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High lithium-ion conducting solid electrolyte thin film of Li1.4Al0.4Ge0.2Ti1.4(PO4)3-TiO2 for aqueous lithium secondary batteries
Solid State Ionics 338 (2019) 127–133
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High lithium-ion conducting solid electrolyte thin film of Li1.4Al0.4Ge0.2Ti1.4(PO4)3-TiO2 for aqueous lithium secondary batteries
T
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Fan Baia,b, , Xuefu Shanga, Daisuke Morib, Sou Taminatob, Mitsuhiro Matsumotob, Shinya Watanabeb, Yasuo Takedab, Osamu Yamamotob, Hiroyoshi Nemoric, Masaya Nomurac, Nobuyuki Imanishib a
Department of Physics, Faculty of Engineering, Jiangsu University, Zhenjiang, China Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan c Suzuki Motor Corporation, Group 2, Component Engineering Department, Hamamatsu, Shizuoka 432-8611, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solid electrolyte Lithium-ion conductor NASICON-type Lithium-secondary battery
The water-impermeable solid electrolyte is the key part to aqueous lithium secondary battery design at it serves as both separator and lithium-ion conducting pass way between the lithium metal anode and the aqueous electrolyte. Up to now, NASICON-type doped LiTi2(PO4)3 solid lithium-ion conductors have been proved to be stable in an aqueous solution with a high content of lithium ions, but its relatively low robustness and high resistance is still a barrier towards fabricating high performance battery. Herein, a Li1.4Al0.4Ge0.2Ti1.4(PO4)3 (LAGTP)-10 wt% TiO2 composite with a greatly reduced 90 μm thickness was managed by the route of tape casting. The lithium-ion conductivity, bending strength and water penetration of the film were systematically investigated in this study. The electrical conduction of the film was 8.70 × 10−4 S cm−1 and the mechanical bending strength of the film was improved more than three times to 200 N mm−2 measured at ambient temperature. Water impermeable function was realized by coupling a small amount of ca. 2 wt% epoxy resin into the open pores of the film. Finally, the assembled Li/Li(FSO2)2N in glyme/LAGTP-10 wt% TiO2-epoxy resin/10 M LiCl-1.5 M LiOH/MnO2, air cell presented a reduced total areal resistance to around 240 Ω cm2 at 25 °C and was successfully cycled at 1.0 mA cm−2 in air.
1. Introduction Rechargeable lithium-air secondary batteries are promising power sources for electrical vehicles (EVs), because they have a far higher theoretical energy density and lower material cost [1–3]. Among these batteries, aqueous lithium-air batteries are more attractive because they have fewer troubles to the non-aqueous lithium-air counterparts, such as electrolyte instability by the reaction products and contamination of the lithium anode with water in air [4]. In the battery structure, lithium-ion conducting solid electrolytes functioned with water impermeability are critical because they provide protection to lithium metal anode from the aqueous electrolyte. Moreover, recent papers from both Goodenough et al. and Imanishi el al. addressed new types of aqueous lithium-metal system depending on sandwiched structure of a water-stable lithium-ion conducting solid electrolyte, a lithium metal anode and an aqueous Fe(CN)63−/Fe(CN)64− redox electrolyte [5], or saturated MCl2 (M = Ni and Co)-LiCl solution [6,7]. To build these lithium-metal batteries, the lithium-ion conducting solid electrolyte is ⁎
also important. The purpose to design highly lithium ion conductive solid electrolytes has been numerous in the last few decades [8]. For instance, the highest room temperature lithium-ion conductivity so far is 2.5 × 10−2 S cm−1 for Li9.54Si1.76P1.44S11.7Cl0.3 [9]. However, sulfide based solid lithium-ion conductors are unstable in contact with water and therefore not suitable for aqueous lithium-secondary batteries. Among the various types of lithium-ion conducting solid electrolytes, the NASICON-type doped LiTi2(PO4)3 solid electrolytes are promising candidates for aqueous lithium batteries as they exhibit good stability in high lithium-ion concentrated aqueous solutions [10] and relatively high lithium-ion conductivity at room temperature [11]. One of the earliest papers about doped LiTi2(PO4)3 is by Aono et al. [11], as they found the enhanced lithium-ion conductivity of 7 × 10−4 S cm−1 by Al and Sc doped Li1.3Al0.3Ti1.7(PO4)3 and Li1.3Sc0.3Ti1.7(PO4)3. Since then, there have been many studies to improve the lithium-ion conductivity of the NASICON-type solid lithiumion conductors. Lee et al. [12] presented an enhanced lithium-ion
Corresponding author at: Department of Physics, Faculty of Engineering, Jiangsu University, Zhenjiang, China. E-mail address: [email protected] (F. Bai).
https://doi.org/10.1016/j.ssi.2019.05.017 Received 14 April 2019; Received in revised form 22 May 2019; Accepted 23 May 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
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conductivity of 9.4 × 10−4 S cm−1 by co-sintering 1.0 wt% Bi2O3 with Li1.4Al0.4Ti1.6(PO4)3 (LATP). The electrical conductivity of LATP was further enhanced from the idea of Ge partial substitution at Ti site and the highest lithium-ion conductivity of 1.15 × 10−3 S cm−1 was achieved for Li1.4Al0.4Ti1.6(PO4)3–3 wt% GeO2 [13]. Kyono et al. [14] reported an increase in the grain boundary conductivity of Li1.4Al0.4Ge0.2Ti1.6(PO4)3 (LAGTP) by the addition of 3 wt% Al2O3, whereby a total conductivity of 1.5 × 10−3 S cm−1 was achieved at room temperature. Although great improvements have been achieved for NASICONtype solid electrolytes, the conductivities are still at least one order lower than conventional non-aqueous electrolytes used in lithium-ion batteries. And this related higher resistance would cause additional energy loss in actual batteries. Parallel to reduce the contribution of the solid electrolyte separator in the aqueous lithium cell to the cell resistance by improving conductivity, the thickness of the solid electrolyte should also be an important factor to concern. However, the fragility nature of solid electrolyte ceramic often hinders this realization. To the best of our knowledge, the thickness of NASICON-type solid electrolyte in assembled aqueous lithium-air batteries are still more than 150 μm [15,16]. In addition, it should be noted that the thickness of solid electrolyte is also an important factor in the battery energy density because it accounts for a large part of the total weight. Utilizing the model of the energy density calculation for a lithium-air battery without packaging as a function of the areal specific capacity by Park et al. [17], the specific energy density of 400 Wh kg−1 at the areal specific capacity of 20 mAh cm−2 for the aqueous lithium-air battery with 300 μm thick LAGTP increases to 660 Wh kg−1 for that with 100 μm thick LAGTP considering a density of 2.95 g/cm3 (Fig. S1). In all, NASICON-type solid electrolyte separator with reduced thickness, water-impermeability and high conductivity is a key issue to develop higher specific energy and power density aqueous lithium secondary battery. In this study, we managed to make an approach to thinner LAGTP by enhancing its mechanical strength with TiO2 and tape casting technique. Both electric and mechanical performances of the solid electrolytes are investigated and the performance of a lithiumair cell prepared with the thin film solid electrolyte is discussed.
