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[Tsuyoshi Ohnishi](https://orcid.org/0000-0002-2333-7752), [Kazutaka Mitsuishi](https://orcid.org/0000-0002-9361-4057), [Kazunori Takada](https://orcid.org/0000-0001-7568-1806)

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[In Situ X-ray Diffraction of LiCoO<sub>2</sub> in Thin-Film Batteries under High-Voltage Charging](https://mdr.nims.go.jp/datasets/cdeaa5c2-8b9c-41cf-8b2e-e6d5b50080bd)

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In Situ X-ray Diffraction of LiCoO2 in Thin-Film Batteries under High-Voltage ChargingIn Situ X‑ray Diffraction of LiCoO2 in Thin-Film Batteries under High-Voltage ChargingTsuyoshi Ohnishi, Kazutaka Mitsuishi, and Kazunori Takada*Cite This: ACS Appl. Energy Mater. 2021, 4, 14372−14379 Read OnlineACCESS Metrics & More Article RecommendationsABSTRACT: LiCoO2 has been used as the cathode material employed inlithium-ion batteries since their development, and efforts to improve itsperformance are still in progress. For example, complete use of lithiumprovides a theoretical capacity as high as 274 mAh g−1; however, charge−discharge cycling with such a high capacity leads to rapid degradation. Thedegradation mechanism has been intensively studied in order to increase thepractical capacity. Although phase transitions taking place in high-voltagecharging have been considered to affect the cycling performance, sidereactions induced by the high-voltage charging always overlap to blur theeffects of phase transitions on the electrode properties. This study hasunveiled the relation between the phase transition and electrode propertiesby employing a solid electrolyte that suppresses the side reactions efficiently. Electrochemical impedance spectroscopy combinedwith in situ X-ray diffraction shows clear correlation between phase transition from O3 to H1−3 and a drastic increase in theelectrode resistance. The increasing resistance is attributable to formation of narrow interlayers with a gallery height of 4.2 Å thatimpede lithium-ion diffusion.KEYWORDS: LiCoO2, thin-film battery, epitaxial film, high-voltage charging, phase transition1. INTRODUCTIONLiCoO2 is the first cathode material employed in lithium-ionbatteries1,2 and has been used for more than 30 years.Tremendous studies have been done on LiCoO2 during the 30years and are still in progress in order to realize its potential inpractical lithium-ion batteries. For example, although completelithium deintercalation from LiCoO2 provides a hightheoretical capacity of 274 mAh g−1, practical capacity hasbeen limited to ca. 160 mAh g−1. Increasing charge voltageincreases the capacity; however, it results in rapid capacityfading. Intensive studies aiming at improving the cyclingperformance have proposed various mechanisms for thedegradation in high-voltage charging: some attribute it toinstability of organic electrolytes,3−5 while others attribute it tostructural instability of LiCoO2.LiCoO2 has O3 structure in which lithium ions reside at itsoctahedral sites, and the CoO2 layers are stacked with three-layer periodicity. Lithium deintercalation brings about somephase transitions in Li1−xCoO2. Lithium ions are orderedaround x = 1/2 to distort the hexagonal crystal lattice tomonoclinic.6 Further lithium deintercalation leads to phasetransitions from O3 to H1−3,7 and then to O1,8 which changethe stacking manner of CoO2 layers. These phase transitionscause internal stress and subsequent mechanical degradation toLiCoO2 particles, resulting in capacity fading. However, therelation between the phase transitions and cyclability has notbeen clarified because such phase transitions occurring at highpotential are always accompanied by decomposition of theelectrolytes. Effects of the phase transitions on the cyclabilityalways overlap that of the electrolyte decomposition, and thus,different values have been reported for the reversible limit ofcharging voltage.9−13 Clear relation will be revealed in solid-state systems because side reactions will hardly occur in solidelectrolytes.2. FABRICATION OF THIN-FILM BATTERIES AND INSITU MEASUREMENTSThis study shows the relation between changes in the crystalstructure and electrode properties of LiCoO2 in thin-film solid-state batteries investigated by in situ X-ray diffraction (XRD)combined with electrochemical measurements. Figure 1schematically illustrates the in situ measurement system andthe structure and dimensions of the thin-film battery. The thin-film battery is fabricated on a sapphire c-plane. A 9.5 mmsquare LiCoO2 cathode film is deposited on a sapphireReceived: September 29, 2021Accepted: November 2, 2021Published: November 16, 2021Articlewww.acsaem.org© 2021 The Authors. Published byAmerican Chemical Society14372https://doi.org/10.1021/acsaem.1c03046ACS Appl. Energy Mater. 