A Deep Reduction and Partial Oxidation Strategy for Fabrication of

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A Deep Reduction and Partial Oxidation Strategy for Fabrication of Mesoporous Si Anode for Lithium Ion Batteries Jianwen Liang, Xiaona Li, Zhiguo Hou, Wanqun Zhang, Yongchun Zhu, and Yitai Qian ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b06995 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 21, 2016

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A Deep Reduction and Partial Oxidation Strategy for Fabrication of Mesoporous Si Anode for Lithium Ion Batteries Jianwen Liang, Xiaona Li, Zhiguo Hou, Wanqun Zhang, Yongchun Zhu,* and Yitai Qian* Dr. J. W. Liang, X. N. Li, Z. G. Hou, W. Q. Zhang, Y. C. Zhu, Prof. Y. T. Qian Hefei National Laboratory for Physical Science at Microscale, Department of Chemistry, University of Science and Technology of China, 96 JinZhai Road, 230026, Hefei, China E-mail:[email protected]; [email protected] Dr. J. W. Liang, Prof. Y. T. Qian School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong, 250100, P. R. China, E-mail:[email protected]

KEYWORDS : silicon; mesoporous; general strategy; lithium ion batteries; high stabilization.

ABSTRACT: A deep reduction and partial oxidation strategy to convert low cost SiO2 into mesoporous Si anode with the yield higher than 90% is provided. This strategy has advantage in efficient mesoporous silicon prouduction and in-situ formation of several nanometer SiO2 layer on the surface of silicon particles. Thus, the resulted silicon anode provides extremely high reversible capacity of 1772 mAh g-1, supior cycling stability with more than 873 mAh g-1 at 1.8 A g-1 after 1400 cycles (corresponding to the capacity decay rate of 0.035% per cycle), and good

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rate capability (~710 mAh g-1 at 18A g-1). These promising results suggest that such strategy for mesoporous Si anode can be potentially commercialized for high energy Li-ion batteries.

Silicon is an exciting anode material of lithium ion batteries (LIBs) with high specific theoretical capacity of 3600 mAh g-1, abundant resource and with a low and flat voltage potential. However, successful applications of silicon anode have been impeded by rapid capacity fading caused by serious volume change (> 300 %) during lithiation-delithiation process.1-5 Moreover, the severe volume expansion/contraction of silicon ruptures and reforms of the solid electrolyte interphase (SEI) layer during cycling. The continuous growth of SEI layer consumes too much lithium ions and electrolyte and causes a significant increase in the cell resistance, thus lead to a large capacity loss and an unstable electrochemical performance. These critical problems are proposed to be alleviated by designing appropriate nanostructures6, 7 or encapsulation with protective coatings.8 Recently, Various silicon nanostructures such as hollow,9-12 nanotube,13, 14 nanowires,15 porous16-19 have been demonstrated with greatly improved performance. This nanostructuring strategy can greatly increase the cycle life to about a few hundred cycles with 80% capacity retention.2 Liu et al. delivered a mesoporous silicon sponge with the retentive reversible capacity of 750 mAh g-1 after 1000 cycles.20 Detailed analysis demonstrated that such silicon sponge could decrease the volumetric expansion at full lithiated state to 30%, thus restraining the corresponding pulverization of silicon particles. Moreover, mechanical protecting coating is also widely used to improve the cycling performance with the attempt to limit/accommodate the volume change and prevent the aggregation/isolation during cycling. Furthermore, such mechanical coating could also modify the surface conductions for a


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stable SEI interface, which is important to realize a long cycle life for silicon anodes.21-24 Recently, works in the use of mechanically rigid such as carbon,13, 25-30 metals,31, 32 polymers33-35 and oxides21, 24, 36 have been shown to stabilize the highly reactive interfaces of Si electrode. Cui et al. 24 demonstrated a silicon nanotube with the outer wall confining of SiOx, which could cycle over 6000 cycle in half cells while retain ~ 500 mAh g-1 under 24 A g-1 at the mass loading of 0.02~0.1 mg cm-2. Recently, using pomegranate-like Si-C yolk-shell anode, a half-cell of 100 stable cycles has also been provided at the high active materials loading (3.7 mAh cm-2). Although the great progress of the silicon anode in recently years, tracking the silicon anode problems still need to combine various of strategies, such as nanostructure designing, functional material supporting or surface coating and modification, which will increase much difficulty in the fabrication process. More challengingly, it is still a tough work to realize high electrochemical performance of silicon anode by low cost, environmentally friendly and scalable production process, all of which may be essential to bring potential applications into reality. Therefore, it is imperative to develop efficient and low cost approaches to improve the practical application of silicon anode. Facing with those challenges, researcher have devoted themselves to exploring more practical approaches to reduce production costs and improve yields of silicon electrodes with good electrochemical performance.17, 26-28, 37-45 For example, the magnesiothermic reduction of silica has been considered as one of the practical processes.38, 41, 42-50 Using this approach, Si can be directly fabricated from SiO2 and the morphologies structure of the original SiO2 is maintained. However, despite the simplicity of such approach, the final extraction yield for silicon under the atmospheric-pressure magnesiothermic reduction process is estimated to be lower than 50%. The low conversion efficiency is mainly come from the significant amount of magnesium silicide


