Karppinen , Maarit Atomic / Molecular Layer Deposition of Lithium Terephthalate Thin Films as High Rate

We demonstrate the fabrication of high-quality electrochemically active organic 9 lithium electrode thin films by the currently strongly emerging combined atomic/molecular layer 10 deposition (ALD/MLD) technique using lithium terephthalate, a recently found anode material for 11 lithium-ion battery (LIB), as a proof-of-the-concept material. Our deposition process for Li12 terephthalate is shown to well comply with the basic principles of ALD-type growth including the 13 sequential self-saturated surface reactions, a necessity when aiming at micro-LIB devices with 3D 14 architectures. The as-deposited films are found crystalline across the deposition temperature range 15 of 200 – 280 °C, which is a trait highly desired for an electrode material but rather unusual for 16 hybrid organic-inorganic thin films. Excellent rate capability is ascertained for the Li-terephthalate 17 films with no conductive additives required. The electrode performance can be further enhanced 18 by depositing a thin protective LiPON solid-state electrolyte layer on top of Li-terephthalate; this 19 yields highly stable structures with capacity retention of over 97 % after 200 charge/discharge 20 cycles at 3.2 C. 21 22 23 The miniaturization of electronic devices demands for energy storage systems of equal 24 dimensions. In order to retain reasonable energy and power densities, all-solid-state thin-film 25 microbatteries based on three-dimensional (3D) microstructured architectures are seen as a viable 26 solution. Compared to 2D thin-film batteries, the increased specific surface area of 3D 27 microstructures provides us with enhanced energy density while the electrodes can still be kept 28 thin enough for short Li diffusion paths and thereby good power density.1–3 Such an approach 29 places an apparent need for a thin-film deposition method capable of manufacturing the electrode 30 and electrolyte materials on high-aspect-ratio substrates. Atomic layer deposition (ALD) is an 31 established thin-film technology for producing conformal coatings on such high-aspect-ratio 32 structures.4,5 It is based on sequential exposure of gaseous precursors on the target substrate where 33 surface-saturation limited reactions allow the layer-by-layer deposition of high-quality thin films 34 with sub-monolayer accuracy.6 However there is an apparent need for broadening the currently 35 rather narrow range of available deposition processes for Li-ion electrode materials. 36 Organic electrode materials would possess several attractive features compared to the current 37 transition-metal based inorganic materials. They are composed of cheap, earth-abundant, 38 environmentally friendly and light elements, and owing to the low molecular mass together with a 39 possibility for multiple redox processes per molecule, organic electrodes display very high 40 theoretical specific capacities of several hundred mAh per gram. Moreover, the redox properties 41 can be tuned by the addition of electron donating/withdrawing functional groups. The biggest 42 obstacles in putting organic electrode materials into practical use in next-generation LIBs are their 43 instability/dissolution in the commonly employed liquid electrolytes and their negligible electronic 44 conductivity which dictates the need for very large amounts of conductive additives resulting in 45 greatly reduced actual capacities.7,8 In an all-solid-state thin-film LIB these obstacles could 46 possibly be circumvented: the dissolution issue would be completely avoided by replacing the 47 liquid electrolyte by a solid one, while the reduced dimensions in thin films should contribute 48 towards mitigating the effect of intrinsically poor electronic conductivity of organics. 49 Owing to the recent progress in combining the ALD technique for inorganic materials with the 50 strongly emerging molecular layer deposition (MLD) technique for organics it has become 51 possible to fabricate inorganic-organic hybrid thin films in a well-controlled atomic/molecular 52 layer-by-layer manner; for a recent review of the combined ALD/MLD technique see ref. 9. A 53 number of ALD/MLD processes with different organic constituents have already been developed. 