Silicon Diphosphide: A Si-Based ThreeDimensional Crystalline Framework as a HighPerformance Li-Ion Battery Anode Hyuk-Tae Kwon,†,§ Churl Kyoung Lee,† Ki-Joon Jeon,*,‡ and Cheol-Min Park*,† †
School of Materials Science and Engineering, Kumoh National Institute of Technology, 61 Daehak-ro, Gumi, Gyeongbuk 39177, Republic of Korea ‡ Department of Environmental Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon 22212, Republic of Korea S Supporting Information *
ABSTRACT: The development of an electrode material for rechargeable Li-ion batteries (LIBs) and the understanding of its reaction mechanism play key roles in enhancing the electrochemical characteristics of LIBs for use in various portable electronics and electric vehicles. Here, we report a three-dimensional (3D) crystalline-framework-structured silicon diphosphide (SiP2) and its interesting electrochemical behaviors for superior LIBs. During Li insertion in the SiP2, a threestep electrochemical reaction mechanism, sequentially comprised of a topotactic transition (0.55−2 V), an amorphization (0.25−2 V), and a conversion (0−2 V), was thoroughly analyzed. On the basis of the three-step electrochemical reaction mechanism, excellent electrochemical properties, such as high initial capacities, high initial Coulombic efficiencies, stable cycle behaviors, and fast-rate capabilities, were attained from the preparation of a nanostructured SiP2/C composite. This 3D crystalline-framework-structured SiP2 compound will be a promising alternative anode material in the realization and mass production of excellent, rechargeable LIBs. KEYWORDS: lithium-ion batteries, anode materials, silicon phosphide, silicon-based compounds, phosphorus-based compounds benignity, and appropriately low operating voltage.8−19 However, Si has some disadvantages, such as a low electronic conductivity, slow Li-ion diffusion rate, and huge volume change (>300%) during Li insertion. The low electronic conductivity and slow Li-ion diffusion rate lead to a slow rate capability, and the huge volume expansion contributes to the pulverization of the active material, resulting in the loss of electrical contacts with the current collector and the deterioration of the cell’s performance. Hence, to overcome such disadvantages of Si-based electrodes, various solutions have been examined. First, syntheses of Si−X compound systems (X = Li-inactive/active element) have been suggested.
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echargeable Li-ion batteries (LIBs) have been considered for use in various portable electronics and electric vehicles due to their large energy densities and high operating voltages.1−5 To date, in conventional, rechargeableLIB systems, graphite (372 mAh g−1) and LiCoO2 (ca. 140 mAh g−1) have been used as anode and cathode materials, respectively. However, to meet the demands of the rapidly increasing market for portable electronics and electric vehicles, various higher-capacity alternatives are being actively researched. Therefore, Li alloys such as Li−Si, Li−Sn, Li−P, and Li−Sb have been proposed as possible anode materials for high-performance LIBs because they react reversibly while possessing large Li ratios.1−7 Silicon has been considered a promising candidate for nextgeneration high-capacity anode materials for rechargeable LIBs because of its high theoretical capacity of 3578 mAh g−1 (Li15Si4) at room temperature, Earth abundance, environmental © 2016 American Chemical Society
Received: April 24, 2016 Accepted: May 31, 2016 Published: May 31, 2016 5701
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Figure 1. Synthesis and electrochemical performance of SiP2. (a) Crystalline structure of the 3D framework structured SiP2 combining cubic Si with orthorhombic P. (b) XRD result of the SiP2. (c) Electrochemical performance of the SiP2 electrode at a current density of 100 mA g−1.
phases has been reported.35 Souza et al. also demonstrated a quasi-topotactic intercalation mechanism between the binary MnP4 phase and the cubic, ternary Li7MnP4 phase.36 Although these electrodes showed long cycle behaviors thanks to their electrochemical reaction mechanism, they also showed limited capacities. Therefore, phosphorus-based compounds having high Li accommodations have been rigorously pursued. In this article, circumventing the demerits of Si and P when they are used solely as electrode materials, we develop a synthetic method to combine Si with P, successfully synthesize a 3D crystalline-framework-structured silicon diphosphide (SiP2) compound, and test its suitability as an anode material for high-performance, rechargeable LIBs. Furthermore, an interesting three-step reaction mechanism between SiP2 and Li is demonstrated on the basis of ex situ X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR), and high-resolution transmission electron microscopy (HRTEM). Additionally, to overcome the problems faced when SiP2 is used as an anode material, we prepare a simple nanostructured SiP2/ C composite and test its suitability for use in high-performance LIBs.
