Minimized Volume Expansion in Hierarchical Porous Silicon upon

Subscriber access provided by WEBSTER UNIV

Energy, Environmental, and Catalysis Applications

Minimized Volume Expansion in Hierarchical Porous Silicon upon Lithiation Fang Dai, Ran Yi, Hui Yang, Yuming Zhao, Langli Luo, Mikhail Gordin, Hiesang Sohn, Shuru Chen, Chongmin Wang, Sulin Zhang, and Donghai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01501 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Minimized Volume Expansion in Hierarchical Porous Silicon upon Lithiation Fang Dai1†, Ran Yi1†, Hui Yang2, Yuming Zhao1, Langli Luo3, Mikhail L. Gordin1, Hiesang Sohn4, Shuru Chen1, Chongmin Wang3*, Sulin Zhang2*, Donghai Wang1* 1Department

of Mechanical Engineering, The Pennsylvania State University, University Park, PA

16802, USA 2Department

of Engineering Science & Mechanics, The Pennsylvania State University, University

Park, PA 16802, USA 3Environmental

Molecular Sciences Laboratory, Pacific Northwest National Laboratory,

Richland, Washington 99352, USA 4Department

of Chemical Engineering, Kwangwoon University, Seoul 01897, Republic of Korea

KEYWORDS: Silicon anode, Lithium-ion battery, Volume expansion, Porous silicon

ABSTRACT: Silicon (Si) remains one of the most promising anode materials for next-generation lithium ion batteries (LIBs). The key challenge for Si anodes is the huge volume change during lithiation-delithiation cycles that leads to electrode pulverization and rapid capacity fading. Here

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

we report a hierarchical porous Si (hp-Si) with tailored porous structure (tunable primary pores (20-200 nm) and secondary nanopores (~ 3-10 nm)) that can effectively minimize the volume expansion. In-situ transmission electron microscopy (TEM) study revealed that the hp-Si material with the same porosity but larger primary pores can more effectively accommodate lithiation induced volume expansion, giving rise to much reduced apparent volume expansion on both material and electrode levels. Chemomechanical modeling revealed that, due to the different relative stiffness of the lithiated and unlithiated Si phases, the primary pore size plays a key role in accommodating the volume expansion of lithiated Si. The higher structural stability of the hpSi materials with larger primary pores also maintains the fast diffusion channels of the connective pores, giving rise to better power capability and capacity retention upon electrochemical cycling. Our findings point towards an optimized hierarchical porous Si material with minimal volume change during electrochemical cycling for next-generation LIBs.

INTRODUCTION Silicon (Si) has been intensively pursued as one of the most promising anode materials for highenergy-density lithium-ion batteries (LIBs) due to its high specific capacity and natural abundance.1-4 Unfortunately, the severe volume change and large mechanical stress generated during lithiation-delithiation cycles of Si anodes can cause pulverization of the electrodes, leading to fast capacity fading and poor cycling performance.5, 6 The lithiation induced stress also retards lithiation rates, and thereby limits the power performance of the Si anodes. In particular, the material-level volume change can cause enormous electrode-level volume expansion, much higher


2

Page 3 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


than the volume expansion (~10%) in current commercial LIBs, resulting in serious safety issues and thus preventing the practical application of Si based anodes.1 A variety of strategies have been explored to mitigate the volume expansion in Si anodes through either external or internal means. External engineering, such as surface coatings7, 8, elastic binders2, 5, 9,

and artificial SEI layers10-12 has led to significantly improved electrochemical performance of

Si-based materials. However, the electrode-level expansion originating from the intrinsic materiallevel expansion has remained a critical issue. On the other hand, internal methods, such as nanostructuring13, 14, nanocompositing15-17 or introducing well-tailored porous structures into the Si materials18-20, 23-25 have proven to be more effective in reducing the volume expansion of Si anodes. It has been demonstrated that nanostructured Si materials can alleviate the lithiationinduced mechanical stress and reduce the fracture resistance of Si anodes. Besides, the nanostructured Si materials can also shorten the ion and electron transport distances, leading to enhanced conductivity and power capability of Si anodes. In Si/C composites, the carbon components serve as a mechanical stiffener and fast ion transport channel, which accordingly improve the cycle life, capacity retention, and rate performance of the electrodes.24 Porous Si materials can accommodate volume change and alleviate massive stress generated during lithiation-delithiation cycles, thereby improving the structural stability and durability of Si anodes. And also, the internal pores provide large surface areas for chemical reaction and fast interfacial ion transport, and hence the enhanced rate performance of Si anodes.18, 20-22 Porosity (i.e., the total volume fraction of the pores) and pore structure (e.g., pore size distribution) are two key parameters in the design of porous Si anodes. While it is straightforward that higher porosity can better accommodate volume expansion of Si anodes, it remains unclear how the pore structure dictates the volume expansion and performance of porous Si. Herein we report the


