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Tape casting and sintering of Li7La3Zr1.75Nb0.25Al0.1O12 with Li3BO3 additions
Solid State Ionics 323 (2018) 49–55
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Tape casting and sintering of Li7La3Zr1.75Nb0.25Al0.1O12 with Li3BO3 additions Robert A. Jonson, Paul J. McGinn
T
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Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Solid-state battery Solid electrolyte Li7La3Zr2O12 Tape casting Sintering aid
An approach for tape casting and sintering of Li7La3Zr1.75Nb0.25Al0.1O12 (LLZNbO) sheets suitable for use in solid-state battery development is described. The use of Li3BO3 as a sintering aid in both pellet and cast tape samples is examined. We find the optimal Li3BO3 content for ionic conductivity in pellets is between 1 and 2 wt% Li3BO3, much less than in prior reports. At this level, ionic conductivity is ~2.5 × 10−4 S cm−1 after sintering in an argon atmosphere at 1000 °C for 6 h. 150–175 micron thick LLZNbO + 0.5% Li3BO3 tapes sintered at 1000 °C for 6 h exhibit ionic conductivity values of 2–3 × 10−4 S cm−1.
1. Introduction Modern electronic devices and electrified vehicles rely heavily on the use of Li-ion batteries. As several accidents have shown, safer batteries are desirable, if not essential for more widespread application. A battery with a solid electrolyte would be a safer alternative to current Li-ion technologies utilizing organic liquid electrolytes. The ideal electrolyte for a solid-state Li-ion battery must have a high lithium ionic conductivity, a negligible electronic conductivity, be stable against elemental lithium and other electrode materials, and allow for operation across a wide electrochemical window. Also desirable is stability against air and water at room temperatures. The ready availability of high energy density bulk solid-state batteries (SSB's) would be a breakthrough advance for both battery safety and performance goals. Investigations show that stuffed garnets may satisfy most of the criteria for a solid-state battery electrolyte [1]. In the cubic form, depending on the substitutions, it has conductivity ranging from 2 to 7 × 10−4 S cm−1 [2]. In addition to its high conductivity, LLZO has also been shown to have high chemical and electrochemical stability with lithium metal [1,2]. LLZO pellets with conductivities > 10−4 S cm−1 are routinely produced, but long sintering times at temperatures between 1100 and 1200 °C are typically required. Significant challenges are posed in trying to process dense bodies from Li-rich oxides because of Li losses that occur above ~900 °C in air. To counteract uncontrolled Li loss in pellets researchers typically add additional Li at various steps of grinding/calcination during solid-state processing. It is also common to surround pellets with the mother powder during sintering to further reduce losses from the pellet. Despite this, Li losses occur [3]. One often ⁎
observes a shell of material on a pellet (e.g. LaZrO3 rich) where there has been significant Li loss. These factors make it difficult to precisely control electrolyte stoichiometry. Reduced sintering temperatures can decrease Li losses, but simply lowering the temperature risks reducing density too. The preferred way to reduce sintering temperature and still get acceptable densities is through the use of sintering aids, such as the introduction of low melting point phases. The use of sintering aids is well established for the processing of technologically important electrolytes, particularly in the SOFC arena [4]. Glasses often serve such a role in sintering of ceramics, but not all glasses are well suited, depending on factors including the reactivity with LLZO, the extent of grain boundary wetting, and the ionic conductivity. In typical liquid phase sintering systems, < 15 vol% liquid used in order to retain shape during sintering [5]. If too much liquid phase is used, or if complete particle wetting occurs, particles will be separated by the additive, and in a solid electrolyte, this means that current must pass through the liquid phase. Most additives to LLZO give undesired reactions but Li3BO3 is a low melting point (~750 °C) compound that is compatible with LLZO [6–16]. Li3BO3 has been demonstrated to permit reduced sintering temperatures, thereby minimizing Li losses due to Li sublimation. Fig. S1 shows a section of the Li2O-B2O3 phase diagram. Li3BO3 has a low melting point (773 °C), and is one component of a lower melting point eutectic (646 °C) at slightly higher B2O3 levels [17]. In addition to Li3BO3, lithium metaborate LiBO2 (LB) and lithium tetraborate Li2B4O7 (LB2) are also highlighted in the diagram. These two compounds are often used together in combinations for fusion sample preparation. Lithium metaborate (LiBO2) has a chain type metaborate (B-O-B) bond, which can be attacked by Li2O [18]. Hence
Corresponding author at: 178 Fitzpatrick Hall, University of Notre Dame, Notre Dame, IN 46556, USA. E-mail address: [email protected] (P.J. McGinn).
https://doi.org/10.1016/j.ssi.2018.05.015 Received 13 March 2018; Received in revised form 10 May 2018; Accepted 15 May 2018 0167-2738/ © 2018 Published by Elsevier B.V.
Solid State Ionics 323 (2018) 49–55
R.A. Jonson, P.J. McGinn
such tapes are used as-cast, and are not sintered after drying. Hanc, et al., tape cast 500 μm thick LLZO tapes with ZnO as a sintering aid [34]. Tapes were sintered at 1250 °C for 5 h while covered with the mother powder. Covering the tapes with powder is undesirable for practical manufacturing. Their tapes achieved conductivity of 8 × 10−5 S cm−1. Rather than a free-standing tape, a Li6.4La3Zr1.4Ta0.6O12 (LLZTO) membrane supported on porous alumina was reported by Liu, et al. [35]. A slurry was prepared from LLZTO powder, ethylene glycol monoethyl ether solvent, a small amount of polyvinyl butyral binder, and glycerol trioleate dispersant. The slurry was coated on an Al2O3 honeycomb, dried and sintered at 1140 °C for 5 h in a covered alumina crucible. Additional Li2CO3 powder was placed in the closed crucible to limit Li losses from the LLZTO. The result was a thin (~70 μm) LLZTO membrane bonded to the porous Al2O3. Tape casting of a La2/3−xLi3xTiO3/Al2O3 composite was reported by Zhang, et al. [36]. They sintered at 1200 °C but did not provide any details about their tape cast formulation. They did document the occurrence of Li-loss during sintering. Ohta, et al., described the fabrication of a solid-state battery where the LiCoO2 active cathode compound is bonded to the Nb-doped LLZO electrolyte by employing Li3BO3 glass in the cathode [7]. In this case the Li3BO3 + LiCoO2 (25:75 wt%) composite material was screen printed as a paste onto the Nb-LLZO. The paste was prepared by mixing LiCoO2 and Li3BO3 powders and ethyl cellulose binder in a solvent. After drying, the paste was annealed at 700 °C for 1 h, melting the Li3BO3. Li, et al. reported on the effect of B2O3 additions on the aqueous tape cast Li2O–Nb2O5–TiO2 compounds [37]. They found that as the B2O3 concentration increased, the slurry viscosity dramatically increased. This was attributed to formation of B(OH)4−, which results from dissolution of B2O3 in the aqueous solution. In this work, we describe the application of tape casting to the fabrication of LLZNbO sheets suitable for use in solid-state battery development. First, dilatometry studies are presented to help define the effect of Li3BO3 additions on LLZO pellet densification. Then challenges posed by the wetting behavior of Li3BO3 with various substrates by Li3BO3 are described. Finally observations on how Li3BO3 additions affect the sintering of LLZNbO in, both pellet and cast tape forms are presented.
LiBO2 extracts Li2O from LLZO during sintering, resulting in the formation of La2Zr2O7. Li3BO3 is composed of orthoborate (BO33−) units, which do not react with Li2O, so that Li3BO3 is relatively inert to LLZO. However, Li3BO3 is a poor lithium ion conductor (2 × 10−6 S cm−1, 25 °C) compared to LLZO and thus composites are likely to have reduced conductivity compared to the pure phase [7]. As a result of these properties, Li3BO3 can act as a sintering aid, enabling LLZO sintering at temperatures > 900 °C, thereby reducing almost all Li loss [6,8]. Li3BO3 has also been shown to be compatible with a common cathode material, LiCoO2 [7] [19]. Co-sintered electrolyte/electrode combinations using Li3BO3 additions in both LLZO and LiCoO2 made by pressing composite layer pellets of LLZO + Li3BO3 and LiCoO2 + Li3BO3 have been reported [8]. Such a process is not easily scalable to larger cross-sectional area cells. Slight modifications to Li3BO3 have also been reported including a 53Li2O-31B2O3-12SiO2-2CaO-2Al2O3 glass [12] and a 65Li2O·27B2O3·8SiO2 glass [20]. Finally, some success was reported in lowering LLZO sintering temperatures by the addition of B2O3 alone, with LLZO-0.3 B2O3 pellets yielding 2.5 × 10−4 S cm−1 conductivity after sintering at 950 °C for 30 h [21]. In all of the above reports the LLZO-Li3BO3 composite was processed as a pellet. Thin electrolyte layers/sheets are desired for development of high-performance solid-state batteries. To date, in the majority of publications exploring the use of LLZO in batteries, the electrolyte has generally been sliced from a sintered pellet and then polished [8,22–30]. While this is suitable for producing small batteries in the lab, it is not an acceptable process for larger scale manufacturing, which requires inexpensive, scalable technology for electrolyte processing. This need can be met if tape casting can be applied to LLZO. Tape casting is widely used in the manufacturing of electronic packaging and solid oxide fuel cells. It is an economical route to produce thin ceramic sheets, so is attractive for processing of solid-state batteries. Tape casting involves dispensing a slurry of powder onto a carrier film (e.g. silicone coated mylar). An adjustable doctor blade and the slurry viscosity determine the dispensed tape thickness height. The viscosity is adjusted by varying the amounts of organic additives (binder, plasticizers, and dispersants) in the slurry relative to the solids. LLZO poses challenges for sintering of tape cast shapes because of the Li losses that occur above ~900 °C in air. Unfortunately, the problem of Li loss in LLZO is exacerbated in sintering of sheets, with their higher surface-volume ratio. A Li-deficient layer at a tape surface is undesired as a high impedance electrode/electrolyte interface will likely result. To date, there have been few reports describing traditional tape casting of LLZO. The most success in fabricating tapes is the work of Laine and colleagues who synthesized flame pyrolyzed LLZO nanoparticles (90 nm in size), and formulated an organic solvent-based slip with a polyvinyl butyral binder. In order to minimize Li losses they sintered for only 1 h at 1090 °C in flowing N2, but because of their small, uniform particle size they were still able to produce films < 30 μm thick that were 94% dense [31,32]. They did not use an additive like Li3BO3. They attributed the short time sintering success to a combination of uniform particle packing, the high surface energy of the nanoparticles, and liquid phase sintering promoted by molten Li2CO3. To account for Li loss they added 50 wt% excess Li in the starting LLZO formulation. In their case, they had an optimal film thickness of 40–50 μm. Films thicker than this were Li-rich while films thinner were Li-poor. In later studies they showed some of the rapid sintering in their samples can be attributed to decomposed LLZO that undergoes reaction driven densification at temperatures below 1000 °C [32]. Extended ball-milling leads to decomposition of the LLZO, with formation of Li2CO3 being important, as it melts at 720 °C, wets oxide particles, as acts as a transient liquid phase while reacting with oxide components and gradually forming LLZO. Tape casting of Al-substituted Li7La3Zr2O12 (LLZO) - poly(ethylene oxide) (P(EO)20-LiClO4) composites has been reported [33]. However
