New Class of LAGP-Based Solid Polymer ... - ACS Publications

Nov 13, 2017 - ABSTRACT: Inorganic solid electrolytes (SEs) possess substantial safety and electrochemical stability, which make them as key component...
0 downloads 0 Views 6MB Size
Download PDF
Research Article www.acsami.org

New Class of LAGP-Based Solid Polymer Composite Electrolyte for Efficient and Safe Solid-State Lithium Batteries Qingpeng Guo,* Yu Han,* Hui Wang, Shizhao Xiong, Yujie Li, Shuangke Liu, and Kai Xie College of Aerospace Science and Engineering, National University of Defence Technology, Changsha, Hunan 410073, China S Supporting Information *

ABSTRACT: Inorganic solid electrolytes (SEs) possess substantial safety and electrochemical stability, which make them as key components of safe rechargeable solid-state Li batteries with high energy density. However, complicated integrally molding process and poor wettability between SEs and active materials are the most challenging barriers for the application of SEs. In this regard, we explore composite SEs of the active ceramic Li1+xAlxGe2−x(PO4)3 (LAGP) as the main medium for ion conduction and the polymer P(VDF-HFP) as a matrix. Meanwhile, for the first time, we choice high chemical, thermal, and electrochemical stability of ionic liquid swelled in polymer, which significantly ameliorate the interface in the cell. In addition, a reduced crystallinity degree of the polymer in the electrolyte can also be achieved. All of these lead to good ionic conductivity of the composite electrolyte (LPELCE), at the same time, good compatibility with the lithium electrode. Especially, high mechanical strength and stable solid electrolyte interphase which suppressed the growth of lithium dendrites and high thermal safety stability can also be observed. For further illustration, the solid-state lithium battery of LiFePO4/LPELCE/Li shows relatively satisfactory performance, indicating the promising potentials of using this type of electrolyte to develop high safety and high energy density solid-state lithium batteries. KEYWORDS: solid composite electrolyte, flexible membrane, ionic liquid, nonflammable, compatibility, ionic conductor been researched. Active ceramic fillers themselves have high ionic conductivity, and the ceramic particles as cross-linking centers can successfully suppress the degree of polymer crystallinity. Moreover, because the transition phase is formed between ceramic particles and the polymer, better ion transmission can be achieved and the large specific surface area of ceramic fillers can enhance salt dissociation, which also enhances the overall performance of the composite SE.7,8 However, large amounts of crystallized polymer regions still exist in the composite SE, together with agglomeration of ceramic particles.6,13−15 Thus, it is generally believed that further improving the ionic conductivity is still a challenge.16−19 To address these concerns, we put forward a strategy to prepare “firmness and flexibility” composite laminate electrolytes via combining ceramic particles Li1+xAlxGe2−x(PO4)3 (LAGP) with polymer P(VDF-HFP) and adding ionic liquid (IL) (room-temperature molten salt with high safety) into the composite electrolyte as plasticizers (LAGP−P(VDF-HFP)− EMITFSI−LiTFSI composite electrolyte, referred to as LPELCE), which can provide high ionic conductivity and ameliorate the interface between the electrode and the SE.

1. INTRODUCTION The development of electronic products and electric vehicles has greatly promoted the development of the electrochemical storage as power storage systems.1−3 Currently, lithium-ion batteries (LIBs) with liquid organic electrolytes (LOEs) have dominated the portable electronics market; however, the potential security issues of LOEs restrict their widespread applications in the new-type high-power and high-capacity batteries.4−7 Exploring solid-state electrolytes can address the issues of battery safety. Despite the fact that inorganic solid electrolytes (SEs) always have high conductivity at room temperature, good thermal stability, and wide electrochemical stability, these types of electrolytes still have some problems such as complicated integrally molding process and poor wettability between the electrode material and the electrolyte.5,8−10 Solid polymer electrolytes (SPEs) also have an excellent film-forming property with good wettability and low flammability; however, the practical use of SPEs is severely impeded by their relatively low ionic conductivity at room temperature (10−7 to 10−8 S cm−1) for the inherently high polymer crystallinity degree.6 Therefore, on the basis of the characteristics and defects of the two types of SEs, several alternative strategies,7−12 such as exploring composite SEs of the active ceramic as the main medium for ion conduction and polymers as film formers, have © 2017 American Chemical Society

Received: August 13, 2017 Accepted: November 13, 2017 Published: November 13, 2017 41837

DOI: 10.1021/acsami.7b12092 ACS Appl. Mater. Interfaces 2017, 9, 41837−41844

Research Article

ACS Applied Materials & Interfaces

8:1:1 in the N-methylpyrrolidone (NMP) solvent. The composite cathode was dried in a vacuum oven for 12 h. The loading density of the LiFePO4 cathode was maintained at 2.2 mg cm−2. Li/LPEL/ LiFePO4 coin cells were assembled as solid-state batteries; the charge− discharge testing was conducted in the range of 2.7−3.85 V at room temperature, using the LAND CT2001A instrument.

