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Synthesis of silicon nanosheets from kaolinite as a high-performance anode material for lithium-ion batteries
Journal of Physics and Chemistry of Solids 137 (2020) 109227
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Synthesis of silicon nanosheets from kaolinite as a high-performance anode material for lithium-ion batteries Haoji Wang a, Wei Tang a, Lianshan Ni a, Wei Ma b, Gen Chen a, *, Ning Zhang a, Xiaohe Liu a, **, Renzhi Ma c, *** a b c
School of Materials Science and Engineering, Central South University, Changsha, Hunan, 410083, PR China School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, 450001, PR China International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Kaolinite Si nanosheet Lithium-ion batteries Magnesiothermic reduction
Because of the high theoretical capability surpassing other anode materials, silicon (Si) has been regarded as the most potential anode material for the next-generation lithium-ion batteries (LIBs). In this work, the amorphous SiO2 precursors extracted from earth-abundant and silicon-rich clay mineral, kaolinite, have been applied to prepare Si nanosheets (k-Si) through a magnesiothermic reduction method. Benefiting from the special twodimensional nanostructure, the problem caused by volume expansion can be greatly relieved. As an anode material of LIBs, the Si nanosheets deliver a high reversible specific capacity of 1909 mAh g 1 at a low current density of 0.2 A g 1 after 50 cycles, 1156 mAh g 1 at a high current density of 2 A g 1 after 500 cycles and excellent rate capability of 889 mAh g 1 at a current density of 4 A g 1, remarkably preceding the performance of Si material prepared by commercial SiO2 powders. The strategy paves a promising way to the efficient pro duction of electrode materials from the natural minerals.
1. Introduction The increasing demands in energy storage devices constantly stim ulate the development of high-performance lithium-ion batteries (LIBs) for application in portable electronics and electric vehicles [1,2]. As is well known, LIBs have various superiorities of high energy density, light weight and environmental amity. Currently, graphite is widely used as the anode of Li-ion batteries [3,4]; nevertheless, it only possesses a low theoretical capacity of 372 mAh g 1, which is unable to meet the busi ness needs [5]. Hence, it is imperative to explore a new-type battery with larger specific capacity and higher power density [6–8]. Under this circumstance, silicon, with the highest theoretical specific capacity (4200 mAh g 1), irritates researchers’ tremendous interests [9–12]. Unfortunately, large volume change (>300%), which is caused by the alloy reaction during the insertion/extraction process of Liþ ions, in duces easy pulverization of the electrode, ultimately leading to rapid capacity fading [13–15]. Furthermore, its intrinsic low conductivity and slow diffusivity for Liþ ions also set obstacles to the application as
commercial anode materials. It is an effective way to relieve the above-mentioned drawbacks to decrease the silicon size to sub micrometer or nanometers. Plentiful studies are devoted to fabricating nanostructured Si materials, such as nanowires [16–18], nanotubes [19–21] and nanoparticles [22–24], etc. As the second richest element in the Earth’s crust, silicon is mainly distributed in rocks, clays and sands as complicated forms of poly silicate, and is rarely found in the form of pure substance [25]. Previous Si anode materials mostly stem from artificial SiO2 that is generally synthesized via the hydrolysis reaction of organic containing silicon element [26,27]. But to a large extent, these methods are not suitable for large-scale production. Simultaneously, the magnesiothermic reduction method has presently gotten approvals as a promising synthetic route for nanosized Si. For instance, Bao et al. presented that SiO2 can be reduced to Si with the participation of Mg (SiO2 (s) þ 2Mg (g)⟶Si (s) þ 2MgO (s)) [28,29]. Nie et al. synthesized graphene caging silicon particles through magnesiothermic reduction for high-performance LIBs [30]. The natural kaolinite (Al4(OH)8(Si4O10)) has a 1:1 layer structure, first
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (G. Chen), [email protected] (X. Liu), [email protected] (R. Ma). https://doi.org/10.1016/j.jpcs.2019.109227 Received 15 July 2019; Received in revised form 10 September 2019; Accepted 12 October 2019 Available online 14 October 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
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Journal of Physics and Chemistry of Solids 137 (2020) 109227
proposed by Pauling [31], whose elementary unit comprises a tetrahe dral sheet of SiO4 and an octahedral sheet with Al3þ as the octahedral cation [32]. Abundantly available resources on the earth determine the latent application value of kaolinite. On account of its plasticity, vis cosity and fire resistance, kaolinite clay has been primarily used in paper, ceramic and refractory materials, respectively. As far as we know, few reports use kaolinite as a source of silica to prepare Si by magne siothermic reduction [33–35]. Hence, this work has demonstrated that Si nanosheets (k-Si) can be extracted from natural kaolinite clay minerals. The schematic illustra tion of the synthetic processes and the crystal structure of kaolinite and k-Si are both shown in Fig. 1. After a simple two-step acid pickling to acquire the precursor of SiO2 nanosheets, the k-Si was synthesized through a representative magnesiothermic reduction method. The k-Si nanosheets derived from the kaolinite with large specific surface areas were directly employed as an active anode material, exhibiting an outstanding long-term cycling stability and high-rate specific capacity.