54 However, the range of the metal components is yet limited, and as far as we know no ALD/MLD 55 processes for lithium-organic thin films have been reported. Here we demonstrate that the 56 ALD/MLD technique indeed is commendably suited for the deposition of organic LIB electrodes; 57 our proof-of-the-concept data are for lithium terephthalate (Li2C8H4O4 or LiTP). The 58 electrochemical activity of bulk LiTP was discovered by Tarascon et al.;10 their data revealed high 59 gravimetric capacity of 300 mAh/g associated with a flat reduction potential at around 0.8 V vs. 60 Li+/Li. As such LiTP is indeed an attractive anode material as it offers considerably higher specific 61 energy compared to other electrode material candidates for thin-film LIBs including TiO2 and 62 Li4Ti5O12. Moreover, computational predictions indicate that the volume change of LiTP during 63 (de)lithiation is relatively small, i.e. ~6 %.11 Hence LiTP would be preferable also over the 64 traditional high-capacity anode materials such as silicon and transition-metal oxides for which the 65 large volume expansion during lithiation results in poor cycle life of the material.12 The major 66 findings of the present study are three-fold. First, we succeeded in fabricating crystalline organic 67 LIB electrode thin films for the first time through a gas-phase deposition technique. Secondly, the 68 films were found to be electrochemically active with excellent rate capability without relying on 69 any conductive additives thus demonstrating that LiTP can perform as a high-rate anode material 70 if the electronic conductivity issue can be overcome. Lastly we show that by applying a protective 71 layer consisting of the lithium phosphorus oxynitride (LiPON) solid-state electrolyte deposited by 72 ALD, the LiTP electrodes can be stabilized even in LiPF6-based liquid electrolytes resulting in an 73 excellent cycle life. 74 The LiTP thin films were deposited using Li(thd) (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) 75 and terephthalic acid (TPA) as precursors. The defining feature of an ALD/MLD process is the 76 self-limiting film growth, that is, after a certain threshold value, the growth-per-cycle (GPC) 77 calculated from the resultant film thickness value (determined in our case by spectroscopic 78 ellipsometry) becomes constant regardless the pulsing times of the precursors. To verify the 79 ALD/MLD-type film growth, the deposition rate for our Li(thd)–TPA process was studied as a 80 function of the precursor pulse lengths at a deposition temperature of 200 °C using 200 ALD/MLD 81 cycles. As shown in Figure 1a, in the case of Li(thd), saturation is achieved with a pulse length of 82 4 s whereas TPA requires a longer pulse length of 10 s. With these optimized pulse lengths the 83 saturation-limited growth rate at 200 °C is ~3.0 Å/cycle. The saturation limited growth was further 84 investigated by depositing LiTP thin films on microstructured silicon substrates consisting of 85 trenches ~50 μm deep and 7.5 μm wide using the aforementioned pulse and purge lengths with 86 400 deposition cycles. As shown in Figure S1 in Supporting Information, essentially conformal 87 films are achieved even with parameters optimized for planar substrates. 88 Using the same pulse lengths, the growth rate was further studied in a temperature range of 200 89 – 280 °C using 400 ALD/MLD cycles. From Figure 1b, no region of constant GPC value, i.e. a 90 so-called ALD window, is observed; instead there is a rather monotonous decrease in GPC with 91 increasing deposition temperature which is actually a common feature for a majority of ALD/MLD 92 processes.9,13,14 The density of the films, as obtained from the X-ray reflectivity (XRR) data, 93 appears to remain essentially constant in the deposition temperature range of 200 – 240 °C (Figure 94 1b). At temperatures higher than this, the decrease in density might arise from thermal 95 decomposition of either of the precursors resulting in inclusion of carbon impurities. Within the 96 uniform density region, the resultant film density of ~1.4 g/cm3 is in a rather good agreement with 97 the ideal density of bulk LiTP (1.6 g/cm3) calculated from its crystal structure proposed in ref. 18. 98 As shown in Figure 1c, instead of the expected linear relationship between the film thickness and 99 the number of deposition cycles, the growth rate increases with increasing number of deposition 100 cycles. The growth rate nears constant after 200 deposition cycles. Simultaneously, an opposite 101 trend is seen in the film density which decreases until reaching a stable value of 1.4 g/cm3 after 102 200 deposition cycles. The film thicknesses up to 100 deposition cycles were crosschecked using 103 X-ray reflectivity (XRR) measurements (Table S1). As the roughness of the samples prevented the 104 use of XRR on the thicker samples, the thickness of the sample with 400 deposition cycles was 105 measured also from a scanning electron microscopy (SEM) cross-section image. 