Li0.3La0.557TiO3 Interpenetrating Composite

All of the superior properties above made the 3D-CPEs with the LLTO .... The tensile test of the membrane with a size of 15 mm × 45 mm × 0.1 mm was ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 24791−24798

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

ACS Appl. Mater. Interfaces 2018.10:24791-24798. Downloaded from pubs.acs.org by DURHAM UNIV on 07/28/18. For personal use only.

S Supporting Information *

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 poly(ethylene 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 3D-CPEs was systematically studied, and an ionic conductivity as high as 1.8 × 10−4 S·cm−1 was achieved at room temperature. The electrochemical window of the 3DCPEs was 4.5 V vs Li/Li+. More importantly, the 3D-CPE membranes could suppress the growth of Li dendrite and reduce 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 h. All of the superior properties above made the 3D-CPEs with the LLTO nano-backbone a promising electrolyte candidate for flexible solid-state lithium-ion batteries. KEYWORDS: LLTO, PEO, nano-backbone, composite polymer electrolyte, quenching

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owns chain flexibility to promote ion hopping.12,13 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 because of its semicrystallinity below its melting point (∼60 °C).14 To enlarge the amorphous fraction in PEO-based SPEs at room temperature (RT), several effective strategies are adopted as follows. First, PEO chains can be modified by synthesizing block copolymers, 15,16 crosslinking,17,18 or branching PEO chains via chemical reactions19 to increase the amorphous proportion and promote the conduction pathway. Second, the motion ability of EO units can be improved by introducing additives in SPEs such as low-latticeenergy Li salts with large-sized anions,13 organic plasticizer,20 and blend polymer.21 Among them, Li salts have been intensively investigated, and phase diagrams of different PEO-Li salt systems have been provided. The effects of Li salts on SPEs were well illustrated.12,22

INTRODUCTION Under the demand of high-energy-density storage and in the wave of electric vehicle revolution over 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 use of flammable organic liquid electrolytes.3 To solve these issues, replacing liquid electrolytes with solid ones is a promising strategy,4−6 which also allows the use of a 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 investigated.8,9 Poly(ethylene oxide) (PEO) has attracted most and continuous attention because it was first 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 high donor number of Li ions and the PEO polymer © 2018 American Chemical Society

Received: April 24, 2018 Accepted: July 4, 2018 Published: July 4, 2018 24791

DOI: 10.1021/acsami.8b06658 ACS Appl. Mater. Interfaces 2018, 10, 24791−24798

Research Article

ACS Applied Materials & Interfaces

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

conductivity. With the rational designs on materials, structure, and synthetic processes above, a flexible and self-standing 3DCPE membrane with an ionic conductivity of 1.8 × 10−4 S· cm−1 at RT, a 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 3DCPE is a promising candidate for high-performance solid electrolytes used for all-solid-state LIBs.

Nevertheless, the degradation of mechanical properties of SPEs often accompanies the improvement of ionic conductivity by the methods above. Addition of ceramic fillers in the polymer matrix is a prevailing solution that can enhance the conductivity and mechanical strength at the same time because of the combination of the merits of both matrix and fillers. Active ionic conducting fillers such as perovskite-type Li0.3La0.557TiO3 (LLTO)23,24 and garnet-type Li7La3Zr2O12 (LLZO)25−27 or inert ones like SiO228 and TiO229 are added into the 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 co-workers in situ synthesized zero-dimensional (0D) SiO2 nanoparticles in the 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 Afterward, 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 2 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 One-dimensional nanofibers can not only reduce the crystallinity of PEO or PAN matrix but also serve as ion conducting pathways because of 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 a three-dimensional (3D) 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 3DCPE membranes with a good interfacial contact between fillers and matrix. Quenching is effective for maintaining the dense structure resulting from hot-pressing and for making selfstanding membranes with amorphous matrix, and it can be generally applied for other composite electrolytes. Third, the effects of Li salt concentration 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

