Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 534−540
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Lithium Titanate Matrix-Supported Nanocrystalline Silicon Film as an Anode for Lithium-Ion Batteries Zhaozhe Yu,†,‡ Bingbing Tian,*,† Ying Li,†,§ Dianyuan Fan,†,§ Daoguo Yang,‡ Guisheng Zhu,‡ Miao Cai,‡ and Dong Liang Yan‡
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SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, International Collaborative Laboratory of 2D Materials for Optoelectronic Science and Technology of Ministry of Education, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China ‡ Guangxi Key Laboratory of Manufacturing Systems and Advanced Manufacturing Technology, Guilin University of Electronic Technology, Guilin 541004, PR China § Engineering Technology Research Center for 2D Material Information Function Devices and Systems of Guangdong Province, Shenzhen University, Shenzhen 518060, China S Supporting Information *
ABSTRACT: A facile preparation method of a Si-based anode with excellent cycling property is urgently required in the process of preparing lithium-ion batteries (LIBs). Here, lithium titanate (LTO) matrix-supported nanocrystalline Si film is prepared by radio frequency (RF) magnetron cosputtering utilizing LTO and silicon (Si) targets as the sputtering source. LTO-supported nanocrystalline Si film electrodes revealed a repeatable specific capacity of 1200 mA h g−1 at 150 mA g−1 with a maintenance of more than 75% even after 800 cycles. The remarkable electrochemical properties of the LTO−Si composite films could be attributed to the LTO matrix, preventing the electrolyte from directly making contact with the nanocrystalline Si materials, alleviating the stress of the periodic volume change and further providing efficient and rapid pathways for lithium-ion transport. The results suggest that Si-based LTO composite films are prospective anodes for LIBs, with high capacities and long cycling stabilities. KEYWORDS: nanocrystalline Si, lithium titanate, anode, Li4Ti5O12−Si composite, lithium-ion batteries, long cycling performance
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INTRODUCTION Si-based anodes on LIBs have attracted remarkable attention recently because of their unique high capacity (≈4200 mA h g−1), environmental friendliness, and suitable electrochemical potential (≈0.4 V vs Li+/Li).1−3 However, the huge volume expansion rate of Si (>300%) during cycling generally causes anode cracking and a constant formation of the irreversible solid electrolyte interphase (SEI) on the Si anode surface.4−7 Additionally, the low electron conductivity of Si also leads to poor electrochemical properties. Therefore, the cycle characteristic of the Si-based anodes is still not suitable for commercial applications. Nanosized Si and Si-based composite materials show good prospects as future anode candidates due to their enhanced resistance to fractures and improved cycling stability during the process of lithiation/delithiation. Recently, several methods have been exploited to enhance the overall electrochemical performances of Si anodes, including design-appropriate Si nanostructures,8−10 a cover with a protective layer,11−14 and fabricated nanocomposites.15−18 Significant efforts have focused on the development of effective protective layers to optimize nanosized Si materials. Graphene, carbon nanotubes, © 2018 American Chemical Society
and graphite shells can strengthen the electrical conductivity of Si materials and be used as buffer layers to slow down the enormous stress of periodic volume change.19−24 Additionally, the carbon coating shell also prevents the electrolyte from directly contacting the nano-Si materials. In this case, stable SEI may only be formed on the surface or interface of the carbon shell, with the Coulombic efficiency and cycling stability of Si materials being significantly improved.25−27 However, in practical applications, achieving a uniform dispersion of nanosized Si by simple mechanical blending is still challenging due to the easy agglomeration properties of the nanosized Si materials. Therefore, it is urgent to explore novel Si-based anodes with excellent electrochemical properties to satisfy the demand from industries. Herein, we propose a simple method of preparing lithium titanate matrix (LTO) supported nanocrystalline Si film by RF magnetron cosputtering, which is effective for the industrial production of a thin-film electrode. As a “zero strain” anode Received: August 13, 2018 Accepted: December 10, 2018 Published: December 10, 2018 534
DOI: 10.1021/acsami.8b13878 ACS Appl. Mater. Interfaces 2019, 11, 534−540
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic illustration of the fabrication of LTO−Si composite film. (b) The mechanism of lithium intercalation/deintercalation in LTO−Si composite film.