2. Experimental methods Li7La3Zr1.75Nb0.25Al0.1O12 (LLZNbO) is produced via a solid-state reaction of LiOH·H2O, La2O3, ZrO2, Nb2O5 and Al2O3. This composition was chosen since it was reported as optimal in published work [38]. 7.5% excess LiOH·H2O is added by weight. The La2O3 is dried overnight at 900 °C before weighing. The precursors are ball milled together with ZrO2 media for ~24 h in ethanol. After drying the mixture is reacted twice at 1100 °C for 6 h, with an intermediate mortar and pestle grinding. The resulting calcined powder is milled again in ethanol for 48 h. The final dry powder is annealed at 650 °C for 2 h to drive off any residual organics and promote uniform surface hydration. Li3BO3 is produced from stoichiometric amounts of LiOH·H2O and H3BO3. The precursors are mixed by hand with mortar and pestle and then reacted at 600 °C for 2 h in air. X-ray diffraction shows complete conversion to Li3BO3.·The reacted powder is then milled in a zirconia jar in a Spex high energy mill for 15 min. Pellets for dilatometry studies were 5 mm in diameter and approximately 5 mm tall as pressed. The pellets were pressed at 340 MPa. A Linseis Model L75 dilatometer was used for the studies. The pellets were positioned between two Al 2 O 3 supports. Samples were typically heated at 10 °C/min to 700–1000 °C. In some cases, intermediate hold temperatures were employed as is discussed below. The theoretical density of LLZO-Li 3BO 3 mixtures were calculated by following the rule of mixtures and using the theoretical density values for LLZO (5.1 g/cm 3) and Li 3BO 3 (2.16 g/cm 3). The 50
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densities of the sintered samples were compared to the calculated theoretical values to give a percent theoretical density. Knowledge gained from the dilatometer studies was used in the sintering of tapes. In tape casting of ceramics, there are a number of established binders that work well across a wide range of powders, particularly vinyl-based binders (Polyvinyl Alcohol, Polyvinyl Butyral) for both aqueous and non-aqueous systems. Unfortunately, these are subject to gelling in the presence of boron, so any dissolution Li3 BO 3 significantly alters the rheological behavior of the slurry [39]. Boron ions dissolved in the solvent will react with the hydroxyl groups of vinyl-based binders leading to the formation of gel-type structures [40]. Hence, ethylcellulose was chosen as the binder for use with LLZO-Li 3 BO 3 composites. Tape slurry for casting is prepared in a two-step process. First LLZNbO and Li 3BO 3 are dispersed through milling in a solution of menhaden fish oil, ethanol, and toluene. The milling is performed in a teflon jar with ZrO 2 media for 24 h. This slurry is then mixed with a premade solution of ethylcellulose in ethanol, polyethylene glycol 400 and dibutyl phthalate along with additional ethanol and toluene in a centrifugal mixer for 1 h. The slurry is de-aired for 2 min by stirring while subject to a vacuum before being cast on a silicone-coated mylar sheet with a doctor blade set to 200 μm. Typical slip component ratios are shown in Table 1. After drying, the tapes are ~60 μm thick. Square samples approximately 15 mm × 15 mm are cut from the tapes and laminated. Typically, 6 layers are stacked in-between two silicone-coated mylar sheets in a pellet die. The die assembly is heated to 190 °C (above the ethyl cellulose glass transition) and pressed at approximately 120 Mpa for 5 min. After lamination, 6-layer tapes are typically ~200–250 μm thick and have a green density of 60–62%. Organics in the green tapes are burned out at 650 °C for 1 h before being sintered. Tapes are sintered in a flowing argon atmosphere. After sintering, the tapes are typically 150–170 μm thick. Ionic conductivity was characterized by electrochemical impedance spectroscopy (EIS) (Solartron 1260 impedance analyzer with a Solartron 1296 dielectric interface). The frequency range was 10 MHz to 10 Hz with an AC amplitude of 10 mV. Pellets were lightly sanded with 600 grit SiC paper to remove any surface irregularities. Tape surfaces were not sanded but were used in as-sintered condition. Silver paint (SPI) was applied on each side of the electrolyte pellets or tapes. For pellets, the entire end surface was painted. Self-adhesive mylar masks were applied to tapes to provide electrode alignment and a controlled 9mm2 area. After painting, samples were annealed in flowing argon at 750 °C for 3 h. Impedance spectra were collected immediately after removal from the argon ambient to minimize interaction with the lab air. Ionic conductivity values were calculated from the complex impedance plots using ZView2 (Scribner Associates) for analysis. The impedance spectra can be modeled with an equivalent circuit of (R b CPEb )(RgbCPE gb )(R eCPE e) where R b is the bulk resistance, R gb is the grain boundary resistance, R e is the electrode resistance and CPE is a constant phase element.
Fig. 1. Shrinkage of LLZNbO pellets with Li3BO3 content varying from 4 to 8% resulting from heating up to 900 °C at 10 °C/min followed by a 5 hour hold.
3. Results and discussion 3.1. Dilatometry Dilatometry was used to examine effects of Li3BO3 content on pellet densification. Fig. 1 shows the results from a series of dilatometry runs in flowing air on LLZNbO pellets with Li3BO3 content varying from 4 to 8%. These Li3BO3 levels were chosen based on earlier literature reports. These plots show the shrinkage (percent change in length) with temperature as the sample is heated up to 900 °C at 10 °C/min and then held there for 5 h. The onset of sintering is near 710 °C. 6 wt% Li3BO3 additions showed the greatest shrinkage. Due to variations in pressed density, additional samples were sintered in the dilatometer to better verify the trend. Fig. 2 shows the trend in sintered density (% of theoretical density) as a function of Li3BO3 content. Although there is some sample-to-sample variability, the general trend suggests there is a maximum in density around 5% Li3BO3. However, the density at the 5% level is still only 77–80%. The effect of pressing pressure on densification was also examined by dilatometry for pellets with 6% Li3BO3 that were heated up to 1000 °C at 10 °C/min with the results shown in Fig. S2. It was observed that higher pressure (higher green density) is associated with lower shrinkage but slightly higher final densities (see the figure inset). It should be noted that the dilatometer pellets are all pressed at substantially higher pressures than is usually used in experiments with larger furnace sintered pellets. The final relative densities of these pellets are not significantly affected by the pressing pressure.
Table 1 Casting slip component ratios. Slurry component
Weight percent
LLZNbO and Li3BO3 powder Menhaden fish oil dispersant Dibutyl phthalate plasticizer Polyethylene glycol 400 Ethyl cellulose binder (Dow Ethocell 45) Ethanol/toluene solvent
40–45% 1–2% 1–2% 0.5–1% 2–3% 45–55%
Fig. 2. Trend in sintered density (% of theoretical density) as a function of Li3BO3 content of LLZNbO dilatometry pellets heated up to 900 °C at 10 °C/min followed by a 5 hour hold. 51
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Finally, the effect of an intermediate hold, such as reported by Tadanaga, et al. [6], was examined. A series of 7 wt% Li3BO3 LLZNbO pellets were subjected to several variations in heating schedule including i) Ramp 10 °C/min to 900 °C, hold at 900 for 5 h; ii) Ramp 10 °C/min to 700 °C, ramp 0.66 °C/min to 900, hold at 900 for 5 h; iii) Ramp 10 °C/min to 700 °C, hold at 700 °C for 5 h., ramp 10 °C/min to 800, hold at 800 for 5 h. iv) Ramp 10 °C/min to 700 °C, hold at 700 °C for 5 h., ramp 10 °C/min to 900, hold at 900 for 5 h. Overall (see Fig. S3), intermediate hold steps in the temperature ramp appear to have minimal impact on densification relative to the final hold temperature (e.g. 900 vs. 800 °C). With Li3BO3 additions, the higher the sintering temperature the greater shrinkage that was observed. It should be noted that because the dilatometer pellets were positioned between two Al2O3 support disks, there was the possibility of interaction (wetting) with the Al2O3 by the Li3BO3 liquid phase. Although 5% was identified as the optimal level of Li3BO3, interaction with the substrate may lead to possible liquid loss from the pellet. This will be discussed further with regard to sintering of tapes. 3.2. Tape sintering Tapes were sintered in a stacked arrangement with a thin plate (e.g. Al2O3) below and on top of the substrate. The top plate was used to separate a small steel setter weight from the substrate. Tapes sintered without a setter weight tended to curl even if the green piece was relatively flat. The setters typically weighed 9 g. In unreported work, a variety of substrates were examined for sintering of LLZO-Li3BO3 pellets in air, including Al2O3, stabilized ZrO2, La2Zr2O7, and Li2ZrO3. All of them showed varying signs degrees of wetting/reaction with the liquid phase. Similarly, LLZO-Li3BO3 tapes demonstrate significant sensitivity to substrate materials that are contacted during sintering. Fig. S4 shows two LLZO tapes containing 6 wt% Li3BO3 that were sintered in an argon atmosphere at 950 °C for 15 h sandwiched between two substrates: pyrolytic graphite sheets and Al2O3 substrates. Al2O3 contact proves to be detrimental, as the tape contacting Al2O3 curled to a much more significant extent than the one in contact with the graphite sheets. Additionally, XRD of the tape surfaces (Fig. 3) shows formation of both La2Zr2O7 and LaAlO3 in the tape contacting Al2O3, while the tape in contact with graphite appears to be pure LLZO. Moreover, there appears to be no wetting of the graphite by the Li3BO3 rich liquid, whereas wetting of the alumina by the liquid is always observed, as evidenced by subtle stains or color changes to the alumina and the LLZO tape where they contact.
Fig. 4. SEM cross-section of LLNbZO + 6 wt% Li3BO3 tape sintered at 950 °C for 36 h.
3.3. Optimization of Li3BO3 content and sintering conditions At the start of the work Li3BO3 levels in the 6–8% range were examined based on earlier literature reports. SEM inspection of crosssections (Fig. 4) of 6% Li3BO3-LLZO tape sintered at 950 °C for 36 h show large, dark, semi-translucent inclusions that appear to be Li3BO3rich regions. It appears Li3BO3 at the 6wt% level is excessive, leading to liquid pooling, and suggests that enhanced properties can be obtained through lower amounts of liquid. Hence, tapes with Li3BO3 contents from 0 to 5% were examined as described below and higher levels of Li3BO3 were not considered. The combination of the graphite interface and argon atmosphere also appears to mitigate Li-loss at high temperatures. Tapes were sintered at 1000 °C for times up to 48 h. without demonstrating signs of Liloss as usually typified by La2Zr2O7 formation at the sample surface. Higher temperatures lead to improved sintering (e.g., 1000 °C vs. 900 °C). As an example, Table 2 compares 5 wt% Li3BO3 tapes sintered at 950 °C and 1000 °C for 6 h. In Table 2, where more than one conductivity value is given for a particular tape, the values represent measurements made through different regions of the tape. It is not always observed that the highest density results in the highest conductivity. It should be noted that in some cases precise determination of density is challenging due to slight curvature in tapes. Sections are cut from larger cast green tapes and are laminated together to form substrates for sintering. Non-uniformities in the tapes (pores, irregular Li3BO3 distribution, and irregularities in the green tapes) are thought to be the primary cause for variability in the tape conductivity and densities. Table 2 Comparison 5 wt% Li3BO3 tapes sintered at 950 °C and 1000 °C for 6 h. Temperature (°C)
Tape
Density (%)
Conductivity (S cm−1)
1000
1 2 3 Average 1 2 3 Average
89.63 87.55 89.98 89.05 79.14 84.77 88.69 84.20
4.37E-05 1.05E-04 7.77E-05 7.54E-05 6.20E-05 4.16E-05 4.63E-05 5.00E-05
950
Fig. 3. XRD spectra of LLNbZO + 6 wt% Li3BO3 tapes sintered in argon at 950 °C for 15 h. One tape (bottom) was sintered between graphite sheets while the other (top) was sintered between Al2O3 substrates. 52
Solid State Ionics 323 (2018) 49–55
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Table 3 The effect of sintering time at 1000 °C LLZO + 4% Li3BO3 tapes.
