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A Simple Synthesis Route for High-Capacity SiOx Anode Materials with Tunable Oxygen Content for Lithium-ion Batteries Yidan Cao, J. Craig Bennett, R.A. Dunlap, and M.N. Obrovac Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02977 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018
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Chemistry of Materials
A Simple Synthesis Route for High-Capacity SiOx Anode Materials with Tunable Oxygen Content for Lithium-ion Batteries Yidan Cao,1,2 J. Craig Bennett,3 R.A. Dunlap,1,4,5 and M.N. Obrovac1,2,4* 1Department
of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada 3Department of Physics, Acadia University, Wolfville, Nova Scotia, B4P 2R6, Canada 4Clean Technologies Research Institute, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada 5College of Sustainability, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada 2Department
ABSTRACT: SiOx (x ~ 1) is a promising negative electrode material used in commercial Li-ion cells. However, SiOx is commonly synthesized by relatively expensive methods. Here, SiOx was synthesized by mechanically milling bulk Si powder in air. Using this simple and inexpensive approach, SiOx with a unique microstructure containing nanometer Si regions in an amorphous silicon oxide matrix was synthesized. Furthermore, the oxygen content is easily tuned by changing the air exposure time during the ball milling process, enabling lower oxygen contents (x ≤ 0.37). This results in SiOx materials with higher reversible capacities and lower irreversible capacities than a commercial SiO. The resulting materials have volumetric capacities up to 1800 Ah/L, an average delithiation voltage of about 0.40 V with a high reversible specific capacity of 1648 mAh/g at 2C.
1. Introduction Driven by emerging demands for high energy density batteries in customer electronics and electric vehicles, extensive research has been conducted to develop highcapacity electrode materials for lithium ion batteries (LIBs).1–4 When used as an anode material, silicon has a 2194 Ah/L theoretical volumetric capacity, which is much higher than its conventional graphite counterpart (719 Ah/L).5 However, the commercial use of pure Si anodes is hindered by poor cycleability resulting from the huge volume expansion of Si (up to 280%).6 Extensive work has been carried out to address this problem. Improvements have been made by reducing the Si grain size,7 controlling crystallization,5,8–10 forming alloys/composites with other elements,11,12 and carbon coating,13 etc. Amorphous SiOx is a promising silicon-based anode material that is already present in commercial Li-ion cells. Here, commercial SiOx will be referred to simply as SiO. The structure of SiO has been described as consisting of nanometer regions of SiO2 and nano-clusters of Si surrounded by a sub-oxide (SiO2-δ) matrix.14 Amorphous silicon monoxide powder is typically produced from the simultaneous evaporation of silicon and silicon dioxide (Si(s) + SiO2(s) → 2SiO(g)) in vacuum at approximately 1400 °C.15–17 This method of producing SiO is energy consuming and difficult to realize because it is a high-
temperature vacuum process in which the oxygen content and microstructure need to be carefully controlled. SiO's low initial coulombic efficiency (ICE) is considered one of its main drawbacks. The major contributor to the low ICE of SiO is the irreversible reaction of Li with oxygen in the suboxide matrix to produce Li4SiO4.18 One method to increase the performance of SiO is to lower its oxygen content. This would result in a lower ICE and higher reversible capacity. It has been reported that Si and SiO2 can be ball milled together to make SiOx.19 In principle, SiOx with any oxygen content between 0 ≤ x ≤ 2 can be prepared by this method, depending on the starting ratio of Si and SiO2. However, we found this method results in high iron contamination from the ball mill, due to the abrasiveness of the SiO2 precursor. A simple, less costly, and efficient method of making SiOx with good cycling characteristics, high purity and controllable oxygen content, would be highly desirable. In this study, we show that amorphous SiOx negative electrode materials with an easily controlled oxygen content between 0 ≤ x ≤ 0.37 can be prepared by simply ball milling Si powder in air. To our knowledge, bulk amorphous SiOx with x < 0.6 has not previously been reported. The resulting material has a lower ICE and higher reversible capacity than a commercial SiO sample. The material also has a unique microstructure in which nano-Si regions are fully surrounded by amorphous silicon oxide. The enhanced SiOx 1