Through a systematic and comprehensive experiment, we observed that the prepared composite electrolyte combines the advantages of both the inorganic SE (with mechanical stability, electrochemical stability, and high thermal stability) and the polymer electrolyte (with good film-forming property). The great advantage is that LPELCE can increase the compatibility of the lithium anode, which suppresses the growth of lithium dendrites. Thus, these advantages can establish the theoretical and technological foundation for the development of safe, reliable, and good overall performing solid-state lithium batteries.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Physical Characterization of the Electrolyte. Figure 1 shows a schematic diagram of the

2. EXPERIMENTAL SECTION 2.1. Preparation of Inorganic Electrolytes LAGP Powder. Four materials such as Li2CO3, GeO2, Al2O3, and NH4H2PO4 were weighed in stoichiometry and ground in an agate jar evenly, and then they were heated to 700 °C and placed for 1 h in a platinum crucible. Subsequently, the mixture was heated up to 1500 °C at a heating rate of 5 °C min−1 and melted for 2 h. The homogenous lava was quenched in water to obtain glass blocks. The glass block was placed in a tube furnace and heated at 850 °C for 12 h and then cooled to room temperature naturally, and LAGP glass−ceramic blocks were obtained. Finally, the glass−ceramic blocks were subjected to milling for 8 h in a zirconia jar to obtain a fine powder with screening particle size between 850 and 950 nm (see Figure S1), which was dried under vacuum at 120 °C for 24 h prior to use. 2.2. LPELCE Synthesis. An appropriate amount of P(VDF-HFP) was dissolved in 1-methyl-2-pyrrolidone (NMP) and magnetic-stirred for 30 min at room temperature to obtain 6.7 wt % polymer clear solution. Then, appropriate ratios of LAGP powder, lithium salt, and IL (the values of mPVDF‑HFP/mLiTFSI/mEMITFSI/mLAGP changed from 5:5:7:5 to 5:5:7:9) were added to the solution in an argon-filled glovebox. The mixed solution was continuously stirred for 3 h and left for 20 min to obtain a homogeneous casting solution. The mixed solution was cast onto a Teflon mold to obtain composite electrolyte films. At last, the films were placed in a vacuum oven at 100 °C for 24 h to completely remove the solvent. 2.3. Characterization. The morphology, microstructures, and phase were characterized by Hitachi S-4800 scanning electron microscopy (SEM) and X-ray power diffraction (Siemens D-500, 2θ = 10°−70°). Thermal properties were characterized by 209 TG F1 at a heating rate of 10 °C min−1 under an N2 atmosphere. The thermal safety performance values of the limiting oxygen index (LOI) of electrolytes were measured by an Atlas Limiting Oxygen Index Chamber. Thus, the lowest percentage of O2 in the mixed gas which allows the test samples to burn out reflects the value of the LOI. 2.4. Electrochemical Measurements. The solid composite electrolyte was sandwiched between two stainless steel (SS) electrodes in a blocking-type cell; then, the ionic conductivities of LPELCEs were measured by electrochemical impedance spectroscopy at various temperatures (25−100 °C), with an ac amplitude of 10 mV from 0.01 to 105 Hz. The ionic conductivity (σ) was calculated by the formula σ = d/(Rb·S). The electrochemical stability of LPELCE was measured by linear sweep voltammetry (LSV) with the cell structure of Li/LPEL/SS at the scanning rate of 0.1 mV S−1 from 0 to 6 V. The cyclic voltammetry (CV) was performed at a sweep rate of 0.1 mV S−1 between −3 and 3 V using a symmetric Li/Li cell. The symmetric Li/Li analog battery with the electrolyte was used to evaluate the stability between the electrolyte and the Li electrode. A current density of 0.2 mA cm−2 was applied to promote lithium plating/stripping on the Li electrode at 25 °C. The cycled battery was disassembled in an Ar-filled glovebox, and the removed lithium electrode was washed with the solvent of dimethyl carbonate, dried in vacuum for 12 h, and used as the samples for the SEM or X-ray photoelectron spectroscopy (XPS) test. The LiFePO4 electrode was prepared by the mixed slurries coated on Al foils; the slurries contain LiFePO4 powders, carbon black, and P(VDF-HFP), which are the ingredients of LPEL at a weight ratio of