2.3. Synthesis of Si nanosheets by magnesiothermic reduction Typically, 2 g white SiO2 and 6 g NaCl were dissolved in 20 mL deionized water under stirring for 30 min, and then directly dried at 80 � C. The obtained mixture was homogenously hybridized with 1.6 g Mg powders in a mortar. Next, the hybrid was calcined at 650 � C for 1 h in a tube furnace with argon atmosphere flow after compressed under a press of 20 MPa. Afterwards, the resultant products were immersed in deionized water, heated 4 M H2SO4 solution, and diluted 2 wt% HF in sequence for the sake of removing NaCl as an adiabatic agent, MgO and Mg2Si byproducts as well as residual Mg and SiO2. In contrast, the commercial SiO2 powders were treated via the same conditions. 2.4. Characterization The crystallographic structure of the as-prepared specimens was characterized by X-ray powder diffraction (XRD) carried out on a Rigaku D/max 2500 with Cu Kα Radiation (λ ¼ 1.54184 Å) in a 2θ range from 5� to 80� . The morphology, structure, and chemical composition were investigated by field emission scanning electron microscopy (FE-SEM, Sirion 200, 15 kV), transmission electron microscopy (TEM, Tecnai G2 F20, 200 kV), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and energy disper sive X-ray spectrometer (EDX).
2. Materials and methods 2.1. Materials The kaolinite used in this work was commercially common. Other chemicals without further purification were all analytical grade. It was worth noting that the size of magnesium (Mg) powders was less than 200 μm.
2.5. Electrochemical tests The electrochemical characterizations of silicon directly used as active materials were performed by coin cells (CR2025 type) with pure Li metal as the counter and reference electrode, Celgard 2400 polymer membrane as separator, 1 M LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC, 1:1:1 in volume) as the electrolyte in which 10 vol% fluoroethylene carbonate (FEC) was added as an additive, respectively. The coin cells were assembled in the glo vebox (Mikrouna, China) filled with argon under a condition of H2O and O2 contents below 0.01 ppm. For preparing working electrode, the slurry, which was obtained by blending with Si materials, acetylene black, sodium carboxymethyl cellulose (CMC, Sigma-Aldrich, �98%, average molecular weight of 250,000) as well as polyacrylic acid (PAA, Alfa Aesar) at a weight ratio of 6:2:1:1 in water solvent, was agitated for
2.2. Synthesis of SiO2 from the raw kaolinite The amorphous SiO2 was prepared via two-step acid pickling treat ment. A certain amount of kaolinite was annealed at 700 � C for 3 h in air followed by dispersing in 200 mL deionized water. 4 M H2SO4 (100 mL) was slowly added into the kaolinite suspension with continuous stirring and heating for 30 min. After cooling down to room temperature, the milk white suspension was centrifuged. Then the precipitate was dispersed in the pre-prepared 70 mL H2SO4 solution (4 M) followed by transferring into a 100 mL Teflon-lined stainless steel autoclave and maintaining at 140 � C for 12 h. The amorphous SiO2 was collected by centrifugation, washed with deionized water and ethanol three times, and dried at 60 � C.
Fig. 1. Schematic illustration of the synthetic processes for the silicon nanosheets from kaolinite clay and corresponding crystal structure. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 2
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24 h and then pasted onto a Cu foil substrate. The foil was dried at 80 � C for 12 h in a vacuum oven. The amount of active material was approx imately 0.69 mg cm 2. The cells’ electrochemical performance was tested using a multichannel battery measurement system (Land, China) in the voltage range of 0.01–1.5 V at room temperature. Cyclic vol tammetry (CV) tests were carried out on a CHI660 D electrochemical workstation between 0.01 V and 1.5 V vs Liþ/Li at a scan rate of 0.1 mV s 1. For the electrochemical impedance spectroscopy (EIS), the frequency range was selected from 100 kHz to 0.01 Hz at an amplitude of 5 mV.