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EXPERIMENTAL SECTION

Synthesis of LLTO Nano-Backbone. The LLTO nanofiber network was fabricated by electrospinning 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 the stoichiometric ratio with excess 15 wt % LiNO3 to compensate for its loss during heat treatment. Dimethylformamide (Sigma-Aldrich), acetic acid (CH3COOH, Sigma-Aldrich), and acetylacetone (Sigma-Aldrich) were mixed in a 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 poly(vinylpyrrolidone) (PVP, Mw = 1 300 000, Sigma-Aldrich) was added into the solution to form a yellow, transparent, and viscous precursor. The precursor was loaded into a 20 mL plastic capillary and spilled from a stainless steel (SS) 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. 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, 750, 850, 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 ∼ 600 000, SigmaAldrich) and bis(trifluoromethane)sulfonamide lithium (LiTFSI, Sigma-Aldrich, 99.95%) with different weight ratios from 10 to 80 wt % 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 a poly(tetrafluoroethylene) (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 was 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 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 24792

DOI: 10.1021/acsami.8b06658 ACS Appl. Mater. Interfaces 2018, 10, 24791−24798

Research Article

ACS Applied Materials & Interfaces

Figure 2. Photographs of the 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.

Figure 3. XRD pattern of SPEs (a) and 3D-CPEs (b) with different LiTFSI concentrations after 1 month aging.

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then stored in a vacuum oven at RT for further use. Figure 1 shows the schematic picture of the synthesis procedure. Characterization of Structure and Properties. A scanning electron microscope (SEM, Zeiss Merlin field-emission) and an X-ray diffractometer (Rigaku D/max-2500 with Cu Kα, 40 kV and 200 mA) were used for the morphology examination and phase identification. Thermogravimetric analysis (TGA, TGA-Q500 instrument) was conducted in a 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 was investigated by electrochemical impedance spectroscopy (EIS) and recorded by an impedance analyzer (ZAHNERelecktrik 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 to 90 °C. The electrochemical stability was shown by linear sweep voltammetry (LSV) and recorded by a potentiogalvanostat (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 min.

RESULTS AND DISCUSSION Structure and Morphology. Figure 2a shows a selfstanding network made of as-electrospun LLTO nanofibers, which can be rolled up like a nonwoven cloth. This LLTO nano-backbone can be large-scale-producible. Small pieces of the LLTO fiber mats were calcined at 650, 750, 850, or 950 °C for 2 h. Figure S1a−d (Supporting Information) shows the SEM images of the LLTO fibers in the calcined mats. The average diameters of the fibers (Figure 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, whereas 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. The phase structure was analyzed from the X-ray diffraction (XRD) patterns of LLTO nanofibers calcined at different temperatures, as shown in Figure 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 24793

DOI: 10.1021/acsami.8b06658 ACS Appl. Mater. Interfaces 2018, 10, 24791−24798

Research Article

ACS Applied Materials & Interfaces

Figure 4. Stress−strain curves (a) and TGA curves (b) of the 3D-CPE and SPE with 40 wt % LiTFSI. The flammability test of the 3D-CPE (c) and SPE (d). Heating experiments operated at 150 °C for 3 h on the 3D-CPE (e) and SPE (f).

Figure 2c shows a flexible 3D-CPE film. Figure 2f,i shows the top-view and cross-sectional morphology of the 3D-CPE film. No macro-sized defect such as void and crack is observed in this dense electrolyte film. The XRD spectra of the 3D-CPEs with 10, 40, and 70 wt % LiTFSI after 1 month aging are shown in Figure 3b. All three samples show some crystalline peaks related to the LLTO backbone. The sample with 40 wt % LiTFSI possesses a broad peak of PEO at around 2θ = 20°, indicating that the PEO matrix in this 3D-CPE is amorphous. The samples with 10 and 70 wt % LiTFSI show some crystalline peaks of PEO. Therefore, a LiTFSI concentration of 40 wt % is optimal for the 3D-CPEs. Mechanical and Thermal Properties. The mechanical properties of solid electrolytes are important for suppressing lithium dendrite growth and retaining mechanical integrity when the batteries encounter accidental collision.36 The mechanical properties of our electrolytes were characterized by the tensile test at RT. Figure 4a shows the stress−strain curves of the SPE and 3D-CPE membranes with 40 wt % LiTFSI. Stress, δ, and strain, ε, can be calculated by δ = P/A and ε = (L − L0)/L0, respectively, where P is the load, A is the cross-sectional area, L0 is the original length, and L is the length after deformation. The tensile strength was taken as the maximum stress value in the curve.37 Young’s modulus was determined by the ratio of stress to strain in the elastic deformation region of the stress−strain curve according to Hooke’s law.37,38 With the assistance of the LLTO nanofiber network, the 3D-CPE exhibits a tensile strength of 16.18 MPa, Young’s modulus of 0.98 GPa, elongation of over 200%, and an apparent yield point. The mechanical properties of the 3DCPE were significantly enhanced in comparison with those of the SPE, which is attributed to the good adhesion between the matrix and filler and the strong support of the inorganic LLTO backbone.39