2021, 4, 14372−14379Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on December 29, 2021 at 07:03:32 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tsuyoshi+Ohnishi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kazutaka+Mitsuishi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kazunori+Takada"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsaem.1c03046&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=abs1&ref=pdfhttps://pubs.acs.org/toc/aaemcq/4/12?ref=pdfhttps://pubs.acs.org/toc/aaemcq/4/12?ref=pdfhttps://pubs.acs.org/toc/aaemcq/4/12?ref=pdfhttps://pubs.acs.org/toc/aaemcq/4/12?ref=pdfwww.acsaem.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsaem.1c03046?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://www.acsaem.org?ref=pdfhttps://www.acsaem.org?ref=pdfhttps://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://acsopenscience.org/open-access/licensing-options/substrate kept at 600 °C under an Ar/O2 atmosphere in whichflow rates of Ar and O2 are 20 and 10 sccm, respectively, by RFmagnetron sputtering and then annealed at 700 °C for 5 h inair.Grain size in the LiCoO2 film is of an order of 100 nm, asshown in the cross-sectional transmission electron microscope(TEM) image in Figure 2a, and the electron diffraction patterntaken from the area covering the LiCoO2 film and sapphiresubstrate in Figure 2b indicates their epitaxial relation: LiCoO2is epitaxially grown on the sapphire (0001) plane with the(001) orientation and in-plane alignment of LiCoO2 [11̅0] ∥sapphire [112̅0]. On the other hand, the surface scanningelectron microscope (SEM) image in Figure 2c shows that theLiCoO2 grains are much larger at the surface. Triangular shapecan be recognized in the image, and the grain size is of theorder of micrometers, which originates from lateral overgrowthat the end of the deposition.After annealing the LiCoO2 film, Pt is deposited by DCmagnetron sputtering as a current collector, which is a squarefilm with 10.0 mm on a side with a circular opening with adiameter of 10 mm. Li3PO4 is formed as the solid electrolytelayer with lateral dimensions of 9.5 mm × 9.5 mm by RFmagnetron sputtering. Finally, a circular Li anode with adiameter of 8.5 mm is formed above the opening of the Ptcurrent collector by thermal evaporation.The electrochemical measurement system connected to thebattery sealed in an airtight sample holder controls the state-of-charge (SOC) of LiCoO2 and measures electrochemicalimpedance, while XRD patterns from the thin-film batteryare taken in situ in parallel-beam geometry using Cu-Kαradiation. The (001) orientation of the LiCoO2 film isbeneficial to detect the phase transitions. The phase transitionsaccompany clear changes in the c-lattice constant, which can berecognized in the basal reflections strengthened by the (001)orientation with high accuracy on the parallel-beam geometry.In addition, the in situ XRD measurement in combination withthe electrochemical measurement easily gives structural andelectrochemical data at various SOC values to provide detailedrelation between the phase transition and electrode properties.Electrochemical impedance spectra are taken in the frequencyrange from 5 × 105 to 0.1 Hz with an AC amplitude of 20 mV.3. INFLUENCE OF CHARGING VOLTAGE ON CYCLINGPERFORMANCEFigure 3a shows the charge−discharge curves of the thin-filmbatteries cycled in the voltage range of 2.5−4.6 V and 2.5−4.2V in which the thin-film batteries are cycled at a constantcurrent of 11.9 μA. The first discharge capacity is 6.79 μAh,when the thin-film battery is cycled between 2.5 and 4.2 V,while that cycled between 2.5 and 4.6 V is 10.67 μAh; that is,raising the charging cutoff voltage from 4.2 to 4.6 V increasesthe capacity by 57%; however, it results in the rapid capacityfading: capacity retentions after the 100 cycles are 99% and79% for the 4.2 and 4.6 V charging, respectively, as shown inFigure 3b. The observed capacity (q) is converted to thespecific capacity (Qactual) in the right vertical axis in Figure 3band SOC (x in Li1−xCoO2) in the left vertical axis in Figure 3con the basis of the “actual” mass of the LiCoO2 filmscontributing to the electrode reactions in which the actualmass is estimated as