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## Creator

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

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[Sputter-Deposited Amorphous Li<sub>3</sub>PO<sub>4</sub> Solid Electrolyte Films](https://mdr.nims.go.jp/datasets/3d69d5c5-6502-4edd-ac95-c60a40519d67)

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Sputter-Deposited Amorphous Li3PO4 Solid Electrolyte FilmsSputter-Deposited Amorphous Li3PO4 Solid Electrolyte FilmsTsuyoshi Ohnishi* and Kazunori TakadaCite This: ACS Omega 2022, 7, 21199−21206 Read OnlineACCESS Metrics & More Article RecommendationsABSTRACT: This paper reports the thin-film synthesis of Li3PO4 solid electrolytes by RFmagnetron sputtering. A relatively high ionic conductivity of more than 1 × 10−6 S cm−1 isachieved. It is revealed that the crystallization of Li3PO4 impedes ionic conduction, and amoderate amount of O2 addition to Ar suppresses the crystallization and guarantees long-term deposition. Another important finding in this study is that when Li3PO4 is deposited ona LiCoO2 film to construct a thin-film battery, the LiCoO2 film can be damaged dependingon the substrate bias potential relative to the cathode potential propagated through thesputtering plasma. Active control of the bias potential to avoid the damage realizes negligibleinterface resistance in the thin-film battery.■ INTRODUCTIONSolid-state Li-ion batteries are promising next-generationpower supplies to replace current Li-ion batteries because oftheir superior features such as high energy density, long cyclelife, and safety. However, it is reported that the interfacebetween a cathode material and a solid electrolyte in a solid-state battery can be rate-determining and thus governs thepower density.1 Converting solid-state batteries into the thin-film form is an effective way to investigate the interfaceproperties since it simplifies the geometry and providesimportant information about the interfaces.2−5Lithium phosphorus oxynitride (LiPON) is widely used as asolid electrolyte layer in thin-film batteries because of itsrelatively high ionic conductivity (∼3 × 10−6 S cm−1). LiPONwas first developed by Bates et al.6 by sputtering a Li3PO4target in pure N2. Their Li/LiPON/LiCoO2 thin-film batteriesoperate for more than 30,000 cycles with a capacity fading ofless than 5%.2 Since then, a number of thin-film batteries withLiPON have been reported.3,7,8 However, recent first-principles calculations indicate that LiPON is not thermody-namically stable, but kinetically stabilized, upon contact with Limetal and LiCoO2,9 and the calculation results are consistentwith experimental results.10,11 Although partial replacement ofO with N (and Li uptake) improves the ionic conductivity, andthe conductivity reaches 6.4 × 10−6 S cm−1 with simultaneousLi enrichment in the target,12 the incorporation of N intoLi3PO4 narrows its electrochemical stability window accordingto the aforementioned calculations.9Li3PO4 itself is also used as a solid electrolyte layer in thin-film batteries. Bates et al. examined it along with LiPON bysputtering a Li3PO4 target with 40% O2 in Ar. However, theconductivities of their Li3PO4 films were as low as 7 × 10−8 Scm−1;6 another group also reported similar values,7 and in bothof these studies, Li3PO4 films were deposited by radio-frequency (RF) magnetron sputtering. Meanwhile, Li3PO4films prepared by pulsed laser deposition (PLD) using ahigh-photon-energy ArF excimer laser showed a relativelyhigher ionic conductivity of ∼5 × 10−7 S cm−1,13,14 and thin-film batteries made with the PLD Li3PO4 operate ratherwell.14−16Here, we report the Li3PO4 solid electrolyte film synthesisby RF magnetron sputtering with a much improved ionicconductivity. Although there are difficulties in LiPONsynthesis in terms of controlling the amount of N incorporatedand the simultaneous Li addition to achieve charge neutrality,Li3PO4 synthesis is much simpler. We also report thin-filmbatteries constructed by depositing Li anodes and Li3PO4 onLiCoO2 epitaxial thin films.■ RESULTS AND DISCUSSIONA schematic configuration of our specially designed RFmagnetron sputtering system is shown in Figure 1. Sincemultiple sputter cathodes with 2″-diameter targets areequipped, each cathode is oriented to the center of a 2″-diameter substrate holder with a 60° incident angle. Thesubstrate holder is continuously rotated during deposition, andReceived: April 5, 2022Accepted: May 26, 2022Published: June 8, 2022Articlehttp://pubs.acs.org/journal/acsodf© 2022 The Authors. Published byAmerican Chemical Society21199https://doi.org/10.1021/acsomega.2c02104ACS Omega 2022, 7, 21199−21206Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on June 23, 2022 at 13:26:06 (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="Kazunori+Takada"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsomega.2c02104&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=abs1&ref=pdfhttps://pubs.acs.org/toc/acsodf/7/24?ref=pdfhttps://pubs.acs.org/toc/acsodf/7/24?ref=pdfhttps://pubs.acs.org/toc/acsodf/7/24?ref=pdfhttps://pubs.acs.org/toc/acsodf/7/24?ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsomega.2c02104?