Figure 2. (a) SEM images of LTO−Si composite film, (b) cross-sectional SEM images of LTO−Si composite film, (c) and (g) TEM images of LTO−Si composite film, (d) pseudocolor TEM images of LTO−Si composite film, (e) and (f) HRTEM images of the LTO−Si composite film, with the corresponding fast Fourier transformation patterns (inset) from the marked regions, and (h) and (i) EDS mapping images of silicon and titanium collected from (g). measurements are depicted and presented in the Supporting Information.
material, spinel LTO not only prevents the electrolyte ingress and further contact with nanosized Si but also alleviates the strain on the Si anode during lithiation.28−31 The strategy is simple and effective for preparing Si-based composite materials, accelerating the preparation techniques for promising Si-based materials with superior cycling stabilities and high energy densities.
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RESULTS AND DISCUSSION The preparation process of LTO−Si film is shown pictorially in Figure 1a. The volume variations of embedded Si during the process of lithiation/delithiation are displayed in Figure 1b. In this construction, the LTO matrix prevents the electrolyte from making direct contact with the nano-Si particles and then alleviates the stress of the periodic volume change while maintaining the structural stability during the process of lithiation/delithiation. The surface morphology of LTO−Si composite film is displayed in Figure 2a. The composite film exhibits a homogeneous morphology. The cross-section image (Figure 2b) shows that the composite film thickness is approximately 1 μm, where the section is dense and smooth at the film thickness. As shown in Figure 2c and 2d, the low-
EXPERIMENTAL SECTION
Li4Ti5O12 powders were synthesized using a facile sol−gel process. The LTO target was made by Li4Ti5O12 powder pressed into a copper holder. The Si target is a commercial ceramic target. The Li4Ti5O12supported nanocrystalline Si film (LTO−Si) electrodes were deposited using a RF magnetron cosputtering system. LTO−Si composite film was prepared by cosputtering using a power ratio of LTO:Si = 2:1. For comparison, the LTO film and Si film were also prepared under identical conditions. More experimental details, the characterization of the material structure, and electrochemical 535
DOI: 10.1021/acsami.8b13878 ACS Appl. Mater. Interfaces 2019, 11, 534−540
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a) XRD patterns of the LTO−Si composite film, Si film, LTO film, and LTO powder target; (b) enlarged diffraction peaks of the LTO− Si composite film; and (c) and (d) XPS spectra of Si2p and Ti2p of the LTO−Si composite film.
Figure 4. CV curves: (a) comparison of the LTO−Si composite film with LTO film electrodes and (b) first three cycles of LTO−Si composite film electrode. EIS profiles: (c) comparison of LTO−Si composite film with LTO film electrodes and (d) fitting to the impedance spectra of the LTO− Si composite film electrode and corresponding equivalent model (inset). Rs: the electrolyte Ohmic resistance. Rct: charge transfer resistance.
magnification transmission electron microscopy (TEM) and pseudocolor images of the LTO−Si composite film indicate
that Si nanoparticles are dispersed uniformly in the LTO matrix. A high-resolution transmission electron microscopy 536
DOI: 10.1021/acsami.8b13878 ACS Appl. Mater. Interfaces 2019, 11, 534−540
Research Article
ACS Applied Materials & Interfaces
Figure 5. Charge/discharge curves: (a) comparison of the LTO−Si composite film with LTO film electrodes, (b) energy density schematic diagram of LTO−Si composite film and LTO film electrodes, where area I is the energy density with LTO film as the anode and area I + II is the energy density with the LTO−Si composite film as the anode, (c) first three charge/discharge cycles of the LTO−Si composite film electrode; and (d) comparison of the 3rd, 30th, and 300th charge/discharge cycles.