below.Figure 4a shows the charge−discharge curves in the secondcycle obtained from the thin-film batteries cycled in thedifferent voltage ranges. The observed discharge capacity(indicated by q in the uppermost horizontal axis) is 6.79 μAh,when the battery is cycled between 2.5 and 4.2 V, while thatcycled between 2.5 and 4.6 V is 10.67 μAh. The apparentspecific capacities (Qapp in the second horizontal axis)calculated from the observed values (q) and the entire massof the LiCoO2 films measured using an electric balance (86−87 μg) are only 78 and 123 mAh g−1, respectively, which aremuch lower than those expected from the relationship betweenFigure 1. Schematic drawing of the in situ XRD measurement.Figure 2. (a) Cross-sectional TEM image, (b) electron diffractionpattern taken along LiCoO2 [11̅0], and (c) surface SEM image of theLiCoO2 film. The diffraction spots from the LiCoO2 film and sapphiresubstrate are labeled with Miller indices colored in yellow and red,respectively.ACS Applied Energy Materials www.acsaem.org Articlehttps://doi.org/10.1021/acsaem.1c03046ACS Appl. Energy Mater. 2021, 4, 14372−1437914373https://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig2&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.1c03046?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asthe charge capacity and the open-circuit voltage of Li1−xCoO2reported in ref 14: the specific capacities at 4.2 and 4.6 Vcharging are expected to be 130 and 200 mAh g−1, respectively.The underestimation of the specific capacity comes from theoverestimation of active electrode mass.In the solid-state batteries in this study, the circular Li anodewith a diameter of 8.5 mm is formed on the square LiCoO2cathode with 9.5 mm on a side with a solid electrolyte film ofLi3PO4 in between, as illustrated in Figure 1. Therefore, only63% of the cathode faces the Li anode, and it is considered thatonly this part contributes to the electrode reactions; that is, theactual mass of LiCoO2 for the electrode reactions will be 63%of the measured value. In fact, the specific capacities calculatedfrom the “actual” mass (Qactual in the third horizontal axis) are124 and 195 mAh g−1 for the 4.2 and 4.6 V charging,respectively, which agree with the above expected values.Moreover, transformation to the monoclinic phase that can berecognized in the charge−discharge curves guarantees thevalidity of the estimation.The O3 hexagonal structure of LiCoO2 transforms tomonoclinic and returns to the hexagonal phase duringcharging, and the phase transition appears in the potentialprofile as small plateaus at ca. 4.1 V between 120 and 135 mAhg−1 in the charging capacity14 and more clearly as a pair ofpeaks around 4.1 V in the differential capacity (dQ/dV) plot.6The differential capacity plots obtained for the present thin-film battery in Figure 4b also have the pair of peaks originatingfrom the phase transition, and the charge−discharge curves inFigure 4a have small plateaus around 4.1 V and Qactual = 120mAh g−1. That is, transition between hexagonal andmonoclinic phases observed in this study falls within thereported composition range of the monoclinic phase, when thespecific capacity is calculated from the “actual” mass for theelectrode reactions.The discussion on the “actual” mass of the LiCoO2 filmleads to the conclusion that increasing the charging cutoffvoltage from 4.2 to 4.6 V increases the specific capacity from124 to 195 mAh g−1 while leading to rapid capacity fading. ItFigure 3. Cycling properties of the thin-film batteries. Charge−discharge curves during the cycling are shown in (a), and changes inthe discharge capacities (q) are plotted against the cycle number in(b). The observed capacities in the left vertical axis are converted tospecific capacities based on the actual mass for the electrode reactions(Qactual) on the right vertical axis. Changes in SOC values during thecycling between 2.5 and 4.6 V are plotted against the cycle number in(c). Closed and open circles indicate the SOC values at the end ofcharging and discharging, respectively. The composition range of theH1−3 phase determined by Monte Carlo simulations20 is also shownby vertical bars.Figure 4. (a) Charge−discharge curves of the thin-film batteries inthe second cycle with the charging cutoff voltages of 4.2 and 4.6 V and(b) corresponding differential capacity plots. The vertical axis in (a)indicates the cell voltage, and the horizontal axes indicate observedcapacity (q), specific capacity based on the entire mass of the LiCoO2film (Qapp), that based on the actual mass for the electrode reactions(Qactual), and change in the lithium content (x) estimated from theQactual from the uppermost to the lowermost, respectively. Thereported composition ranges for monoclinic,14 H1−3, and O1phases20 are indicated by horizontal bars.ACS Applied Energy Materials www.acsaem.org Articlehttps://doi.org/10.1021/acsaem.1c03046ACS Appl. Energy Mater. 