3D Fiber-Network-Reinforced Bicontinuous Composite Solid

Feb 7, 2018 - Numerous efforts have been devoted to improve rechargeable lithium batteries, in terms of the ever-increasing demand for high energy den...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 7069−7078

3D Fiber-Network-Reinforced Bicontinuous Composite Solid Electrolyte for Dendrite-free Lithium Metal Batteries Dan Li, Long Chen, Tianshi Wang, and Li-Zhen Fan* Beijing Advanced Innovation Center for Materials Genome Engineering, Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: Replacement of flammable organic liquid electrolytes with solid Li+ conductors is a promising approach to realize excellent performance of Li metal batteries. However, ceramic electrolytes are either easily reduced by Li metal or penetrated by Li dendrites through their grain boundaries, and polymer electrolytes are also faced with instability on the electrode/electrolyte interface and weak mechanical property. Here, we report a three-dimensional fiber-network-reinforced bicontinuous solid composite electrolyte with flexible Li+-conductive network (lithium aluminum titanium phosphate (LATP)/polyacrylonitrile), which helps to enhance electrochemical stability on the electrode/electrolyte interface by isolating Li and LATP and suppress Li dendrites growth by mechanical reinforcement of fiber network for the composite solid electrolyte. The composite electrolyte shows an excellent electrochemical stability after 15 days of contact with Li metal and has an enlarged tensile strength (10.72 MPa) compared to the pure poly(ethylene oxide)−bistrifluoromethanesulfonimide lithium salt electrolyte, leading to a long-term stability and safety of the Li symmetric battery with a current density of 0.3 mA cm−2 for 400 h. In addition, the composite electrolyte also shows good electrochemical and thermal stability. These results provide such fiber-reinforced membranes that present stable electrode/electrolyte interface and suppress lithium dendrite growth for high-safety all-solid-state Li metal batteries. KEYWORDS: 3D fiber-network, bicontinuous composite solid electrolyte, interface, dendrite-free, all-solid-state lithium metal batteries

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INTRODUCTION

Targeted at the above problems of the Li anode, extensive efforts have been made, including tuning the compositions of liquid electrolytes (i.e., solvents, lithium salts, and additives7−9), improving mechanical property of the separator,10−14 surface modification of the lithium metal,15 and designing for lithium metal and current collectors with various three-dimensional (3D) structures.16−18 Among all above methods, application of solid electrolytes is of particular interest because solid electrolytes can avoid the intrinsic drawbacks of liquid electrolytes (e.g., flammability, volatility, and limited electrochemical windows) from the root with their resistance to lithium dendrite growth.19−21 The solid-state lithium electrolytes include two classes of materials: polymer electrolytes and inorganic ceramic electrolytes.22−25 Inorganic solid electrolytes are rigid and nonflammable, which is generally considered to be the ultimate solution to the safety issues associated with lithium-ion batteries.26 However, utilizing inorganic electrolytes triggers problems such as low ionic conductivity and high interfacial resistance to electrodes (both cathode and anode). In

Numerous efforts have been devoted to improve rechargeable lithium batteries, in terms of the ever-increasing demand for high energy density and safety for portable electronics, electric vehicles, and grid-energy storage systems.1,2 Lithium metal, with the advantages of its ultrahigh theoretical specific capacity (3860 mA h g−1) and extremely low potential (−3.040 V vs standard hydrogen electrode), is regarded as the most promising anode which can maximize energy density of stateof-the-art lithium-ion batteries.3,4 However, challenges for applying metallic lithium anode in liquid electrolytes in view of cell performance and safety have long prevented the development of the good-performance Li metal batteries. In general, the spontaneous chemical reaction between highly reactive Li metal and organic liquid electrolytes generates an incompact solid electrolyte interphase on the surface of the Li metal, leading to consecutive consumption of Li metal and electrolyte, accompanied by the degradation of coulombic efficiency.5 Another major challenge for Li metal anode in conventional liquid battery is that Li dendrites originated from uneven lithium deposition will pierce through the separator and cause internal short circuit, resulting in severe safety issues.6 © 2018 American Chemical Society

Received: November 30, 2017 Accepted: February 7, 2018 Published: February 7, 2018 7069

