Li3BO3âLi2SO4 Interface and Application to Bulk

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Letter

Highly Stable Li/Li3BO3-Li2SO4 Interface and Application to Bulk-Type All-Solid-State Lithium Metal Batteries Kenji Nagao, Motoshi Suyama, Atsutaka Kato, Chie Hotehama, Minako Deguchi, Atsushi Sakuda, Akitoshi Hayashi, and Masahiro Tatsumisago ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00470 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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ACS Applied Energy Materials

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Highly Stable Li/Li3BO3-Li2SO4 Interface and

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Application to Bulk-Type All-Solid-State Lithium

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Metal Batteries

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Kenji Nagao1, Motoshi Suyama1, Atsutaka Kato1,2, Chie Hotehama1, Minako Deguchi1,

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Atsushi Sakuda1, Akitoshi Hayashi1,*, Masahiro Tatsumisago1

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1

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University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan

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2

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Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture

Morinomiya Center, Osaka Research Institute of Industrial Science and Technology, 1-6-50,

Morinomiya, Joto-ku, Osaka, Osaka, Japan

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Corresponding Author

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*Akitoshi Hayashi, E-mail: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT

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All-solid-state batteries (ASSBs) are potentially safe energy storage devices. The

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90Li3BO3·10Li2SO4 (mol%) glass-ceramic is the one of the promising oxide electrolytes due to its

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high ductility and ionic conductivity. Utilization of Li metal negative electrode enhances the

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energy density of ASSBs. Herein, the high electrochemical stability of the 90Li3BO3·10Li2SO4

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electrolyte against Li metal negative electrode was demonstrated. The symmetric cells using a

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dense electrolyte body with relative density of 99% synthesized by the hot-pressing technique

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showed excellent cycle performance for the Li dissolution and deposition reactions. Finally, the

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all-solid-state (Li/80LiNi0.5Mn0.3Co0.2O2·20Li2SO4) full-cell operated as a secondary battery at

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100 °C.

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Keywords:

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electrolytes, Interface

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TOC GRAPHICS

All-solid-state batteries, Li metal negative electrode, Li3BO3, glass-ceramic

75

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Symmetric cell of Li/90Li3BO3·10Li2SO4/Li

0 Enlarged

Voltage / mV

-150

Electrolyte

-75

75

Lithium

Voltage / mV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5"

-2

0.25 mA cm ,1 h

50 25 0 -25 -50 -75

150

152

154

156

158

Time / h

-225 0

50

100

150 Time / h

200

250


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All-solid-state batteries have attracted great interest due to their potential for the construction

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of safe energy storage systems with high energy density1. However, highly ion-conducting solid

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electrolytes are necessary for their fabrication. In recent times, various sulfide-based and oxide-

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based solid electrolytes have been developed and applied to all-solid-state batteries. On the other

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hand, negative and positive electrodes are also important for achieving high energy density in all-

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solid-state batteries. Li metal is the best negative electrode active material due to its high

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theoretical capacity (3861 mAh g-1) and low electrode potential (-3.045 V vs. SHE)1,2. Hence, all-

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solid-state batteries with high energy density are potentially fabricated using Li metal negative

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electrode. However, it is important to obtain high electrochemical stability of solid electrolytes

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against Li metal. Several analyses on the interface between solid electrolytes and Li metal have

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been done to investigate the electrochemical stability of the electrolytes by calculations and

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experiments3–7. Among the solid electrolytes, garnet-type Li7La3Zr2O12 is one of the most

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attracting solid electrolytes in terms of the electrochemical stability against Li metal7,8. Akimoto

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et al. reported that denser electrolytes can prevent Li dendrite formation in the sintered body of the

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garnet-type Li7La3Zr2O129,10. However, the construction of a well-contacted interface between

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electrode and electrolyte particles is challenging due to the poor deformability of typical crystalline

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oxide electrolytes in bulk-type all-solid-state batteries11. Hence, high temperature sintering and

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densification are necessary to achieve a good contact and decrease interfacial resistance12. A large

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amount of interfacial resistance occurs when adverse side reactions occur at the interfaces due to

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high temperatures13,14. Therefore, the fabrication and operation of bulk-type all-solid-state

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batteries using typical crystalline oxide electrolytes are difficult due to large resistance.