2021, 4, 14372−1437914374https://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig4&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.1c03046?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asshould be noted that the first charge capacities are quitedifferent from those in the following cycles, as shown in Figure3a. In fabrication of thin-film batteries, deposition of solidelectrolytes onto cathode layers sometimes changes the lithiumcontents in the cathodes. For example, deposition of Li3PO4onto LiNi0.5Mn1.5O4 increases its lithium content to convert itto Li2Ni0.5Mn1.5O4.15 On the contrary, deposition of Li3PO4onto LiCoO2 in this study decreases the lithium content. Wehave found that the decrease in the lithium content iscontrollable by substrate bias in the Li3PO4 deposition by RFsputtering, and the details will be reported elsewhere. Thedifferences between the first charge and discharge capacities(Δq) correspond to the amounts of lithium ions extracted inthe deposition of Li3PO4. Both batteries show the same valueof Δq = 5.8 μAh, which are converted to ΔQactual = 106 mAhg−1 and Δx = 0.39, and thus, it is concluded that the Li3PO4deposition partially charges the LiCoO2 to Li0.61CoO2.Therefore, the thin-film batteries must be dischargedcompletely in order to adjust the lithium content toLi1.0CoO2 before investigating the relationship between theelectrode properties and crystal phase of LiCoO2 and theirdependence on the SOC.4. RELATION BETWEEN THE ELECTRODERESISTANCE AND PHASE TRANSITIONSFigure 5 shows typical Nyquist plots obtained for the thin-filmbattery obtained at various SOC values. The Nyquist plotsconsist of two semicircles and a straight line. The diameter ofthe semicircles in the higher frequencies is independent of theSOC and agrees with that estimated from the conductivity anddimensions of the Li3PO4 film, and thus, it is attributable to theresistance of the electrolyte layer (Rele). The battery shows alittle higher resistance at shallow SOC values, e.g., at x = 0.03,as indicated in Figure 5a. It is because the cathode in thisbattery has a current collector only on its edge, and LiCoO2 isa semiconductor. Because it is resistive before lithiumdeintercalation provides metallic conduction,16 the batteryshows somewhat high resistance at shallow SOC values. Thesecond semicircle shows SOC dependence, indicating that it isattributable to the charge-transfer resistance at the cathode(Rct). Rct observed in Figure 5a is ca. 14 Ω cm2, which iscomparable to that observed for a (001)-oriented LiCoO2/Lipon interface.17 On the other hand, Rct noticeably starts toincrease at x = 0.67 and reaches 5.6 × 105 Ω cm2 at x = 0.81, asshown in Figures 5b and 6.Oxidative decomposition of electrolytes caused by high-voltage charging generally increases Rct in liquid electrolytesystems;5 however, the increase in Rct in this battery will notcome from decomposition of Li3PO4. First-principles calcu-lations show that the oxidation potential of Li3PO4 is 4.2 V vsLi+/Li,18 whereas some experimental results suggest that itspractical electrochemical window is much wider than thethermodynamic value due to slow kinetics of the decom-position reactions. In fact, experimental results indicate thatLi3PO4 and a related material, Lipon, are stable against a 5 Vcathode, LiNi0.5Mn1.5O4. Interfacial resistance between Li-Ni0.5Mn1.5O4 and Li3PO4 is as low as 7.6 Ω cm2, and a Li/Li3PO4/LiNi0.5Mn1.5O4 battery shows stable cycling even at anelevated temperature of 70 °C.15 In addition, a Li/Lipon/LiNi0.5Mn1.5O4 battery delivers a remarkable capacity retentionof 90% over 10,000 cycles with coulombic efficiency higherthan 99.98%.19 These results strongly suggest that Li3PO4 isnot oxidatively decomposed at 4.7 V vs Li+/Li, which is theelectrode potential of LiNi0.5Mn1.5O4 in its charged state, toincrease the electrode resistance. On the other hand, Rct in thisthin-film battery starts to increase at x = 0.67 obviously, eventhough the cathode potential is still 4.55 V there. The reasonfor the different stability against high-voltage charging from theabove studies will be found