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performance coupled with its simple synthesis method are highly attractive. 2. Experimental Materials Synthesis: SiOx powders were synthesized by ball milling in a high energy ball mill (SPEX Model 8000-D, Spex CertiPrep, Metuchen, NJ). 0.5 mL of Si powder (Sigma Aldrich, -325 mesh, 99%) with 180 g of 0.125" stainless steel balls in a 65 ml hardened steel vial equipped with an o-ring seal, shown in Figure 1(a). The ball milling was conducted in either an air or argon atmosphere. For argon milling, the milling vial was sealed in an Ar-filled glovebox with the o-ring seal in place. For air milling, the vial was sealed in air with the o-ring seal removed. A small hole was drilled in the clamping ring at the same level as the vial/cover interface to facilitate the entry of air during milling, as shown in Figure 1(a) and (b). All samples were ball milled for a total time of 20 hours, but with different air exposure times during milling. Figure 1(c) schematically illustrates the milling procedure for each sample. Some samples were subsequently heated for 3 hours at 800 °C in an Ar atmosphere. Characterization: X-ray diffraction (XRD) patterns were measured using a Rigaku Ultima IV diffractometer equipped with a Cu Kα X-ray source and a graphite diffracted beam monochromator. True sample densities were measured with He gas using a Micromeritics AccuPyc II 1340 gas pycnometer. Oxygen contents were determined by the LECO method (NSL Analytical Services, Inc, Cleveland OH). Specific surface area was determined by the single-point Brunauer-Emmett-Teller (BET) method using a Micromeritics Flowsorb II2300 surface area analyzer. Scanning electron microscope (SEM) images were measured using a TESCAN MIRA 3 LMU Variable Pressure Schottky Field Emission Scanning Electron Microscope. Transmission electron microscopy (TEM) images were taken using a Philips CM30 TEM. Electrochemistry: Electrode slurries were prepared by mixing active materials, carbon black (Imerys Graphite and Carbon, Super C65) and a 10 weight % aqueous solution of lithium polyacrylate (LiPAA) in distilled water, so that the final dried coating had an active/carbon black/LiPAA volumetric ratio of 70/5/25, which is equivalent to a mass ratio of ~77/5/18 (exact ratio depends on the density of the alloy). Either the as prepared SiOx samples or as received commercial SiO (Aldrich, -325 mesh) were used as active materials. Slurries were mixed for one hour in a Retsch PM200 planetary mill at 100 rpm with three 13 mm tungsten carbide balls and then spread onto copper foil (Furukawa Electric, Japan) with a 0.004 inch gap coating bar. Then the coatings were dried in air for 1 hour at 120 °C and cut into 1.3 cm disks and then heated under vacuum for 1 hour at 120 °C with no further air exposure. The mass loadings of electrodes were 1~2 mg/cm2 which corresponds to an area specific capacity of 2~2.5 mAh/cm2. The porosity of electrodes were 60~65 vol. %. Electrodes were assembled in 2325-type coin cells with a lithium foil counter/reference electrode. Two layers of Celgard 2300 separator were used in each cell. 1 M LiPF6 (BASF) in a solution of ethylene carbonate, diethyl
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carbonate and monofluoroethylene carbonate (volume ratio 3:6:1, all from BASF) was used as electrolyte. Cell assembly was carried out in an Ar-filled glove box. Cells were cycled galvanostatically at 30.0 ± 0.1 °C between 5 mV and 0.9 V using a Maccor Series 4000 Automated Test System at a C/20 rate and C/40 trickle discharge for the 1st cycle and a C/5 rate for the following cycles with a C/20 trickle discharge (lithiation). Electrodes were also cycled at various rates (C/10, C/5, C/2, 1C, 2C, and C/10) for 10 cycles respectively to test rate capability. 3. Results and Discussion All samples were ball milled for a total time of 20 hours, but with different air exposure times during milling, according to Figure 1(c). These samples are referred to here as SOh, where h (0 ≤ h ≤ 16) is the number of hours of air exposure during milling. All ball milled products consist of ~1 μm particles and some 3~5 μm flakes, as observed by SEM (Figure S1). The amount of aggregated flakes increases with increasing milling time in air, which is reasonable since oxygen has been reported to increase grain size during ball milling.20,21 The BET surface area of all the samples are similar (21 ± 1 m2/g). Therefore, air exposure during milling increased grain size, but did not appreciably change sample surface area. Figure 2(a) shows the oxygen content as a function of the milling time in air. The oxygen content initially increases with increasing milling time in air, but then surprisingly does not increase after a composition of SiO0.37 has been reached. This composition is well below the theoretical oxygen content for full reaction to SiO2. The Si in SiO0.37 must be in a form that it is no longer available for reaction with oxygen, as is further confirmed below by TEM. In contrast, a commercial sample of SiO had a composition of SiO0.97 as measured by the LECO method. Figure 2(a) demonstrates that the oxygen content can be controlled by limiting the air exposure time during milling. The steadystate oxygen content that is ultimately reached can be beneficial for a stable manufacturing process of the saturated oxygen composition. (a)
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Figure 1. (a) An image and (b) an illustration of the mechanical milling vial with a hole in the ring clamp to allow oxygen to enter during ball milling. (c) Schematic illustration of the SiOx synthesis procedure.
