Edited by Yi Cui, Stanford University, Stanford, CA, and accepted by Editorial Board Member Tobin J. Marks May 4, 2016 (received for review January 10, 2016)
This work describes a flexible, solid-state, lithium-ion–conducting membrane based on a 3D ion-conducting network and polymer electrolyte for lithium batteries. The 3D ion-conducting network is based on percolative garnet-type Li6.4La3Zr2Al0.2O12 solid-state electrolyte nanofibers, which enhance the ionic conductivity of the solid-state electrolyte membrane at room temperature and improve the mechanical strength of the polymer electrolyte. The membrane has shown superior electrochemical stability to high voltage and high mechanical stability to effectively block lithium dendrites. This work represents a significant breakthrough to enable high performance of lithium batteries.
Beyond state-of-the-art lithium-ion battery (LIB) technology with metallic lithium anodes to replace conventional ion intercalation anode materials is highly desirable because of lithium’s highest specific capacity (3,860 mA/g) and lowest negative electrochemical potential (∼3.040 V vs. the standard hydrogen electrode). In this work, we report for the first time, to our knowledge, a 3D lithium-ion–conducting ceramic network based on garnet-type Li6.4La3Zr2Al0.2O12 (LLZO) lithium-ion conductor to provide continuous Li+ transfer channels in a polyethylene oxide (PEO)-based composite. This composite structure further provides structural reinforcement to enhance the mechanical properties of the polymer matrix. The flexible solid-state electrolyte composite membrane exhibited an ionic conductivity of 2.5 × 10−4 S/cm at room temperature. The membrane can effectively block dendrites in a symmetric Li | electrolyte | Li cell during repeated lithium stripping/plating at room temperature, with a current density of 0.2 mA/cm2 for around 500 h and a current density of 0.5 mA/cm2 for over 300 h. These results provide an all solid ion-conducting membrane that can be applied to flexible LIBs and other electrochemical energy storage systems, such as lithium–sulfur batteries.
High capacity, high safety, and long lifespan are three of the most important key factors to developing rechargeable lithium batteries for applications in portable electronics, transportation (e.g., electrical vehicles), and large-scale energy storage systems (1⇓⇓⇓–5). Based on state-of-the-art lithium-ion battery (LIB) technology, metallic lithium anode is preferable to replace conventional ion intercalation anode materials because of the highest specific capacity (3,860 mAh/g) of lithium and the lowest negative electrochemical potential (∼3.040 V vs. the standard hydrogen electrode), which can maximize the capacity density and voltage window for increased battery energy density (1). Moreover, the success of beyond LIBs, such as lithium–sulfur and lithium–oxygen, will strongly rely on lithium metal anode designs with good stability to achieve their targeted goals of high energy density and long cycle life.
Using lithium metal in organic liquid electrolyte systems faces many challenges in terms of battery performance and safety. For example, lithium–sulfur batteries suffer from the dissolution of intermediate polysulfides in the organic electrolyte that causes severe parasitic reactions on lithium metal surfaces, leading to lithium metal degradation and low lithium cycling efficiency (6). Lithium–oxygen batteries have the challenge of chemically instable liquid electrolytes on the oxygen electrode that cause limited battery cycling (7). All of these challenges are associated with the use of lithium metal in liquid electrolyte battery systems. Another major associated challenge is lithium dendrite growth on lithium metal anodes, which causes internal short circuits after lithium dendrites penetrate through the separator and touch the cathode. In addition, solid–electrolyte interphase (SEI) formation during the uneven lithium deposition will continuously consume Li metal and dry up the electrolyte, leading to an increase of cell resistance and decrease of cell Coulombic efficiency (1, 8). Although extensive studies have been performed to address these challenges, Li dendrite and SEI formation are inevitable and mainly caused by the intrinsic problems of the thermodynamically unstable Li with low-molecular weight organic solvents and the poor strength of formed SEI layers (1).
