Characterization of lithium-rich garnet-type Li6.5La2.5Ba0.5ZrTaO12 for beyond intercalation chemistry-based lithium-ion batteries

Solid State Ionics 318 (2018) 71–81

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Characterization of lithium-rich garnet-type Li6.5La2.5Ba0.5ZrTaO12 for beyond intercalation chemistry-based lithium-ion batteries

T

Kyle Hofstetter, Alfred Junio Samson, Venkataraman Thangadurai⁎ Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada

A R T I C L E I N F O

A B S T R A C T

Keywords: Li-O2 Li-garnet interface, chemical stability of garnets Li charge transfer impedance Li-symmetrical cell

Li-rich garnet-type Li6.5La2.5Ba0.5ZrTaO12 (LLBZT) electrolyte is characterized as a Li protecting layer for potential application in aqueous Li-O2 battery. AC impedance spectroscopy and DC electrical measurements, high temperature powder X-ray diffraction (HT-PXRD), scanning electron microscopy (SEM) and thermogravimetic analysis (TGA) were used to investigate the electrochemical and chemical properties of Li/LLBZT and LLBZT/ aqueous interfaces. Stable open circuit voltage (OCV) of ~3 V was observed for Li/LLBZT/0.1 M LiOH, Li/ LLBZT/1 M LiOH and Li/LLBZT/1 M LiCl at 25 °C. DC galvanostatic Li plating/stripping cycle at varying current density was performed and the area specific polarization resistance (ASR) for Li+ ion charge transfer was found to be 473 Ω cm2 at 25 °C. The impedance of LLBZT was found to be improved after treating the samples with 1 M LiOH, and 1 M LiCl, and retains its crystal structure and electrochemical stability with Li; thus, Li-rich LLBZT garnet can be successfully employed in next generation beyond Li-ion batteries.

1. Introduction At present, energy storage from renewable sources such as solar, wind and hydro energy plays a vital role in the protection of planet earth from global warming due to greenhouse gas emissions. Various electrochemical devices, including fuel cells (energy conversion from fuels), battery (energy storage in the form of chemical energy), capacitors (directly stores electricity in the form electrical charge) and reverse fuel cells (convert the fuel cells by-products such as CO2 and water to fuels) have been developed. Among them, batteries have drawn much attention due to their ability to provide power, ranging from a pacemaker, portable electronics (10–100 Wh), long-range electric vehicles (20–90 kWh) and support peak power demand at the grid (MWh) [1]. Currently, several battery chemistries, including Ni-Cd, NiMH, lead-acid and C-LiCoO2 are being considered. Lead-acid, Ni-MH and Ni-Cd batteries lack the specific energy density due to low energy output [2]. State-of-the-art Li-ion batteries, based on organic polymer electrolytes, lack the ability for high cyclability, chemical and electrochemical stability, and pose safety concerns due to flammability and prone to explosion [3–5]. To overcome these hurdles of organic polymer-membrane based Li-ion batteries, high-temperature stable battery chemistry needs to be developed, and are targeted to reach energy density comparable to that of gasoline. Replacing flammable organic polymers with solid-state (ceramic) electrolytes can eliminate the long-term safety issues in the current Li-ion battery chemistry [6,7].

⁎

Conventional Li-ion batteries rely on intercalation mechanism, i.e., the insertion and removal of lithium ions from anode and cathode structures. Although, there are continuous increases in the conventional Li-ion batteries' energy densities in the last decades, since Sony Corporation's commercialization of Li-ion batteries in 1990's, it is widely assumed that the current lithium ion technology will reach its fundamental/intrinsic limits in terms of specific energies and energy densities soon. Thus, in recent years, there is intensive evaluation and development of Li-ion batteries beyond intercalation chemistry. This chemistry is based on conversion chemistry rather than traditional intercalation chemistry, which can allow for increased energy densities. For example, Li-O2 and Li-S cells can provide theoretical energy densities that can compete with gasoline [8,9]. Li-O2 cells have theoretical energy density of about 10 times that of state-of-the-art Li-ion polymermembrane, graphite anode and LiCoO2 cathode based batteries [10]. This high specific energy density is a combination of oxygen not being stored on board and metallic lithium as the anode material rather than lithiated graphite [11]. The battery chemistry of Li-O2 typically relies on a four-electrons process. However, various chemistry processes can occur depending on the chemical composition and pH of the electrolyte of the cell [6,9,12]. Furthermore, their practical capacity is also found to be much lower than expected theoretical values. To improve the performance of Li-O2 and Li-S cells, novel electrode materials with high electrochemical activity and huge surface area are being investigated [13,14]. One of the

