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Cite This: ACS Appl. Energy Mater. 2019, 2, 3042−3048
Highly Stable Li/Li3BO3−Li2SO4 Interface and Application to BulkType All-Solid-State Lithium Metal Batteries Kenji Nagao,† Motoshi Suyama,† Atsutaka Kato,†,‡ Chie Hotehama,† Minako Deguchi,† Atsushi Sakuda,† Akitoshi Hayashi,*,† and Masahiro Tatsumisago*,† †
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Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan ‡ Morinomiya Center, Osaka Research Institute of Industrial Science and Technology, 1-6-50, Morinomiya, Joto-ku, Osaka, Osaka Japan S Supporting Information *
ABSTRACT: All-solid-state batteries (ASSBs) are potentially safe energy storage devices. The 90Li3BO3·10Li2SO4 (mol %) glass-ceramic is one of the promising oxide electrolytes due to its high ductility and ionic conductivity. Utilization of Li metal negative electrode enhances the energy density of ASSBs. Herein, the high electrochemical stability of the 90Li3BO3·10Li2SO4 electrolyte against Li metal negative electrode was demonstrated. The symmetric cells using a dense electrolyte body with relative density of 99% synthesized by the hot-pressing technique showed excellent cycle performance for the Li dissolution and deposition reactions. Finally, the all-solid-state (Li/80LiNi0.5Mn 0.3 Co0.2O 2· 20Li2SO4) full cell operated as a secondary battery at 100 °C. KEYWORDS: all-solid-state batteries, Li metal negative electrode, Li3BO3, glass-ceramic electrolytes, interface
A
tion are necessary to achieve good contact and decreased interfacial resistance.12 A large amount of interfacial resistance occurs when adverse side reactions occur at the interfaces due to high temperatures.13,14 Therefore, the fabrication and operation of bulk-type all-solid-state batteries using typical crystalline oxide electrolytes are difficult due to large resistance. On the other hand, oxide glassy materials can also be used as an electrolyte rather than crystalline materials. Glassy materials have better ductility and higher conductivity than crystalline materials due to their open and random structure.15 Moreover, super-ion-conducting thermodynamically metastable phases, which are difficult to obtain via conventional solid-state reactions, can be precipitated by crystallization of glass electrolytes.16−18 Furthermore, glassy electrolytes are better suited for the construction of interfaces in bulk-type all-solidstate batteries. Generally, glass transforms into supercooled liquid and softens around its transition temperature (Tg). Thus, a dense compact is possibly obtained by pressing the glass powder around its Tg. Moreover, the viscous flow of the supercooled liquid around Tg leads to the formation of a liquid−solid interface, which cools to room temperature to achieve a close solid−solid contact.19 In our previous studies, we developed several Li3BO3-based glass and glass-ceramic
ll-solid-state batteries have attracted great interest due to their potential for the construction of safe energy storage systems with high energy density.1 However, highly ionconducting solid electrolytes are necessary for their fabrication. In recent times, various sulfide-based and oxide-based solid electrolytes have been developed and applied to all-solid-state batteries. On the other hand, negative and positive electrodes are also important for achieving high energy density in allsolid-state batteries. Li metal is the best negative electrode active material due to its high theoretical capacity (3861 mAh g−1) and low electrode potential (−3.045 V vs SHE).1,2 Hence, all-solid-state batteries with high energy density are potentially fabricated using Li metal negative electrode. However, it is important to obtain high electrochemical stability of solid electrolytes against Li metal. Several analyses on the interface between solid electrolytes and Li metal have been done to investigate the electrochemical stability of the electrolytes by calculations and experiments.3−7 Among the solid electrolytes, garnet-type Li7La3Zr2O12 is one of the most attractive solid electrolytes in terms of the electrochemical stability against Li metal.7,8 Akimoto et al. reported that denser electrolytes can prevent Li dendrite formation in the sintered body of the garnet-type Li7La3Zr2O12.9,10 However, the construction of a well-contacted interface between electrode and electrolyte particles is challenging due to the poor deformability of typical crystalline oxide electrolytes in bulk-type all-solid-state batteries.11 Hence, high temperature sintering and densifica© 2019 American Chemical Society
Received: March 4, 2019 Accepted: May 8, 2019 Published: May 8, 2019 3042
DOI: 10.1021/acsaem.9b00470 ACS Appl. Energy Mater. 2019, 2, 3042−3048
Letter
ACS Applied Energy Materials
Figure 1. (a) Galvanostatic cycling test for the symmetric cell (Li/Li2.9B0.9S0.1O3.1/Li) operated at constant current densities of 0.13 and 0.25 mA cm−2 at 100 °C. The cold-pressed pellet was used as a solid electrolyte. (b) Complex impedance plots of the symmetric cell before and after the galvanostatic test for 100 cycles. (c) Cross-sectional SEM images of the symmetric cells before and after the galvanostatic cycling test. The left and right figures are the secondary electron images and the backscattering electron images, respectively.