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On the other hand, oxide glassy materials can also be used as an electrolyte as well as rather

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than crystalline materials. Glassy materials have better ductility and higher conductivity than


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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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crystalline materials due to their open and random structure15. Moreover, super ion-conducting

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thermodynamically metastable phases, which are difficult to obtain via conventional solid-state

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reactions, can be precipitated by crystallization of glass electrolytes16-18. Furthermore, glassy

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electrolytes are better suited for the construction of interfaces in bulk-type all-solid-state batteries.

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Generally, glass transforms into supercooled liquid and softens around its transition temperature (Tg).

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Thus, a dense compact is possibly obtained by pressing the glass powder around its Tg. Moreover, the

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viscous flow of the supercooled liquid around Tg leads to the formation of a liquid-solid interface, which

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cools to the room temperature to achieve a close solid–solid contact19. In our previous studies, we

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developed several Li3BO3-based glass and glass-ceramic electrolytes with high ductilities and

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ionic conductivities20-22. From these electrolytes, Li2.9B0.9S0.1O3.1 (90Li3BO3·10Li2SO4 in mol%)

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glass-ceramic electrolyte was well-suited for bulk-type all-oxide solid-state batteries due to its high

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ductility and ionic conductivity (~10-5 S cm-1) at room temperature20-22. Furthermore, we

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demonstrated the stable operation of bulk-type all-oxide-solid-state batteries using the Li3BO3-

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based glass-ceramic electrolytes23. Previously, Li-In alloy has been used as a model negative

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electrode model in all-solid-state batteries. Utilization of Li metal enhances the energy density of

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the battery, but the electrochemical stability of Li3BO3-based glass-ceramic electrolytes has not

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been investigated in detail. In this study, the interfacial stability of Li2.9B0.9S0.1O3.1 glass-ceramic

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against Li metal is evaluated by electrochemical analyses, X-ray photoelectron spectroscopy

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(XPS), and scanning electron microscopy (SEM). The construction of oxide-type all-solid-state Li

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metal cells is also demonstrated using hot-pressing technique.

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The electrochemical stability of Li2.9B0.9S0.1O3.1 against Li metal was investigated. Figure 1

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shows the results of the galvanostatic cycling test of the symmetric cell (Li/Li2.9B0.9S0.1O3.1/Li)

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operated at 100 °C. The cycling test was carried out at constant current densities of 0.13 mA cm-2


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and 0.25 mA cm-2 at 100 °C. Relatively stable voltage plateaus were observed for 100 cycles in

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Figure 1a, indicating that the Li dissolution and deposition reactions occurred without any increase

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in the interfacial resistance. Figure 1b describes the complex impedance plots of the symmetric

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cells before and after the cycling test. The inset figures show the enlarged plots at the lower

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frequency region. Moreover, small and large semicircles were observed at the lower and higher

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frequency regions, respectively. The large semicircles are based on the electrolyte (separator)

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resistance, while the small semicircles are based on the interfacial resistance between the Li metal

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and Li2.9B0.9S0.1O3.1 electrolyte. A similar interfacial resistance was also observed in the symmetric

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cells of the lower frequency region using a sulfide Li3PS4 glass electrolyte24. Hence, we determined

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the small resistances to the interfacial ones (Rint) for the symmetric cell using the Li2.9B0.9S0.1O3.1

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electrolyte. The results indicate that the resistance of the reduction products was not large even if

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the electrolytes are partially reduced and generated some interphases. Moreover, the interfacial

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resistance did not increase after the cycle, indicating that the reduction product kinetically

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passivated further reduction reaction. Meanwhile, the synthesized glass-ceramic electrolyte was

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kinetically stable with Li metal negative electrode, which is favorable for the fabrication of all-

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solid-state batteries with high energy density. Furthermore, SEM observation was carried out to

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directly evaluate the interfacial contact between Li metal and the electrolyte. Figure 1c shows the

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cross-sectional SEM images of the symmetric cells before and after the galvanostatic test. The

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interface between the Li2.9B0.9S0.1O3.1 and Li metal was well-contacted even after the cycle.

