Reaction-Ball-Milling-Driven Surface Coating Strategy to Suppress

Jun 1, 2018 - (16,17) From a practical point of view, therefore, designing and fabricating ... (15,18−26) Of these, amorphous silicon oxides (SiOx, ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 20591−20598

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Reaction-Ball-Milling-Driven Surface Coating Strategy to Suppress Pulverization of Microparticle Si Anodes Yaxiong Yang,† Xiaolei Qu,† Lingchao Zhang,† Mingxia Gao,† Yongfeng Liu,*,†,‡ and Hongge Pan† †

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China

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S Supporting Information *

ABSTRACT: In this work, we report a novel reaction-ballmilling surface coating strategy to suppress the pulverization of microparticle Si anodes upon lithiation/delithiation. By energetically milling the partially prelithiated microparticle Si in a CO2 atmosphere, a multicomponent amorphous layer composed of SiOx, C, SiC, and Li2SiO3 is successfully coated on the surface of Si microparticles. The coating level strongly depends on the milling reaction duration, and the 12 h milled prelithiated Si microparticles (BM12h) under a pressure of 3 bar of CO2 exhibit a good conformal coating with 1.006 g cm−3 of tap density. The presence of SiC remarkably enhances the mechanical properties of the SiOx/C coating matrix with an approximately 4-fold increase in the elastic modulus and the hardness values, which effectively alleviates the global volume expansion of the Si microparticles upon lithiation. Simultaneously, the existence of Li2SiO3 insures the Li-ion conductivity of the coating layer. Moreover, the SEI film formed on the electrode surface maintains relatively stable upon cycling due to the remarkably suppressed crack and pulverization of particles. These processes work together to allow the BM12h sample to offer much better cycling stability, as its reversible capacity remains at 1439 mAh g−1 at 100 mA g−1 after 100 cycles, which is nearly 4 times that of the pristine Si microparticles (381 mAh g−1). This work opens up new opportunities for the practical applications of micrometer-scale Si anodes. KEYWORDS: lithium-ion battery, anode materials, microparticle silicon, amorphous coating, chemical prelithiation, ball milling

1. INTRODUCTION Developing high-energy and high-power lithium-ion batteries (LIBs) is of critical importance for satisfying the ever-growing demands of portable electronics, electric vehicles (EVs), hybrid electric vehicles (HEVs), and grid-scale energy storage.1−5 In the past 20 years, a large number of research efforts have been devoted to the design and fabrication of new promising materials for cathodes and anodes. For example, silicon (Si) is widely recognized as a highly attractive alternative to graphite as an anode material for LIBs because of its ten times higher theoretical capacity (3578 mAh g−1 for Li3.75Si), low lithiation potential, environmental friendliness, and natural abundance.6,7 Unfortunately, the extremely high lithiation capacity is inevitably associated with a huge volume expansion (>300%), which induces severe particle pulverization upon cycling and, subsequently, the loss of mechanical/electrical contact, the repeated formation of solid−electrolyte interphases (SEIs), and eventual fast capacity fading.8−12 A frequently used strategy to accommodate the huge volume expansion of Si anodes is to fabricate various nanostructures and nanocomposites, including nanoparticles, nanowires, nanospheres, nanotubes, and nanocrystals.7,13−15 However, some newly emerged challenges, including a serious side reaction caused by high surface area, low tap density, complicated preparation process, and so on, © 2018 American Chemical Society

have to be overcome prior to widespread use of nanoscale Si as anodes.16,17 From a practical point of view, therefore, designing and fabricating micron-sized Si anode materials with high tap density, high power, and low cost is highly desired. Alternatively, creating a thin functional coating layer is also a feasible approach to retard the volume expansion and increase the lifetime of a Si anode. Well-studied coating materials include carbon, silicon oxide, silicon oxycarbide, aluminum oxide, titanium dioxide, zinc oxide, lithium titanate, and even organic polymers.15,18−26 Of these, amorphous silicon oxides (SiOx, 1 ≤ x ≤ 2) or silicon oxycarbides (SiOC) have been proven to be quite effective in alleviating volume expansion and stabilizing the surface SEIs of Si nanoparticles due to their low volume expansion level (300%) and poor contact between the active materials and current collectors for the 5-cycled pristine microparticle Si electrode (Figure 6a). For the prepared microparticle Si electrodes, the thickness increase of the active materials was prevented, and the contact performance between the active materials and current collectors was remarkably improved by extending the ball milling time (Figure 6b−f). For the BM12h sample, the thickness increase of the active materials was determined to be only 50.4% (from 12.59 to 20595

