NSG Found. Mat. Sci. Eng. Rep.

北海道大学

大学院工学

ロゼロナバロナタリーカロリーナ

Nano Glass-Ceramic Lithium-Ion Conductive Buffer Layers for High Li-Ion Transport atElectrode/Electrolyte Interface for All-Solid-State Batteries
Nataly Carolina Rosero-Navarro
Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Japan
Current Affiliation. Institute of Ceramic and Glass ICV-CSIC, Spain
Nano glass-ceramic lithium-ion conductive buffer layers with controlled composition andcrystallization were prepared to generate stable Li-ion transport at the Li|solid electrolyte
interface under high current densities. In particular, Li 2 O-B 2 O 3 glass was used as a sintering
additive and interfacial buffer layer to modify the microstructure of Li 6 . 5 La 3 Zr 1 . 5 Ta 0 . 5 O 12(LLZT) and enhance the Li|LLZT interfacial resistance. The effect of the Li 2 O-B 2 O 3 addition
as sol on the sintering behaviour of LLZT was investigated and compared with those using
Li 2 O-B 2 O 3 as powder. The ionic conductivity of the LLZT sintered with addition of Li 2 OB 2 O 3 sol and Li 2 O-B 2 O 3 powder achieves 8 × 10−4 S cm−1 ( 94 %) and 6 . 3 × 10−5 S cm−1
( 78 %) at room temperature, respectively. Moreover, Li|LLZT interfacial resistance became
lower than those without the use of Li 3 BO 3 , achieving a critical current density of
10 mA cm−2 . The results suggest that the interfacial inorganic layer can favour not only thesintering of solid oxide-type electrolytes but also the inter facial electrode/electrolyte
resistances.
1．Introduction
All-solid-state batteries are one of the most promising candidates for energy storage
systems with high energy density. Nowadays, several solid electrolytes with exceptional highionic conductivities and wide electrochemical windows have been obtained, making them not
only compatible with high-potential electrodes but also a safe alternative for energy storage.
Although the advantages of the solid electrolytes, the realization of all-solid-state batteries
still remains elusive mainly due to high electrode/electrolyte interface resistance. All-solidstate batteries based on oxide-type solid electrolytes (Li 6 . 5 La 3 Zr 1 . 5 Ta 0 . 5 O 12 , LLZT) are ofparticular interest because of chemical compatibility with lithium metal as anode. In thisregard, the main challenge is to reduce the interfacial Li/solid electrolyte resistance andprevent the dendrite formation at high current densities.
Li alloys [ 1 , 2 ] are one of the most used strategies to reduce the interfacial resistance
between LLZ (Li 7 La 3 Zr 2 O 12 ,) solid electrolyte and lithium metal (Li|LLZ interface). Ourgroup[ 3 ] have proposed the use of an amorphous Li 2 O-SiO 2 buffer coating. These strategies
significantly reduce the Li|LLZ interfacial resistance. However, the current limit density, atwhich the cell is shorted by lithium dendrite formation, is still low and far from the current
density required for the fast-charge goal ( 10 mA cm−2 )[ 4 ]. Ideally, new alternatives to handle
free-dendrites Li|solid electrolyte interface at high current density should have adequate− 195 −