106 Atomic force microscopy (AFM) image taken after 70 deposition cycles (Figure 2a) shows 107 distinct granular shapes with voids in between. The average feature height of 20 nm matches well 108 with the film thickness obtained with ellipsometry. In the AFM image taken after 400 deposition 109 cycles (Figure 2b) the granular features have gained in size and coalesced forming a more 110 continuous film. In between the granules there appear to be deep voids and the overall roughness 111 of the sample is quite high. As such, the nonlinearity of could possibly be explained by the island 112 growth model of crystalline films.15,16 The initial nucleation and growth is not uniform; instead, 113 distinct islands are formed, which grow in size as the deposition proceeds. A constant-growth 114 regime is achieved only after the islands have coalesced and formed a continuous layer. As the 115 growth rate along different lattice planes varies, during the coalescence parts of the film can 116 become inaccessible forming voids within the films thus explaining the decrease in the apparent 117 density. 118 Grazing incidence X-ray diffraction (GIXRD) data do indeed confirm that the as-deposited films 119 are highly crystalline at all deposition temperatures which is unusual for ALD/MLD inorganic120 organic hybrid thin films.9 To our best knowledge, thus far only one report exists on crystalline 121 ALD/MLD thin films.17 The GIXRD patterns could be indexed in space group P21/c according to 122 the crystal structure proposed by Kaduk18 for LiTP with no additional reflections (Figure 2c). The 123 lattice parameters were determined to be, a = 8.36 Å, b = 5.12 Å, c = 8.46 Å, β = 93.08 °, in an 124 excellent agreement with those reported for bulk LiTP.10,18 The FWHM value of the 011 peak 125 (Figure S2a and S2b) decreases with increasing number of deposition cycles indicating an increase 126 in the crystallite size. Such an observation is in line with the proposed island-type growth mode. 127 As with the GPC and density values, the FWHM values appear to level off after 200 deposition 128 cycles. Additional X-ray diffraction measurements conducted in the Bragg-Brentano configuration 129 revealed only the most prominent 110 and 102 peaks indicating that the films are polycrystalline 130 without any evident orientation effect. 131 Fourier transform infrared spectroscopy (FTIR) studies were carried out to further elucidate the 132 structure of our LiTP thin films. In the FTIR spectrum (Figure 2d) the dominant absorption peaks 133 at 1392 and 1570 cm-1 arise from the symmetric and asymmetric stretching of the carboxylate 134 group, respectively. The values are in good agreement with those reported for bulk LiTP.10,19 The 135 peak separation, i.e. 178 cm-1, indicates towards a structure where Li is in a bridging position,20,21 136 in accordance with the crystal structure proposed for LiTP. Additionally, at ~523 cm-1 a 137 characteristic peak of Li-O bond can be observed. The lack of characteristic absorption bands due 138 to -OH stretching in the region of 2500 3000 cm-1 indicates that during the deposition process 139 TPA has fully reacted as intended with no inclusions of the unreacted precursor in the films.22 Also 140 no traces of Li2CO3 and LiOH were detected and no changes were seen in the FTIR spectrum even 141 after extended storage (6 months) of the films in ambient atmosphere (Figure S3). The surface 142 composition for a fresh and aged sample were probed with X-ray photoelectron spectroscopy 143 (XPS). As shown in Figure S4, the long storage time has not resulted in significant changes in the 144 spectrum. While the similarities of the chemical environments of carbonate and carboxylate group 145 make the analysis somewhat ambiguous, we conclude that Li2CO3 is not formed on the films 146 surface based on the following observations. For lithium carbonate, the CO3 signal should be 147 seen at 290 eV. Instead, a peak at 289.0 eV is detected, which matches with values reported for 148 the carboxylate group.23 Furthermore, the Li 1s peak is detected at 55.8 eV, while for Li2CO3 the 149 peak should be located at 55.2 55.4 eV. While the shift is relatively minor, it is in the opposite 150 direction as compared to the carboxylate/carbonate peak thus ruling out a systematic error in the 151 measurements. Lastly, the ratio of oxygen vs. carboxylate-type carbon is 1.9:1, which matches 152 quite well with the expected ratio of 2:1 153 The electrochemical performance of the LiTP films was evaluated using conventional LiPF6 154 based liquid electrolyte coin