sheet was punched into round pieces with a typical diameter of 20 mm and the thickness is about 40 μm each piece. To prepare actual green sheet for battery use, several pieces were then isostatically pressed together at 150 MPa and 80 °C for 30 min. The sheets were sintered at 500 °C to remove organic additives and then at 950 °C for a dense structure on Au sheets in an air atmosphere. The solid electrolyte with epoxy composite were made by dipping into a tetrahydrofuran solution containing of 0.05 M 1,3-phenylene diamine (Sigma-Aldrich) and 2,2-bis(4-glycidyloxy-phenyl)propane (Sigma Aldrich) and polymerized at 170 °C for 24 h. The absorption amount of epoxy resin was estimated from the weight change by thermogravimetric analysis (Rigaku Thermo-Plus TG8128). The aqueous lithium-air test cell was a laminate-type cell, as reported in our previous paper [19]. From the anode side of the cell was lithium metal with a thickness of around 200 μm (Honjo Metal, Japan), an organic buffer layer of 4.5 M lithium bis(fluorosulfonyl)imide (LiFSI) in ethylene glycol dimethyl ether (DME) and as-prepared 90 μm thick LAGTP-10 wt% TiO2-epoxy solid electrolyte. The air cathode consisted of activated MnO2 (Japan Metal and Chemicals, Japan), conductive Ketjen black (Akzo Noble) and polytetrafluoroethylene (PTFE) in a weight ratio of 4:76:20, while the aqueous electrolyte was made from the mixture of 10 M LiCl with 1.5 M LiOH. The contact areas of the lithium electrode and solid electrolyte with the buffer electrolyte were 1.0 cm2, and that of the air electrode with the aqueous electrolyte was 1.7 cm2. The applied charge and discharge current was depending on the area of lithium electrode. The phases of the LAGTP-10 wt% TiO2 composite were measured by X-ray diffraction analysis (XRD; Bruker D8) with Cu Kα radiation in the 2θ range from 10 to 90°, together with Rietveld refinement by attached TOPAS 4.2 software. The morphology of the sintered films were characterized using scanning electron microscopy (SEM; Hitachi S4800) equipped with energy dispersive X-ray spectroscopy (EDX). The electrical conductivities of the sintered films (ca. 7 mm diameter and ca. 0.1 mm thick) were measured by the impedance spectra of sandwichtype Au/LAGTP/Au (Solartron 1260) within the frequency range from 1 MHz to 0.1 Hz at biased voltage of 10 mV. The 3-point bending strength of the films was measured by cutting them into bars and measured by a material tester (Shimadzu Ez-SX-500 N). Water penetration tests were conducted by an H-type cell with a sandwiched structure of saturated aqueous LiCl solution, solid electrolyte and distilled water. The water permeation rate through the film was estimated from the change of the chloride concentration in the distilled water over time using a chloride meter (Kasahara Chemical Instruments, Japan).
2. Experimental The route of LAGTP powders preparation can be summarized by two-step solid-state reaction method as reported in our previous paper with slight modifications [13,18]. The typical preparation route is as follows: analytical grade Li2CO3, TiO2, Al2O3, NH4H2PO4 (Nacalai Tesque Japan) and GeO2 powder (Sigma Aldrich) were added to isopropanol according to the chemical formula of Li1.4Al0.4Ge0.2Ti1.4(PO4)3 and sealed in a zirconia vessel for 2 h ball milling at a speed of 400 rpm (Fritsch Pulverisette 7). The mixture was transferred into an oven at 80 °C for 3 h and then 200 °C for 12 h to totally evaporate solvent before meshed and sintered at 600 °C for 5 h to remove CO2, NH3 and H2O. The obtained powder was again ball-milled for additional 5 h and heated at 800 °C for 4 h to obtain refined LAGTP powder. The slurry for tape-casting films was prepared as follows. 3.6 g of the as-prepared LAGTP powder was mixed with 10 wt% TiO2 (Kojundo Chemicals, Japan; 2 μm particle size), and 0.1 g Menhaden fish oil (Sigma Aldrich) and ball-milled in toluene and ethanol mixed solvent with a weight ratio of 1:1 for 24 h. Afterwards, reasonable amount of butvar B-98 (Sigma Aldrich) and benzyl-butyl-phthalate (Wako Chemical Japan) were added to the slurry as binder and plasticizer and ball-milled at low speed to get a homogeneous slurry for tape cast. The slurry was debubbled in a planetary vacuum mixer (THINKY, Japan) before casting on a silicone coated polyethylene substrate using homemade two doctor blade apparatus. The as-prepared green tapecast sheets were immediately transferred into fridge and stored at 4 °C to slow the dry process of as-prepared films. Then the green tape-cast
3. Results and discussion Fig. 1 compares XRD patterns of the LAGTP-10 wt% TiO2 composite and LAGTP without TiO2 sintered at 950 °C for 7 h. The main diffraction peaks indicated both materials were NASICON-type structure (R3C). Impurity phases of LiTiOPO4 and TiO2 were emerged for LAGTP-10 wt % TiO2 with addition of TiO2, while GeO2 and AlPO4 were related to the sintering process of LAGTP and observed in both cases. Rietveld refinement in Fig. S2 reveals that the c lattice parameter for LAGTP increased and the a lattice decreased slightly by the addition of 10 wt% TiO2. These results suggest that large ionic radius Ti4+ (0.745 Å) was substituted for small ionic radius Al3+ (0.675 Å) and/or Ge4+ (0.670 Å) with TiO2 addition. Thermal analysis of the tape cast green sheet in Fig. S3 showed a weight decrease up to 450 °C and then no change of weight. Therefore, the solvent, binder and plasticizer in the green sheets were decomposed at 500 °C for 5 h and then sintered at 950 °C for densification. Both sintering temperature and period were selected to obtain the highest lithium-ion conductivity of the film before sample characterization. The obtained LAGTP-10 wt% TiO2 film had no cracks, as shown in Fig. 2 (a) and (b), but small pores was observed. The grain size was in a range of 128
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Fig. 1. XRD patterns of LAGTP (red) and LAGTP-10 wt% TiO2 (black) tape cast films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
strength, (6) ease of thin film preparation. The stability of the NASICON-type doped LiTi2(PO4)3 in water was already confirmed in aqueous solution with a high content of Li+ [10,20,21]. Although the lithium-ion solid conductor with Ti4+ was proven to be unstable in contact with lithium metal due to the reduction of Ti4+ by lithium [22], a lithium-stable lithium-ion conducting electrolyte buffer layer, such as a non-aqueous liquid electrolyte [5,6] or polymer electrolyte [23] can be served a separator between lithium metal and the solid electrolyte. Water impermeability through the lithium-ion conducting thin solid electrolyte film is an important requirement for the separator in aqueous lithium batteries. Fig. 3 shows the water permeation test results obtained at room temperature for the 90 μm thick films of LAGTP-10 wt % TiO2 and LAGTP-10 wt% TiO2 with around 2 wt% epoxy resin, where