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://http://pubs.acs.org/journal/acsodf?ref=pdfhttps://http://pubs.acs.org/journal/acsodf?ref=pdfhttps://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://acsopenscience.org/open-access/licensing-options/10 mm square or 10 mm diameter substrates are locatedaround the middle radius position of the 2″ inconel holder forsimultaneous multiple deposition. Ar as well as O2 gases can beintroduced through mass flow controllers. The chamber isevacuated by a turbo molecular pump, and a conductance-controllable gate valve is equipped between the chamber andthe pump to adjust the chamber pressure independent of thegas flow rate. The substrate holder potential can be adjusted bya bipolar direct current (DC) power supply. Stainless steel and0.5 wt % Nb-doped SrTiO3 substrates were used, and theywere electronically connected to the substrate holder duringLi3PO4 deposition. For the Nb:SrTiO3 substrates, a piece ofmetal Mg was bridged to make a low-resistance connectionwith the inconel holder. The sputtering plasma potentialaround the substrate position can be measured by a substrateshutter. Besides, the cathode DC potential can be measuredthrough a low-pass filter during RF sputtering.Improvement of Li3PO4 Conductivity. It is obvious fromFigure 2 that the crystallization of Li3PO4 film drasticallydecreases the ionic conductivity. The figures show thesubstrate temperature (Tsub) dependence of the film depositionrate, ionic conductivity, and X-ray diffraction (XRD) patternsof 2 h-deposited Li3PO4 films on mirror-polished stainless steelsubstrates. The 10 mm square and 0.5 mm-thick stainless steelsubstrate, which works as the bottom blocking electrode, wasvacuum-annealed before use to remove the insulative oxidationlayer on the surface. The deposition rate was evaluated by thefilm thickness measured with X-ray reflectance measurement,and ionic conductivity was estimated by alternating-current(AC) impedance measurements with 2 mm diameter Ptblocking electrodes deposited by DC magnetron sputtering.The AC impedance data were obtained in the frequency rangeof 5 × 105−0.01 Hz with an AC amplitude of 20 mV, and ionicconductivity was estimated from the diameter of the semicircleat a higher-frequency region by fitting. The XRD patterns weremeasured by the surface-sensitive grazing-incidence method(GIXRD), where the X-ray incident angle is fixed at 0.25° andthe intensity is recorded with 2θ scanning. Ar and O2 gas flowrates were 20 and 5 sccm, respectively, and the total gaspressure was controlled to be 0.6 Pa during deposition. A RFpower of 150 W was used, and 200 W data are also plotted inthe left panel of Figure 2 for a comparison.The deposition rate, i.e., the film thickness, is almostconstant and independent of Tsub, but the ionic conductivity issensitive to Tsub; a higher Tsub results in lower conductivity.According to the GIXRD results, when Tsub is lower than 150°C, the Li3PO4 film is in an amorphous state, showing a halocentered at 2θ = 23°. On the other hand, when Tsub is higher,sharp diffraction peaks appear, which correspond to the Li3PO4crystal phase, and at 300 °C, additional peaks at 2θ ≈ 14 and28° appear, which are attributable to the Li4P2O7 crystal phase.Because the Tsub at the starting of crystallization coincides wellwith that during the conductivity drop, it is concluded that thecrystallization of Li3PO4 impedes ionic conduction, and anamorphous state is essential for high ionic conductivity. Theactivation energy estimated from the temperature dependenceof ionic conductivity of the amorphous films in the range of200−350 K was 0.53−0.55 eV. The obtained activation energyand frequency dependence of the impedance were similar tothose reported for a PLD-deposited film under an O2atmosphere (0.58 eV),15 but quite different from that of aRF-sputtered film deposited under pure Ar (0.38 eV),7Figure 1. Schematic configuration of a specially designed RFmagnetron sputtering system.Figure 2. Left panels: Tsub dependences of the film deposition rate (top) and ionic conductivity (bottom). Lines are visual guides. Right panel: Tsubdependence of GIXRD patterns of 2 h-deposited Li3PO4 films on stainless steel substrates with an incident angle of 0.25°. A GIXRD pattern from asubstrate without Li3PO4 deposition is also shown at the bottom. Deposition conditions are as follows: RF power, 150 W; Ar, 20 sccm; O2, 5 sccm;total pressure, 0.6 Pa; target−substrate distance, 150 mm; and substrate bias potential, +0.5 V.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.2c02104ACS Omega 2022, 7, 21199−2120621200https://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig2&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.2c02104?