C materials (e.g., nanospheres, nanowires, yolk-shell structured materials) have been researched as anode materials.9-13 Undoubtedly, these nanostructured materials have improved cycling stability and high-rate durability by effectively accommodating large stress and shortening the diffusion length of Li+. For instance, a Si/Carbon yolk-shell composite synthesized by etching SiO2 layer with hydrofluoric acid shows a capacity retention of 83% after cycling 430 at 0.46 A g-1.14 However, synthesis strategies for those welldesigned hierarchically-structured Si-anodes are often elaborate as they often involve complicated treatment like pre-coating or selective etching.15, 16 They inevitably require the use of hazardous chemicals like HF and severe conditions like high temperature.17,

18

Moreover,

those complicated synthesis procedure would add the production costs and diminish the commercial attractiveness of Si materials.19, 20 Furthermore, these carbon-based hosts have poor mechanical stability, as they often tend to fracture so that continued fresh surface of the Si core would be exposed to the electrolyte, which will cause sustained capacity loss after numerous cycles.21 Although the introducing of a protective metal oxide layer (e.g., SiOx, TiO2, TiO2-xFx and Al2O3) can increase the interfacial and structural stability of Si-based composites in the lowpotential range, those coating materials are usually poor electrode kinetics.22-26 Hence, it is of critical importance to design an ideal Si-based composite functionalized with a coating layer that is mixed conducting (ionically and electrically) via a facile and green approach. Recently, metal-organic frameworks (MOFs), as a new class of coordination compounds, have attracted a tremendous amount of interest.27,

28

In particular, prussian blue and its analogues

(PBAs), as a class of MOFs, have been widely used in watercolor, catalyst, and recently studied


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as the possible candidates in energy storage owing to their low cost, easy to synthesize, tunable composition, stable crystal structure for highly reversible insertion and extraction of ions. 29-36 For example, KMFe(CN)6 compounds (M = Mn, Fe, Co, Ni and Zn) synthesized by Goodenough et al. show no capacity losing after 30 cycles as cathodes for SIB.37 Recently, MnFe(CN)6 as the anode for LIBs were synthesized via a simple co-precipitation route and delivered a specific capacity of 295.7 mAh g-1 after 200 cycles.38 However, such PBAs anode is limited in low capacity,39,

40

which inspires us to explore new materials with core-shell Si-based

nanocomposites in LIBs for confining the volumetric change and enhancing the electronic conductivity of Si anode. In this work, we synthesize a novel architecture of PBAs@Si-450 nanocomposites via a facile co-precipitation strategy at room temperature and followed by thermal treatment at 450 oC. The results reveal that PBAs derived shell can accommodate a large volume expansion of Si core by coating a mixed and strengthened conducting layer (ionically and electrically) and protect the electrolyte from incessant decomposition on the Si core. The electrochemical measurements show that the PBAs@Si-450 anodes exhibit an improved cycle stability and a high-rate durability by effective accommodation of large stress and allowing highly reversible intercalation and extraction of Li+ inside their open framework structure. Moreover, our synthesized PBAs@Si-450 nanocomposites have two advantageous features: 1) facile and scalable preparation of composites via co-precipitation strategy, 2) tunable PBAs composition and precursors. Therefore, the combination of PBAs and Si is a better design of anode materials for LIBs system with a facile/scalable synthetic process, high cycling stability and high rate performance. 2. Experimental Section