the spinel LTO. No obvious Si−O component corresponding to SiOx is detected at ∼532 eV in the O 1s core level spectrum.1 According to the XPS data presented above, the LTO matrix is nanocrystalline or amorphous, and the monatomic Si nanoparticles are distributed in the LTO matrix. Figure S2 presented the Raman spectra of the LTO−Si composite film. The crystalline Si−Si stretching modes are observed at ∼520 cm−1. The peak centered at ∼550 cm−1 is caused by the Ti−O stretching vibrational mode of the “TiO6” octahedron.27,35 Figure 4a displays the cyclic voltammetry (CV) experiment curves of the LTO−Si composite film and LTO film at a scanning rate of 0.1 mV/s. The normalized CV curves show that the LTO−Si composite film has a larger enclosed geometric area than that of the LTO film, demonstrating a better electrochemical property of the LTO−Si film electrode. The first three cycles of the LTO−Si film electrode (in Figure 4b) exhibits a broad redox peak at approximately 0.5 V, which displays similar lithiation/delithiation characteristics to nanocrystalline Si. The slight difference in the initial cathodic peak with the second and third cathodic peaks implies a good cyclic reversibility of the LTO−Si composite film electrode. This is related to the supporting role of LTO film (in Figure S3). The Nyquist plots of the LTO−Si film and LTO film electrodes after three cycles at 0.1 C and charged to 1.0 V are presented in Figure 4c. The equivalent model fitting impedance spectra of the LTO−Si film is displayed in Figure 4d. The small intercept in the high-frequency region is related with the electrolyte Ohmic resistance (Rs).36 The depressed semicircle is denoted as charge transfer resistance (Rct).37 The internal resistance of the LTO−Si composite film electrode (Rs = 4 Ω) is lower than that of the LTO film electrode (Rs = 5 Ω),
(HRTEM) image is recorded to explore more details on the structure (Figure 2e and 2f). The Si is composed of spherical particles with a diameter of 20−30 nm, and the inner particles are crystalline with the lattice of ∼0.30 nm in accordance with the (111) planes of crystalline Si. Moreover, energy-dispersive spectrum (EDS) elemental mapping is performed to further identify Si distribution in the LTO−Si composite film (Figure 2g−i). Obviously, the Si is evenly distributed in the LTO matrix. The inductively coupled plasma optical emission spectrometry (ICP-OES) results shown in Table S1 indicated that the mass ratio of titanium to silicon is approximately 2 to 1. Figure 3 shows the X-ray diffraction (XRD) patterns and Xray photoelectron spectroscopy (XPS) spectra of the LTO−Si composite film. Figure 3a compares the XRD patterns of the LTO−Si film on stainless-steel substrate with those of the Si film, LTO film, and LTO powders used as targets. No obvious peaks of spinel LTO and crystalline Si are found in the LTO− Si composite film. Only a weak (111) peak of Si and LTO is found in the enlarged view of the LTO−Si composite film (Figure 3b). Figure 3c shows Si2p XPS spectra of LTO−Si composite films. Two deconvoluted peaks, Si2p1/2 at ∼100.2 eV and Si2p3/2 at ∼99.6 eV, are ascribed to the spin−orbit doublet of elemental Si (Si−Si bonds).20,32 The signal assigned to the Si−O bonds in the SiOx matrix at approximately 103 eV is not detected, further indicating a monatomic silicon presence.20 Figure 3d presents the XPS Ti2p spectra of LTO−Si composite films. The two broad peaks centered at ∼464.3 and ∼458.6 eV are ascribed to Ti2p1/2 and Ti2p3/2 of the spinel LTO.33,34 The XPS O 1s core level spectra of the LTO−Si composite film is also recorded in Figure S1. The obvious single O 1s peak located at ∼530.2 eV is attributed to the Ti−O component in 537
DOI: 10.1021/acsami.8b13878 ACS Appl. Mater. Interfaces 2019, 11, 534−540
Research Article
ACS Applied Materials & Interfaces
Figure 6. (a) Rate performance and (b) cycling performance of the LTO−Si composite film electrodes.
curve shape of the LTO−Si film electrode is well preserved, and the discharge capacity is still higher than 995 mA h g−1 even after 300 cycles (in Figure 5d). Although the initial charge capacity of the pure Si film is higher than 2500 mA h g−1, it is decayed by half after three cycles and further decreased to ∼300 mA h g−1 after 30 discharge/charge cycles (in Figure S4). Figure 6 presented the electrochemical property of rate and cycling of the LTO−Si films. As shown in Figure 6a, the rate performance of the LTO−Si composite film electrode shows that the charge/discharge capacities decrease with an increase in the rates from 150 to 3000 mA g−1. However, the charge and discharge curves are still preserved well (in Figure S5), and the capacities increased gradually from 690 to 1020 mA h g−1 with the decrease of the current from 3000 to 300 mA g−1. Moreover, at 3000 mA g−1, the capacity maintains 63% of the value achieved at 150 mA g−1, demonstrating outstanding capacity retention of the LTO−Si composite film electrode. The cycling performances of the LTO−Si composite film electrode at 150 mA g−1 are also displayed in Figure 6b. The LTO−Si composite film electrode retained approximately 900 mA h g−1 even after 800 cycles, with a capacity retention ratio more than 75%. Moreover, the Coulombic efficiency was about 99% during the 800 cycles. The LTO−Si composite film electrode surface is flat, and there are no obvious cracks after 800 charge/discharge cycles (in Figure S6). The LTO−Si composite film, lithium titanate matrix-supported nanocrystalline Si, can alleviate the stress during the periodic discharge and charge, prevent the electrolyte from making direct contact with the nanocrystalline Si, and provide efficient and rapid pathways for lithium transport, thus resulting in an improved lithium storage capability and superior cycling performance.