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-assuggesting the importance of O2 introduction. Although lowTsub is preferable to make the films amorphous, the substrate isheated up by sputtering plasma during much longer depositionprocesses, resulting in an unstable Tsub in our depositionsystem, because it does not have a substrate cooler. Heating atmoderate temperatures between 50 and 150 °C is reliable tokeep Tsub constant throughout the deposition and to depositamorphous films. Although the 200 W data show higherdeposition rates (almost double), the higher cathode powertends to damage the target surface severely and quickly (e.g.,by causing cracking and color change); thus, a lower RF poweris preferred for long-term deposition.Figure 3 shows the Tsub dependence of the mixing ratio ofO2 and Ar gases, in the same manner as in Figure 2, under atotal pressure of 0.6 Pa, which was controlled by theconductance valve. According to the results of Figure 2, aTsub of 100 °C is selected, and the room-temperaturedeposition is also examined without O2 introduction. It isobvious that the deposition rate is higher when none or a smallamount of O2 is introduced. However, the ionic conductivity islow (less than 10−6 S cm−1) when no O2 is introduced. GIXRDresults indicate that Li3PO4 is crystallized clearly when the O2ratio is 1% or less and only slightly when it is 50%. The latterconditions seem similar to those examined by Bates et al.reporting low conductivity;6 thus, it can be concluded that O2is necessary to avoid crystallization; however, excess of O2 alsoresults in crystallization and decreases the conductivity. Inaddition, a thin film deposited at room temperature reveals theimportance of O2 introduction. The Bragg peaks indicatingcrystallization are observed for the thin film deposited withoutO2 introduction, even though the film is deposited withoutsubstrate heating, and the film shows low ionic conductivity. Itmeans that O2 gas is anyway needed to suppress thecrystallization of Li3PO4. Because sputtering is a vacuumprocess and the deposited film is an oxide, the film tends tobecome oxygen-deficient. In the field of thin-film growth ofhigh Tc superconducting and other functional oxides, it is well-known that the melting point (Tmelt) and crystallizationtemperature of oxide materials tend to be lowered whenoxygen is deficient, and thus, high-crystallinity thin films ofoxide materials with high Tmelt can be obtained via a vacuumprocess,17 e.g., molecular beam epitaxy and PLD, under muchlower Tsub relative to their Tmelt values. Besides, oxygendeficiency is introduced not only in the deposited films butalso in the sputtering target, resulting in serious target damage.O2 gas introduction is therefore necessary to suppress filmcrystallization and to avoid target damage in the long-termdeposition for a thick solid electrolyte layer. O2 introductionFigure 3. Left panels: O2 and Ar gas ratio dependences of the film deposition rate (top) and ionic conductivity (bottom). Lines are visual guides.Right panel: the same dependence of GIXRD patterns of 2 h-deposited Li3PO4 films on stainless steel substrates with an incident angle of 0.25°.Deposition conditions are as follows: Tsub, 100 °C; RF power, 150 W; total pressure, 0.6 Pa; target−substrate distance, 150 mm; and substrate biaspotential, +0.5 V.Figure 4. Left panels: thickness dependences of the film deposition rate (top) and ionic conductivity (bottom). Lines are visual guides. Right panel:deposited thickness dependence of AC impedance Nyquist plots. Deposition conditions were as follows: Tsub, 100 °C; RF power, 150 W; Ar, 20sccm; O2, 1 sccm; total pressure, 0.6 Pa; target−substrate distance, 150 mm; and substrate bias potential, +0.5 V. The AC impedance data weretaken in the frequency range of 5 × 105−0.01 Hz with an AC amplitude of 20 mV.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.2c02104ACS Omega 2022, 7, 21199−2120621201https://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig4&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.2c02104?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asalso makes the deposited films stable in air; otherwise, thetransparent films devitrify after long-term storage in air,probably because of the humidity.Figure 4 shows the film thickness dependence of thedeposition rate, ionic conductivity, and AC impedance Nyquistplots. Deposition conditions are as follows: Tsub, 100 °C; RFpower, 150 W; Ar, 20 sccm; O2, 1 sccm; and total pressure, 0.6Pa. When the thickness fringes were unclear in X-rayreflectance curves for films thicker than the typical 200 nm,the thickness was evaluated with a stylus profilometer, i.e.,measuring the step height made by peeling off masking tapes.It appears that films with a thickness of 50 nm or more areneeded to guarantee ionic conduction because all six Pt padsprepared on a 44 nm or thicker Li3PO4 film were electronicallyopen and showed similar conductivities, but on a 25 nm orthinner Li3PO4 film, the conductivity was low and some Ptpads were short-circuited already. The constant deposition rateand ionic conductivity observed in the thickness range beyond50 nm demonstrate that long-term and stable deposition ispossible under the current conditions. For the precedingexperiments, including this thickness dependence, the substratepotential was kept at +0.5 V by the DC power supply, and thetarget−substrate distance was fixed to be 150 mm duringdeposition.Damage to the Underlying LiCoO2 Film in BatteryDevices. Li3PO4 films were deposited on PLD-grown epitaxialLiCoO2 thin films. The substrates were 0.5 wt % Nb-dopedSrTiO3 (111) single crystals with a 10 mm square or 10 mmdiameter and 0.5 mm thickness, and LiCoO2 grew in the c-axisorientation with a thickness of 100−200 nm. Details of theLiCoO2 thin-film synthesis are described elsewhere.18,19 Figure5 shows the 2θ−ω scan XRD patterns of 15 h Li3PO4-deposited (2−2.5 μm thick) LiCoO2 thin films under differentsubstrate bias potentials during