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2.1 Materials preparation PBAs@Si nanocomposites were precipitated by a facile co-precipitation method using Na4Fe(CN)6•10H2O and MnCl2•1H2O as the ion source at room temperature. First, Na4Fe(CN)6•10H2O (2.5 mmol) and polyvinyl pyrrolidone (PVP, K-30, 3 g) were dissolved in 25 ml deionized water to obtain a homogeneous solution (solution A). The commercial Si nanoparticles (Si NPs, 0.3 g, 30 ~ 50 nm) dispersed in 25 ml ethanol by sonication for 0.5 h were then added to solution A with a stirring speed of 600 rpm, and then adjusted to pH ~3.0 with hydrochloric acid. The mixture A was carried out in air for 2 h. Second, under vigorous stirring (3000 rpm), mixture A was added dropwise using a syringe into solution B, which contains MnCl2•1H2O (1.25 mmol) with pH ~3.0 in the ethanol (25 ml) and deionized water (25 ml) mixture solution. After reaction for another 3 h, the solution was allowed to stand in the dark without any interruption for 12 h, and its color changed from pale yellow to yellow-green, indicating precipitation of Na4Fe(CN)6 and MnCl2 to form the PBAs@Si nanocubes. After 12 h, the resulting yellow-green solution was collected by centrifugation and washed with deionized water several times to obtain PBAs@Si nanocomposites. Finally, under the protection of Ar, the PBAs@Si nanocomposites were heated at two steps to obtain PBAs@Si-450 nanocomposites: 250 oC ( 5 oC min-1 ) for 2 h and subsequent 450 oC (2 oC min-1) for 2 h. For the fabrication of pure PBAs, mixsture A without Si NPs was used. 2.2 Materials characterization The morphologies and structures of the as-synthesized samples were characterized by highresolution field-emission scanning electron microscopy (FESEM) (JEM-2100F, JEOL, Japan) and transmission electron microscopy (TEM) (JEM-2100F, JEOL, Japan). The X-ray


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photoelectron spectroscopy (XPS) measurements (ESCALAB 250XI) equipped with an Al Kα achromatic X-ray source were performed to analyze the surface species and their chemical states. The powder X-ray diffraction (XRD) spectrum was collected on a Bruker D8 Advance diffractometer in steps of 0.02

o

over the 2θ range of 10 ~ 80 o. Thermogravimetric analysis

(TGA) was performed on a TA-Q60 instrument from 40 to 800 oC at 10 oC min-1 in air atmosphere. Brunauere Emmette Teller (BET) was determined by N2 adsorption/desorption using automatic specific surface area measuring equipment (Kubo, X1000). 2.3 Electrochemical measurements All the electrochemical tests were carried out in coin cells (CR 2032) in which a Li foil was used as the counter electrode. The working electrode was obtained by casting the aqueous slurry on copper foil consisted of active materials (pure Si, PBAs@Si, or PBAs@Si-450), super P and sodium carboxymethyl cellulose with a weight ratio of 70: 15: 15. The loading electrode weight was 0.8 ~ 1 mg cm-2. After casting, the slurry was dried at 100 oC overnight in a vacuum oven. Thereafter, the electrode was further dried at 105 oC for 4 h before coin cell assembly. The electrolyte was 1.0 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 in a volume ratio) with 2 wt% vinylene carbonate (VC) as the additive. The cells were assembled in an argon-filled glove box (O2 and H2O levels < 0.1 ppm). The specific capacity was calculated based on the mass of the active materials. Discharge-charge measurements were conducted in the voltage window of 0.01~2 V (vs. Li/Li+) on a NEWWARE CT-4008-5V5mA battery testing system at 25

o

C using the

galvanostatic mode. The electrochemical impedance spectroscopy (EIS) were carried out on PARSTAT 2273 Electrochemical System (Princeton). The impedance spectra was measured


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under an open-circuit state by applying a sine wave with an amplitude of 5 mV over frequency range from 10-2 to 105 Hz to pure Si, PBAs@Si and PBAs@Si-450 electrodes after 1 and 200 cycles. 3 Results and discussion The synthesis procedure of PBAs@Si nanocomposites is precipitated by a facile coprecipitation method using Na4Fe(CN)6 and MnCl2 as the ion sources in the presence of Si NPs suspension at room temperature (Figure 1a). Since its high surface area, Si NPs may easily be oxidized into SiO2 around their external surface, which may be helpful for [Fe(CN)6]4- anions to chemisorb onto the surface of Si NPs via the binding of FeⅡ(CN)5-C≡N-SiⅣ. The FESEM of Si NPs shows uniform spherical shapes and well distributed behavior with diameter of 20 ~ 50 nm (Figure 1b). During the precipitation reaction, the Si NP acts as a nucleation site, the reaction of [Fe(CN)6]4--modified Si with Mn2+ cations results in the formation PBAs on the surface of the Si NPs. Subsequent reaction with [Fe(CN)6]4- results in the further growth of PBAs on the PBAs@Si crystal. During the co-precipitation reaction, the sample color gradually changes from pale yellow to yellow-green (Figure S1, Supporting Information). FESEM image of PBAs@Si nanocomposite suggests that the as-prepared PBAs@Si nanocomposite has a relatively uniform cubic shape with the similar morphology to the pure PBAs nanocube,38 and with a very narrow size distribution and an average size of ~ 250 nm (Figure 1c). Moreover, the TEM image confirms that Si NPs are well encapsulated by the PBAs crystal framework (Figure 1d). However, the Si NPs cannot be encapsulated entirely when the Si NPs are unevenly dispersed or aggregated (Figure S2). After thermal treatment at 450 oC, the obtained PBAs@Si-450 nanocomposites significantly change their morphology into elliptical shape (Figure 1e), meaning