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Meanwhile, secondary phase was not observed at the interface although Li metal was observed in

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the voids on the surface of the green compacts, indicated by yellow dotted circles in the figure.

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The results imply that the glass-ceramic electrolyte has a high electrochemical stability against Li


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metal negative electrode kinetically. Thus, the interfacial resistance is not increased because a

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highly stable interface is maintained, as shown in Figure 1b.

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Moreover, XPS was performed on the Li/Li2.9B0.9S0.1O3.1 interface to investigate the presence

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of interphases. Figures S1a-d show the XPS spectra for the Li/Li2.9B0.9S0.1O3.1 interface with

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various etching times. The etching was carried out from the surface of the Li metal to the

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electrolyte bulk. Figures 2a,b show the depth profile of atomic ratios for the sample with

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Li/Li2.9B0.9S0.1O3.1 interface while Figures 2c-f show B1s, S2p, O1s, and Li1s XPS spectra for the

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Li/Li2.9B0.9S0.1O3.1 respectively at the outermost surface, interface, and electrode bulk regions. At

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the outermost surface, two peaks corresponding to Li2CO3 (531.4 eV) and Li2O (528.7 eV) in the

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O1s spectrum were observed25. Moreover, two peaks were observed for Li2O and Li2CO3 in the

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Li1s spectrum, with binding energies of 54 eV and 55 eV, respectively25. These results reveal that

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the outermost surface of the Li metal thin-film was slightly oxidized after the deposition. When

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the surface was etched for a while, the peaks of the oxidized components in the O1s and Li1s

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spectra disappeared while the peak of metallic Li appeared in the Li1s spectra with binding energy

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52.6 eV26, indicating that the surface oxidized layer was not thick. However, no peak was observed

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for the B1s and S1p spectra in the outermost region. When the etching time was extended, an

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intermediate phase was confirmed. Meanwhile, weak (185.0 eV) and strong peaks due to the non-

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bridging BO33- unit in the Li3BO3 (191.8 eV) were observed at the interface region in the B1s

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spectra27. This indicates that only a slight BO33- anion was reduced while the additional peak at

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185.0 eV corresponds to the lithium boride (LixB)28. Hensley et al. investigated the interfacial

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stability of LiBO2 glass electrolyte against Li metal negative electrode by XPS analysis28. They

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observed a weak peak (186 eV) and a main borate peak (191.8 eV) at the Li/LiBO2 interface,

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indicating that the lithium meta-borate glass was partially reduced while lithium boride species


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were generated20. In the S2p spectra, S2- doublet peaks for Li2S4,29 and SO32- species30 (160.3 eV

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and 167.4 eV, respectively) and for SO42- (169.4 eV) appeared27, indicating that the Li2SO4

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component in the glass-ceramic electrolyte was partially reduced by the Li metal negative

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electrode. Furthermore, Li2O was also confirmed in the O1s and Li1s spectra. Thus, the

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intermediate reacted layer was composed of Li2O, Li2S, and LixB. When the etching time was

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increased, the peak intensities of the reduced species decreased while the peak intensities of

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Li3BO3 and Li2SO4 components increased. This clearly reveals that the reduction reaction at the

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interface was kinetically suppressed. The reduced species at the interface passivated further

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reduction reaction due to their insulating property, thereby reducing the thickness of the reduced

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interphase. Furthermore, a stable interfacial contact was maintained after the galvanostatic cycling

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test as shown in Figure 1c. These are the reasons for maintaining the lower interfacial resistance.