Figure 1. Schematic illustration for the synthesis of LPELCE and photograph of the membrane.

reaction steps involved in the synthesis of LPELCE. LAGP nanoparticles were prepared by the method of solid-phase reaction and then filtered to obtain the LAGP powder (for details, see Experimental Section). X-ray diffraction (XRD) patterns of the LAGP powder (Figure 2a) show that the diffraction peak position is consistent with the NASICON LiGe2(PO4)3 (JCPDS 80-1924) structure. Meanwhile, a number of glass−ceramic continuous phases were examined by SEM, as shown in Figure 2b. The grains are coated and connected with the adhesive phase. There are no obvious grain boundaries, indicating the successful formation of the NASICON type of glass ceramic LAGP. Figure 2c−f indicates the surface topography of LPELCE with various contents of LAGP nanoparticles (L-x %-PELCE). SEM micrographs of the composite electrolyte with 50 wt % LAGP in Figure 2c (surface) and Figure 2d (cross section) clearly show that nanoparticles are uniformly embedded inside the gel state of P(VDF-HFP)−EMITFSI−LiTFSI. Meanwhile, ceramic particles, as a cross-linking center, were wrapped by the amorphous tissue of the gel polymer. A better transitional phase can be obtained between ceramic particles and polymer (Figure 2e). In addition, when the content of LAGP is higher than 60 to 80 wt %, the ceramic particles agglomerated easily and gaps appeared between the particles, resulting in a poor film-forming property (Figure 2f). XRD patterns were taken at room temperature for P(VDF-HFP), P(VDF-HFP)−LiTFSI, and the composite electrolytes with different contents of LAGP (Figure 2g). The bottom curve, which is the spectrum of pure P(VDFHFP), contains two strong characteristic peaks, showing that this polymer has high degree of crystallinity. It can also be seen that the diffraction peaks of P(VDF-HFP) can be weakened by the complexation between LiTFSI and P(VDF-HFP) but not obvious. A dramatic decrease on the peak intensity can be observed with the increase of LAGP. When the content of LAGP is more than 50% and with further increase, notably, the polymer shows almost indiscernible characteristic peaks; characteristic peaks of the solid composite electrolyte are consistent with the pure LAGP characteristic spectrum peak. 41838

DOI: 10.1021/acsami.7b12092 ACS Appl. Mater. Interfaces 2017, 9, 41837−41844

Research Article

ACS Applied Materials & Interfaces

Figure 2. Phase structure and morphological characterizations of LAGP and LPELCEs: (a) XRD patterns of LAGP. (b) SEM micrographs of LAGP. (c) Surface of L-50%-PELCE. (d) Cross-sectional morphology of L-50%-PELCE. (e) TEM image of L-50%-PELCE. (f) SEM image of LPELCE with 80 wt % LAGP content. (g) XRD patterns of the polymer matrix P(VDF-HFP) and LPELCEs. (h) Strain−stress curve of L-50%-PELCE.

Figure 3. Thermal properties and flammability tests of LPELCE: (a) with different kinds of electrolytes with four basic electrolyte materials and the state of the electrolyte from liquid to all solid, the security becomes better. (b) TGA curves of electrolytes with different ingredients. (c) Flammability tests of commercial Celgard membrane, L-50%-PLCE (without IL in the composite electrolyte), and L-50%-PELCE. (d) LOI of different electrolytes.

41839

DOI: 10.1021/acsami.7b12092 ACS Appl. Mater. Interfaces 2017, 9, 41837−41844

Research Article

ACS Applied Materials & Interfaces

Figure 4. Electrical properties of LPEL composite electrolytes: (a) Arrhenius plots of the composite electrolytes with various LAGP concentrations, together with the schematic illustration for Li+ migration in the electrolyte. (b) ESW of LPELCE.