Si−X compounds employing Li-inactive metals include CoSi2, FeSi2, NiSi2, etc.,20−24 while Si−X compounds employing Liactive metals include Mg2Si, CaSi2, SiO, and SiO2.25−30 These compounds have shown better electrochemical performances than Si alone due to inactive matrix roles or enhanced electrical conductivities. However, the former have demonstrated a drawback of low reversible capacities, caused by large overpotentials used to break the Si−X bonds, while the latter have shown poor initial Coulombic efficiencies of the irreversible phases, such as Li2O, Li−Si−O, etc. As another feasible method, nanoarchitecturing of Si-based materials, such as crystallite-size reduction, carbon coating, thin-film technique, porous structures, nanotube/nanowire arrays, etc., has been recommended.8−19 Although materials formed by these approaches exhibit improved electrochemical performances compared to pure-Si electrode, the complex synthetic procedures and high fabrication costs prevent their commercialization. Recently, black phosphorus and its compounds have been utilized as anode materials for rechargeable LIBs.31−36 These materials have shown interesting electrochemical behaviors related to their two-dimensional (2D) and three-dimensional (3D) crystalline structures. In the case of black P, owing to the puckered, layered, 2D crystalline structure, our group demonstrated that a quasi-intercalation reaction is possible between the black-P and LiP phases.32 Additionally, a topotactic reaction mechanism between the VP and LiVP
RESULTS AND DISCUSSION Si and P can be alloyed into stoichiometric silicon phosphide compounds, such as SiP, SiP2, and Si6P2.54, which are all known as high-pressure phases except for SiP.37−40 Among them, SiP2 has a cubic crystalline structure (Pa3̅, a = 5.681 Å) with a 3D 5702
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Figure 2. Electrochemical reaction mechanism of the SiP2 electrode. (a) DCP result of the first cycle. (b) Ex situ XRD results during the first cycle. (c) 29Si NMR spectrum of the first cycle. (d) HRTEM image combined with SAED patterns at 0.55 V during initial discharge. (e) HRTEM image combined with SAED patterns at 0.25 V during initial discharge. (f) HRTEM image combined with SAED patterns at 2 V of initial full charge state. (g) Schematic representation of the electrochemical reaction mechanism between SiP2 and Li.
and P into their high-pressure-compound phase SiP2 at ambient temperature and pressure. Figure 1b shows the XRD pattern of the SiP2 phase, which exactly corresponds to cubic SiP2 (JCPDS no. 77-1191). HRTEM images combined with selected-area electron diffraction (SAED) patterns confirmed well-developed, crystalline SiP2 particles consisting of agglomerated ca. 20−30 nm nanocrystallites (Figure S1). To evaluate the electrochemical performances, voltage profiles of Si, blackP, and SiP2 electrodes are compared in Figures S2a, S2b, and 1c, respectively. Although Si and black-P electrodes showed high initial discharge and charge capacities, respectively, they
framework for facile Li diffusion and accommodation, as shown in Figure 1a. SiP2 with 6-fold coordination of Si favors a Zintllike description. Electron transfer from less electronegative Si atoms to more electronegative P atoms results in Si4+ and [P2]4− ions.41,42 This interesting 3D framework structure was attained by combining cubic Si with the puckered-layerstructured orthorhombic P. Bearing in mind the crystalline structure of SiP2, high-energy ball milling (HEBM) was applied to synthesize SiP2 because the pressure and temperature generated in the vial during HEBM could rise to 6 GPa and 200 °C,43 respectively, conditions that are sufficient to transform Si 5703
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Figure 3. Preparation and morphological characteristics of the nanostructured SiP2/C composite. (a) TEM bright-field image. (b) HRTEM image. (c) SAED patterns. (d) FT patterns corresponding to the selected regions in the HRTEM image. (e) STEM image and its corresponding EDS mapping images. (f) Schematic representation of the preparation of the nanostructured SiP2/C composite.