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As discussed earlier, the Li2.9B0.9S0.1O3.1 glass-ceramic electrolyte has a high electrochemical

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stability kinetically. However, some voids and grain boundaries exist in the green compacts, which

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permit the Li dendrite formation as shown in Figure 1c. Hence, a denser electrolyte layer is

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necessary to prevent the dendrite formation and obtain better cycling properties as in the case of

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Li7La3Zr2O12 crystal. Therefore, we fabricated a more densified pellet with relative density of 99%

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around Tg by hot-pressing technique.

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Figure 3a shows the cross-sectional SEM images of the cold-pressed and hot-pressed

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Li2.9B0.9S0.1O3.1 glass-ceramic electrolyte compacts. The density of the hot-pressed densified pellet

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(99%) was higher than that of the cold-pressed pellet (88%) when it was pressed under 720 MPa

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at room temperature. These relative densities were calculated using the bulk densities of the

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compacts and the powder density (2.09 g cm-3) measured by the Ar gas pychnometer22. Hot-

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pressing technique is useful for obtaining a dense electrolyte compact with no grain boundaries


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and voids. Figure 3b shows the temperature dependence of the conductivity for both compacts.

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The ionic conductivities of the glass-ceramic electrolytes had a similar value of 1.0×10-5 S cm-1

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at 25 °C, indicating that the influence of voids and grain boundary on the ionic conductivity was

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negligible. Thus, the influence of microstructure on the compacts can be evaluated without the

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influence of ionic conductivity on the Li dissolution and deposition properties. Figures 3c-d show

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the results of the galvanostatic cycling test for the symmetric cells using cold-pressed and hot-

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pressed electrolytes at 100 °C, respectively. The applied current density was gradually increased,

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and the critical current density (CCD) for the Li metal dissolution and deposition reactions was

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determined for these symmetric cells. A stable cycle occurred for the hot-pressed cell even at

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higher current densities. The CCDs of the cold-pressed and hot-pressed cells were 1.15 and 1.53

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mA cm-2, respectively. Hence, the denser electrolyte body prevents Li dendrite formation during

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the cycling processes, thereby enabling the symmetric cells to be operated at higher current

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densities. Figure S2 shows the galvanostatic cycling test for the hot-pressed symmetric cell at 100

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°C. A constant current of 0.25 mA cm-2 was applied for 2, 5, and 10 hours, and stable voltage

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plateaus were observed for all cycles. The obtained capacity was 2.55 mAh cm-2, which was

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sufficient to be applied to the Li metal of all-solid-state cells. Electrochemical performances of all-

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solid-state Li symmetric cells using the Li2.9B0.9S0.1O3.1 electrolyte with a comparison of typical

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solid electrolytes of Li7La3Zr2O12 and Li3PS4 are summarized in Table S1.

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All-oxide-solid-state Li metal cells were fabricated and their charge-discharge performances

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were evaluated. Amorphous 80LiNi0.5Mn0.3Co0.2O2·20Li2SO4 was used as a positive electrode

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active material due to its high ductility and mixed conductivity23. The all-solid-state cells were

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fabricated by cold-pressing or hot-pressing the constitute powders. Figure S3 shows the charge-

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discharge performance of the cold-pressed all-solid-state cells at 100 °C. The cell with the


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amorphous 80LiNi0.5Mn0.3Co0.2O2·20Li2SO4 gave the charge-discharge curves with a gentle

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slope23, which is somewhat different with the behavior of typical crystalline LiNi0.5Mn0.3Co0.2O2

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having a layered rock-salt structure. The operation of the cold-pressed cell was stable at lower

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current densities, with a capacity of ~160 mAh g-1. However, when the applied current density was

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increased to 1.27 mA cm-2, the operation of the cell was not stable. Meanwhile, a higher capacity

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was obtained at a high current density during the charging process than in previous cycles with

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lower current densities. A similar phenomenon was observed in all-solid-state cells using sulfide

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electrolytes and Li metal negative electrode24. This phenomenon was probably caused by an

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internal short-circuit in the cell due to Li dendrite formation during the charging process30. Thus,

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a stable charge-discharge was achieved in the hot-pressed cell. Figure 4 shows the charge-