Celgard membrane is ignited easily once in contact with the flame and quickly burned into ashes. Low flammability is observed for the L-50%-PLCE membrane, and there are many pores left in the membrane after the polymer component is burned out. In comparison, no flammability is observed for L50%-PELCE even upon prolonged ignition time with an acetylene flame where the outer flame temperature is up to 1300 °C, which proved that LPELCE can enhance the safety of solid-state lithium batteries. In addition, to further describe the flame-retardant degree of the solid polymer composite electrolyte, the LOI test was administered, which is a useful method to quantitatively evaluate the minimum concentration of oxygen to judge whether a substance is flammable.25 Figure 3d shows the LOI values of different electrolytes; generally, the value higher than 27 indicates that the material is flame-retardant.26 Thus, we can say that L-50%-PLCE exhibits flame-retardancy, and with the addition of ILs, the LOI value of L-50%-PELCE can reach 56, suggesting that ILs can improve the nonflammablility of the composite electrolyte. 3.3. Electrochemical Performance Analysis. 3.3.1. Ionic Conductivity. Ionic conductivity is a key parameter to evaluate the performance of the electrolyte.27−30 Arrhenius plots of ionic conductivity with various concentrations of IL and LAGP as a function of temperature are shown in Figures S2 and 4a. As can be seen, the ionic conductivity of LPELCE indicates that the increase of temperature leads to the increase of ionic conductivity. In addition, the composite electrolyte containing 50−60 wt % LAGP displays the highest ionic conductivity of 0.92 × 10−3 to 0.96 × 10−3 S cm−1 at room temperature. Furthermore, the conductivity has a sharp turn at 70 wt % LAGP and then drops progressively as the LAGP content increases, and as expected, when the content of LAGP is 80%, the ionic conductivity of the electrolyte is close to the pure LAGP SE of 1.9 × 10−4 S cm−1. Thus, we can deduce that the dispersion of IL in the composite electrolyte results in multidirectional swelling of the polymer molecular chains, forming more amorphous regions. The swollen polymer

The polymer crystallization regions are squeezed by the LAGP amorphous area, resulting in neat molecular chains in the crystal region to become more disordered, which implies that LAGP can reduce the degree of crystallinity of P(VDF-HFP). On the other hand, IL swells the molecular chains of P(VDFHFP), making it almost amorphous. Meanwhile, a SPE must have sufficient mechanical strength for practical use; therefore, the mechanical strength is also one of the important indicators to evaluate the merit of the electrolyte membrane. Figure 2h is a demonstration of the mechanical properties of L-50%-PELCE. It can be seen that the electrolyte showed certain mechanical properties such as a tensile strength of 4.90 MPa and Young’s modulus of 13.96 MPa. Thus, we can say that the special mechanical properties and tenacity of L-50%-PELCE can enable easy formation of films during the production process and effectively mitigate dendrite penetration when used in lithium batteries.6,20−22 3.2. Thermal Safety Analysis. Figure 3a lists four main components of polymeric-based electrolytes. Among these, only ILs and inorganic solids are nonflammable, which reflects good thermal stability.3,23,24 Here, we select inorganic solid as a main component of the composite electrolyte and add a small amount of IL as the modifier, its main purpose is to make an electrolyte with good thermal stability and electrochemical performance. Thus, thermogravimetric analysis (TGA) and flammability tests are implemented to describe the thermal safety stability of LPELCE. The TGA curves of L-50%-PLCE (without IL in the composite electrolyte) and L-50%-PELCE show two different processes taking place within the sample during heating (Figure 3b). Before 300 °C, a minimal loss of mass can be observed, and it is noteworthy to mention that the weight loss of the electrolyte is nearly from the lithium salt and the polymer; a second degradation process takes place above 300 °C, which is related to the decomposition of the IL. These results confirm that LAGP and IL showed the intrinsic thermal safety and improved the thermal stability of the electrolyte. Flammability tests for Celgard membrane, L-50%-PLCE, and L50%-PELCE are depicted in Figure 3c. The commercial 41840

DOI: 10.1021/acsami.7b12092 ACS Appl. Mater. Interfaces 2017, 9, 41837−41844

Research Article

ACS Applied Materials & Interfaces

Figure 5. Compatibility of LPELCE and lithium anode: (a) cyclic voltammograms of L-50%-PELCE in Li/Li symmetrical cells at 25 °C at a scan rate of 0.1 mV s−1. (b) Time evolution of the impedance response of the Li/L-50%-PELCE/Li cell. (c) Voltage profile of the lithium plating/striping cycling with a current density of 0.2 mA cm−2. (d,e) SEM images of the Li metal anode in the Li/L-50%-PELCE/Li cell after 400 h of cycling with different scale bars. (f) XPS patterns of the cycled lithium anode with L-50%-PELCE.

molecular chains provide an effective channel for the Li+ migration; eventually, LPELCE exhibits a relatively high ionic conductivity compared to other SPE systems (see Table S1). Therefore, introducing appropriate amounts of IL in the allsolid polymer composite electrolyte is an effective way to improve the conductivity; meanwhile, besides the superior conductivity, in our case, LPELCE generally has better mechanical strength and electrochemical stability. 3.3.2. Electrochemical Stability. The electrochemical stability window (ESW) of the electrolyte is critical for the development of high energy density lithium batteries. Currently, some high potential cathode materials have been developed; however, electrolytes that are stable enough to endure cathode decomposition at high potential versus Li+/Li remain rare.8,12−15 With inorganic solids and IL as the main ingredient of the electrolyte, we are able to develop a composite electrolyte, which helps to improve the electrochemical stability. Figure 4b shows linear sweep voltammograms of LPELCE with various concentrations of LAGP. When the potential is less than 4.8 V, the current is found to be more stable with respect to LPELCE, whereas the current began to increase when the potential exceeds 4.8 V, which is related to the decomposition of LPELCE. Thus, LPELCE displays an ESW nearly exceeding 4.8 V (vs Li+/Li). We can conclude that