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further thermal fusing occurred apparently on the surfaces of the PBAS@Si nanocubes. It is worth noting that the Si cores (mark by a pale green circle) are surrounded by a dense layer (mark by a blue circle) (Figure 1f). Significantly, this dense layer with a mainly pore size of 9.7 nm is mesoporous (Figure S3) and the Si core is crystalline (Figure 1g).

Figure 1 (a) Scheme for the synthesis of PBAs@Si-450 nanocomposite. (b) FESEM image of pure Si. (c) FESEM and (d) TEM images of the as-synthesized PBAs@Si nanocomposite, respectively. (e) FESEM and (f, g) TEM images of the as-synthesized PBAs@Si-450 nanocomposite. The rough edges of PBAs derived transition metal atoms and N doped carbon


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shell and Si core in (f) are marke by blue and pale green circle, respectively. The measured lattice distance in (g) is 0.32 nm, corresponding to the (111) plane of the pure Si. The core-shell structured PBAs@Si-450 nanocomposite is further proved via TEM mapping (Figure 2). The elemental mapping of Fe, Mn, C, N (Figure 2b-2e) are uniformly dispersed in the outer part of the PBAs@Si-450 composite. Moreover, the Si elemental mapping image shows that Si is distributed within the ligament of the PBAs@Si-450 composite, suggesting that Si core is packed in the framework. Based on the above analyses, the structure of the PBAs@Si-450 nanocomposite can be expressed by a PBAs derived framework (shell) incorporated with Si nanoparticles (core). To further elucidate the possible process of the growth of PBAs-like crystal on the surface of the Si nanoparticle, TEM images of nanoparticles during the precipitation reaction at 0, 3, and 15 h are taken (Figure S4). Uniform spherical nanoparticles with an average diameter of ~30 nm could be observed before precipitation reaction (Figure S4a). When the reaction time is increased to 3 h, most of spherical nanoparticles evolve into nanocubes (Figure S4b), indicating Si NPs can act as nucleation sites for the crystallization of PBAs via the reaction of [Fe(CN)6]4- anions and Mn2+ cations. And those nanocubes show wide range sizes (Figure S4b). After reaction for 15 h, the formed samples grow into monodisperse nanocubes with a larger edges size of ~250 nm (Figure S4c). In addition, it is clearly seen from the TEM images that the Si core is encapsulated by the PBAs layer (Figure S4d). Based on the experimental results, a possible mechanism of the growth of PBAs-like crystal on the Si nanoparticle surface is elucidated, as shown in Figure S4e. There are two stages on the formation of the PBAs-like crystal on the Si nanoparticle surface: (1) in the initial stage, the Si NP acts as a nucleation site, the precipitation reaction of [Fe(CN)6]4-modified Si nanoparticle and Mn2+ cations results in the formation nanocubic PBAs-like on the


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surface of the Si nanoparticle; this stage is “nucleation” process, and nanocubes with various sizes are formed. (2) At a longer reaction time, subsequent reaction with [Fe(CN)6]4- results in further growth of PBAs on the nanocubic PBAs@Si crystals, and those nanocubic crystals tend to stack or merge to form nanocubes with larger sizes; this stage is related to the “further growth and stacking” process, and nanocubes with perfect and uniform structure are formed.