2.5–1.0 μm. While the LAGTP without TiO2 showed extremely large grain and the contact between grains was insufficient, as shown in Fig. 2(c).This phenomenon suggests that the added TiO2 could suppress LAGTP grain growth at relatively high temperature. EDX maps of the Ti, Al, Ge, and P contents showed a homogeneous distribution of all elements over the cross-section of the LAGTP-10 wt% TiO2 film, as shown in Fig. 2(d). It indicates that the added TiO2 in the LAGTP-10 wt% TiO2 composite were dispersed homogeneously on the grain boundary or in the lattice sites in LAGTP. The requirements for the separator between the lithium electrode and the aqueous electrolyte in aqueous lithium batteries are as follows: (1) stable in water, (2) resist to lithium reduction, (3) water-impermeable, (4) high lithium-ion conductivity, (5) high mechanical
Fig. 2. (a, b) Cross-sectional SEM images for the 90 μm thick LAGTP-10 wt% TiO2 film (c) cross-sectional SEM images of pure LAGTP film (d) EDX maps of the 90 μm thick LAGTP-10 wt% TiO2 film. 129
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Fig. 3. Water penetration test results for the 90 μm thick LAGTP-10 wt% TiO2 (red line) and LAGTP-10 wt% TiO2-epoxy (black line) films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
TiO2 film with 2 wt% epoxy resin are presented in Fig. 5. The total conductivities of LAGTP-10 wt% TiO2 and LAGTP-10 wt% TiO2 with 2 wt% epoxy resin at 60 °C were 2.28 × 10−3 and 1.78 × 10−3 S cm−1, respectively. The activation energy of the bulk conductivity for both electrolytes was 12.1 kJ mol−1, which is less than 20.2 kJ mol−1 for LAGTP reported previously [13]. The activation energy of the grain boundary conductivity for LAGTP-10 wt% TiO2 with epoxy resin was higher than that of LAGTP-10 wt% TiO2 and that of LAGTP reported previously, which may result from higher content of impurity phases at grain boundary [16]. The contribution of an electron and/or hole conduction in the LAGTP-10 wt% TiO2 film was also estimated using the potentiostatic polarization method [27,28]. A bias voltage of 0.25 V was applied on the Au/LAGTP-10 wt% TiO2/Au cell and the steady current of 10 nA cm−2 was recorded in Fig. S4. The result suggests that the contribution of the electronic conductivity may be negligibly small to the ionic conductivity. Another important requirement to emphasize is the mechanical strength of the solid electrolyte, as its robustness accounts for the ability of preventing water leakage and battery safety. Previously, the highest 3-point bending strength of a 200–300 μm thick tape cast LAGTP film in aqueous lithium air battery was reported to be 110 N mm−2 [16]. Fig. 6(a) shows the average 3-point bending strength of the several tape-casted films of LAGTP-x wt% TiO2 as a function of the TiO2 content, where the thickness of the tape cast films was approximately 500 μm. The bending strength increased with the TiO2 content and 210 N mm−2 was recorded for LAGTP-10 wt% TiO2. This bending strength is comparable to that of the sample sintered from a pressed tablet of LAGTP-10 wt% TiO2 by sol-gel method, indicating the modified solid state method and tape casting are feasible to the mechanical strength improvement of LAGTP [14]. Fig. 6(b) shows dependence of the bending strength on the thickness of the LAGTP-10 wt% TiO2 tape cast film. No significant dependence on thickness was observed in the range from 50 to 500 μm. The bending strength of LAGTP-10 wt% TiO2 is comparable with that of beta-alumina (140–240 N mm−2), which is used as a separator for sodium sulfur rechargeable batteries [29]. To demonstrate the advantages of 90 μm solid electrolytes in actual aqueous lithium-air batteries, the LAGTP-10 wt% TiO2–epoxy composite film was assembled in laminate-type cell, of which isostatic pressing and vacuum zipping were used to ensure the close contact of each part and water-tightness. The impedance profile of the Li/4.5 M LiFSI in
the contact area with the liquid was 3.14 cm2. LAGTP-10 wt% TiO2 without epoxy resin exhibited a linear increase of chloride ion content with time, while LAGTP-10 wt% TiO2 with epoxy resin showed no increase of chloride ion content for 24 h in regardless of thin thickness of less than 100 μm. This result confirms the 90 μm thick LAGTP-10 wt% TiO2 film with around 2 wt% epoxy resin is water-impermeable. One of the other important requirements for the separator in lithium aqueous batteries is high lithium-ion conductivity. To study the influence of TiO2 and epoxy on conductivity, gold electrode was sputtered on both sides of LAGTP-10 wt% TiO2 and those with 2 wt% epoxy resin. The impedance spectra of both pellets are presented at 25 °C in Fig. 4, where the resistances were normalized with the gold surface area of 0.38 cm2. Both spectra show a high frequency semicircle followed by a straight line at low frequency. The semicircles are attributed to the grain boundary resistance and the straight line to the Warburg diffusion process according to the analysis in previous reports [24]. As the semicircle that represents the bulk resistance was out of the frequency of the analyzer, the left intercept of the semicircle simulated can be considered to be the bulk resistance of the samples for simplicity. The estimated bulk, grain boundary and total conductivities are listed in the table of Fig. 4. A total conductivity of 8.70 × 10−4 S cm−1 at 25 °C for the LAGTP-10 wt% TiO2 is comparable to that of LAGTP prepared by solid state method at 900 °C [18]. The lithium-ion conductivity of LAGTP-10 wt% TiO2 was decreased to 4.45 × 10−4 S cm−1 by the addition of epoxy resin but it is still reasonable compared to Ohara LATP of ~1 × 10−4 S cm−1. As the bulk conductivities of LAGTP-10 wt% TiO2 with and without epoxy resin were almost the same, the low conductivity of the film with epoxy resin may be due to the formation of a highly resistive phase with epoxy resin at the grain boundaries. Owing to the reduced thickness, the area specific resistance of the 90 μm thick LAGTP-10 wt% TiO2-epoxy film was as low as 21.0 Ω cm2, which is even lower than that of the solid electrolyte interphase (SEI) between lithium metal and a conventional electrolyte in a lithium-ion battery, such as with LiPF6 in EC-DEC at 25 °C (around 100 Ω cm2) [25] and between lithium metal and polymer electrolyte at 60 °C (around 100 Ω cm2) [26]. Therefore, the resistance of the LAGTP-10 wt% TiO2 film with epoxy resin may contribute to only a small part of the total cell resistance and less energy loss for the aqueous lithium batteries. The temperature dependence of the total, bulk, and grain boundary conductivity of the LAGTP-10 wt% TiO2 film and the LAGTP-10 wt% 130
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Sample
Bulk
Grain boundary
Total conductivity
conductivity
conductivity (S cm-1)
(S cm-1)
(S cm-1) LAGTP-10 wt% TiO2
1.8410-3
1.6810-3
8.7010-4
1.8410-3
5.8710-4
4.4510-4
LAGTP-10 wt%TiO2 -epoxy
Fig. 4. Impedance profiles for the Au/LAGTP-10 wt% TiO2/Au and LAGTP-10 wt% TiO2 with epoxy resin (ca. 2 wt%)/Au cells at 25 °C. The thickness of the electrolyte was 90 μm.
