Effects of Fluorine Doping on Structural and Electrochemical

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 2042−2049

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Effects of Fluorine Doping on Structural and Electrochemical Properties of Li6.25Ga0.25La3Zr2O12 as Electrolytes for Solid-State Lithium Batteries Yao Lu,†,‡ Xiaoyi Meng,†,‡ José A. Alonso,§ María T. Fernández-Díaz,∥ and Chunwen Sun*,†,‡,⊥

ACS Appl. Mater. Interfaces 2019.11:2042-2049. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/17/19. For personal use only.

†

CAS Center for Excellence in Nanoscience, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P. R. China ‡ School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Instituto de Ciencia de los Materiales de Madrid, CSIC, E-28049, Cantoblanco-Madrid, Spain ∥ Institut Laue-Langevin, B.P. 156, F-38042 Grenoble Cedex 9 France ⊥ Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning 530004, P. R. China S Supporting Information *

ABSTRACT: Solid-state lithium batteries (SSLBs) are promising technologies with great potential in improving safety and energy density, compared with the traditional liquid based lithium ion batteries. However, the bottleneck of SSLBs lies in the issues of poor interface contact and low electrolyte conductivity. In this work, the crystal structure of garnet-type Li6.25Ga0.25La3Zr2O12 (LGLZO) was engineered more rigidly with subdued atoms displacement by fluorine doping and thus smoother and faster lithium ion diffusion paths are formed. The ionic conductivity of garnet-type electrolyte is significantly increased from 5.43 × 10−4 S/cm to 1.28 × 10−3 S/cm at 25 °C and the activation energy is reduced from 0.33 to 0.28 eV. The solid-state symmetric cell consisting of F doped Li6.25Ga0.25La3Zr2O12 electrolyte and lithium metal has lower resistance and displays stable lithium plating/stripping for over 650 h with smaller overpotentials than those on LGLZO electrolyte. Moreover, the all solid-state lithium battery with F-LGLZO electrolyte and LiFePO4 composite electrode exhibits an improved rate capability, which can still keep 132.9 mAh/g at 1 C. Fluorine substitution in garnet-type electrolyte opens new avenues to design new solid-state electrolytes for practical applications of SSLBs. KEYWORDS: Li6.25Ga0.25La3Zr2O12, F-doping, solid electrolyte, neutron powder diffraction, solid-state lithium batteries

1. INTRODUCTION Solid-state lithium batteries (SSLBs) have attracted everincreasing attention in recent years and are regarded as one of the most promising next generation batteries with great potential for improving safety and energy density, in contrast to the traditional liquid based lithium ion batteries (LIBs).1−5 Because of the high mechanical strength of solid-state electrolytes (SSEs), especially inorganic lithium ion conductors, the lithium dendrite formation and the internal short circuit hazard would be prevented. Therefore, the application of lithium metal anodes could be greatly promoted and thus facilitate the development of energy storage market, with taking the great advantages of high theoretical energy densities (3860 mAh/g) and low electrochemical potential (−3.04 V).6,7 Additionally, the safety issue arising from flammable liquid electrolyte could be simultaneously prevented by using SSEs. As one part of the SSLBs, the SSEs play a crucial role in advancing this novel technology. Desired SSEs are required to satisfy several requirements, including high ionic conductivity, © 2018 American Chemical Society

low interface resistances between electrolyte and electrodes, wide electrochemical window, good chemical stability with lithium metal, and so on.8 In the past decades, many kinds of SSEs are developed, such as perovskites, LIPON, sulfides, garnets, etc.9−12 Among them, the garnet-type lithium ion conductor is a promising candidate because of its excellent chemical stability against Li metal, favorable ionic conductivity, and high mechanical strength.13 While, various problems facing the garnets still need to be solved and more efforts have to be made on advancing the application of garnet-type electrolytes in SSLBs. Because of the poor wettability with melting lithium metal and high hardness of garnet-type conductors, there are always poor interface contact and huge impedance between the electrolyte and electrode.14 Therefore, many artificial interface structures have been developed to increase surface contacting area by a 3D ion-conductive framework15 or improving Received: October 9, 2018 Accepted: December 18, 2018 Published: December 18, 2018 2042