A fundamental strategy to address Li dendrite penetration and SEI formation is to develop a solid-state electrolyte to mechanically suppress the lithium dendrite and intrinsically eliminate SEI formation (9⇓⇓⇓⇓–14). Among the different types of solid-state electrolytes (inorganic oxides/nonoxides and Li salt-contained polymers), solid-state polymer electrolytes have been the most extensively studied (15⇓⇓⇓⇓⇓–21). Polyethylene oxide (PEO)-based composite electrolytes have attracted the most interest (22, 23). In PEO-based composite, powders are incorporated into a host PEO polymer matrix to influence the recrystallization kinetics of the PEO polymer chains to promote local amorphous regions, thereby increasing the Li salt–polymer system’s ionic conductivity (15). The addition of powders will also improve the electrochemical stability and enhance the mechanical strength. As studied in previous work, the fillers can be either non–Li+-conductive nanoparticles, such as Al2O3 (15), SiO2 (24), TiO2 (25), ZrO2 (26), and organic polymer spheres (23), or Li+-conductive nanoparticles, such as Li0.33La0.557TiO3 (22), tetragonal Li7La3Zr2O12 (27), and Li1.3Al0.3Ti1.7(PO4)3 (28). Developing nanostructured fillers is an essential approach to increase the ionic conductivity of polymer composite electrolytes because of the increased surface area of the amorphous region and improved interface between fillers and polymers. Typically, 1D nanowire fillers, based on perovskite-type lithium-ion–conducting Li0.33La0.557TiO3 material, were shown by Cui and coworkers (22) to enhance the ionic conductivity of the polymer composite electrolyte. This enhanced ionic conductivity was because the nanowire fillers provide extended ionic transport pathways in the polymer matrix instead of an isolated distribution of nanoparticle fillers in the polymer electrolyte (22). However, the agglomeration of ceramic fillers may remain, and it will become a challenge for its mixing with polymer to fabricate uniform solid polymer electrolyte on a large scale. To solve this challenge, in situ synthesis of ceramic filler particles with high monodispersity in polymer electrolyte was recently reported (29). By in situ synthesizing nanosized SiO2 particles into PEO–Li salt polymer, the reported solid polymer electrolyte exhibited an ionic conductivity of 4.4 × 10−5 S/cm at 30 °C, which needs additional improvement to achieve a higher ionic conductivity at room temperature. Based on our understanding, therefore, creating a continuous nanosized network with interconnected long-range ion transport and controlling a minimum/nonfiller agglomeration are the main directions to design high ionic-conductive polymer composite electrolytes.
In this work, we have successfully developed a 3D ceramic network based on garnet-type Li6.4La3Zr2Al0.2O12 (LLZO) nanofibers to provide continuous Li+ transfer channels in PEO-based composite electrolytes as all solid ion-conducting membranes for lithium batteries. Here, we select garnet-type lithium-ion–conducting ceramic as the inorganic component because of several desired physical and chemical properties, including (i) high ionic conductivity approaching 10−3 S/cm at room temperature with optimized element substitution, (ii) good chemical stability against lithium metal, and (iii) good chemical stability against air and moisture (11, 30, 31). Fig. 1 shows the schematic structure of the 3D LLZO–polymer composite membrane. The LLZO porous structure consists of randomly distributed and interconnected nanofibers, creating a continuous lithium-ion–conducting network. The Li salt–PEO polymer is then filled into the porous 3D ceramic networks, forming the 3D garnet–polymer composite membrane. Different from conventional methods to prepare polymer electrolytes, the 3D garnet–polymer composite membrane does not need to mechanically mix fillers with polymers; instead, we can directly soak a preformed 3D ceramic structure into Li salt–polymer solutions to get the desired polymer composite electrolyte hybrid structure, thus simplifying fabrication process and avoiding the agglomeration of fillers.
Schematic of the hybrid solid-state composite electrolyte, where ceramic garnet nanofibers function as the reinforcement and lithium-ion–conducting polymer functions as the matrix. The interwelded garnet nanofiber network provides a continuous ion-conducting pathway in the electrolyte membrane.
Fig. 2 schematically shows the procedure to synthesize flexible solid-state garnet LLZO nanofiber-reinforced polymer composite electrolytes. As shown in Fig. 2A, garnet LLZO nanofibers were prepared by electrospinning of polyvinylpyrrolidone (PVP) polymer mixed with relevant garnet LLZO salts followed by the calcination of the as-prepared nanofibers at 800 °C in air for 2 h. On the drum collector of the electrospinning setup, a thin nonwoven fabric was covered to collect the nanofibers.
