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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Extremely Low Resistance of Li3PO4 Electrolyte/Li(Ni0.5Mn1.5)O4 Electrode Interfaces Hideyuki Kawasoko,*,†,§ Susumu Shiraki,† Toru Suzuki,† Ryota Shimizu,‡ and Taro Hitosugi†,‡ †
Advanced Institute for Material Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, Miyagi 980-8577, Japan School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8550, Japan
‡
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S Supporting Information *
ABSTRACT: Solid-state Li batteries containing Li(Ni0.5Mn1.5)O4 as a 5 V-class positive electrode are expected to revolutionize mobile devices and electric vehicles. However, practical applications of such batteries are hampered by the high resistance at their solid electrolyte/electrode interfaces. Here, we achieved an extremely low electrolyte/ electrode interface resistance of 7.6 Ω cm2 in solid-state Li batteries with Li(Ni0.5Mn1.5)O4. Furthermore, we observed spontaneous migration of Li ions from the solid electrolyte to the positive electrode after the formation of the electrolyte/ electrode interface. Finally, we demonstrated stable fast charging and discharging of the solid-state Li batteries at a current density of 14 mA/cm2. These results provide a solid foundation to understand and fabricate low-resistance electrolyte/electrode interfaces. KEYWORDS: solid-state Li batteries, interface resistance, Li(Ni0.5Mn1.5)O4, solid electrolyte, activation energy, epitaxial thin film
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batteries using LNMO electrodes.5,6 The activation energy (Ea) of Ri was found to be ∼0.3 eV, which is as low as that for ionic migration in Li superionic conductors.10 The fabricated batteries with extremely low Ri exhibited fast charging and discharging. Furthermore, half of the theoretical capacity (147 mA h/g) was charged and discharged at a current density of 14 mA/cm2 (3600C; the theoretical capacity is fully charged and discharged in 1 s). These studies pave the way to develop fastcharging solid-state LBs with 5 V-class positive electrodes.
INTRODUCTION Solid-state Li batteries (LBs) show great promise as energystorage devices because of their high energy density and good safety.1−3 In particular, solid-state LBs with a 5 V-class positive electrode consisting of Li(Ni0.5Mn1.5)O4 (LNMO) have been extensively investigated to further increase their energy density.4 However, in solid-state LBs with LNMO, the high interface resistance (charge-transfer resistance, Ri) of ∼200− 2000 Ω cm2 at the LNMO electrode/solid electrolyte interface hinders fast charging and discharging.5,6 The interface resistance of solid-state LNMO LBs is reported to be an order of magnitude higher than that of liquid electrolyte-based batteries using LNMO.5,6 Thus, it remains highly desirable to lower the interface resistance of solid-state LNMO LBs. Investigation of structurally defined interfaces without impurities is an effective approach to develop low-resistance interfaces. Electrolyte/electrode interfaces fabricated without exposing the electrode surfaces to air provide an ideal platform to clarify intrinsic battery performance. Indeed, we have shown that the resistance at the interface between an Li3P(O,N)4 electrolyte and an LiCoO2 electrode was very low when fabricated without exposing the electrode surfaces to air by using thin-film deposition chambers under ultrahigh vacuum (UHV) conditions.7−9 In addition, clean electrolyte/electrode interfaces can reveal the intrinsic Li-ion distribution around the interface. In this work, we observed an extremely low Ri value and spontaneous Li-ion transfer across the interface between the solid electrolyte Li3PO4 (LPO) and an LNMO positive electrode. To precisely estimate Ri of the electrolyte/electrode interface, the thin-film batteries were never exposed to air during fabrication. The Ri value was less than 7.6 Ω cm2, which is smaller than that observed for liquid electrolyte-based Li-ion © XXXX American Chemical Society