Fig. 5. Temperature dependence of the bulk, grain boundary and total conductivity for (a) LAGTP-10 wt% TiO2 and (b) LAGTP-10 wt% TiO2 with 2 wt% epoxy resin.
electrolyte. These values were derived by the simulation of the equivalent circuit shown in Fig. 7(a). The electrolyte resistance (R1 + R2) of 34.6 Ω cm2 is significantly lower than previous reported aqueous lithium-air batteries with a 210 μm thick Li1.5Al0.5Nb0.2Ti1.3(PO4)3 (~120 Ω cm2) [19], a 250 μm Ohara LATP (~400 Ω cm2) [31], and a 180 μm thick LAGTP (~180 Ω cm2) [16]. The charge transfer resistance of 47.5 Ω cm2 was comparable to that of 40 Ω cm2 for Li/LiFSI in tetraethylene glycol dimethyl ether (G4) [25]. Owing to the reduced thickness of solid electrolyte, the total cell
DME/LAGTP-10 wt% TiO2-epoxy/10 M LiCl-1.5 M LiOH/MnO2, air cell at 25 °C is shown in Fig. 7(a). The impedance profile consists of three semicircles at different frequencies; the semicircle at high frequency can be assigned to the grain boundary response of the solid electrolyte (R2) and middle frequency range one to the interface resistance between the lithium metal electrode, liquid electrolyte and solid electrolyte (R3), while the one in the low frequency range may relate to the charge transfer resistance (R4) [30]. And the series resistance R1 is the total resistance of the solid electrolyte's bulk and the liquid buffer 131
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Fig. 6. (a) Three-point bending strength of ca. 500 μm thick LAGTP-x TiO2 film as a function of TiO2 content, and (b) 3-point bending strength of the LAGTP-10 wt% TiO2 film as a function of thickness. Fig. 7. (a) Impedance profile of the Li/ 4.5 M LiFSI in DME/90 μm thick LAGTP10 wt% TiO2-epoxy resin/10 M LiCl-1.5 M LiOH aqueous solution/MnO2, air cell at 25 °C. (b) Short term and (c) long term charge and discharge cyclic performance of the Li/4.5 M LiFSI in DME/90 μm thick LAGTP-10 wt% TiO2-epoxy resin/10 M LiCl1.5 M LiOH aqueous solution/MnO2, air cell at 25 °C and 1.0 mA cm2.
resistance of 240 Ω cm2 was achieved, which is compared with that of 320 Ω cm2 reported previously [16]. This cell resistance is acceptable for a high specific power density lithium-air battery because the contribution of the cell potential drop by the resistance at 1.0 mA cm2 is less than 10%. Moreover, we note that the cell resistance could be further reduced by selecting a buffer electrolyte with a lower SEI resistance [25]. In addition, the cyclic performance of the aqueous lithium cell using the 90 μm thick LAGTP-10 wt% TiO2 with epoxy resin at 1.0 mAh cm−2 and 25 °C under open air is shown in Fig. 7(b) and (c). An excellent cyclic performance was observed in a short cycle. The round trip cell voltage change is only 0.5 V at 1.0 mAh cm−2, which is almost same to the ohmic drop regarding to the impedance spectra. This overpotential is also significantly lower, compared to 0.6 V at 0.4 mAh cm−2 reported previously for the lithium-air battery with 180 μm thick
LAGTP [16]. The charging potentials were gradually increased by cycling, of which may be due to degradation of the lithium electrode and carbon based air cathode [31,32]. The cell performance was considerably improved by the use of a high conductivity thin solid electrolyte. This result suggests that the 90 μm thick LAGTP-10 wt% TiO2 film with epoxy resin could be acceptable as the separator in aqueous lithium batteries because of its high lithium-ion conductivity and enhanced mechanical strength. 4. Conclusions A 90 μm thick LAGTP-10 wt% TiO2 lithium-ion conducting solid electrolyte was fabricated using the tape-casting method. The thin film with around 2 wt% epoxy resin was confirmed to be water132
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impermeable. The specific area resistances of the LAGTP composite film without and with epoxy resin were 9.5 and 20.5 Ω cm2, respectively. The mechanical strength of the LAGTP film was significantly improved by the addition of TiO2. The 3-point bending strength of the film was increased from 50 N mm−2 for the LAGTP film without TiO2 to 210 N mm−2 for the LAGTP with 10 wt% TiO2. The feasibility of the thin film electrolyte for an aqueous lithium-air cell was demonstrated. These optimized features enables its wide application in the field of aqueous lithium secondary batteries, such as aqueous lithium-air batteries and also aqueous Li/MCl2 (M = Co, Ni) batteries (Fig. S5).