(0 < x < 0.167) fast-ion conductor series (when x = 0.11, the bulk ionic conductivity is 1.0 × 10−3 S·cm−1 at RT).23 Therefore, the LLTO fiber mat calcined at 850 °C (Figure 2b) was chosen for the preparation of 3D-CPEs in the following experiments. Figure 2d,e shows the surface morphology of aselectrospun and calcined LLTO nanofibers, respectively, and Figure 2g,h shows individual ones, respectively. The lithium salt and polymer matrix are the two components in SPEs. The lithium salt would bring low crystallinity and high ionic conductivity in PEO complexes after it is dissolved in PEO and dissociated as free conducting ions. PEO and LiTFSI mixtures with an EO/Li molar ratio of 8:1 to 10:1 preserve a “crystalline gap” that intrinsically keeps amorphous state endurable at RT, according to the PEOLiTFSI phase diagram.22 Here, we varied the LiTFSI concentration in the SPEs from 10 to 80 wt %. The XRD spectra of as-synthesized SPEs are shown in Figure S3 (Supporting Information). The sharp crystalline peaks corresponding to LiTFSI are absent in the SPEs, indicating the full dissolution of LiTFSI in the polymer matrix. Compared with pure PEO with peaks at 2θ = 19.0 and 23.5°, the SPEs with a LiTFSI concentration of ≤30 wt % have peaks with reduced relative intensity, confirming that LiTFSI lowers the crystallinity of PEO.35 For the samples with 40−60 wt % LiTFSI, a characteristic amorphous diffraction peak exists; therefore, the crystallinity gap is 40−60 wt % for LiTFSI. After these samples were aged for 1 month at RT, only that sample with 40 wt % LiTFSI maintained an amorphous phase, as shown in Figure 3a. When the LiTFSI concentration is more than 50 wt %, some crystalline peaks appear, which are attributed to crystalline compounds (PEO) 6 LiTFSI 1 , (PEO)3LiTFSI1, and (PEO)2LiTFSI1.22 Nevertheless, all amorphous SPEs were rubbery and sticky and could not be peeled from the PTFE substrate for practical use. Therefore, the LLTO nano-backbone was mixed with PEO to obtain 3D-CPE films by hot-pressing and quenching. 24794

DOI: 10.1021/acsami.8b06658 ACS Appl. Mater. Interfaces 2018, 10, 24791−24798

Research Article

ACS Applied Materials & Interfaces

Figure 5. EIS profile (a) and conductivity (b) of 3D-CPEs with different hot-pressing times. The inset in (a) shows the equivalent circuit and the Nyquist plot before hot-pressing. (c) EIS profiles of SPEs and 3D-CPEs at 25 and 40 °C. (d) Conductivity of SPEs and 3D-CPEs at room temperature as a function of the LiTFSI concentration.

interfaces.41 The ionic conductivity, σ, is calculated by σ = L/RS, where L and S are the thickness and area of the electrolyte membrane, respectively. Figure 5b shows the ionic conductivity, σ, calculated from the Nyquist plots for 3D-CPEs with different hot-pressing times. The ionic conductivity increases when the hot-pressing time increases. The bulk resistance of the 3D-CPE before hotpressing and quenching is 6900 Ω (inset in Figure 5a), and it significantly decreases to 148 Ω after 90 min hot-pressing and quenching. During hot-pressing, the polymer matrix becomes liquid, fully covers nanofibers, and fills in the pores among the fibers. The following quenching procedure not only can maintain the dense structure resulting from the hot-pressing but also is helpful for peeling off the membranes completely from the PTFE substrate to get self-standing 3D-CPEs. Figure 5c compares the impedance of the SPE and 3D-CPE with that of 40 wt % LiTFSI at 25 and 40 °C. It is clear that the resistance of the 3D-CPE is much less than that of the SPE. Figure 5d summarizes the σ of 3D-CPEs and SPEs as a function of the LiTFSI concentration. The dependency of σ on Li salt is similar for 3D-CPEs and SPEs, both reaching the maximum value at 40 wt % LiTFSI. The highest σ values of the SPE and 3D-CPE are 1.1 × 10−5 and 1.8 × 10−4 S·cm−1 at RT, respectively. At 40 wt % LiTFSI, the SPE possesses intrinsically high conductivity because of its endurable amorphous structure, as shown in Figure 2a. For the 3D-CPE at 40 wt % LiTFSI, the amorphous matrix is sticky and rubbery, which enhances the interfacial bonding between the LLTO nanobackbone and the polymer matrix; therefore, the ionic conductivity is further improved. When the weight fraction of the Li salt increases from 0 to 40 wt %, the σ of the