in the differences in the cathodes,for instance, in the crystal structure. LiCoO2 has a two-dimensional layered structure, while LiNi0.5Mn1.5O4 has athree-dimensional spinel structure.Figure 6 shows the XRD patterns obtained at various SOCvalues along with Rct values derived from the Nyquist plots inFigure 5. The XRD pattern before charging consists of basalreflections from LiCoO2 indicating the (001) orientation ofthe film, 110 and 200 reflections from the Li metal anode, and0003 and 0009 reflections from the sapphire substrate, whereforbidden sapphire 0003 reflection appears due to the epitaxialstrain. The basal reflections shift to the lower angles uponcharging, which indicates expansion of the c axis caused by theweakening coulombic attraction between CoO2 and Li layerswith the lithium deintercalation. When the SOC comes to x =0.67, a new series of diffractions emerges at 2θ = 9.7, 19.5,29.7, 39.7, 50.5, and 61.3 °, which are assignable to 003n (n =1−6) reflections of the H1−3 phase.20 Further lithiumdeintercalation gives a rise of new reflections at 2θ = 20.96and 41.78 °, which are 001 and 002 reflections from the O1phase, i.e., lithium-depleted CoO2.21 The right panel of Figure6 shows SOC dependence of Rct taken with the XRD patterns.The Rct is almost unchanged before the emergence of the H1−3 phase and then starts to increase. The coincidence of onsetof increasing Rct and emergence of the H1−3 phase stronglysuggests that the formation of the H1−3 phase increases theRct.The H1−3 phase is a second stage structure in Li1−xCoO2,which is composed of CoO2 layers piled up with two kinds ofinterlayers alternately: one is O3-type with an interlayerdistance of 4.8 Å, and the other is O1-type with an interlayerdistance of 4.2 Å, as illustrated in Figure 7a.7 A kinetic MonteCarlo simulation has revealed that the chemical diffusionFigure 5. Typical Nyquist plots for the thin-film battery at variousSOC values (a) below x = 0.35 and (b) above x = 0.58. Theimpedance spectra are taken in a frequency range from 5 × 105 to 0.1Hz with an AC amplitude of 20 mV.ACS Applied Energy Materials www.acsaem.org Articlehttps://doi.org/10.1021/acsaem.1c03046ACS Appl. Energy Mater. 2021, 4, 14372−1437914375https://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig5&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.1c03046?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ascoefficient in Li1−xCoO2 decreases by five orders of magnitudebeyond x = 0.6 due to the high activation barrier for Li+-ionicdiffusion.22 The high activation barrier is attributed to twofactors. One is a drop of the c-lattice constant, whichcompresses the tetrahedral sites along the conduction paths.The second factor is increasing effective charge on the Co ionthat exerts a large electrostatic repulsion to the Li+ ion in thetetrahedral sites. Because the effective charge on the Co ionwill increase gradually during charging, the drastic increase ofRct with the emergence of the H1−3 phase implies that theformer is the predominant factor in this case; that is, the dropof the interlayer distance to 4.2 Å makes the gallery heightinsufficient for fast ionic diffusion.The LiCoO2 film grown on the sapphire c-plane has the(001) orientation in which the interlayers are parallel to thesubstrate and perpendicular to the current direction. Althoughlithium ions are considered to hardly diffuse along the cdirection, (001)-oriented films have been reported to showsmooth charge−discharge reactions.23−25 It is deduced thatlithium ions can diffuse along the c direction through defects,e.g., grain boundaries, antiphase boundaries, and pairs ofantisite Li and oxygen vacancies acting as the conductionchannels. Figure 7b schematically presents lithium extractionfrom (001)-oriented LiCoO2. Lithium extraction to x ≈ 0.65 inLi1−xCoO2 is reported to expand the interlayer spacing from4.7 to 4.8 Å, maintaining its O3 structure.14 Further extractiondepletes lithium ions in the alternate interlayers to decreasetheir height to 4.2 Å and form the H1−3 phase, and completeextraction of lithium ions leads to the formation of the O1phase in which all the CoO2 layers are piled up with aninterlayer distance of 4.2 Å. Since the charge reaction willpreferentially extract lithium ions neighboring to the“conduction channel along the c direction,” 4.2-Å-high gallerieswill be formed to surround the “channel” and impede thesupply of lithium ions to the “channel.” In addition, lithiumions must undergo long lateral diffusion to reach the “channel”in the present epitaxial film, as indicated in Figure 2c.Therefore, lithium ions around the “channel” will be depletedto make the channel inactive and increase Rct right after theemergence of the H1−3 phase. The XRD patterns on the topFigure 6. XRD patterns and Rct measured for the thin-film battery during charging. Diffraction intensity (I) and Rct are plotted on the logarithmicscales. Some XRD patterns referred in the text are labeled with the SOC values.Figure 7. (a) Crystal structure of the H1−3 phase7 and (b) schematicrepresentation of lithium deintercalation from the (001)-orientedLiCoO2 film. Crystal structure is presented in a space-filling structuremodel drawn by VESTA.34ACS Applied Energy Materials www.acsaem.org Articlehttps://doi.org/10.1021/acsaem.1c03046ACS Appl. Energy Mater. 2021, 4, 14372−1437914376https://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig7&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.1c03046?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asin Figure 6 support this scheme. They consist of three basalreflection series, indicating the coexistence of O3, H1−3, andO1 phases, although the H1−3 phase should coexist only witheither the O3 or O1 phase thermodynamically.26,27 Despite thethermodynamic instability, the three-phase coexistence will beallowed due to the slow kinetics. The 4.2 Å galleries willimpede the movement of lithium ions from O3 to H1−3 andO1 phases that vanishes the O3 phase.5. EFFECTS OF PHASE TRANSITIONS ON THECYCLING PERFORMANCELithium reintercalation recovers the original O3 structure, asshown in Figure 8; however, the recovery is somewhatsluggish. In Figure 8, Li1−xCoO2 is charged up to x = 0.70 andthen discharged. Reflections from the H1−3 phase appear at x= 0.65 in the charging while surviving until x = 0.62 in thedischarging. In addition, Rct shown in the right panel starts toincrease rapidly at x = 0.65 in the charging while decreasingduring the discharging slowly. The sluggish recovery will bedue to the slow kinetics: lithium ions must prop up thegalleries from 4.2 to 4.8 Å in height in order to recover the O3phase. In fact, the decreasing Rct value is accelerated to someextent after the H1−3 phase disappears. However, the Rct doesnot go back to its original value: The Rct is 16 Ω cm2 before thecharging, while it is ca. 70 Ω cm2 at the end of discharging.The irreversible increase in Rct should be the reason for therapid capacity fading observed in the cycling with 4.6 Vcharging because the charge−discharge curves overlap thecomposition range for the H1−3 phase determined by MonteCarlo simulations20 at the end of charging to 4.6 V, as shownin Figure 4a. In fact, the formation of the H1−3 phase appearsas a potential plateau in Figure 4a, which gives a peak around4.6 V in the corresponding differential capacity plot in Figure4b, and diffractions from the H1−3 phase are detected in theXRD patterns in Figures 6 and 8. The irreversible increase inRct will be caused by the large change in the gallery height inthe transformation between the O3 and H1−3 phases. Thedifference in the interlayer distance between O3 and O1-typeexceeds 10%. In addition, the transformation between O3 andO1-type galleries accompanies lateral shift of the CoO2 layers.Such structure changes will degrade the LiCoO2 film toincrease the resistance and lead to the capacity fading in thecycling with high-voltage charging. On the other hand, thecycling between 2.5 and 4.2 V is very stable because it changesthe SOC between x = 0 and 0.46, where the O3 structure iskept, and the corresponding change in the interlayer distance isbetween 4.68 and 4.80 Å, i.e., only 2%.Phase transitions taking place in high-voltage charging havebeen considered to affect the cycling performance of LiCoO2;however, their direct relation has not been clarified becausehigh-voltage charging also brings about side reactions that veilthe direct relation, e.g., formation of resistive LiF films on thesurface,28 pitting corrosion,29 or dissolution of Co.30 Such sidereactions make the relation between the phase transitions andcycling performance unclear, and an increase in electrodeimpedance has been always attributed to the electrode/electrolyte interface.31,32 On the other hand, Li3PO4 used asthe solid electrolyte in this study has unveiled the relationbetween the phase transition and electrode properties becauseit suppresses the side reactions and provides stable and low-resistive interface even to LiNi0.5Mn1.5O4 that shows anelectrode