3h 6h
10 h
Table 4 The effect of Li3BO3 concentration in LLZNbO tapes sintered at 1000 °C for 6 h.
Tape
Density (%)
Conductivity (S cm−1)
Average
Li3BO3 wt%
Tape
Density (%)
Conductivity (S cm−1)
Average
1 2 1
87.6 89.6 91.8
1.19E-04
0% 0.50%
92.3 89.6 90.8
89.3
1
91.1
2 3 4
86.2 87.7 –
2
91.3
1.50%
1
83.0
3%
2 1 2
87.5 85.9 85.0
3
85.4
9.24E-05 1.63E-04 2.27E-04 2.55E-04 – – 2.57E-04 3.89E-04 9.68E-05 7.04E-05 1.05E-04 7.82E-05 1.37E-04 5.74E-05 4.43E-05 4.36E-05
1.28E-04
1.11E-04
1 2 1
2
5.56E-05 1.82E-04 1.25E-04 1.18E-04 1.10E-04 9.07E-05 6.52E-05 3.88E-05 8.45E-05
6.29E-05
2.82E-04
9.08E-05
7.21E-05
(Fig. S5). The reduced conductivity is attributed to an increased grain boundary resistance. Fig. 6 shows a series of EIS spectra that illustrate growth of the grain boundary related arc with increased sintering time. Tapes sintered for 48 h and longer appear to have only one arc corresponding to ionic conductivity in the Nyquist plot. However, examination of the Bode plots (Fig. S6) indicates the (RgbCPEgb) circuit element associated with the grain boundary impedance still exists between approximately 1 MHz and 10 kHz with resistance values that place these tapes' conductivities in the 10−6 S cm−1 range. It is hypothesized that inter-grain contact area is reduced by the growth of large grains when there is no corresponding increase in overall tape density. For example, reduced grain boundary area could lead to more grains being separated by a thin glassy phase after the liquid phase solidifies, such as seen in Fig. S5. The studies on 4 and 5% Li3BO3 tapes suggested the level of Li3BO3 additive was still too high. Thus, even lower Li3BO3 levels were examined in a series of tapes, with Li3BO3 concentrations varying from 0 to 3 wt% that were sintered at 1000 °C for 6 h. The results are summarized in Table 4, with typical EIS spectra seen in Fig. 7. The maximum conductivity is observed at 0.5 wt% Li3BO3. The borate-free tapes have higher densities but also demonstrate larger grain boundary attributed resistance than the 0.5 wt% tapes. The decreased conductivity in 1.5 and 3 wt% tapes comes predominantly from increased grain boundary resistance suggesting an increased presence of Li3BO3 at grain boundaries. These higher concentrations also have reduced bulk conductivities. Due to processing variability, tape properties are non-
Fig. 5. SEM cross-section of LLNbZO + 4 wt% Li3BO3 tape sintered at 1000 °C for 48 h.
The effect of sintering time at 1000 °C was examined in a series of 4% Li3BO3 tapes, with results for 3, 6, and 10 h. shown in Table 3. Longer sintering times can result in some very large grains (100+ μm) as depicted in Fig. 5, and a slight density improvement, but that does not necessarily lead to enhanced conductivity performance. Many areas still have small grains (2–4 μm) and traces of liquid phase are evident
Fig. 6. (a) EIS spectra of LLNbZO + 4 wt% Li3BO3 tapes sintered at 1000 °C for a range of times between 6 and 96 h. (b) Enlargement of the box-enclosed region in (a). 53
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Fig. 8. Density and conductivity of LLZNbO pellets sintered on graphite in an argon atmosphere at 1000 °C for 6 h as a function of Li3BO3 content.
Fig. 7. EIS spectra of LLNbZO tapes sintered at 1000 °C for 6 h with Li3BO3 content varying from 0 to 3 wt%.
~14.5% to ~16.5% as Li3BO3 was increased from 0% to 4 wt%. Ionic conductivity reached a peak (~2.5 × 10−4 S cm−1) between 1 and 2 wt % Li3BO3, suggesting an optimal concentration in that range. This is in the same general composition range as the optimum in tapes where 0.5 wt% was found to be the best. It is important to note that the dilatometry pellets (Fig. 4) were in direct contact with Al2O3, while these pellets were contacted by graphite. It is observed that pellets contacting alumina will lose liquid by substrate wetting, but that is not an issue with graphite. Hence the peak at 5% Li3BO3 seen in Fig. 1 is not present for the pellets sintered on graphite. Li3BO3 content was also found to reduce the rate of ionic conductivity degradation in LLZNbO pellets due to air exposure. Pellets with no Li3BO3 and 4 wt% Li3BO3 sintered at 1000 °C for 6 h were examined periodically over 120 h of air exposure. As seen in Fig. S8, after 72 h of air exposure the ionic conductivity pure LLZNbO pellets fell to 20% or less of their initial values while pellets with 4% Li3BO3 fell to only 60% over the same period. The optimal Li3BO3 seen in this work is lower than the preferred values reported by Ohta et al. [8], and Rosero-Navarro et al. [13], and is also less than that reported by Matsuyama, et al. [26] and Shin, et al. [14] Ohta reports an optimal level of 5–10 vol%. At 7 vol% for example, this is equivalent to 0.331 mol Li3BO3 to 1 mol LLZO. Rosero-Navarro finds 0.8 mol Li3BO3 to 1 mol LLZO as optimal, while Matsuyama (0.72 mol Li3BO3) and Shin (0.844 mol Li3BO3) are similar. Our ~1.5 wt % pellet optimum, is equivalent to 0.16 mol Li3BO3 to 1 mol LLZO, while the value in tapes (0.5 wt%) is equivalent to 0.053 mol Li3BO3 to 1 mol LLZO. The difference may be the result of Li3BO3 - substrate interactions. When Li3BO3-LLZO pellets were processed on Al2O3, ZrO2, LLZO, or LaZrO2 substrates, there was wetting or interaction of the substrate by the Li3BO3 observed to varying degrees. In such cases, larger amounts of Li3BO3 may be optimal to compensate for losses of Li3BO3 to the substrate. Similar to Li losses to the atmosphere during high-temperature pellet sintering, this Li3BO3 loss is difficult to control in a repeatable fashion. When using graphite, there is no interaction with Li3BO3. Hence, lower levels of Li3BO3 are needed as there is no compensation for liquid loss that is required. Because the graphite does not react and is not oxidized, the substrates can be reused many times. Argon is necessary for sintering of LLZO with graphite plates, but it has been shown to be beneficial for improved LLZO conductivity compared to processing in air [41]. The tape casting apparatus used in this work limits the cast area to approximately 25 cm × 10 cm. The laminated and sintered tapes are restricted to the ~15 mm × 15 mm sheets described in the experimental methods section due to the size of the press die equipment.
uniform, with the ionic conductivities of two separate areas of the same tape sample sometimes varying by more than a factor of two, as seen for example in Table 4 for 3 wt% tape #2. Visual inspection of the sintered tapes (Supplementary Fig. S7) demonstrates non-uniformity in microstructure, particularly with regard to grain size. 15 mm × 15 mm tapes without Li3BO3 have edge regions with a fine-grained structure (5–10 μm). The center region of the tapes (differentiated in the figures by contrast) consists of a roughly 50/50 mixture of both fine (2–10 μm) and large (~100 μm) grains. The region between the edge and the center is primarily made up of large grains (~100 μm) with only a few small areas of remnant fine grains. This microstructure is likely the result of a ripening process with the small grains undergoing dissolution and reprecipitation on the large grains. Both the 0.5 and 1.5 wt% tapes are similar to the 0 wt% tapes but the central fine-grained area increases with increased Li3BO3 concentration. The 3 wt% tapes are fine-grained throughout with only isolated large grains appearing at the very edges of the tape. A darkened region at the center appears to be due to surface roughness. Interestingly, at 1000 °C the presence of the Li3BO3 appears to slow down the coarsening process compared to the 0 wt% case. Variation in tape properties is attributed, at least in part, to the nonuniformity of grain structure. For example, both Tables 2 and 3 provide examples where conductivity in two different regions varied, in some cases by a factor of two. If the measured region contains a larger proportion of the fine-grained structure, its grain boundary resistance will be higher. The radial nature of the pattern suggests that grain growth in LLZNbO tapes may be promoted by loss of excess lithium (or inhibited by the presence of excess lithium). Since the tapes are covered by graphite on both the top and bottom, the edges of the tape have the most exposure to the surrounding atmosphere and should have a correspondingly higher rate of Li loss. Hence the region at the periphery is where the finest grains are found. In order to discern if lamination density gradients might be responsible, green tapes were sliced in half before sintering. In the sintered half-tapes, what would be the center region in a full tape (and now was at an edge) again showed finer grain structure. At present, more microstructural uniformity is achieved in pellets than in tapes due to processing challenges. Hence the variation in sintered density and conductivity with Li3BO3 for pelletized samples sintered on graphite in an argon atmosphere at 1000 °C for 6 h was also examined, with the results summarized in Fig. 8. Density is seen to decrease somewhat with addition of more Li3BO3. Weight loss of these pellets during sintering also increased with Li3BO3 content going from 54
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However the approach is easily scalable to larger sizes, suggesting an economical route to production of LLZO sheets. No mechanical testing has been performed on sintered tapes but at 150 μm thickness they stand up well to the handling required for examination. Samples with larger grains are observed to be more likely to fracture with handling. Thinner tapes (e.g., 50 μm) can be made, but prove more difficult to handle.