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EXPERIMENTAL SECTION
All the processes including thin-film fabrication and electrochemical characterization were performed in UHV chambers.7,8 The thin-film LBs were fabricated on an Nb-doped (0.5 wt %) SrTiO3(100) substrate and consisted of an LaNiO3 current collector, LNMO positive electrode, LPO electrolyte, and Li anode, that is, Li/LPO/ LNMO/LaNiO3, as shown in Figure 2a. LaNiO3 and LNMO films were deposited by pulsed laser deposition (PLD) with a KrF excimer laser (wavelength of 248 nm, laser fluence of 1 J/cm2, and repetition rate of 5 Hz). During the deposition of LaNiO3 and LNMO, the substrate temperatures were maintained at 650 and 600 °C, respectively, at an oxygen pressure of 100 mTorr. The typical deposition rates of LaNiO3 and LNMO films were 0.5 and 0.8 nm/min, respectively. To fabricate LNMO, a ceramic target with a Li-rich composition of Li1.2Ni0.5Mn1.5Ox (Toshima Manufacturing Co. Ltd., Japan) was used to compensate for the loss of Li. Subsequently, the solid electrolyte LPO was deposited on the LNMO film by PLD with an ArF excimer laser (wavelength of 193 nm, laser fluence of 1 J/cm2, and repetition rate of 20 Hz). Finally, a metallic Li negative electrode was deposited on the electrolyte by conventional thermal evaporation. Both LPO and Li Received: May 22, 2018 Accepted: July 10, 2018 Published: July 10, 2018 A
DOI: 10.1021/acsami.8b08506 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces layers were deposited at room temperature under an UHV of ∼1 × 10−9 Torr. The typical deposition rates of LPO and Li films were 0.8 and 200 nm/min, respectively. Batteries with MgO inserted at the LPO/LNMO and Li/LPO interfaces were fabricated by depositing MgO on the LNMO and LPO layers before depositing the LPO and Li films, respectively. We deposited MgO using PLD with an ArF excimer laser (wavelength of 193 nm, laser fluence of 1 J/cm2, and repetition rate of 10 Hz) while maintaining the substrate at room temperature under an UHV of ∼1 × 10−9 Torr. The typical deposition rate of MgO was 2 nm/min. The crystal structures of the thin films were evaluated by X-ray diffraction (XRD; Rigaku, SmartLab with a Cu Kα source) and Raman spectroscopy (HORIBA, LabRAM HR-800). We determined film thickness using X-ray reflection (Rigaku, SmartLab with a Cu Kα source) and a stylus profiler (Veeco, Dektak150). The incidence angle of X-rays was from 0 to 0.3° in the X-ray reflection measurements. The obtained curves were fitted with the software “GlobalFit” (Rigaku). The typical thicknesses of the LaNiO3, LNMO, and Li films in the batteries were 20, 60, and 600 nm, respectively. The active area of each battery was circular with a 0.25 mm radius and an area of 0.2 mm2 (see refs 7 and 8 for details). The electrochemical properties of the fabricated lithium-ion batteries were measured with a potentiostat/galvanostat with a frequency-response analyzer (Solartron Analytical, ModuLab). Impedance spectra were measured by applying an ac voltage with an amplitude of 5 mV at frequencies of 0.5 MHz to 1 Hz.
[010] (Figure S1b). The full-width at half maximum (fwhm) values in the rocking curves for the LNMO 004 and LaNiO3 002 peaks were 0.81° and 0.12°, respectively. These XRD patterns indicate that high-quality LNMO/LaNiO3 thin films were epitaxially grown on Nb-doped SrTiO3(001) substrates. Furthermore, we confirmed the growth of single-phase LNMO epitaxial thin films using Raman spectroscopy. The Raman spectra of LNMO/LaNiO3 epitaxial thin films (Figure S1c) showed peaks at 500, 608, and 639 cm−1, which is consistent with Ni−O and Mn−O stretching vibrations in the Ni/Mndisordered Fd3̅m phase of LNMO.11 To understand the formation processes at the interface between LPO and LNMO, we investigated the crystal structure of LPO/LNMO thin films. Figure 1 shows an XRD pattern of an LNMO/LaNiO3 epitaxial thin film after depositing LPO on an LNMO surface at room temperature. The LPO surface was capped with MgO to prevent degradation. Notably, two peaks were observed in addition to the LNMO 004 and 