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formation due to the uncontrollable process. The removal of these undesirable components (Mg2Si and unreacted SiO2) by the use of HF acid results in a substantial loss in production yield. It is also harmed for the environment and hard to large-scale production. Moreover, the silicon production from the magnesiothermic reduction approach only maintains electrochemical active and capacity retention after carbon modification. Thus, it still needs to further improve such strategies to further application in LIBs. Previously, we have demonstrated that Mg2Si can be converted into the silicon materials under the air atmosphere by controlling the reaction condition.51 It is an efficient strategy to fabricate silicon and avoid the use of HF acid but limited by the stiff morphology and the solid structure of the silicon previous. Here, we present an efficient approach to convert various SiO2 from commercial bulk sand, Silica adiatom frustule, industrial waste silica aerogel and silica zeolite into mesoporous silicon with the conversion yield higher than 90%. Such approach is based on the metallothermic deep reduction of silica and partial air oxidation in the subsequent steps. Since the oxidation reaction of silicon is a dynamic restriction process, a passivated SiOx layer will be formed in the interface between silicon and oxygen, which will slow down the reaction rate, thus allow us to prepare silicon materials. The conversion of SiO2 into Si based on the deep reduction and partial oxidation strategy is illustrate as: 4Mg + SiO2 = 2MgO + Mg2Si Mg2Si + O2 = MgO + Si 2Mg + O2 = 2MgO

(3)

(2)

(1)

∆Gθ = -338.6 kJ mol-1 ∆Gθ = -565.1 kJ mol-1 ∆Gθ = -569.6 kJ mol-1

Different from the traditional metallothermic reduction approach, our strategy has efficient silicon fabrication and in-situ formation of several nanometer SiO2 layer on the surface of silicon particles. Thus, the as-prepared mesoporous silicon provides extremely high capacity of 1772


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mAh g-1 and maintains more than 873 mAh g-1 at 1.8 A g-1 for 1400 cycles (corresponding to the capacity decay rate of 0.035% per cycle), even at the current density high as 18 A g-1, it still stable remain ~710 mAh g-1. In general, more than the high cycling stability and rate performance, our mesoporous Si anode is easy and efficient to fabrication, which will well be aligned with practical application. Moreover, diluted HCl was used instead of toxic HF to remove the impurity substance for the silicon fabrication, suggesting that our strategy is also more environmentally friendly. These promising results suggest that such efficient strategy for mesoporous silicon fabrication can potentially be commercialized for high energy LIBs. Results and Discussion Fabrication of mesoporous silicon materials A schematic illustration of the process for fabricating mesoporous silicon materials via the deep reduction and partial oxidation strategy is shown in Figure 1a. We used industrial waste silica aerogel as an example and such strategy can be expand to the commercial bulk sand, silica adiatom frustule and even silica zeolite. In brief, industrial waste silica aerogel are deeply reduced by excuss Mg powder through a magnesiothermic deep reduction reaction under 500 °C, forming Mg2Si/MgO/Mg intermediate in the first step. The XRD peaks of the silica aerogel previous can be indexed to the amorphous structure (Figure 1b). After the magnesiothermic deep reduction reaction, the XRD patterns of the intermediate products clearly shows the composition of Mg2Si (JPCDS No. 65-0690) and MgO (JPCDS No.75-1525). The magnesiothermic deep reduction reaction equation is shown in chemical equation (1). A trace amount of Mg (JPCDS No. 01-1141) is also detected from the XRD pattern, which is associated with the excess of Mg (Figure 1c). Afterward, when transform the Mg2Si/MgO/Mg intermediate


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to air atmosphere under 600 °C, the partial oxidation stage takes place and continues to formation of Si/MgO due to the the oxidation of Mg2Si into Si/MgO and the excess Mg into MgO. Surprisingly, our experimental results suggest that there are no significant production of SiO2 by controlling the annealing temperature (Figure 1d). The oxidation rate of Si by O2 at the partial oxidation process is slow enough for us to obtain the production and, at the same time, to in-situ form of a thin SiOx layer in the surface of the resulting silicon. Eventually producing porous and nanoparticle combine structure of silicon materials in the third step through an selective etching process using HCl solution to remove MgO. XRD pattern of the resulting nanosilicon material is shown in Figure 1e, all XRD peaks of the materials can well be indexed to the cubic phase of silicon (JPCDS 27-1402). The conversion yield of Si from this strategy is calculated and declared to be higher than 90% (the repeated experiments for conversion yield calculation is shown in Table S1 in supporting information).

Figure 1. a) Schematic illustration of the deep reduction and partial oxidation process. b-e) XRD pattern of the products obtained at different steps during the fabrication process.