Li2.9B0.9S0.1O3.1/Li) operated at 100 °C. The cycling test was carried out at constant current densities of 0.13 and 0.25 mA cm−2 at 100 °C. Relatively stable voltage plateaus were observed for 100 cycles in Figure 1a, indicating that the Li dissolution and deposition reactions occurred without any increase in the interfacial resistance. Figure 1b describes the complex impedance plots of the symmetric cells before and after the cycling test. The inset figures show the enlarged plots at the lower frequency region. Moreover, small and large semicircles were observed at the lower and higher frequency regions, respectively. The large semicircles are based on the electrolyte (separator) resistance, while the small semicircles are based on the interfacial resistance between the Li metal and Li2.9B0.9S0.1O3.1 electrolyte. A similar interfacial resistance was also observed in the symmetric cells of the lower frequency region using a sulfide Li3PS4 glass electrolyte.24 Hence, we determined the small resistances to the interfacial ones (Rint) for the symmetric cell using the Li2.9B0.9S0.1O3.1 electrolyte. The results indicate that the resistance of the reduction products was not large even if the electrolytes are partially reduced and generated some interphases. Moreover, the
electrolytes with high ductilities and ionic conductivities.20−22 From these electrolytes, Li2.9B0.9S0.1O3.1 (90Li3BO3·10Li2SO4; mol %) glass-ceramic electrolyte was well-suited for bulk-type all-oxide solid-state batteries due to its high ductility and ionic conductivity (∼10−5 S cm−1) at room temperature.20−22 Furthermore, we demonstrated the stable operation of bulktype all-oxide-solid-state batteries using the Li3BO3-based glass-ceramic electrolytes.23 Previously, Li−In alloy has been used as a model negative electrode model in all-solid-state batteries. Utilization of Li metal enhances the energy density of the battery, but the electrochemical stability of Li3BO3-based glass-ceramic electrolytes has not been investigated in detail. In this study, the interfacial stability of Li2.9B0.9S0.1O3.1 glassceramic against Li metal is evaluated by electrochemical analyses, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The construction of oxide-type all-solid-state Li metal cells is also demonstrated using hot-pressing technique. The electrochemical stability of Li2.9B0.9S0.1O3.1 against Li metal was investigated. Figure 1 shows the results of the galvanostatic cycling test of the symmetric cell (Li/ 3043
DOI: 10.1021/acsaem.9b00470 ACS Appl. Energy Mater. 2019, 2, 3042−3048
Letter
ACS Applied Energy Materials
Figure 2. (a) Correlation between the atomic ratio of the constituent elements and etching time. (b) Enlarged profile for the region of the low atomic ratio. (c) XPS spectra of B 1s, (d) XPS spectra of S 2p, (e) XPS spectra of O 1s, and (f) XPS spectra of Li 1s, for the outermost surface, interface, and electrolyte bulk regions.