To determine the electrochemical reaction mechanism between SiP2 and Li, ex situ XRD, solid-state NMR, and HRTEM analyses were performed at the selected potentials indicated in the DCP, and the results are presented in Figure 2b, c, and d−f, respectively. When the potential was lowered from the opencircuit potential to 0.55 V (corresponding to ca. 2 mol of Li per mole of SiP2), the XRD peaks of the SiP2 phase were amorphized (t1 in Figure 2b). Therefore, to obtain the structural changes of the lithiated SiP2 electrode, solid-state 29 Si NMR was analyzed (Figure 2c). The 29Si NMR results of SiP2 (t0 in Figure 2c) showed two peaks comprised of a large peak at 16.5 ppm, corresponding to the SiP2 phase, and a small impurity SiO2 peak at −114 ppm.44 The 29Si NMR peak of SiP2 (16.5 ppm) was slightly shifted to the right (2.1 ppm, t1 in Figure 2c) at 0.55 V. Additionally, HRTEM results, combined with SAED, also showed that the d-spacings of (2 0 0), (2 1 0), (2 1 1), and (2 2 0) were slightly increased (Figure 2d). The 29 Si NMR and HRTEM results demonstrated that Li was inserted into the 3D-framework-structured SiP2 with very little structural variation at 0.55 V. Considered the capacity for solid
demonstrated very poor cycling durability. The poor cycling performances of the Si and black-P electrodes were caused by the large volume expansion and contraction that occurred during the formation of the Li−Si or Li−P alloying phases, respectively. In Figure 1c, the first discharge and charge capacities of the SiP2 electrode were 2812 and 2044 mAh g−1, respectively, with a Coulombic efficiency of 72.7%. Considering the theoretical capacity (2902 mAh g−1, calculated on the basis of the final phases of Li15Si4 and Li3P) of SiP2 at room temperature, we could conclude that the SiP2 significantly reacted with Li. However, the SiP2 electrode also exhibited poor capacity retention, corresponding to approximately 22.8% of the initial charge capacity after the 10th cycle. The poor capacity retention of the SiP2 electrode might have been caused by the large volume change through the formation of the Li−Si and Li−P alloying phases associated with the pulverization of the active material and its electrical isolation from the current collector. The differential capacity plot (DCP) of the first cycle of the SiP2 electrode is shown in Figure 2a, with three and two large peaks during the discharge and charge reactions, respectively. 5704
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Figure 4. Electrochemical performance of the nanostructured SiP2/C composite electrode. (a) Voltage profile within the potential range of the conversion step (0−2 V) at a current density of 100 mA g−1. (b) Voltage profile within the potential range of the amorphization step (0.25−2 V) at a current density of 100 mA g−1. (c) Voltage profile within the potential range of the topotactic-transition step (0.55−2 V) at a current density of 100 mA g−1. (d) Cycling performances of the SiP2/C nanocomposite electrode at various potential ranges and MCMB graphite electrode at a cycling rate of 100 mA g−1 (ICE = initial Coulombic efficiency). (e) Plot of the Coulombic efficiency vs cycle number at the various potential ranges. (f) Plot of the discharge and charge capacities vs cycle number at various C rates (amorphization step, 1 C, 1100 mAh g−1; topotactic-transition step, 1 C, 500 mAh g−1).