[12] N.C. Rosero-Navarro, T. Yamashita, A. Miura, M. Higuchi, K. Tadanaga, Effect of sintering additives on relative density and Li-ion conductivity of Nb-doped Li7La3ZrO12 solid electrolyte, J. Am. Ceram. Soc. 100 (2017) 276–285. [13] N.C. Rosero-Navarro, A. Miura, M. Higuchi, K. Tadanaga, Optimization of Al2O3 and Li3BO3 content as sintering additives of Li7−xLa2.95Ca0.05ZrTaO12 at low temperature, J. Electron. Mater. (2016) 1–5. [14] R.H. Shin, S.I. Son, Y.S. Han, Y.D. Kim, H.T. Kim, S.S. Ryu, W. Pan, Sintering behavior of garnet-type Li7La3Zr2O12-Li3BO3 composite solid electrolytes for all-solidstate lithium batteries, Solid State Ionics 301 (2017) 10–14. [15] R.H. Shin, S.I. Son, S.M. Lee, Y.S. Han, Y.D. Kim, S.S. Ryu, Effect of Li3BO3 additive on densification and ion conductivity of garnet-type Li7La3Zr2O12 solid electrolytes of all-solid-state lithium-ion batteries, J. Korean Ceram. Soc. 53 (2016) 712–718. [16] L.C. Zhang, J.F. Yang, Y.X. Gao, X.P. Wang, Q.F. Fang, C.H. Chen, Influence of Li3BO3 additives on the Li+ conductivity and stability of Ca/Ta-substituted Li6.55(La2.95Ca0.05)(Zr1.5Ta0.5)O12 electrolytes, J. Power Sources 355 (2017) 69–73. [17] H. Yu, Z.P. Jin, Q. Chen, M. Hillert, Thermodynamic assessment of the lithiumborate system, J. Am. Ceram. Soc. 83 (2000) 3082–3088. [18] M. Tatsumisago, M. Takahashi, T. Minami, M. Tanaka, N. Umesaki, N. Iwamoto, Structural investigation of rapidly quenched Li2O-B2O3 glasses by Raman spectroscopy, Journal of the Ceramic Association, Japan 94 (1986) 464–469. [19] K. Chen, Y. Shen, Y.B. Zhang, Y.H. Lin, C.W. Nan, High capacity and cyclic performance in a three-dimensional composite electrode filled with inorganic solid electrolyte, J. Power Sources 249 (2014) 306–310. [20] E.A. Il'ina, A.A. Raskovalov, N.S. Saetova, B.D. Antonov, O.G. Reznitskikh, Composite electrolytes Li7La3Zr2O12-glassy Li2O-B2O3-SiO2, Solid State Ionics 296 (2016) 26–30. [21] L.L. Feng, L. Li, Y.Q. Zhang, H.J. Peng, Y.P. Zou, Low temperature synthesis and ion conductivity of Li7La3Zr2O12 garnets for solid state Li ion batteries, Solid State Ionics 310 (2017) 129–133. [22] C.W. Ahn, J.J. Choi, J. Ryu, B.D. Hahn, J.W. Kim, W.H. Yoon, J.H. Choi, J.S. Lee, D.S. Park, Electrochemical properties of Li7La3Zr2O12-based solid state battery, J. Power Sources 272 (2014) 554–558. [23] T. Kato, T. Hamanaka, K. Yamamoto, T. Hirayama, F. Sagane, M. Motoyama, Y. Iriyama, In-situ Li7La3Zr2O12/LiCoO2 interface modification for advanced allsolid-state battery, J. Power Sources 260 (2014) 292–298. [24] F.M. Du, N. Zhao, Y.Q. Li, C. Chen, Z.W. Liu, X.X. Guo, All solid state lithium batteries based on lamellar garnet-type ceramic electrolytes, J. Power Sources 300 (2015) 24–28. [25] C. Hansel, S. Afyon, J.L.M. Rupp, Investigating the all-solid-state batteries based on lithium garnets and a high potential cathode - LiMn1.5Ni0.5O4, Nano 8 (2016) 18412–18420. [26] T. Matsuyama, R. Takano, K. Tadanaga, A. Hayashi, M. Tatsumisago, Fabrication of all-solid-state lithium secondary batteries with amorphous TiS4 positive electrodes and Li7La3Zr2O12 solid electrolytes, Solid State Ionics 285 (2016) 122–125. [27] T. Liu, Y. Ren, Y. Shen, S.-X. Zhao, Y. Lin, C.-W. Nan, Achieving high capacity in bulk-type solid-state lithium ion battery based on Li6.75La3Zr1.75Ta0.25O12 electrolyte: interfacial resistance, J. Power Sources 324 (2016) 349–357. [28] J. van den Broek, S. Afyon, J.L.M. Rupp, Interface-engineered all-solid-state Li-ion batteries based on Garnet-type fast Li+ conductors, Adv. Energy Mater. 6 (2016) 1600736. [29] X. Han, Y. Gong, K.K. Fu, X. He, G.T. Hitz, J. Dai, A. Pearse, B. Liu, H. Wang, G. Rubloff, Y. Mo, V. Thangadurai, E.D. Wachsman, L. Hu, Negating interfacial impedance in garnet-based solid-state Li metal batteries, Nat. Mater. 16 (2017) 572–579. [30] M. Shoji, H. Munakata, K. Kanamura, Fabrication of all-solid-state lithium-ion cells using three-dimensionally structured solid electrolyte Li7La3Zr2O12 pellets, Frontiers in Energy Research 4 (2016). [31] E.Y. Yi, W.M. Wang, J. Kieffer, R.M. Laine, Flame made nanoparticles permit processing of dense, flexible, Li+ conducting ceramic electrolyte thin films of cubicLi7La3Zr2O12 (c-LLZO), J. Mater. Chem. A 4 (2016) 12947–12954. [32] E. Yi, W. Wang, J. Kieffer, R.M. Laine, Key parameters governing the densification of cubic-Li7La3Zr2O12 Li+ conductors, J. Power Sources 352 (2017) 156–164. [33] F. Langer, I. Bardenhagen, J. Glenneberg, R. Kun, Microstructure and temperature dependent lithium ion transport of ceramic–polymer composite electrolyte for solid-state lithium ion batteries based on garnet-type Li7La3Zr2O12, Solid State Ionics 291 (2016) 8–13. [34] E. Hanc, W. Zajac, L. Lu, B.G. Yan, M. Kotobuki, M. Ziabka, J. Molenda, On fabrication procedures of Li-ion conducting garnets, J. Solid State Chem. 248 (2017) 51–60. [35] K. Liu, C.A. Wang, Honeycomb-alumina supported garnet membrane: composite electrolyte with low resistance and high strength for lithium metal batteries, J. Power Sources 281 (2015) 399–403. [36] H. Zhang, X.B. Liu, Y. Qi, V. Liu, On the La2/3−xLi3xTiO3/Al2O3 composite solidelectrolyte for Li-ion conduction, J. Alloys Compd. 577 (2013) 57–63. [37] S.C. Li, Q.L. Zhang, T.J. Zhao, H. Yang, Effect of B2O3 on the aqueous tape casting and microwave properties of Li2O-Nb2O5-TiO2 ceramics, Prog. Nat. Sci. 23 (2013) 152–156. [38] S. Ohta, T. Kobayashi, T. Asaoka, High lithium ionic conductivity in the garnet-type oxide Li7−X La3(Zr2−X, NbX)O12 (X = 0–2), J. Power Sources 196 (2011) 3342–3345. [39] C. Wang, H.M. Ji, J. Wang, Effects of solvent system on tape casting of the BaSrTiO3-MgO-based dielectric ceramics containing B2O3, J. Mater. Sci. 47 (2012) 2486–2491. [40] C.W. Cho, Y.S. Cho, J.G. Yeo, S.C. Choi, J. Kim, U. Paik, The role of 2-methyl-2,4pentanediol modifier and its interaction with poly(vinyl butyral) binder in BaTiO3 and Li2O-B2O3-BaO-SiO2 glass suspensions, Colloids Surf. A Physicochem. Eng. Asp. 224 (2003) 83–91. [41] L. Cheng, H.M. Hou, S. Lux, R. Kostecki, R. Davis, V. Zorba, A. Mehta, M. Doeff, Enhanced lithium ion transport in garnet-type solid state electrolytes, J. Electroceram. 38 (2017) 168–175.
4. Conclusions Li7La3Zr1.75Nb0.25Al0.1O12 with Li3BO3 additions was sintered in pellet and cast tape form. Dilatometric analysis shows the onset of sintering with Li3BO3 additions at 710 °C. Graphite substrates are found to be important for eliminating wetting of the substrate by Li3BO3, but necessitate sintering in an inert environment. The feasibility of producing LLZNbO electrolytes by tape casting without the use of sacrificial powder covering was demonstrated using Li3BO3 as a sintering aid. The optimal Li3BO3 content for ionic conductivity in LLZO pellets sintered at 1000 °C was between 1 and 2 wt% Li3BO3, much less than in prior studies, likely due to elimination of Li3BO3-substrate interactions. At this Li3BO3 level ionic conductivity in pellets was ~2.5 × 10−4 S cm−1 after sintering in an argon atmosphere at 1000 °C for 6 h without packing of any extra powder around the pellets. 150–175 μm thick LLZO + 0.5% Li3BO3 tapes sintered at 1000 °C for 6 h exhibited ionic conductivity values up to 2.83 × 10−4 S cm−1 and density of ~90%. These tapes showed grain size non-uniformity that may be related edge effects of Li loss/interaction with the furnace atmosphere during sintering. Acknowledgements The authors gratefully acknowledge partial funding of this work provided by a grant from the Ford University Research Program. The authors thank Mr. Liam McDermott for performing the dilatometry analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2018.05.015. References [1] R. Murugan, V. Thangadurai, W. Weppner, Fast lithium ion conduction in garnettype Li7La3Zr2O12, Angew. Chem. Int. Ed. 46 (2007) 7778–7781. [2] V. Thangadurai, D. Pinzaru, S. Narayanan, A.K. Baral, Fast solid-state Li ion conducting garnet-type structure metal oxides for energy storage, J. Phys. Chem. Lett. 6 (2015) 292–299. [3] M. Huang, T. Liu, Y. Deng, H. Geng, Y. Shen, Y. Lin, C.-W. Nan, Effect of sintering temperature on structure and ionic conductivity of Li7−xLa3Zr2O12−0.5x (x = 0.5–0.7) ceramics, Solid State Ionics 204–205 (2011) 41–45. [4] P. Babilo, S.M. Haile, Enhanced sintering of yttrium-doped barium zirconate by addition of ZnO, J. Am. Ceram. Soc. 88 (2005) 2362–2368. [5] R.M. German, Sintering With a Liquid Phase, Sintering: From Empirical Observations to Scientific Principles, Butterworth-Heinemann, Boston, 2014, pp. 247–303. [6] K. Tadanaga, R. Takano, T. Ichinose, S. Mori, A. Hayashi, M. Tatsumisago, Low temperature synthesis of highly ion conductive Li7La3Zr2O12-Li3BO3 composites, Electrochem. Commun. 33 (2013) 51–54. [7] S. Ohta, S. Komagata, J. Seki, T. Saeki, S. Morishita, T. Asaoka, All-solid-state lithium ion battery using garnet-type oxide and Li3BO3 solid electrolytes fabricated by screen-printing, J. Power Sources 238 (2013) 53–56. [8] S. Ohta, J. Seki, Y. Yagi, Y. Kihira, T. Tani, T. Asaoka, Co-sinterable lithium garnettype oxide electrolyte with cathode for all-solid-state lithium ion battery, J. Power Sources 265 (2014) 40–44. [9] R. Takano, K. Tadanaga, A. Hayashi, M. Tatsumisago, Low temperature synthesis of Al-doped Li7La3Zr2O12 solid electrolyte by a sol-gel process, Solid State Ionics 255 (2014) 104–107. [10] N. Janani, C. Deviannapoorani, L. Dhivya, R. Murugan, Influence of sintering additives on densification and Li+ conductivity of Al doped Li7La3Zr2O12 lithium garnet, RSC Adv. 4 (2014) 51228–51238. [11] N.C. Rosero-Navarro, T. Yamashita, A. Miura, M. Higuchi, K. Tadanaga, Preparation of Li7La3 (Zr2−x,Nbx)O12 (x = 0–1.5) and Li3BO3/LiBO2 composites at low temperatures using a sol–gel process, Solid State Ionics 285 (2016) 6–12.