Figure 2 (a) TEM image of as-synthesized PBAs@Si-450 nanocomposite, and elemental mapping of (b) Fe, (c) Mn, (d) C, (e) N, (f) Si, respectively.

The TGA plot of PBAs@Si shows two weight loss (Figure 3a): the first weight loss from room temperature to 200 oC is 7.71 %, which is generally caused by the loss of coordinated water and the crystal water in the product; the second weight from 250 to 350 oC reaches 4.61 %, which can be assigned to the decomposition and carbonization of the metal-organic frameworks. On the base of this TGA data, the PBAs@Si nanocubes are heated at 450 oC to increase their ionic conductivity.41 The crystallinity and phase information of pure Si, pure PBAs, PBAs@Si and PBAs@Si-450 are confirmed by XRD measurements (Figure 3b). For PBAs@Si, sharp diffraction peaks at 2θ = 28.6 o, 47.4 o, and 56.3 o are indexed to the (111), (220), (311), (400)


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and (331) planes of crystal Si (JCPDS card, no. 27-1402), indicating Si NPs encapsulated in PBAs@Si nanocubes maintain their crystalline structure after precipitation treatment. Moreover, the other reflections of PBAs@Si are well assigned to the pure PBAs (ICSD 240909) and no peaks from other phases,38 indicating high purity of PBAs. After thermal treatment at 450 oC, the diffraction peaks in the PBAs@Si-450 are similar to PBAs@Si, while the intensities of the diffraction peaks corresponding to PBAs are clearly weakened, suggesting PBAs shell in the PBAs@Si-450 is partly decomposed or degraded. Moreover, for the peaks assignable to crystal Si in the diffraction patterns, the relative intensity of PBAs@Si-450 is the same to that of the PBAs@Si, reflecting this decomposition of PBAs shell has no influence on the structure of Si core. In addition, a broad and weak peak of C (002) (2θ = 22.6o) is detected, confirming the existence of a carbon layer in the [email protected], 16 Therefore, PBAs@Si-450 nanocomposite is composed of a PBAs derived transition metal (Mn, Fe) atoms and N doped carbon shell and Si core. This formed transition metal atoms and N doped carbon layer is beneficial to prevent agglomeration of the inside Si NPs to protect the inside Si core from fracture, enhance both the ionic and electronic conductivity, making it beneficial for improving the electrochemical performances.42-44


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Figure 3 Structure characterization of the samples. (a) TGA curve of PBAs@Si. (b) XRD spectrums of the pure Si, pure PBAs, PBAs@Si and PBAs@Si-450. The surface compositions and their chemical states of pure Si, PBAs@Si and PBAs@Si-450 are further analyzed via the XPS spectrum ranging from 1350 to 0 eV (Figure 4). Compared with the fully scanned spectra of pure Si, additional peaks for N, Mn, Fe and O elements are existed in the PBAs@Si and PBAs@Si-450 (Figure 4a). Moreover, the intensity of the Si characteristic peaks are sharply decreased. These results suggest that Si NPs are encapsulated by the PBAs layer. For the pure Si (Figure 4b), the peaks at ca. 102.5 eV are related to the silicon oxide, suggesting that there is a silicon oxide layer on the exterior surface of Si nanoparticles.22 This silicon oxide layer is helpful for [Fe(CN)6]4- anions to chemisorb onto the surface of Si NPs. After precipitation reaction with PBAs (Figure 4c), a new and strong peak at ca. 101.8 eV is attributed to the Si-N bond on the surface of PBAs@Si,45 and this peak intensity is significantly increased after thermal treatment. This Si-N bond is coming from the interfacial Si–N–C bonds, and confirms the strong interaction between Si and C. This strong interaction may result in superior electrochemical performance. According to the literature, the obvious shift of Si-Si bond in PBAs@Si-450 compared with the PBAs@Si may be caused by the possible charge transfer, or