that of the Li-extraction reaction from LixSi, the process demonstrated that the Li13Si4 was transformed to Si. In the fully charged state at 2 V (t5 in Figure 2a), although XRD results indicated an amorphous phase (t5 in Figure 2b), 29Si NMR (t5 in Figure 2c), and HRTEM (Figure 2f) results definitely showed the reappearance of the SiP2 phase, demonstrating that the SiP2 phase was recombined during the charge step. This recombination reaction phenomenon was quite interesting and similar to that observed with nanosized transition-metal oxides, ZnP2, Cu6Sn5, SnSb, etc.47−49 Based on the ex situ XRD, solidstate 29Si and 7Li NMR, and HRTEM results, the first cycle of the reaction mechanism of the SiP2 electrode consisted of the following steps. During the first cycle,
electrolyte interphase formation around 0.8 V, the corresponding capacity was approximately 530 mAh g−1, corresponding to approximately 1.8 mol of Li and meaning that SiP2 was topotactically transformed to LixSiP2 (x ≤ 1.8). At a further discharged state of 0.25 V (t2 in Figure 2a), the LixSiP2 (x ≤ 1.8) was transformed to an amorphous structure on the basis of the XRD (t2 in Figure 2b), and 29Si NMR (t2 in Figure 2c) results. Additionally, HRTEM results (Figure 2e) also showed an amorphous structure possessing blurred, concentric rings. When the potential was fully discharged at 0 V (t3 in Figure 2a), the XRD pattern confirmed the presence of only the Li3P (JCPDS no. 74-1160) phase (t3 in Figure 2b). Although the 29 Si NMR spectrum showed no distinctive characteristics, that of the 7Li NMR (t3 in Figure S3) definitely showed the formation of Li13Si4 (11.5 ppm),45 even if 3.75 mol of Li reacted with Si, forming the Li15Si4 phase at room temperature,10 which may be originated from the overpotential owing to the poor conductivity of SiP2. Considering the first discharge capacity of 2812 mAh g−1 of the SiP2 electrode, capacity loss for the solid electrolyte interphase layer formation reaction,46 and its theoretical capacity of 2753 mAh g−1 (calculated on the basis of the final phases of Li13Si4 and Li3P), we could conclude that the SiP2 was fully converted to its final discharged phases comprised of Li3P and Li13Si4. In short, during the discharge reaction, the SiP2 electrode showed an interesting three-step electrochemical reaction comprised of a topotactic transition, amorphization, and conversion in sequence. On the other hand, during the charge reaction, when the potential reached 0.4 V (t4 in Figure 2a), the Li3P phase still remained. Considering that the potential region of the DCP peak was well matched with
Discharge: SiP2 → LixSiP2 (x ≤ 1.8) → amorphous Li‐Si‐P → Li13Si4 + Li3P Charge:
Li13Si4 + Li3P → Si + Li3P → SiP2
SiP2 has the following distinctive advantages compared with Si or Si-based materials. First, the Li-reacted products of Li13Si4 and Li3P are reversible phases, thus contributed to high capacity, which is contrary to irreversible Li2O formation in SiO or SiO2 electrodes. Second, the 3D crystalline framework structure of SiP2 serves as a Li intercalation framework, followed by alloying and dealloying to circumvent the demerits of Si and P. Third, SiP2 electrode has a relatively wide reaction 5705
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ACS Nano potential of 0−0.7 V (vs Li+/Li), as shown in the voltage profile. Therefore, voltage control is possible along with control of crystalline structure of SiP2 during Li insertion, which is contrary to Si electrode, which has a very narrow reaction potential of 0−0.2 V (vs Li+/Li). To further enhance the electrochemical performance of the SiP2 electrode, we produced a SiP2/C nanocomposite using the HEBM method. Similar to the cases of a nanostructured S/C composite cathode in a Li/S battery system and an amorphized Sn−Co−C composite anode in commercial Nexelion batteries, the nanocomposites were also prepared using HEBM because this method produces evenly distributed metal or alloy nanocrystallites within an amorphous carbon matrix.50−52 As shown in Figure S4, the XRD pattern of the SiP2/C nanocomposite confirmed that no other crystalline phases were present. Furthermore, HRTEM