LPELCE can be used in combination with high-voltage cathodes for high-energy lithium batteries. Figure 5a shows the CV of the lithium electrode with L-50%-PELCE. A typical couple of reversible redox peaks appeared at −0.5 and 0.5 V (vs Li+/Li), showing the typical plating and dissolution of lithium, respectively. These peaks are very sharp and fairly symmetrical to each other, and the consistency of CV curves is very good after cycling several times, indicating an excellent reversibility of Li+ pass through L-50%-PELCE with the fast kinetics electrochemical process, and the plating/stripping process on the electrode is reversible. Interface compatibility between L-50%-PELCE and lithium electrode can directly affect the specific capacity and cycling stability of the solid-state battery. Time evolution of the interfacial resistance in the symmetrically nonblocking Li/L50%-PELCE/Li cell was tested in Figure 5b. The bulk electrolyte resistance (Rb) and the electrode/electrolyte interfacial resistance (Rf) can be obtained from the spectrum at the whole frequency range. During storage, the Rb of L-50%PELCE is almost constant, indicating that the ionic conductivity of the electrolyte does not change with extended storage time variation. However, there is a significant variation of Rf from the beginning to 24 h, and then Rf slightly increases and tends to remain steady. Hence, the result demonstrated 41841

DOI: 10.1021/acsami.7b12092 ACS Appl. Mater. Interfaces 2017, 9, 41837−41844

Research Article

ACS Applied Materials & Interfaces

Figure 6. Performance of LPELCE in the solid-state battery of LiFePO4/Li: (a) schematic diagram representing the solid-state battery of LiFePO4/ Li. (b) Cycling performance of LiFePO4/L-50%-PELCE/Li, insets indicate charge and discharge profiles. (c) C-rate discharge performance of LiFePO4/Li cells. (d) Photographs of button battery after cycles.

1s spectra show that the chemical composition is consistent with that of −SO2CF3 (170.36 eV) and S−O (168.8 eV); it reveals the evidence of the reaction of these groups from TFSI− breakdown. Thus, we can deduce that all of these ingredients may result in the formation of the SEI layer and these ingredients are formed by the decomposition between theIL and Li anode. 3.4. Effect of L-50%-PELCE in LFP/Li Cells. Although the related properties of L-50%-PELCE imply a promising applicability in the cell, the actual effect in LIBs should still be proven. Therefore, the performance of L-50%-PELCE in the solid-state battery of LiFePO4/Li was investigated in detail. Particularly, we mixed a small amount of the composition of the electrolyte as a bridge to connect the active material of the cathode and the electrolyte (schematic illustration of the cell design in Figure 6a); the aim of this design is to effectively ameliorate the interface between the electrode and the electrolyte, reducing barriers of ion conductivity in the cell. Setting the interval of voltage between 2.7 and 3.85 V, the first specific discharge capacity is as high as 157.8 mA h g−1 at a current rate of 0.05 C and maintains 141.3 mA h g−1 after the 50th cycle with a capacity retention of 89.5% (Figure 6b). More importantly, apart from the initial irreversible capacity, the Coulombic efficiency is nearly 100% after the first cycle. In addition, the C-rate discharge performance of LiFePO4/Li cells assembled with L-50%-PELCE is evaluated, where the cell is charged at a current rate of 0.05 C and discharged with different current densities ranging from 0.05 to 2 C. As shown in Figure 6c, they can achieve capacities of 156.18, 154.06, 153.29, 144.56, 138.48, and 53.62 mA h g−1 at current rates of 0.05, 0.1, 0.2, 0.5, 1, and 2 C, respectively. When the current rate returns to 0.05 C, there exhibits a reversible capacity of 152.4 mA h g−1, which still show a certain rate capability. Finally, a proper