Fabrication of the flexible solid-state FRPC electrolyte. (A) Schematic setup of electrospinning garnet–PVP nanofibers. (B) Schematic procedure to fabricate the FRPC lithium-ion–conducting membrane. (C) SEM image of the as-spun nanofiber network. (D) Diameter distribution of the as-spun nanofibers. (E) SEM image of the garnet nanofiber network. (F) Diameter distribution of the garnet nanofibers. (G) Photo image to show the flexible and bendable FRPC lithium-ion–conducting membrane.
The schematic fabrication of fiber-reinforced polymer composite (FRPC) lithium-ion–conducting membrane using the 3D porous garnet nanofiber network is shown in Fig. 2B. A PEO polymer mixture with Li salt, such as bis(trifluoromethane)sulfonimide lithium salt (LiTFSI), is prepared. Then, the Li salt–PEO polymer is reinforced by the 3D nanofibers to form a composite electrolyte, which can be called FRPC electrolyte membrane. Compared with filler-containing polymer electrolyte, the FRPC electrolyte membrane maintains the framework of 3D garnet nanofiber networks and is believed to have a better mechanical property because of the continuous nanofiber structure that enhances the integrity of polymer electrolyte.
Morphologies of the as-spun PVP–garnet salt nanofibers and calcinated garnet nanofibers were characterized by SEM as shown in Fig. 2 C and E. Before calcination, the PVP–garnet salt nanofibers have smooth surfaces, and nanofibers have a diameter of 256 nm on average. The corresponding diameter distribution is shown in Fig. 2D. After the calcination at 800 °C in air, PVP polymers were removed, and garnet LLZO nanofibers were obtained. The average diameter of the nanofibers decreased to 138 nm. Their diameter distribution is given in Fig. 2F. We can see that, after annealing, garnet nanofibers were “interwelded” with each other, forming cross-linked 3D garnet nanofiber networks. The large volume of interspace between nanofibers can facilitate Li salt–polymer infiltration to form the composite membrane. The flexibility of the membrane is shown in Fig. 2G. The bendable electrolyte membrane can then be used to construct flexible solid-state lithium batteries. Note that the design of flexible 3D ion-conducting networks mainly depends on ceramic garnet nanofibers, which require a thin and mechanically stable structure for good ionic conductivity and feasible battery fabrication. To achieve a thinner polymer composite electrolyte while maintaining a good mechanical stability, some key parameters need to be considered, which include electrospinning process (collecting time, drum rotating speed, and syringe moving speed), precursor solution preparation (garnet salt concentration, polymer concentration, polymer molecular weight, and solvent selection), and thermal annealing optimization (heating rate, temperature, time, and cooling rate). Therefore, we believe that additional development of 3D ceramic nanofiber networks as well as polymer–salt optimization become important and necessary in the future understanding of mechanical properties and electrochemical performance of the 3D ion-conducting network-based solid-state electrolyte for lithium batteries and beyond.
Fig. 3 shows the morphological characterization of the garnet nanofibers and resulting FRPC electrolyte. As shown in Fig. 3A, garnet nanofibers were bonded together at their intersection points, forming a cross-linked network. These interconnected garnet nanofibers offer a continuous ion-conducting pathway because of the extended long-range lithium transport channels, which should be superior to the isolated particle fillers that are distributed in typical polymer matrixes (22). Fig. 3 B and C shows the transmission electron microscopy (TEM) images of the garnet nanofibers. The garnet nanofiber has a polycrystalline structure consisting of interconnected small crystallites to form the long, continuous nanofiber (Fig. 3B). Fig. S1 shows the magnified TEM image of a garnet nanofiber with an average grain size of 20 nm in diameter. Fig. 3C indicates the highly crystalized structure of the garnet grain.
Morphological characterizations of garnet nanofiber reinforcement and the solid-state FRPC electrolyte. (A) SEM image showing the interwelded garnet nanofibers. (B) TEM image of polycrystalline garnet nanofiber. (C) High-resolution TEM image of an individual garnet nanofiber. (D) SEM image of FRPC electrolyte membrane surface. (E) Cross-sectional SEM image of the membrane. (F) Magnified SEM image of the cross-section morphology. The free space of garnet 3D porous structure was filled with polymer.