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Tape casting and sintering of Li7La3Zr1.75Nb0.25Al0.1O12 with Li3BO3 additions Robert A. Jonson, Paul J. McGinn
T
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Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Solid-state battery Solid electrolyte Li7La3Zr2O12 Tape casting Sintering aid
An approach for tape casting and sintering of Li7La3Zr1.75Nb0.25Al0.1O12 (LLZNbO) sheets suitable for use in solid-state battery development is described. The use of Li3BO3 as a sintering aid in both pellet and cast tape samples is examined. We find the optimal Li3BO3 content for ionic conductivity in pellets is between 1 and 2 wt% Li3BO3, much less than in prior reports. At this level, ionic conductivity is ~2.5 × 10−4 S cm−1 after sintering in an argon atmosphere at 1000 °C for 6 h. 150–175 micron thick LLZNbO + 0.5% Li3BO3 tapes sintered at 1000 °C for 6 h exhibit ionic conductivity values of 2–3 × 10−4 S cm−1.
1. Introduction Modern electronic devices and electrified vehicles rely heavily on the use of Li-ion batteries. As several accidents have shown, safer batteries are desirable, if not essential for more widespread application. A battery with a solid electrolyte would be a safer alternative to current Li-ion technologies utilizing organic liquid electrolytes. The ideal electrolyte for a solid-state Li-ion battery must have a high lithium ionic conductivity, a negligible electronic conductivity, be stable against elemental lithium and other electrode materials, and allow for operation across a wide electrochemical window. Also desirable is stability against air and water at room temperatures. The ready availability of high energy density bulk solid-state batteries (SSB's) would be a breakthrough advance for both battery safety and performance goals. Investigations show that stuffed garnets may satisfy most of the criteria for a solid-state battery electrolyte [1]. In the cubic form, depending on the substitutions, it has conductivity ranging from 2 to 7 × 10−4 S cm−1 [2]. In addition to its high conductivity, LLZO has also been shown to have high chemical and electrochemical stability with lithium metal [1,2]. LLZO pellets with conductivities > 10−4 S cm−1 are routinely produced, but long sintering times at temperatures between 1100 and 1200 °C are typically required. Significant challenges are posed in trying to process dense bodies from Li-rich oxides because of Li losses that occur above ~900 °C in air. To counteract uncontrolled Li loss in pellets researchers typically add additional Li at various steps of grinding/calcination during solid-state processing. It is also common to surround pellets with the mother powder during sintering to further reduce losses from the pellet. Despite this, Li losses occur [3]. One often ⁎
observes a shell of material on a pellet (e.g. LaZrO3 rich) where there has been significant Li loss. These factors make it difficult to precisely control electrolyte stoichiometry. Reduced sintering temperatures can decrease Li losses, but simply lowering the temperature risks reducing density too. The preferred way to reduce sintering temperature and still get acceptable densities is through the use of sintering aids, such as the introduction of low melting point phases. The use of sintering aids is well established for the processing of technologically important electrolytes, particularly in the SOFC arena [4]. Glasses often serve such a role in sintering of ceramics, but not all glasses are well suited, depending on factors including the reactivity with LLZO, the extent of grain boundary wetting, and the ionic conductivity. In typical liquid phase sintering systems, < 15 vol% liquid used in order to retain shape during sintering [5]. If too much liquid phase is used, or if complete particle wetting occurs, particles will be separated by the additive, and in a solid electrolyte, this means that current must pass through the liquid phase. Most additives to LLZO give undesired reactions but Li3BO3 is a low melting point (~750 °C) compound that is compatible with LLZO [6–16]. Li3BO3 has been demonstrated to permit reduced sintering temperatures, thereby minimizing Li losses due to Li sublimation. Fig. S1 shows a section of the Li2O-B2O3 phase diagram. Li3BO3 has a low melting point (773 °C), and is one component of a lower melting point eutectic (646 °C) at slightly higher B2O3 levels [17]. In addition to Li3BO3, lithium metaborate LiBO2 (LB) and lithium tetraborate Li2B4O7 (LB2) are also highlighted in the diagram. These two compounds are often used together in combinations for fusion sample preparation. Lithium metaborate (LiBO2) has a chain type metaborate (B-O-B) bond, which can be attacked by Li2O [18]. Hence
Corresponding author at: 178 Fitzpatrick Hall, University of Notre Dame, Notre Dame, IN 46556, USA. E-mail address: [email protected] (P.J. McGinn).
https://doi.org/10.1016/j.ssi.2018.05.015 Received 13 March 2018; Received in revised form 10 May 2018; Accepted 15 May 2018 0167-2738/ © 2018 Published by Elsevier B.V.
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such tapes are used as-cast, and are not sintered after drying. Hanc, et al., tape cast 500 μm thick LLZO tapes with ZnO as a sintering aid [34]. Tapes were sintered at 1250 °C for 5 h while covered with the mother powder. Covering the tapes with powder is undesirable for practical manufacturing. Their tapes achieved conductivity of 8 × 10−5 S cm−1. Rather than a free-standing tape, a Li6.4La3Zr1.4Ta0.6O12 (LLZTO) membrane supported on porous alumina was reported by Liu, et al. [35]. A slurry was prepared from LLZTO powder, ethylene glycol monoethyl ether solvent, a small amount of polyvinyl butyral binder, and glycerol trioleate dispersant. The slurry was coated on an Al2O3 honeycomb, dried and sintered at 1140 °C for 5 h in a covered alumina crucible. Additional Li2CO3 powder was placed in the closed crucible to limit Li losses from the LLZTO. The result was a thin (~70 μm) LLZTO membrane bonded to the porous Al2O3. Tape casting of a La2/3−xLi3xTiO3/Al2O3 composite was reported by Zhang, et al. [36]. They sintered at 1200 °C but did not provide any details about their tape cast formulation. They did document the occurrence of Li-loss during sintering. Ohta, et al., described the fabrication of a solid-state battery where the LiCoO2 active cathode compound is bonded to the Nb-doped LLZO electrolyte by employing Li3BO3 glass in the cathode [7]. In this case the Li3BO3 + LiCoO2 (25:75 wt%) composite material was screen printed as a paste onto the Nb-LLZO. The paste was prepared by mixing LiCoO2 and Li3BO3 powders and ethyl cellulose binder in a solvent. After drying, the paste was annealed at 700 °C for 1 h, melting the Li3BO3. Li, et al. reported on the effect of B2O3 additions on the aqueous tape cast Li2O–Nb2O5–TiO2 compounds [37]. They found that as the B2O3 concentration increased, the slurry viscosity dramatically increased. This was attributed to formation of B(OH)4−, which results from dissolution of B2O3 in the aqueous solution. In this work, we describe the application of tape casting to the fabrication of LLZNbO sheets suitable for use in solid-state battery development. First, dilatometry studies are presented to help define the effect of Li3BO3 additions on LLZO pellet densification. Then challenges posed by the wetting behavior of Li3BO3 with various substrates by Li3BO3 are described. Finally observations on how Li3BO3 additions affect the sintering of LLZNbO in, both pellet and cast tape forms are presented.
LiBO2 extracts Li2O from LLZO during sintering, resulting in the formation of La2Zr2O7. Li3BO3 is composed of orthoborate (BO33−) units, which do not react with Li2O, so that Li3BO3 is relatively inert to LLZO. However, Li3BO3 is a poor lithium ion conductor (2 × 10−6 S cm−1, 25 °C) compared to LLZO and thus composites are likely to have reduced conductivity compared to the pure phase [7]. As a result of these properties, Li3BO3 can act as a sintering aid, enabling LLZO sintering at temperatures > 900 °C, thereby reducing almost all Li loss [6,8]. Li3BO3 has also been shown to be compatible with a common cathode material, LiCoO2 [7] [19]. Co-sintered electrolyte/electrode combinations using Li3BO3 additions in both LLZO and LiCoO2 made by pressing composite layer pellets of LLZO + Li3BO3 and LiCoO2 + Li3BO3 have been reported [8]. Such a process is not easily scalable to larger cross-sectional area cells. Slight modifications to Li3BO3 have also been reported including a 53Li2O-31B2O3-12SiO2-2CaO-2Al2O3 glass [12] and a 65Li2O·27B2O3·8SiO2 glass [20]. Finally, some success was reported in lowering LLZO sintering temperatures by the addition of B2O3 alone, with LLZO-0.3 B2O3 pellets yielding 2.5 × 10−4 S cm−1 conductivity after sintering at 950 °C for 30 h [21]. In all of the above reports the LLZO-Li3BO3 composite was processed as a pellet. Thin electrolyte layers/sheets are desired for development of high-performance solid-state batteries. To date, in the majority of publications exploring the use of LLZO in batteries, the electrolyte has generally been sliced from a sintered pellet and then polished [8,22–30]. While this is suitable for producing small batteries in the lab, it is not an acceptable process for larger scale manufacturing, which requires inexpensive, scalable technology for electrolyte processing. This need can be met if tape casting can be applied to LLZO. Tape casting is widely used in the manufacturing of electronic packaging and solid oxide fuel cells. It is an economical route to produce thin ceramic sheets, so is attractive for processing of solid-state batteries. Tape casting involves dispensing a slurry of powder onto a carrier film (e.g. silicone coated mylar). An adjustable doctor blade and the slurry viscosity determine the dispensed tape thickness height. The viscosity is adjusted by varying the amounts of organic additives (binder, plasticizers, and dispersants) in the slurry relative to the solids. LLZO poses challenges for sintering of tape cast shapes because of the Li losses that occur above ~900 °C in air. Unfortunately, the problem of Li loss in LLZO is exacerbated in sintering of sheets, with their higher surface-volume ratio. A Li-deficient layer at a tape surface is undesired as a high impedance electrode/electrolyte interface will likely result. To date, there have been few reports describing traditional tape casting of LLZO. The most success in fabricating tapes is the work of Laine and colleagues who synthesized flame pyrolyzed LLZO nanoparticles (90 nm in size), and formulated an organic solvent-based slip with a polyvinyl butyral binder. In order to minimize Li losses they sintered for only 1 h at 1090 °C in flowing N2, but because of their small, uniform particle size they were still able to produce films < 30 μm thick that were 94% dense [31,32]. They did not use an additive like Li3BO3. They attributed the short time sintering success to a combination of uniform particle packing, the high surface energy of the nanoparticles, and liquid phase sintering promoted by molten Li2CO3. To account for Li loss they added 50 wt% excess Li in the starting LLZO formulation. In their case, they had an optimal film thickness of 40–50 μm. Films thicker than this were Li-rich while films thinner were Li-poor. In later studies they showed some of the rapid sintering in their samples can be attributed to decomposed LLZO that undergoes reaction driven densification at temperatures below 1000 °C [32]. Extended ball-milling leads to decomposition of the LLZO, with formation of Li2CO3 being important, as it melts at 720 °C, wets oxide particles, as acts as a transient liquid phase while reacting with oxide components and gradually forming LLZO. Tape casting of Al-substituted Li7La3Zr2O12 (LLZO) - poly(ethylene oxide) (P(EO)20-LiClO4) composites has been reported [33]. However