potential of 4.7 V.15,19The high-voltage cycling properties of LiCoO2 have beengenerally investigated in liquid electrolytes, whereas they wereonce reported for an Li/Lipon/LiCoO2 thin-film battery.33The battery in the paper shows very stable cycling below 4.4 V,while increasing the cutoff voltage higher than 4.4 V results incapacity fading and an increase in the cell resistance. Thisstudy provides detailed explanations of the results. The phasetransition leading to the degradation of the battery will be thatbetween O3 and H1−3, although it was assigned to thatbetween O3 and O1 in the paper because it takes place justFigure 8. XRD patterns and Rct measured for the thin-film battery during the charging up to x = 0.7 and discharging. Diffraction intensity (I) isplotted on the logarithmic scales. Some XRD patterns referred in the text are labeled with the SOC values. Closed and open circles in the rightpanel indicate Rct in the charging and discharging process, respectively.ACS Applied Energy Materials www.acsaem.org Articlehttps://doi.org/10.1021/acsaem.1c03046ACS Appl. Energy Mater. 2021, 4, 14372−1437914377https://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaem.1c03046?fig=fig8&ref=pdfwww.acsaem.org?ref=pdfhttps://doi.org/10.1021/acsaem.1c03046?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asabove 4.5 V, where the H1−3 phase is detected in this study.The thin-film battery cycled with an upper cutoff voltage of 5.0V in the paper shows quicker capacity fading than that with 4.6V in this study, which is due to the different cutoff voltages.The SOC values at the end of charging and discharging in thisstudy are estimated from the accumulated capacity and plottedin Figure 3c. The SOC values at the end of charging becomeshallower upon the cycling because increasing Rct enlarges theoverpotential. The increased overpotential makes LiCoO2never reach the composition for the O3/H1−3 phasetransition and thus lowers the degradation rate in theprolonged cycle.6. CONCLUSIONSThis study has revealed the clear relation between electrodekinetics and phase transition in high-voltage charging ofLiCoO2. Combination of impedance spectroscopy and in situXRD shows that a drastic increase in electrode resistancecoincides with phase transition from O3 to H1−3 phases. Thecoincidence strongly suggests that O1-type interlayers with agallery height of 4.2 Å formed in the H1−3 phase impede ionicdiffusion to reduce the electrode kinetics of LiCoO2 charged athigh voltage. Once LiCoO2 has experienced the phasetransition, the increased electrode resistance never comesback to its original value in the following discharging eventhough it recovers its O3 phase. It will be the large differencein the interlayer distance between the O3 and O1-typegalleries, which leads to the irreversible increase in theelectrode resistance and capacity fading.■ AUTHOR INFORMATIONCorresponding AuthorKazunori Takada − Center for Green Research on Energy andEnvironmental Science, National Institute for MaterialsScience, Tsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0001-7568-1806; Phone: +81 29 8604317;Email: takada.kazunori@nims.go.jpAuthorsTsuyoshi Ohnishi − Center for Green Research on Energy andEnvironmental Science, National Institute for MaterialsScience, Tsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0002-2333-7752Kazutaka Mitsuishi − Research Center for AdvancedMeasurement and Characterization, National Institute forMaterials Science, Tsukuba, Ibaraki 305-0047, Japan;orcid.org/0000-0002-9361-4057Complete contact information is available at:https://pubs.acs.org/10.1021/acsaem.1c03046Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was partly supported by the Advanced Low CarbonTechnology Research and Development Program, SpeciallyPromoted Research for Innovative Next Generation Batteries(ALCA-SPRING) of the Japan Science and TechnologyAgency (JST), Japan; a Materials Processing Science project(“Materealize”) of Ministry of Education, Culture, Sports,Science and Technology, Japan (MEXT); a KAKENHI Grant-in-Aid for Scientific Research on Innovative Areas “InterfaceIONICS” (grant number JP19H05813) from the Japan Societyfor the Promotion of Science (JSPS); and JST grant numberJPMJPF2016. We thank Rigaku Corporation for letting us touse the air-tight sample holder.■ REFERENCES(1) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B.LixCoO2 (0<x≤1): a new cathode material for batteries of highenergy density. Mater. Res. 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