Corresponding author. E-mail address: [email protected] (V. Thangadurai).

http://dx.doi.org/10.1016/j.ssi.2017.09.005 Received 22 February 2017; Received in revised form 16 August 2017; Accepted 13 September 2017 Available online 18 October 2017 0167-2738/ © 2017 Elsevier B.V. All rights reserved.

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2. Experimental Li-rich garnet-type Li6.5La2.5Ba0.5ZrTaO12 (LLBZT) was prepared using conventional ceramic method using stoichiometric quantities of LiNO3 (99%, Alfa Aesar), La2O3 (99.99%, Alfa Aesar) (dried at 900 °C for 12 h), Ta2O5 (99%, Alfa Aesar) ZrO2 (99%, Alfa Aesar) and Ba (NO3)2 (98%, Alfa Aesar). 10 wt% excess LiNO3 was added to compensate for lithium oxide volatilization during high-temperature sintering treatment. This composition has previously been synthesized and characterized in our laboratory [24,25]. The synthesis process involved the conventional heating and ball milling steps. Planetary milling (Pulverisette, Fritsch, Germany) was used at a spinning rate of 200 rpm for 6 h using 2-propanol to ensure homogeneous mixing of the powders. Milling was performed before and after decomposition of metal nitrates. Nitrates were burned off by firing powder at 700 °C for 6 h. The resultant powders were pressed into pellets using an isostatic press and placed on a powder bed and covered with mother powder in a clean alumina crucible. Final sintering process involved 2 steps, 900 °C for 24 h and a final sintering of 1100 °C for 6 h in ambient atmosphere. Ex-situ Powder X-ray diffraction (Powder X-ray Diffractometer, Model: Bruker D8 Advance) (Cu Kα, 40 kV, 40 mA) confirms the formation of garnet-type LLBZT. Typically, measurements were performed from 2θ range 10° to 80° at a count rate of 4 s per step of 0.025° at room temperature. In-situ PXRD measurements using a high-temperature reactor chamber (Anton Paar XRK 900) in air were acquired from 2θ range 10° to 80° at a count rate of 3 s per step of 0.02°. Fig. 1 shows the schematic representation of the electrochemical cell used to investigate the stability of Li-rich LLBZT in various media and also with elemental Li. AC impedance spectroscopy (Solartron Model No: 1260; 0.1 Hz1 MHz; 100 mV) was used to investigate the electrical conductivity of the samples. Highly porous gold blocking electrodes were used as current collectors. Commercially available gold paste obtained from Heraeus Inc., Germany (LP A88-11S) was coated on the surface of pellets and cured at 700 °C for 1 h to remove the organic binder. Porosity of gold layer was confirmed with scanning electron microscopy (Zeiss Sigma VP), with pores on the order of 10–20 μm (Fig. S1 Supporting information). The stability of LLBZT in contact with Li metal was investigated under an argon-filled glove box (Innovative Technology, Inc.). A crucible shaped sample of LLBZT was fabricated by isostatically pressing a powder sample of LLBZT inside a polymer mold made in-house, with a load of 200 kN. The powder was pre-sintered at 900 °C for 12 h in air and then ball milled for 6 h. The crucible shaped sample was covered with the same powder and sintered at 1100 °C for 12 h. A schematic of the sample and the setup for stability experiments is shown in Fig. 1a. Lithium granules (99%, Alfa Aesar) were softened on top of a stainlesssteel foil at ca. 180 °C and then the crucible shaped LLBZT filled with Li granules for melting was placed on top of the softened Li. To ensure that the surface of LLBZT was free of interference from surface contaminants during the preparation, several cycles of melting, removal, and refill of fresh lithium was performed prior to the measurements. DC measurements (galvanostatic cycling), specifically Li plating/stripping at varying constant current at room temperature were performed using a Solartron 1287 electrochemical interface. Impedance spectroscopy was performed at open circuit voltage (OCV) before and after Li plating/ stripping using a Solartron 1260 impedance analyzer from 0.1 Hz–1 M Hz at an amplitude of 100 mV. For Li/LLBZT/aqueous, crucible shape LLBZT was used and outside of the crucible was coated with porous gold layer as shown in Fig. 1b. Conducting carbon electrode was immersed in the solution. OCV between Li and carbon electrode was measured using a potentiostat (PARSTAT 4000, Princeton applied research). Adhering 1 mm (thickness) pellet to hollow quartz tube cylinders was used to perform aqueous stability of LLBZT in deionized water, D2O, 1 M LiOH and 1 M LiCl. Porous gold electrodes were coated at the surface of a pellet to serve as a current collector (Fig. S1–Supporting information). These tubes were suspended in a 20 ml