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discharge performance of the hot-pressed all-solid-state cell. Meanwhile, some voids and grain

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boundaries existed in the cold-pressed composite electrode layer23. However, these voids and grain

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boundaries were hardly observed in the composite electrode layer when a hot-pressing technique

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was used. Hence, a favorable electrode-electrolyte interface with large contact area was obtained

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as shown in Figure 4c. The all-oxide-solid-state cell (Li/80NMC532·20Li2SO4) operated as a

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secondary battery with a high specific capacity of ~150 mAh g-1 at a constant current density of

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0.25 mA cm-2 at 100 °C, as shown in Figure 4a. When the cell was cycled under the low current

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density of 0.25 mA cm-2 at 100 °C, the slight capacity fading was observed in a few initial cycles.

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XPS results for the Li symmetrical cell after the galvanostatic cycling test at 100 °C are shown in

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Fig. S4. Almost the same result as shown in Fig. S1 was obtained, meaning that the stable interface

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was maintained due to the passivating effect of the reduction materials. Moreover, an obvious

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resistance increase was not observed in the symmetric cells even after the cycling test like in Fig.

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1b. These results suggest that the capacity fading comes not from the negative electrode side but


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from the positive electrode side. The cells were stably charged and discharged at a high current

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density of 1.27 mA cm-2 at 100 °C, as opposed to the cold-pressed cell (Fig. S3). Figure 4d shows

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the cross-sectional SEM images at the interface between the Li metal negative electrode and hot-

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pressed electrolyte in the all-solid-state cells after charge-discharge measurements. An excellent

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interfacial contact without any thick intermediate phase was maintained even after the charge-

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discharge. The dense electrolytes suppressed the internal short-circuit due to dendritic formation,

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thereby enabling the high current operation of the all-solid-state Li cells.

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In conclusion, we have evaluated the interfacial stability of the Li2.9B0.9S0.1O3.1 glass-ceramic

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electrolyte against Li metal negative electrode. Although the reduced compounds were confirmed

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at the interfacial region by XPS, the interphase kinetically passivated further reduction reactions

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due to its insulating property. Hence, no obvious increase in the interfacial resistance was

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observed. On the other hand, stable dissolution and deposition reactions occurred in the hot-

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pressed symmetric cells of Li/Li2.9B0.9S0.1O3.1/Li at a constant current density of 1.27 mA cm-2 at

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100 °C. Finally, we fabricated a bulk-type all-oxide solid-state Li metal cells which were stable

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operated as a secondary battery. Therefore, hot-pressing technique is an effective means for the

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fabrication of bulk-type all-solid-state batteries.

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(a)

0 0.075

-0.075

-0.15

0.075

0.13 mA cm -2,1 h

0.050

Enlarged

Cell potential / V

Enlarged

Cell potential / V

Cell potential / V

0.075

0.025 0.0 -0.025 -0.050 -0.075

6

8

10

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0.025 0.0 -0.025 -0.050 -0.075

14

0.25 mA cm -2,1 h

0.050

150

152

Time / h

-0.225

0

154

156

158

Time / h

50

100

150

200

250

Time / h (c)

0 164 165 166 167 168

Z’ /

-2 -1 0 164 165 166 167 168 Z’ /

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160

Z’ /

180

200

220

After cycling

-40 -30 -20 -10 0 120

Z’’ /

After the 100th cycles at 100o C -4 50 mV, 106-10-1 Hz -3

Electrolyte

R int

-1

Lithium

-2

Back-scattering electron images

5 "m

5 "m

5 "m

5 "m

Electrolyte

-3

Secondary electron images

Lithium

-4

Before cycling at 100o C 50 mV, 106-10-1 Hz Z’’ /

-50 -40 -30 -20 -10 -500

Before cycling

Z’’ /

(b)

Z’’ /

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Figure 1: (a) Galvanostatic cycling test for the symmetric cell (Li/Li2.9B0.9S0.1O3.1/Li) operated at

2

constant current densities of 0.13 and 0.25 mA cm-2 at 100 °C. The cold-pressed pellet was used

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as a solid electrolyte. (b) Complex impedance plots of the symmetric cell before and after the

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galvanostatic test for 100 cycles. (c) Cross-sectional SEM images of the symmetric cells before

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and after the galvanostatic cycling test. The left and right figures are the secondary electron

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images and the back-scattering electron images, respectively.