Thermogravimetric analysis (TGA) was conducted to evaluate the thermal stability of the electrolytes as shown in Figure 4b. It can be seen that the 3D-CPE is stable until 400 °C. Typically, commercial organic liquid electrolytes have potential explosion hazard or thermal failure over 100 °C.40 Compared to the unstable organic liquid electrolytes, the 3DCPE shows much better thermal stability. Combustion experiment was conducted. The SPE was rapidly ignited and burned to ashes (Figure 4d). On contrast, the inorganic nanobackbone maintained after the polymer matrix in the 3D-CPE was burnt (Figure 4c). The residual mass for the LLTO nanobackbone is about 20 wt % according to the TGA curve in Figure 4b. The nano-backbone serves as a barrier between the electrodes that can reduce short-circuit risk. In addition, both the membranes were heated at 150 °C for 3 h. The edge of the SPE shrinked obviously after heating (Figure 4f), whereas the shape of the 3D-CPE maintained (Figure 4e). Thus, 3D-CPEs possess much better mechanical and thermal properties in comparison to those of SPEs. Electrochemical Properties. The ionic conductivity test was conducted by AC electrochemical impedance spectroscopy (EIS) with SPEs or 3D-CPEs sandwiched in stainless steel symmetrical cells. The Nyquist plots shown in Figure 5a can be well fit into two parts: a suppressed semicircle at middle-high frequency and a linear part at a low frequency zone. The semicircle represents the bulk electrolyte resistance, and the curve tail is ascribed to the double-layer capacitance in between the electrolyte and electrodes. The equivalent circuit is modeled by R as resistance to fit bulk impedance, CPE as the constant-phase element to fit bulk capacitance, and Zw as Warburg impedance to mimic ion diffusion behavior at 24795

DOI: 10.1021/acsami.8b06658 ACS Appl. Mater. Interfaces 2018, 10, 24791−24798

Research Article

ACS Applied Materials & Interfaces

backbone, no obvious decomposition occurs for the 3D-CPE below 4.5 V, which is suitable for most high-voltage cathode materials like LiNixMnyCozO2 (x + y + z = 10).45 The reason why the oxidation voltage is increased lies in the removal of impurities such as vapor from the polymer matrix by the LLTO ceramic network.46 The repeated Li stripping and plating were tested under 0.1 mA·cm−2 current density at 25 °C for periodic 20 min charging and discharging. Figure 6c shows the constant current galvanostatic cycles for the SPE and 3D-CPE. During the 200 h cycling in a Li|SPE|Li cell, the SPE shows a fluctuating voltage of over 2 V because of its high impedance, and the cell is short-circuited after 200 h because of lithium dendrite piercing.46 For the Li|3D-CPE |Li cell, the polarized voltage is as small as 0.2 V and very stable over 800 h. The comparison clearly demonstrates that the LLTO nano-backbone can effectively reduce the local current density at the interface between 3D-CPE and Li, hence stabilizing Li striping.47,48 We assembled a LiFePO4|3D-CPE|Li solid-state cell. The cell can be operated at 25 °C and 0.3 C-rate. Figure S5a (Supporting Information) shows its discharge capacity and Coulombic efficiency in the first 30 cycles. The capacity decay is possibly due to the LiFePO4 composite cathode, which remains to be optimized in our future work.