Fig. 1. Schematic representation of experimental setup (a) Li/Li6.5La2.5Ba0.5ZrTaO12 (LLBZT)/Li; (b) Li/LLBZT/aqueous; and (c) aqueous/LLBZT/aqueous solution cell used for characterization of Li-rich garnet-type LLBZT for Li-aqueous battery. The garnet phase was used in the form of crucible for Li symmetrical cell (a) and for EMF tests (b). In (c) the Au current collector is coated on the surface of the pellet and Au wire is attached. The Au layers are highly porous based on SEM imaging.

key challenges is instability of elemental lithium towards the electrolyte in the Li-O2 and Li-S batteries. Lithium reacts with organic polymer membranes in non-aqueous [15] and aqueous electrolyte Li-O2 cells [11]. Reactions with polysulphide discharge products have been wellknown in the Li-S batteries [16,17]. Development of suitable solid electrolytes will be the key to allowing for successful electrochemical reactions to occur in both Li-O2 and Li-S batteries. For aqueous Li-O2 batteries, the electrolyte needs to have high Li-ion conductivity and stability in aqueous solutions and in contact with elemental lithium [6]. Among the various known solid Li-ion electrolytes, perovskite-type (Li,La)TiO3, NASICON-type Li1 + (x + y)Ti2 − xAlxP3 − ySiyO12, LISICON-type Li2 + 2xZn1 − xGeO4 and garnet-type Li5La3Ta2O12 and Li7La3Zr2O12 have been considered for all-solid-state batteries [7,18,19]. However, Ti-based NASICON and perovskites structure solid electrolytes lack the stability against lithium metal due to the reduction of Ti4 + to Ti3 + [6,11,20], which leads to electronic conduction in the electrolyte. LISICON structured Li2 + 2xZn1 − xGeO4 are also not stable with elemental lithium due to the reduction of Ge4 + [6]. Recently developed, Zr and Ta-based garnet-type structure metal oxides are found to be stable against direct contact with elemental Li, making them a strong candidate for the use in Li-O2 battery chemistry [21,22]. Here, we report garnet-type structure Li6.5La2.5Ba0.5ZrTaO12 (LLBZT) as a separator for elemental lithium and an aqueous electrolyte in a typical aqueous Li-O2 battery architecture. LLBZT is referred as Lirich or Li-stuffed garnets because it contains more lithium than that can be accommodated in a classical garnet Li3Ln3Te2O12 (Ln = Y, Pr, Nd, Sm-Lu) [23]. Chemical and electrochemical stability of LLBZT were investigated using electrochemical AC impedance and DC methods under various solutions including deionized water, D2O, 1 M LiOH, and 1 M LiCl, to explore its application in Li-aqueous battery.