The etching was carried out from the surface of the Li metal to the electrolyte bulk. Panels a and b of Figure 2 show the depth profile of atomic ratios for the sample with Li/Li2.9B0.9S0.1O3.1 interface, while panels c−f of Figure 2 show B 1s, S 2p, O 1s, and Li 1s XPS spectra for Li/Li2.9B0.9S0.1O3.1 respectively at the outermost surface, interface, and electrode bulk regions. At the outermost surface, two peaks corresponding to Li2CO3 (531.4 eV) and Li2O (528.7 eV) in the O 1s spectrum were observed.25 Moreover, two peaks were observed for Li2O and Li2CO3 in the Li 1s spectrum, with binding energies of 54 and 55 eV, respectively.25 These results reveal that the outermost surface of the Li metal thin film was slightly oxidized after the deposition. When the surface was etched for a while, the peaks of the oxidized components in the O 1s and Li 1s spectra disappeared while the peak of metallic Li appeared in the Li 1s spectra with binding energy 52.6 eV,26 indicating that the surface oxidized layer was not thick. However, no peak was observed for the B 1s and S 1p spectra in the outermost region. When the etching time was extended, an intermediate phase was confirmed. Meanwhile, weak (185.0 eV) and strong peaks due to the nonbridging BO33− unit in Li3BO3 (191.8 eV) were observed at the interface region in the B 1s spectra.27 This indicates that only a slight BO33− anion was reduced while the
interfacial resistance did not increase after the cycle, indicating that the reduction product kinetically passivated further reduction reaction. Meanwhile, the synthesized glass-ceramic electrolyte was kinetically stable with Li metal negative electrode, which is favorable for the fabrication of all-solidstate batteries with high energy density. Furthermore, SEM observation was carried out to directly evaluate the interfacial contact between Li metal and the electrolyte. Figure 1c shows the cross-sectional SEM images of the symmetric cells before and after the galvanostatic test. The interface between the Li2.9B0.9S0.1O3.1 and Li metal was well-contacted even after the cycle. Meanwhile, a secondary phase was not observed at the interface, although Li metal was observed in the voids on the surface of the green compacts, indicated by yellow dotted circles in the figure. The results imply that the glass-ceramic electrolyte has a high electrochemical stability against Li metal negative electrode kinetically. Thus, the interfacial resistance is not increased because a highly stable interface is maintained, as shown in Figure 1b. Moreover, XPS was performed on the Li/Li2.9B0.9S0.1O3.1 interface to investigate the presence of interphases. Panels a−d of Supporting Information Figure S1 show the XPS spectra for the Li/Li2.9B0.9S0.1O3.1 interface with various etching times. 3044
DOI: 10.1021/acsaem.9b00470 ACS Appl. Energy Mater. 2019, 2, 3042−3048
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ACS Applied Energy Materials
Figure 3. (a) Temperature dependence of the conductivity of the cold-pressed and hot-pressed Li2.9B0.9S0.1O3.1 glass-ceramic electrolytes. (b) Cross-sectional SEM images for the cold-pressed and hot-pressed electrolytes. Galvanostatic cycling test for the symmetric cells using the (c) coldpressed and (d) hot-pressed Li2.9B0.9S0.1O3.1 glass-ceramic electrolytes at 100 °C for various current densities.
pellet with relative density of 99% around Tg by hot-pressing technique. Figure 3a shows the cross-sectional SEM images of the coldpressed and hot-pressed Li2.9B0.9S0.1O3.1 glass-ceramic electrolyte compacts. The density of the hot-pressed densified pellet (99%) was higher than that of the cold-pressed pellet (88%) when it was pressed under 720 MPa at room temperature. These relative densities were calculated using the bulk densities of the compacts and the powder density (2.09 g cm−3) measured by the Ar gas pycnometer.22 Hot-pressing technique is useful for obtaining a dense electrolyte compact with no grain boundaries and voids. Figure 3b shows the temperature dependence of the conductivity for both compacts. The ionic conductivities of the glass-ceramic electrolytes had a similar value of 1.0 × 10−5 S cm−1 at 25 °C, indicating that the influence of voids and grain boundary on the ionic conductivity was negligible. Thus, the influence of microstructure on the compacts can be evaluated without the influence of ionic conductivity on the