Magnified TEM of garnet fiber to show the polyscrystaline structure.
The morphologies of FRPC electrolyte were examined by SEM (Fig. 3 D–F). The FRPC electrolyte exhibited a smooth surface, which came from the PEO–LiTFSI polymer (Fig. 3D). Inside of the FRPC electrolyte, we can see that the 3D porous garnet nanofiber network supported the main structure of the composite and that the PEO–LiTFSI polymer was infiltrated into the porous garnet membrane and filled the interspace between garnet nanofibers. The cross-section image of the FRPC electrolyte showed a thickness of 40–50 μm (Fig. 3E). To increase interphase contact between garnet nanofibers and PEO–LiTFSI polymer, the FRPC electrolyte was thermally treated at 60 °C, which is slightly above the polymer melting temperature (Tm), to enable the melted PEO–LiTFSI polymer to fully infiltrate the 3D porous garnet nanofiber network. As shown in Fig. 3F, after thermal treatment, PEO–LiTFSI polymer was fully embedded with garnet nanofibers. We can see that garnet nanofibers increased to an average diameter of 500 nm because of the PEO–LiTFSI polymer coating. The interconnected pores were filled with polymer to maintain good lithium-ion transfer. The FRPC electrolyte membrane is proposed to have three ion-conducting pathways: the first one is the interwelded ceramic garnet nanofiber network, the second one is the continuous garnet fiber–polymer interface, and the third one is the Li salt-containing polymer matrix. Because of the higher ionic conductivity of garnet-type electrolytes than that of Li salt-containing polymer electrolyte, we believe that the former two ion-conducting pathways are the dominant factors to provide improved ionic conductivity to the electrolyte membrane.
Thermogravimetric analysis (TGA) was used to study the garnet nanofiber formation during the calcination process. The TGA was carried out under airflow with a rapid heating rate of 10 °C/min. Fig. 4A shows the TGA profile of the as-spun nanofibers containing PVP polymer and garnet precursor. The result shows that, above 750 °C, the weight became stable, indicating that stable garnet nanofibers were formed. Fig. 4B compares the TGA profiles of the PEO–LiTFSI and the FRPC electrolyte. Both electrolytes were thermally stable to around 200 °C. In the rapid heating process, polymers began to decompose above 200 °C and showed a significant weight loss at around 400 °C because of the almost complete decomposition of the polymer. The slope at 400 °C was the decomposition of LiTFSI. For the FRPC electrolyte, the weight was stable at 500 °C, and the remaining was the garnet nanofiber membrane caused by the superior stability of garnet material in air. For the polymer electrolyte, the weight was stable at 650 °C, leaving with decomposed LiTFSI salt.
Thermal properties and flammability tests of the solid-state FRPC electrolyte. (A) TGA curve of the as-spun nanofibers. (B) TGA curves of Li salt–PEO polymer and FRPC electrolyte membrane. (C) Flammability test of Li salt–PEO polymer mixed with garnet nanoparticles. (D) Flammability test of FRPC electrolyte membrane.
Thermal stability is an important consideration for using solid-state electrolytes, especially polymer electrolyte. Traditional liquid electrolytes, such as carbonate electrolytes, tend to cause thermal runaway when batteries are under extreme conditions of short circuits, overcharge, and high temperature (19). Because of its relatively high thermal stability, polymer electrolyte becomes a safer choice compared with liquid electrolyte. Because traditional polymer electrolytes are built on their own polymer structure and fillers cannot offer sufficient mechanical support for the electrolyte, the polymer electrolyte inevitably melts and shrinks at high temperature, especially above the polymer thermal decomposition temperature, which may cause direct contact between cathode and anode and is a significant safety concern. The FRPC electrolyte is able to address this concern, because the garnet nanofiber membrane within the polymer electrolyte provides a ceramic barrier to physically block cathode and anode contact, even after loss of the polymer.