that L-50%-PELCE can form a dense and stable solid electrolyte interphase (SEI) layer on the Li electrode surface in a short time. This SEI layer inhibits the chemical reactions that continue to occur on the surface of the active Li electrode and thus helps to improve related properties of the cell. The other stability analysis of lithium electrodeposition was evaluated through the method of “strip plate test” in symmetric Li/Li cells.31 Figure 5c shows the time-dependent voltage profile of the cell with L-50%-PELCE cycled at a current density of 0.2 mA cm−2; we can see that the voltage tends to be stable with increasing cycle time. The voltage changes in the initial mainly ascribed to the nonuniform lithium deposition and the interfacial film formed by the reaction of the electrolyte with the lithium metal. The cells show stable voltage profiles during subsequent cycles, indicating that the SEI layer can remain stable during cycling. In addition, the surface morphology of lithium metal after 400 h of cycling was analyzed by SEM. As can be seen in Figure 5d,e, the surface has become less and less smooth after cycling several times; however, there are no lithium dendrites or mossy lithium distributions on the surface of the lithium metal, only a small amount of SE particles can be discovered, indicating that the composite electrolyte can effectively inhibit the formation of lithium dendrites. To explore whether the proposed SEI is influenced by the composition of L-50%-PELCE, XPS is used to study the chemical composition and element distribution of the SEI layer symmetry, which is attached on the cycled Li anode surface after cycling. Figure 5f shows F 1s and S 2p XPS spectra; F and S are the main elements to promote the formation of the SEI layer, which have been demonstrated in many previous studies.32,33 F 1s spectrum reveals that the surface layer composition contains components of S−F, and LiF are dominated by the decomposition of TFSI−. S 2p and O 41842

DOI: 10.1021/acsami.7b12092 ACS Appl. Mater. Interfaces 2017, 9, 41837−41844

Research Article

ACS Applied Materials & Interfaces

(2) Zeng, X.-X.; Yin, Y.-X.; Li, N.-W.; Du, W.-C.; Guo, Y.-G.; Wan, L.-J. Reshaping Lithium Plating/Stripping Behavior via Bifunctional Polymer Electrolyte for Room-Temperature Solid Li Metal Batteries. J. Am. Chem. Soc. 2016, 138, 15825. (3) Wu, F.; Chen, N.; Chen, R.; Zhu, Q.; Tan, G.; Li, L. SelfRegulative Nanogelator Solid Electrolyte: A New Option to Improve the Safety of Lithium Battery. Adv. Sci. 2016, 3, 1500306. (4) Huang, Y.; Zhong, M.; Huang, Y.; Zhu, M.; Pei, Z.; Wang, Z.; Xue, Q.; Xie, X.; Zhi, C. A Self-Healable and Highly Stretchable Supercapacitor based on a Dual Crosslinked Polyelectrolyte. Nat. Commun. 2015, 6, 10310. (5) Lee, K. S.; Jun, Y.; Park, J. H. Controlled Dissolution of Polystyrene Nanobeads: Transition from Liquid Electrolyte to Gel Electrolyte. Nano Lett. 2012, 12, 2233−2237. (6) Duan, H.; Yin, Y.-X.; Zeng, X.-X.; Li, J.-Y.; Shi, J.-L.; Shi, Y.; Wen, R.; Guo, Y.-G.; Wan, L.-J. In-Situ Plasticized Polymer Electrolyte with Double-Network for Flexible Solid-State Lithium-Metal Batteries. Energy Storage Mater. 2018, 10, 85−91. (7) Liu, W.; Liu, N.; Sun, J.; Hsu, P.-C.; Li, Y.; Lee, H.-W.; Cui, Y. Ionic Conductivity Enhancement of Polymer Electrolytes with Ceramic Nanowire Fillers. Nano Lett. 2015, 15, 2740−2745. (8) Lin, D.; Liu, W.; Liu, Y.; Lee, H. R.; Hsu, P.-C.; Liu, K.; Cui, Y. High Ionic Conductivity of Composite Solid Polymer Electrolyte Via in Situ Synthesis of Monodispersed SiO2 Nanospheres in Poly(ethylene oxide). Nano Lett. 2016, 16, 459−465. (9) Choudhury, S.; Mangal, R.; Agrawal, A.; Archer, L. A. A Highly Reversible Room-Temperature Lithium Metal Battery based on Crosslinked Hairy Nanoparticles. Nat. Commun. 2015, 6, 10101. (10) Fu, K.; Gong, Y.; Dai, J.; Gong, A.; Han, X.; Yao, Y.; Wang, C.; Wang, Y.; Chen, Y.; Yan, C.; Li, Y.; Wachsman, E. D.; Hu, L. Flexible, Solid-State, Ion-Conducting Membrane with 3D Garnet Nanofiber Networks for Lithium Batteries. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 7094−7099. (11) Oh, D. Y.; Nam, Y. J.; Park, K. H.; Jung, S. H.; Cho, S.-J.; Kim, Y. K.; Lee, Y.-G.; Lee, S.-Y.; Jung, Y. S. Excellent Compatibility of Solvate Ionic Liquids with Sulfide Solid Electrolytes: Toward Favorable Ionic Contacts in Bulk-Type All-Solid-State Lithium-Ion Batteries. Adv. Energy Mater. 2015, 5, 1500865. (12) Suo, L.; Hu, Y.-S.; Li, H.; Armand, M.; Chen, L. A New Class of Solvent-in-Salt Electrolyte for High-Energy Rechargeable Metallic Lithium Batteries. Nat. Commun. 2013, 4, 1481. (13) Peng, X.; Liu, H.; Yin, Q.; Wu, J.; Chen, P.; Zhang, G.; Liu, G.; Wu, C.; Xie, Y. A Zwitterionic Gel Electrolyte for Efficient Solid-State Supercapacitors. Nat. Commun. 2016, 7, 11782. (14) Zeng, Z.; Liang, W.-I.; Liao, H.-G.; Xin, H. L.; Chu, Y.-H.; Zheng, H. Visualization of Electrode−Electrolyte Interfaces in LiPF6/ EC/DEC Electrolyte for Lithium Ion Batteries via in Situ TEM. Nano Lett. 2014, 14, 1745−1750. (15) Appetecchi, G. B.; Kim, G. T.; Montanino, M.; Alessandrini, F.; Passerini, S. Room Temperature Lithium Polymer Batteries based on Ionic Liquids. J. Power Sources 2011, 196, 6703−6709. (16) Wilken, S.; Xiong, S.; Scheers, J.; Jacobsson, P.; Johansson, P. Ionic Liquids in Lithium Battery Electrolytes: Composition Versus Safety and Physical Properties. J. Power Sources 2015, 275, 935−942. (17) Kim, J.-K.; Ahn, J.-H.; Jacobsson, P. Influence of Temperature on Ionic Liquid-based Gel Polymer Electrolyte Prepared by Electrospun Fibrous Membrane. Electrochim. Acta 2014, 116, 321−325. (18) Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S. Rechargeable Lithium−Sulfur Batteries. Chem. Rev. 2014, 114, 11751. (19) Luntz, A. C.; Voss, J.; Reuter, K. Interfacial Challenges in SolidState Li Ion Batteries. J. Phys. Chem. Lett. 2015, 6, 4599−4604. (20) Guo, Q.; Han, Y.; Wang, H.; Hong, X.; Zheng, C.; Liu, S.; Xie, K. Safer Lithium Metal Battery based on Advanced Ionic Liquid Gel Polymer Nonflammable Electrolytes. RSC Adv. 2016, 6, 101638− 101644. (21) Sun, H.; You, X.; Jiang, Y.; Guan, G.; Fang, X.; Deng, J.; Chen, P.; Luo, Y.; Peng, H. Self-Healable Electrically Conducting Wires for Wearable Microelectronics. Angew. Chem., Int. Ed. 2014, 53, 9526− 9531.