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the electrostatic charge effect during the thermal treatment. Specifically, the C1s XPS spectrum (Figure 4d) can be divided into three peaks (C-C (284.6 eV), C=N (285.7 eV), C=O (288.5 eV)), suggesting that the bonding forming of doped nitrogen atoms correspond to sp2-C atoms.46 Moreover, the intensity of the C=N (285.9 eV) shows a considerable increase after the thermal treatment at 450

o

C, thus confirming the effective N doping into the PBAs@Si-450

nanocomposite. In addition, the Mn2p XPS spectrum clearly exhibits that the binding energy peaks of Mn2p3/2 and Mn2p1/2 are centered at 641.6 eV and 653.9 eV (Figure 4e). 38 The Fe 2p spectrum (Figure 4f) shows characteristic peaks for Fe2p3/2 located at 721.4 eV, which is coming from the Fe3+ of PBAs shell.38 Moreover, an additional XPS peak at 708.3 eV can be corresponded to the Fe2p3/2 of Fe2+ in PBAs nanocubes.47 However, the binding energy of Fe2p3/2 at 721.4 eV in the PBAs@Si-450 is decreased, and a significant peak at 708.5 eV is assigned to Fe2p3/2 of Fe2+, indicating that most Fe2+ is converted to Fe3+. The above results confirm the formation of a transition metal (Mn, Fe) atoms and N doped carbon layer on the surface of PBAS@Si-450.


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Figure 4 XPS spectrum of pure Si, PBAs@Si and PBAs@Si-450. (a) Survey spectrum. (b) Si 2p spectrum of Pure Si. (c) Si 2p spectrum, (d) C 1s spectrum, (e) Mn 2p spectrum, (f) Fe 2p spectrum of PBAs@Si and PBAs@Si-450. The electrochemical performances of PBAs@Si and PBAs@Si-450, as well as pure Si and pure PBAs-450 for comparison, are investigated in 2303 coin-type cells. The specific capacities are calculated on the total loading mass of the active material. The galvanostatic discharge/charge curves for the initial first cycle are tested at 0.07 A g-1 in a potential range of 0.01~2 V. As shown in Figure 5a, the first discharge and charge capacities of pure Si, PBAs@Si and

PBAs@Si-450

are

3510.55/3018.38

mAh

g-1,

1899.45/1415.58

mAh

g-1

and

2155.67/1479.25 mAh g-1, respectively. Moreover, the initial coulombic efficiency of pure Si, PBAs@Si and PBAs@Si-450 reaches 85.98 %, 74.53 % and 68.62 %, respectively. Compared with the pure Si electrode, the initial capacities and initial coulombic efficiencies of PBAs@Si and PBAs@Si-450 are decreased, which can be attributed to their reduced Si content and the low


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initial coulombic efficiency of PBAs-450 (53.17 %) (Figure S5a). Compared with the PBAs@Si electrode, the initial coulombic efficiency of PBAs@Si-450 electrode is lower, which can be ascribed to that electrolyte decomposition is much easily to happen on the surface of a PBAs derived transition metal (Mn, Fe) atoms and N doped carbon layer. This irreversible capacity loss can be further confirmed by the SEI peak in the dQ/dV plots (Figure 5b). An apparent oxidation peak at ~0.46 V indicates the occurrence of irreversible reactions, which corresponds to the delithiation of amorphous LixSi alloys to yield crystalline Si and the irreversible formation of a SEI layer.18 Evidently, in the first cycle, there is a higher reduction peak, resulting from the formation of a SEI film and the polarization of the active materials, which can lead to an irreversible capacity loss and a low coulombic efficiency. Compared with pure PBAs-450 (Figure S5b), PBAs@Si-450 shows two distinguished difference reduction peaks at ~0.1 V, ~0.25 V. The peak at ~0.1 V is corresponding to reversible lithiation of Si, and the peak at ~0.25 V is attributed to reversible lithiation of carbon and transition metal atoms in PBAs derived shell.48 Owing to the PBAs shell, the electrolyte is isolated from the Si core, and lithiation of the Si core occurs by lithium diffusion through the PBAs shell, and this can be demonstrated in the form of declining peak intensity below 0.2 V.49 Besides, a broad oxidation peak in PBAs@Si appears over 0.6~0.8 V in the first cycle and disappears in the succeeding two cycles (Figure S6), owing to the formation of a SEI film on the surface of the active materials.50 Moreover, the dQ/dV curves of the subsequent two cycles are basic coincident (Figure S6), indicating a good cycling reversibility of the PBAs@Si and PBAs@Si-450.