sputtering. Diffraction patternsbefore Li3PO4 deposition are also shown in blue curves.Deposition conditions were as follows: Tsub, 100 °C; RFpower, 120 W; Ar, 20 sccm; O2, 7 sccm; and total pressure, 0.6Pa. Because the deposition rate of Li3PO4 was stable undercertain conditions, the deposited thickness was controlled bythe deposition time. It is obvious that there is a clear substratebias potential dependence of LiCoO2 crystallinity after Li3PO4deposition. When the potential is lower than −3 V or higherthan +0.5 V, LiCoO2 diffraction peaks disappear or theintensity decreases drastically, i.e., the LiCoO2 crystal lattice isdestroyed. Besides, it seems that there is an optimal substratepotential, and −2.0 V is close to the optimal value in thesedepositions as the intensity decrease of LiCoO2 diffraction isminimal. It should be noted here that the conductivity ofLi3PO4 films deposited on stainless steel substrates within thispotential range is almost constant (≈1.2 × 10−6 S cm−1),independent of the substrate bias potential.It appears that an optimal substrate bias potential exists;however, it is not constant but varies gradually with eachdeposition. Figure 6 shows a plot of the cathode potentialwhen the RF power is set at 100 W before each 5 h deposition.Other deposition conditions were as follows: Ar, 20 sccm; O2,10 sccm; total pressure, 0.8 or 1.0 Pa; and the target−substratedistance was shortened to 95 mm from and after thisexperiment to approximately double the deposition rate. Thetarget was changed between depositions #525 and #526. Thetarget change alters the various deposition conditions, but thebiggest change is in the DC cathode potential during RFFigure 5. Red curves: 2θ−ω scan XRD patterns of 15 h Li3PO4-deposited (2−2.5 μm thick) LiCoO2 thin films under different substrate biaspotentials during sputtering. Deposition conditions were as follows: Tsub, 100 °C; RF power, 120 W; Ar, 20 sccm; O2, 7 sccm; total pressure, 0.6 Pa;and target−substrate distance, 150 mm. XRD patterns before Li3PO4 deposition are also shown in blue curves, and the LiCoO2 thickness is givenon the right top of each panel. Right panels are magnified views around LiCoO2 003 in a linear intensity scale at potentials of 0.0, −1.0, and −2.0 V.Diffraction peaks marked by “*” in the left top panel indicate the O1 phase 00l.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.2c02104ACS Omega 2022, 7, 21199−2120621202https://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig5&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.2c02104?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-assputtering. When the target is being worn out, the cathodepotential increases and reaches around −250 V under theseconditions, and finally, the rear indium bond and Cu backingplate appear around the eroding part of the Li3PO4 target. Inthe case of a new target, the cathode potential is as low asaround −400 V. Scattering of the cathode potential is mainlycaused by the total pressure change. It should also be notedthat the total pressure is related to the deposition rate as wellas the shutter potential. When the pressure is increased in therange of 0.6−1.0 Pa, the deposition rate slightly decreasesmainly due to stronger gas scattering, and the shutter potentialincreases and approaches 0 V. It should be noted that thecathode potential has a strong correlation to the shutterpotential around the substrate position as shown in Figure 6(top). Although the shutter potential is always negative, itchanges and correlates to the cathode potential. It can beassumed that the optimal substrate bias potential that preservesthe LiCoO2 crystal quality after the Li3PO4 deposition isrelated to the cathode potential, and it changes gradually withthe change in the target surface state.The change in the optimum bias potential can be seen inFigure 7. It shows 2θ−ω scan XRD patterns of two 15 hLi3PO4-deposited LiCoO2 thin films under the same substratebias potential of −0.5 V, but before and after the target change.Other deposition conditions were as follows: Tsub, 100 °C; RFpower, 100 W; Ar, 20 sccm; O2, 10 sccm; and total pressure,0.6 Pa. Between the two depositions, the Li3PO4 target waschanged for a new one because the target was worn out. TheLiCoO2 crystallinity of the top one is almost preserved,whereas that of the bottom one is degraded severely. It meansthat the optimal substrate bias potential was around −0.5 V,but it has shifted after the target change. Thin-film batteriesmade by ∼500 nm-thick Li anode deposition with vacuumthermal evaporation on Li3PO4 films support the tendency; anopen-circuit voltage (ocv) of the thin-film battery with the topsample just after cell construction was 3.9 V, whereas that ofthe bottom one was 4.3 V.Here, we discuss what happens when the substrate biaspotential is not optimal. Because Li3PO4 and LiCoO2 do notreact with each other at substrate temperatures as low as 100°C, there must be other reasons that relate to the potential. Ina battery, the LiCoO2 cathode can be damaged by over-charging and over-discharging, with too high and too lowcutoff voltages, respectively. In the sputtering process, thecathode DC potential always becomes negative to sputter thetarget material by positively ionized Ar gases. Even though itdepends on the total pressure, the