DOI: 10.1021/acsami.8b17656 ACS Appl. Mater. Interfaces 2019, 11, 2042−2049

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) XRD patterns of as-prepared LGLZO and F-LGLZO samples and (b) local view of XRD patterns at the range of 24° to 40°. Electron micrographs of sintered (c, e) LGLZO and (d, f) F-LGLZO pellets. SEM images of (c, d) cross sections and (e, f) surfaces. (g−j) Elemental mappings of F-LGLZO pellet’s surface.

interface contact by adding solid polymer electrolyte (SPE) interlayer,16 coating lithophilic surface layers,17 or surface chemistry modification.18 By means of these approaches, the interface resistances can be significantly reduced from 104 to 10 Ω cm2.7 Additionally, great efforts have also been made to improve the lithium ion conductivity and the sinterability of garnet oxide. The phase evolution during sintering was investigated in detail by in situ neutron powder diffraction (NPD) analysis to figure out rational synthesis procedures and several preparation methods with devised sintering processes were reported to obtain high dense garnet-type electrolytes.19 Various dopants, such as Al, Ga, Ta, Nb, and Mo, have been introduced into the lattice to stabilize the cubic structure with a 3D lithium ion pathway, and the lithium ion conductivity was dramatically increased from 10−7 to 10−4 S/cm.20−24 Nevertheless, the lithium ion conductivity of garnet type electrolyte is still lower than that of the commercial liquid electrolyte, resulting in a worse electrochemical performance, especially at ambient temperature and high charge/discharge current density. In this work, we try to substitute oxygen anions by fluorine anions in garnet-type Li6.25Ga0.25La3Zr2O12 (LGLZO) to engineer the 3D transport pathway for lithium ions. Since fluorine anions have a high electronegativity compared with that of oxygen ions, a favorable lithium ion diffusion path may form in LGLZO via partial substitution of O by F. The garnettype electrolyte materials were synthesized and high dense pellets were prepared successfully by solid state reaction method. The effects of fluorine anions doping on crystal structure, lattice parameters and ionic conductivity of LGLZO were systematically investigated by NPD, transmission electron

microscopy (TEM), electrochemical impedance spectra (EIS), and X-ray diffraction (XRD). By doping fluorine into the crystal lattice of LGLZO, the atoms displacements were reduced, which means a more rigid structure and smoother and faster lithium ion diffusion paths would be formed. Consequently, a high ionic conductivity of F doped LGLZO electrolyte (F-LGLZO) is obtained as 1.28 × 10−3 S/cm at 25 °C and the activation energies are also reduced. Moreover, the solid-state F-LGLZO symmetric cell was more stable during lithium plating/stripping at different current densities for over 650 h with smaller overpotentials than those of LGLZO symmetric cell. In addition, the Li|SPE-F-LGLZO-SPE|LFP full cell performs improved rate capability with a high capacity of 132.9 mAh/g even at 1 C and a good stability for over 400 cycles with a reversible capacity of 113 mAh/g at 1 C and 60 °C.

2. EXPERIMENTAL SECTION 2.1. Preparation of Materials. Li6.05Ga0.25La3Zr2O11.8F0.2 and Li6.25Ga0.25La3Zr2O12 ceramics were prepared via conventional solidstate reaction. LiOH·H2O (Aladdin, 99.9%), La2O3 (Aladdin, 99.99%), ZrO2 (Aladdin, 99.9%), Ga2O3 (Aladdin, 99.99%), and NH4F (Sinopharm, 99%) were used as the starting materials. La2O3 and ZrO2 were preheated at 1000 °C for 12 h to fully dehydrate. Stoichiometric amounts of starting materials were mixed together by a wet grinding process in a ball milling machine at 300 rpm for 8 h with yttrium stabilized zirconium oxide (YSZ) balls and isopropanol (IPA) as the grinding media. Ten weight percent excess of LiOH·H2O was added to compensate for lithium loss during the calcination process. The mixture was dried and calcined at 950 °C for 12 h, and then the calcined powder was ground and pressed into a pellet with a diameter of 13 mm, followed by sintering at 1160−1200 °C. To prevent the lithium loss and reaction with aluminum spacer during sintering 2043