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73

Ceramic synthesis; 1200 °C for 36 h Ceramic synthesis; 1000 °C for 20 h Ceramic synthesis; 1000 °C for 20 h Ceramic synthesis; 1230 °C Sol gel; 1150 °C for 36 h

Ceramic synthesis; 1180 °C for 36 h

Ceramic synthesis; 1180 °C for 36 h

Ceramic synthesis; 1100 °C for 12 h

Ceramic synthesis, 1100 °C for 12 h Ceramic synthesis, 1050 °C for 12 h Ceramic synthesis, 1050 °C for 12 h Ceramic synthesis; 1100 °C for 12 h

Li6.75La3(Zr1.75Nb0.25)O12, (d = 1.3 cm, t = 0.2 cm) (Li| garnet | Au) Li6.625La3Zr1.625Ta0.375O12 with 29 mol% Al content (d = 2.0 cm, t = 0.5 cm) Li6La3ZrTaO12 (d = 2.0 cm, t = 0.5 cm) Li7La3Zr2O12 with 28 mol % Al, (d = 2.0 cm, t = 0.5 cm)

LLZ (d = 1.0 cm, t = 0.1 cm)

0.5 wt% Al2O3–doped LLZ (d = 1.0 cm, t = 0.1 cm)

Li5.98Al0.33La3Zr1.95O11.89 (d = 0.78 cm, t = 0.1 cm) (Grain size: 20–40 μm)

Li5.98Al0.33La3Zr1.95O11.89 (d = 0.78 cm, t = 0.1 cm) (Grain size: 100–200 μm) Li6.85La2.9Ca0.1Zr1.75Nb0.25O12

Li6.5La2.5Ba0.5ZrTaO12 (LLBZT)

Si-coated Li6.85La2.9Ca0.1Zr1.75Nb0.25O12

Diameter; tthickness;

d

Ceramic synthesis; 1230 °C for 36 h

Li7La3Zr2O12 (LLZ), pellet dimensions are not reported

Li6.75 − xLa3Zr1.75Nb0.25 O12 − 0.5x with 0.46 wt% Al2O3 (d = 1.0 cm, t = 0.1 cm)

Solid electrolyte preparation condition

Li | garnet | Li (pellet dimension) CV (−0.2 to 0.4 V, 10 mV min− 1); dissolution and deposition reactions of Li were observed reversibly. Chronopotentiometry (10 to 50 μA cm− 2); Up to 10 μA cm− 2, the dissolution and deposition curves gave the mirrored relationship at least until 600 s. CV (− 0.5 to 9 V, 1 mV s− 1); Li deposition and dissolution peaks are observed near 0 V vs. Li+/Li, CV (−0.1 to 0.1 V, 1 mV s− 1); Linear behavior indicates reversibility of the electrode process. CV (−0.1 to 0.1 V, 1 mV s− 1); Linear behavior indicates reversibility of the electrode process. CV (−0.1 to 0.1 V, 1 mV s− 1); Linear behavior indicates reversibility of the electrode process. EIS monitoring for long-term stored samples (up to 5 months); increase in interfacial resistance suggests that the Nb in the compound that is in contact with Li may be reduced slightly. EIS monitoring for long term stored samples (up to 1 month); The resistance of the cell decreased with storage period for the first one week and then became stable for one month at room temperature Chronopotentiometry (0.5 mA cm− 2); Abrupt drop in cell voltage after 122 s of polarization EIS monitoring for long term stored samples (up to 1 month); The resistance of the cell decreased with storage period for the first one week and then became stable for one month at room temperature Chronopotentiometry (0.5 mA cm− 2); Abrupt drop in cell voltage after 1000 s of polarization Galvanostatic cycling (up to 134 μA cm− 2); The potential of the cell remains constant at different current densities and increased linearly at higher current densities up to 134 μA cm− 2. Above this value, the cell exhibited voltage instability and shortcircuited. Galvanostatic cycling (up to 90 μA cm− 2); The cell shorted during the 2 h period at the current density of 90 μA cm− 2. Galvanostatic cycling (0.05 mA cm− 2); Voltage hysteresis is large and the plating/ stripping curves are unstable Galvanostatic cycling (up to 0.2 mA cm− 2); Voltage profiles exhibited flat and stable plating and stripping curves with small over-potential. Voltage profile remained stable after cycling for 225 h (0.1 or 0.05 mA cm− 2) Galvanostatic cycling up to 169 μA cm− 2

1.8 × 10− 4

1.5 × 10− 4

2.5 × 10− 4

2.5 × 10

−4

2.0 × 10− 4

2.5 × 10− 4

4.12 × 10− 4

2.33 × 10− 4

5.69 × 10− 4

3.5 × 10

−4

2.6 × 10− 4

5.2 × 10− 4

8.0 × 10− 4

Stability testing and comments

σ25 °C (S cm− 1)

Table 1 Summary of Li/Garnet/Li cell, solid electrolyte preparation, electrochemical stability tests and ASR.