target−substrate distance,and the on-axis/off-axis geometry,7 the target and substrate areconnected to each other by plasma, which is an electron-conductive gas; thus, the substrate holder is subjected to thecathode potential to some extent, as shown in Figure 6. SinceLiCoO2 is underneath the previously deposited Li3PO4, whichis connected to the plasma, LiCoO2 can be charged ordischarged depending on the substrate bias potential relative tothe plasma potential. In Figure 7 (bottom), the LiCoO2 filmappears to be overcharged as the X-ray diffraction intensity isconsiderably decreased, and additional reflections appearing at2θ ≈ 21.6 and 44.0° are attributable to the 001 and 002diffractions of the O1 phase, respectively, which is anovercharged phase of LiCoO2.20,21 Besides, the ocv of the as-constructed battery was 4.3 V, which is higher than thestandard charging cutoff voltage of 4.2 V. In Figure 5, the topfilm is also overcharged, but the bottom one seems over-discharged as LiCoO2 cannot take up extra Li,22 unlike theLiNi0.5Mn1.5O4 cathode,16,23 and thus the LiCoO2 crystalcollapses.In the literature, disordered LiCoO224 and Li2MnO425cathode layers are observed at the interface with LiPON incross-sectional images obtained by a scanning transmissionelectron microscope. Although LiPON is deposited bysputtering the Li3PO4 target with pure N2 instead of an Arand O2 mixture, as in our study, effects of the plasma potentialarise likewise: if the substrate potential is not adequate, thecathode layer can be damaged from the LiPON interface.Related to the latter Li2MnO4, the same authors reportLixMnO2 cathode formation from a MnO2−x layer bydepositing a LiPON layer on top of it.26 They claim that theLiPON deposition infuses Li ions into MnO2−x. Consideringthe Li2MnO4 disordering and formation of LixMnO2 fromMnO2−x, when the substrate potential is lower than that of theFigure 6. Top: shutter potential; bottom: cathode potential; RFpower was set at 100 W before each 5 h deposition. Depositionconditions were as follows: Ar, 20 sccm; O2, 10 sccm; total pressure,0.8 or 1.0 Pa; and target−substrate distance: 95 mm. The target waschanged between depositions #525 and #526.Figure 7. Red curves: 2θ−ω scan XRD patterns of two 15 h Li3PO4-deposited LiCoO2 thin films under a substrate bias potential of −0.5V. Other deposition conditions were as follows: Tsub, 100 °C; RFpower, 100 W; Ar, 20 sccm; O2, 10 sccm; total pressure, 0.6 Pa; andtarget-substrate distance: 95 mm. XRD patterns before Li3PO4deposition are also shown in blue curves; the LiCoO2 thickness isgiven on the right top of each panel. Diffraction peaks marked by “*”in the bottom panel indicate the O1 phase 00l.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.2c02104ACS Omega 2022, 7, 21199−2120621203https://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig7&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.2c02104?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asplasma, discharging (Li-ion infusion) takes place, probably dueto the charge build-up of negative plasma by an electronicallyisolated substrate holder.The shutter potential can be a good reference to determinethe optimal substrate bias potential; however, it is not stableenough for long-term Li3PO4 sputtering, e.g., 10 h deposition;it can shift due to changes in the target surface state, e.g.,sudden target cracking, oxygen deficiency introduction, and soon. Therefore, even if the substrate bias potential is determinedonce from the shutter potential before the deposition, the shiftof the plasma potential deviates the substrate bias potentialfrom the optimum value during deposition, which results inlow experimental reproducibility. Here, we introduce a strategyto solve the reproducibility problem rather easily. DuringLi3PO4 deposition, the substrate bias potential is adjusted inreal time so that the current meter between the substrateholder and DC power supply (Figure 1) shows zero current.Ideally, no current flows when the substrate bias potential andpotential induced from the plasma are balanced. With thisactive control, we successfully fabricated 10 mm-scale Li/Li3PO4/LiCoO2 thin-film batteries exhibiting high perform-ance; some results have been published already,21,27 andanother example is described below.Figure 8 shows our thin-film battery structure and room-temperature performances of the battery in which Li3PO4 (ca.1.1 μm thick) was deposited under the consideration of theabove-described over-charging/discharging processes. Li3PO4deposition conditions were as follows: Tsub, 50 °C; RF power,150 W; Ar, 20 sccm; O2, 10 sccm; total pressure, 0.8 Pa; and 5h deposition. The increased total pressure was to reduce theabsolute value of the plasma potential. The RF magnetronsputtering-grown LiCoO2 was a (104)-oriented epitaxial filmon a 10 mm square, 0.5 mm-thick 0.5 wt % Nb-doped SrTiO3(100) substrate, with a film weight of 561 μg (ca. 1.4 μmthick), which was measured with an electronic balance. Detailsof LiCoO2 sputtering growth are reported elsewhere. Beforethe Li3PO4 deposition, an ∼100 nm-thick Pt current collectorwas deposited on a LiCoO2 film by DC magnetron sputtering,which is a 10 mm square film with a circular opening with adiameter of 10 mm,21 and connected to the substrate holderelectronically during Li3PO4 