2. Experimental methods Li7La3Zr1.75Nb0.25Al0.1O12 (LLZNbO) is produced via a solid-state reaction of LiOH·H2O, La2O3, ZrO2, Nb2O5 and Al2O3. This composition was chosen since it was reported as optimal in published work [38]. 7.5% excess LiOH·H2O is added by weight. The La2O3 is dried overnight at 900 °C before weighing. The precursors are ball milled together with ZrO2 media for ~24 h in ethanol. After drying the mixture is reacted twice at 1100 °C for 6 h, with an intermediate mortar and pestle grinding. The resulting calcined powder is milled again in ethanol for 48 h. The final dry powder is annealed at 650 °C for 2 h to drive off any residual organics and promote uniform surface hydration. Li3BO3 is produced from stoichiometric amounts of LiOH·H2O and H3BO3. The precursors are mixed by hand with mortar and pestle and then reacted at 600 °C for 2 h in air. X-ray diffraction shows complete conversion to Li3BO3.·The reacted powder is then milled in a zirconia jar in a Spex high energy mill for 15 min. Pellets for dilatometry studies were 5 mm in diameter and approximately 5 mm tall as pressed. The pellets were pressed at 340 MPa. A Linseis Model L75 dilatometer was used for the studies. The pellets were positioned between two Al 2 O 3 supports. Samples were typically heated at 10 °C/min to 700–1000 °C. In some cases, intermediate hold temperatures were employed as is discussed below. The theoretical density of LLZO-Li 3BO 3 mixtures were calculated by following the rule of mixtures and using the theoretical density values for LLZO (5.1 g/cm 3) and Li 3BO 3 (2.16 g/cm 3). The 50
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densities of the sintered samples were compared to the calculated theoretical values to give a percent theoretical density. Knowledge gained from the dilatometer studies was used in the sintering of tapes. In tape casting of ceramics, there are a number of established binders that work well across a wide range of powders, particularly vinyl-based binders (Polyvinyl Alcohol, Polyvinyl Butyral) for both aqueous and non-aqueous systems. Unfortunately, these are subject to gelling in the presence of boron, so any dissolution Li3 BO 3 significantly alters the rheological behavior of the slurry [39]. Boron ions dissolved in the solvent will react with the hydroxyl groups of vinyl-based binders leading to the formation of gel-type structures [40]. Hence, ethylcellulose was chosen as the binder for use with LLZO-Li 3 BO 3 composites. Tape slurry for casting is prepared in a two-step process. First LLZNbO and Li 3BO 3 are dispersed through milling in a solution of menhaden fish oil, ethanol, and toluene. The milling is performed in a teflon jar with ZrO 2 media for 24 h. This slurry is then mixed with a premade solution of ethylcellulose in ethanol, polyethylene glycol 400 and dibutyl phthalate along with additional ethanol and toluene in a centrifugal mixer for 1 h. The slurry is de-aired for 2 min by stirring while subject to a vacuum before being cast on a silicone-coated mylar sheet with a doctor blade set to 200 μm. Typical slip component ratios are shown in Table 1. After drying, the tapes are ~60 μm thick. Square samples approximately 15 mm × 15 mm are cut from the tapes and laminated. Typically, 6 layers are stacked in-between two silicone-coated mylar sheets in a pellet die. The die assembly is heated to 190 °C (above the ethyl cellulose glass transition) and pressed at approximately 120 Mpa for 5 min. After lamination, 6-layer tapes are typically ~200–250 μm thick and have a green density of 60–62%. Organics in the green tapes are burned out at 650 °C for 1 h before being sintered. Tapes are sintered in a flowing argon atmosphere. After sintering, the tapes are typically 150–170 μm thick. Ionic conductivity was characterized by electrochemical impedance spectroscopy (EIS) (Solartron 1260 impedance analyzer with a Solartron 1296 dielectric interface). The frequency range was 10 MHz to 10 Hz with an AC amplitude of 10 mV. Pellets were lightly sanded with 600 grit SiC paper to remove any surface irregularities. Tape surfaces were not sanded but were used in as-sintered condition. Silver paint (SPI) was applied on each side of the electrolyte pellets or tapes. For pellets, the entire end surface was painted. Self-adhesive mylar masks were applied to tapes to provide electrode alignment and a controlled 9mm2 area. After painting, samples were annealed in flowing argon at 750 °C for 3 h. Impedance spectra were collected immediately after removal from the argon ambient to minimize interaction with the lab air. Ionic conductivity values were calculated from the complex impedance plots using ZView2 (Scribner Associates) for analysis. The impedance spectra can be modeled with an equivalent circuit of (R b CPEb )(RgbCPE gb )(R eCPE e) where R b is the bulk resistance, R gb is the grain boundary resistance, R e is the electrode resistance and CPE is a constant phase element.
Fig. 1. Shrinkage of LLZNbO pellets with Li3BO3 content varying from 4 to 8% resulting from heating up to 900 °C at 10 °C/min followed by a 5 hour hold.
3. Results and discussion 3.1. Dilatometry Dilatometry was used to examine effects of Li3BO3 content on pellet densification. Fig. 1 shows the results from a series of dilatometry runs in flowing air on LLZNbO pellets with Li3BO3 content varying from 4 to 8%. These Li3BO3 levels were chosen based on earlier literature reports. These plots show the shrinkage (percent change in length) with temperature as the sample is heated up to 900 °C at 10 °C/min and then held there for 5 h. The onset of sintering is near 710 °C. 6 wt% Li3BO3 additions showed the greatest shrinkage. Due to variations in pressed density, additional samples were sintered in the dilatometer to better verify the trend. Fig. 2 shows the trend in sintered density (% of theoretical density) as a function of Li3BO3 content. Although there is some sample-to-sample variability, the general trend suggests there is a maximum in density around 5% Li3BO3. However, the density at the 5% level is still only 77–80%. The effect of pressing pressure on densification was also examined by dilatometry for pellets with 6% Li3BO3 that were heated up to 1000 °C at 10 °C/min with the results shown in Fig. S2. It was observed that higher pressure (higher green density) is associated with lower shrinkage but slightly higher final densities (see the figure inset). It should be noted that the dilatometer pellets are all pressed at substantially higher pressures than is usually used in experiments with larger furnace sintered pellets. The final relative densities of these pellets are not significantly affected by the pressing pressure.
Table 1 Casting slip component ratios. Slurry component
Weight percent
LLZNbO and Li3BO3 powder Menhaden fish oil dispersant Dibutyl phthalate plasticizer Polyethylene glycol 400 Ethyl cellulose binder (Dow Ethocell 45) Ethanol/toluene solvent
40–45% 1–2% 1–2% 0.5–1% 2–3% 45–55%
Fig. 2. Trend in sintered density (% of theoretical density) as a function of Li3BO3 content of LLZNbO dilatometry pellets heated up to 900 °C at 10 °C/min followed by a 5 hour hold. 51
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Finally, the effect of an intermediate hold, such as reported by Tadanaga, et al. [6], was examined. A series of 7 wt% Li3BO3 LLZNbO pellets were subjected to several variations in heating schedule including i) Ramp 10 °C/min to 900 °C, hold at 900 for 5 h; ii) Ramp 10 °C/min to 700 °C, ramp 0.66 °C/min to 900, hold at 900 for 5 h; iii) Ramp 10 °C/min to 700 °C, hold at 700 °C for 5 h., ramp 10 °C/min to 800, hold at 800 for 5 h. iv) Ramp 10 °C/min to 700 °C, hold at 700 °C for 5 h., ramp 10 °C/min to 900, hold at 900 for 5 h. Overall (see Fig. S3), intermediate hold steps in the temperature ramp appear to have minimal impact on densification relative to the final hold temperature (e.g. 900 vs. 800 °C). With Li3BO3 additions, the higher the sintering temperature the greater shrinkage that was observed. It should be noted that because the dilatometer pellets were positioned between two Al2O3 support disks, there was the possibility of interaction (wetting) with the Al2O3 by the Li3BO3 liquid phase. Although 5% was identified as the optimal level of Li3BO3, interaction with the substrate may lead to possible liquid loss from the pellet. This will be discussed further with regard to sintering of tapes. 3.2. Tape sintering Tapes were sintered in a stacked arrangement with a thin plate (e.g. Al2O3) below and on top of the substrate. The top plate was used to separate a small steel setter weight from the substrate. Tapes sintered without a setter weight tended to curl even if the green piece was relatively flat. The setters typically weighed 9 g. In unreported work, a variety of substrates were examined for sintering of LLZO-Li3BO3 pellets in air, including Al2O3, stabilized ZrO2, La2Zr2O7, and Li2ZrO3. All of them showed varying signs degrees of wetting/reaction with the liquid phase. Similarly, LLZO-Li3BO3 tapes demonstrate significant sensitivity to substrate materials that are contacted during sintering. Fig. S4 shows two LLZO tapes containing 6 wt% Li3BO3 that were sintered in an argon atmosphere at 950 °C for 15 h sandwiched between two substrates: pyrolytic graphite sheets and Al2O3 substrates. Al2O3 contact proves to be detrimental, as the tape contacting Al2O3 curled to a much more significant extent than the one in contact with the graphite sheets. Additionally, XRD of the tape surfaces (Fig. 3) shows formation of both La2Zr2O7 and LaAlO3 in the tape contacting Al2O3, while the tape in contact with graphite appears to be pure LLZO. Moreover, there appears to be no wetting of the graphite by the Li3BO3 rich liquid, whereas wetting of the alumina by the liquid is always observed, as evidenced by subtle stains or color changes to the alumina and the LLZO tape where they contact.
Fig. 4. SEM cross-section of LLNbZO + 6 wt% Li3BO3 tape sintered at 950 °C for 36 h.
3.3. Optimization of Li3BO3 content and sintering conditions At the start of the work Li3BO3 levels in the 6–8% range were examined based on earlier literature reports. SEM inspection of crosssections (Fig. 4) of 6% Li3BO3-LLZO tape sintered at 950 °C for 36 h show large, dark, semi-translucent inclusions that appear to be Li3BO3rich regions. It appears Li3BO3 at the 6wt% level is excessive, leading to liquid pooling, and suggests that enhanced properties can be obtained through lower amounts of liquid. Hence, tapes with Li3BO3 contents from 0 to 5% were examined as described below and higher levels of Li3BO3 were not considered. The combination of the graphite interface and argon atmosphere also appears to mitigate Li-loss at high temperatures. Tapes were sintered at 1000 °C for times up to 48 h. without demonstrating signs of Liloss as usually typified by La2Zr2O7 formation at the sample surface. Higher temperatures lead to improved sintering (e.g., 1000 °C vs. 900 °C). As an example, Table 2 compares 5 wt% Li3BO3 tapes sintered at 950 °C and 1000 °C for 6 h. In Table 2, where more than one conductivity value is given for a particular tape, the values represent measurements made through different regions of the tape. It is not always observed that the highest density results in the highest conductivity. It should be noted that in some cases precise determination of density is challenging due to slight curvature in tapes. Sections are cut from larger cast green tapes and are laminated together to form substrates for sintering. Non-uniformities in the tapes (pores, irregular Li3BO3 distribution, and irregularities in the green tapes) are thought to be the primary cause for variability in the tape conductivity and densities. Table 2 Comparison 5 wt% Li3BO3 tapes sintered at 950 °C and 1000 °C for 6 h. Temperature (°C)
Tape
Density (%)
Conductivity (S cm−1)
1000
1 2 3 Average 1 2 3 Average
89.63 87.55 89.98 89.05 79.14 84.77 88.69 84.20
4.37E-05 1.05E-04 7.77E-05 7.54E-05 6.20E-05 4.16E-05 4.63E-05 5.00E-05
950
Fig. 3. XRD spectra of LLNbZO + 6 wt% Li3BO3 tapes sintered in argon at 950 °C for 15 h. One tape (bottom) was sintered between graphite sheets while the other (top) was sintered between Al2O3 substrates. 52
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Table 3 The effect of sintering time at 1000 °C LLZO + 4% Li3BO3 tapes.