23.7% to 0.55% in atomic ratio) after acid etching. The conglomerated sheets of raw kaolinite clay can be clearly observed from the SEM image, as depicted in Fig. 2(b), (c). Fig. 2(d) exhibits a typical bright-field TEM image of raw kaolinite clay with thick layers. The inset in Fig. 2(d) de picts a selected area electron diffraction (SAED) pattern. The concentric diffraction circle rings imply the polycrystalline structure of kaolinite and the rings can be corresponded to the (101), (022), (131), (051), (104) and (260) lattice plane in terms of the measured radius. As can be seen in Fig. 2(f), the acid pickling process usually damages original smooth surface with many visible holes, but SiO2 powders still maintain the overall nanostructure of slice. Meanwhile, no obvious diffraction rings can be observed in the SAED pattern, which is consistent with the conclusion of XRD pattern. It further reveals the amorphous structure. Moreover, a SEM image of SiO2 precursor after etching in Fig. 2(e) shows a similar morphology compared to raw kaolinite. Fig. 3(a) displays the XRD pattern, which reveals the crystal structure of products after magnesiothermic reduction process under a high temperature. The peaks of resulted samples are well indexed to the cubic Si phase (PDF#65–1060, a ¼ 5.43086 Å). It should be mentioned that pure Si was obtained without any other impurities when NaCl, MgO, Mg2Si and residual SiO2 were removed via water wash, acid treatment and HF etching, as shown in Figure S2. The structure of k-Si nanosheets can be proved in Fig. 3(b), showing that the products miraculously inherit the slice and nanoscale size features of SiO2 precursor. To further
3. Results and discussion Generally, Fig. 2(a) provides the XRD patterns of the pristine lamellar kaolinite before and after pickling treatment. It is obvious that the primitive clay minerals well match with the standard card of triclinic kaolinite phase (PDF #99–0067, a ¼ 5.149 Å, b ¼ 8.9335 Å and c ¼ 7.3844 Å), corresponding to the natural kaolinite. Additionally, a broad peak range from 10� to 40� in the acid posttreatment XRD pattern illustrates the white powders mainly might be indexed as amorphous SiO2 with few impurities, which makes no difference to obtain pure k-Si. The energy dispersive X-ray spectrometer (EDX), as shown in Figure S1, also testifies the coexistence of Si and O (the signals of Cu and C come from the grid substrate) and the depressed contents of Al element (from
Fig. 2. (a) XRD patterns for the kaolinite clay before and after pickling treatment; (b, c) low and high magnification SEM images of raw kaolinite; (d) TEM image of raw kaolinite. Inset, SAED of raw kaolinite; (e, f) SEM and TEM images of SiO2 nanosheets. Inset, SAED of SiO2 nanosheets. 3
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Journal of Physics and Chemistry of Solids 137 (2020) 109227
Fig. 3. (a) XRD pattern (b) TEM (c) HRTEM (d) SAED images of Si nanosheets.
obtain the information about the pores, the k-Si nanosheets were investigated by the Brunauer-Emmett-Teller (BET) analysis (see Figure S3) with a relatively high BET specific surface area of 38.3 m2 g 1, pore volume of 0.31 cm3 g 1 and pore size of 87.8 nm [36–39]. Besides, the HRTEM image (Fig. 3(c)) shows regular and clear lattice fringes, which are attributed to high crystallinity of Si nanosheets. The lattice spacing of 0.31 nm is owe to the (111) crystal face. A selected area electron diffraction (SAED) of Si nanosheets is displayed in Fig. 3 (d). The diffraction rings can be well indexed to (111), (220) and (311) crystal planes, indicating polycrystalline structure of the final products. The investigation of the original electrochemical process about the kSi anode by the cyclic voltammetry curves as described in Fig. 4a. The test conditions were set at a voltage range from 0.01 V to 1.5 V (vs Liþ/ Li) at a scan rate of 0.1 mV s 1. During the first cathodic sweep, a tiny reductive peak at 1.14 V can be observed which disappear in the following cycles. This irreversible peak is ascribed to the formation of the solid electrolyte interface (SEI) film caused by secondary reaction between the electrode materials and electrolyte. It can be surmised that the SEI film accomplishes itself growth in the first loop on account of no peaks being observed in the same place in subsequent curves. Further more, it can be speculated that the lithiation peak in the initial cycle should be lower than 0.1 V related to the lithiation peak of high crystal silicon [23]. The cathodic peak at low potential (around 0.21 V) is assigned to the insertion reaction of Liþ ions to form Li–Si alloy. Upon discharge, the oxidative peaks located at about 0.37 V and 0.5 V repre sent the de-lithiation process, which is consistent with previous exper iments on silicon anode [40]. It’s worth mention that the magnitude of current peaks increased with circulation which is related to more active materials activated to react with Liþ in each cycle [17]. Fig. 4(b) presents the charge and discharge profiles of 1st, 2nd, 20th and 50th about the k-Si electrode using a coin-type half-cell at a constant current density of 0.5 A g 1. The initial charge and discharge specific capacities of the k-Si are 2973 mAh g 1 and 3850 mAh g 1 with a relatively high original coulombic efficiency (CE) of 77.23%. The loss of irreversible capacities are likely ascribed to the formation of a solid electrolyte interface (SEI) film [41] and the irreversible insertion of Liþ [42]. The 2nd charge and discharge capacities are respectively 2924 mAh g 1 and 3052 mAh g 1, corresponding to the CE of 95.8%.