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Electrolyte bulk

Li

60 40

O B

20 C

BO33-

0

500 1000 1500 2000 2500 Etching time / s

(b)

Thickness / nm SiO2 200 400 600 800 1000 1200

0

Li xB Outermost surface

195

190 185 Binding energy / eV

180

Electrolyte bulk

SO32-

Interface S2- (in Li 2S) 2p 3/2 2p 1/2 Outermost surface

175

170 165 160 Binding energy / eV

Intensity (arb.unit)

B

2 S

BO33SO42-

Interface Li 2O Li 2O

CO32-

1

155

(f) Li1s

(e) O1s Electrolyte bulk

C

3

2p 3/2

2p 1/2

Interface

Interface

4

SO42-

Electrolyte bulk

S

0

5

(d) S2p


Atomic ratio / %

80

Outermost surface Interface

(c) B1s

Electrolyte bulk


100

0


Thickness / nm SiO2 200 400 600 800 1000 1200

(a)

Atomic ratio / %

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Page 12 of 21

Outermost surface

Li 3BO3 or Li 2SO4

Interface

Li 2O

Li

Li 2O Li 2CO3

Outermost surface

0 0

500 1000 1500 2000 2500 Etching time / s

536

534

532 530 528 526 Binding energy / eV

524

60

58

56 54 52 50 Binding energy / eV

48

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Figure 2: (a) Correlation between the atomic ratio of the constituent elements and etching time.

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(b) Enlarged profile for the region of the low atomic ratio. (c) XPS spectra of B1s, (d) XPS

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spectra of S2p, (e) XPS spectra of O1s, and (f) XPS spectra of Li1s, for the outermost surface,

4

interface, and electrolyte bulk regions.

5 6


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EXPERIMENTAL SECTION

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Synthesis of Li3BO3-Li2SO4 glass-ceramic electrolytes. The Li2.9B0.9S0.1O3.1 (90Li3BO3·10Li2SO4

3

in mol%) glass electrolyte was synthesized using a mechanochemical technique21,22. The

4

synthesized glass powder was cold-pressed into a pellet under 720 MPa at room temperature.

5

Moreover, the glass powder was hot-pressed under 540 MPa at its transition temperature (250 °C)

6

for 2 hours to obtain a more densified pellet. Meanwhile, the molding temperatures of the glass

7

samples were determined from the DTA curve21,22. The diameter and thickness of the pellet were

8

10 mm and ~0.7 mm, respectively. The glass pellets were then heat-treated at 290 °C for 1 hour to

9

crystallize the highly ion-conducting phase.

10

Electrochemical measurement. Ionic conductivity of the compacts was measured by an alternating

11

current (AC) impedance technique. A gold thin-film electrode was deposited onto both surfaces of

12

the compacts using a vacuum evaporation technique with a sputter apparatus (Quick Coater SC-

13

701; Sanyu Electron Corp.) while the AC impedance measurements were obtained using an

14

impedance analyser (SI-1260; Solartron Analytical). The frequency range and applied voltage

15

were 1 MHz ~ 0.1 Hz and 50 mV, respectively. Furthermore, the electrochemical stability of the

16

Li2.9B0.9S0.1O3.1 glass-ceramic electrolyte was evaluated against Li metal negative electrode using

17

galvanostatic cycling test. Li metal foils (99.99%; 9 mm , 0.25 mm thickness; Furuuchi Chem.