DOI: 10.1021/acsami.7b18123 ACS Appl. Mater. Interfaces 2018, 10, 7069−7078

Research Article

ACS Applied Materials & Interfaces addition, although many ceramic solid Li+ electrolytes have been investigated, they are easily reduced by Li metal and/or they have failed to block dendrite formation and growth into their grain boundaries.27 Polymer electrolytes are flexible, but they have disadvantages of the narrow operating temperatures, weak mechanical stability, and the instability of the electrode/ electrolyte interface. Taking consideration of these characteristics, composite polymer electrolytes developed by the integration of non-Li + -conductive (SiO 2 , TiO 2 , and Al 2 O 3 ) 28−30 or Li + -conductive (Li6.4 La 3 Zr 2 Al 0.2 O 12 and Li1.5Al0.5Ge1.5(PO4)3)31,32 fillers into the host polymer matrix are proven to be an effective strategy to enhance the performance of all-solid-state Li batteries. The drawbacks of inorganic electrolytes on electrolyte/electrode interfacial resistance can be markedly offset, and the advantages of polymer electrolytes on flexibility and interfacial wetting property can be exhibited in the solid composite electrolyte. Among different types of fillers, the Li+-conductive ceramic fillers have attracted extensive interest for providing extra Li+ from crystal structure to further improve the ionic conductivity when compared to those passive fillers.33 Lithium aluminum titanium phosphate (LATP) with the sodium superionic conductor (NASICON)34-type structure has been widely investigated as a competitive Li+ conductor because of its high ionic conductivity (>10−3 S cm−1),35 good chemical stability in ambient atmosphere,36 and low cost.37 In addition, LATP can be synthesized more simply and with higher reproducibility than other electrolytes such as garnet-type electrolytes because of lower temperature preparation routes.35 However, LATP delivers a chemical instability against Li anode because of the reduction of Ti4+ on contact with lithium metal, which impedes its direct contact with lithium anode.38 Recently, Li1.4Al0.4Ti1.6(PO4)3 (LATP) was used as fillers to prepare composite solid polymer electrolytes, and the particlefillers composite (poly(ethylene oxide) (PEO)−LATP) was reported with unsatisfactory electrochemical stability in Li metal batteries, which indicated that LATP probably reacted with Li metal when directly mixed with PEO as particle-fillers.39 Here, we report a 3D fiber-network-reinforced bicontinuous composite solid electrolyte with high stability and Li dendrites suppression against Li metal for all-solid-state lithium metal batteries. As a network filler, LATP/ polyacrylonitrile (PAN) composite fiber by electrospinning helps to improve the mechanical property of PEO-based polymer matrix and enhance ionic conductivities by decreased segmental reorientations of polymers. Meanwhile, the reaction of LATP with Li anode is effectively prevented from utterly isolating chemically active Ti4+ with Li metal because LATP particles are wellenveloped within PAN polymeric chains. The LATP/PAN fiber-network-reinforced composite electrolyte presents greatly improved electrochemical performance, mechanical properties, and thermal stability. The as-assembled Li symmetric and allsolid-state Li|LiFePO4 batteries also exhibit excellent electrochemical performance.