The reversible capacities gradually rise in comparison with the initial cycle. According to the 20th and 50th curves, the specific discharge capacities still maintain at a relative high level with slight reduction and both of the CE are close to 100% indicated a highly reversible feature for the as-prepared k-Si. As can be clearly seen from Fig. 4(c), it exhibits the superior galvanostatic charge-discharge behaviors of the k-Si electrode operated on a low current density of 0.2 A g 1, which retains a reversible capacity of 1909 mAh g 1 after 50 cycles. In contrast, the chaotic Si materials (c-Si) synthesized by commercial SiO2 powders (see Figure S4) have an initial reversible capacity of 2674 mAh g 1 while dropping rapidly to 450 mAh g 1 after 50 cycles. There is an apparent gap be tween the first ten cycles and the last ten cycles at the same current density of 0.2 A g 1, as seen from Fig. 4(d), which might be resulted by the amplification in polarization voltage [43]. Profiting from the pecu liar structure, the k-Si electrode exhibits a good rate capacity at diverse current densities (see Fig. 4(e)). The k-Si electrode delivers average discharge capacities of ~2637, 2275, 1767, 1227, 934 and 889 mAh g 1, respectively corresponding to 0.2, 0.4, 1, 2, 3 and 4 A g 1. When the cycling rate recovers to the current density of 0.2 A g 1, the discharge capacity can also restore to 2196 mAh g 1. In order to further certify the outstanding electrochemical durability and high reversible capacity of the k-Si electrode, a long-term measurement was tested at a large cur rent density of 2 A g 1 after the first five cycles, as indicated in Fig. 4(f). The specific capacity still remains at around 1156 mAh g 1 after 500 cycles and the corresponding CE rapidly increases in the second cycle, which implies the potential to apply this material as a high-performance commercial anode material. The dynamical characteristics of the k-Si electrode were explored by the representative Nyquist plots, as illustrated in Fig. 5(a). The corre sponding analog circuit diagram is exhibited in Fig. 5(b). The plot is divided into two parts containing a depressed semicircle at the high frequency region and a sloping line at the low frequency region. Before testing, the initial charge-transfer resistance (Rct) of the k-Si and c-Si is respectively corresponded to 203.1 Ω and 582.1 Ω, which can be infer red that the c-Si electrode has the higher ohmic resistance and polari zation resistance compared with the k-Si electrode. Interestingly, the Rct drastically reduces to 14.7 Ω at the fifth cycle, which might be attributed to the SEI film covering the surface of active materials. The dynamic 4
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Journal of Physics and Chemistry of Solids 137 (2020) 109227
Fig. 4. (a) First five CV curves of the k-Si nanosheets in the voltage window of 0.0–1.5 V at a scan rate of 0.1 mV s 1; Lithium-storage performance of the k-Si nanosheets: (b) Discharge-charge profiles of the k-Si nanosheets at a current density of 0.5 A g 1; (c) Cycling of the k-Si nanosheets at current of 0.2 A g 1; (d) Discharge-charge profiles of the k-Si nanosheets at various current density; (e) Rate capacity of the kSi nanosheets at various current density; (f) Cycling of the k-Si nanosheets at current of 2 A g 1.
activation is in agreement with the previous experiment results [23]. Moreover, the slope of the sloping line at the fifth cycle is larger than that in the pristine cycle, revealing more difficult diffusion of lithium ions through the electrode/electrolyte interface. 4. Conclusions In summary, Si nanosheets were successfully synthesized via a magnesiothermic reduction of SiO2 nanosheet powders at 650 � C. The SiO2 precursors were extracted from abundant and low-cost kaolinite clay minerals after a series of processes about acid pickling. In addition, the kaolinite is often used as an adsorbent, but it is rarely used as a raw material for LIBs. Benefiting from the large specific surface areas of the nanosheets to accelerate the transmission of ions and electrons, the k-Si displays a high specific capacity of 1909 mAh g 1 at a low current density of 0.2 A g 1 after 50 cycles, 1156 mAh g 1 at a high current density of 2 A g 1 after 500 cycles and excellent rate capability of 889 mAh g 1 at a current density of 4 A g 1 when used as anodes, remarkably superior to the Si materials prepared by commercial SiO2 powders. All of the results disclose that the k-Si with particular morphology is an appropriate material to store lithium ions with low cost, while retaining good performance. Acknowledgment The authors acknowledge the financial support by National Natural Science Foundation of China (51874357, 51872333), Hunan Provincial Natural Science Foundation of China (13JJ1005). X. L. acknowledges support from Shenghua Scholar Program of Central South University. R. M. acknowledges support from JSPS KAKENNHI (18H03869).
Fig. 5. (a) Nyquist plots of the pristine, the 5th cycles for k-Si nanosheets electrode and the pristine c-Si in a resistance range of 0–1600 Ohm. Inset is the Nyquist plot of the k-Si nanosheets electrode for the 5th cycles in a resistance of 0–30 Ohm; (b) The corresponding fitted equivalent circuit.