images combined with SAED and Fourier transform (FT) patterns confirmed the presence of approximately 10−15 nm SiP2 nanocrystallites within the amorphous carbon matrix (Figure 3a−d). In addition, the scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS) mapping images showed that SiP2 nanocrystallites were dispersed uniformly within the amorphous carbon matrix (Figure 3e). The simple one-pot method for preparation of the SiP2/C nanocomposite is schematically illustrated in Figure 3f. To evaluate of the SiP2/C nanocomposite as an anode material for rechargeable LIBs, we tested the electrochemical properties of the nanocomposite, and the results are shown in Figure 4. The voltage profiles of the SiP2/C nanocomposite electrode, according to the various potential ranges, are shown in Figure 4a−c. In Figure 4a, the SiP2/C nanocomposite electrode showed high discharge and charge capacities of 1999 and 1661 mAh g−1, with a high Coulombic efficiency of approximately 83% within a potential range between 0 and 2 V. Considering the first irreversible capacity of the ball-milled carbon (Super P, 40 wt %, 142 mAh g−1) within the SiP2/C nanocomposite electrode (Figure S5) and the capacity loss that occurred via a subreaction between the electrolyte and the electrode surface, the SiP2 within the composite was highly reversible with regard to Li. The high Li reversibility of the SiP2/C nanocomposite electrode resulted from the enhanced electrical conductivity due to the uniform distribution of 10−15 nm SiP2 nanocrystallites within the conducting amorphous carbon buffering matrix. Although the SiP2/C nanocomposite electrode demonstrated high reversibility, the capacity decreased to less than 738 mAh g−1 after 100 cycles due to mechanical cracking and crumbling caused by the large volume change originating from the conversion reaction that formed the Li3P and Li13Si4 phases, as illustrated in Figure 2g. The DCP result of the SiP2/C nanocomposite electrode coincided well with that of SiP2 electrode (Figure S6), which demonstrates that the SiP2/C nanocomposite electrode also has the three-step electrochemical reaction mechanism. Therefore, bearing in mind the three-step electrochemical reaction mechanism of the SiP2 electrode, the SiP2/C nanocomposite electrode was cycled at each step, e.g., the amorphization step (0.25−2 V) and the topotactic-transition step (0.55−2 V), while controlling the potential ranges, as shown in Figure 4b,c. This revealed high initial discharge and charge capacities of 1317 and 1137 mAh g−1, respectively, with a high initial Coulombic efficiency of approximately 86% (amorphization step, 0.25−2 V). Additionally, the cycle behavior was greatly enhanced, showing high charge capacity of 980 mAh g−1 after
the 100th cycle with an excellent capacity retention of 86.2% of the initial charge capacity (Figure 4b,d). The enhanced cycle behavior of this potential range might be attributable to the amorphous Li−Si−P formation. To utilize the topotactictransition step, the SiP2/C nanocomposite electrode was cycled in a potential range between 0.55 and 2 V, demonstrating excellent electrochemical performances, as shown in Figure 4c. The first discharge and charge capacities were 625 and 517 mAh g−1, respectively, with a high initial Coulombic efficiency of approximately 83% at a current density of 100 mA g−1. Additionally, the nanocomposite electrode showed an excellent capacity retention of 98.8% of the initial charge capacity after the 100th cycle (Figure 4d). Furthermore, the average Coulombic efficiency per cycle was above 99.9%, except for several of the initial cycles (Figure 4e). We attribute these excellent electrochemical performance results to the highly reversible topotactic-transition reaction between SiP2 and LixSiP2 (x ≤ 1.8). Rate capability tests of the SiP2/C nanocomposite electrode were also performed within the potential ranges of the amorphization and topotactic-transition steps. Figure 4f shows the cyclability of the SiP2/C nanocomposite electrode as a function of the C rate, where C is defined as the full use of the restricted charge capacity of 1100 mAh g−1 (amorphization step, 0.25−2 V) and 