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In order to understand and well control of the oxidation reactions between Mg2Si and O2 in the air oxidation process, a thermogravimetric analysis (TGA) measurement was employed to be guidance here. The TGA curves of Mg2Si/MgO/Mg intermediate from the deep reduction of silica aerogel previous and commercial Mg powder are presented in Figure 2. The Mg2Si/MgO/Mg intermediate undergo continuous weight increase in three steps, the onset of which occurs at about 400 °C. Compared with the TGA curve of the Mg powder, the Mg2Si/MgO/Mg intermediate show a total weignt increase of 23% between 400 to 650 °C, which is corresponds to the air oxidation of Mg2Si and Mg as shown in chemical equation (2) and (3). When the temperature is higher than 650 °C, the weight of the intermediate sample is sitll continuous increased with a uniform rates. This mainly come from the reaction between silicon and oxygen. Total amount of TGA weight increase of the Mg2Si/MgO/Mg intermediate sample comes from three contributions: mass increase from the oxidation of Mg2Si, mass increase from the oxidation of Mg powder and mass increase from the oxidation of silicon production. The contributions are independent and significant different of each other. Moreover, TGA curve of the commercial 200 mesh Si powder was also shown in Figure S1 for comparison. In air atmosphere, there is a slight weight decrease (4 %) from 100 to 800 °C, indicating that the oxidation reaction of bulk silicon by air below 800 °C is passivation process. Furthermore, we also measure the TGA curve of the prepared A-Si powder. From the TGA curve of the A-Si powder in Figure S2a, we can find that the prepared A-Si powder also undergo a slightly weight decrease from room temperature to 550 °C. This mainly come from the desorption of water etc. from the power and oxidation the surface layer of Si-OH into SiO2. When the temperature is over to 550 °C, the weight decrease become more slower. Moreover, the sample weight become increase when the temperature further up to 700 °C, indicating the reaction between silicon and


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oxygen. As for more detail information, we also employ HF solution as cleaner to remove the surface passivated SiO2 layer of A-Si sample for more understand of the silicon oxidation process. The TGA curve of A-Si powder with HF pre-treated demonstrated three steps reaction process during the temperature increased (Figure S2b). The first step is slightly weight decrease (~ 2%) from room temperature to about 250 °C, which is mainly come from the water etc. desorption process. The A-Si powder with HF pre-treated show a slow and steady weight increase process from 250 to ~ 600 °C. When the temperature is higher than 600 °C, the sample surffer from a higher weight increase. Such weight increase process in mainly come from the oxidation of Si to SiO2. Such TGA curve indicate that the silicon oxidation reaction by air is a dynamic restriction process, a passivated SiOx layer will be formed in the interface between silicon and oxygen, which will slow down the reaction rate. Then, it can provided a space and enough time for us to prepare Si materials by using the air oxidation without excessive reaction. Generally, in the traditional magnesiothermic reduction process, there are two competition reaction as 1) 2Mg + SiO2 = 2MgO + Si and 2) 4Mg + SiO2 = 2MgO + Mg2Si. The low conversion efficiency is mainly come from the significant formation of Mg2Si, along with large amount of unreacted silica, due to the uncontrollable process. Compared with the traditional magnesiothermic reduction process, our deep reduction and partial oxidation strategy to the preparation of silicon is divided into two controllable steps. The first step is deep reduction of SiO2 with excess Mg to formation of Mg2Si/MgO/Mg intermediate product as 4Mg + SiO2 = 2MgO + Mg2Si. All of the SiO2 have been reducted to Mg2Si due to the existence of excess Mg in the reaction process. In the second step, the as-prepared Mg2Si/MgO/Mg intermediate product has been diretly oxidated in air atmoshpere to producte of silicon. The reaction equationes in this process are Mg2Si + O2 = 2MgO + Si and 2Mg + O2 = 2MgO. Since the silicon oxidation


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reaction with O2 is a dynamic restriction process, a passivated SiOx layer will be in-situ formed in the interface between silicon and oxygen, which will slow down the reaction rate, thus allow us to prepare silicon materials with high yield.

Figure 2.The TGA curves of Mg2Si/MgO intermediate from a), the deep reduction of silica aerogel previous and b), commercial Mg powder under the air atmosphere from roomtemperature to 800 °C at a heating rate of 2.5 °C min-1.

To comfirm the general of our strategy, different silica previous with various morphologies and porous distribution (such as silica aerogel, commercial bulk sand, silica adiatom frustule, and even silica zeolite) have been employed. Figure S3 in the supproting information shows SEM images of these silica prevous. The bulk sand and Silica adiatom frustule shown solid macrostructure morphology and the silica aerogel is uneven sized of nanoball composed with some aggregation. In contrast, the silica zeolite provided a uniform porous structure composed in nanorods structure. Silicon production is synthesised by using these silica previous based on the deep reduction and partial oxidation method (where A-Si for the resulting production form the previous silica aerogel, C-Si for the production from commercial bulk sand, D-Si for the production from silica adiatom frustule and S-Si for the production from the silica zeolite). All