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Figure 5 (a) Galvanostatic voltage profiles and (b) dQ/dV curves of pure Si, PBAs@Si, and PBAs@Si-450 for the first cycle at 0.07 A g-1. To evaluate the cycling stability of PBAs@Si and PBAs@Si-450, the electrodes are cycled between 2.0 and 0.01 V at 0.42 A g-1 (Figure 6a). After activation process at 0.07 A g-1 for the first cycle and 0.14 A g-1 for subsequent four cycles, the charge capacities of PBAs@Si and PBAs@Si-450 can achieve 1121.87 and 1191.22 mAh g-1 at the first cycle, and fade to 435.13 and 725.02 mAh g-1 at 200 cycles, respectively. As a result, the charge capacity of pure Si electrode fades fast (Figure S7), and only 1.87 mAh g-1 remain after 150 cycles. Such a poor cycling stability of pure Si is mainly caused by the deactivation of Si and the loss of electrical contact.11 In contrast, the PBAs@Si electrode exhibits an improved capacity retention ( 38.79 %). For the PBAs@Si-450, an enhanced capacity retention of 60.86 % at 200 cycles is obtained. This enhanced cycling stability of PBAs@Si-450 is related to the introduction of a strengthened and mesoporous PBAs derived transition metal atoms and N doped carbon layer (ionically and electrically). Specifically, in the dQ/dV curves of 200 cycles (Figure 6b,c), no significant peak shift is observed, illustrating that the loss mechanism for the PBAs@Si and PBAs@Si-450 is mechanical loss of active material.51 During the same cycles, the PBAs@Si and PBAs@Si-450 electrode continue to exhibit remarkable stability, both with regards to peak intensity and


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position. Interesting, with the cycle proceeding, the characteristic peaks of Si in PBAs@Si and PBAs@Si-450 become sharper, indicating the improvement of the electrode kinetics through an activation process.


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Figure 6 Electrochemical performance of pure Si, PBAs@Si and PBAs@Si-450. (a) Cycling performance comparison and the corresponding coulombic efficiency of pure Si, PBAs@Si and PBAs@Si-450 at 0.42 A g−1. (b, c) dQ/dV curves of PBAs@Si and PBAs@Si-450 at 1st, 50th, 100th, 150th, 200th cycle. (d) Rate performance of pure Si, PBAs@Si and PBAs@Si-450 at


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various current densities ranging from 0.14, to 14 A g−1 and finally back to 0.42 A g−1. (e) EIS plots of pure Si, PBAs@Si and PBAs@Si-450 after 1 cycle and after 200 cycles. Moreover, the PBAs@Si-450 also shows an excellent rate capacity. As shown in Figure 6d, reversible charge capacities of 1450, 1203, 949, 613, 263 mAh g-1 are achieved at current densities of 0.14, 0.42, 1.4, 4.2, 14 A g-1, respectively. Significantly, a charge capacity of 1136 mAh g-1 is recovered when the current density goes back to 0.42 A g-1 and maintained well in the subsequent cycles. This illustrates the robust structural stability of PBAs@Si-450. In addition, the relationship between the SEI film and kinetics of electrode reaction are explored by the electrochemical impedance spectroscopy (EIS) spectrum as shown in Figure 6e. EIS spectrum of all electrodes after 1 and 200 cycles under an open-circuit voltage (2.0 V) are composed of a depressed semicircle in the high-frequency region and a sloped line in the low-frequency region, which are corresponded to the charge-transfer process and the Warburg diffusion process.52 Obviously, the electrolyte resistance and charge transfer resistance of the pure Si electrode is the highest. Although the sizes of the semicircles of PBAs@Si-450 is larger than that of PBAs@Si after 1 cycle, the sizes of the semicircles of PBAs@Si-450 decreases after 200 cycles, indicating the decreased resistance. The reasons for this decreased semicircle may be caused by the cracking of the electrode or active particles, and thus increasing the active sites of electrode reaction. This superior cycling stability and rate capacity of PBAs@Si-450 electrodes are due to the highly conducting and stable PBAs derived transition metal atoms and N doped carbon layers, as demonstrated by the SEM and TEM images. The morphological changes of these electrodes are shown in Figure 7 and Figure S8. Before cycling, both pure Si (Figure S8a, S8d) and PBAs@Si (Figure S8b, S8e) show uniform and compact surface with negligible cracks, Si NPs and PBAs@Si NPs disperse separately and