2

473 Ω cm2

127 Ω cm2

925 Ω cm

2

130 Ω cm2

37 Ω cm2

Not mentioned

Not mentioned

300 to 492 Ω cm2

1398 Ω cm

551 Ω cm2

99 Ω cm2

Not mentioned

4400 Ω

Interfacial resistance between the Li and electrolyte

This study

[29]

[29]

[31]

[31]

[30]

[30]

[28]

[32]

[22]

[22]

[27]

[26]

Reference

K. Hofstetter et al.

Solid State Ionics 318 (2018) 71–81

Solid State Ionics 318 (2018) 71–81

K. Hofstetter et al.

Fig. 2. (a) Plating and stripping Li using a symmetrical cell: Li/Li6.5La2.5Ba0.5ZrTaO12 (LLBZT)/ Li at different current densities at 25 °C, (b) Arrhenius plot of bulk Li ion conductivity for LLBZT, and (c) open circuit AC impedance (plots obtained for symmetrical cell before and after the cycling test.

A linear fit of V vs. I of the data in Table 2 shows total resistance (R) of 3200 Ω at 25 °C. The bulk resistance of the solid electrolyte LLBZT was obtained from Li+ ion conductivity (Fig. 2). The bulk resistance of the electrolyte was found to be 1308 Ω at 25 °C. Thus, the ASR for Li ion charge transfer, for symmetrical cell with area of 0.5 cm2, can be estimated as: {(3200–1308)/2}0.5 = 473 Ω cm2. The factor 2 divided since a symmetrical cell was used. Another way to estimate the ASR is by looking at the difference in the total resistance, obtained through electrochemical ac impedance spectroscopy of Li non-blocking cell: Li| LLBZT |Li (Fig. 2c). The total cell resistance of ca. 3500 Ω is found to be comparable to the estimated resistance from the galvanostatic cycling. By subtracting the bulk resistance of the electrolyte (1308 Ω), and dividing by a factor of 2, the interfacial resistance of Li| LLBZT is 1096 Ω, and the ASR is 548 Ω cm2 which is slightly higher than that of DC ASR value. Fig. 2b shows that the interfacial resistance significantly increased after galvanostatic cycling. The higher charge transfer ASR value in the present study could be attributed to potential interface reaction products and the contact resistance between Li and LLBZT. Recent studies using interfacial chemical modification by employing Si [29], ZnO [33], Al2O3 [34] and LiF [35] show much lower Li+ ion charge transfer ASR value between Li metal and Li-garnet LLZ or LLZ and polymer composites. The present value is comparable to that literature value where no surface modification was performed on solid electrolyte surface (Table 1). Further decrease in the Li+ ion charge transfer impedance can also be obtained using a thin solid electrolyte, while the present work utilized rather thick sample (about 2–3 mm).

Table 2 DC electrical properties of Li | LLBZT| Li symmetrical cell at 25 °C. Current density (A cm− 2)

Voltage (V)

Current (A)

39 × 10− 6 84 × 10− 6 169 × 10− 6

0.08 0.17 0.25

1.98 × 10− 5 4.24 × 10− 5 8.48 × 10− 5

vial then; the desired solution was poured into the tube as well as into the vial, as shown in Fig. 1c. AC impedance of symmetrical aqueous cells was recorded for 10 days. 3. Results and discussion 3.1. Chemical and electrochemical stabilities of Li6.5La2.5Ba0.5ZrTaO12 (LLBZT) with elemental Li Several groups have investigated the stability of various garnet-type materials, especially with Li-rich Li7La3Zr2O12 (LLZ) and related structure oxides, in contact with Li metal [22,26–31]. Table 1 shows a summary of selected Li-rich garnet-type compositions and their chemical stability and interfacial Li+ ion charge transfer area specific polarization resistance (ASR) between Li and solid electrolyte [22,26–32]. Li garnets can have wide electrochemical stability window (ESW) up to 6–9 V/Li [7,27]. In one of the particular study of the composition, Li6.75La3Zr1.75Nb0.25O12, cyclic voltammetry reveals Li deposition and dissolution peaks near 0 V vs. Li+/Li but indicates no other electrochemical reactions up to 9 V vs. Li+/Li [27]. Fig. 2a shows the charging and discharging cycles of presently studied Li-rich garnet-type LLBZT under different current densities at 25 °C. The Li plating and or stripping time were set to one minute. A similar voltage hysteresis and the unstable plating/stripping curves were reported by Luo et al. [29]. The ASR value for Li+ ion charge transfer can be estimated from the initial over-potentials associated with each current density.