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into the above-mentioned acidic solution under vigorous stirring. When NH4H2PO4 was added subsequently, a sol was immediately formed. Starting from the sol heated at 80 °C in drying oven for about 10 h, amorphous powders were prepared by heat-treatment at 500 °C for 3 h in an alumina crucible, and LATP powders were obtained by heat-treating the amorphous product at 800 °C in air for 3 h. Planetary ball milling using ZrO2 balls and alcohol as solvent with 650 rpm for 4 h was introduced to relieve the agglomerations of powders. LATP dense pellets (14 mm in diameter) were obtained by cold pressing (250 MPa) the as-prepared powders and then heating at 900 °C for 4 h. Composite solid electrolytes were fabricated according to the following procedure. First, LATP/PAN composite fibers were prepared by electrospinning of LATP nanoparticles, PAN, and dimethylformamide (DMF) mixture. First, 10 wt % of PAN was added into DMF. After it was dissolved completely, LATP was added into the above-mixed solution under intense stirring. Four precursor DMF solutions with different mass ratios of LATP to PAN (for 1:1, 2:1, and 3:1) and pure PAN were prepared by heating the mixture at 60 °C for 10 h. For the electrospinning procedure, the precursor solution was loaded into a 10 mL plastic injection syringe with a stainless steel (SS) needle. A high voltage of 15 kV was applied during the process of electrospinning, and an aluminum foil was placed 18 cm beneath the needle to collect the as-spun composite fibers. After electrospinning, the as-spun LATP/PAN composite fibers were peeled off the aluminum collector and then dried in vacuum at 80 °C for 24 h to remove the residual solvent. The prepared composite fibers can respectively represent xLATP/PAN (x: the mass ratios of LATP to PAN, for 1, 2, and 3) and PAN (for pure PAN). Bistrifluoromethanesulfonimide lithium salt (LiTFSI) (99.99%, Sigma-Aldrich) and PEO (MW ≈ 600 000, Sigma-Aldrich) were dried at 80 °C under vacuum for 24 h before use. The composite solid electrolytes were prepared via the solution-casting technique. PEO and LiTFSI (molar ratio of ethylene oxide [EO] to LiTFSI is 8:1) were first dissolved in acetonitrile (99.8%, Sigma-Aldrich) with a concentration of 12 wt % of PEO. The mixture of PEO and LiTFSI was mechanically stirred at 60 °C for 10 h and then cast on the above LATP/PAN composite fiber membranes. The concentration ratio of polymer PAN to PEO was fixed at 1. The fiber-network-reinforced composite electrolytes {xLATP/PAN−[PEO8−LiTFSI] (x: the mass ratios of LATP to PAN, for 1, 2, and 3) and PAN−[PEO8−LiTFSI]} were prepared after being dried under N2 protection at 25 °C for 24 h and heated at 60 °C in a vacuum oven for extra 24 h. The crystal structure of the samples was characterized on a Rigaku/ max diffractometer via X-ray diffraction (XRD) with Cu Kα (λ = 0.154 nm) radiation with 2θ in the range of 10−80°. The morphologies of composite solid electrolytes were observed by field-emission scanning electron microscopy (Zeiss SUPRA 55 microscope). Differential scanning calorimetry (DSC) measurements were carried out under argon atmosphere on a TA Instruments equipment (Q5000IR) with a heating rate of 10 °C min−1. The mechanical property of membranes was investigated by an Instron-3300 universal testing machine (USA) at a stretching speed of 1.66 mm s−1. The impedance spectra of the electrolytes were recorded from 25 to 90 °C by Solartron electrochemical station (1287 + 1260) in a frequency range from 1 MHz to 0.01 Hz with an alternating current (ac) amplitude of 10 mV. The electrochemical window of the electrolytes was examined by cyclic voltammetry (CV) of the SS|Li coin battery at a scan rate of 1 mV s−1 from −0.5 to 5 V at 60 °C. The variation of current with time and the impedance spectra before and after polarization for the composite solid electrolyte were measured to calculate the lithium transference number (tLi+). The lithium periodic stripping/plating tests as well as the interface stability against Li metal electrode were investigated by Li symmetrical batteries. To fabricate the cathodes for full battery testing, the LiFePO4 powders were mixed with carbon black and polyvinylidene fluoride at a weight ratio of 8:1:1 with N-methyl-2-pyrrolidone as the solvent. The obtained slurry was coated on the surface of aluminum foil and dried in a vacuum oven at 100 °C for 12 h. CR2032-type coin batteries were fabricated in an argon-filled glove box, and the charge/discharge tests

EXPERIMENTAL SECTION

Nanosized LATP particles were prepared via an aqueous-assisted sol− gel method. Stoichiometric amounts of Ti(OC4H9)4, LiNO3·H2O, Al(NO3)3·9H2O, and NH4H2PO4 were used as starting materials. Ti(OC4H9)4 was dissolved in deionized water followed by nitric acid added dropwise into the titanium hydroxide precipitate under vigorous stirring. Then, citric acid monohydrate was added into the solution to condition the pH degree at 2. LiNO3 and Al(NO3)3·9H2O were added 7070

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the mixture of PEO8−LiTFSI acetonitrile solution was cast onto the above composite fiber membrane and then dried to obtain the fiber-reinforced composite solid electrolytes. Li1+xAlxTi2−x(PO4)3 with x = 0.5 was chosen as one of the most promising compounds of the LATP family to prepare fiber-reinforced composite electrolytes. The crystalline phase of as-prepared LATP particles has been examined by XRD measurements (Figure 1a), which indicates that all peaks could be indexed to a primary NASICON structure with the rhombohedral lattice with a space group R3c̅ . Figure 1a inset showed the microstructure of the LATP powders calcined at 800 °C for 3 h after ball milling, and the particles were of homogeneous distribution with an average size of ∼200 nm. Morphologies of the xLATP/PAN composite fibers were examined by SEM (Figures 1b and S1). The fibers exhibited smooth surface with an average diameter of about 400 nm, and the diameter of fibers grew slightly finer with the increase of the mass ratio of LATP to PAN. LATP particles could uniformly disperse within PAN fibers in the above SEM images, whereas the particle-aggregation was observed when the mass ratio of LATP to PAN increased to 3 (Figure S1b). The microstructure morphologies of the fiber-reinforced composite solid electrolyte (2LATP/PAN−[PEO8−LiTFSI]) was also presented in Figure 1c, and the LATP/PAN composite fibers were free of aggregates and uniformly embedded inside the PEO matrix. The optical photos of LATP/PAN composite porous membrane and LATP/PAN−PEO8−LiTFSI composite solid electrolyte film were also shown in the insets of Figure 1b,c respectively. It is noted that the optical photo of the PEOinfiltrated sample turned slightly transparent in comparison with the pure spun LATP/PAN composite membrane, and both samples displayed excellent flexibility without any fold and crack when bent and twisted. In contrast, uncompact structure

of the batteries were performed between 2.9 and 3.8 V on a LAND CT2001A testing system (Wuhan Jinnuo Electronics, Ltd.).