The XRD patterns of the resulting powders after 0h, 8h and 16h of air exposure during milling are comparatively shown with a commercial SiO in Figure 2(b). The SO0 sample (i.e. Si milled 20 h in argon) comprises amorphous (a-Si) and nanocrystalline Si (n-Si) components. These two components and peaks from an amorphous silicon oxide were fit using pseudo-Voigt peak shape functions. The relative integrated intensity of each phase is plotted in Figure 2(c). Also, TEM images of SO0 are shown in Figure S2. This sample consists of ~5-10nm silicon crystallites in 2
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an amorphous silicon matrix, which is typical of ball milled Si.22–24 As the air exposure time during milling is increased, the amount of silicon oxide phase increases linearly. With increased oxygen content, the n-Si content reduces substantially. After 16 h of air exposure during milling, the Si phase becomes essentially X-ray amorphous, similar to that of commercial SiO. However, there is more X-ray intensity from silicon oxide in SiO which is reasonable due to its high oxygen content. Figure S3 shows the relative amount of Si that is in the nanocrystalline phase and the oxygen content plotted as a function of milling time. Both are roughly correlated. As the oxygen content increases, the amount of n-Si decreases. Both the oxygen content and the average Si grain size become constant after 8 hours of air milling. Figure 3(a) shows a high-resolution TEM image of the SO16 sample. The material is mostly amorphous, but also contains ~5-10 nm embedded nanocrystallites with lattice spacings corresponding to Si. The Si nanocrystallites are completely isolated from one another and are completely surrounded by an amorphous phase. All of the samples have similar TEM images with crystalline domains (a few nanometers in size) and grain boundaries. Therefore, according to XRD and TEM results, the SiOx samples consist of a-Si, amorphous silicon oxide matrix , isolated Si single nanocrystallites and grain boundaries, which may contribute positively to Li ion diffusion and lithiation/delithiation process during cycling.23 Raw data Background Crystalline Si Amorphous Si Amorphous SiO2-δ Fitted data
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Figure 3 (a) TEM images of (a) the as milled SO16 sample and (b) the SO12 sample after heating at 800°C.
a-Si c-Si SiO2-δ
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resulting microstructure is that of small Si regions dispersed in an amorphous silicon oxide matrix. To confirm this model, a sample of SO12 was heated to 800 °C to crystallize all of the Si phase. TEM images of this sample are shown in Figure 3(b). The heated sample now comprises many isolated Si nanocrystallites, none of which are greater than 10 nm in size. This demonstrates that all of the Si in the samples milled in air for 8 hours or more is well isolated by the silicon oxide matrix, which eliminates Si aggregation and the formation of large crystallites during heating. Such a microstructure is highly desirable, since the active Si phase should be well protected from reaction with electrolyte from the surrounding silicon oxide matrix phase. The silicon oxide matrix phase should also suppress Si aggregation during cycling. In addition, the ability to heat the sample to high temperature without Si aggregation makes it compatible with carbon coating by CVD methods, which is an important method to increase the cycle life of SiO electrodes further.13
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Chemistry of Materials
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Figure 2 (a) Oxygen content in SiOx samples as a function of air exposure time during milling. (b) X-ray diffraction pattern fitting results of SiOx samples. (c) Relative integrated XRD intensities of different phases in SiOx as a function of milling time in air.