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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2019.05.017. References [1] P. Zhang, Y. Zhao, X. Zhang, Functional and stability orientation synthesis of materials and structures in aprotic Li–O2 batteries, Chem. Soc. Rev. 47 (2018) 2921–3004. [2] D. Safanama, S. Adams, High efficiency aqueous and hybrid lithium-air batteries enabled by Li1.5Al0.5Ge1.5(PO4)3 ceramic anode-protecting membranes, J. Power Sources 340 (2017) 294–301. [3] J. Lu, l. Li, Y.-K. Sun, F. Wu, K. Amine, Aprotic and aqueous Li–O2 batteries, Chem. Rev. 114 (2014) 5611–5640. [4] N. Imanishi, O. Yamamoto, Rechargeable lithium–air batteries: characteristics and prospects, Mater. Today 17 (2014) 24–30. [5] Y. Lu, J.B. Goodenough, Y. Kim, Aqueous cathode for next-generation alkali-ion batteries, J. Am. Chem. Soc. 133 (2011) 5756–5795. [6] Y. Morita, S. Watanabe, P. Zhang, H. Wang, D. Mori, Y. Matsuda, O. Yamamoto, Y. Takeda, N. Imanishi, High specific energy density aqueous lithium-metal chloride rechargeable batteries, J. Electrochem. Soc. 164 (2017) A1958–A1964. [7] Y. Morita, S. Watanabe, D. Mori, Y. Takeda, O. Yamamoto, N. Imanishi, High-energy-density rechargeable lithium–nickel chloride aqueous solution batteries, ACS Omega 3 (2018) 5558–5562. [8] J. Bachman, S. Muy, A. Grimaud, H.-H. Chang, N. Pour, S.F. Lux, O. Paschos, F. Maglia, S. Lupart, P. Lamp, L. Giordano, Y. Shao-Hon, Inorganic solid-state electrolytes for lithium batteries: mechanisms and properties governing ion conduction, Chem. Rev. 116 (2016) 140–162. [9] Y. Kato, S. Hori, T. Saito, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, High-power all-solid-state batteries using sulfide superionic conductors, Nat. Energy 1 (2016) 1603–1609. [10] S. Hasegawa, N. Imanishi, T. Zhang, J. Xie, A. Hirano, Y. Takeda, O. Yamamoto, Study on lithium/air secondary batteries—stability of NASICON-type lithium ion conducting glass–ceramics with water, J. Power Sources 189 (2008) 371–377. [11] H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka, G. Adachi, Ionic conductivity of solid electrolytes based on lithium titanium phosphate, J. Electrochem. Soc. 137 (1990) 1023–1027. [12] S.D. Lee, K.-N. Jung, H. Kim, H.S. Shin, S.-W. Song, M.-S. Park, Composite electrolyte for all-solid-state lithium batteries: low-temperature fabrication and conductivity enhancement, ChemSusChem 10 (2017) 2175–2181. [13] F. Bai, X. Shang, H. Nemori, M. Nomura, D. Mori, M. Matsumoto, N. Kyono, Y. Takeda, O. Yamamoto, N. Imanishi, Lithium-ion conduction of Li1.4Al0.4Ti1.6(PO4)3-GeO2 composite solid electrolyte, Solid State Ionics 329 (2019)
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High lithium-ion conducting solid electrolyte thin film of Li1.4Al0.4Ge0.2Ti1.4(PO4)3-TiO2 for aqueous lithium secondary batteries
T
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Fan Baia,b, , Xuefu Shanga, Daisuke Morib, Sou Taminatob, Mitsuhiro Matsumotob, Shinya Watanabeb, Yasuo Takedab, Osamu Yamamotob, Hiroyoshi Nemoric, Masaya Nomurac, Nobuyuki Imanishib a
Department of Physics, Faculty of Engineering, Jiangsu University, Zhenjiang, China Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan c Suzuki Motor Corporation, Group 2, Component Engineering Department, Hamamatsu, Shizuoka 432-8611, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solid electrolyte Lithium-ion conductor NASICON-type Lithium-secondary battery
The water-impermeable solid electrolyte is the key part to aqueous lithium secondary battery design at it serves as both separator and lithium-ion conducting pass way between the lithium metal anode and the aqueous electrolyte. Up to now, NASICON-type doped LiTi2(PO4)3 solid lithium-ion conductors have been proved to be stable in an aqueous solution with a high content of lithium ions, but its relatively low robustness and high resistance is still a barrier towards fabricating high performance battery. Herein, a Li1.4Al0.4Ge0.2Ti1.4(PO4)3 (LAGTP)-10 wt% TiO2 composite with a greatly reduced 90 μm thickness was managed by the route of tape casting. The lithium-ion conductivity, bending strength and water penetration of the film were systematically investigated in this study. The electrical conduction of the film was 8.70 × 10−4 S cm−1 and the mechanical bending strength of the film was improved more than three times to 200 N mm−2 measured at ambient temperature. Water impermeable function was realized by coupling a small amount of ca. 2 wt% epoxy resin into the open pores of the film. Finally, the assembled Li/Li(FSO2)2N in glyme/LAGTP-10 wt% TiO2-epoxy resin/10 M LiCl-1.5 M LiOH/MnO2, air cell presented a reduced total areal resistance to around 240 Ω cm2 at 25 °C and was successfully cycled at 1.0 mA cm−2 in air.
1. Introduction Rechargeable lithium-air secondary batteries are promising power sources for electrical vehicles (EVs), because they have a far higher theoretical energy density and lower material cost [1–3]. Among these batteries, aqueous lithium-air batteries are more attractive because they have fewer troubles to the non-aqueous lithium-air counterparts, such as electrolyte instability by the reaction products and contamination of the lithium anode with water in air [4]. In the battery structure, lithium-ion conducting solid electrolytes functioned with water impermeability are critical because they provide protection to lithium metal anode from the aqueous electrolyte. Moreover, recent papers from both Goodenough et al. and Imanishi el al. addressed new types of aqueous lithium-metal system depending on sandwiched structure of a water-stable lithium-ion conducting solid electrolyte, a lithium metal anode and an aqueous Fe(CN)63−/Fe(CN)64− redox electrolyte [5], or saturated MCl2 (M = Ni and Co)-LiCl solution [6,7]. To build these lithium-metal batteries, the lithium-ion conducting solid electrolyte is ⁎
also important. The purpose to design highly lithium ion conductive solid electrolytes has been numerous in the last few decades [8]. For instance, the highest room temperature lithium-ion conductivity so far is 2.5 × 10−2 S cm−1 for Li9.54Si1.76P1.44S11.7Cl0.3 [9]. However, sulfide based solid lithium-ion conductors are unstable in contact with water and therefore not suitable for aqueous lithium-secondary batteries. Among the various types of lithium-ion conducting solid electrolytes, the NASICON-type doped LiTi2(PO4)3 solid electrolytes are promising candidates for aqueous lithium batteries as they exhibit good stability in high lithium-ion concentrated aqueous solutions [10] and relatively high lithium-ion conductivity at room temperature [11]. One of the earliest papers about doped LiTi2(PO4)3 is by Aono et al. [11], as they found the enhanced lithium-ion conductivity of 7 × 10−4 S cm−1 by Al and Sc doped Li1.3Al0.3Ti1.7(PO4)3 and Li1.3Sc0.3Ti1.7(PO4)3. Since then, there have been many studies to improve the lithium-ion conductivity of the NASICON-type solid lithiumion conductors. Lee et al. [12] presented an enhanced lithium-ion
Corresponding author at: Department of Physics, Faculty of Engineering, Jiangsu University, Zhenjiang, China. E-mail address: [email protected] (F. Bai).
https://doi.org/10.1016/j.ssi.2019.05.017 Received 14 April 2019; Received in revised form 22 May 2019; Accepted 23 May 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.