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ACS Applied Materials & Interfaces process, pellets were placed on and covered by some powder with the same composition. 2.2. Characterization of Materials. The crystal structure of asprepared samples was studied by X-ray diffraction (XRD, X’pert3 powder) at a scan rate of 1 °/min in the 2θ range from 10° to 130° with Cu Kα radiation. The morphology of the samples was characterized by a field-emission scanning electron microscope (SEM, SU8020), a transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN TMP), and also in a high-resolution mode (HRTEM). The distribution of elements was probed by SEM with an energy dispersive X-ray spectroscopy (EDX) detector. The chemical compositions were determined by combining the induced coupled plasma optical emission spectrometer (ICP-OES, Agilent 730) and X-ray fluorescence (XRF, XRF-1800). The ionic conductivity of dense pellets with a thickness of 1 mm was measured from −20 to 80 °C by AC impedance analysis (Autolab PGSTAT302N). The frequency applied was in the range from 10 Hz to 1 MHz with an amplitude of 10 mV. A thin layer of Au was sputtered on each surface of the pellets as blocking electrodes. Good quality NPD patterns were collected at 295 K at the highresolution D2B neutron diffractometer of ILL-Grenoble, with the high-flux mode and a counting time of 2 h. The samples were contained in vanadium holders. A wavelength of 1.594 Å was selected from a Ge monochromator. The patterns were refined by the Rietveld method,25 using the FULLPROF refinement program.26 A pseudoVoigt function was chosen to generate the line shape of the diffraction peaks. No regions were excluded in the refinement. In the final run the following parameters were refined from the high-resolution D2B data: scale factor, background coefficients, zero-point error, unit-cell parameters, pseudo-Voigt corrected for asymmetry parameters, positional coordinates, anisotropic displacement factors for O and F atoms, and isotropic for the metal atoms. The coherent scattering lengths for Li, Ga, La, Zr, O, and F atoms were −1.90, 7.288, 8.24, 7.16, 5.803, and 5.654 fm, respectively. 2.3. Electrochemical Measurements and Cells Assembly. The electrochemical behaviors of the electrolytes were measured by using CR2032 coin cells. All of the sintered pellets with a diameter about 12 mm for cell assembling were polished to a thickness of 300 μm. Both sides of the ceramic electrolyte were coated with a solid polymer electrolyte (SPE) interlayer with a diameter about 10 mm. Thickness of the SPE is about 13 μm as shown in Figure S1. The SPE precursor solution was prepared by dissolving poly(ethylene oxide) (PEO, Aladdin, MW 100000), succinonitrile (SCN, Aladdin, 99%), and Li(CF3SO2)2N (LiTFSI, Aladdin, 99%) in acetonitrile at a weight ratio of 7:7:6 and stirred for 12 h. And then the precursor solution was coated on the surface of ceramic pellets by doctor blade and dried in vacuum oven at 60 °C for 12 h. The cathode electrodes were prepared by mixing LiFePO4 (LFP, 60 wt %), super P (10 wt %), and SPE (30 wt %) in acetonitrile to form homogeneous slurry, which was then coated onto aluminum foils and vacuum-dried at 60 °C for 12 h. The dried layers were punched into round discs with a diameter of 8 mm. The loading of composite cathode is 2−3 mg cm−2. Lithium foil was used as the anode electrode. CR2032-type LFP|SPE-LLZO-SPE| Li full cells and Li|SPE-LLZO-SPE|Li symmetric cells were assembled in an argon filled glovebox. Electrochemical charge/discharge tests were performed on a LAND CT2001A battery testing instrument in a voltage range of 2.5−4.0 V at 60 °C and room temperature. In addition, electrochemical stable window of electrolytes was examined by cyclic voltammetry (CV) on the stainless steel (SS)|LLZO|Li cell in the voltage range of −0.4 to 6 V at a scanning rate of 0.2 mV/s.