The above results suggest a mechanism for ball milling Si in argon and air. When Si is ball milled in Ar, much of the sample remains nanocrystalline and the grain size cannot be reduced further, regardless of milling time. This is likely because of cold-welding processes that can continually reform nanocrystalline Si during the milling process. When Si is milled in air, the fresh Si surfaces react with oxygen to form an amorphous silicon oxide phase, which would prevent any cold welding. Thus the Si grain size can be reduced substantially. As the grain size is reduced, fresh Si surfaces are exposed to oxygen, resulting in the formation of more silicon oxide. Eventually the Si regions in the sample become completely isolated by the silicon oxide matrix phase and become so small that they can no longer fracture in the ball mill, as shown in Figure S2. At this point no fresh Si surfaces can be formed during milling and further reaction with oxygen can no longer take place. The
Figure 4(a) shows the voltage curves of SO0, SO8, and SiO. Voltage curves, differential capacity curves, and cycling performance of all samples are shown in Figure S4-S6. The average delithiation voltages of all samples are about 0.40 V. As the air exposure time during milling is increased, the capacity accordingly decreases and the formation of Li15Si4, as indicated by a 0.43 V delithiation plateau, is suppressed. Air exposure during milling also causes a small plateau to appear at voltages greater than 0.5 V during the first lithiation, which can clearly be seen in the SO8 voltage curve in Figure 4(a). This high voltage initial lithiation plateau is not present in the following cycles. As the oxygen content is increased, the amount of Li associated with the high voltage initial lithiation plateau increases linearly, as shown in Figure 4(b). Therefore, this plateau is directly related to the irreversible reaction of Li with oxygen in the sample. For commercial SiO, a high voltage initial lithiation plateau does not appear, however, it is present after ball milling commercial SiO, as shown in Figure 4(b) (and Figure S7(d)), and also fits the linear trend with oxygen content. Therefore, the high voltage initial lithiation plateau represents oxygen that is easily available for reaction with Li (as indicated by its relatively high voltage) and is present only when defects are introduced by ball milling. As shown in Figure S7(a) and (b), the particle sizes in c-SiO decrease after ball milling. Therefore, the appearance of high voltage initial lithiation plateau in milled c-SiO is understandable, since the introduction of defects would cause the formation of free radical oxygen, which would react readily with Li. The slope of the linear trendline in Figure 4(b) is 0.95, 3
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Chemistry of Materials
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Figure 4 (a) Representative voltage curves of SO0, SO8 and SiO samples. (b) The relation between the amount of Li inserted per mole of SiOx during the initial ~0.5V voltage plateau and the oxygen content, x.
The very low initial coulombic efficiency (ICE) of commercial SiO (measured here to be ~55%) is one of its major drawbacks. The ball milled SiOx samples made here have much higher reversible capacities (>1500 mAh/g, i.e. 1800 Ah/L) and higher ICE values (>70%). The cycling performance of SiOx is improved with increasing oxygen content, as shown in Figure S6. The oxygen saturated samples, i.e. SO8, SO12, SO16, have similar cycling performance to the commercial SiO. This trend is likely due to the increasing amount of inactive Li4SiO4 phase in the samples, which can inhibit the expansion of silicon and capacity fade. In addition, the surface composition of SiOx may also be increasing in oxygen content with x. The criticality of the surface of Si alloys and its interactions with binders for achieving good cycling has been shown previously.25,26 In this regard, the increased oxygen content may improve the connectivity of the surface with the binder, also improving cycling. Figure 5(a) compares the cycling performance of SO12 and the commercial SiO sample. Since these samples have different particle sizes, cycling performance is also shown for the ball milled commercial SiO sample. Samples SO12 and the commercial SiO sample have similar cycling performance, but the capacity of the SO12 sample is much larger. Ball milling improves the cycling performance of the commercial SiO sample. However, it should be noted that the ball milled commercial SiO sample may not represent the behavior of commercial SiO. Ball milling commercial SiO results in a high level of Fe contamination (Figure S7(c)), likely due to the high abrasiveness of SiO. Its electrochemistry has also been significantly altered by the ball milling process, as shown in Figure S7(d). Indeed, after milling it more closely resembles Si ball milled in air, having