comparison on the performance is recommended in Figure S3, which is a comparative study to illustrate the effect of L-50%PELCE on the performance of the solid-state battery. We attribute the good performance of LiFePO4/L-50%-PELCE/Li to the better compatibility between the electrode and the electrolyte, which makes Li+ intercalation and deintercalation easier; certainly, this is also closely related to the higher ionic conductivity, the electrochemical stability of the electrolyte itself, and the stability of the interface in the cell. However, impedance spectra (see Figure S4) of the cell were still increased after cycling, which might be due to the part of the reaction happened between the electrolyte and the lithium metal.

4. CONCLUSIONS In summary, firmness and flexibility LPELCE was fabricated via combining ceramic nanoparticles with the IL gel polymer. This method not only provides the slurry easy to film-forming and ensures the mechanical strength of the composite electrolyte but also largely reduces the degree of crystallinity of the polymer, which further improves the ionic conductivity of the composite electrolyte. Meanwhile, LPELCE has good compatibility with the lithium electrode, which can endure up to 4.8 V versus Li+/Li without significant anodic decomposition. Moreover, the electrolyte displayed good interface stability with the lithium electrode after cycling for extended time, which can effectively inhibit the formation of lithium dendrites. Thus, with these advantages of LPELCE, the fabrication of the solid-state metallic lithium battery is enabled with high safety and appreciably enhanced performance. This opens up an opportunity to use this type of electrolyte for developing high energy density and high security solid-state metallic lithium batteries.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12092. Size distribution of massive LAGP nanoparticles; ionic conductivity of the polymer electrolyte; performance of the PEO/LAGP (LiTFSI) composite electrolyte; and impedance spectra of the coin-type solid-state cell (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-0731-84573149. Fax: +86-0731-84573149 (Q.G.). *E-mail: [email protected] (Y.H.). ORCID