3.2. Chemical and electrochemical stabilities of Li6.5La2.5Ba0.5ZrTaO12 (LLBZT) with aqueous solutions Using AC impedance spectroscopy, we have studied the bulk ionic conductivity stability of Li-rich LLBZT in deionized water, D2O and aqueous Li+ solutions at room temperature to further explore its chemical stability and its application in beyond Li ion intercalation electrode-based battery. Fig. 3 shows the variation of conductivity of LLBZT 74

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time. After 10 days, the bulk impedance of the samples was found to follow the order: 1 M LiOH < 1 M LiCl < D2O < H2O at room temperature. The bulk impedance was found to have a small variation in LiCl and LiOH compared to H2O and D2O. It is important to note that Li-rich garnets are known to undergo fast proton exchange in water and in aqueous LiOH/LiCl and deuterium exchange in D2O [36–38]. A slight increase in the impedance in D2O compared to water indicates potential proton migration in water since the mobility of ions depends on charge and mass of the mobile species. The improvement in the bulk ionic conductivity for LLBZT in the aqueous medium with time may be considered due to increase in mobile charge carriers. We believe that either partial exchange of protons in Li garnets may change the mobile path of Li ions that seem to increase the electrical mobility of Li ions in the garnet-type structure. However, proton contribution to total conductivity cannot be ruled out. Truong et al. have previously shown the change in electrical conductivity of Li5La3Nb2O12 garnet structure in humidified N2 and D2O + N2 atmospheres and suggested proton migration in Li-stuffed Li5La3Nb2O12 garnet [36]. Li-rich garnet structures are known to show reversible Li+/H+ ionexchange in water and organic acids [37,39,40]. Similar ion-exchange reaction was also reported by Chi et al., where a 63.6% proton exchange in LLZ was fully replaced by Li + after immersing in a 2 M LiOH [41]. Truong and Thangadurai also showed the reversible Li + ion exchange for proton in Li6La2BaNb2O12 using statured LiNO3 [42]. Shimonishi et al. reported that LLZ is stable in saturated LiCl for 1 week using PXRD and AC electrochemical impedance spectroscopy [43]. Similarly, Ishiguro et al. for Nb-doped LLZ, where PXRD and AC impedance profiles showed rather small change for pellets immersed in saturated LiOH and LiCl compared to pristine sample [28]. It should be noted that most of the studies report the samples after exposed to the solution, while the current studies report the real-time change in the AC impedance in various solutions. Nonetheless, the slight change in the impedance under aqueous Li solution is found to be very consistent with literature on similar Li-rich garnets [27,39,40,41]. To further understand the chemical/structural stability of Li-rich

Fig. 3. Variation of electrical conductivity obtained at 1 MHz for Li6.5La2.5Ba0.5ZrTaO12 under H2O, D2O, 1 M LiOH and 1 M LiCl at room temperature.

obtained using a 1 MHz impedance value in H2O, D2O, 1 M LiOH, and 1 M LiCl for 10 days (see experimental set up in Fig. 1c). The shape of the impedance plots is found to be typical for Li-garnets with Au blocking electrodes study (Fig. S2 – Supporting information). In all the cases, low-frequency regime shows a tail due to blocking nature of the electrodes. Under the investigated condition, the bulk impedance decreased in all solutions after the first day. 1 M LiOH and 1 M LiCl solutions showed a gradual decrease in bulk impedance with increasing

Fig. 4. (a) Variation of open circuit voltage for LiOH, Li/ Li/Li6.5La2.5Ba0.5ZrTaO12/0.1 M Li6.5La2.5Ba0.5ZrTaO12/1 M LiOH and Li/ Li6.5La2.5Ba0.5ZrTaO12/1 M LiCl as a function of time. (b) Shows stability of the OCV of cell (a) before and after short circuit testing. (c) Shows the impedance of cell (a). Same garnet sample was used in all three measurements, varying solutions in the sequence 0.1 M LiOH, 1 M LiOH and 1 M LiCl.