3


Page 4 of 23

preparation of hierarchical porous Si (hp-Si) materials with tunable pore structures through a novel bottom-up wet synthesis route. The hp-Si contains large primary pores of tens to hundreds of nanometers in a nanoporous matrix with secondary pores. Lithiation-induced volume expansion in hp-Si with different pore size distributions is characterized at both material and electrode levels. Our in-situ and ex-situ experiments demonstrated that given the same porosity, larger secondary pores can accommodate lithiation induced volume expansion more effectively. Chemo-mechanical modeling revealed that larger primary pores promote inward volume expansion, supporting the experimental observation. Further, the hp-Si with larger primary pores has better structure preservation and more stable surface SEI formation. This yielded a reversible capacity of 1013 mAh g-1 after 200 cycles at 1 A g-1 (83% retention) and a good rate performance of 566 mAh g-1 at 12.8 A g-1, which exceeded that of the hp-Si with smaller primary pores and the samples with only secondary pores. EXPERIMENTAL SECTION Preparation of hp-Si materials. The as-prepared NaK alloy (6g) and SiO2 templates (220 mg, Aldrich) were added to 120 mL of toluene solution of anhydrous SiCl4 (4 mL, 34 mmol, Aldrich 99%) in an Ar filled glovebox. This mixture was heated under reflux (~110 °C) for 4 h. Then 20 mL of diethyl ether solution of hydrogen chloride (2M, Aldrich) was added slowly with stirring under the Ar. The raw products were collected by filtration and annealed at 700 °C for 30 min under Ar atmosphere. The final product was obtained by removing the salt by-products with DI water and HF aqueous solution, and then dried in a vacuum oven before use. Characterizations: The XRD measurement was carried out on a Rigaku Dmax-2000 Xray powder diffractometer with Cu Ka radiation (1.5418Å). The operating voltage and current were kept at 40


4

Page 5 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kV and 30 mA, respectively. The microscopic analyses were conducted with a JEOL-1200 TEM, JEOL-2010 TEM and HRTEM, and FEI Nova NanoSEM 630 scanning electron microscope (SEM and EDS mapping). The Brunauer–Emmett–Teller (BET) specific surface area of the samples were determined by a Micrometrics ASAP 2020 physisorption analyzer using the standard N2 adsorption and desorption isotherm measurements at 77 K. Xray photoelectron spectroscopy (XPS) was conducted with a Kratos Analytical Axis Ultra XPS using monochromatic Al Ka radiation. High-resolution spectra were recorded with a pass energy of 20 eV in 0.1 eV steps. Binding energies were corrected with respect to the binding energy of the C 1s peak at 284.8 eV. Raman spectroscopy was collected with a WITec CMR200 confocal Raman instrument. The insitu TEM observation of the lithiation of hp-Si was carried out using an open-cell configuration as schematically illustrated in Supporting Information Fig. S3. The hp-Si samples were loaded on a Pt rod, which was affixed to one side of a TEM holder (Nanofactory Instrument, AB). Lithium metal was loaded on a tungsten rod which was actuated by a piezoelectric system on the other side of the holder. The lithium metal was moved to contact the hp-Si. The lithium oxide layer on lithium metal surface served as the electrolyte. To drive Li diffusion through the Li2O and lithiation of the hp-Si, an overpotential of -3 V was applied to the hp-Si against the Li metal. The microstructure changes were recorded using a charge-couple device (CCD) attached to the TEM. Modeling: In the model, the total strain 𝜀𝑖𝑗 consists of three components, 𝜀𝑖𝑗 = 𝜀𝑒𝑖𝑗 + 𝜀𝑝𝑖𝑗 + 𝜀𝑐𝑖𝑗, where 𝜀𝑒𝑖𝑗 is the elastic strain, 𝜀𝑝𝑖𝑗 is the plastic strain, and 𝜀𝑐𝑖𝑗 = 𝛽𝑐𝛿𝑖𝑗 is the chemical strain. Here is the local Li concentration that varies from 0 (representing the unlithiated 𝑐 porous Si) to 1 (representing the fully lithiated phase), 𝛽 is the expansion coefficient, and 𝛿𝑖𝑗 is the stress. The hpSi is modeled as an elasto-plastic material. The Young’s modulus of hpSi is estimated by (𝑃) = (1 ‒𝑃)3, where A = 169 GPa, and 𝑃 is the porosity. For 𝑃 ≈ 0.70, the Young’s modulus of is