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RESULTS AND DISCUSSION The schematic illustration (Scheme 1) indicates the procedure to synthesize the 3D fiber-network bicontinuous reinforced Scheme 1. Schematic Illustration for the Preparation of the Fiber-Reinforced Membranes; (a) Nanosized Li1.5Al0.5Ti1.5(PO4)3 Particles Were Obtained from Sol−Gel Method, (b) PAN Polymer with LATP Particles Were Blended Together into DMF to Prepare Solution for Electrospinning, (c) LATP/PAN Network Was Got by Electrospinning, and (d) Casting Process of PEO−LiTFSI Solution onto the Fiber-Network

composite solid electrolytes. Nanosized Li1.5Al0.5Ti1.5(PO4)3 particles were first prepared via an aqueous-assisted sol−gel method. PAN polymer and LATP particles of different mass ratio were added into the DMF in sequence under intense stirring to obtain the homogeneous precursor solution. Subsequently, LATP/PAN composite fibers were prepared by electrospinning of the as-prepared precursor solution. Finally,

Figure 1. (a) XRD pattern of nanosized LATP powders with an inset SEM image. SEM images of the (b) 2LATP/PAN and (c) 2LATP/PAN− [PEO8−LiTFSI] with inset flexibility exhibition images. 7071

DOI: 10.1021/acsami.7b18123 ACS Appl. Mater. Interfaces 2018, 10, 7069−7078

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Figure 2. (a) Arrhenius plots of the fiber-reinforced composite electrolytes with various weight ratios of LATP to PAN, together with the data for pure PEO8−LiTFSI electrolyte. (b) DSC curves of 2LATP/PAN−[PEO8−LiTFSI] and PEO8−LiTFSI.

Figure 3. (a) CV for the 2LATP/PAN−[PEO8−LiTFSI] and PEO8−LiTFSI electrolytes at 60 °C, (b) impedance change of Li|SS battery when applying a continuous bias voltage of 4.5 V at 60 °C on 2LATP/PAN−[PEO8−LiTFSI] and PEO8−LiTFSI, (c) optical photos of Celgard membrane and 2LATP/PAN−[PEO8−LiTFSI] (FCSE) after heating at 160 °C, and (d) stress−strain curves of 2LATP/PAN−[PEO8−LiTFSI] and PEO8−LiTFSI.

(Figure S2a,b) was delivered when the mass ratio of PAN to PEO was 2 because low weight ratio of PEO polymer cannot fill into the porous spun mat completely. The temperature dependence of the ionic conductivities for fiber-reinforced composite electrolytes was obtained by ac impedance spectroscopy measurements conducted at different temperatures (Figure 2a). With the increase of LATP, the ionic conductivity gradually rose and reached maximum (for x = 2) and then decreased. The initial increase was because the contribution of Li+ from active nanofiller LATP to the enhanced ionic conductivities of the fiber-reinforced composites and the decrease (for x = 3) was because aggregation of particles blocked segmental reorientations and increased the impedance between particles. It can be seen clearly that the ionic conductivity of the 2LATP/PAN−[PEO8−LiTFSI] electrolyte is higher than that of PAN−[PEO8−LiTFSI], which may be because of the percolation behavior arising

from the adsorption effect at the second-phase interface. The Li+ in LATP can be absorbed by the PAN matrix without lithium salt. This increases lithium vacancies which serve as mobile carriers on the LATP surface, thus increasing the ionic conductivities of 2LATP/PAN fiber and 2LATP/PAN− [PEO8−LiTFSI].40,41 For comparison, the ionic conductivity of PEO8−LiTFSI electrolyte was also shown in this figure. Compared to the PEO8−LiTFSI electrolyte, an appreciable enhancement of conductivity could be observed throughout all fiber-network blending contents. At 60 °C, the ionic conductivity of the 2LATP/PAN−[PEO8−LiTFSI] was about 6.5 × 10−4 S cm−1 and about four times higher than that of the PEO8−LiTFSI. The introduction of LATP/PAN fiber network can facilitate the interaction between neighboring polymer chains and decrease segmental reorientations, resulting in improved conductivity. This result can be verified by DSC profiles of 2LATP/PAN−[PEO8−LiTFSI] and pure PEO8− 7072

