Li0.3La0.557TiO3 Interpenetrating

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Applications of Polymer, Composite, and Coating Materials 0.3

0.557

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A Lithium-Salt-Rich PEO/Li La TiO Interpenetrating Composite Electrolyte with Three-Dimensional Ceramic Nano-Backbone for All-Solid-State Lithium-Ion Batteries Xinzhi Wang, Yibo Zhang, Xue Zhang, Ting Liu, Yuanhua Lin, Liangliang Li, Yang Shen, and Cewen Nan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06658 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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ACS Applied Materials & Interfaces

A Lithium-Salt-Rich PEO/Li0.3La0.557TiO3 Interpenetrating Composite Electrolyte with Three-Dimensional Ceramic Nano-Backbone for All-Solid-State Lithium-Ion Batteries Xinzhi Wang, Yibo Zhang, Xue Zhang, Ting Liu, Yuan-Hua Lin, Liangliang Li*, Yang Shen*, Ce-Wen Nan* State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China.

Keywords: LLTO, PEO, nano-backbone, composite polymer electrolyte, quenching

Abstract

Solid electrolytes with high ionic conductivity and good mechanical properties are required for solid-state lithium-ion batteries. In this work, we synthesized composite polymer electrolytes (CPEs) with a three-dimensional (3D) Li0.33La0.557TiO3 (LLTO) network as a nano-backbone in polyethylene oxide matrix by hot-pressing and quenching. Self-standing 3D-CPE membranes were obtained with the support of the LLTO nano-backbone. These membranes had much better thermal stability and enhanced mechanical strength in comparison with solid polymer electrolytes. The influence of lithium (Li) salt concentration on the conductivity of the 3D-CPEs was systematically studied and an ionic conductivity as 1 / 26

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high as 1.8×10-4 S·cm-1 was achieved at room temperature. The electrochemical window of the 3D-CPEs was 4.5 V vs Li/Li+. More importantly, the 3D-CPE membranes could suppress the growth of Li dendrite and reduce the polarization, therefore a symmetric Li|3D-CPE|Li cell with these membranes was cycled at a current density of 0.1 mA·cm-2 for over 800 hours. All the superior properties above made the 3D-CPEs with LLTO nano-backbone a promising electrolyte candidate for flexible solid-state lithium-ion batteries.

Introduction

Under the demand of high-energy-density storage and in the wave of electric vehicle revolution in the past decades, lithium-ion batteries (LIBs) come to front as one of the most promising rechargeable energy storage devices that attract ever-increasing attention in both academic and industrial research worldwide.1-2 The safety issues haunting over commercial LIBs are attributed to the usage of flammable organic liquid electrolyte3. In order to solve these issues, replacing liquid electrolytes with solid ones is a promising strategy4-6, which also allows the usage of lithium (Li) metal anode (3860 mAh·g-1, -3.040 V vs. standard hydrogen electrode theoretically) and delivers an ultrahigh energy capacity.7 Among diverse types of solid-state electrolytes, solid polymer electrolytes (SPEs) possessing flexibility and good interfacial contact with electrodes are being intensively investigated8,9. Poly(ethyleneoxide) (PEO) has attracted most and continuous attention since it was firstly reported in 1970s.10 It has excellent solubility for various Li salts and PEO-based SPEs have promising ionic conductivity and stability against Li metal.11 The prevailing ion transport mechanism in PEO concludes that ethylene oxide (EO) units have a 2 / 26


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high donor number of Li ions and PEO polymer owns chain flexibility to promote ion hopping.12-13 The ion hopping is assisted by the segmental motion of polymer chains, and thus the amorphous phase of PEO is the preferable region. However, PEO suffers from low ionic conductivity due to its semi-crystallinity below its melting point (~60°C).14 In order to enlarge the amorphous fraction in PEO-based SPEs at room temperature (RT), several effective strategies are adopted as follows. Firstly, PEO chains can be modified by synthesizing block copolymer15-16, crosslinking17-18, or branching PEO chains via chemical reactions19 to increase the amorphous proportion and promote the conduction pathway. Secondly, the motion ability of EO units can be improved by introducing additives in SPEs such as low lattice energy Li salts with large-sized anions13, organic plasticizer20, and blend polymer21. Among them, Li salts have been intensively investigated, and phase diagrams of different PEO-Li salts systems have been provided. The effects of Li salts on SPEs were well illustrated.12, 22 Nevertheless, the degradation of mechanical properties of SPEs often accompanies the improvement of ionic conductivity by the methods above. Adding ceramic fillers in polymer matrix is a prevailing solution that can enhance the conductivity and mechanical strength at the same time due to the combination of the merits of both matrix and fillers. Active ionic conducting fillers like perovskite-type Li0.3La0.557TiO3 (LLTO)23-24 and garnet-type Li7La3Zr2O12 (LLZO) 25-27, or inertia ones like SiO228 and TiO229 are added into polymer matrix to synthesize composite polymer electrolytes (CPEs) with high RT ionic conductivity, excellent mechanical strength, good heat resistance, and high electrochemical stability. For example, Cui and coworkers in-situ synthesized zero-dimensional (0D) SiO2 nanoparticles in 3 / 26