indicating a smaller interface resistance of the composite film electrode. The reason behind this phenomenon is that the Si nanoparticles embedded in the LTO matrix create an enlarged surface area in the composite film. The Rct of the LTO−Si composite film electrode is much smaller (Rct = 98 Ω) than that of the LTO film electrode (Rct = 120 Ω) because of the close integration of the LTO with nanocrystalline Si, then further increasing the ionic conductivity of the composite film electrode. Figure 5a describes the initial constant-current discharge/ charge curves of the LTO−Si and LTO film electrodes in 0.01−3.0 V at 150 mA g−1. In the initial discharge curves of the LTO and LTO−Si film electrodes, the potential drops rapidly to 0.01 V, releasing discharge capacities of ∼300 and ∼1400 mA h g−1, respectively. Upon a subsequent charge, no obvious potential plateau with a continuous increase of the voltage is exhibited, which has been assigned to the delithiation process of the amorphous electrodes. The energy density diagrams, with lithium manganate as cathode and LTO−Si or LTO as anode, are shown in Figure 5b. Compared to area I (the energy density with the LTO film as anode), the area I + II (LTO−Si composite film as the anode) is larger, indicating that the LTO−Si composite film could evidently increase the capacity of the full batteries. Figure 5c exhibits the initial three discharge/charge results of the LTO−Si composite film at 150 mA g−1. The initial discharge/charge capacities reach ∼1410 and ∼1200 mA h g−1, respectively. The formation of the SEI layer on LTO−Si composite film leads to an irreversible capacity loss of ∼210 mA h g−1 at the initial lithiation/ delithiation process.14 The reversible capacity is more than 1100 mA h g−1 in the third cycle. Moreover, the Coulombic efficiency of LTO−Si film electrode is higher than 98%. The 538
DOI: 10.1021/acsami.8b13878 ACS Appl. Mater. Interfaces 2019, 11, 534−540
Research Article
ACS Applied Materials & Interfaces
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CONCLUSION A lithium titanate matrix-supported nanocrystalline Si film is prepared by RF magnetron cosputtering with LTO powder targets and Si ceramic targets. The LTO−Si composite film exhibits a reversible capacity of 1200 mA h g−1 at 150 mA g−1, a remarkable rate capability (∼690 mA h g−1 at 3000 mA g−1), and good cyclic stability (more than 75% capacity maintenance after 800 cycles). The outstanding electrochemical properties of the LTO−Si composite film could be ascribed to the individual structure of lithium titanate matrix-supported nanocrystalline Si. LTO−Si composite film prepared with an LTO supporting matrix and activated nanocrystalline Si is a facile and effective approach for enhancing the capacity and reversibility of LIBs, which is applicable to other electrode materials.
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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.8b13878. XPS, Raman spectrum, CV curves, cycling performance, discharge/charge curves, SEM images, and elemental analysis results (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (B. Tian). ORCID
Bingbing Tian: 0000-0003-1508-6217 Dong Liang Yan: 0000-0002-4148-6487 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 21805055, 61540073, 51564006, 21506126, and 51764011), Guangxi Natural Science Foundation (2018GXNSFAA138064, 2016GXNSFAA380053, 2016GXNSFAA380205, and 2017GXNSFDA198021), Guangxi Key Laboratory of Manufacturing Systems Foundation (Grant Nos. 17-259-05-003Z and 17-259-05-001Z), the Shenzhen Peacock Plan (Grant Nos. 827-000113, 827-000273, and KQTD2016053112042971), and the Department of Science and Technology of Guangxi Province (Project Contract No.: AA17204063). The authors also would like to thank Prof. Huarui Xu and Prof. Aibing Yu for the helpful suggestions and discussions.
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