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XRD peaks including intermediated and the resulting production in the synthetic process can be indexed to the similar results of the A-Si fabrication process Figure S4-6. Figure 3a-c, shows the morphology and structure of the A-Si production. From the scanning electron microscopy (SEM, Figure 3a) and transmission electron microscopy (TEM, Figure 3b) images, it is found that the A-Si production has a ruffle and clustering nanosheet structure. Nitrogen adsorption measurements indicate that the specific BET (Brunauer–Emmett– Teller, Figure S7) surface area of the A-Si is 239.3 m2 g-1. Barrett–Joyner–Halenda (BJH, Figure 3c) curves indicate that such A-Si production possesses a large amount of uniform mesoporous around 4 nm. When the bulk solid sand was used as a previous, we also obtained the compound structure with many primary nanoparticles connection and aggregation (Figure 3d-f). The formation of such nanoparticles aggregation is associated with the previous silica used. When used the silica zeolite as previous, it can obtained a similar morphology frame, and much of mesoporous formation in it (Figure 3g-i). Such morphology evolution can also be found in the silica zeolite previous (Figure 3j-l). The Nitrogen adsorption curves of all sample is shown in Figure S8-10, and BET surface areas of C-Si, D-Si and S-Si is 23.9, 74.13 and 102.6 m2 g-1, respectively. Moreover, the tap density of the A-Si, D-Si, S-Si and C-Si powder was estimated to be 0.375, 0.615, 0.539 and 0.734 g cm−3, respectively.


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Figure 3. The characterization of the silicon production come from different silica previous.a-c), the SEM image, TEM image and BJH porous distribution of A-Si production obtained by using silica aerogel; d-f), the SEM image, TEM image and BJH porous distribution of C-Si production obtained by using commercial bulk sand; g-i), the SEM image, TEM image and BJH porous distribution of S-Si production obtained by using silica zeolite; j-l), the SEM image, TEM image and BJH porous distribution of D-Si production obtained by using silica adiatom frustule. Figure 4a provides the detail structure of the as-producted A-Si, which shown a nanosheets morphology and mesoporous structure. Figure 4b displays the high-resolution TEM image of the A-Si production. Lattice fringe images of 0.31 nm, which can be ascribed to the (111) crystal planes of cubic Si, are commonly observed. Moreover, the high-resolution TEM image of the ASi production shows an amorphous layers with a thick above 3 nm on the surface of Si. The surface contents and valence state of the A-Si are further investigated. Figure 4c shows the Xray photoelectron spectroscopy (XPS) spectrum of A-Si. It can be seen that the Si 2p spectrum of the A-Si exhibits two sinifigance peaks at ~103 eV and ~99 eV. The peak at ~103 eV is assigned to the existence of SiO250 and the peakat ~99 eV is due to the Si. The oxygen concentration is quantified as high as 63.7 at.% (close to the content of pure SiO2 ), indicating the full coating


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layer of SiO2 surface layer, in tune with the HRTEM results. Figure 4d shows the Raman spectra of A-Si material with three peaks at about 300 cm-1, 499 cm-1 and 920 cm-1. These peaks can be index to the Si, further confirming the phase structure of the final product.37, 53

Figure 4. The detail mesoporous structure of the as-producted A-Si. a) TEM image, b) HRTEM image, c) XPS spectrum and d) Raman spectra of A-Si production.

Electrochemical Properties The cyclic voltammogram (CV) curve of A-Si anode was measured in an assembled coin-type half cell in the range between 0.01 and 2.0 V (vs. Li/Li+) at 0.1 mV s-1 (Figure S11). For the first anode scan, one broad reduction peak at around 0.8 V can be ascribed to the SEI layer formation coming from the electrolyte reduction and deposition, which disappeared from the following cycles.54 The main reduction peak at first anode scan curve begins at a potential of about 0.3 V and became to a shape peaks below 0.15 V, corresponding to the formation of LixSi phase from crystal silicon.55 In the following cathodic scan, the oxidation peaks appear at 0.35 V and 0.51 V


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is correspond to the delithiation process from the phase of LixSi to amorphous Si.56 The subsequent CV curves show two discharge peaks at 0.19 V and ~0.03 V, and two charge peaks at 0.35 V and 0.51 V, which is the typical electrochemical behavior of amorphous silicon in the reversible charge/discharge cycles. After the first scan, the CV curves exhibit a good coincidence, which are consistent with the outstanding cyclic performance of the silicon anode. Moreover, The voltage profiles of the A-Si anode for the first five cycles at 0.36 A g-1 are shown in Figure 5a. It can be seen that a slight plateau appears at about 0.8 V in the initial discharge curve and a long and flat plateau appears at around 0.2 V. After that, a long charge plateau from 0.2 to 0.5 V in the charging process have been revealed. Since the second cycle, the dischargecharge curves are almost identical, indicating a stable and reversible electrochemical behaviour of the A-Si anode. These results are also in good agreement with the CV curve of A-Si anode. As a result, the cell delivers a discharge capacity of 2789 mAh g-1 and a charge capacity of 1782 mAh g-1 at 0.36 A g-1 at the first cycle. When cycling up to 100 cycles, the A-Si electrode maintains a reversible capacitly of 1470 mAh g-1, which is 82.5 % of its initial reversible capacity (Figure 5c). The stable capacity during the initial reversible cycles may be attributed to the increasing of the active sites for reversible electrochemical Li storage and effective reduced SEI layer generation, which come from the mesoporous silicon structure combined with the SiO2 coating layer. Moreover, the long-life performance of the A-Si anode at 1.8A g-1 after the activation (0.36 A g-1 for the five cycles) have been provided in Figure 5e. The half-cell delivers a reversible capacity of 1772 mAh g-1 at 1.8A g-1. After cycling up to 1400 cycles, a stable reversible capacity of 873 mAh g-1 is remained, indicating a retention of 49.3%. It means that the cell degrades only about 0.035% per cycle over 1400 total cycles indicates the superior stability of the A-Si anode. Note that, expect for the Si with constant-charge capacity technology