008 peaks (Figure 1). These two additional peaks at 2θ = 41.5° and 90.3° were close to the position of the 004 and 008 peaks of the Lirich spinel compound Li2(Ni0.5Mn1.5)O4 (L2NMO).12,13 There was no peak related to LPO, indicating that the LPO film was in an amorphous state (not shown in the figure). The fwhm values of the rocking curves for the LNMO 004 and L2NMO 004 peaks were 0.78° and 0.62°, respectively, indicating that LNMO and L2NMO have similar crystallinity. In addition, the intensities of these two peaks were almost identical. Accordingly, we roughly estimate that about half of the LNMO converted to L2NMO. These results suggest that Li ions migrated from LPO to LNMO across the LPO/LNMO interface even at room temperature because of the difference in the chemical potentials of LPO and LNMO. The formation of L2NMO was also confirmed by cyclic voltammetry (CV) measurements. We prepared solid-state LBs by depositing Li (negative electrode) on an LPO (solid electrolyte)/LNMO (positive electrode)/LaNiO3 layered structure, as illustrated in Figure 2a. The fabricated batteries showed an initial open-circuit voltage of 2.8 V, and as the voltage was increased, a strong peak appeared at approximately 2.9 V (Figure 2b). This peak was consistent with the conversion of L2NMO to LNMO12,13 and indicative of the formation of L2NMO in the films. The fabricated LNMO batteries showed excellent charging and discharging characteristics above 3.0 V. CV measurements revealed distinct peaks between 3.5 and 5.0 V (vs Li+/Li) (Figure 2b). Two sharp peaks that appeared at 4.65 and 4.75 V during charging and discharging are consistent with the Ni2+/ Ni3+ and Ni3+/Ni4+ redox couples, respectively. The small peak near 4.1 V is attributed to a redox reaction of Mn. The peaks in the CV curve are consistent with the electrochemical properties of the Fd3̅m phase of LNMO.14 The CV curves overlapped in the first three cycles, suggesting that the redox reaction was reversible. Taking the above mentioned findings together, the formation process of an LPO/LNMO interface is believed to occur as follows. Spontaneous migration of Li ions from LPO to LNMO induces the formation of an L2NMO phase within LNMO. In the initial charging process, at ∼2.9 V, the L2NMO phase converted into an LNMO phase through electrochemical reactions and Li ions migrated from L2NMO to LPO. The LPO/LNMO interface with spontaneous Li-ion migration showed an extremely low Ri, as discussed later.
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RESULTS AND DISCUSSION First, we characterized the structure and crystallinity of the LNMO films used as the positive electrodes in solid-state batteries. Figure 1 shows the out-of-plane XRD pattern of
Figure 1. Out-of-plane XRD patterns. (Top) XRD pattern of an LNMO/LaNiO3 film on an Nb-doped SrTiO3 substrate. (Bottom) XRD pattern of an MgO/LPO/LNMO/LaNiO3 film on an Nb-doped SrTiO3 substrate. MgO was used as a capping layer to prevent the spontaneous degradation of the LPO.
LNMO (60 nm)/LaNiO3 (20 nm) films grown on Nb-doped SrTiO3(001) substrates. The XRD pattern showed four peaks (2θ = 23.5°, 47.8°, 75.0°, and 108.3°) and two peaks (2θ = 44.0°, 97.2°) corresponding to (001)-oriented LaNiO3 and LNMO, respectively (Figure 1). An in-plane XRD scan along the [111] direction exhibited (111)-oriented LaNiO3 and LNMO (Figure S1a). In the ϕ-scan of LaNiO3 111 and LNMO 111, four peaks appeared at angles separated by 90°, confirming the epitaxial relationship of SrTiO3[010]//LNMOB
DOI: 10.1021/acsami.8b08506 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. Structure of thin-film LBs and their electrochemical properties. (a) Schematic cross-sectional view (left) and micrograph (right) of a fabricated thin-film LB. tLPO indicates the thickness of the LPO layer. (b) CV curves of the fabricated thin-film LB. (c) Electrochemical impedance spectra of the fabricated thin-film LB measured at 27 °C. The potentials were 3.3 V (top) and 4.7 V (middle). Fitting results at 4.7 V are shown in the bottom panel, where the red line is a fitting curve.