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Figure 4. Stabilities of the electrode/electrolyte interface. (a) Image of the LATP pellet after 7 days in contact with Li metal, (b) impedance plots of Li|LATP|Li symmetric cell taken at different storage time at 60 °C, (c) images of the 2LATP/PAN and 2LATP/PAN−[PEO8−LiTFSI] after 7 days in contact with the Li metal, and (d) impedance plots of Li|2LATP/PAN−[PEO8−LiTFSI]|Li and Li|PEO8−LiTFSI|Li symmetric cell taken at different storage times at 60 °C.

contributions from the Li-conducting NASICON solid electrolyte and anion trapping, which promote the movement of free Li ions. As a requisite parameter for practical application in a battery, the electrochemical working window of 2LATP/PAN− [PEO8−LiTFSI] was tested. The CV curves in Figure 3a of the Li|2LATP/PAN−[PEO8−LiTFSI]|SS battery and the Li| PEO8−LiTFSI|SS battery were obtained at a scan rate of 0.1 mV s−1 from −0.5 to 5 V vs Li/Li+ at 60 °C. In the positive scans of the red curve, no any oxidation signal was observed from the voltage range, indicating high electrochemical stability of fiber-reinforced composite at this working voltage window. It is noted that no current peak was measured around 2.5 V vs Li/ Li+ as the signal for reduction of Ti3+ from Ti4+,38 which can further prove good electrochemical stability of LATP in this structure. A low background current was measured in the potential region between 1.0 and 2.0 V in the curves of PEO8− LiTFSI, which might be attributed to chemical instability. The current was not obviously observed in the composite electrolyte, and the possible reason is that LATP/PAN composite can help to stabilize PEO in the bulk region, leading to a better electrochemical stability.46 To make further evaluation of the electrochemical stability, the impedance spectra for 2LATP/PAN−[PEO8−LiTFSI] and PEO8−LiTFSI after applying a dc bias voltage at 4.5 V for 72 h were recorded (Figure 3b).46,47 The impedance curves of Li| 2LATP/PAN−[PEO8−LiTFSI]|SS only showed a slight change after being subjected to 4.5 V versus Li/Li+ for 72 h, which could indicate that the composite electrolyte had a steady electrochemical stability. The semicircle in the impedance spectra of the PEO8-LiTFSI electrolyte was obvious in the primary state, showing a huge continuous increase. The essential cause of better electrochemical stability of 2LATP/ PAN−[PEO8−LiTFSI] than that of PEO8−LiTFSI may be the presence of the 3D network of PAN/LATP, which can facilitate the chemical bonding strength within the whole composite matrix and result in an enhanced electrochemical stability.

LiTFSI in the DSC results in Figure 2b. As presented in the DSC profiles, endothermic peak was found at 62.8 °C in the black curve, corresponding to the melting temperature (Tm) of PEO8−LiTFSI electrolyte. Meanwhile, the endothermic peaks Tm of the 2LATP/PAN−[PEO8−LiTFSI] shifted to lower temperature (58.2 °C). In addition, incorporating LATP/PAN composite fibers into the PEO matrix could also lower the glass transition temperature (Tg) of PEO polymer. Tg of PEO8− LiTFSI decreased from −34.2 to −37.5 °C because of the incorporation of composite fibers. This result indicates that fraction of the amorphous phase was increased, which implies that the composite fibers can suppress crystallization of PEO polymer at low temperature and speed up the dynamics.42,43 Lithium transference number (tLi+) in polymer-based electrolytes is a crucial factor to evaluate the mobility of Li ions. High value of tLi+ benefits from promoted movement of free Li ions in electrolytes, which can enable fast discharge of batteries. Low tLi+ can adversely affect battery performance, resulting in low energy density and low cycle number. It generally believed that the addition of fillers into the polymer electrolytes could improve the tLi+ because of surface interactions between fillers and polymer or salt. The fillers could promote local relaxation and segmental motion, resulting in an enhanced mobility of Li+ and increased tLi+.44 The tLi+ of 2LATP/PAN−[PEO8−LiTFSI] was estimated by chronoamperometry and ac impedance (Figure S3), and the values were calculated according to eq 1. t Li+ =

IS(ΔV − I0R 0) I0(ΔV − ISR S)