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Li-ion conductivity allowing for the stable distribution of the high current densities andmaintaining the integrity of the solid-solid contacts. In this sense, the lithium alloys seem
impractical as the large volume changes[ 5 ] (e.g. 100 – 400 % for Li–Si and Li–Al alloys) could
certainly threaten the integrity of solid-solid contacts.
In this work, the synthesis of low-temperature oxide-type solid electrolytes by using
sintering glassy Li 2 O-B 2 O 3 additives is presented. The use of amorphous Li 2 O-B 2 O 3interfacial material is used to overcome the rigid nature of ceramic-oxide solid electrolytes.
Inorganic Li 2 O-B 2 O 3 interfacial material is a promising alternative to prepare all-solid-statebatteries producing enough physical contacts between solid electrolyte and electrodes. This
report includes results already partially published. [ 6 , 7 ]2．Experimental Section
Synthesis of materials. The synthesis of Ta-doped LLZ by the sol-gel process was carriedout following a reported procedure[ 8 ]. First, LiNO 3 ( 99 %, Kanto chemicals) and La(NO 3 ) 3・
6 H 2 O ( 99 . 9 %, Kanto chemicals) were dissolved in ethanol. Second, Ta(OC 2 H 5 ) 5 ( 99 . 9 %,High-purity chemicals), Zr(OC 4 H 9 ) 4 ( 85 % in butanol, Wako pure chemicals) and ethyl
acetoacetate (EAcAc, 99 %, Kanto chemicals) were dissolved in ethanol under argon
atmosphere and stirred for 1 hour. Then, both solutions were mixed, and the resulted
transparent solution was stirred at room temperature for 1 to 2 hours to obtain a sol. Theobtained sol was dried under vacuum between 80 and 130℃ to obtain a dried gel that waslater pulverized and heat-treated at 700℃ in an alumina crucible for 5 hours ( 1℃ min- 1 ).
Finally, 2 g of this powder and 4 mL of toluene were placed in a ZrO 2 pot and grounded at
300 RPM for 6 hours. Then, toluene was evaporated under vacuum at 100℃ to obtain final
LLZT calcined powder. The molar ratio of Li:La:Zr:Ta:EAcAc was 7 . 15 : 3 : 1 . 3 : 0 . 5 : 1 . 5 andlithium was added in 10 % excess to compensate for the loss of Li during sintering. The Li 2 OB 2 O 3 sol was synthesized by the sol-gel process using trimethoxyborane (TMB, Shin-Etsu
Chemicals) and lithium ethoxide (High-purity chemicals) in a molar ratio 3 : 1 and total
concentration of 40 gL- 1 . TMB was dissolved in ethanol and stabilized by addition of EAcAc
under argon atmosphere. Then, 0 . 1 M HNO 3 was added dropwise (molar ratio TMB:HNO 3of 1 : 0 . 5 ), and the mixture was stirred for 1 hour. In parallel, lithium ethoxide was dissolvedin ethanol. Then, both solutions, TMB and lithium ethoxide, were mixed and stirred for
1 hour to obtain a precursor LBO solution. The LBO powder was synthesized by the solidstate process following a reported procedure[ 8 , 9 ]. Briefly, the Li 2 O (Wako pure chemicals)
and B 2 O 3 (Wako pure chemicals) in a molar ratio 3 : 1 were ball milled under an argon
atmosphere for 40 hours under 370 RPM. Then, the milled powder was heat-treated at 210℃
for 10 hours and finally to 600℃ for 5 hours to obtain Li 3 BO 3 powder. The Al 2 O 3 sol andpowder were obtained by commercial source with par ticle size ～ 50 nm and ～ 1 µm,respectively. Both materials were used without any further purification or pretreatment.
Sintering of LLZT pellets. LBO and Al 2 O 3 , sols and/or powders, were added to the LLZT
calcined powder and mixed in a mortar using toluene. Toluene was completely removed at
80℃ for approximately 2 hours. The calcinated LLZT powder containing sintering additives− 196 −NSG Found. Mat. Sci. Eng. Rep.

were formed into pellets (Ø= 10 mm) at 100 MPa ( 5 min) using a uniaxial press and then,were sintered at 1000℃ ( 10 h) using a heating rate of 1℃ /min and intermedium step at
700℃ ( 5 h). The pellets were sintered under ambient atmosphere using alumina crucibles.
The pellets were thoroughly buried in identical powder to mitigate losses of lithium andprevent any contamination with crucibles.
Physicochemical Characterization. Cr ystal phase was determined by X-ray diffraction(XRD) using a RINT 2000 Ultima RIGAKU diffractometer. Powder samples were scanned
between 10 º and 60 º at a rate of 2 º/min using Cu-Kα radiation. Morphology of the crosssection pellets were obser ved by scanning electron microscopy (SEM), performed on aMultiFlex 600 and JIB- 4600 F Multibeam SEM-FIB Scanning Electron Microscopes. The
relative density was calculated with a geometrical dimension of the pellets and theoreticaldensity of LLZT ( 5 . 11 g cm- 3 ).
Electrochemical characterization. HS Flat Cell HS- 3 ETK (Hohsen Corporation) was used
to perform the electrochemical measurements. The conductivity of the composite solid
electrolyte pellets was evaluated by the electrochemical impedance spectroscopy (EIS),under Ar atmosphere. The samples were Au-coated and sandwiched between two stainlesssteel (SS, blocking electrodes) disks used as current collectors. Symmetric cells wereconstructed with lithium metal heated at 175℃ . EIS measurements were conducted using
an impedance analyzer (SI 1260 , Solartron) in the frequency range of 1 - 3 MHz to 100 Hz at
the amplitude of 80 - 100 mV. The impedance spectra were analyzed with ZView (ScribnerAssociates) to assess the ohmic total resistance (Rt), which was normalized to the pellet
geometry (t: thickness and A: golden surface area) to calculate the conductivity through theusual formula σ= t/Rt.A.
3．Results and Discussion
The following cases were evaluated: (i) LBO and Al 2 O 3 in the powder states, (ii) LBO inthe sol state and Al 2 O 3 in the powder state, and (iii) LBO and Al 2 O 3 in the sol states (LLZTPP, LLZT-SP, and LLZT-SS, denoting the state of each additive (powder or sol), respectively).
XRD patterns of LLZT-PP, LLZT-SP, and LLZT-SS sintered at 1000℃ confirmed the formationof expected cubic phase with the Ia 3¯d space group (ICSD # 183607 ). LLZT-PP and LLZT-SP
exhibited other peaks attributed to the tetragonal phase with the I 41 /acd space group (ICSD# 238686 ), LaAlO 3 , and unidentified phases. The presence of the tetragonal phase isassociated with the insufficient stabilization of the cubic phase, which suggests a nonuniform
distribution of the sintering additives or insufficient amount of them, which could be themain reason for the promotion of secondary reactions during sintering. Nonetheless, theaddition of solution-derived sintering additives leads to a single-phase cubic garnet structurewith a lattice parameter in good agreement with that of LLZT ( 12 . 94 Å)[ 8 , 9 ]. This confirmsthat the liquid phase of sintering additives formed during the sintering remains almost
unreactive with LLZT grains and is present in the sintered ceramic body as an amorphousphase.
Figure 1 shows cross-sectional SEM images of the LLZT pellets without and with sintering− 197 −