PEO matrix to obtain CPEs. They effectively suppressed the crystallinity of PEO, and achieved an ionic conductivity of 4.4×10-5 S·cm-1 at 30 °C.30 Afterwards, they compared the effects of one-dimensional (1D) and 0D LLTO fillers on the conductivity of polyacrylonitrile (PAN)-based CPEs and found that the conductivity of the CPEs with oriented 1D fillers was two orders of magnitude larger than that of the CPEs with 0D fillers.31,32. More recently, Zhang and co-workers prepared PEO-based CPEs filled with 1D LLTO nanofibers, which possessed a high ionic conductivity of 2.4×10-4 S·cm-1 at RT.33 1D nanofibers can not only reduce the crystallinity of PEO or PAN matrix, but also serve as ion conducting pathways due to their large length-to-diameter ratio.34 However, the bonding among the nanofibers is not tight. There is still room to improve the performance of the CPEs. In this work, we synthesized CPEs with three-dimensional LLTO network as the nano-backbone in the PEO matrix by hot pressing and quenching, and studied the influence of Li salt concentration on the conductivity of the three-dimensional interpenetrating composite polymer electrolytes (3D-CPEs). The novelty and advantages of our work include three aspects. First, an interconnected LLTO network is synthesized for the first time. The ceramic network can not only facilitate the Li ion transport, but also enhance the mechanical properties of the 3D-CPEs to suppress the growth of Li dendrite. Second, an innovative synthetic process combining hot-pressing and quenching is used to fabricate dense and self-standing 3D-CPE membranes with a good interfacial contact between fillers and matrix. The quenching is effective for maintaining the dense structure resulting from the hot-pressing and for making self-standing membranes with amorphous matrix, and it can be generally applied for other composite electrolytes. Third, the effects of Li salt concentration 4 / 26


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on the structure and ionic conductivity of 3D-CPEs are systematically studied and a crystallinity gap where an amorphous PEO phase can endurably exist is determined. The amorphous phase improves the bonding between the LLTO nano-backbone and the polymer matrix, which significantly enhances the ionic conductivity. With the rational designs on materials, structure, and synthetic processes above, a flexible and self-standing 3D-CPE membrane with an ionic conductivity of 1.8×10-4 S·cm-1 at RT, yield strength of 16.18 MPa, Young’s modulus of 0.98 GPa, and electrochemical window up to 4.5 V is obtained. The 3D-CPE shows an excellent stability against Li metal during ion stripping and plating at 0.1 mA·cm-2 over 800 h in a Li|3D-CPE|Li cell. The experimental data show that our 3D-CPE is a promising candidate of high-performance solid electrolytes used for all-solid-state LIBs.

Experimental Section

Synthesis of LLTO nano-backbone LLTO nanofiber network was fabricated by electro-spinning and subsequent calcination processes. Lithium nitrite (LiNO3, Sigma-Aldrich, 99%), lanthanum nitrite hexahydrate (La(NO3)3·6H2O, Sigma-Aldrich, 99%) and titanium butoxide (Ti(OC4H9)4, Sigma-Aldrich, 99%) were weighted according to stoichiometric ratio with excess 15 wt.% LiNO3 to compensate for its loss during heat treatment.

Dimethylformamide (DMF, Sigma-Aldrich),

acetic acid (CH3COOH, Sigma-Aldrich) and acetylacetone (Sigma-Aldrich) were mixed in 5:2:2 volume ratio to prepare a solvent. The inorganic salts were dissolved in the solvent and vigorously stirred for 12 h until a homogeneous solution was obtained. Then, an appropriate amount of polyvinylpyrrolidone (PVP, Mw=1300000, Sigma-Aldrich) was added into the 5 / 26



solution to form a yellow, transparent, and viscous precursor. The precursor was loaded into a 20 mL plastic capililary and spilled from a stainless steel needle to a rolling collector (350 rpm) covered with aluminum foil under 1 kV·cm-1 voltage. A nanofiber network with a thickness of ~60 µm could be peeled off after 3.5 h spinning. In order to form a uniform network, the salt concentration, viscosity and defoaming time of the precursor, voltage and distance between the needle and collector, and rotation speed of the drum need to be carefully controlled. Electrospray generation, beaded fibers, and discontinuous spinning will lead to uneven films, which are not conducive to subsequent processes. An as-spun nanofiber network was sintered on a platform at ~330°C to fully remove the organic binder PVP and then calcined at 650°C, 750°C, 850°C, or 950 °C for 2 h in air with a heating rate of 5 °C·min-1. A silicon wafer was used to cover the network during calcination to obtain a smooth and unrippled ceramic network.