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and the thin-film silicon from CVD (chemical vapor deposition), there have been just a few Si anode materials cycling up to 500 cycles reported before, expecially for the Si anode materials without further carbon modification. Here, we compare these previously reports with ours resulting in Supporting Information (Table S2). Coulombic efficiency (CE) is also a very important concern for practical silicon based anode. For our A-Si electrode, the initial CE was 64.1%, which may due to the formation of an irreversible SEI layer on the surface of the electrode.52 The low initial CE could be improved by pre-lithiation in further studies. During cycling, the CE is up to 92.6% in the second cycle and 94.2% in the three cycle and 96.5% in the six cycle and then go to 99 ~ 100% in the subsequent 7th cycle to 1400th cycle excepted one cycle in 505th. Such specific CE point in 505th is mainly come from the sudden temperature fluctuations in the room temperature. It also points out that the undulate capacity curves is mainly effect by the temperature, which is confirmed by the previous reports.57 The high CE might derive from the stable SEI layer during cycling. The average CE of the A-Si anode from the 2nd to 1400th cycle is 99.95%, and it can probably be improved by carbon composition, surface modifications and electrolyte improvement. Furthermore, the electrochemical performances of different mesoporous silicon production (ASi, C-Si, D-Si and S-Si) have also been evaluated. The cycling stability of the four kinds of silicon sample have been shown in Figure 5c. All of the silicon electrode deliver good electrochemical performance. As comparation, The C-Si sample has better reversible capacity than D-Si and S-Si, while has a lower stability. The reason might relate to their specific morphology. Although the surface area of D-Si and S-Si is much higher, the morphology of D-Si and S-Si sample is mainly aggregated by isolated particle. Due to the surface coating of an insulative SiO2 layer, the structure with isolated particle aggregation is not benefit to the electron


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and ionic conduction during the battery operation. At the same time, the lithiation process for silicon anode is self-limiting,58 an insulative SiO2 layer would further obstacle the lithiation of silicon and result in the inactive of the silicon internal. Thus, the reversible capacity of D-Si and S-Si electrode is lower than C-Si electrode and the cycling stability of D-Si and S-Si electrode is better than C-Si electrode. On the other hand, the discharge-charge voltage profiles of D-Si, S-Si and C-Si anode (Figure S12) display similar electrochemical plateaus with A-Si anode at charge/discharge process, which indicated the same electrochemical reaction mechanism. The different of voltage profiles among A-Si, D-Si, S-Si and C-Si anode is come from the plateaus capacity, which is mainly due to the different surface area and active silicon content in the resulting four kinds of sample. The similar electrochemical performance of A-Si, C-Si, D-Si and S-Si production indicates that the general and efficient strategy for the fabrication of high performance silicon electrode materials. The resulting mesoporous Si anodes also exhibit a decent rate performance (showed in Figure 5b and d). As an example, Figure 5b shows the typical discharge-charge voltage profiles of A-Si anode at various current density. The discharge curves have a flat plateau from ~0.3 V to ~0.1 V and the charge plateau from 0.2 to ~0.5 V at 0.36 A g-1, which is good agreement with the typical charge/discharge behavior of Si. With the increase of current density, there’s only a slight change of the discharge and charge voltage files, indicating a small polarization process. The A-Si anode delivers the reversible capacities of 2100, 1800, 1650, 1400 and 1100 mAh g-1 at 0.36, 0.72, 1.8, 3.6 and 7.2 A g-1, respectively (Figure 5d). Sinificantly, the cell still can be able to maintain about 710 mAh g-1 even at a higher rate of 18 A g-1. Furthermore, after runing the high rates, the cell with A-Si anode could recover back to 1500 mAh g-1 at 1.8 A g-1 and 1800 mAh g-1 at 0.36 A g-1, which is close to the capacity without high rate process, manifesting that the A-Si anode


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has a fine rate performance and good reversibility. On the other hand, the silicon anode such as C-Si, D-Si and S-Si also provide a good rate performance and capacity retention after rate capacity tests (Figure 5d).

Figure 5. Electrochemical performances of mesoporous silicon production anode. a) First five cycles of A-Si electrode at 0.36 A g-1. b) The typical galvanastatic discharge–charge curves of the cell with A-Si electrode at different current densities from 0.36 A g-1 to 18 A g-1 and then back to 1.8 and 0.36 A g-1. c) Cycling performance of the half-cell with A-Si, C-Si, D-Si and SSi anode at the constant current density of 0.36 A g-1. d) The rate performance of A-Si, C-Si, DSi and S-Si anode. e) Cycling property of the A-Si electrode at 1.8 A g-1 for 1400 cycles.