5


Page 6 of 23

approximately to be ~4.56 GPa. The fully lithiated Si phase possesses a Young’s modulus of ~40GPa. We adopt a yield stress of 0.1GPa for the porous Si and 0.5GPa for fully lithiated Si. The relatively low yield stresses reflect the ease of generating plastic flow observed in the experiments. Both the Young’s modulus and the yield stress are set to be nonlinearly dependent on Li concentration. Electrochemical Measurements: CR2016-type coin cells consisting of the h-PSi based electrode and lithium foil anode separated by a Celgard 2400 membrane were used for battery tests. The electrode contained 60 wt% active material, 20 wt% Super P and 20 wt% polyacrylic acid (PAA). The electrolyte was 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC) (1:2, v/v) with 10 wt% fluoroethylene carbonate (FEC) as additive. The average loading of the active material is 1.5 mg cm-2. The cells were assembled in an argon-filled glove box (MBraun GmbH, Germany). The charge-discharge experiments were performed on a BT2000 battery testing system (Arbin Instruments, USA) in the potential range of 0.01-1.5 V using galvostatic charging and discharging method with different current rates. Lithium foil acted as both the reference and counter electrode. Electrochemical tests were performed at room temperature. RESULTS AND DISCUSSION Preparation and characterization of the materials. The synthesis of hp-Si materials is based on reduction of Si tetrachloride (SiCl4) precursors.26 SiO2 particles serve as the external templates for generating the primary pores, while the salt byproducts as the internal templates for generating the secondary pores. The primary pore size can be fine-tuned by using SiO2 templates of different sizes. In this work, we utilized two SiO2 templates with average particle size of 20 nm and 200 nm, respectively. The reduction of SiCl4


6

Page 7 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


gives to an amorphous Si, which was then crystallized by an optimized heat treatment. Two final products, namely, hp-Si with large primary pores (hp-Si-L, 200 nm primary pores) and hp-Si with small primary pores (hp-Si-S, 20 nm primary pores), were obtained by the removal of all salt byproducts and SiO2 templates with HF aqueous solution. Both hp-Si materials have secondary pores with size of ~ 10 nm and ~ 3nm. The bulk particle size of both the hp-Si materials is ~5-10 µm, as shown in the SEM images (Fig. S1). Some large pores with a size of around a few hundred nanometers are clearly observed in the SEM image of hp-Si-L (Fig. 1A and S1B). The TEM images (Fig. 1B and S1C) show that the wall of the large primary pores is composed of nano-sized porous Si units that further contain the secondary pores, rendering the hp-Si-L a hierarchical porous structure. In contrast, the hp-Si-S bulk particles have relatively smoother surfaces in the SEM images (Fig. 1C and S1D). The TEM images of hp-Si-S in Fig. 1D and S1E show a disordered hierarchical porous structure, with the primary pores (~20 nm) mixing with the secondary pores.


7


Page 8 of 23

Figure 1. Microscope images of hp-Si materials. A) SEM image of hp-Si-L; B) TEM image of hp-Si-L; C) SEM image of hp-Si-S; and D) TEM image of hp-Si-S. The XRD patterns show the formation of crystalline Si phases in both hp-Si materials (Fig. 2A). The size of the crystallites in hp-Si is ~15 nm, as estimated by the Debye–Scherrer equation, which is consistent with our previously reported porous Si sample (mPSi-700) using the same crystallization conditions.26 The presence of oxides in both hp-Si materials is evidenced by their Raman spectra (Fig. 2B), in which a peak shoulder appears between 300 and 450 cm-1 corresponding to the amorphous SiOx (0

Minimized Volume Expansion in Hierarchical Porous Silicon upon

Recommend Documents