3h 6h
10 h
Table 4 The effect of Li3BO3 concentration in LLZNbO tapes sintered at 1000 °C for 6 h.
Tape
Density (%)
Conductivity (S cm−1)
Average
Li3BO3 wt%
Tape
Density (%)
Conductivity (S cm−1)
Average
1 2 1
87.6 89.6 91.8
1.19E-04
0% 0.50%
92.3 89.6 90.8
89.3
1
91.1
2 3 4
86.2 87.7 –
2
91.3
1.50%
1
83.0
3%
2 1 2
87.5 85.9 85.0
3
85.4
9.24E-05 1.63E-04 2.27E-04 2.55E-04 – – 2.57E-04 3.89E-04 9.68E-05 7.04E-05 1.05E-04 7.82E-05 1.37E-04 5.74E-05 4.43E-05 4.36E-05
1.28E-04
1.11E-04
1 2 1
2
5.56E-05 1.82E-04 1.25E-04 1.18E-04 1.10E-04 9.07E-05 6.52E-05 3.88E-05 8.45E-05
6.29E-05
2.82E-04
9.08E-05
7.21E-05
(Fig. S5). The reduced conductivity is attributed to an increased grain boundary resistance. Fig. 6 shows a series of EIS spectra that illustrate growth of the grain boundary related arc with increased sintering time. Tapes sintered for 48 h and longer appear to have only one arc corresponding to ionic conductivity in the Nyquist plot. However, examination of the Bode plots (Fig. S6) indicates the (RgbCPEgb) circuit element associated with the grain boundary impedance still exists between approximately 1 MHz and 10 kHz with resistance values that place these tapes' conductivities in the 10−6 S cm−1 range. It is hypothesized that inter-grain contact area is reduced by the growth of large grains when there is no corresponding increase in overall tape density. For example, reduced grain boundary area could lead to more grains being separated by a thin glassy phase after the liquid phase solidifies, such as seen in Fig. S5. The studies on 4 and 5% Li3BO3 tapes suggested the level of Li3BO3 additive was still too high. Thus, even lower Li3BO3 levels were examined in a series of tapes, with Li3BO3 concentrations varying from 0 to 3 wt% that were sintered at 1000 °C for 6 h. The results are summarized in Table 4, with typical EIS spectra seen in Fig. 7. The maximum conductivity is observed at 0.5 wt% Li3BO3. The borate-free tapes have higher densities but also demonstrate larger grain boundary attributed resistance than the 0.5 wt% tapes. The decreased conductivity in 1.5 and 3 wt% tapes comes predominantly from increased grain boundary resistance suggesting an increased presence of Li3BO3 at grain boundaries. These higher concentrations also have reduced bulk conductivities. Due to processing variability, tape properties are non-
Fig. 5. SEM cross-section of LLNbZO + 4 wt% Li3BO3 tape sintered at 1000 °C for 48 h.
The effect of sintering time at 1000 °C was examined in a series of 4% Li3BO3 tapes, with results for 3, 6, and 10 h. shown in Table 3. Longer sintering times can result in some very large grains (100+ μm) as depicted in Fig. 5, and a slight density improvement, but that does not necessarily lead to enhanced conductivity performance. Many areas still have small grains (2–4 μm) and traces of liquid phase are evident
Fig. 6. (a) EIS spectra of LLNbZO + 4 wt% Li3BO3 tapes sintered at 1000 °C for a range of times between 6 and 96 h. (b) Enlargement of the box-enclosed region in (a). 53
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Fig. 8. Density and conductivity of LLZNbO pellets sintered on graphite in an argon atmosphere at 1000 °C for 6 h as a function of Li3BO3 content.
Fig. 7. EIS spectra of LLNbZO tapes sintered at 1000 °C for 6 h with Li3BO3 content varying from 0 to 3 wt%.
~14.5% to ~16.5% as Li3BO3 was increased from 0% to 4 wt%. Ionic conductivity reached a peak (~2.5 × 10−4 S cm−1) between 1 and 2 wt % Li3BO3, suggesting an optimal concentration in that range. This is in the same general composition range as the optimum in tapes where 0.5 wt% was found to be the best. It is important to note that the dilatometry pellets (Fig. 4) were in direct contact with Al2O3, while these pellets were contacted by graphite. It is observed that pellets contacting alumina will lose liquid by substrate wetting, but that is not an issue with graphite. Hence the peak at 5% Li3BO3 seen in Fig. 1 is not present for the pellets sintered on graphite. Li3BO3 content was also found to reduce the rate of ionic conductivity degradation in LLZNbO pellets due to air exposure. Pellets with no Li3BO3 and 4 wt% Li3BO3 sintered at 1000 °C for 6 h were examined periodically over 120 h of air exposure. As seen in Fig. S8, after 72 h of air exposure the ionic conductivity pure LLZNbO pellets fell to 20% or less of their initial values while pellets with 4% Li3BO3 fell to only 60% over the same period. The optimal Li3BO3 seen in this work is lower than the preferred values reported by Ohta et al. [8], and Rosero-Navarro et al. [13], and is also less than that reported by Matsuyama, et al. [26] and Shin, et al. [14] Ohta reports an optimal level of 5–10 vol%. At 7 vol% for example, this is equivalent to 0.331 mol Li3BO3 to 1 mol LLZO. Rosero-Navarro finds 0.8 mol Li3BO3 to 1 mol LLZO as optimal, while Matsuyama (0.72 mol Li3BO3) and Shin (0.844 mol Li3BO3) are similar. Our ~1.5 wt % pellet optimum, is equivalent to 0.16 mol Li3BO3 to 1 mol LLZO, while the value in tapes (0.5 wt%) is equivalent to 0.053 mol Li3BO3 to 1 mol LLZO. The difference may be the result of Li3BO3 - substrate interactions. When Li3BO3-LLZO pellets were processed on Al2O3, ZrO2, LLZO, or LaZrO2 substrates, there was wetting or interaction of the substrate by the Li3BO3 observed to varying degrees. In such cases, larger amounts of Li3BO3 may be optimal to compensate for losses of Li3BO3 to the substrate. Similar to Li losses to the atmosphere during high-temperature pellet sintering, this Li3BO3 loss is difficult to control in a repeatable fashion. When using graphite, there is no interaction with Li3BO3. Hence, lower levels of Li3BO3 are needed as there is no compensation for liquid loss that is required. Because the graphite does not react and is not oxidized, the substrates can be reused many times. Argon is necessary for sintering of LLZO with graphite plates, but it has been shown to be beneficial for improved LLZO conductivity compared to processing in air [41]. The tape casting apparatus used in this work limits the cast area to approximately 25 cm × 10 cm. The laminated and sintered tapes are restricted to the ~15 mm × 15 mm sheets described in the experimental methods section due to the size of the press die equipment.
uniform, with the ionic conductivities of two separate areas of the same tape sample sometimes varying by more than a factor of two, as seen for example in Table 4 for 3 wt% tape #2. Visual inspection of the sintered tapes (Supplementary Fig. S7) demonstrates non-uniformity in microstructure, particularly with regard to grain size. 15 mm × 15 mm tapes without Li3BO3 have edge regions with a fine-grained structure (5–10 μm). The center region of the tapes (differentiated in the figures by contrast) consists of a roughly 50/50 mixture of both fine (2–10 μm) and large (~100 μm) grains. The region between the edge and the center is primarily made up of large grains (~100 μm) with only a few small areas of remnant fine grains. This microstructure is likely the result of a ripening process with the small grains undergoing dissolution and reprecipitation on the large grains. Both the 0.5 and 1.5 wt% tapes are similar to the 0 wt% tapes but the central fine-grained area increases with increased Li3BO3 concentration. The 3 wt% tapes are fine-grained throughout with only isolated large grains appearing at the very edges of the tape. A darkened region at the center appears to be due to surface roughness. Interestingly, at 1000 °C the presence of the Li3BO3 appears to slow down the coarsening process compared to the 0 wt% case. Variation in tape properties is attributed, at least in part, to the nonuniformity of grain structure. For example, both Tables 2 and 3 provide examples where conductivity in two different regions varied, in some cases by a factor of two. If the measured region contains a larger proportion of the fine-grained structure, its grain boundary resistance will be higher. The radial nature of the pattern suggests that grain growth in LLZNbO tapes may be promoted by loss of excess lithium (or inhibited by the presence of excess lithium). Since the tapes are covered by graphite on both the top and bottom, the edges of the tape have the most exposure to the surrounding atmosphere and should have a correspondingly higher rate of Li loss. Hence the region at the periphery is where the finest grains are found. In order to discern if lamination density gradients might be responsible, green tapes were sliced in half before sintering. In the sintered half-tapes, what would be the center region in a full tape (and now was at an edge) again showed finer grain structure. At present, more microstructural uniformity is achieved in pellets than in tapes due to processing challenges. Hence the variation in sintered density and conductivity with Li3BO3 for pelletized samples sintered on graphite in an argon atmosphere at 1000 °C for 6 h was also examined, with the results summarized in Fig. 8. Density is seen to decrease somewhat with addition of more Li3BO3. Weight loss of these pellets during sintering also increased with Li3BO3 content going from 54
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However the approach is easily scalable to larger sizes, suggesting an economical route to production of LLZO sheets. No mechanical testing has been performed on sintered tapes but at 150 μm thickness they stand up well to the handling required for examination. Samples with larger grains are observed to be more likely to fracture with handling. Thinner tapes (e.g., 50 μm) can be made, but prove more difficult to handle.