We now quantitatively evaluate Ri at the electrolyte/ electrode interfaces. The top and middle panels in Figure 2c show the impedance spectra of the fabricated LBs at potentials of 3.3 and 4.7 V (vs Li+/Li), respectively. At 3.3 V, a clear semicircle was observed in the frequency range of 4 × 103 to 5 × 106 Hz. Because this semicircle was also observed in the impedance spectrum obtained at 4.7 V, it originated from impedance of the solid electrolyte LPO. The ionic conductivity of LPO (σLPO) estimated from the impedance spectrum was 5.5 × 10−7 S/cm, which agrees well with the reported value.15 When the potential was increased to 4.7 V, that is, the voltage at which the main redox reaction of Ni occurs, two semicircles emerged in the intermediate- (3 × 102 to 4 × 103 Hz) and lowfrequency regions (4 × 100 to 3 × 102 Hz) (fitting results are shown at the bottom of Figure 3a), in addition to the semicircle observed at high frequencies (4 × 103 to 5 × 106 Hz) discussed earlier. These intermediate- and low-frequency semicircles should be associated with the impedance component either at the electrolyte/electrode interface or inside the LNMO positive electrode. To clarify the origin of the intermediate- and low-frequency semicircles, we modified the interfaces by inserting an ultrathin MgO layer with a thickness of less than 0.2 nm (i.e., less than one unit cell). We expected MgO to serve as a barrier to Li-ion transport at the interface because MgO is a poor Li-ion conductor. We covered roughly half of the surface of LNMO with MgO. Note that the initial open-circuit voltage for the batteries with an MgO layer was 2.8 V. Accordingly, Li ions spontaneously migrated into LNMO despite the insertion of the MgO layer. The upper panels in Figure 3a,b compare the electrochemical impedance spectra of the thin-film LBs with MgO inserted at the LPO/LNMO and Li/LPO interfaces, respectively. In both figure panels, the semicircles in the intermediate-frequency region became larger than the semicircle obtained for the corresponding interface without MgO (Figure 2c top). When MgO was inserted at the LPO/LNMO interface, the enlarged semicircle in the intermediate-frequency region merged with that in the low-frequency region to form a
Figure 3. Electrochemical impedance spectra of fabricated batteries with modified interfaces. tLPO indicates the thickness of the LPO layer. (a) Impedance spectrum of a sample with MgO inserted at the LNMO/LPO interface (4.7 V, top) and the fitting result (bottom). The red line is a fitting curve. (b) Impedance spectrum of a sample with MgO inserted at the Li/LPO interface (4.7 V, top) and the fitting results (bottom). The red line is a fitting curve.
single semicircle. The fitting results indicated that the original Ri of the semicircle in the intermediate-frequency region increased from 3.9 × 103 Ω (7.6 Ω cm2) for a clean interface without MgO to 2.1 × 104 Ω (42 Ω cm2) after MgO insertion C
DOI: 10.1021/acsami.8b08506 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
and 4.8 V at 70 °C. Although the discharge rate increased to 14 mA/cm2 (3600C), the discharge capacity was 73 mA h/g, which is half of the theoretical capacity. At this temperature, the discharge capacity at 0.0039 mA/cm2 (1C) was 145 mA h/ g, agreeing with the theoretical capacity of the LNMO positive electrode. The LNMO thin-film LBs showed excellent cycling stability at 70 °C, even at 14 mA/cm2 (3600C). Figure 4b,c shows the cycling performance of the batteries between 2.8 and 5.8 V at 14 mA/cm2 (3600C) for 100 cycles. The charge−discharge capacity did not decay even after 100 cycles. In addition, the shape of the charge−discharge curves did not change over the 100 cycles, indicating that the stable interface suppressed the generation of Li dendrite18 and formation of interphases during charging and discharging at high current density (the battery properties at 27 °C are shown in Figure S2). In addition, we note that it would be possible to charge the battery even faster if we used a solid electrolyte with higher conductivity because the charging rate of our LBs was limited by the low ionic conductivity of the solid electrolyte.