deposition. Meanwhile, all of ourLiCoO2 films were air-exposed after synthesis for all purposesincluding weighing, XRD measurement, and Pt deposition.After Li3PO4 deposition, a circular Li anode with a diameter of8.5 mm was formed above the opening of the Pt currentcollector.21 Although the LiCoO2 cathode layer was relativelythick, the battery showed a rather high rate capability. Here, acharging/discharging rate of 1 C is defined as 137 mA g−1,which is based on the expected capacity when LiCoO2 ischarged up to Li0.5CoO2 at 4.2 V.28 The thin-film battery wascharged at a 1 C constant current up to a cutoff voltage of 4.2V, followed by a constant-voltage charging at 4.2 V for 1 hbefore each discharge to guarantee a fully charged state. Thecapacity of low-rate discharge with 7.69 μA (0.1 C) was 108mAh g−1, whereas the capacity of high-rate discharge with 7.69mA (100 C) was 60 mAh g−1, maintaining >55% of the low-rate discharge. Figure 8 (bottom) shows a Nyquist plot at the4.2 V charged state. The semicircle at the higher-frequencyregion originates from the resistance of Li3PO4, and its ionicconductivity calculated from the x-intercept of the semicircle ata lower frequency is 1.1 × 10−6 S cm−1. It should be noted thatother resistances, e.g., the interface resistance between the Lianode and Li3PO4, or Li3PO4 and the LiCoO2 cathode, are notclearly observed, which is different from previously reportedresults.7,14 Most recently, the reduced interface resistance wasreported to be 10.3 Ω cm2 between LiCoO2 and RF-sputteredLi3PO4,29 and also a further reduced resistance of 8.6 Ω cm2was reported between LiCoO2 and LiPON.7 As the active areaof our battery was 0.567 cm2 (8.5 mm-diameter Li anode), atleast 15 Ω of interface resistance should appear in the Nyquistplots, if such a resistance exists; however, it cannot be seen inFigure 8 (bottom). The IR drop at 7.69 mA discharge is 1.37 V(Figure 8 middle) and thus R becomes 178 Ω. The resistanceof Li3PO4 in Figure 8 (bottom) is 179 Ω, which is almost thesame value, i.e., no additional resistance exists, even though theLiCoO2 film is air-exposed. It strongly suggests that theintrinsic interface resistance of Li/Li3PO4 or Li3PO4/LiCoO2 ismuch smaller than that reported and is almost negligible, andthis can be achieved by the active control of the substratepotential during Li3PO4 deposition. In other words, sputtering7Figure 8. Room-temperature performances of a Li (∼500 nm)/Li3PO4 (1.1 μm)/LiCoO2 (561 μg) thin-film battery. Top: the batterydevice structure; middle: discharge curves for different C-rates;bottom: AC impedance Nyquist plot at 4.2 V charged state, where thetop x-axis is areal resistance calculated with an 8.5 mm-diameter Lianode. AC impedance data were obtained in the frequency range of 5× 105−0.01 Hz, with an AC amplitude of 20 mV. Li3PO4 depositionconditions were as follows: Tsub, 50 °C; RF power, 150 W; Ar, 20sccm; O2, 10 sccm; total pressure, 0.8 Pa; and target−substratedistance, 95 mm.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.2c02104ACS Omega 2022, 7, 21199−2120621204https://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig8&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.2c02104?fig=fig8&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.2c02104?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asor bombardment of high-energy ablated species14 during theLi3PO4 deposition has been proposed to cause damages to theLiCoO2 layer, which results in a large interface resistance, andeliminating these causes lowered the interface resistance to ca.10 Ω cm2. However, the interface resistance has not reachedthe intrinsic value yet due to remaining damages underuncontrolled bias potential. In fact, we also observed such aninterfacial resistance (≈10 Ω cm2) when the bias potentialcontrol was not adequate, and the details are reported in aseparate paper.■ CONCLUSIONSThis study revealed that RF magnetron sputtering-depositedLi3PO4 can have a relatively high ionic conductivity of morethan 1 × 10−6 S cm−1, close to that of LiPON, by avoidingLi3PO4 crystallization; the addition of a certain amount of O2gas into Ar as well as a low substrate temperature are effectiveto suppress the crystallization. Besides, this study points outthe importance of substrate potential control during Li3PO4deposition on LiCoO2 films, which has been a hidden, butpredominant, process parameter for achieving high perform-ance in thin-film batteries.■ AUTHOR INFORMATIONCorresponding AuthorTsuyoshi Ohnishi − Center for Green Research on Energy andEnvironmental Materials, National Institute for MaterialsScience (NIMS), Tsukuba, Ibaraki 305-0044, Japan;orcid.org/0000-0002-2333-7752;Email: ohnishi.tsuyoshi@nims.go.jpAuthorKazunori Takada − Center for Green Research on Energy andEnvironmental Materials, National Institute for MaterialsScience (NIMS), Tsukuba, Ibaraki 305-0044, Japan;orcid.org/0000-0001-7568-1806Complete contact information is available at:https://pubs.acs.org/10.1021/acsomega.2c02104Author ContributionsThis manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript and contributed equally.