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conductivity of 9.4 × 10−4 S cm−1 by co-sintering 1.0 wt% Bi2O3 with Li1.4Al0.4Ti1.6(PO4)3 (LATP). The electrical conductivity of LATP was further enhanced from the idea of Ge partial substitution at Ti site and the highest lithium-ion conductivity of 1.15 × 10−3 S cm−1 was achieved for Li1.4Al0.4Ti1.6(PO4)3–3 wt% GeO2 [13]. Kyono et al. [14] reported an increase in the grain boundary conductivity of Li1.4Al0.4Ge0.2Ti1.6(PO4)3 (LAGTP) by the addition of 3 wt% Al2O3, whereby a total conductivity of 1.5 × 10−3 S cm−1 was achieved at room temperature. Although great improvements have been achieved for NASICONtype solid electrolytes, the conductivities are still at least one order lower than conventional non-aqueous electrolytes used in lithium-ion batteries. And this related higher resistance would cause additional energy loss in actual batteries. Parallel to reduce the contribution of the solid electrolyte separator in the aqueous lithium cell to the cell resistance by improving conductivity, the thickness of the solid electrolyte should also be an important factor to concern. However, the fragility nature of solid electrolyte ceramic often hinders this realization. To the best of our knowledge, the thickness of NASICON-type solid electrolyte in assembled aqueous lithium-air batteries are still more than 150 μm [15,16]. In addition, it should be noted that the thickness of solid electrolyte is also an important factor in the battery energy density because it accounts for a large part of the total weight. Utilizing the model of the energy density calculation for a lithium-air battery without packaging as a function of the areal specific capacity by Park et al. [17], the specific energy density of 400 Wh kg−1 at the areal specific capacity of 20 mAh cm−2 for the aqueous lithium-air battery with 300 μm thick LAGTP increases to 660 Wh kg−1 for that with 100 μm thick LAGTP considering a density of 2.95 g/cm3 (Fig. S1). In all, NASICON-type solid electrolyte separator with reduced thickness, water-impermeability and high conductivity is a key issue to develop higher specific energy and power density aqueous lithium secondary battery. In this study, we managed to make an approach to thinner LAGTP by enhancing its mechanical strength with TiO2 and tape casting technique. Both electric and mechanical performances of the solid electrolytes are investigated and the performance of a lithiumair cell prepared with the thin film solid electrolyte is discussed.
sheet was punched into round pieces with a typical diameter of 20 mm and the thickness is about 40 μm each piece. To prepare actual green sheet for battery use, several pieces were then isostatically pressed together at 150 MPa and 80 °C for 30 min. The sheets were sintered at 500 °C to remove organic additives and then at 950 °C for a dense structure on Au sheets in an air atmosphere. The solid electrolyte with epoxy composite were made by dipping into a tetrahydrofuran solution containing of 0.05 M 1,3-phenylene diamine (Sigma-Aldrich) and 2,2-bis(4-glycidyloxy-phenyl)propane (Sigma Aldrich) and polymerized at 170 °C for 24 h. The absorption amount of epoxy resin was estimated from the weight change by thermogravimetric analysis (Rigaku Thermo-Plus TG8128). The aqueous lithium-air test cell was a laminate-type cell, as reported in our previous paper [19]. From the anode side of the cell was lithium metal with a thickness of around 200 μm (Honjo Metal, Japan), an organic buffer layer of 4.5 M lithium bis(fluorosulfonyl)imide (LiFSI) in ethylene glycol dimethyl ether (DME) and as-prepared 90 μm thick LAGTP-10 wt% TiO2-epoxy solid electrolyte. The air cathode consisted of activated MnO2 (Japan Metal and Chemicals, Japan), conductive Ketjen black (Akzo Noble) and polytetrafluoroethylene (PTFE) in a weight ratio of 4:76:20, while the aqueous electrolyte was made from the mixture of 10 M LiCl with 1.5 M LiOH. The contact areas of the lithium electrode and solid electrolyte with the buffer electrolyte were 1.0 cm2, and that of the air electrode with the aqueous electrolyte was 1.7 cm2. The applied charge and discharge current was depending on the area of lithium electrode. The phases of the LAGTP-10 wt% TiO2 composite were measured by X-ray diffraction analysis (XRD; Bruker D8) with Cu Kα radiation in the 2θ range from 10 to 90°, together with Rietveld refinement by attached TOPAS 4.2 software. The morphology of the sintered films were characterized using scanning electron microscopy (SEM; Hitachi S4800) equipped with energy dispersive X-ray spectroscopy (EDX). The electrical conductivities of the sintered films (ca. 7 mm diameter and ca. 0.1 mm thick) were measured by the impedance spectra of sandwichtype Au/LAGTP/Au (Solartron 1260) within the frequency range from 1 MHz to 0.1 Hz at biased voltage of 10 mV. The 3-point bending strength of the films was measured by cutting them into bars and measured by a material tester (Shimadzu Ez-SX-500 N). Water penetration tests were conducted by an H-type cell with a sandwiched structure of saturated aqueous LiCl solution, solid electrolyte and distilled water. The water permeation rate through the film was estimated from the change of the chloride concentration in the distilled water over time using a chloride meter (Kasahara Chemical Instruments, Japan).
2. Experimental The route of LAGTP powders preparation can be summarized by two-step solid-state reaction method as reported in our previous paper with slight modifications [13,18]. The typical preparation route is as follows: analytical grade Li2CO3, TiO2, Al2O3, NH4H2PO4 (Nacalai Tesque Japan) and GeO2 powder (Sigma Aldrich) were added to isopropanol according to the chemical formula of Li1.4Al0.4Ge0.2Ti1.4(PO4)3 and sealed in a zirconia vessel for 2 h ball milling at a speed of 400 rpm (Fritsch Pulverisette 7). The mixture was transferred into an oven at 80 °C for 3 h and then 200 °C for 12 h to totally evaporate solvent before meshed and sintered at 600 °C for 5 h to remove CO2, NH3 and H2O. The obtained powder was again ball-milled for additional 5 h and heated at 800 °C for 4 h to obtain refined LAGTP powder. The slurry for tape-casting films was prepared as follows. 3.6 g of the as-prepared LAGTP powder was mixed with 10 wt% TiO2 (Kojundo Chemicals, Japan; 2 μm particle size), and 0.1 g Menhaden fish oil (Sigma Aldrich) and ball-milled in toluene and ethanol mixed solvent with a weight ratio of 1:1 for 24 h. Afterwards, reasonable amount of butvar B-98 (Sigma Aldrich) and benzyl-butyl-phthalate (Wako Chemical Japan) were added to the slurry as binder and plasticizer and ball-milled at low speed to get a homogeneous slurry for tape cast. The slurry was debubbled in a planetary vacuum mixer (THINKY, Japan) before casting on a silicone coated polyethylene substrate using homemade two doctor blade apparatus. The as-prepared green tapecast sheets were immediately transferred into fridge and stored at 4 °C to slow the dry process of as-prepared films. Then the green tape-cast