(Figure 3a bottom). When MgO was inserted at the Li/LPO interface, the resistance of the intermediate-frequency semicircle increased to 1.1 × 104 Ω (21 Ω cm2) (Figure 3b bottom). These results strongly suggest that Ri of both the LPO/LNMO and Li/LPO interfaces contributed to the resistance of the intermediate-frequency semicircle. Therefore, the estimated Ri at the LPO/LNMO interface was less than 7.6 Ω cm2. This value is two orders of magnitude smaller than that previously reported for LNMO-based solid-state LBs.6 Furthermore, this value was 20% smaller than that observed in liquid-electrolytes-based Li-ion batteries using LNMO.5 We also assessed the Ea of Ri at 4.7 V using the Arrhenius equation (Figure S3). The obtained Ea of 0.31 ± 0.01 eV for Ri was smaller than the reported value of 0.62 eV at the electrolyte/LNMO interface of solid-state LBs16 and comparable with that of Li migration in Li superionic conductors (0.25 eV).10 The estimated Ea of σLPO was 0.50 ± 0.03 eV, which agreed well with the reported value.15 The Ea value of RLNMO was 0.31 ± 0.02 eV, which was almost the same as that of Ri. Because Ea of Ri was smaller than that of σLPO, these results indicate that Ri at the electrolyte/LNMO interface did not limit the battery performance. The batteries displayed fast charging and discharging at a current density of 14 mA/cm2 (3600C; 1C is equal to a current of 146.7 mA h/g17). This current density was much higher than the 0.67 mA/cm2 (10C) used in a previous study of solid-state LNMO batteries.6 Figure 4a shows the discharge capacity after charging at 0.0039 mA/cm2 (1C) between 3.8
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CONCLUSIONS We demonstrated extremely low interface resistance in solidstate LBs with LNMO electrodes by fabricating an electrolyte/ electrode interface without impurities. We found that the Li ions migrated spontaneously at the clean LPO/LNMO interface. Ea for Li-ion transfer across the interface was ∼0.3 eV, which was almost the same as that of Li superionic conductors. The solid-state LNMO LBs with extremely low interface resistance showed fast charging and discharging at 14 mA/cm2 (3600C, 70 °C); half of the capacity was charged and discharged within 1 s. These results clearly demonstrate the promise of solid-state LBs using 5 V-class positive electrode in terms of fast charging and discharging through low-resistance interfaces.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08506. Equivalent circuit model for the analysis of impedance spectra; in-plane XRD pattern, ϕ-scan, and Raman spectrum of an LNMO/LaNiO3 film; temperature dependence of interface resistance, ionic conductivity of LPO, and resistance component of LNMO in fabricated LBs; charge−discharge curves, rate capability, and rate dependence of discharge curves at 27 °C (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hideyuki Kawasoko: 0000-0001-9069-3784 Present Address §
Department of Chemistry, Faculty of Science, Tohoku University, 6-3 Azaaoba, Aramaki, Aoba, Sendai, 980-8578 Japan (H.K.).
Author Contributions
H.K., S.S., R.S., and T.H. designed the experiment and wrote the paper. H.K. fabricated the thin-film batteries, measured the electrochemical properties, and analyzed the experimental data.
Figure 4. Performance of the fabricated thin-film LBs at 70 °C. (a) Rate dependence of the discharge curves. (b) Charge−discharge curves and (c) cycling performance at 14 mA/cm2 (3600C). D
DOI: 10.1021/acsami.8b08506 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
(14) Xia, H.; Lu, L. Li diffusion in spinel LiNi0.5Mn1.5O4thin films prepared by pulsed laser deposition. Phys. Scr., T 2007, 129, 43−48. (15) Kuwata, N.; Iwagami, N.; Tanji, Y.; Matsuda, Y.; Kawamura, J. Characterization of Thin-Film Lithium Batteries with Stable ThinFilm Li[sub 3]PO[sub 4] Solid Electrolytes Fabricated by ArF Excimer Laser Deposition. J. Electrochem. Soc. 2010, 157, A521− A527. (16) Kaneko, M.; Ishii, Y.; West, W. C.; Motoyama, M.; Iriyama, Y. The Effect of Ferroelectric BaTiO3 Particles on Interfacial Resistance Between the Li-Ni-Mn-(Cr) Oxide (LNM) Spinel Cathode and Lipon. ECS Meeting Abstracts, 2015, MA2015−01, 402. (17) Liu, D.; Zhu, W.; Trottier, J.; Gagnon, C.; Barray, F.; Guerfi, A.; Mauger, A.; Groult, H.; Julien, C. M.; Goodenough, J. B.; Zaghib, K. Spinel Materials for High-Voltage Cathodes in Li-Ion Batteries. RSC Adv. 2014, 4, 154−167. (18) Li, N.-W.; Yin, Y.-X.; Yang, C.-P.; Guo, Y.-G. An Artificial Solid Electrolyte Interphase Layer for Stable Lithium Metal Anodes. Adv. Mater. 2016, 28, 1853−1858.
T.S., S.S., and R.S. helped the fabrication of thin-film batteries and the measurement of electrochemical properties. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the World Premier International Research Center Initiative (WPI Initiative), the Toyota Corporation, and the “Applied and Practical LiB Development for Automobile and Multiple Application” project of the New Energy and Industrial Technology Development Organization (NEDO). We also acknowledge the support of Grants-in-Aid for Scientific Research (nos. 26106502, 26108702, 26246022, and 26610092), JST-ALCA, and JST-CREST. The authors thank Dr. Toshiya Saito, Dr. Shigeto Okada, Dr, Kazunori Nishio, and Shigeru Kobayashi for fruitful discussions. We thank Natasha Lundin, PhD, from Edanz Group (www. edanzediting.com/ac) for editing a draft of this manuscript.
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REFERENCES
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DOI: 10.1021/acsami.8b08506 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