(1)

where I0 and Is are the initial and steady-state currents, ΔV is the potential applied across the battery, and R0 and Rs are the initial and steady-state resistance of the passivation layer. The calculated tLi+ obtained by comparing the maximum and equilibrium current values was 0.32 for 2LATP/PAN− [PEO8−LiTFSI], which was higher than PEO8−LiTFSI polymer electrolyte (0.12).45 This may be because of 7073

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Figure 5. Electrochemical compatibility and stability of electrolytes with the Li metal. (a) Voltage profiles of the Li|2LATP/PAN−[PEO8−LiTFSI]| Li and Li|PEO8−LiTFSI|Li batteries for the lithium plating/stripping experiment with a current density of 0.3 mA cm−2 at 60 °C. (b) Impedance spectra of the symmetric batteries measured at different cycle times (0, 200, and 400 h). (c) SEM image of the 2LATP/PAN−[PEO8−LiTFSI] after cycling the symmetric battery for 400 h at 0.3 mA·cm−2. SEM images for the surface of the Li electrodes obtained from (d) Li|2LATP/PAN− [PEO8−LiTFSI]|Li and (e) Li|PEO8−LiTFSI|Li batteries after cycling for 400 h at 0.3 mA·cm−2.

verified by the mechanical test in Figure 3d. Compared to the polymer PEO8−LiTFSI electrolyte, the fiber-reinforced composite (2LATP/PAN−[PEO8−LiTFSI]) with an excellent ionic conductivity also possessed favorable mechanical properties (higher tensile strength of 10.72 MPa) and decreased elongation, which can be attributed to the adhesion effect between the LATP/PAN fiber network and PEO polymer chains. Good mechanical property is critical for solid electrolyte membranes used in lithium metal batteries because they should withstand big stress during cell packaging and prevent dendrite formation and growth during plating of an alkali metal anode. The improved mechanical property of our fiber-reinforced composite solid electrolyte could offer a notable safetyreinforcement against short circuit in Li metal batteries. The stability of the solid electrolyte on contact with a lithium anode is a critical issue in lithium batteries. To explore the electrochemical compatibility and stability of the prepared fiberreinforced composites with Li metal, the long-term Li symmetric battery tests were conducted. For comparison, pure LATP pellet and pure PEO8−LiTFSI were also tested.

From the above electrochemical stability results, the wide electrochemical window of fiber-reinforced composite electrolyte is favorable for its practical applications in high-voltage Li batteries and lithium metal batteries. As a significantly necessary character for Li batteries, thermal stability of the electrolytes was represented in Figure 3c. In shrinkage comparison tests against a commercial Celgard membrane, good thermal stability was observed for the 2LATP/PAN−[PEO8−LiTFSI] when heated at 160 °C for 30 min, whereas a severe shrinkage from both horizontal and vertical appeared for Celgard. During this thermal stability test, no shrinkage and crack from thermal stress was observed in the 2LATP/PAN−[PEO8−LiTFSI], demonstrating good thermal stability of fiber-reinforced composite electrolytes for significantly eliminating the risk of short circuit. Good electrochemical and thermal properties benefited from the composite fiber-network blending. The interconnected fiber network played an important part in effectively interacting with the clusters of chains of PEO along the whole matrix, feasibly performing a robust substrate. These results can be firmly 7074

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Figure 6. Electrochemical performance of all-solid-state batteries. (a) Charge and discharge curves of Li|2LATP/PAN−[PEO8−LiTFSI]|LiFePO4, (b) rate performance of the Li|LiFePO4 batteries at 60 °C, (c) cycling stability of Li|LiFePO4 batteries at 0.2 C and 60 °C, (d) impedance spectra of Li|LiFePO4 batteries at 60 °C, and (e,f) proposed lithium plating/stripping processes and lithium surfaces in Li|PEO8−LiTFSI|LiFePO4 and Li| 2LATP/PAN−[PEO8−LiTFSI]|LiFePO4.