Li dissolution and deposition properties. Panels c and d of Figure 3 show the results of the galvanostatic cycling test for the symmetric cells using cold-pressed and hot-pressed electrolytes at 100 °C, respectively. The applied current density was gradually increased, and the critical current density (CCD) for the Li metal dissolution and deposition reactions was determined for these symmetric cells. A stable cycle occurred for the hotpressed cell even at higher current densities. The CCDs of the cold-pressed and hot-pressed cells were 1.15 and 1.53 mA cm−2, respectively. Hence, the denser electrolyte body prevents Li dendrite formation during the cycling processes, thereby enabling the symmetric cells to be operated at higher current
additional peak at 185.0 eV corresponds to the lithium boride (LixB).28 Hensley and Garofalini investigated the interfacial stability of LiBO2 glass electrolyte against Li metal negative electrode by XPS analysis.28 They observed a weak peak (186 eV) and a main borate peak (191.8 eV) at the Li/LiBO2 interface, indicating that the lithium meta-borate glass was partially reduced while lithium boride species were generated.20 In the S 2p spectra, S2− doublet peaks for Li2S4,29 and SO32− species30 (160.3 and 167.4 eV, respectively) and for SO42− (169.4 eV) appeared,27 indicating that the Li2SO4 component in the glass-ceramic electrolyte was partially reduced by the Li metal negative electrode. Furthermore, Li2O was also confirmed in the O 1s and Li 1s spectra. Thus, the intermediate reacted layer was composed of Li2O, Li2S, and LixB. When the etching time was increased, the peak intensities of the reduced species decreased while the peak intensities of Li3BO3 and Li2SO4 components increased. This clearly reveals that the reduction reaction at the interface was kinetically suppressed. The reduced species at the interface passivated further reduction reaction due to their insulating property, thereby reducing the thickness of the reduced interphase. Furthermore, a stable interfacial contact was maintained after the galvanostatic cycling test, as shown in Figure 1c. These are the reasons for maintaining the lower interfacial resistance. As discussed earlier, the Li2.9B0.9S0.1O3.1 glass-ceramic electrolyte has a high electrochemical stability kinetically. However, some voids and grain boundaries exist in the green compacts, which permit the Li dendrite formation as shown in Figure 1c. Hence, a denser electrolyte layer is necessary to prevent the dendrite formation and obtain better cycling properties as in the case of Li7La3Zr2O12 crystal. Therefore, we fabricated a more densified 3045
DOI: 10.1021/acsaem.9b00470 ACS Appl. Energy Mater. 2019, 2, 3042−3048
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ACS Applied Energy Materials
Figure 4. (a) Charge−discharge curves of the hot-pressed all-solid-state lithium metal cell (Li/80NMC532·20Li2SO4) at the 1st and the 2nd cycles operated under a constant current density of 0.25 mA cm−2 at 100 °C. (b) Correlation between charge/discharge capacities, Coulombic efficiency, and cycle number. (c) Cross-sectional SEM images of the composite positive electrode layer in the hot-pressed all-solid-state cell and the corresponding energy dispersive X-ray (EDX) mappings for nickel and carbon. (d) Cross-sectional SEM images for the interface between Li metal negative electrode and the glass-ceramic electrolyte. The left and right figures are the secondary electron image and the backscattering electron image, respectively.
enon was probably caused by an internal short circuit in the cell due to Li dendrite formation during the charging process.30 Thus, a stable charge−discharge was achieved in the hot-pressed cell. Figure 4 shows the charge−discharge performance of the hot-pressed all-solid-state cell. Meanwhile, some voids and grain boundaries existed in the cold-pressed composite electrode layer.23 However, these voids and grain boundaries were hardly observed in the composite electrode layer when a hot-pressing technique was used. Hence, a favorable electrode−electrolyte interface with large contact area was obtained as shown in Figure 4c. The all-oxide-solidstate cell (Li/80NMC532·20Li2SO4) operated as a secondary battery with a high specific capacity of ∼150 mAh g−1 at a constant current density of 0.25 mA cm−2 at 100 °C, as shown in Figure 4a. When the cell was cycled under