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Fig. 5. TGA curves of LLBZT after the chemical stability test using the solutions employed in AC impedance spectroscopy study. Samples heated at 5 °C min− 1 under 50/50 N2/Air. For comparison, TGA of as-prepared sample is also shown. Asterisks (*) indicate temperatures at which insitu variable temperature PXRD were performed.

Cathode side reaction:

garnet-type LLBZT with LiOH and LiCl, variation of open circuit voltage (OCV) of the Li-aqueous cells: Li/LLBZT/0.1 M LiOH, Li/LLBZT/1 M LiOH, and Li/LLBZT/1 M LiCl, was measured as a function of time at room temperature (25 °C), and results are shown in Fig. 4a. Interestingly, the OCV of the cell (~ 3 V vs. Li) was found to be constant over the recorded time and it was found to be highly reproducible and also found to be very reliable when replacing different solutions and returned to the original value after intentional short-circuit test (Fig. 4b). Based on the literature [44], we suggest the following possible half-cell chemical reactions and net reaction to describe the observed voltage: Anode side reaction:

2Li → 2Li+ + 2e−

O2 + H2 O + 2e− → OH− + HO2−

(2)

Overall reaction:

2Li + O2 + H2 O → LiOH + Li+ + HO2−

(3)

Depending upon pH and nature of electrode catalysts, the oxygen reduction reaction (ORR) appears to follow two- electron and/or fourelectron paths in alkaline solution, leading to different reaction products such as Li2O2 (12). The former show OCV of ~3V/Li (Eq. 3) while the latter show ~3.45 V/Li (Eq. 6). The four-electron reaction step may involve the following half-cell chemical reactions and net reaction in alkaline solution [44]:

(1) 76

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Fig. 6. Shown are typical scanning electron microscopy (SEM) images of Li6.5La2.5Ba0.5ZrTaO12 after and before the chemical stability tests. (1) as-prepared, (2) water, (3) D2O, (4) 1 M LiOH (3), and (5) 1 M LiCl. Images (1a)–(5a) show the higher magnification of (1)–(5), respectively.

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3.3. TGA, SEM, and HT-PXRD analysis of Li6.5La2.5Ba0.5ZrTaO12 (LLBZT) after treatment in aqueous solutions The structural stability of LLBZT after exposure to H2O, D2O, 1 M LiOH, and 1 M LiCl for 5 days was investigated using TGA (Fig. 5), SEM (Fig. 6), and powder X-ray diffraction (Fig. 7). All the samples, including pristine, show progressive weight loss suggesting that successive decomposition reactions occur with increasing temperature. Generally, several hydrated metal oxides show catastrophic weight loss over narrow temperature range, below or around 100 °C. The later weight loss shows that there will be more than one-type of structural/ hydrated water, but the former decomposition mechanism can be used to describe potential different sites for hydration. It is known that the lithium ions in garnet occupy tetrahedral and octahedral sites [7,23,40]. We believe that protons in these sites may have different dehydration energy. For compete replacement of Li by protons, i.e., Li+ H+

Li6.5La2.5Ba 0.5ZrTaO12 ⎯⎯⎯⎯⎯⎯⎯→ H6.5 La2.5Ba 0.5ZrTaO12 + 6.5LiOH

(7)

the total proton exchange is anticipated 6.6 wt% loss for the loss of 3.25 mol of water. As previously reported by Yow et al., [36], Ta-doped garnets experience adsorbed water loss around 250 °C, H+ release in the form of H2O around 400–450 °C and CO2 loss above 550 °C. LiOH at the surface of the materials may interact with CO2 to form carbonate, i.e.,

2LiOH + CO2 → Li2 CO3 + H2 O

(8)

We have taken all weight loss up to 550 °C to be from H2O. Eqs. (9)–(12) show the anticipated ion-exchange reaction after 5 days, based on the TGA weight loss experiments. H2 O 5 Days

Li6.5La2.5Ba 0.5ZrTaO12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Li3.13H3.37 La2.5Ba 0.5ZrTaO12

Fig. 7. Powder X-ray diffraction (PXRD) patterns of Li6.5La2.5Ba0.5ZrTaO12 sample after soaking in (a) as-prepared, (b) water, (c) D2O, (d) 1 M LiOH, and (e) 1 M LiCl for 5 days. For comparison simulated PXRD of the parent Li5La3Nb2O12 garnet phase is shown in (f) [25].