Li2CO3, LaAlO3, La2Zr2O7, LiGaO2, and Li2ZrO2, especially F contained compounds, were detected under the limitation of XRD instrument. Also, the chemical compositions of LGLZO and F-LGLZO materials are well consistent with the nominal ones, and only tiny amount of Al element is detected as listed in Table S1 (Supporting Information). Typical microstructures of the cross sections and surfaces of LGLZO and F-LGLZO pellets were identified by SEM, as shown in Figure 1c−f. Few small pores can be observed in the cross-sectional images (Figure 1c and d) implying that high density of the garnet-type ceramic pellets can be obtained through the described procedure in experimental section. The grains size for both materials is very close within the range of 5−20 μm. There are no impurity phases located at the grain boundary as observed from the high magnification SEM images of surface, as shown in Figure 1e and f. From the EDX mapping results in Figure 1g−j for F-LGLZO pellet and Figure S2 for LGLZO pellet, it can be seen that La, Zr, Ga, and F elements exhibit uniform distribution with no obvious chemical segregation on the grain boundary. The above results suggest that the F anions are introduced into the lattice. It should be mentioned that the garnet materials are extremely sensitive to moisture and carbon dioxide, so to eliminate these undesired reactions, EDX testing are performed on the samples as soon as possible after being calcined in high temperature furnace. To gain insight into the effects of F doping on cubic garnet oxide, the detailed crystal structure of F-LGLZO was further investigated by HRTEM and EDX, as shown in Figure 2. The

Figure 2. (a, b) HRTEM images and (c−h) EDX elemental mappings of F-LGLZO material.

3. RESULTS AND DISCUSSIONS The crystal structures of the as-prepared LGLZO pellets with and without F doping were examined with XRD by grinding their sintered dense pellets and the measured XRD patterns are shown in Figure 1a. Both samples are indexed to a cubic garnet structure (JCPDS No. 80-0457) with the space group of Ia3̅d. As observed from the high quality of XRD patterns and their enlarged view in Figure 1a and b, no impurity phases, such as

sample for HRTEM characterization was prepared from a sintered F-LGLZO pellet. As shown in Figure 2a and b, the interplanar spacings of 0.36 and 0.32 nm correspond to the (312) and (400) crystal planes of LLZO, respectively, which is consistent with the interslab distance calculated by the following refined crystallographic data in Tables S2 and S3. From the EDX mapping results, it is found that the La, Zr, Ga, 2044


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Figure 3. Electrochemical impedance spectra of LGLZO and F-LGLZO pellets with Au blocking electrodes tested at (a) 0 °C and (b) 25 °C and (c) Arrhenius plots of temperature dependent lithium ion conductivity in the temperature range from −20 to 80 °C.

F, and O elements are also uniformly distributed in the bulk of F-LGLZO grain. Combined the above results, it can be concluded that the F anions are successfully doped into LLZO lattice, and dense garnet type pellets are obtained with the same cubic Ia3̅d structure. The complex impedance plots of LGLZO and F-LGLZO pellets are shown in Figures 3 and S3. Au blocking electrodes were sputtered on both sides and tails were found in the impedance plots at low frequencies indicating that the sputtered Au electrode blocked mobile lithium ions. As the testing temperature is decreased below 0 °C, semicircles can be observed in high frequency region as shown in Figure S3. The plots tested at 0 °C were fitted with the conventional equivalent circuit consisting of (Rb)(RgbCPEgb)(CPEel) in solid lines as shown in Figure 3a, where Rb, Rgb, CPEgb, and CPEel donate the bulk resistance, grain boundary resistance, constant phase element for grain boundary and electrodes, respectively.27 The capacitances of the semicircles were fitted at ∼10−9 F, which is attributed to the grain boundary.7,28 According to the fitted bulk and grain boundary resistances, the bulk conductivities are calculated as 4.77 × 10−4 and 2.72 × 10−4 S/cm, and grain boundary conductivities are 3.66 × 10−3 and 4.2 × 10−4 S/cm, for F-LGLZO and LGLZO, respectively. While, with increasing the temperature, the semicircle is decreased and then disappears as higher than 25 °C. Only a line is left as shown in Figures 3b and S3, which is hard to recognize the grain boundary and bulk resistance at all, and thus, the intercept of the plots is approximately regarded as the total resistances. So, the total ionic conductivities at different temperatures are calculated based on the total resistances for LGLZO and F-LGLZO, respectively, and plotted in Figure 3c. It is found that the lithium ion conductivity of F-LGLZO is higher than the LGLZO’s (e.g., 1.28 × 10−3 S/cm for F-LGLZO and 5.43 × 10−4 S/cm for LGLZO at 25 °C), with smaller activation energies calculated from the Arrhenius plots, which is one of the best values in compared with some typical published results as listed in Table 1. Since the other factors that may influence conductivities, such as grain size, density, and phase purity have been carefully controlled during preparation, it suggests that doping F anions into the crystal structure could improve the lithium ionic conductivities with lower activation energies. To reveal the effects of doping F anions on the improvement of lithium ion conductivity, it is necessary to further explore the changes of lattice parameters and phase composition. Given the weak scattering factor for Li+ ions, the determination of crystal structure and Li position in fast Li-conductors is difficult by XRD; hence neutron diffraction measurements are essential. Besides that, our aim was to unveil extremely subtle structural differences responsible for the improvement of the