（公財）日本板硝子材料工学助成会，42（2024）

additives, including therelative densities. The relative
densities of LLZT, LLZT-PP,LLZT-SP, and LLZT-SS were
50 % , 78 % , 94 % , a n d 88 % ,respectively. LLZT sintered in
the absence of sintering
additives exhibited a porous
microstr ucture with single
par ticles with a size of
approximately 1 µm,indicating an insuf ficientdensification. In contrast, thesintering additives positively
contributed to the LLZT
densification, where larger Figure 1 . SEM image of cross section (a) LLZT without sintering
grains (above 20 µm) wereadditive, (b) LLZT-SS, (c) LLZT-SP, (d) LLZT-SS
observed as a clear signal of
the grain growth and enhanced sintering. Notably, the relative densities of LLZT-PP andLLZT-SP were over- and under-estimated because these samples contained secondar yphases as verified by XRD.
Figure 2 (top) how Nyquist plots of the electrochemical impedance of the of LLZT-PP,LLZT-SP and LLZT-SS pellets measured at 25℃ . The semicircles at high frequencies(> 10 kHz) are associated to the resistance of the LLZT composite electrolyte involving grains
and grain boundaries. The tail at low frequencies is attributed the stainless-steel blocking
electrodes. The total ionic conductivity (grain + grain boundary) calculated by the fitting ofimpedance profiles to the equivalent circuits was 0 . 063 , 0 . 16 , and 0 . 8 mS cm−1 for LLZT-PP,LLZT-SP, and LLZT-SS, respectively. Figure 2 (bottom) shows an Arrhenius plot of the LLZTFigure 2 .

(top) Impedance profiles of LLZT-PP, LLZT-SP and LLZT-SS cells using blocking Au electrodeat 25℃ . (bottom) Arrhenius plot.

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pellets obtained by the total ionic conductivity in the temperature range of 25 – 125℃ . The
activation energy was 14 , 12 , and 9 kJ mol−1 for LLZT-PP, LLZT-SP, and LLZT-SS, respectively.
The lowest activation energy obtained when LLZT is sintered with LBO and Al 2 O 3 sols is
attributed to the low resistance of the grain boundaries. In other words, the ion mobility
through the grain boundaries, formed by a more uniform distribution of LBO presumably
located at the grain boundaries, is facilitated by the good contact between the LLZT grainsand LBO. LBO in the solid electrolyte microstr ucture distributes the cur rent
homogeneously and prevents dendrite nucleation and propagation, suppressing lithium
dendrites up to 10 mA cm−2 (unpublished).
4．Conclusions
The ef fects of Li 2 O–B 2 O 3 and Al 2 O 3 solution-derived sintering additives on thesinterability of LLZT solid electrolyte were studied and compared to their powder-state
counterparts. The use of solution-derived sintering additives leads to a single-phase cubic
garnet structure and enhanced densification. The use of in Li 2 O–B 2 O 3 and Al 2 O 3 sinteringof LLZT ceramic solid electrolyte achieves a high ionic conductivity of 0 . 8 mS cm−1 at 25 ºwith low activation energy ( 9 kJ mol−1 ) and almost negligible contribution of the grainboundary resistance ( 10 %). The designed microstructure of the solid electrolytes plays acritical role in governing the electrochemical properties, particularly the CCD.
5．Acknowledgments
We would like to express our sincere gratitude to the grant organization of Nippon Sheet
Glass Foundation for Materials Science and Engineering of Japan in 2021 for their kind
support for the research activities of the current project.
References[ 1 ] X. Han et al. Nature Materials 16 , 572 , 2016 .
[ 2 ] K. Karthik, et al. Materials Today Energy 16 , 100389 , 2020 .
[ 3 ] N.C. Rosero-Navarro et al. ACS Applied Energy Materials 3 , 6 , 5533 – 5541 , 2020 .
[ 4 ] P. Albertus et al. Nature Energy 3 , 16 – 21 , 2018 .
[ 5 ] D Rehnlund et al. Energy Environ. Sci. 10 , 1350 – 1357 , 2017 .
[ 6 ] N.C. Rosero-Navarro et al. Journal of the European Ceramic Society 41 , 6767 – 6771 ,2021 .
[ 7 ] N.C. Rosero-Navarro. Journal of Sol-Gel Science and Technology 103 ( 2 ), 680 - 689 ,2022 .
[ 8 ] N.C. Rosero-Navarro et al. Journal of Electronic Materials, 46 , 497 - 501 , 2017 .
[ 9 ] N.C. Rosero-Navarro et al. Solid State Ionics, 285 , 6 - 12 . 2016 .

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