D2 O 5 Days

Li6.5La2.5Ba 0.5ZrTaO12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Li 4.5D2 La2.5Ba 0.5ZrTaO12 1 M LiOH 5 Days

Li6.5La2.5Ba 0.5ZrTaO12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Li3.59 H2.91La2.5Ba 0.5ZrTaO12

Anode side reaction:

1 M LiCl 5 Days

4Li → 4Li+ + 4e−

Li6.5La2.5Ba 0.5ZrTaO12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Li1.39 H5.11La2.5Ba 0.5ZrTaO12

(4)

(5)

Overall reaction:

4Li + O2 + 2H2 O → 4LiOH

(10) (11) (12)

It is also noted that as-prepared samples also show ca. 0.5 wt% which is most likely due to carbonate and moisture adsorption during the sample preparation. The second heating and cooling cycles do not show any weight loss, which further support the adsorption of CO2 and moisture (Fig. 5) Furthermore, particle size of the investigated garnet, temperature, and duration of the ion-exchange reaction may also play important role in amount of proton exchange. Scanning electron microscopy images of 1 M LiOH and 1 M LiCl treated samples were found to have different morphology than that of the as-prepared and H2O/D2O soaked samples (Fig. 6). Further experimental work is required to deeply understand the change the morphology and composition resulted phases. The structural stability of the solution treated samples was studied with in-situ PXRD in the temperature range of 30–650 °C. In all cases, the garnet-type structure was retained even up to the maximum temperature of 650 °C (Fig. 8). The weak additional peaks in Fig. 8a–d appear due to sample holder contribution. For comparison, PXRD of an empty alumina sample holder used for HT-PXRD is shown in Fig. 8e. Coupling this with the TGA data, this would enforce that the weight loss was due to adsorbed carbonate and moisture rather than the decomposition of the garnet crystal structure.

Cathode side reaction:

O2 + 2H2 O + 4e− → 4OH−

(9)

(6)

In the present study, it seems to follow the two-electron path with OCV of ~ 3 V for all the three Li-aqueous cells (Fig. 4a). Safanama and Adams argued that lower OCV may be limited by oxygen concentration available within the catholyte solution [44]. This two-electron path was also suggested independently by He et al. [12]. With a lack of an ORR catalyst on the air cathode and/or low oxygen concentration near the cathode/catalyst, a net two-electron electrochemical reduction seems to occur in the present work. Further research is required to confirm the proposed hypothesis in this study. It should also be noted from the steady OCV of ~3 V/Li, potential Li+/H+ exchange in LiOH and LiCl does not affect the thermodynamic potential of the Li cell. Also, absence of short-circuit voltage clearly indicates that the investigated LLBZT garnet sample is free from open porous and is chemically compatible in aqueous phase as well as metallic Li. AC impedance plots of aqueous Li cells are compared with as-prepared sample with similar sample dimension in Fig. 4c. The bulk impedance of pristine and aqueous Li cells were found to be similar, while second semicircle due to grainboundary impedance was dominant in the aqueous cells.

4. Conclusions The present study shows that Li-rich garnet-type Li6.5La2.5Ba0.5ZrTaO12 (LLBZT) was found to be structurally stable after exposure to H2O, D2O, 1 M LiOH, and 1 M LiCl at room temperature. TGA analysis showed a partial exchange of Li ions by protons in LLBZT 78

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Fig. 8. In-situ variable temperature powder X-ray diffraction (PXRD) patterns of Li6.5La2.5Ba0.5ZrTaO12 sample after soaking in (a) deionized water, (b) D2O, (c) 1 M LiOH, and (d) 1 M LiCl for 5 days. For comparison, empty sample holder XRD pattern is shown in Fig. 8e.

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