Table 1. Comparison of Ionic Conductivities for GarnetType Solid Electrolytes σionic (mS/cm)

temperature (°C)

Ea (eV)

Li6.05Ga0.25La3Zr2O11.8F0.2

1.28

25

0.28

Li7La3Zr2O12 Li6.25Al0.25La3Zr2O12 Li7La2.75Ca0.25Zr1.75Nb0.25O12 Li6.75La3Zr1.75Nb0.25O12 Li6.4La3Zr1.4Ta0.6O12 Li6.5La3Zr1.75W0.25O12 Li6.5La3Zr1.75Mo0.25O12 Li6.375La3Zr1.375Nb0.625O12

0.10 0.21 0.22 0.8 1.0 0.49 0.34 1.37

25 25 22 25 25 25 25 25

0.43 0.33 0.35 0.31 0.35 0.35 0.45 0.25

composition

ref this work 29 30 7 21 12 31 24 32

properties observed for the F specimens. In spite of the absorbing character of Li atoms for neutrons, we obtained good quality NPD patterns (Figure 4a and b), with a sufficiently high peak-to-background ratio. The NPD diagrams were perfectly indexed to the cubic Ia3̅d symmetry; no impurities or additional reflections that indicate a departure from this symmetry were observed. Rietveld refinements were performed based on the structural model given by Xie et al.,33 in which La are located at 24c (1/8,0,1/4) sites, Zr at 16a (0,0,0), and O1 were placed at 96h (x,y,z) positions. In the Fcontaining sample, F atoms were distributed at random at 96 h together with O1 atoms. According to the Xie’s results, Li1 atoms were located at 24d (3/8,0,1/4) sites and Li2 at 96h (x,y,z) positions. Ga was distributed at random with Li1 atoms and its occupancy was fixed to the nominal one. Our neutron data allowed us the access to a wide region of the reciprocal space enabling the successful refinement of the anisotropic displacement factors for O/F atoms, minimizing the correlation with the occupancy factors. The final atomic parameters and occupancy factors together with atomic displacement factors after the full refinement of the crystal structures are included in Tables S2 and S3 for LGLZO and FLGLZO, respectively. Figures 4a and b show the goodness of the neutron fits. Table S4 summarizes the main bond distances after the refinement from NPD data. Figure 4c shows a simplified view of the crystal structure, highlighting the anisotropic displacement factors for O/F drawn with 95% probability ellipsoids. The unit-cell parameter for the fluorinated F-LGLZO structure (a = 12.9689(1) Å) is slightly smaller than that of the pristine LGLZO oxide (a = 12.97188(1) Å), as it corresponds to the smaller ionic radius of F− versus O2−. As expected for the minor compositional difference between both samples, of 0.2 F per formula among 12 anionic positions, the 2045


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Figure 4. Room-temperature NPD patterns with Rietveld refinements results of (a) LGLZO and (b) F-LGLZO. (c) Schematic view of the crystal structure of garnet LLZO which was created using VESTA,34 showing the anisotropic ellipsoids of O/F and highlighting the diffusion path for Li ions.

Figure 5. (a) Cyclic voltammetry curve of Li|F-LGLZO|Fe cell within the voltage range of −0.4 to 6 V at a scanning rate of 0.2 mV/s. (b) Impedance spectra of the Li|SPE-LLZO-SPE|Li symmetric cells tested at 60 °C. (c, d) Long-term cycling performance of the symmetric cells at various current densities.