DOI: 10.1021/acsami.8b05609 ACS Appl. Mater. Interfaces 2018, 10, 20591−20598

Research Article

ACS Applied Materials & Interfaces (Figure 7a−c and Figure S5a−d). The average particle size was diminished from 2.466 to 0.372 μm (in Ar), 0.364 μm (in O2), and 0.387 μm (in CO2) (Figure S5e). Unlike the milled prelithiated sample, fully oxidized SiOx submicroparticles and pure Si nanoparticles in isolated states were distinguished in the EDS mapping of the milled pristine Si microparticles in either the O2 or CO2 atmospheres (Figure S5f,g). In other words, ball milling the pristine Si microparticles in an O2 and CO2 atmosphere only gave rise to the formation of amorphous SiOx or SiOx/C instead of a coating layer (Figure S5f−i). This may be ascribed to the poor mechanical properties of the amorphous SiOx and crystalline Si, which causes them to continuously fracture and pulverize much easier due to the energetic collisions that occur during ball milling. This was confirmed by measuring their elastic moduli and hardness values with a nanoindentation technique (Figure S6). As shown in Figure 7d and e, the milled pristine Si microparticles from Ar, O2, or CO2 atmospheres exhibited a slight decrease in their elastic moduli and hardness compared to the untreated sample. However, it is worthy highlighting that an approximately 4-fold increase was obtained for the milled prelithiated Si microparticles in CO2 atmosphere, as the values of elastic modulus and hardness increased from 9.2/0.33 to 35.4/1.57 GPa. This also explains why the milled prelithiated Si microparticles in CO2 atmosphere still maintained their micron sizes. Careful comparison of the sample composition and structure implies that the strengthened mechanical properties were primarily attributed to the presence of SiC, which has a high strength, elastic modulus, hardness, and corrosion resistance.50,51 Further electrochemical investigations showed minimal improvement in the cycling stability for ball milling pristine Si microparticles in either an Ar, O2, or CO2 atmosphere, although their particle size was distinctly reduced (Figure 7f). This also rules out the size effect for the improved cyclability of the prelithiated Si microparticles milled in CO2. As a result, the in situ formed multicomponent amorphous coating layer, composed of SiOx/C, SiC, and Li2SiO3 and exhibiting high mechanical properties, should be the most important reason for the remarkably improved cycling stability of the prelithiated Si microparticles. The milling in CO2 effectively alleviated the volume expansion upon lithiation, as observed in Figure 6. It is known that the amorphous SiOx/C coating layer can accommodate to some extent the volume expansion of the nanoscale Si upon lithiation,19,27−32 but it is ineffective for Si microparticles. Here, the in situ formation of SiC played a key role in enhancing the hardness and toughness of the amorphous SiOx/C species. Meanwhile, the formation of Li2SiO3 guarantees good Li-ion conduction of the coating layer due to its high Li-ion conductivity.52 Additionally, the multicomponent amorphous coating layer also induced the formation of a stable SEI film, which protects the active materials from continuous corrosion by the electrolyte. After only one discharge/charge cycle, the surface of the electrode using the prelithiated Si microparticles milled in CO2 is completely covered by an SEI film, as no Si XPS signals were detected (Figure 8a). In addition, the F 1s XPS peak of LiF, which is one of the important SEI components in the LiPF6based electrolyte,23 remained nearly constant after the first 5 cycles (Figure 8b). In contrast, for the pristine Si sample, the XPS peaks of Si were clearly observed with considerable intensity after 1 cycle due to the exfoliation of the unstable SEI film (Figure 8a). Moreover, the F 1s peak of the LixPOyFz/ LixPFy species first dominated the XPS profile and then

Figure 8. (a) Si 2p XPS spectra of the BM12h and pristine Si electrodes obtained after the first cycle. F 1s XPS spectra of the (b) BM12h and (c) pristine Si electrodes recorded at different cycles.

decreased with cycling, further confirming the repeated formation of an unstable SEI film (Figure 8c). Thus, the in situ formed multicomponent amorphous coating layer during ball milling the partially prelithiated Si microparticles in CO2 atmosphere induced a limited volume expansion upon lithiation, high Li-ion conductivity, and a stable SEI film. These properties work together to remarkably improve the cycling stability of the microparticle Si electrode.

4. CONCLUSION In conclusion, ball milling the partially prelithiated microparticle Si in a CO2 atmosphere in situ created a multicomponent amorphous coating layer on the surface of the Si microparticles. The component of the amorphous coating layer was identified to be SiOx, C, SiC, and Li2SiOx, and there is a strong dependence of the coating degree on the ball milling time. Near complete coverage was achieved for the BM12h sample with an average particle size of 0.993 μm, which delivers optimal overall electrochemical properties. The available specific capacity of the electrode using the BM12h sample stabilized at 1439 mAh g−1 at 100 mA g−1 after 100 cycles, although its initial specific capacity was reduced to 1924 mAh g−1 from 2651 mAh g−1 for the pristine Si caused by the presence of the low capacity SiOx and inactive Li2SiO3 and SiC. The remarkably improved cycling stability was attributed to the in situ formed multicomponent amorphous coating layer composed of SiOx, C, SiC, and Li2SiOx. First, the presence of SiC induced a dramatic strengthening in the mechanical properties of the SiOx/C coating matrix, as the elastic modulus and hardness values were increased by approximately 4-fold from 9.2/0.33 GPa to 35.4/1.57 GPa, which effectively alleviated the global volume expansion of the Si microparticles upon lithiation. Second, the formation of Li2SiO3 contributed good Li-ion conduction to the coating layer due to its high Liion conductivity. Third, the multicomponent amorphous coating layer also induced the formation of a stable SEI film 20596

DOI: 10.1021/acsami.8b05609 ACS Appl. Mater. Interfaces 2018, 10, 20591−20598

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ACS Applied Materials & Interfaces

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on the electrode surface upon cycling, which protected the active materials from continuous corrosion by the electrolyte. These factors worked together to significantly improve the cycling stability of microparticle Si electrodes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05609. Experimental data and Figures S1−S6 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mingxia Gao: 0000-0002-4719-6747 Yongfeng Liu: 0000-0002-4002-8265 Hongge Pan: 0000-0002-0787-5470 Author Contributions

Y.Y. and Y.L. proposed the concept and designed the research. Y.Y. performed the experiments. All authors contributed to the data analyses and discussion. Y.Y. and Y.L. wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (51471152, 51571178), the National Materials Genome Project (2016YFB0700600), and the National Program for Support of Top-notch Young Professionals. We appreciate the help in nanoindentation measurement from Professor Lin Zhang of Zhejiang University of Technology.

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DOI: 10.1021/acsami.8b05609 ACS Appl. Mater. Interfaces 2018, 10, 20591−20598