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, grant no. JPMJAL1301) of the JapanScience and Technology Agency (JST), Japan; a MaterialsProcessing Science project (“Materealize”) of the Ministry ofEducation, Culture, Sports, Science and Technology, Japan(MEXT); a KAKENHI Grant-in-Aid for Scientific Research onInnovative Areas “Interface IONICS” (grant no. JP19H05813)from the Japan Society for the Promotion of Science (JSPS);and JST grant no. JPMJPF2016.■ ABBREVIATIONSLiPON, lithium phosphorus oxynitride; RF, radiofrequency;PLD, pulsed-laser deposition; Tsub, substrate temperature;XRD, X-ray diffraction; AC, alternating current; GIXRD,grazing-incidence X-ray diffraction; Tmelt, melting point; ocv,open circuit voltage■ REFERENCES(1) Ohta, N.; Takada, K.; Zhang, L.; Ma, R.; Osada, M.; Sasaki, T.Enhancement of the High-Rate Capability of Solid-State LithiumBatteries by Nanoscale Interfacial Modification. Adv. Mater. 2006, 18,2226−2229.(2) Wang, B.; Bates, J. B.; Hart, F. X.; Sales, B. C.; Zuhr, R. A.;Robertson, J. D. Characterization of Thin-Film Rechargeable LithiumBatteries with Lithium Cobalt Oxide Cathodes. J. Electrochem. Soc.1996, 143, 3203−3213.(3) Iriyama, Y.; Nishimoto, K.; Yada, C.; Abe, T.; Ogumi, Z.;Kikuchi, K. Charge-Transfer Reaction at the Lithium PhosphorusOxynitride Glass Electrolyte/Lithium Manganese Oxide Thin-FilmInterface and Its Stability on Cycling. J. Electrochem. Soc. 2006, 153,A821−A825.(4) Yamamoto, K.; Iriyama, Y.; Asaka, T.; Hirayama, T.; Fujita, H.;Fisher, C.A.J..; Nonaka, K.; Sugita, Y.; Ogumi, Z. DynamicVisualization of the Electric Potential in an All-Solid-StateRechargeable Lithium Battery. Angew. Chem., Int. Ed. 2010, 49,4414−4417.(5) Song, J.; Jacke, S.; Cherkashinin, G.; Schmid, S.; Dong, Q.;Hausbrand, R.; Jaegermann, W. Valence Band Offsets of LiPON/LiCoO2 Hetero-Interfaces Determined by X-ray PhotoelectronSpectroscopy. Electrochem. Solid-State Lett. 2011, 14, A189−A191.(6) Bates, J. B.; Dudney, N. J.; Gruzalski, G. R.; Zuhr, R. A.; AChoudhury, A.; Luck, C. F.; Robertson, J. D. Fabrication andcharacterization of amorphous lithium electrolyte thin films andrechargeable thin-film batteries. J. Power Sources 1993, 43, 103−110.(7) Haruta, M.; Shiraki, S.; Suzuki, T.; Kumatani, A.; Ohsawa, T.;Takagi, Y.; Shimizu, R.; Hitosugi, T. Negligible “Negative Space-Charge Layer Effects” at Oxide-Electrolyte/Electrode Interfaces ofThin-Film Batteries. Nano Lett. 2015, 15, 1498−1502.(8) Bates, J. B.; Dudney, N. J.; Neudecker, B.; Ueda, A.; Evans, C. D.Thin-film lithium and lithium-ion batteries. Solid State Ionics 2000,135, 33−45.(9) Zhu, Y.; He, X.; Mo, Y. Origin of Outstanding Stability in theLithium Solid Electrolyte Materials: Insights from ThermodynamicAnalyses Based on First-Principles Calculations. ACS Appl. Mater.Interfaces 2015, 7, 23685−23693.(10) Schwöbel, A.; Hausbrand, R.; Jaegermann, W. InterfaceReactions between LiPON and Lithium Studied by in-Situ X-RayPhotoemission. Solid State Ionics 2015, 273, 51−54.(11) Yu, X.; Bates, J. B.; Jellison, G. E.; Hart, F. X. A Stable Thin-Film Lithium Electrolyte: Lithium Phosphorus Oxynitride. J. Electro-chem. Soc. 1997, 144, 524−532.(12) Suzuki, N.; Inaba, T.; Shiga, T. Electrochemical properties ofLiPON films made from a mixed powder target of Li3PO4 and Li2O.Thin Solid Films 2012, 520, 1821−1825.(13) Kuwata, N.; Iwagami, N.; Kawamura. ArF excimer laserdeposition of wide-band gap solid electrolytes for thin film batteries.Solid State Ionics 2009, 180, 644−648.(14) Shiraki, S.; Shirasawa, T.; Suzuki, T.; Kawasoko, H.; Shimizu,R.; Hitosugi, T. Atomically Well-Ordered Structure at SolidElectrolyte and Electrode Interface Reduces the Interfacial Resistance.ACS Appl. Mater. Interfaces 2018, 10, 41732−41737.(15) Kuwata, N.; Iwagami, N.; Tanji, Y.; Matsuda, Y.; Kawamura, J.Characterization of thin-film lithium batteries with stable thin-filmLi3PO4 solid electrolytes fabricated by ArF excimer laser deposition. J.Electrochem. Soc. 2010, 157, A521−A527.(16) Kawasoko, H.; Shirasawa, T.; Nishio, K.; Shimizu, R.; Shiraki,S.; Hitosugi, T. Clean Solid-Electrolyte/Electrode Interfaces Doublethe Capacity of Solid-State Lithium Batteries. ACS Appl. Mater.Interfaces 2021, 13, 5861−5865.(17) Yun, K. S.; Choi, B. D.; Matsumoto, Y.; Song, J. H.; Kanda, N.;Ito, T.; Kawasaki, M.; Koinuma, H.; et al. Vapor-liquid-solid tri-phaseACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.2c02104ACS Omega 2022, 7, 21199−2120621205https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tsuyoshi+Ohnishi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-2333-7752https://orcid.org/0000-0002-2333-7752mailto:ohnishi.tsuyoshi@nims.go.jphttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kazunori+Takada"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-7568-1806https://orcid.org/0000-0001-7568-1806https://pubs.acs.org/doi/10.1021/acsomega.2c02104?ref=pdfhttps://doi.org/10.1002/adma.200502604https://doi.org/10.1002/adma.200502604https://doi.org/10.1149/1.1837188https://doi.org/10.1149/1.1837188https://doi.org/10.1149/1.2178647https://doi.org/10.1149/1.2178647https://doi.org/10.1149/1.2178647https://doi.org/10.1002/anie.200907319https://doi.org/10.1002/anie.200907319https://doi.org/10.1002/anie.200907319https://doi.org/10.1149/2.006112eslhttps://doi.org/10.1149/2.006112eslhttps://doi.org/10.1149/2.006112eslhttps://doi.org/10.1016/0378-7753(93)80106-Yhttps://doi.org/10.1016/0378-7753(93)80106-Yhttps://doi.org/10.1016/0378-7753(93)80106-Yhttps://doi.org/10.1021/nl5035896?