structural refinements show only subtle modifications of the garnet arrangement. There are no significant variations of the bond distances (Table S4), and hence, the size of the different coordination polyhedra remains virtually unchanged within the standard deviations. For O/F atoms, the thermal ellipsoids show a superior anisotropy for the fluorinated sample, with the flattened thermal ellipsoids perpendicular to the chemical bonds to (La, Zr), as shown in Figure 4c. The root-meansquare (RMS) displacements of O/F are ∼0.10 Å along the c axis and 0.06 Å perpendicular to it. However, a main difference that should be highlighted is that all the atoms have smaller

displacement (thermal) parameters in the F sample (even Li atoms); for instance 0.41(3) Å2 versus 0.60(4) Å2 for La, 0.27(3) Å2 versus 0.46(4) Å2 for F-LGLZO versus LGLZO, respectively. This suggests that the atoms are more tightly bound in the fluorinated structure. In the case of O/F, this may be more convenient for Li motion, since the windows they have to cross are less impeded, as the thermal displacements of the anionic sublattice are significantly smaller. In the case of cations, it may be a symptom of a more rigid structure; our results suggest that this is, in general, more favorable for Li diffusion. It seems that the introduction of 1.67% of F in the 2046


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Figure 6. Electrochemical performance of all solid-state Li metal battery. (a) Schematic illustration of the solid-state Li metal batteries. (b) Rate performance at different current densities (0.1, 0.2, 0.5, and 1 C) at 60 °C. (c) Charge/discharge voltage profiles of the cell with F-LGLZO electrolyte at different current density rates from 0.1 to 1 C at 60 °C. (d) Charge/discharge voltage profiles of the cells with Ga and F-LGLZO electrolytes at 1 C and at 60 °C. (e, f) Capacity retention and Coulombic efficiency of F-LGLZO based cell at RT and 60 °C, respectively.

lower bulk resistance than that of the symmetric cell with LGLZO electrolyte. This indicates to the increased lithium ionic conductivity of the F-LGLZO electrolyte, and thus the overpotential of the Li|SPE-F-LGLZO-SPE|Li symmetric cell is decreased at different current densities during long-term cycling as shown in Figure 5c and d. Nevertheless, a weaker voltage hysteresis was also observed for the Li|SPE-F-LGLZOSPE|Li symmetric cell than the symmetric cell with LGLZO electrolyte. With increasing the current density from 0.05 to 0.2 mA/cm2, the Li|SPE-F-LGLZO-SPE|Li symmetric cell can still run well for up to 650 h and show stable lithium platingstripping overpotential. In contrast, the Li|SPE-LGLZO-SPE|Li cell shows gradually increased overpotential and then exhibits short-circuited after 590 h at 0.1 mA/cm2. The stable low overpotential of the Li|SPE-F-LGLZO-SPE|Li symmetric cell performs great potential in solid state battery, which is attributed to the high ionic conductivity of the garnet type ion conductor by F doping. To further examine the effects of improved lithium ion conductivity by F substitution on applied all solid-state lithium batteries, we then tested the electrochemical performance of the batteries with LGLZO and F-LGLZO electrolytes as illustrated in Figure 6a. It should be noted that except the electrolytes, all the other conditions for both cells were carefully kept the same, for instance electrolyte thickness, electrode, and interlayer, to guarantee a reasonable comparison. Figure 6b compares the rate capabilities of the solid-state LiFePO4/Li cells with LGLZO and F-LGLZO electrolytes tested at 60 °C. The cell with LGLZO electrolyte exhibits

anionic sublattice gives rise to stronger La-(O,F) and Zr-(O,F) chemical bonds, even if a more ionic character could be forecasted from the higher electronegativity of F versus O. The more anisotropic character of the O/F ellipsoids, vibrating as disks in a perpendicular way to the metal−oxygen chemical bonds also gives a clue about their stronger bonding. This gives a hint for the improvement of the properties of other ionic conductors that might be achieved via the partial fluorination of oxides. To examine the durability and electrochemical performance of the F-doped garnet electrolyte, a SS|F-LGLZO|Li cell was fabricated and used for cyclic voltammetry (CV) measurement to evaluate the electrochemical stability window of F-LGLZO electrolyte. As shown in Figure 5a, two peaks near 0 V versus Li+/Li are observed, which correspond to the Li metal plating and stripping, while, no other redox peaks are observed up to 6 V, indicating a wide and stable electrochemical window of the F-LGLZO electrolyte. Moreover, Li|SPE-LLZO-SPE|Li symmetric cells were also fabricated to investigate the electrochemical performance of F-doped LGLZO as a solid-state electrolyte for lithium batteries. As shown in Figure 5b, the Nyquist plots of the symmetrical cells intercept the real axis at the high frequency, and the values of the intercepts correspond to the bulk electrolyte resistances. In the following high and intermediate frequency regions, the plots exhibit perfect semicircles representing the response from the interfaces. The interfacial resistances for both symmetrical cells are similar due to the identical SPE interlayer, but the total resistance of the symmetric cell with F-LGLZO electrolyte is decreased with 2047