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Appendix A. Supplementary data
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Synthesis of silicon nanosheets from kaolinite as a high-performance anode material for lithium-ion batteries Haoji Wang a, Wei Tang a, Lianshan Ni a, Wei Ma b, Gen Chen a, *, Ning Zhang a, Xiaohe Liu a, **, Renzhi Ma c, *** a b c
School of Materials Science and Engineering, Central South University, Changsha, Hunan, 410083, PR China School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, 450001, PR China International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Kaolinite Si nanosheet Lithium-ion batteries Magnesiothermic reduction
Because of the high theoretical capability surpassing other anode materials, silicon (Si) has been regarded as the most potential anode material for the next-generation lithium-ion batteries (LIBs). In this work, the amorphous SiO2 precursors extracted from earth-abundant and silicon-rich clay mineral, kaolinite, have been applied to prepare Si nanosheets (k-Si) through a magnesiothermic reduction method. Benefiting from the special twodimensional nanostructure, the problem caused by volume expansion can be greatly relieved. As an anode material of LIBs, the Si nanosheets deliver a high reversible specific capacity of 1909 mAh g 1 at a low current density of 0.2 A g 1 after 50 cycles, 1156 mAh g 1 at a high current density of 2 A g 1 after 500 cycles and excellent rate capability of 889 mAh g 1 at a current density of 4 A g 1, remarkably preceding the performance of Si material prepared by commercial SiO2 powders. The strategy paves a promising way to the efficient pro duction of electrode materials from the natural minerals.
1. Introduction The increasing demands in energy storage devices constantly stim ulate the development of high-performance lithium-ion batteries (LIBs) for application in portable electronics and electric vehicles [1,2]. As is well known, LIBs have various superiorities of high energy density, light weight and environmental amity. Currently, graphite is widely used as the anode of Li-ion batteries [3,4]; nevertheless, it only possesses a low theoretical capacity of 372 mAh g 1, which is unable to meet the busi ness needs [5]. Hence, it is imperative to explore a new-type battery with larger specific capacity and higher power density [6–8]. Under this circumstance, silicon, with the highest theoretical specific capacity (4200 mAh g 1), irritates researchers’ tremendous interests [9–12]. Unfortunately, large volume change (>300%), which is caused by the alloy reaction during the insertion/extraction process of Liþ ions, in duces easy pulverization of the electrode, ultimately leading to rapid capacity fading [13–15]. Furthermore, its intrinsic low conductivity and slow diffusivity for Liþ ions also set obstacles to the application as
commercial anode materials. It is an effective way to relieve the above-mentioned drawbacks to decrease the silicon size to sub micrometer or nanometers. Plentiful studies are devoted to fabricating nanostructured Si materials, such as nanowires [16–18], nanotubes [19–21] and nanoparticles [22–24], etc. As the second richest element in the Earth’s crust, silicon is mainly distributed in rocks, clays and sands as complicated forms of poly silicate, and is rarely found in the form of pure substance [25]. Previous Si anode materials mostly stem from artificial SiO2 that is generally synthesized via the hydrolysis reaction of organic containing silicon element [26,27]. But to a large extent, these methods are not suitable for large-scale production. Simultaneously, the magnesiothermic reduction method has presently gotten approvals as a promising synthetic route for nanosized Si. For instance, Bao et al. presented that SiO2 can be reduced to Si with the participation of Mg (SiO2 (s) þ 2Mg (g)⟶Si (s) þ 2MgO (s)) [28,29]. Nie et al. synthesized graphene caging silicon particles through magnesiothermic reduction for high-performance LIBs [30]. The natural kaolinite (Al4(OH)8(Si4O10)) has a 1:1 layer structure, first
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (G. Chen), [email protected] (X. Liu), [email protected] (R. Ma). https://doi.org/10.1016/j.jpcs.2019.109227 Received 15 July 2019; Received in revised form 10 September 2019; Accepted 12 October 2019 Available online 14 October 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
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proposed by Pauling [31], whose elementary unit comprises a tetrahe dral sheet of SiO4 and an octahedral sheet with Al3þ as the octahedral cation [32]. Abundantly available resources on the earth determine the latent application value of kaolinite. On account of its plasticity, vis cosity and fire resistance, kaolinite clay has been primarily used in paper, ceramic and refractory materials, respectively. As far as we know, few reports use kaolinite as a source of silica to prepare Si by magne siothermic reduction [33–35]. Hence, this work has demonstrated that Si nanosheets (k-Si) can be extracted from natural kaolinite clay minerals. The schematic illustra tion of the synthetic processes and the crystal structure of kaolinite and k-Si are both shown in Fig. 1. After a simple two-step acid pickling to acquire the precursor of SiO2 nanosheets, the k-Si was synthesized through a representative magnesiothermic reduction method. The k-Si nanosheets derived from the kaolinite with large specific surface areas were directly employed as an active anode material, exhibiting an outstanding long-term cycling stability and high-rate specific capacity.