electrolytes increases because of the increase of the amorphous PEO phase with high conductivity shown by the XRD patterns in Figure 3a.13 When the Li salt concentration increases from 40 to 80 wt %, the σ decreases because of the formation of crystallized PEO-LiTFSI complexes, as shown in Figure S5 (Supporting Information), with low conductivity.12 It is worth noting that the weight ratio of the LLTO nano-backbone is ∼20 wt % in the 3D-CPEs, which is close to the optimal quantity reported in the previous work.32,33 The advantages of our LLTO nano-backbone in comparison with discrete 0D and 1D fillers are that the nano-backbone is self-standing and effectively avoids agglomeration as the 0D and 1D fillers often do. In addition, the Li ion transference number, t+, of the 3DSPE was obtained using the equation t+ = Is(ΔV − I0R0)/ I0(ΔV − IsRs), where ΔV is the applied voltage, Is and I0 are the steady and initial current, respectively, and Rs and R0 are the steady and initial resistance, respectively.42 The t+ value of the 3D-SPE was measured to be 0.33 (Figure S4 in Supporting Information), which was comparable to those of other solidstate electrolytes.43 The Arrhenius plots of ionic conductivity in a temperature range from RT to 90 °C are shown in Figure 6a. The activation

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CONCLUSIONS In summary, we have fabricated a scale-producible, long-time endurable, three-dimensional interpenetrating composite polymer electrolyte with the LLTO nano-backbone by hot-pressing and quenching. The hot-pressing and quenching processes can be generally applied to other kinds of composite polymer electrolytes for property optimization. The effects of the LiTFSI concentration on the structure of the PEO matrix are studied. The PEO matrix remains amorphous after 1 month aging with 40 wt % LiTFSI. The LLTO nano-backbone can not only enhance the mechanical properties and thermal stability of the electrolyte membrane but also increase its ionic conductivity. The tensile strength, Young’s modulus, ionic conductivity at RT, and electrochemical window of the 3DCPE membrane are as high as 16.18 MPa, 0.98 GPa, 1.8 × 10−4 S·cm−1, and 4.5 V, respectively. The 3D-CPE membrane also demonstrates excellent stability against Li metal in an 800 h cycling test in a symmetrical Li|3D-CPE|Li cell at 0.1 mA· cm−2. All of these mechanical properties, thermal stability, and excellent electrochemical performance prove that the 3D-CPE is a promising candidate as a solid electrolyte for all-solid-state flexible LIBs. The unique structure design with a ceramic nanobackbone and novel synthetic processes with hot-pressing and quenching provide a new paradigm for high-performance flexible solid electrolytes.

Figure 6. (a) Arrhenius plots of the SPE and 3D-CPE with their activation energy indicated beside the curves. (b) LSV curves of the SPE and 3D-CPE. (c) Voltage profiles of Li plating and stripping cycling for the SPE and 3D-CPE. The inset shows four cycles for the Li|3D-CPE|Li cell.

energy, Ea, can be calculated by classical Arrhenius equation σ(T) = A exp(−Ea/RT), where T is the absolute temperature and A is a pre-exponential factor. For both the SPE and 3DCPE, the slope change at ∼60 °C ascribes to the PEO softening.29 The Ea values of the SPE are 0.584 and 0.284 eV below and above 60 °C, respectively, whereas those of the 3DCPE decrease to 0.305 and 0.218 eV, respectively. The evenly distributed interfaces between the LLTO ceramic network and PEO matrix provide a fast pathway for ion transport and reduce the barrier for ion hopping.44,45 Polymer electrolytes with a small electrochemical window cannot sustain a high voltage, which limits the energy density of lithium-ion batteries; thus, it is necessary to raise the electrochemical window. The LSV test was carried out to measure the electrochemical stability of our electrolytes, as shown in Figure 6b. With the addition of the LLTO nano-

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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.8b06658. SEM images, diameter distribution, and XRD patterns of LLTO nano-backbone calcined at 650, 750, 850, and 950 °C; XRD patterns of as-synthesized SPEs; characterization of Li ion transference number of the 3D-CPE; and cycling performance of a LiFePO4|3D-CPE|Li cell (PDF) 24796

DOI: 10.1021/acsami.8b06658 ACS Appl. Mater. Interfaces 2018, 10, 24791−24798

Research Article

ACS Applied Materials & Interfaces

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-1062797162. Fax: +86-10-62771160 (L.L.). *E-mail: [email protected]. Phone: +86-1062773300. Fax: +86-10-62771160 (Y.S.). *E-mail: [email protected]. Phone: +86-1062773587. Fax: +86-10-62771160 (C.-W.N.). ORCID

Xue Zhang: 0000-0002-2531-4263 Liangliang Li: 0000-0001-7808-7052 Yang Shen: 0000-0002-1421-0629 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Basic Science Center Project of National Natural Science Foundation of China (NSFC) under Grant No. 51788104 and NSFC projects under Grant Nos. 51572149 and 51532002.

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