Qingpeng Guo: 0000-0001-5241-0974 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was financially supported by the Research Project of National University of Defense Technology. REFERENCES

(1) Piper, D. M.; Evans, T.; Leung, K.; Watkins, T.; Olson, J.; Kim, S. C.; Han, S. S.; Bhat, V.; Oh, K. H.; Buttry, D. A.; Lee, S.-H. Stable Silicon-Ionic Liquid Interface for Next-Generation Lithium-Ion Batteries. Nat. Commun. 2015, 6, 6230. 41843

DOI: 10.1021/acsami.7b12092 ACS Appl. Mater. Interfaces 2017, 9, 41837−41844

Research Article

ACS Applied Materials & Interfaces (22) Li, L.; Peng, S.; Wu, H. B.; Yu, L.; Madhavi, S.; Lou, X. W. D. A Flexible Quasi-Solid-State Asymmetric Electrochemical Capacitor based on Hierarchical Porous V2O5 Nanosheets on Carbon Nanofibers. Adv. Energy Mater. 2015, 5, 1500753. (23) Kang, Y. J.; Chun, S.-J.; Lee, S.-S.; Kim, B.-Y.; Kim, J. H.; Chung, H.; Lee, S.-Y.; Kim, W. All-Solid-State Flexible Supercapacitors Fabricated with Bacterial Nanocellulose Papers, Carbon Nanotubes, and Triblock-Copolymer Ion Gels. ACS Nano 2012, 6, 6400−6406. (24) Shi, Y.; Ha, H.; Al-Sudani, A.; Ellison, C. J.; Yu, G. Smart Electrolytes: Thermoplastic Elastomer-Enabled Smart Electrolyte for Thermoresponsive Self-Protection of Electrochemical Energy Storage Devices. Adv. Mater. 2016, 28, 7810. (25) Kuo, P.-L.; Tsao, C.-H.; Hsu, C.-H.; Chen, S.-T.; Hsu, H.-M. A New Strategy for Preparing Oligomeric Ionic Liquid Gel Polymer Electrolytes for High-Performance and Nonflammable Lithium Ion Batteries. J. Membr. Sci. 2016, 499, 462−469. (26) Chen, B.; Gao, W.; Shen, J.; Guo, S. The Multilayered Distribution of Intumescent Flame Retardants and Its Influence on the Fire and Mechanical Properties of Polypropylene. Compos. Sci. Technol. 2014, 93, 54−60. (27) Zhang, R.; Chen, Y.; Montazami, R. Ionic Liquid-Doped Gel Polymer Electrolyte for Flexible Lithium-Ion Polymer Batteries. Materials 2015, 8, 2735−2748. (28) Yu, X.-Y.; Xiao, M.; Wang, S.-J.; Zhao, Q.-Q.; Meng, Y.-Z. Fabrication and Characterization of PEO/PPC Polymer Electrolyte for Lithium-Ion Battery. J. Appl. Polym. Sci. 2010, 115, 2718−2722. (29) Wu, H.; Chan, G.; Choi, J. W.; Ryu, I.; Yao, Y.; Mcdowell, M. T.; Lee, S. W.; Jackson, A.; Yang, Y.; Hu, L.; Cui, Y. Stable Cycling of Double-Walled Silicon Nanotube Battery Anodes through Solid− Electrolyte Interphase Control. Nat. Nanotechnol. 2012, 7, 310. (30) Hou, J.; Zhang, Z.; Madsen, L. A. Cation/Anion Associations in Ionic Liquids Modulated by Hydration and Ionic Medium. J. Phys. Chem. B 2011, 115, 4576−4582. (31) Li, N.-W.; Yin, Y.-X.; Li, J.-Y.; Zhang, C.-H.; Guo, Y.-G. Passivation of Lithium Metal Anode via Hybrid Ionic Liquid Electrolyte toward Stable Li Plating/Stripping. Adv. Sci. 2017, 4, 1600400. (32) Wu, F.; Chen, N.; Chen, R.; Zhu, Q.; Qian, J.; Li, L. “Liquid-inSolid” and “Solid-in-Liquid” Electrolytes with High Rate Capacity and Long Cycling Life for Lithium-Ion Batteries. Chem. Mater. 2016, 28, 848−856. (33) Wu, F.; Zhu, Q.; Chen, R.; Chen, N.; Chen, Y.; Li, L. Ionic Liquid Electrolytes with Protective Lithium Difluoro(oxalate)borate for High Voltage Lithium-Ion Batteries. Nano Energy 2015, 13, 546− 553.

41844

DOI: 10.1021/acsami.7b12092 ACS Appl. Mater. Interfaces 2017, 9, 41837−41844