Fig. 4 C and D compares the combustion tests of a traditional polymer electrolyte and the FRPC electrolyte developed in this work. The traditional polymer electrolyte was prepared using the same recipe used to prepare the PEO–LTFSI polymer but using garnet nanopowders (vs. the 3D garnet network) as fillers. The mass ratio of polymer and filler was controlled at 4:1. In Fig. 4C, the polymer electrolyte caught fire instantly when it came close to the ignited lighter and was quickly burned off into ashes. This high flammability indicates poor thermal stability of the polymer electrolyte. In comparison, the FRPC electrolyte exhibited an outstanding thermal stability; although the polymer component was gone, the garnet nanofiber membrane still retained its3 structure (Fig. 4D). This low-flammability FRPC electrolyte can provide enhanced safety for all lithium metal and LIBs.
Powder X-ray diffraction (XRD) patterns of LLZO garnet nanofibers that were calcined at 800 °C for 2 h are shown in Fig. 5A. Almost all of the diffraction peaks match very well with those of cubic-phase garnet Li5La3Nb2O12 (Joint Committee on Powder Diffraction Standards card 80-0457). Li5La3M2O5 (M = Nb, Ta) is the first example, to our knowledge, of a fast lithium-ion–conductive processing garnet-like structure, which is the typical structure that has been widely used as a model to study the garnet structure of LLZO material. Here, we use the standard Li5La3Nb2O12 XRD profile to identify the synthesized garnet nanofiber structure. A small amount of La2Zr2O7 was identified, but other impurities were below detection limit. According to the thermogravimetric results, decomposition of precursors to oxide was completed at ∼750 °C. Additional heating at 800 °C resulted in reaction of the oxides and formation of cubic-phase LLZO garnet structure. However, the small amount of La2Zr2O7 phase could also be formed by lithium loss at elevated temperature.
Phase structure of garnet fiber and electrical properties of solid-state FRPC electrolyte. (A) XRD pattern of the garnet nanofibers and the powder diffraction file (PDF) of Li5La2Nb2O12. (B) EIS profiles of the FRPC electrolyte membrane at different temperatures (25 °C, 40 °C, and 90 °C). (C) Arrhenius plot of the FRPC electrolyte membrane at elevated temperatures (from 20 °C to 90 °C and record every 10 °C increase). (D) LSV curve of the FRPC electrolyte membrane to show the electrochemical stability window in the range of 0–6 V. OCV, open-circuit voltage.
The total lithium-ion conductivity of FRPC electrolyte was characterized by electrochemical impedance spectroscopy (EIS). Fig. 5B shows the typical Nyquist plots of FRPC electrolyte sandwiched between stainless steel blocking electrodes in the frequency range from 1 Hz to 1MHz. Each impedance profile shows a real axis intercept at high frequency, a semicircle at intermediate frequency, and an inclined straight tail at low frequency. The intercept of the extended semicircle on the real axis and the semicircle in the high- and intermediate-frequency range represent the bulk relaxation of FRPC electrolyte. The low-frequency tail is caused by the migration of lithium ions and the surface inhomogeneity of the blocking electrodes. Fig. 5C shows the Arrhenius plot of the FRPC electrolyte. Lithium-ion conductivity was calculated based on the thickness of FRPC electrolyte and diameter of stainless electrodes. As reported, lithium-ion conductivity of the cubic-phase LLZO garnet pellet would reach as high as 10−3 S/cm, whereas lithium salt-stuffed PEO is generally on the order of 10−6–10−9 S/cm at room temperature (15, 22). Our FRPC electrolyte combining conductive cubic LLZO garnet and lithium–PEO could exhibit reasonably high ionic conductivity of 2.5 × 10−4 S/cm at room temperature.
A large electrochemical window is another key factor to determine the polymer electrolyte application for high-voltage lithium batteries. Fig. 5D shows the result of the linear sweep voltammetry (LSV) profile of the FRPC electrolyte using lithium metal as the counter and reference electrode and stainless steel as the working electrode. The FRPC electrolyte exhibits a stable voltage window up to 6.0 V vs. Li/Li+, indicating that this ion-conducting membrane can satisfy the requirement of most high-voltage lithium batteries.