3. Results and discussion Fig. 1 compares XRD patterns of the LAGTP-10 wt% TiO2 composite and LAGTP without TiO2 sintered at 950 °C for 7 h. The main diffraction peaks indicated both materials were NASICON-type structure (R3C). Impurity phases of LiTiOPO4 and TiO2 were emerged for LAGTP-10 wt % TiO2 with addition of TiO2, while GeO2 and AlPO4 were related to the sintering process of LAGTP and observed in both cases. Rietveld refinement in Fig. S2 reveals that the c lattice parameter for LAGTP increased and the a lattice decreased slightly by the addition of 10 wt% TiO2. These results suggest that large ionic radius Ti4+ (0.745 Å) was substituted for small ionic radius Al3+ (0.675 Å) and/or Ge4+ (0.670 Å) with TiO2 addition. Thermal analysis of the tape cast green sheet in Fig. S3 showed a weight decrease up to 450 °C and then no change of weight. Therefore, the solvent, binder and plasticizer in the green sheets were decomposed at 500 °C for 5 h and then sintered at 950 °C for densification. Both sintering temperature and period were selected to obtain the highest lithium-ion conductivity of the film before sample characterization. The obtained LAGTP-10 wt% TiO2 film had no cracks, as shown in Fig. 2 (a) and (b), but small pores was observed. The grain size was in a range of 128
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Fig. 1. XRD patterns of LAGTP (red) and LAGTP-10 wt% TiO2 (black) tape cast films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
strength, (6) ease of thin film preparation. The stability of the NASICON-type doped LiTi2(PO4)3 in water was already confirmed in aqueous solution with a high content of Li+ [10,20,21]. Although the lithium-ion solid conductor with Ti4+ was proven to be unstable in contact with lithium metal due to the reduction of Ti4+ by lithium [22], a lithium-stable lithium-ion conducting electrolyte buffer layer, such as a non-aqueous liquid electrolyte [5,6] or polymer electrolyte [23] can be served a separator between lithium metal and the solid electrolyte. Water impermeability through the lithium-ion conducting thin solid electrolyte film is an important requirement for the separator in aqueous lithium batteries. Fig. 3 shows the water permeation test results obtained at room temperature for the 90 μm thick films of LAGTP-10 wt % TiO2 and LAGTP-10 wt% TiO2 with around 2 wt% epoxy resin, where
2.5–1.0 μm. While the LAGTP without TiO2 showed extremely large grain and the contact between grains was insufficient, as shown in Fig. 2(c).This phenomenon suggests that the added TiO2 could suppress LAGTP grain growth at relatively high temperature. EDX maps of the Ti, Al, Ge, and P contents showed a homogeneous distribution of all elements over the cross-section of the LAGTP-10 wt% TiO2 film, as shown in Fig. 2(d). It indicates that the added TiO2 in the LAGTP-10 wt% TiO2 composite were dispersed homogeneously on the grain boundary or in the lattice sites in LAGTP. The requirements for the separator between the lithium electrode and the aqueous electrolyte in aqueous lithium batteries are as follows: (1) stable in water, (2) resist to lithium reduction, (3) water-impermeable, (4) high lithium-ion conductivity, (5) high mechanical
Fig. 2. (a, b) Cross-sectional SEM images for the 90 μm thick LAGTP-10 wt% TiO2 film (c) cross-sectional SEM images of pure LAGTP film (d) EDX maps of the 90 μm thick LAGTP-10 wt% TiO2 film. 129
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Fig. 3. Water penetration test results for the 90 μm thick LAGTP-10 wt% TiO2 (red line) and LAGTP-10 wt% TiO2-epoxy (black line) films. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
TiO2 film with 2 wt% epoxy resin are presented in Fig. 5. The total conductivities of LAGTP-10 wt% TiO2 and LAGTP-10 wt% TiO2 with 2 wt% epoxy resin at 60 °C were 2.28 × 10−3 and 1.78 × 10−3 S cm−1, respectively. The activation energy of the bulk conductivity for both electrolytes was 12.1 kJ mol−1, which is less than 20.2 kJ mol−1 for LAGTP reported previously [13]. The activation energy of the grain boundary conductivity for LAGTP-10 wt% TiO2 with epoxy resin was higher than that of LAGTP-10 wt% TiO2 and that of LAGTP reported previously, which may result from higher content of impurity phases at grain boundary [16]. The contribution of an electron and/or hole conduction in the LAGTP-10 wt% TiO2 film was also estimated using the potentiostatic polarization method [27,28]. A bias voltage of 0.25 V was applied on the Au/LAGTP-10 wt% TiO2/Au cell and the steady current of 10 nA cm−2 was recorded in Fig. S4. The result suggests that the contribution of the electronic conductivity may be negligibly small to the ionic conductivity. Another important requirement to emphasize is the mechanical strength of the solid electrolyte, as its robustness accounts for the ability of preventing water leakage and battery safety. Previously, the highest 3-point bending strength of a 200–300 μm thick tape cast LAGTP film in aqueous lithium air battery was reported to be 110 N mm−2 [16]. Fig. 6(a) shows the average 3-point bending strength of the several tape-casted films of LAGTP-x wt% TiO2 as a function of the TiO2 content, where the thickness of the tape cast films was approximately 500 μm. The bending strength increased with the TiO2 content and 210 N mm−2 was recorded for LAGTP-10 wt% TiO2. This bending strength is comparable to that of the sample sintered from a pressed tablet of LAGTP-10 wt% TiO2 by sol-gel method, indicating the modified solid state method and tape casting are feasible to the mechanical strength improvement of LAGTP [14]. Fig. 6(b) shows dependence of the bending strength on the thickness of the LAGTP-10 wt% TiO2 tape cast film. No significant dependence on thickness was observed in the range from 50 to 500 μm. The bending strength of LAGTP-10 wt% TiO2 is comparable with that of beta-alumina (140–240 N mm−2), which is used as a separator for sodium sulfur rechargeable batteries [29]. To demonstrate the advantages of 90 μm solid electrolytes in actual aqueous lithium-air batteries, the LAGTP-10 wt% TiO2–epoxy composite film was assembled in laminate-type cell, of which isostatic pressing and vacuum zipping were used to ensure the close contact of each part and water-tightness. The impedance profile of the Li/4.5 M LiFSI in