2.3. Synthesis of Si nanosheets by magnesiothermic reduction Typically, 2 g white SiO2 and 6 g NaCl were dissolved in 20 mL deionized water under stirring for 30 min, and then directly dried at 80 � C. The obtained mixture was homogenously hybridized with 1.6 g Mg powders in a mortar. Next, the hybrid was calcined at 650 � C for 1 h in a tube furnace with argon atmosphere flow after compressed under a press of 20 MPa. Afterwards, the resultant products were immersed in deionized water, heated 4 M H2SO4 solution, and diluted 2 wt% HF in sequence for the sake of removing NaCl as an adiabatic agent, MgO and Mg2Si byproducts as well as residual Mg and SiO2. In contrast, the commercial SiO2 powders were treated via the same conditions. 2.4. Characterization The crystallographic structure of the as-prepared specimens was characterized by X-ray powder diffraction (XRD) carried out on a Rigaku D/max 2500 with Cu Kα Radiation (λ ¼ 1.54184 Å) in a 2θ range from 5� to 80� . The morphology, structure, and chemical composition were investigated by field emission scanning electron microscopy (FE-SEM, Sirion 200, 15 kV), transmission electron microscopy (TEM, Tecnai G2 F20, 200 kV), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and energy disper sive X-ray spectrometer (EDX).
2. Materials and methods 2.1. Materials The kaolinite used in this work was commercially common. Other chemicals without further purification were all analytical grade. It was worth noting that the size of magnesium (Mg) powders was less than 200 μm.
2.5. Electrochemical tests The electrochemical characterizations of silicon directly used as active materials were performed by coin cells (CR2025 type) with pure Li metal as the counter and reference electrode, Celgard 2400 polymer membrane as separator, 1 M LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC, 1:1:1 in volume) as the electrolyte in which 10 vol% fluoroethylene carbonate (FEC) was added as an additive, respectively. The coin cells were assembled in the glo vebox (Mikrouna, China) filled with argon under a condition of H2O and O2 contents below 0.01 ppm. For preparing working electrode, the slurry, which was obtained by blending with Si materials, acetylene black, sodium carboxymethyl cellulose (CMC, Sigma-Aldrich, �98%, average molecular weight of 250,000) as well as polyacrylic acid (PAA, Alfa Aesar) at a weight ratio of 6:2:1:1 in water solvent, was agitated for
2.2. Synthesis of SiO2 from the raw kaolinite The amorphous SiO2 was prepared via two-step acid pickling treat ment. A certain amount of kaolinite was annealed at 700 � C for 3 h in air followed by dispersing in 200 mL deionized water. 4 M H2SO4 (100 mL) was slowly added into the kaolinite suspension with continuous stirring and heating for 30 min. After cooling down to room temperature, the milk white suspension was centrifuged. Then the precipitate was dispersed in the pre-prepared 70 mL H2SO4 solution (4 M) followed by transferring into a 100 mL Teflon-lined stainless steel autoclave and maintaining at 140 � C for 12 h. The amorphous SiO2 was collected by centrifugation, washed with deionized water and ethanol three times, and dried at 60 � C.
Fig. 1. Schematic illustration of the synthetic processes for the silicon nanosheets from kaolinite clay and corresponding crystal structure. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 2
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24 h and then pasted onto a Cu foil substrate. The foil was dried at 80 � C for 12 h in a vacuum oven. The amount of active material was approx imately 0.69 mg cm 2. The cells’ electrochemical performance was tested using a multichannel battery measurement system (Land, China) in the voltage range of 0.01–1.5 V at room temperature. Cyclic vol tammetry (CV) tests were carried out on a CHI660 D electrochemical workstation between 0.01 V and 1.5 V vs Liþ/Li at a scan rate of 0.1 mV s 1. For the electrochemical impedance spectroscopy (EIS), the frequency range was selected from 100 kHz to 0.01 Hz at an amplitude of 5 mV.