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ACS Applied Materials & Interfaces discharge capacities of 137.2, 129.3, 113.5, and 95.9 mAh/g at 0.1, 0.2, 0.5, and 1 C rates, while the cell with F doped FLGLZO electrolyte delivers higher capacities of 160.7, 152.4, 144.3, and 132.9 mAh/g at 0.1, 0.2, 0.5, and 1 C rates, respectively. The typical long potential plateaus for LFP cathode material are observed at 3.40 and 3.45 V (0.1 C) in the F-LGLZO electrolyte based cell with respect to discharge and charge, respectively, as shown in Figure 6c. And the polarization increases from 0.05 to 0.34 V with increasing rates. The cell with F-LGLZO electrolyte still keeps a higher discharge capacity of 132.9 mAh/g at 1 C rate with a smaller polarization of 0.34 V, while the cell with LGLZO electrolyte only shows a low capacity of 95.9 mAh/g and a bigger polarization of 0.45 V, as shown in Figure 6d. This improved rate performance and lower polarization can be attributed to the enhanced ionic conductivity of the electrolyte by F doping. The cycle performances of the solid-state cell with F-LGLZO electrolyte at 0.1 and 1 C tested at RT and 60 °C are presented in Figure 6e and f. It can be seen that the capacity maintains at 113 mAh/g stably even after 400 cycles with a high Coulombic efficiency of 99.9%. When the operating temperature is decreased to room temperature, the solid-state cell could also deliver a stable capacity of 113.0 mAh/g at 0.1 C with a Coulombic efficiency of 99.8% after 160 cycles. In fact, capacity degradation is observed during the initial 20 cycles at 1 C, which is probably due to the directly coming fast high current density causing a more serious expanding of electroactive particles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

José A. Alonso: 0000-0001-5329-1225 Chunwen Sun: 0000-0002-3610-9396 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Science Foundation of China (Nos. 51672029, 51372271), National Key R & D Project from Ministry of Science and Technology, China (2016YFA0202702), and China Postdoctoral Science Foundation (2018M631416). We acknowledge the financial support of the Spanish Ministry of Science and Innovation to the project MAT2017-84496-R. We are grateful to ILL for making all facilities available.

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4. CONCLUSIONS In conclusion, garnet-type LGLZO with and without F doping are prepared by a solid-state reaction method. The effects of doping fluorine anions on the crystal structure and electrochemical performances were systematically investigated by TEM, NPD, SEM, EDX, and ICP-OES. These results indicate that the fluorine anions cause a more rigid structure with smaller thermal displacements of the anionic sublattice because of its high electronegativity and, thus, result in less resistance to Li ion diffusion. Therefore, the ionic conductivity is improved and the activation energy is reduced by fluorine doping. As a result, the solid-state symmetric cell based on F-LGLZO electrolyte with lithium metal is characterized with lower resistance and displays stable lithium plating/stripping for over 650 h with smaller overpotentials than those of LGLZO electrolyte. The fabricated all solid-state Li|SPE-LLZO-SPE| LFP full cell shows improved rate capability with a high capacity of 132.9 mAh/g even at 1 C. In addition, the capacity of the cell can maintain at 113 mAh/g at 1 C and 60 °C even after 400 cycles with a high Coulombic efficiency of 99.9%. Even reducing the operating temperature to room temperature, the solid-state cell can deliver a stable capacity of 113.0 mAh/g at 0.1 C with a Coulombic efficiency of 99.8% after 100 cycles. The fluorine substitution in garnet-type electrolyte opens new avenues to design new solid-state electrolytes for practical applications of SSBs.

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LGLZO materials; electrochemical impedance spectra of F-LGLZO and LGLZO pellets; and structural parameters and main interatomic distances for LGLZO and FLGLZO from NPD data (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b17656. SEM image and Elemental mappings of the surface of LGLZO pellet; chemical composition of LGLZO and F2048


Research Article

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Effects of Fluorine Doping on Structural and Electrochemical

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