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an initial high voltage plateau associated with oxygen in a form that readily reacts with lithium. Figure 5(b) shows the rate performance of the oxygen saturated SO12 alloy, commercial SiO, and ball milled commercial SiO at different rates. Rate capability of the ball milled SO12 sample synthesized here is excellent, which 3000maintains a reversible capacity of ~1648 mAh/g at 2C; (c) 2700much superior to the SiO sample (~600 mAh/g at 2C). Rate capability of the commercial SiO sample is improved by ball 2400 milling (~ 801 mAh/g at 2C), but is still lower than the SO12 2100 C/10 C/5 sample. C/2 C/10 1C 1800 2C Thermal treatment is a known method of improving the 27. Figure 5(a) also shows the Reactive 0.37 1500cycling of milled SiOx SiO materials cycling performance of SO12, pristine SiO and ball milled 1200 SiO after thermal treatment. The capacity of ball milled SiO 900 reduces significantly after annealing. This may be due to the 600presence of Fe contamination (as shown in Figure S7(c)) in Commercial SiO 300the sample, which can react with Si during heating. Nevertheless, its cycling performance is improved. The 0 0 10 20performance 30 40 50 60 cycling of both SO12 and SiO are also Cycle number improved by annealing at 800 °C. The SO12 electrodes have little fade, comparable to the commercial SiO tested here, however the capacity and initial coulombic efficiency of SO12 is significantly higher. These results show that cycling can be improved further by thermal treatment and that the ball milled SiOx materials are compatible with high temperatature processing, such as CVD carbon coating, that can additionally improve cycling. The effect of thermal treatment on the microstructure and further improvements to cycling will be described in future publications. Because of the ease of manufacture, good electrochemical characteristics and thermal stability of these materials, we believe that they could be rapidly adopted by commercial cell makers to improve Li-ion cell energy density.
Capacity (mAh/gSiOx)
indicating it is associated with a reaction between lithium and oxygen in a 1:1 Li:O molar ratio. This is consistent with the irreversible formation a Li4SiO4 matrix, which is known to occur during the lithiation of commercial SiO.18 Further details on the lithiation/delithiation process and capacity model in SiOx will be discussed in an upcoming publication.
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Figure 5 (a) Cycling performance of SO12 and a commercial SiO as made at room temperature (RT) and of the samples after annealing at 800 ºC. (b) Rate capability of the SO12 (reactive milled SiO0.37) and a commercial SiO.
4. Conclusion It was found that amorphous SiOx with variable oxygen content could be made by simply ball milling Si powder in air. As the air milling time increased, the oxygen content 4
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Chemistry of Materials also increased, but reached a maximum at a composition of SiO0.37. At this composition the sample consisted of ~10 nm Si regions surrounded completely by a silicon oxide matrix. This proved to be an excellent microstructure for cycling performance and thermal stability. The microstructure protected the Si from reaction with electrolyte, suppressed Si phase fracture and inhibited Si aggregation during cycling and thermal annealing. Thermal annealing resulted in the cycling performance being improved further. As a result, SiO0.37 made here was found to have the same cycling performance of a commercial SiO, but with much larger volumetric capacity (1800 Ah/L vs 1400 Ah/L), much larger initial coulombic efficiency (70% vs 55%) and better rate capability. Considering the higher capacity, initial efficiency and ease of synthesis, of SiO0.37 compared to a commercial SiO, we believe this material has great promise to be developed for practical use in Li-ion cells. The synthesis method described here also opens up many research paths to new, inexpensive negative electrode materials, as the method can be readily adopted to include variations in composition and surface chemistry.
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ASSOCIATED CONTENT
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Supporting Information. SEM images, voltage curves, differential capacity curves and cycling performance of all the samples are shown in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION (17)
Corresponding Author * M. N. Obrovac:
[email protected] (18)
Funding Sources Any funds used to support the research of the manuscript should be placed here (per journal style).
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ACKNOWLEDGMENT The authors would like to acknowledge funding from NSERC and 3M Canada, Co. under the auspices of the Industrial Research Chair program. We also would like to acknowledge Dr. Xiang Yang at Saint Mary’s University for his assistance in acquiring SEM images, Dr. Jeff Dahn for use of pycnometer and BET surface area analyzer and Dr. Andrew George for his assistance in acquiring XPS profiles. Yidan Cao acknowledges financial support from the Killam Trusts.
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