interfacial resistance change between PEO and Li metal was also revealed in Li|PEO8−LiTFSI|Li symmetrical battery in Figure 4d. With the increase of storage time, the interfacial resistance for the PEO8−LiTFSI and Li metal first grew from 250 to 317 Ω (∼3 day), then quickly decreased to 217 Ω. Afterward, the interfacial resistance gradually increased to 282 Ω on the 10th day. Compared to Li|PEO8−LiTFSI|Li, the decreased initial interfacial resistance in Li|2LATP/PAN− [PEO8−LiTFSI]|Li may be because the nanosized fibers in the composite electrolyte is favorable to increase the interface contact area between electrolyte and Li electrode.45 The reason for better electrochemical stability of LATP/PAN−[PEO8− LiTFSI] than pure PEO8−LiTFSI is that the reinforcement of LATP/PAN composite fibers helps to stabilize PEO in the bulk region.38 The lower resistance and higher stability on the electrode/electrolyte interface of the Li|2LATP/PAN−[PEO8− LiTFSI]|Li battery make 2LATP/PAN−[PEO8−LiTFSI] a more suitable Li+ solid electrolyte than LATP pellet and pure PEO8−LiTFSI. Generally, the PEO-based polymer Li metal batteries suffer from decomposition of the polymer electrolyte by an alkali metal anode under an electric field.41 To investigate the electrochemical compatibility of electrolytes with Li metal,

After 7 days in contact with the lithium metal, the LATP pellet turned into dark blue and generated fracture, indicating the reaction of lithium metal and LATP (Figure 4a). Furthermore, the ever-increasing interface resistance between LATP pellet and Li anode in Li|LATP|Li symmetrical battery was presented at Figure 4b, which indicate that LATP was reduced by the lithium metal. After 7 days of contact with lithium metal, the 2LATP/PAN and 2LATP/PAN−[PEO8−LiTFSI] remained white (Figure 4c), indicating that the fiber-reinforced structure provided a stable chemical stability for Ti4+ in LATP because of the isolation of LATP and Li metal. With the increase of storage time, the Li|2LATP/PAN−[PEO8−LiTFSI]|Li delivered a perfect stability with no obvious change for both bulk and interface resistance after 12 days (Figure 4d). Whereas LATP dense pellet can have a higher bulk ionic conductivity, the interface of a symmetric Li|LATP|Li battery created a large resistance to Li+ transfer between the anode and the electrolyte (>3500 Ω, 60 °C) that dominated the total resistance of the electrolyte. In contrast, resistance value between 2LATP/ PAN−[PEO8−LiTFSI] and Li anode became smaller (∼158 Ω, 60 °C) in comparison with that of LATP pellet, which was profited from the good wetting ability and flexibility on interface of 2LATP/PAN−[PEO8−LiTFSI] and Li metal. The 7075

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ACS Applied Materials & Interfaces

state batteries from 2.9 to 3.8 V at 0.2 C rate and 60 °C were shown in Figure 6c. The Li|2LATP/PAN−[PEO8−LiTFSI]| LiFePO4 battery presented an initial discharge capacity of 144 mA h g−1 with a tremendously stable coulombic efficiency of 99.5 ± 0.5% over 100 cycles, which indicates an outstanding interface stability between electrolyte and Li anode during cycling. On the contrary, both capacity and coulombic efficiency for LiFePO4|PEO8−LiTFSI|Li delivered recession, possibly on account of the decomposition of PEO on Li metal. The interface resistance was observed by impedance spectra after different cycles (Figure 6d). The intersection with the real axis got smaller after 10 cycles in comparison with the initial one because of the electrochemical activation, and then it increased gradually and achieved a steady state in the subsequent cycles. The electrochemical performance of Li| 2LATP/PAN−[PEO8−LiTFSI]|LiFePO4 was much better than batteries using pure PEO8−LiTFSI, which could benefit from the stable interface between electrode and electrolyte without significant dendrite growth during charge−discharge long-term cycles. The proposed effect of the introduction of LATP/PAN composite fiber network on lithium dendrite formation and interface of Li metal with electrolyte was presented in Figure 6e,f. Because of the strong mechanical ability and enhanced stability on interface to inhibit the lithium dendrite formation and growth, the addition of LATP/PAN-reinforced 3D bicontinuous structure can effectively inhibit ununiform Li deposition and short circuit to extend the cycle life of full cell.