the low current density of 0.25 mA cm−2 at 100 °C, the slight capacity fading was observed in a few initial cycles. XPS results for the Li symmetrical cell after the galvanostatic cycling test at 100 °C are shown in Figure S4. Almost the same result as shown in Figure S1 was obtained, meaning that the stable interface was maintained due to the passivating effect of the reduction materials. Moreover, an obvious resistance increase was not observed in the symmetric cells even after the cycling test such as that in Figure 1b. These results suggest that the capacity fading comes not from the negative electrode side but from the positive electrode side. The cells were stably charged and discharged at a high current density of 1.27 mA cm−2 at 100 °C, as opposed to the cold-pressed cell (Figure S3). Figure 4d shows the cross-sectional SEM images at the interface between
densities. Figure S2 shows the galvanostatic cycling test for the hot-pressed symmetric cell at 100 °C. A constant current of 0.25 mA cm−2 was applied for 2, 5, and 10 h, and stable voltage plateaus were observed for all cycles. The obtained capacity was 2.55 mAh cm−2, which was sufficient to be applied to the Li metal of all-solid-state cells. Electrochemical performances of all-solid-state Li symmetric cells using the Li2.9B0.9S0.1O3.1 electrolyte with a comparison of typical solid electrolytes of Li7La3Zr2O12 and Li3PS4 are summarized in Table S1. All-oxide-solid-state Li metal cells were fabricated and their charge−discharge performances were evaluated. Amorphous 80LiNi0.5Mn0.3Co0.2O2·20Li2SO4 was used as a positive electrode active material due to its high ductility and mixed conductivity.23 The all-solid-state cells were fabricated by coldpressing or hot-pressing the constituent powders. Figure S3 shows the charge−discharge performance of the cold-pressed all-solid-state cells at 100 °C. The cell with the amorphous 80LiNi0.5Mn0.3Co0.2O2·20Li2SO4 gave the charge−discharge curves with a gentle slope,23 which is somewhat different with the behavior of typical crystalline LiNi0.5Mn0.3Co0.2O2 having a layered rock-salt structure. The operation of the cold-pressed cell was stable at lower current densities, with a capacity of ∼160 mAh g−1. However, when the applied current density was increased to 1.27 mA cm−2, the operation of the cell was not stable. Meanwhile, a higher capacity was obtained at a higher current density during the charging process than in previous cycles with lower current densities. A similar phenomenon was observed in all-solid-state cells using sulfide electrolytes and Li metal negative electrode.24 This phenom3046
DOI: 10.1021/acsaem.9b00470 ACS Appl. Energy Mater. 2019, 2, 3042−3048
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ACS Applied Energy Materials
were calibrated by setting the measured binding energy of the C 1s peak to 284.7 eV of adventitious carbon accumulated in the analysis chamber. Fabrication of All-Solid-State Lithium Metal Cells. All-solidstate cells were fabricated, and their charge−discharge properties were evaluated. Amorphous 80LiNi0.5Mn0.3Co0.2O2·20Li2SO4 was used as a positive electrode active material.23 The active material (68.4 wt %), Li2.9B0.9S0.1O3.1 glass electrolyte (28.8 wt %), and acetylene black carbon (3.8 wt %) powders were then mixed using a mortar and pestle and used as a composite positive electrode. The three-layered pellets of the stainless steel (ca. 200 mg), composite electrode (ca. 10 mg), and glass electrolyte (ca. 50 mg) were obtained by cold-pressing these powders at room temperature under 720 MPa or hot-pressing them at 250 °C under 540 MPa for 2 h. The obtained pellets were then heattreated at 290 °C to increase their conductivity by crystallizing the Li2.9B0.9S0.1O3.1 electrolyte. A Li metal foil and a stainless steel foil were attached to the electrolyte surface of the pellets as current collectors, and the fabricated cells were pressed by CIP under 80 MPa. The cells were then charged and discharged using a charge− discharge measuring device (BTS-2004; Nagano Co. Ltd.) at 100 °C. Moreover, cross-sections of the cells for the scanning electron microscopy (SEM) observations were prepared using an Ar+ ion milling system (E-3500; Hitachi High-Technologies Corp.). A field emission SEM (FE-SEM, SU8220; Hitachi High-Technologies Corp.) was used to investigate the electrode/electrolyte interfacial contacts.