500 mAh g−1 (topotactic-transition step, 0.25−2 V) in 1 h. In the case of the potential range of the amorphization step, at rates of 1C and 3C, the SiP2/C nanocomposite electrode showed very high charge capacities of 990 and 820 mAh g−1, respectively, corresponding to approximately 88% and 73% of the charge capacity at a rate of 0.1C with stable cycling behavior (Figures 4f and S7a). Additionally, in the case of the potential range of the topotactictransition step, the SiP2/C nanocomposite electrode also showed very stable charge capacities of 455 (1C rate) and 395 mAh g−1 (3C rate), corresponding to approximately 93% and 81%, respectively, of the charge capacity at a rate of 0.1C with stable cycling behavior (Figures 4f and S7b). We ascribed the excellent rate capability of the SiP2/C nanocomposite electrode to the presence of the approximately 10−15 nm nanocrystalline SiP2 within the amorphous carbon matrix that in turn contributed to short Li-diffusion paths. Additionally, the interesting structural variations, such as the amorphization and topotactic-transition reactions, also contributed to the fast rate capability. Considering its excellent electrochemical properties, such as its high initial capacity, high initial Coulombic efficiency, long cyclability, and fast rate capability, this SiP2/C nanocomposite electrode could be significantly utilized as anodes in rechargeable LIBs. It is noteworthy that the SiP2/C nanocomposite electrodes were prepared using a conventional binder (PVDF) and electrolyte (1 M LiPF6 in ethylene carbonate (EC)/diethylcarbonate (DEC)) with no additive for a simple comparison with other Si-based electrodes. Recently, various binders and electrolytes containing various additives have been suggested as alternative solutions for high performance Si-based anodes.53−56 Therefore, if the binders and electrolytes developed recently are applied to SiP2 anode, we believe that its electrochemical performances will be much better.
CONCLUSIONS By introducing a 3D crystalline-framework-structured SiP2 compound material into a LIB system, we successfully developed a high-performance Si-based compound anode. 5706
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containing an anhydrous ethyl alcohol solution. After ultrasonic treatments, droplets of the anhydrous ethyl alcohol solution containing the dispersed lithiated and delithiated SiP2 particles were dropped onto a carbon-coated TEM grid and dried. For the solid-state NMR, all spectra were obtained using a 400 MHz solid-state NMR at KBSI Daegu center in Korea, operating at 79.488 MHz for 29Si and 155.5 MHz for 7Li. All XRD, TEM, and NMR sample-preparation processes were conducted in a glovebox under an Ar atmosphere. Electrochemical Measurement. For the electrochemical evaluation of the SiP2 and the SiP2/C nanocomposite, the electrodes were prepared via coating with a slurry, consisting of the active powdered material (70 wt %), carbon black (15 wt %; Denka) as a conducting agent, and polyvinylidene fluoride (PVDF, 15 wt %) dissolved in Nmethyl-2-pyrrolidone as a binder, onto the copper-foil substrates. Samples of each mixture were vacuum-dried at 120 °C for 3 h and pressed (electrode; thickness: ca. 0.045 mm, area: 0.79 cm2, weight of active material: ca. 2.5 mg). Coin-type electrochemical cells were assembled in an Ar-filled glovebox using Celgard 2400 as the separator, Li foil as the counter and reference electrodes, and 1 M LiPF6 in EC/ DEC (1:1 by volume, Panax STARLYTE) as the electrolytes. All the cells were tested galvanostatically between 0 and 2 V (vs Li+/Li) at a current density of 100 mA g−1 using a Maccor automated tester, except for the rate capability tests. Discharge was defined as Li insertion reaction into a working electrode, whereas charge was defined as Li extraction reaction from the working electrode. The gravimetric capacity was calculated from the weight of the active materials, and the volumetric capacity can be calculated by multiplying the gravimetric capacity by the tap density (SiP2, 1.3 g cm−3; SiP2/C nanocomposite, 1.15 g cm−3), which was measured using a powder tap density tester (BT-301, Bettersize).