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/nl5035896?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/nl5035896?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1016/S0167-2738(00)00327-1https://doi.org/10.1021/acsami.5b07517?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsami.5b07517?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsami.5b07517?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1016/j.ssi.2014.10.017https://doi.org/10.1016/j.ssi.2014.10.017https://doi.org/10.1016/j.ssi.2014.10.017https://doi.org/10.1149/1.1837443https://doi.org/10.1149/1.1837443https://doi.org/10.1016/j.tsf.2011.08.107https://doi.org/10.1016/j.tsf.2011.08.107https://doi.org/10.1016/j.ssi.2008.09.010https://doi.org/10.1016/j.ssi.2008.09.010https://doi.org/10.1021/acsami.8b08926?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsami.8b08926?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1149/1.3306339https://doi.org/10.1149/1.3306339https://doi.org/10.1021/acsami.0c21586?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsami.0c21586?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1063/1.1432111http://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.2c02104?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aspulsed-laser epitaxy of RBa2Cu3O7‑y single-crystal films. Appl. Phys.Lett. 2002, 80, 61−63.(18) Okada, K.; Ohnishi, T.; Mitsuishi, K.; Takada, K. Epitaxialgrowth of LiCoO2 thin films with (001) orientation. AIP Adv. 2017, 7,No. 115011.(19) Nishio, K.; Ohnishi, T.; Akatsuka, K.; Takada, K. Crystalorientation of epitaxial LiCoO2 films grown on SrTiO3 substrates. J.Power Sources 2014, 247, 687−691.(20) Amatucci, G. G.; Tarascon, J. M.; Klein, L. C. CoO2, the endmember of the LixCoO2 solid solution. J. Electrochem. Soc. 1996, 143,1114−1123.(21) Ohnishi, T.; Mitsuishi, K.; Takada, K. In Situ X-ray Diffractionof LiCoO2 in Thin-Film Batteries under High-Voltage Charging. ACSAppl. Energy Mater. 2021, 4, 14372−14379.(22) Godshall, N. A.; Raistrick, I. D.; Huggins, R. A. Relationshipsamong Electrochemical, Thermodynamic, and Oxygen PotentialQuantities in Lithium-Transition Metal-Oxygen Molten Salt Cells. J.Electrochem. Soc. 1984, 131, 543−549.(23) Kawasoko, H.; Shiraki, S.; Suzuki, T.; Shimizu, R.; Hitosugi, T.Extremely low resistance of Li3PO4 electrolyte/Li(Ni0.5Mn1.5)O4electrode interfaces. ACS Appl. Mater. Interfaces 2018, 10, 27498−27502.(24) Wang, Z.; Santhanagopalan, D.; Zhang, W.; Wang, F.; Xin, H.L.; He, K.; Li, J.; Dudney, N.; Meng, Y. S. In Situ STEM-EELSObservation of Nanoscale Interfacial Phenomena in All-Solid-StateBatteries. Nano Lett. 2016, 16, 3760−3767.(25) Xia, Q.; Sun, S.; Xu, J.; Zan, F.; Yue, J.; Zhang, Q.; Gu, L.; Xia,H. Self-Standing 3D Cathodes for All-Solid-State Thin Film LithiumBatteries with Improved Interface Kinetics. Small 2018, 14,No. 1804149.(26) Xia, Q.; Zhang, Q.; Sun, S.; Hussain, F.; Zhang, C.; Zhu, X.;Meng, F.; Liu, K.; Geng, H.; Xu, J.; Zan, F.; Wang, P.; Gu, L.; Xia, H.Tunnel Intergrowth LixMnO2 Nanosheet Arrays as 3D Cathode forHigh-Performance All-Solid-State Thin Film Lithium Microbatteries.Adv. Mater. 2021, 33, No. 2003524.(27) Kawashima, K.; Ohnishi, T.; Takada, K. High-rate capability ofLiCoO2 cathodes. ACS Appl. Energy Mater. 2020, 3, 11803−11810.(28) Ohzuku, T.; Ueda, A. Solid-State Redox Reactions of LiCoO2(R3m) for 4 Volt Secondary Lithium Cells. J. Electrochem. Soc. 1994,141, 2972−2977.(29) Kobayashi, S.; Arguelles, E. F.; Shirasawa, T.; Kasamatsu, S.;Shimizu, K.; Nishio, K.; Watanabe, Y.; Kubota, Y.; Shimizu, R.;Watanabe, S.; Hitosugi, T. Drastic Reduction of the Solid Electro-lyte−Electrode Interface Resistance via Annealing in Battery Form.ACS Appl. Mater. Interfaces 2022, 14, 2703−2710.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://doi.org/10.1021/acsomega.2c02104ACS Omega 2022, 7, 21199−2120621206https://doi.org/10.1063/1.1432111https://doi.org/10.1063/1.4999833https://doi.org/10.1063/1.4999833https://doi.org/10.1016/j.jpowsour.2013.08.132https://doi.org/10.1016/j.jpowsour.2013.08.132https://doi.org/10.1149/1.1836594https://doi.org/10.1149/1.1836594https://doi.org/10.1021/acsaem.1c03046?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsaem.1c03046?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1149/1.2115624https://doi.org/10.1149/1.2115624https://doi.org/10.1149/1.2115624https://doi.org/10.1021/acsami.8b08506?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsami.8b08506?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.nanolett.6b01119?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.nanolett.6b01119?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acs.nanolett.6b01119?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1002/smll.201804149https://doi.org/10.1002/smll.201804149https://doi.org/10.1002/adma.202003524https://doi.org/10.1002/adma.202003524https://doi.org/10.1021/acsaem.0c01973?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsaem.0c01973?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1149/1.2059267https://doi.org/10.1149/1.2059267https://doi.org/10.1021/acsami.1c17945?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://doi.org/10.1021/acsami.1c17945?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://doi.org/10.1021/acsomega.2c02104?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-as