The mechanical stability of the FRPC electrolyte membrane against Li dendrites was evaluated by using a symmetric Li | FRPC electrolyte | Li cell. During charge and discharge processes at a constant current, lithium ions are plating/stripping the lithium metal electrode to mimic the operation of charging and discharging lithium metal batteries. Fig. 6A represents the schematic of the symmetric cell setup. The FRPC electrolyte membrane was sandwiched between two lithium metal foils and sealed in coin cell. Fig. 6B shows the time-dependent voltage profile of the cell with FRPC electrolyte membrane cycled over 230 h at a constant current density of 0.2 mA/cm2 and a temperature of 15 °C. The symmetric cell was periodically charged and discharged for 0.5 h. The positive voltage is the Li stripping, and the negative voltage value refers to the Li plating process. In the first 70 h, the cell’s voltage slightly increased from 0.3 to 0.4 V and then, stabilized at 0.4 V.
Electrochemical performance of the FRPC electrolyte membrane measured in the symmetric Li | FRPC electrolyte | Li cell. (A) Schematic of the symmetric cell for the lithium plating/stripping experiment. (B) Voltage profile of the lithium plating/striping cycling with a current density of 0.2 mA/cm2 at 15 °C. (C) Voltage profile of the continued lithium plating/stripping cycling with a current density of 0.2 mA/cm2 at 25 °C. (D) The impedance spectra of the symmetric cell measured at different cycle times (300, 500, and 700 h). (E) Magnified EIS spectra in the high-frequency region. (F) Voltage profile of the continued lithium plating/stripping cycling with a current density of 0.5 mA/cm2 at 25 °C.
When the testing temperature increased to 25 °C, the voltage dropped to 0.3 V because of the improved ionic conductivity at elevated temperature as shown in Fig. 6C. In the following long-time cycles, the voltage kept decreasing to 0.2 V with increasing cycle time to 700 h (Fig. S2). The fluctuation of voltage was caused by the surrounding environmental temperature change. Two voltage profiles of the symmetric cell at two different stripping/plating process times were compared as shown in Fig. S3. The voltage hysteresis apparently decreased with increase of cycle time. This decrease in voltage is quite different from the liquid electrolyte system, in which the voltage normally increases with the increase of time and is mainly ascribed to the nonuniform Li deposition and severe electrolyte decomposition that cause impedance increase (32). Similar voltage decrease has been observed in recent polymer electrolyte studies, but the reason why voltage keeps decreasing with the increasing cycle time has not yet been explained (23, 33). Based on our understanding, the decrease in voltage might be because of the improved interface between the electrolyte membrane and lithium metal during the repeated Li electrodeposition, which is confirmed by the EIS spectra of the symmetric cell measured at 300, 500, and 700 h (Fig. 6D). The depressed semicircles at lower frequency indicate decreased interfacial impedance between electrolyte membrane and lithium metal during cycling. At high frequency (Fig. 6E), the semicircle also decreased with the increased cycle time, indicating the decreased bulk impedance of the electrolyte membrane. When the current density increased to 0.5 mA/cm2, the voltage increased to 0.3 V, and the cell also exhibited slight decrease in voltage with increasing time to 1,000 h (Fig. 6F), showing good cycling stability with long cycle life.
Voltage profile of the continued lithium plating/stripping cycling to 700 h at a current density of 0.2 mA/cm2 at 25 °C.
Voltage profiles of the symmetric cells at two different stripping/plating process times. (A) The voltage plateau of the cell is stabilized at 0.3 V after 300 h of cycling. (B) The voltage plateau of the cell is decreased to 0.2 V after around 700 h of cycling.
In conclusion, all solid ion-conducting membranes of 3D garnet–polymer composite were synthesized for lithium batteries. 3D garnet nanofiber networks were prepared by electrospinning and high-temperature annealing. The garnet nanofibers constructed an interwelded 3D structure that provides long-range lithium-ion transfer pathways and further provides structural reinforcement to enhance the polymer matrix. This flexible solid-state electrolyte composite membrane exhibited an ionic conductivity of 2.5 × 10−4 S/cm at room temperature. The membrane can effectively block dendrites in a symmetric Li | electrolyte | Li cell during repeated lithium stripping/plating at room temperature, with a current density of 0.2 mA/cm2 around 500 h and a current density of 0.5 mA/cm2 over 300 h. The decrease of voltage with increasing cycle time is observed for the symmetric cell, which is possibly because of the improved interfaces during repeated lithium electrodeposition. Our work is the first report, to our knowledge, of the development of 3D lithium-ion–conducting ceramic materials in solid-state electrolytes, which can be potentially applied to flexible LIBs and other electrochemical energy storage systems, such as lithium–sulfur batteries.