[12] N.C. Rosero-Navarro, T. Yamashita, A. Miura, M. Higuchi, K. Tadanaga, Effect of sintering additives on relative density and Li-ion conductivity of Nb-doped Li7La3ZrO12 solid electrolyte, J. Am. Ceram. Soc. 100 (2017) 276–285. [13] N.C. Rosero-Navarro, A. Miura, M. Higuchi, K. Tadanaga, Optimization of Al2O3 and Li3BO3 content as sintering additives of Li7−xLa2.95Ca0.05ZrTaO12 at low temperature, J. Electron. Mater. (2016) 1–5. [14] R.H. Shin, S.I. Son, Y.S. Han, Y.D. Kim, H.T. Kim, S.S. Ryu, W. Pan, Sintering behavior of garnet-type Li7La3Zr2O12-Li3BO3 composite solid electrolytes for all-solidstate lithium batteries, Solid State Ionics 301 (2017) 10–14. [15] R.H. Shin, S.I. Son, S.M. Lee, Y.S. Han, Y.D. Kim, S.S. Ryu, Effect of Li3BO3 additive on densification and ion conductivity of garnet-type Li7La3Zr2O12 solid electrolytes of all-solid-state lithium-ion batteries, J. Korean Ceram. Soc. 53 (2016) 712–718. [16] L.C. Zhang, J.F. Yang, Y.X. Gao, X.P. Wang, Q.F. Fang, C.H. Chen, Influence of Li3BO3 additives on the Li+ conductivity and stability of Ca/Ta-substituted Li6.55(La2.95Ca0.05)(Zr1.5Ta0.5)O12 electrolytes, J. Power Sources 355 (2017) 69–73. [17] H. Yu, Z.P. Jin, Q. Chen, M. Hillert, Thermodynamic assessment of the lithiumborate system, J. Am. Ceram. Soc. 83 (2000) 3082–3088. [18] M. Tatsumisago, M. Takahashi, T. Minami, M. Tanaka, N. Umesaki, N. Iwamoto, Structural investigation of rapidly quenched Li2O-B2O3 glasses by Raman spectroscopy, Journal of the Ceramic Association, Japan 94 (1986) 464–469. [19] K. Chen, Y. Shen, Y.B. Zhang, Y.H. Lin, C.W. Nan, High capacity and cyclic performance in a three-dimensional composite electrode filled with inorganic solid electrolyte, J. Power Sources 249 (2014) 306–310. [20] E.A. Il'ina, A.A. Raskovalov, N.S. Saetova, B.D. Antonov, O.G. Reznitskikh, Composite electrolytes Li7La3Zr2O12-glassy Li2O-B2O3-SiO2, Solid State Ionics 296 (2016) 26–30. [21] L.L. Feng, L. Li, Y.Q. Zhang, H.J. Peng, Y.P. Zou, Low temperature synthesis and ion conductivity of Li7La3Zr2O12 garnets for solid state Li ion batteries, Solid State Ionics 310 (2017) 129–133. [22] C.W. Ahn, J.J. Choi, J. Ryu, B.D. Hahn, J.W. Kim, W.H. Yoon, J.H. Choi, J.S. Lee, D.S. Park, Electrochemical properties of Li7La3Zr2O12-based solid state battery, J. Power Sources 272 (2014) 554–558. [23] T. Kato, T. Hamanaka, K. Yamamoto, T. Hirayama, F. Sagane, M. Motoyama, Y. Iriyama, In-situ Li7La3Zr2O12/LiCoO2 interface modification for advanced allsolid-state battery, J. Power Sources 260 (2014) 292–298. [24] F.M. Du, N. Zhao, Y.Q. Li, C. Chen, Z.W. Liu, X.X. Guo, All solid state lithium batteries based on lamellar garnet-type ceramic electrolytes, J. Power Sources 300 (2015) 24–28. [25] C. Hansel, S. Afyon, J.L.M. Rupp, Investigating the all-solid-state batteries based on lithium garnets and a high potential cathode - LiMn1.5Ni0.5O4, Nano 8 (2016) 18412–18420. [26] T. Matsuyama, R. Takano, K. Tadanaga, A. Hayashi, M. Tatsumisago, Fabrication of all-solid-state lithium secondary batteries with amorphous TiS4 positive electrodes and Li7La3Zr2O12 solid electrolytes, Solid State Ionics 285 (2016) 122–125. [27] T. Liu, Y. Ren, Y. Shen, S.-X. Zhao, Y. Lin, C.-W. Nan, Achieving high capacity in bulk-type solid-state lithium ion battery based on Li6.75La3Zr1.75Ta0.25O12 electrolyte: interfacial resistance, J. Power Sources 324 (2016) 349–357. [28] J. van den Broek, S. Afyon, J.L.M. Rupp, Interface-engineered all-solid-state Li-ion batteries based on Garnet-type fast Li+ conductors, Adv. Energy Mater. 6 (2016) 1600736. [29] X. Han, Y. Gong, K.K. Fu, X. He, G.T. Hitz, J. Dai, A. Pearse, B. Liu, H. Wang, G. Rubloff, Y. Mo, V. Thangadurai, E.D. Wachsman, L. Hu, Negating interfacial impedance in garnet-based solid-state Li metal batteries, Nat. Mater. 16 (2017) 572–579. [30] M. Shoji, H. Munakata, K. Kanamura, Fabrication of all-solid-state lithium-ion cells using three-dimensionally structured solid electrolyte Li7La3Zr2O12 pellets, Frontiers in Energy Research 4 (2016). [31] E.Y. Yi, W.M. Wang, J. Kieffer, R.M. Laine, Flame made nanoparticles permit processing of dense, flexible, Li+ conducting ceramic electrolyte thin films of cubicLi7La3Zr2O12 (c-LLZO), J. Mater. Chem. A 4 (2016) 12947–12954. [32] E. Yi, W. Wang, J. Kieffer, R.M. Laine, Key parameters governing the densification of cubic-Li7La3Zr2O12 Li+ conductors, J. Power Sources 352 (2017) 156–164. [33] F. Langer, I. Bardenhagen, J. Glenneberg, R. Kun, Microstructure and temperature dependent lithium ion transport of ceramic–polymer composite electrolyte for solid-state lithium ion batteries based on garnet-type Li7La3Zr2O12, Solid State Ionics 291 (2016) 8–13. [34] E. Hanc, W. Zajac, L. Lu, B.G. Yan, M. Kotobuki, M. Ziabka, J. Molenda, On fabrication procedures of Li-ion conducting garnets, J. Solid State Chem. 248 (2017) 51–60. [35] K. Liu, C.A. Wang, Honeycomb-alumina supported garnet membrane: composite electrolyte with low resistance and high strength for lithium metal batteries, J. Power Sources 281 (2015) 399–403. [36] H. Zhang, X.B. Liu, Y. Qi, V. Liu, On the La2/3−xLi3xTiO3/Al2O3 composite solidelectrolyte for Li-ion conduction, J. Alloys Compd. 577 (2013) 57–63. [37] S.C. Li, Q.L. Zhang, T.J. Zhao, H. Yang, Effect of B2O3 on the aqueous tape casting and microwave properties of Li2O-Nb2O5-TiO2 ceramics, Prog. Nat. Sci. 23 (2013) 152–156. [38] S. Ohta, T. Kobayashi, T. Asaoka, High lithium ionic conductivity in the garnet-type oxide Li7−X La3(Zr2−X, NbX)O12 (X = 0–2), J. Power Sources 196 (2011) 3342–3345. [39] C. Wang, H.M. Ji, J. Wang, Effects of solvent system on tape casting of the BaSrTiO3-MgO-based dielectric ceramics containing B2O3, J. Mater. Sci. 47 (2012) 2486–2491. [40] C.W. Cho, Y.S. Cho, J.G. Yeo, S.C. Choi, J. Kim, U. Paik, The role of 2-methyl-2,4pentanediol modifier and its interaction with poly(vinyl butyral) binder in BaTiO3 and Li2O-B2O3-BaO-SiO2 glass suspensions, Colloids Surf. A Physicochem. Eng. Asp. 224 (2003) 83–91. [41] L. Cheng, H.M. Hou, S. Lux, R. Kostecki, R. Davis, V. Zorba, A. Mehta, M. Doeff, Enhanced lithium ion transport in garnet-type solid state electrolytes, J. Electroceram. 38 (2017) 168–175.
4. Conclusions Li7La3Zr1.75Nb0.25Al0.1O12 with Li3BO3 additions was sintered in pellet and cast tape form. Dilatometric analysis shows the onset of sintering with Li3BO3 additions at 710 °C. Graphite substrates are found to be important for eliminating wetting of the substrate by Li3BO3, but necessitate sintering in an inert environment. The feasibility of producing LLZNbO electrolytes by tape casting without the use of sacrificial powder covering was demonstrated using Li3BO3 as a sintering aid. The optimal Li3BO3 content for ionic conductivity in LLZO pellets sintered at 1000 °C was between 1 and 2 wt% Li3BO3, much less than in prior studies, likely due to elimination of Li3BO3-substrate interactions. At this Li3BO3 level ionic conductivity in pellets was ~2.5 × 10−4 S cm−1 after sintering in an argon atmosphere at 1000 °C for 6 h without packing of any extra powder around the pellets. 150–175 μm thick LLZO + 0.5% Li3BO3 tapes sintered at 1000 °C for 6 h exhibited ionic conductivity values up to 2.83 × 10−4 S cm−1 and density of ~90%. These tapes showed grain size non-uniformity that may be related edge effects of Li loss/interaction with the furnace atmosphere during sintering. Acknowledgements The authors gratefully acknowledge partial funding of this work provided by a grant from the Ford University Research Program. The authors thank Mr. Liam McDermott for performing the dilatometry analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ssi.2018.05.015. References [1] R. Murugan, V. Thangadurai, W. Weppner, Fast lithium ion conduction in garnettype Li7La3Zr2O12, Angew. Chem. Int. Ed. 46 (2007) 7778–7781. [2] V. Thangadurai, D. Pinzaru, S. Narayanan, A.K. Baral, Fast solid-state Li ion conducting garnet-type structure metal oxides for energy storage, J. Phys. Chem. Lett. 6 (2015) 292–299. [3] M. Huang, T. Liu, Y. Deng, H. Geng, Y. Shen, Y. Lin, C.-W. Nan, Effect of sintering temperature on structure and ionic conductivity of Li7−xLa3Zr2O12−0.5x (x = 0.5–0.7) ceramics, Solid State Ionics 204–205 (2011) 41–45. [4] P. Babilo, S.M. Haile, Enhanced sintering of yttrium-doped barium zirconate by addition of ZnO, J. Am. Ceram. Soc. 88 (2005) 2362–2368. [5] R.M. German, Sintering With a Liquid Phase, Sintering: From Empirical Observations to Scientific Principles, Butterworth-Heinemann, Boston, 2014, pp. 247–303. [6] K. Tadanaga, R. Takano, T. Ichinose, S. Mori, A. Hayashi, M. Tatsumisago, Low temperature synthesis of highly ion conductive Li7La3Zr2O12-Li3BO3 composites, Electrochem. Commun. 33 (2013) 51–54. [7] S. Ohta, S. Komagata, J. Seki, T. Saeki, S. Morishita, T. Asaoka, All-solid-state lithium ion battery using garnet-type oxide and Li3BO3 solid electrolytes fabricated by screen-printing, J. Power Sources 238 (2013) 53–56. [8] S. Ohta, J. Seki, Y. Yagi, Y. Kihira, T. Tani, T. Asaoka, Co-sinterable lithium garnettype oxide electrolyte with cathode for all-solid-state lithium ion battery, J. Power Sources 265 (2014) 40–44. [9] R. Takano, K. Tadanaga, A. Hayashi, M. Tatsumisago, Low temperature synthesis of Al-doped Li7La3Zr2O12 solid electrolyte by a sol-gel process, Solid State Ionics 255 (2014) 104–107. [10] N. Janani, C. Deviannapoorani, L. Dhivya, R. Murugan, Influence of sintering additives on densification and Li+ conductivity of Al doped Li7La3Zr2O12 lithium garnet, RSC Adv. 4 (2014) 51228–51238. [11] N.C. Rosero-Navarro, T. Yamashita, A. Miura, M. Higuchi, K. Tadanaga, Preparation of Li7La3 (Zr2−x,Nbx)O12 (x = 0–1.5) and Li3BO3/LiBO2 composites at low temperatures using a sol–gel process, Solid State Ionics 285 (2016) 6–12.
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