the contact area with the liquid was 3.14 cm2. LAGTP-10 wt% TiO2 without epoxy resin exhibited a linear increase of chloride ion content with time, while LAGTP-10 wt% TiO2 with epoxy resin showed no increase of chloride ion content for 24 h in regardless of thin thickness of less than 100 μm. This result confirms the 90 μm thick LAGTP-10 wt% TiO2 film with around 2 wt% epoxy resin is water-impermeable. One of the other important requirements for the separator in lithium aqueous batteries is high lithium-ion conductivity. To study the influence of TiO2 and epoxy on conductivity, gold electrode was sputtered on both sides of LAGTP-10 wt% TiO2 and those with 2 wt% epoxy resin. The impedance spectra of both pellets are presented at 25 °C in Fig. 4, where the resistances were normalized with the gold surface area of 0.38 cm2. Both spectra show a high frequency semicircle followed by a straight line at low frequency. The semicircles are attributed to the grain boundary resistance and the straight line to the Warburg diffusion process according to the analysis in previous reports [24]. As the semicircle that represents the bulk resistance was out of the frequency of the analyzer, the left intercept of the semicircle simulated can be considered to be the bulk resistance of the samples for simplicity. The estimated bulk, grain boundary and total conductivities are listed in the table of Fig. 4. A total conductivity of 8.70 × 10−4 S cm−1 at 25 °C for the LAGTP-10 wt% TiO2 is comparable to that of LAGTP prepared by solid state method at 900 °C [18]. The lithium-ion conductivity of LAGTP-10 wt% TiO2 was decreased to 4.45 × 10−4 S cm−1 by the addition of epoxy resin but it is still reasonable compared to Ohara LATP of ~1 × 10−4 S cm−1. As the bulk conductivities of LAGTP-10 wt% TiO2 with and without epoxy resin were almost the same, the low conductivity of the film with epoxy resin may be due to the formation of a highly resistive phase with epoxy resin at the grain boundaries. Owing to the reduced thickness, the area specific resistance of the 90 μm thick LAGTP-10 wt% TiO2-epoxy film was as low as 21.0 Ω cm2, which is even lower than that of the solid electrolyte interphase (SEI) between lithium metal and a conventional electrolyte in a lithium-ion battery, such as with LiPF6 in EC-DEC at 25 °C (around 100 Ω cm2) [25] and between lithium metal and polymer electrolyte at 60 °C (around 100 Ω cm2) [26]. Therefore, the resistance of the LAGTP-10 wt% TiO2 film with epoxy resin may contribute to only a small part of the total cell resistance and less energy loss for the aqueous lithium batteries. The temperature dependence of the total, bulk, and grain boundary conductivity of the LAGTP-10 wt% TiO2 film and the LAGTP-10 wt% 130
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Sample
Bulk
Grain boundary
Total conductivity
conductivity
conductivity (S cm-1)
(S cm-1)
(S cm-1) LAGTP-10 wt% TiO2
1.8410-3
1.6810-3
8.7010-4
1.8410-3
5.8710-4
4.4510-4
LAGTP-10 wt%TiO2 -epoxy
Fig. 4. Impedance profiles for the Au/LAGTP-10 wt% TiO2/Au and LAGTP-10 wt% TiO2 with epoxy resin (ca. 2 wt%)/Au cells at 25 °C. The thickness of the electrolyte was 90 μm.
Fig. 5. Temperature dependence of the bulk, grain boundary and total conductivity for (a) LAGTP-10 wt% TiO2 and (b) LAGTP-10 wt% TiO2 with 2 wt% epoxy resin.
electrolyte. These values were derived by the simulation of the equivalent circuit shown in Fig. 7(a). The electrolyte resistance (R1 + R2) of 34.6 Ω cm2 is significantly lower than previous reported aqueous lithium-air batteries with a 210 μm thick Li1.5Al0.5Nb0.2Ti1.3(PO4)3 (~120 Ω cm2) [19], a 250 μm Ohara LATP (~400 Ω cm2) [31], and a 180 μm thick LAGTP (~180 Ω cm2) [16]. The charge transfer resistance of 47.5 Ω cm2 was comparable to that of 40 Ω cm2 for Li/LiFSI in tetraethylene glycol dimethyl ether (G4) [25]. Owing to the reduced thickness of solid electrolyte, the total cell
DME/LAGTP-10 wt% TiO2-epoxy/10 M LiCl-1.5 M LiOH/MnO2, air cell at 25 °C is shown in Fig. 7(a). The impedance profile consists of three semicircles at different frequencies; the semicircle at high frequency can be assigned to the grain boundary response of the solid electrolyte (R2) and middle frequency range one to the interface resistance between the lithium metal electrode, liquid electrolyte and solid electrolyte (R3), while the one in the low frequency range may relate to the charge transfer resistance (R4) [30]. And the series resistance R1 is the total resistance of the solid electrolyte's bulk and the liquid buffer 131
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Fig. 6. (a) Three-point bending strength of ca. 500 μm thick LAGTP-x TiO2 film as a function of TiO2 content, and (b) 3-point bending strength of the LAGTP-10 wt% TiO2 film as a function of thickness. Fig. 7. (a) Impedance profile of the Li/ 4.5 M LiFSI in DME/90 μm thick LAGTP10 wt% TiO2-epoxy resin/10 M LiCl-1.5 M LiOH aqueous solution/MnO2, air cell at 25 °C. (b) Short term and (c) long term charge and discharge cyclic performance of the Li/4.5 M LiFSI in DME/90 μm thick LAGTP-10 wt% TiO2-epoxy resin/10 M LiCl1.5 M LiOH aqueous solution/MnO2, air cell at 25 °C and 1.0 mA cm2.
resistance of 240 Ω cm2 was achieved, which is compared with that of 320 Ω cm2 reported previously [16]. This cell resistance is acceptable for a high specific power density lithium-air battery because the contribution of the cell potential drop by the resistance at 1.0 mA cm2 is less than 10%. Moreover, we note that the cell resistance could be further reduced by selecting a buffer electrolyte with a lower SEI resistance [25]. In addition, the cyclic performance of the aqueous lithium cell using the 90 μm thick LAGTP-10 wt% TiO2 with epoxy resin at 1.0 mAh cm−2 and 25 °C under open air is shown in Fig. 7(b) and (c). An excellent cyclic performance was observed in a short cycle. The round trip cell voltage change is only 0.5 V at 1.0 mAh cm−2, which is almost same to the ohmic drop regarding to the impedance spectra. This overpotential is also significantly lower, compared to 0.6 V at 0.4 mAh cm−2 reported previously for the lithium-air battery with 180 μm thick
LAGTP [16]. The charging potentials were gradually increased by cycling, of which may be due to degradation of the lithium electrode and carbon based air cathode [31,32]. The cell performance was considerably improved by the use of a high conductivity thin solid electrolyte. This result suggests that the 90 μm thick LAGTP-10 wt% TiO2 film with epoxy resin could be acceptable as the separator in aqueous lithium batteries because of its high lithium-ion conductivity and enhanced mechanical strength. 4. Conclusions A 90 μm thick LAGTP-10 wt% TiO2 lithium-ion conducting solid electrolyte was fabricated using the tape-casting method. The thin film with around 2 wt% epoxy resin was confirmed to be water132
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impermeable. The specific area resistances of the LAGTP composite film without and with epoxy resin were 9.5 and 20.5 Ω cm2, respectively. The mechanical strength of the LAGTP film was significantly improved by the addition of TiO2. The 3-point bending strength of the film was increased from 50 N mm−2 for the LAGTP film without TiO2 to 210 N mm−2 for the LAGTP with 10 wt% TiO2. The feasibility of the thin film electrolyte for an aqueous lithium-air cell was demonstrated. These optimized features enables its wide application in the field of aqueous lithium secondary batteries, such as aqueous lithium-air batteries and also aqueous Li/MCl2 (M = Co, Ni) batteries (Fig. S5).
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