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Inspired by the successful fabrication of the silicon anode with long-term cycling stability performance, we further inverstigate its action mechanism on the electrochemical Li-storage process during cycling. To this end, A. C. impedance in-situ measurement technology is applied to investigate the SEI layer at different lithiation/delithiation state during the cycling. Figure 6a and b show the Nyquist plot of A-Si electrode in the full lithiation (0.05V) and de-lithiation (1.2V) state in the first cycle at 0.36 A g-1. The plots can be well modeled by an commonly adopted R(CR)(CR)W equivalent circuit (Figure S13, R1 is the electrolyte resistance, R2 is the ohmic resistance in the interface of the SEI layer, R3 is the charge transfer resistance which is related to the actived Si electrode particles. While, C is the constant phase elements (CPE1 and CPE2), and W is the the Warburg impedance for the solid state diffusion of Li+ in the A-Si electrode). The simulated kinetic parameter is summerized in Table 1. The A-Si anode shows the similar and low R1 (~3.5 Ω) and R2 (7-10 Ω) value at the lithiation and de-lithiation states, indicating that the A-Si electrode has a similar and high Li+ and electron conduction in electrolyte and SEI layer at both lithiation and de-lithiation states. These analysis suggest that the A-Si elelctrode being a stable SEI layer during lithiation and delithiation process, without significant cracking and thickening. Moreover, the R3 of the A-Si elelctrode at the lithiation situation (9.12Ω) is much lower than that of the de-lithiation situation (66.58 Ω), which is attributed to the different diffusion rate of the electron and Li-ion on the interface of the A-Si electrode between lithiation and de-lithiation situation. Such behaviour is coincident with and previous reports in several anode materials including silicon and germanium anode.57, 59-60 To further confirm the stable SEI layer, we performed cell impedance measurements of the ASi electrode at full delithiation situation after different cycles. The Nyquist plot is shown in Figure 6c and the simulated kinetic data is collected in Table 1. The impedance increase


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obviously between the fresh cell and the subsequent cycling cell, which may be attributed to the major reason of the SEI formation. As shown in Table 1, no obvious impedance change is observed after cycling for 700 cycles, especially for R1 and R2, indicating limited growth of SEI layer during cycling. Therefore, it's speculated that the SEI layer formed here is thin enough for ensuring the stable cylcing stability of A-Si anode. To further study the morphology evolution of the Si anode after cycling, we disassembled the cells in the fully delithiation state after 100 cycles at 1.8 A g-1 and monitored the morphology of the electrode by TEM. Figure S14 shows the representative TEM images of the A-Si, C-Si, S-Si and D-Si anode after 100 cycles. It is apparent that the A-Si anode remains the initial ruffle and clustering nanosheet structure in addition to more aggregation. Such aggregation might come from the electrode assemble process which with the using of binder and also may be come from the electrochemical agglomeration process during cycling. Compared with A-Si materials, C-Si, D-Si and S-Si samples after 100 cycles suffer from much aggregation and structure collapse (Figure S15 (b)-(d))

a)

b)

c)

Figure 6. a-b) Nyquist plots obtained for the cell of the A-Si electrode in lithiation and delithiation situation. c) Nyquist plots obtained for the cell of the A-Si electrode at the full delithiation situation after different cycles.


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Table 1. The simulated kinetic parameter for the cell of A-Si electrode. Statement Fresh Lithiation, 1

st

Delithiation, 1

st

R2(Ω)

R3(Ω)

CPE1(F) a)

CPE2(F) a)

0.90

---

48.41

---

3.17E-5

3.401

9.11

9.12

2.3E-5

4.82E-4

3.81

7.84

66.58

6.6E-6

7.11E-4

th

3.81

7.01

83.42

4.9E-6

8.52E-4

st

3.85

7.06

83.39

4.5E-6

8.84E-4

Delithiation, 300th

4.39

7.37

78.82

5.5E-6

8.89E-4

th

4.64

7.93

84.92

5.6E-6

8.18E-4

th

4.88

8.69

92.27

4.6E-6

7.98E-4

st

4.33

7.92

92.59

5.1E-6

7.77E-4

Delithiation, 700th

4.11

7.19

86.43

4.3E-6

8.30E-4

Delithiation, 100

Delithiation, 101

Delithiation, 400 Delithiation, 500 Delithiation, 501 a)

R1(Ω)

CPE1 and CPE2: represent the two resistors with constant phase elements.

It is a very challenging key point to further evaluate the full-cell performance based on high capacity of anode materials for particle application. Then, we established a full cell based on our prepared A-Si electrode as negative (N) and LiCoO2 (LCO) electrode as positive (P). Firstly, the electrolyte containing FEC (fluoroethylene carbonate) was employed for the silicon anode to form a more stable SEI layer during cycling. Therefore, the LCO cathode was also tested and evaluated by using the same electrolyte. The LCO cathode under such electrolyte delivered an initial charge capacity of 138.6 mAh g-1 at 1 C as well as a Coulombic efficiency of 91.0% and retained its capacity very well for 100 charge/discharge cycles (Figure S15). Thus, we confirm that FEC can be adding into the electrolyte for Si-LCO full cell. On the other hand, before completing the Li-ion full cell assembly, the silicon electrode was pretreated by electrochemical pre-lithiation in order to suppress the first irreversible capacity loss. Such strategies have been widely investigated and suggested to overcome the low ICE electrode materials in full-cell assemble. The electrochemical pretreated silicon anode and LCO cathode were assembled in CR2016 coin-type cell with the N/P (negative electrode capacity/positive electrode capacity)