Specifically, we propose a facile synthesis method of a highpressure SiP2 compound by combining Si with P powders, using an HEBM technique at ambient temperature and pressure. Various analytical tools were applied to reveal the reaction mechanism between the SiP2 electrode and Li, and a three-step electrochemical reaction mechanism sequentially comprised of a topotactic transition, amorphization, and conversion was clearly demonstrated. Additionally, a nanostructured SiP2/C composite was prepared and evaluated for its suitability for use as an anode in rechargeable LIBs. When the SiP2/C nanocomposite electrode was electrochemically tested within the potential range corresponding to the reaction of conversion step (0−2 V), it possessed a highly reversible capacity of 1661 mAh g−1, while it demonstrated poor cycle behaviors. However, when it was cycled within the potential ranges corresponding to the topotactic-transition (0.55−2 V) and amorphization (0.25−2 V) steps, it showed greatly enhanced cycle behaviors. Particularly, when the SiP2/C nanocomposite electrode was cycled within the topotactictransition step, it demonstrated an excellent capacity retention of 98.8% of the initial charge capacity after the 100th cycle with high Coulombic efficiency per cycle and a fast rate capability, which were ascribed to the topotactic transition from SiP2 to LixSiP2 (x ≤ 1.8) with very little structural variation. In summary, a 3D crystalline-framework-structured SiP2 and its nanostructured composite were successfully synthesized, and when they were applied as anode materials for rechargeable LIBs, a distinctive reaction mechanism, comprised of topotactic transition, amorphization, and conversion steps, was demonstrated. The nanostructured SiP2/C composite electrodes, when tested within each step by controlling the potential ranges, showed excellent electrochemical performances, proving that they could be easily used for applications requiring highperformance anodes. We anticipate that this nanostructured SiP2/C composite will be a promising alternative anode material for high-performance LIB systems.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02727. Additional HRTEM image of SiP2; voltage profiles of black P and Si electrodes; 7Li NMR of SiP2 electrode during Li reactions; and XRD, DCP, and voltage profiles at various C-rates of SiP2/C nanocomposites, including Figures S1−S7 (PDF)
METHODS Sample Preparation. SiP2 was synthesized via the following solidstate synthetic route. Stoichiometric amounts of Si (99%, average size: ca. 100 μm; Sigma-Aldrich) and P (99%, average size: ca. 75 μm; Kojundo Chemical) powders and stainless steel balls (with diameters of 3/8 and 3/16 in.) were placed in a hardened-steel vial with a of capacity 80 cm3 at a ball-to-powder weight ratio of 20:1. The vial was assembled in an Ar-filled glovebox, and an HEBM process (8000 M Mixer/Mill; SPEX SamplePrep) was conducted for 20 h. To obtain the SiP2/C composite, the same HEBM process was carried out using the synthesized SiP2 powder and carbon (Super P; Timcal) for 6 h. The preliminary electrochemical tests showed that in terms of the electrochemical performance, including such factors as initial capacity, initial Coulombic efficiency, and cycle performance, the optimal amounts of SiP2 and C were 60% and 40% by weight, respectively. The SiP2 and SiP2/C composite samples have been handled in dry condition owing to their hygroscopic properties. Material Characterization. The SiP2 and its nanostructured SiP2/ C composite samples were characterized using XRD (DMAX2500-PC; Rigaku), HRTEM (FEI F20; Tecnai, operating at 200 kV), EDS in conjunction with HRTEM, and solid-state NMR (Avance II+; Bruker). Ex situ XRD analyses were used to observe the structural changes occurring in the active material of the SiP2 electrode during Li insertion/extraction. The electrodes were detached from the cell, washed with DEC, and coated with Kapton tape, which served as a protective film against the moisture in the air. For the TEM analyses of the lithiated and delithiated SiP2 electrodes, the electrode samples scratched from the Cu substrate were introduced into a glass vial
AUTHOR INFORMATION Corresponding Authors
*
[email protected] *
[email protected] Present Address §
H.-T.K.: Functional Composites Department, Composites Research Division, Korea Institute of Materials Science, Changwon 51508, Republic of Korea Author Contributions
C.-M.P. and K.-J.J. initiated the study and outlined the experiments. H.-T.K. synthesized the samples and performed various analyses. C.-K.L. provided assistance in the analyses of electrochemical data. C.-M.P. and K.-J.J. supervised the research work and wrote the manuscript. All authors contributed to the discussion of the results reported in the manuscript. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (NRF-2014R1A2A1A11053057). 5707
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DOI: 10.1021/acsnano.6b02727 ACS Nano 2016, 10, 5701−5709
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