18

Corp.) and stainless steel foils (10 mm , 20 N

19

electrolyte pellets as current collectors, and the symmetric cells were pressed by cold isostatic

20

pressing (CIP) under 80 MPa. The galvanostatic cycling test was then carried out on the symmetric

21

cells at 100 °C using charge-discharge measuring devices (BTS-2004, Nagano Co.). The

22

symmetric cells operated at a constant current density along a negative potential direction for 30

23

min to 1 h and then switched to a positive potential direction.

thickness) were attached to both surfaces of the


15


of

the

Li/Li2.9B0.9S0.1O3.1

interface.

Page 16 of 21

1

Characterization

XPS

was

conducted

on

the

2

Li/90Li3BO3·10Li2SO4 interface by using a spectrometer (K-Alpha, Thermo Fisher Scientific).

3

The Li metal thin-film was deposited onto the surface of the Li2.9B0.9S0.1O3.1 glass-ceramic pellets

4

by a thermal evaporator placed in an Ar-filled glove box, using a vacuum evaporation technique.

5

The samples were then transferred to the analysis chamber of the spectrometer by a vessel

6

containing dry Ar gas. Monochromatic

7

The samples were neutralized by a flood gun during the measurement to reduce the influence of

8

charging effect on the spectra. The samples were then etched by an Ar+ ion monomer and depth

9

analyses were carried out from the top of the Li thin-film to the Li2.9B0.9S0.1O3.1 electrolyte bulk.

10

The obtained spectra were calibrated by setting the measured binding energy of the C1s peak to

11

284.7 eV of adventitious carbon accumulated in the analysis chamber.

12

Fabrication of all-solid-state Li metal cells. All-solid-state cells were fabricated and their charge-

13

discharge properties were evaluated. An amorphous 80LiNi0.5Mn0.3Co0.2O2·20Li2SO4 was used as

14

a positive electrode active material23. The active material (68.4 wt.%), Li2.9B0.9S0.1O3.1 glass

15

electrolyte (28.8 wt.%), and acetylene black carbon (3.8 wt.%) powders were then mixed using a

16

mortar and pestle and used as a composite positive electrode. The three-layered pellets of the

17

stainless steel (ca. 200 mg), composite electrode (ca. 10 mg), and glass electrolyte (ca. 50 mg)

18

were obtained by cold-pressing these powders at room temperature under 720 MPa or hot-pressing

19

them at 250 °C under 540 MPa for 2 h. The obtained pellets were then heat-treated at 290 °C to

20

increase their conductivity by crystallizing the Li2.9B0.9S0.1O3.1 electrolyte. A Li metal foil and a

21

stainless steel foil were attached to the electrolyte surface of the pellets as current collectors, and

22

the fabricated cells were pressed by CIP under 80 MPa. The cells were then charged and discharged

23

using a charge-discharge measuring device (BTS-2004; Nagano Co. Ltd.) at 100 °C.

)EO radiation (1486.6 eV) was used as the X-ray source.


16

Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Moreover, cross-sections of the cells for the scanning electron microscopy (SEM) observations

2

were prepared using Ar+ ion milling system (E-3500; Hitachi High-Technologies Corp.). A field

3

emission SEM (FE-SEM, SU8220; Hitachi High-Technologies Corp.) was used to investigate the

4

electrode/electrolyte interfacial contacts.

5 6

ASSOCIATED CONTENT

7

Supporting Information

8

Supporting Information is available free of charge on the ACS Publications website.

9

Summary of XPS spectra, charge-discharge performance of all-solid-state cells, SEM images

10 11

AUTHOR INFORMATION

12

Corresponding author:

13

*E-mail address: [email protected]

14

ORCID

15

Kenji Nagao: 0000-0002-2462-7620

16

Atsushi Sakuda: 0000-0002-9214-0347

17

Akitoshi Hayashi: 0000-0001-9503-5561

18

Masahiro Tatsumisago: 0000-0002-4836-0158


17


1

Notes

2

The authors declare no competing financial interest.

Page 18 of 21

3 4

ACKNOWLEDGMENTS

5

This work was supported by JSPS KAKENHI Grant Numbers JP18H05255 and JP18J14547.

6 7

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