the Li metal negative electrode and hot-pressed electrolyte in the all-solid-state cells after charge−discharge measurements. An excellent interfacial contact without any thick intermediate phase was maintained even after the charge−discharge. The dense electrolytes suppressed the internal short circuit due to dendritic formation, thereby enabling the high current operation of the all-solid-state Li cells. In conclusion, we have evaluated the interfacial stability of the Li2.9B0.9S0.1O3.1 glass-ceramic electrolyte against Li metal negative electrode. Although the reduced compounds were confirmed at the interfacial region by XPS, the interphase kinetically passivated further reduction reactions due to its insulating property. Hence, no obvious increase in the interfacial resistance was observed. On the other hand, stable dissolution and deposition reactions occurred in the hotpressed symmetric cells of Li/Li2.9B0.9S0.1O3.1/Li at a constant current density of 1.27 mA cm−2 at 100 °C. Finally, we fabricated bulk-type all-oxide solid-state Li metal cells which were stable operated as a secondary battery. Therefore, hotpressing technique is an effective means for the fabrication of bulk-type all-solid-state batteries.
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EXPERIMENTAL SECTION
Synthesis of Li3BO3−Li2SO4 Glass-Ceramic Electrolytes. The Li2.9B0.9S0.1O3.1 (90Li3BO3·10Li2SO4 in mol %) glass electrolyte was synthesized using a mechanochemical technique.21,22 The synthesized glass powder was cold-pressed into a pellet under 720 MPa at room temperature. Moreover, the glass powder was hot-pressed under 540 MPa at its transition temperature (250 °C) for 2 h to obtain a more densified pellet. Meanwhile, the molding temperatures of the glass samples were determined from the DTA curve.21,22 The diameter and thickness of the pellet were 10 and ∼0.7 mm, respectively. The glass pellets were then heat-treated at 290 °C for 1 h to crystallize the highly ion-conducting phase. Electrochemical Measurement. Ionic conductivity of the compacts was measured by an alternating current (AC) impedance technique. A gold thin-film electrode was deposited onto both surfaces of the compacts using a vacuum evaporation technique with a sputter apparatus (Quick Coater SC-701; Sanyu Electron Corp.), while the AC impedance measurements were obtained using an impedance analyzer (SI-1260; Solartron Analytical). The frequency range and applied voltage were 1 MHz to 0.1 Hz, and 50 mV, respectively. Furthermore, the electrochemical stability of the Li2.9B0.9S0.1O3.1 glass-ceramic electrolyte was evaluated against Li metal negative electrode using galvanostatic cycling test. Li metal foils (99.99%; ϕ 9 mm; 0.25 mm thickness; Furuuchi Chem. Corp.) and stainless steel foils (ϕ 10 mm, 20 μm thickness) were attached to both surfaces of the electrolyte pellets as current collectors, and the symmetric cells were pressed by cold isostatic pressing (CIP) under 80 MPa. The galvanostatic cycling test was then carried out on the symmetric cells at 100 °C using charge−discharge measuring devices (BTS-2004, Nagano Co.). The symmetric cells operated at a constant current density along a negative potential direction for 30 min to 1 h and then switched to a positive potential direction. Characterization of the Li/Li2.9B0.9S0.1O3.1 Interface. XPS was conducted on the Li/90Li3BO3·10Li2SO4 interface by using a spectrometer (K-Alpha, Thermo Fisher Scientific). The Li metal thin film was deposited onto the surface of the Li2.9B0.9S0.1O3.1 glassceramic pellets by a thermal evaporator placed in an Ar-filled glovebox, using a vacuum evaporation technique. The samples were then transferred to the analysis chamber of the spectrometer by a vessel containing dry Ar gas. Monochromatic Al Kα radiation (1486.6 eV) was used as the X-ray source. The samples were neutralized by a flood gun during the measurement to reduce the influence of charging effect on the spectra. The samples were then etched by an Ar+ ion monomer, and depth analyses were carried out from the top of the Li thin film to the Li2.9B0.9S0.1O3.1 electrolyte bulk. The obtained spectra
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00470.
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Summary of XPS spectra, charge−discharge performance of all-solid-state cells, and plot of galvanstatic cycling test (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (A.H.). *E-mail:
[email protected] (M.T.). ORCID
Atsushi Sakuda: 0000-0002-9214-0347 Akitoshi Hayashi: 0000-0001-9503-5561 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Nos. JP18H05255 and JP18J14547. REFERENCES
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ACS Applied Energy Materials
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DOI: 10.1021/acsaem.9b00470 ACS Appl. Energy Mater. 2019, 2, 3042−3048