23.7% to 0.55% in atomic ratio) after acid etching. The conglomerated sheets of raw kaolinite clay can be clearly observed from the SEM image, as depicted in Fig. 2(b), (c). Fig. 2(d) exhibits a typical bright-field TEM image of raw kaolinite clay with thick layers. The inset in Fig. 2(d) de picts a selected area electron diffraction (SAED) pattern. The concentric diffraction circle rings imply the polycrystalline structure of kaolinite and the rings can be corresponded to the (101), (022), (131), (051), (104) and (260) lattice plane in terms of the measured radius. As can be seen in Fig. 2(f), the acid pickling process usually damages original smooth surface with many visible holes, but SiO2 powders still maintain the overall nanostructure of slice. Meanwhile, no obvious diffraction rings can be observed in the SAED pattern, which is consistent with the conclusion of XRD pattern. It further reveals the amorphous structure. Moreover, a SEM image of SiO2 precursor after etching in Fig. 2(e) shows a similar morphology compared to raw kaolinite. Fig. 3(a) displays the XRD pattern, which reveals the crystal structure of products after magnesiothermic reduction process under a high temperature. The peaks of resulted samples are well indexed to the cubic Si phase (PDF#65–1060, a ¼ 5.43086 Å). It should be mentioned that pure Si was obtained without any other impurities when NaCl, MgO, Mg2Si and residual SiO2 were removed via water wash, acid treatment and HF etching, as shown in Figure S2. The structure of k-Si nanosheets can be proved in Fig. 3(b), showing that the products miraculously inherit the slice and nanoscale size features of SiO2 precursor. To further
3. Results and discussion Generally, Fig. 2(a) provides the XRD patterns of the pristine lamellar kaolinite before and after pickling treatment. It is obvious that the primitive clay minerals well match with the standard card of triclinic kaolinite phase (PDF #99–0067, a ¼ 5.149 Å, b ¼ 8.9335 Å and c ¼ 7.3844 Å), corresponding to the natural kaolinite. Additionally, a broad peak range from 10� to 40� in the acid posttreatment XRD pattern illustrates the white powders mainly might be indexed as amorphous SiO2 with few impurities, which makes no difference to obtain pure k-Si. The energy dispersive X-ray spectrometer (EDX), as shown in Figure S1, also testifies the coexistence of Si and O (the signals of Cu and C come from the grid substrate) and the depressed contents of Al element (from
Fig. 2. (a) XRD patterns for the kaolinite clay before and after pickling treatment; (b, c) low and high magnification SEM images of raw kaolinite; (d) TEM image of raw kaolinite. Inset, SAED of raw kaolinite; (e, f) SEM and TEM images of SiO2 nanosheets. Inset, SAED of SiO2 nanosheets. 3
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Fig. 3. (a) XRD pattern (b) TEM (c) HRTEM (d) SAED images of Si nanosheets.
obtain the information about the pores, the k-Si nanosheets were investigated by the Brunauer-Emmett-Teller (BET) analysis (see Figure S3) with a relatively high BET specific surface area of 38.3 m2 g 1, pore volume of 0.31 cm3 g 1 and pore size of 87.8 nm [36–39]. Besides, the HRTEM image (Fig. 3(c)) shows regular and clear lattice fringes, which are attributed to high crystallinity of Si nanosheets. The lattice spacing of 0.31 nm is owe to the (111) crystal face. A selected area electron diffraction (SAED) of Si nanosheets is displayed in Fig. 3 (d). The diffraction rings can be well indexed to (111), (220) and (311) crystal planes, indicating polycrystalline structure of the final products. The investigation of the original electrochemical process about the kSi anode by the cyclic voltammetry curves as described in Fig. 4a. The test conditions were set at a voltage range from 0.01 V to 1.5 V (vs Liþ/ Li) at a scan rate of 0.1 mV s 1. During the first cathodic sweep, a tiny reductive peak at 1.14 V can be observed which disappear in the following cycles. This irreversible peak is ascribed to the formation of the solid electrolyte interface (SEI) film caused by secondary reaction between the electrode materials and electrolyte. It can be surmised that the SEI film accomplishes itself growth in the first loop on account of no peaks being observed in the same place in subsequent curves. Further more, it can be speculated that the lithiation peak in the initial cycle should be lower than 0.1 V related to the lithiation peak of high crystal silicon [23]. The cathodic peak at low potential (around 0.21 V) is assigned to the insertion reaction of Liþ ions to form Li–Si alloy. Upon discharge, the oxidative peaks located at about 0.37 V and 0.5 V repre sent the de-lithiation process, which is consistent with previous exper iments on silicon anode [40]. It’s worth mention that the magnitude of current peaks increased with circulation which is related to more active materials activated to react with Liþ in each cycle [17]. Fig. 4(b) presents the charge and discharge profiles of 1st, 2nd, 20th and 50th about the k-Si electrode using a coin-type half-cell at a constant current density of 0.5 A g 1. The initial charge and discharge specific capacities of the k-Si are 2973 mAh g 1 and 3850 mAh g 1 with a relatively high original coulombic efficiency (CE) of 77.23%. The loss of irreversible capacities are likely ascribed to the formation of a solid electrolyte interface (SEI) film [41] and the irreversible insertion of Liþ [42]. The 2nd charge and discharge capacities are respectively 2924 mAh g 1 and 3052 mAh g 1, corresponding to the CE of 95.8%.