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ratio of ~1.2 using the same separator and electrolyte as the silicon anode used in its half-cell test. The cell was cathode limited and was charge and discharge between 2.5 and 4.0 V at the current density of 1.0 C based on the weight of cathode active materials. Figure 7 shows a typical charge/discharge voltage profile of the Si-LiCoO2 full cell between 2.5 and 4.0 V. The practical working voltage of the Si-LCO full-cell is in the range of 3.2-3.8 V. The first charge and discharge capacities were 136.2 and 119.4 mAh g-1, respectively, yielding a Coulombic efficiency of 87.7%. The cell realized a slight decayed capacity at the following cycling. The highest reversible capacity of the full-cell at 1 C is ~140 mAh g-1 (based on the weight of cathode active materials) and energy density of ~487 Wh kg-1 (based on the total weight of anode and cathode materials). Furthermore, the full-cell displays at about 80 mAh g-1 (~292 Wh kg-1) after 100 cycles, with capacity retention of 60%.

Figure 7. (a) Galvanostatic charge-discharge profiles of the Si-LiCoO2 full cell between 2.5 and 4.0 V at the current density rate of 1 C. (b) Cycling property and coulombic efficiency of the SiLiCoO2 full cell between 2.5 and 4.0 V at 1C.


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Conclusion A deep reduction and partial oxidation strategy to convert low cost SiO2 into mesoporous Si anode with the yield higher than 90% is provided. Such approach can provided efficient silicon fabrication and in-situ formation of several nanometer SiO2 layer on the surface of silicon particles. Thus, the result Si anode shown extremely high capacity of 1772 mAh g-1 and maintain more than 873 mAh g-1 at the current density of 1.8 A g-1 for 1400 cycles, even at the current density high as 18A g-1, it still stable remain ~710 mAh g-1. Such excellent performance is mainly come from the mesoporous nanostructures and a full mechanically rigid SiO2 coatings. At the same time, A. C. impedance measurement confirm a stabilizing effect of the resulting silicon electrode in enabling a stable SEI during the lithiation-delithiation cycling. In general, more than the high cycling stability and rate performance, our Si anode is easy and efficient to fabrication, which will well be aligned with practical application. Moreover, based on the high yield production of our strategies, diluted HCl was used instead of toxic HF to remove the impurity substance for the silicon fabrication,suggesting that our strategy is also more environmentally friendly. These promising results suggest that such efficient strategy for mesoporous silicon fabrication can potentially be commercialized for high energy Li-ion batteries. Experimental Section Materials: HCl (37%), ethanol (99.5%), Mg (99%, 100-200 mesh powder) and commercial sand (50-75 mesh) were derived from Sinopharm Chemical Reagent Co., Ltd (China). Silica adiatom frustule was purchased from Wuxi Daxin Enviorment Protected Material Co., Ltd (China). Industrial waste silica aerogel is come from ZTT International group (China).


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Mesoporous-silicon preparation: Typically, 5g of commercial sand and 10g of Mg were mixed and added into the stainless autoclave (50 mL) before sealed. After that, the autoclave was heat-treated at 500 °C for 10 h to prepare the Mg2Si intermediate. Then the Mg2Si intermediate powder was annealed at 600 °C under the air atmosphere for 5 h. Annealing at the 600 °C led to the oxidation of Mg2Si into Si/MgO and the excess Mg into MgO. The as-resulting production was immersed in hydrochloric acid (1 mol L-1) for 30 minute to remove MgO. The resulting pale-yellow precipitate was further collected by filter and washed with deionized water and ethanol and then dried for 10 hours at 60 °C in vacuum oven. The mass production of the final nano-silicon is about 2.3g, which means the yield of nano-silicon from the sand conversion is high than 90%. As an extension, different kinds of silica previous such as adiatom frustule, fumed silica and SBA-15 were directly applied for the synthesis of nano-silicon with different morphology and structure via the same deep reduction and partial oxidation process, thus getting several kinds of nano-silicon. Material Characterization:The structure of the as-prepared nano-silicon samples were characterized by X-ray diffractometer (XRD, Philips X’ Pert Super diffract meter), Raman spectra (Renishaw inVia Raman microscope, laser wavelength = 514.5 nm), scanning electron microscopy (SEM, JEOL-JSM-6700F), transmission electron microscopy (TEM, Hitachi H7650) and high resolution transmission electron microscopy (HRTEM, JEOL 2010). The BET (Brunauer-Emmett-Teller) surface areas and the BJH (Barrett-Joyner-Halenda) pore distribution of the resulting nano-silicon samples were measured by the Nitrogen adsorption-desorption method (ASAP 2020 M+C, Micromeritics). Thermogravimetric analysis (TGA) curves of Mg2Si/MgO intermediate, 200 mesh Mg powder and A-Si materials with HF or without solution treatement were provided by DTG-60H (Shimadzu) under the air atmosphere. The tap density of