Preparation of SPE and 3D-CPE PEO (Mw~600000, Sigma-Aldrich) and bis(trifluoromethane)sulfonamide lithium (LiTFSI, Sigma-Aldrich, 99.95%) with different weight ratios from 10 wt.% to 80 wt.% and were dissolved into acetonitrile (CH3CN, Sigma-Aldrich). The equivalent EO:Li ratio varies from 32 to 3.2. The solutions were defoamed before use, and then they were slurry cast onto polytetrafluoroethylene (PTFE) substrate. For SPEs, the cast films were dried in a glove box. For the 3D-CPE, a LLTO nanofiber network was placed on the top of the cast films, and a small amount of the PEO-LiTFSI solution were coated on the LLTO network. The samples were dried at RT in a glove box filled with argon for 4 h and further baked in a vacuum oven 6 / 26


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for 12 h to remove solvent. Finally, composite membranes with a thickness of 80-120 µm were obtained. Next, the membranes were sandwiched between two PTFE plates, hot-pressed at 75°C under 2 N·cm-2 for 15, 30, 45, or 90 min, and quenched in liquid N2. The quenched membranes with a thickness of 70-100 µm were immediately peeled from the PTFE substrates and then stored in a vacuum oven at RT for further use. Fig. 1 shows the schematic picture of synthesis procedure.

Figure 1. Schematic picture of synthetic processes of 3D-CPEs.

Characterization of structure and properties Scanning electron microscope (SEM, Zeiss Merlin field-emission) and X-ray diffractometer (XRD, Rigaku D/max-2500 with Cu-Kα, 40 kV and 200 mA) were used for morphology examination and phase identification. Thermo-gravimetric analysis (TGA, TGA-Q500 instrument) was conducted in N2 atmosphere with a heating rate of 10 °C·min-1. The tensile test of the membrane with a size of 15 mm×45 mm×0.1 mm was done by a Zwick testing machine at a stretching speed of 5 mm·min-1. SPEs and 3D-CPEs were sandwiched between stainless steel (SS) substrates for ionic conductivity measurement. Three samples were measured to obtain an average value. Electrochemical stability and cycling performance measurements were conducted on Li|SPE or 3D-CPE|SS cells and symmetrical Li|SPE or 3D-CPE|Li cells, respectively. The cells were all assembled in a glove box. Ionic conductivity 7 / 26



was investigated by electrochemical impedance spectroscopy (EIS) and recorded by an impedance analyzer (ZAHNER-elecktrik IM6) with an AC amplitude of 50 mA within the frequency range from 1 Hz to 8 MHz. To measure the Arrhenius plots, the temperature was controlled by a chamber (Cincinnati Sub-Zero MCB-1.2-AC) from 25°C to 90°C. The electrochemical stability was shown by linear sweep voltammetry (LSV) and recorded by potentio-galvanostat (CHI760, Chenhua, China) at a scan rate of 1 mV·s-1 from open circuit voltage to 6 V. The cycling properties were tested using a battery test system (C2001A, LAND, China). Galvanostatic cycles with a constant current density of 0.1 mA·cm-2 were periodically charged-discharged for 20 minutes.

Results and discussion

Structure and morphology

Figure 2. Photographs of rolled as-electrospun LLTO nanofiber network (a), as-calcined LLTO nano-backbone (b), and a 3D-CPE (c). SEM images of as-electrospun (d, g) and calcined (e, h) LLTO nanofibers. Top-view (f) and cross-sectional (i) SEM images of the 3D-CPE. 8 / 26


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Fig. 2a shows a self-standing network made of as-electrospun LLTO nanofibers, which can be rolled up like a non-woven cloth. This LLTO nano-backbone can be scale-producible. Small pieces of the LLTO fiber mats were calcined at 650, 750, 850 or 950°C for 2 h. Fig. S1a-d (Supporting Information) show the SEM images of the LLTO fibers in the calcined mats. The average diameters of the fibers (Fig. S1e-h in Supporting Information) were 383, 309, 216, and 281 nm for 650, 750, 850, and 950°C calcination, respectively. When the calcination temperature increases from 650 to 850°C, the average diameter decreases due to the shrinkage of the calcined fibers, while the diameter increase for the nanofibers calcined at 950°C may be due to the irregular growth of ceramic grains.32 Additionally, the surface of the fibers becomes coarser as the calcination temperature rises. Phase structure was analyzed from the XRD patterns of LLTO nanofibers calcined at different temperatures, as shown in Fig. S2 (Supporting Information). The sharp diffraction peaks of the samples calcined at 850°C and above are well matched to a tetragonal-phase P4/mmm perovskite-structure Li0.33La0.557TiO3 (Joint Committee on Powder Diffraction Standards card #87-0935). It is one of the representatives of A-site deficient Li3xLa2/3-x□1:3-2xTiO3 (0

Li0.3La0.557TiO3 Interpenetrating

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