The reversible capacities gradually rise in comparison with the initial cycle. According to the 20th and 50th curves, the specific discharge capacities still maintain at a relative high level with slight reduction and both of the CE are close to 100% indicated a highly reversible feature for the as-prepared k-Si. As can be clearly seen from Fig. 4(c), it exhibits the superior galvanostatic charge-discharge behaviors of the k-Si electrode operated on a low current density of 0.2 A g 1, which retains a reversible capacity of 1909 mAh g 1 after 50 cycles. In contrast, the chaotic Si materials (c-Si) synthesized by commercial SiO2 powders (see Figure S4) have an initial reversible capacity of 2674 mAh g 1 while dropping rapidly to 450 mAh g 1 after 50 cycles. There is an apparent gap be tween the first ten cycles and the last ten cycles at the same current density of 0.2 A g 1, as seen from Fig. 4(d), which might be resulted by the amplification in polarization voltage [43]. Profiting from the pecu liar structure, the k-Si electrode exhibits a good rate capacity at diverse current densities (see Fig. 4(e)). The k-Si electrode delivers average discharge capacities of ~2637, 2275, 1767, 1227, 934 and 889 mAh g 1, respectively corresponding to 0.2, 0.4, 1, 2, 3 and 4 A g 1. When the cycling rate recovers to the current density of 0.2 A g 1, the discharge capacity can also restore to 2196 mAh g 1. In order to further certify the outstanding electrochemical durability and high reversible capacity of the k-Si electrode, a long-term measurement was tested at a large cur rent density of 2 A g 1 after the first five cycles, as indicated in Fig. 4(f). The specific capacity still remains at around 1156 mAh g 1 after 500 cycles and the corresponding CE rapidly increases in the second cycle, which implies the potential to apply this material as a high-performance commercial anode material. The dynamical characteristics of the k-Si electrode were explored by the representative Nyquist plots, as illustrated in Fig. 5(a). The corre sponding analog circuit diagram is exhibited in Fig. 5(b). The plot is divided into two parts containing a depressed semicircle at the high frequency region and a sloping line at the low frequency region. Before testing, the initial charge-transfer resistance (Rct) of the k-Si and c-Si is respectively corresponded to 203.1 Ω and 582.1 Ω, which can be infer red that the c-Si electrode has the higher ohmic resistance and polari zation resistance compared with the k-Si electrode. Interestingly, the Rct drastically reduces to 14.7 Ω at the fifth cycle, which might be attributed to the SEI film covering the surface of active materials. The dynamic 4
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Fig. 4. (a) First five CV curves of the k-Si nanosheets in the voltage window of 0.0–1.5 V at a scan rate of 0.1 mV s 1; Lithium-storage performance of the k-Si nanosheets: (b) Discharge-charge profiles of the k-Si nanosheets at a current density of 0.5 A g 1; (c) Cycling of the k-Si nanosheets at current of 0.2 A g 1; (d) Discharge-charge profiles of the k-Si nanosheets at various current density; (e) Rate capacity of the kSi nanosheets at various current density; (f) Cycling of the k-Si nanosheets at current of 2 A g 1.
activation is in agreement with the previous experiment results [23]. Moreover, the slope of the sloping line at the fifth cycle is larger than that in the pristine cycle, revealing more difficult diffusion of lithium ions through the electrode/electrolyte interface. 4. Conclusions In summary, Si nanosheets were successfully synthesized via a magnesiothermic reduction of SiO2 nanosheet powders at 650 � C. The SiO2 precursors were extracted from abundant and low-cost kaolinite clay minerals after a series of processes about acid pickling. In addition, the kaolinite is often used as an adsorbent, but it is rarely used as a raw material for LIBs. Benefiting from the large specific surface areas of the nanosheets to accelerate the transmission of ions and electrons, the k-Si displays a high specific capacity of 1909 mAh g 1 at a low current density of 0.2 A g 1 after 50 cycles, 1156 mAh g 1 at a high current density of 2 A g 1 after 500 cycles and excellent rate capability of 889 mAh g 1 at a current density of 4 A g 1 when used as anodes, remarkably superior to the Si materials prepared by commercial SiO2 powders. All of the results disclose that the k-Si with particular morphology is an appropriate material to store lithium ions with low cost, while retaining good performance. Acknowledgment The authors acknowledge the financial support by National Natural Science Foundation of China (51874357, 51872333), Hunan Provincial Natural Science Foundation of China (13JJ1005). X. L. acknowledges support from Shenghua Scholar Program of Central South University. R. M. acknowledges support from JSPS KAKENNHI (18H03869).
Fig. 5. (a) Nyquist plots of the pristine, the 5th cycles for k-Si nanosheets electrode and the pristine c-Si in a resistance range of 0–1600 Ohm. Inset is the Nyquist plot of the k-Si nanosheets electrode for the 5th cycles in a resistance of 0–30 Ohm; (b) The corresponding fitted equivalent circuit.
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Appendix A. Supplementary data
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