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the resulting four kinds of silicon powder was analyzed with a Powder Autotap Density Meter (JT-1, Chengdu Jingxin Powder Analyses Instrument Co., LTD). Electrochemical Measurement of the prepared Si materials: The silicon anode slurry were fabricated by ball-milling mixing of 60% as-prepared nano-silicon active materials (A-Si, C-Si, D-Si, S-Si), 20% super P carbon black, 20% sodium alginate(SA) binder in water solvent. The slurry were further pasted on one side of the Cu foil by using a doctor blade (200 µm) and then dried in a vacuum oven at 80 °C for 12 h. After drying, the Cu foil were pressed by rolling and cut for a wafer (Ф = 12 mm) to fabricate the silicon anode electrode. The mass loading of nanosilicon active material for each electrode was determined to be 0.7-1.2 mg cm-2 (about 2.0-3.4 mAh cm-2). The coin-type (2016 R-type) half-cells were carefully assemble in the Ar-filled glove box (H2O, O2 < 1 ppm). Li foil was employed as counter electrode, and commercial polyolefin membrane was provided to be the separator (Celgard 2300). LiPF6 (1M) dissolved inethylene carbonate/dimethylcarbonate/fluoroethylene carbonate (EC/DMC/FEC; 1:1:0.1 by Volume) mixture solvent was used as the electrolyte. The half-cells were galvanostatic discharge-charge under the voltage between 0.005 V and 1.50 V versus Li+/Li at the current density rate of o.1 C-5 C (1 C = 0.36 A g-1) by using a LAND-CT2001A instrument. Electrochemical impedance spectroscopy (EIS) was performed with the CHI660E electrochemical workstation. Electrochemical Measurement of the commercial LiCoO2: The commercial LiCoO2 as cathode materials is come from Ningbo Veken Battery Company (5th Gangxi Avenue, West Bonded Zone, Ningbo, China). The preparation process of LCO electrode is similar to Si anode. By ball-milling of the 80 wt.% LCO material, 10 wt.% carbon black, and 10 wt.% poly(vinyl difluoride) (PVDF) binder with N-Methyl-2-pyrrolidone (NMP) solvent, a homogeneous slurry had been obtain. Then, the resulting slurry was paint onto an Al foil with 300 mm scraper and


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dried at 120 °C for 10 h. After drying and pressing, the Al foil was cut to a dish (12 mm in diameter) with activity material loading of 6-8 mg cm-2. The electrochemical performance of LCO cathode was measured by using a CR2016 coin-type cell with metallic Li foil as counter electrode, 1.0 M LiPF6 solution with carbonate/dimethylcarbonate/fluoroethylene carbonate (EC/DMC/FEC; 1:1:0.1 by Volume) mixture solvent as electrolyte and Celgard 2300 polyolefin as separator. The cells were galvanostatic charge-discharge under the voltage between 2.8 V and 4.2 V versus Li+/Li at different current density rate by using a LAND-CT2001A instrument at room temperature Electrochemical Measurement for Si-LCO full cell: The Si-LCO full cell was assembling with our prepared A-Si electrode as negative (N) and LiCoO2 electrode as positive (P). Before completing the Li-ion full cell assembly, the silicon electrode was pretreated by electrochemical pre-lithiation in order to suppress the first irreversible capacity loss. The electrochemical pretreated silicon anode and LCO cathode were assembled in CR2016 coin-type cell with the N/P (negative electrode capacity/positive electrode capacity) ratio of ~1.2 using the same separator and electrolyte as the silicon anode used in its half-cell test. The cell was cathode limited and was charge and discharge between 2.5 and 4.0 V at the current density of 1.0 C based on the weight of cathode active materials. Supporting Information: Additional data associated with this article are included: SEM images of these silica prevous; XRD peaks including intermediated and the resulting production in the synthetic process of C-Si, D-Si, S-Si; Nitrogen adsorption-desorption isotherms of the resulting silicon materials; The contrastive data of long-life stable Si anode between previous reports and ours resulting.This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *Yongchun Zhu and Yitai Qian University of Science and Technology of China, 96 JinZhai Road, 230026, Hefei, China E-mail:[email protected]; [email protected]. Shandong University, Jinan, Shandong, 250100, P. R. China, E-mail:[email protected]; Author Contributions Yongchun Zhu and Yitai Qian supervised the project. Jianwen Liang conceived the idea, experimentally realized the idea and wrote the manuscript. Jianwen Liang, Xiaona Li, ZhiguoHou and Wanqun Zhang carried out the examples synthesis, characterization and data analysis. All of the authors discussed the results, commented and revised the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the 973 Project of China (No. 2011CB935901), the National Natural Science Fund of China (No. 91022033, 21201158), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), ABBREVIATIONS LIBs, Lithium-ion batteries; SEI, solid-electrolyte interphase. REFERENCES


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A deep reduction and partial oxidation strategy to convert SiO2 into mesoporous Si anode with the yield higher than 90% is provided. Such approach can provided efficient silicon fabrication and in-situ formation of several nanometer SiO2 layer on the surface. Thus, the resulting Si anode showed extremely high capacity stability and rate performance for lithium ion batteries. Keyword: silicon; mesoporous; general strategy; lithium ion batteries; high stabilization. Jianwen Liang, Xiaona Li, ZhiguoHou,Wanqun Zhang,Yongchun Zhu,* Yitai Qian* Title: A Deep Reduction and Partial Oxidation Strategy for Preparation of a Mesoporous Si Anode for Lithium Ion Batteries


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A Deep Reduction and Partial Oxidation Strategy for Fabrication of

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