An LLZO composition was selected for study. This Al concentration was chosen, because it was in the range where highly ion-conductive cubic phase would be formed (34). Stoichiometric amounts of LiNO3 (99%; Alfa Aesar), La(NO3)3·6H2O (99.9%; Alfa Aesar), ZrO(NO3)2·6H2O (99.9%; Alfa Aesar), and Al(NO3)3·9H2O (99.9%; Alfa Aesar) were dissolved in dimethylformamide with 15 vol % acetic acid. PVP (molecular weight, ∼1,300,000) was dissolved in dimethylformamide with a concentration of 10 wt%. The salt solution and PVP solution were mixed at a 1:1 ratio in volume to prepare the precursor solution for electrospinning. During electrospinning, a high voltage of 20 kV was applied between the needle and the drum collector. The drum collector was connected to the ground. The distance between the needle and the drum collector was 10 cm. The drum collector was covered with a thin nonwoven fabric to collect the as-spun nanofibers. The rotation speed was controlled, and the humidity of the electrospinning chamber was no more than 20%. After electrospinning, the as-spun nanofibers were peeled off the nonwoven fabric and kept in a vacuum oven for 24 h to dry completely. The free-standing nanofiber mat was then calcinated at 800 °C for 2 h in air at a heating rate of 10 °C/min. After thermal annealing, a garnet nanofiber mat was obtained.
The Li salt–polymer composite solution was prepared by dissolving LiTFSI (Sigma) and PEO (molecular weight, ∼600,000) in acetone nitrile. Then, the solution was dropped onto the garnet nanofiber membrane that was fixed by tweezers. The wetted garnet nanofiber membrane was first dried in a flow of dry air environment for 1 h and then, dried in a vacuum oven for an additional 2 h to further remove the solvent. This wetting and drying process was repeated several times until the garnet nanofiber membrane was fully embedded in the PEO polymer matrix. The solid-state membrane was kept in the vacuum oven before assembling batteries in a glovebox.
The morphology of the samples was examined by a field emission scanning electron microscope (JEOL 2100F). The crystallographic and chemical structures were studied by wide-angle X-ray diffraction (Bruker C2 Discover X-Ray Powder Diffraction System). A transmission electron microscope (JEOL 2100F) was used to examine the crystal structure of garnet nanofibers. TGA (STAR System) was conducted to determine the thermal properties of samples.
The symmetric Li | solid-state electrolyte | Li full cell was assembled in a glovebox. The electrolyte membrane was sandwiched between two surface polished lithium metal foils and sealed in 2,032 coin cells. The ionic conductivity was tested by sandwiching the electrolyte membrane in the between of two stainless steel plates for the EIS measurement at elevated temperatures. The EIS was performed in a frequency range of 1 MHz to 100 MHz. The electrochemical stability of the electrolyte membrane was measured using LSV mode at a sweep rate of 1 mV/s in the range of 0–6 V. In the LSV test, the electrolyte membrane was sandwiched between stainless steel as a working electrode and lithium metal foil as a counter and reference electrode.
This work was supported by EERE, which is funded by the US Department of Energy. We acknowledge the support of the Maryland NanoCenter and its FabLab and NispLab.
↵1K.(K.)F. and Y.G. contributed equally to this work.
Author contributions: K.(K.)F., Y.G., E.D.W., and L.H. designed research; K.(K.)F., Y.G., J.D., A.G., and Y.W. performed research; K.(K.)F., Y.G., X.H., Y.Y., C.W., Y.C., C.Y., Y.L., E.D.W., and L.H. analyzed data; and K.(K.)F. and Y.G. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. Y.C. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1600422113/-/DCSupplemental.