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evenly without any agglomeration. In comparison, PBAs@Si -450 electrode (Figure S8c, S8f) before cycling shows a relative rough surface with a litter nanoparticle agglomeration. The possoble reason for this agglogated PBAs@Si -450 NPs is PBAs@Si NPs are more easily to cluster during the thermal treatment. After 200 cycling, the pure Si electrode undergoes a serious cracks of surface and dramatic agglomeration of Si NPs (Figure 7a,d), suggesting a serious structural degradation. In contrast, the PBAs@Si electrode shows slight cracks, and the active particle morphology can be maintained (Figure 7b,e). Due to the existance of the PBAs shell, the Si expasion of PBAs@Si can be alleviated to some extent. While, this PBAs shell is fragile and can not endure deep cycling. As a result, slight cracks are appeared after 200 cycles. Significantly, the PBAs@Si-450, does not show great structural changes after 200 cycles, only negligible surface cracks and active particle agglomeration are appeared (Figure 7c). Moreover, the PBAs@Si-450 nanocomposite still maintains the elliptical shapes (Figure 7f), illustrating exceptional structural stability. This exceptional structural stability could be explained by the following reasons: (i) the PBAs derived layer in the PBAs@Si-450 nanocomposite can prevent direct contract between the Si core and the electrolyte, inhibite the successive formation of SEI film on the Si surface. (ii) This PBAs derived shell is composed of transition metal (Mn, Fe) atoms and N doped carbon, benifits to strenghthen the structure integrity upon lithiation and delithiation, and also helps to the formation of a thin SEI film on the PBAs@Si-450 surface, and avoids the irreversible reaction between Si core and the electrolyte. These advantages significantly improve the stable cyclability of PBAs@Si-450 nanoparticles.


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Figure 7 (a, d) SEM images of pure Si electrode, (b, e) PBAs@Si electrode and (c, f) PBAs@Si450 electrode after 200 cycles at 0.42 A g-1. 4 Conclusions In summary, we have developed a green and low-cost two steps approach, i.e. a coprecipitation process and a thermal treatment process, to fabricate a new Si-based nanocomposite with a multifunctional PBAs derived transition metal atoms and N doped carbon layer. This multifunctional layer is ionically and electrically conducting, and beneficial for the formation of stable SEI and the integration of structure. The resulting PBAs@Si-450 nanocomposite exhibits an enhanced cycling stability and rate capability: it shows a reversible charge capacity of 725.02 mAh g-1 after 200 cycles, with 60.68 % retention. This work may open a new opportunity to


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design a promising Si-based nanocomposite with excellent structural stability via a green, lowcost, and facile scalability synthetic route. ASSOCIATED CONTENT Supporting Information Supporting information associated with this article is available free of charge on the ACS Publications website at DOI: Optical images of the reaction solution containing PBAs@Si nanocomposites during coprecipitation reaction, TEM image of PBAs@Si nanocomposite, N2 adsorption/desorption isotherm and BET data of PBAs@Si-450, the schematic growth and TEM images of PBAs@Si nanocomposite during the co-precipitation reaction, galvanostatic voltage profiles and dQ/dV curves of pure PBAs-450 for the first cycle, dQ/dV curves of PBAs@Si, PBAs@Si-450 and pure Si for the initial three cycles, SEM images of pure Si electrode, PBAs@Si electrode and PBAs@Si-450 electrode before cycling.

AUTHOR INFORMATION Corresponding Author *Yong Xiang, [email protected] ORCID Fang Wu: 0000-0002-8165-7332 Yong Xiang: 0000-0002-6667-3473


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (G0501200151472044), Postdoctoral Science Foundation of University (Y02006023601890), the Program for New Century Excellent Talents of China (A1098524023901001067), the Innovation Founding from Interstellar, Shenzhen (H04012001W2015000189) and from Sichuan Lvran Science and Technology Group (H04012001W2015000572). References 1.

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