time-dependent voltage profiles of symmetric batteries were performed by charging for 1 h and discharging for 1 h at a current density of 0.3 mA cm−2 to get the cyclic behaviors. As shown in Figure 5a, the Li plating/stripping behaviors of Li| 2LATP/PAN−[PEO8−LiTFSI]|Li, Li|PEO8−LiTFSI|Li for 400 h were regularly reflected by the negative and positive voltage plateaus. The black curve for Li|PEO8−LiTFSI|Li battery led to a larger voltage polarization and became fluctuant along with the time of charge−discharge, which was attributed to the lower ionic conductivity and electrochemical instability of the PEO8− LiTFSI electrolyte. The voltage of Li|2LATP/PAN−[PEO8− LiTFSI]|Li battery became relatively steady at 120 mV, and no short circuit appeared during charge−discharge for 400 h, indicating that the improved interfacial property between 2LATP/PAN−[PEO8−LiTFSI] and Li anode can promote a uniform Li deposition and effectively suppress the formation of Li dendrites. Furthermore, the impedance spectra of the symmetric batteries after cycled 0, 200, and 400 h were also measured. As presented in Figure 5b, the impedance of Li| 2LATP/PAN−[PEO8−LiTFSI]|Li only experienced a slight change whereas the battery with PEO electrolyte suffered from a huge continuous increase of resistance on the electrode/ electrolyte interface, indicating that a highly enhanced interfacial compatibility of 2LATP/PAN−[PEO8−LiTFSI] and Li anode was constructed by the reinforcement of the composite fibrous framework. The SEM image of 2LATP/ PAN−[PEO8−LiTFSI] in the symmetric cell after 400 h of cycling was also shown in Figure 5c, representing a complete and analogous morphology in comparison with the initial state. To visually validate the assumption for ability of 2LATP/ PAN−[PEO8−LiTFSI] against dendrite growth in Li metal batteries, the surface morphologies of the Li anodes harvested from the cycled Li|2LATP/PAN−[PEO8−LiTFSI]|Li batteries were scrutinized. Before examination, the coated electrolytes were removed by rinsing Li metal in the dimethyl carbonate for 7 days. Compared with the smooth surface of the Li electrode from Li|2LATP/PAN−[PEO8−LiTFSI]|Li in Figure 5d, nonuniform Li deposition and Li dendrites were clearly observed on the surface of Li anode in Figure 5e, which demonstrated excellent ability as lithium dendrites suppressor for fiber-network-reinforced composite solid electrolyte in Li metal batteries. Furthermore, as represented in Figure S4a,b, the Li|1 M LiPF6-EC/EMC/DMC|Li battery suffered from short circuit after 200 h, and the surface of the cycled Li metal generated obvious lithium dendrite growth. Given the satisfactory ability of dendrites suppression, the application of 2LATP/PAN−[PEO8−LiTFSI] in all-solid-state Li metal batteries will be feasible because of both of the chemical and electrochemical stability in the process of charge− discharge. The all-solid-state LiFePO4|Li batteries were implemented by the assembling of cathodes, free-standing 2LATP/PAN−[PEO8−LiTFSI] and Li metal as a sandwich structure. The charge/discharge voltage curves of LiFePO4| 2LATP/PAN−[PEO8−LiTFSI]|Li batteries at various current densities at 60 °C (Figure 6a), exhibiting discharge capacity of around 144, 130, 118, and 90 mA h g−1 of different rates of 0.2, 0.5, 1, and 2 C, respectively. Figure 6b represented the cycling performance of various rates for LiFePO4|Li batteries. It was noted that the batteries armed with 2LATP/PAN−[PEO8− LiTFSI] displayed an improved rate performance in comparison with cells with PEO8−LiTFSI, and the enhanced discharge capacity could be contributed to better ionic transport capability. The long-term cycling performance of all-solid-

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CONCLUSIONS In summary, an LATP/PAN network-reinforced PEO-based solid composite electrolyte for dendrite-free Li anode battery was prepared, exhibiting both excellent stability and improved resistance against lithium dendrites. The reaction between Ti4+ in LATP and Li anode is effectively inhibited by utter isolation of the chemically active Ti4+ and Li metal. Beyond this, improved electrochemical window and enhanced thermal stability than pure PEO8−LiTFSI are attributed to the efficient reinforcement of the fiber network. This work can also extend the application of bicontinuous reinforcement-structure solid electrolyte using flexible fibers constructed by inorganic fast lithium-ion conductor and polymers to support the satisfactory performance of solid electrolyte for Li anode-based energy storage systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18123. SEM images and optical photo of fiber-network membranes; chronoamperometry and ac impedance; and voltage profiles and SEM images of Li metal after cycling in symmetric battery (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +86 10 62334311. ORCID

Li-Zhen Fan: 0000-0003-2270-4458 Author Contributions

D.L. and L.C. contributed equally to this work. L.Z.F. designed the experiments, discussed the interpretation of results, and 7076

DOI: 10.1021/acsami.7b18123 ACS Appl. Mater. Interfaces 2018, 10, 7069−7078

Research Article

ACS Applied Materials & Interfaces

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cowrote the paper. D.L., L.C., and T.S.W. prepared the samples and performed the structural and electrochemical experiments. All authors discussed the results and participated in manuscript revision. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports from the Natural Scientific Foundation of China (51532002), the Beijing Natural Science Foundation (L172023), and the National Basic Research Program of China (2015CB932500) are gratefully acknowledged.

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