cells in order to establish that the electrochemical characteristics 155 match those previously reported10 for bulk LiTP electrodes. LiTP deposited on a stainless steel 156 substrate was employed as the working electrode and Li foil as the counter electrode. In order to 157 shield the electrodes from the liquid electrolyte and to mimic the situation in all-solid-state LIBs 158 regarding the electrode-solid electrolyte interface, samples with 600 ALD cycles of LiPON solid 159 electrolyte layer on the surface were manufactured using our recently developed ALD process for 160 LiPON.24 Assuming a growth rate of ~0.7 Å/cycle, this would add up to a layer of ~40 nm. Figures 161 3a and 3b display the cyclic voltammograms recorded for the bare and LiPON-coated LiTP films, 162 respectively. The as-deposited films are indeed electrochemically active but for the bare LiTP film 163 the initial cycle differs greatly from the following ones with a broad reduction peak appearing at 164 around 0.4 V. For the subsequent two cycles, a much sharper peak at 0.76 V is observed but the 165 peak current density decreases from 52 to 42 μA/cm2 between the second and third cycle. The 166 anodic sweep is identical for each cycle with three oxidation peaks appearing at 0.89, 1.00, and 167 1.06 V. For the LiTP-LiPON case, the initial wide reduction peak is not seen, but instead a sharp 168 reduction peak with a constant peak current density of 40 μA/cm2 appears at 0.79 V during each 169 cycle. Also, the anodic peak at 1.06 V is not existing and the one at 0.89 is significantly reduced. 170 Thus the data seem to implicate that a solid electrolyte interface (SEI) layer is formed on the bare 171 LiTP anode either directly due to immersion in the liquid electrolyte or during the initial cathodic 172 scan. Furthermore, during subsequent CV measurements at varying scan rates (data not shown 173 here), the initial broad reduction peak was always observed after the cell had been left to stabilize 174 in between the scans. The LiPON layer however was found to stabilize the LiTP-anode without 175 reducing the peak currents, i.e. not increasing the cell resistance, and the reduction peaks remained 176 sharp and consistent also with the higher scan rates. 177 Similar behavior was observed also in the initial charge/discharge conducted at a voltage range 178 of 0.4 3.0 V vs Li+/Li with a current rate of 0.5 μA/cm2 (Figure 3c). During the initial discharging 179 a distinct plateau at 0.8 V is observed for both the bare and the LiPON-coated LiTP electrodes 180 with the latter one demonstrating a slightly lower polarization consistent with the CV data. 181 However the amount of irreversible capacity is much higher with the bare LiTP film resulting in 182 an initial coulombic efficiency of 0.50 while the LiPON coating increases the efficiency to 0.64. 183 The larger irreversible capacity of the bare LiTP also implies formation of a SEI layer during the 184 cycling. 185 The electrochemical performance was further tested by cycling the electrodes at various current 186 rates at a voltage range of 0.4 3.0 V vs Li+/Li. Because the mass of the electrodes could not be 187 reliably assessed, here the C-rates are calculated assuming the reversible capacity obtained from 188 the initial charge/discharge conducted at a very low current rate of 0.5 μA/cm2 to be equal of the 189 full capacity. Assuming an electrode thickness of 170 nm and density of 1.4 g/cm3 as obtained 190 from ellipsometry and XRR measurements, respectively, for a comparable sample deposited on 191 silicon, the measured reversible capacity would correspond to a specific capacity of approximately 192 350 mAh/g. As we were unable to measure the thickness directly from the stainless steel substrate, 193 the value is only directional due to possible differences in the growth rate. While higher than the 194 theoretical capacity assuming two electron transfer reactions per molecule, it is not unreasonable 195 as recently Lee et al.25 revealed that LiTP may undergo further lithium insertions when cycled 196 below 0.7 V vs Li+/Li bringing the actual specific capacity up to 522 mAh/g when discharged to 197 0.0 V vs. Li/Li+. As shown in Figure 4a, both LiTP and LiTP-LiPON perform very well up to 6.4 198 C retaining approximately 69 and 66 % of the initial capacity, respectively at that current rate. At 199 higher current rates, the bare LiTP electrode fails completely while the LiPON-coated LiTP 200 electrode retains a very respectable performance delivering over 50 % of the initial capacity at ~20 201 C, i.e. charge/discharge in 3 min, and even at 64 C, i.e. charge/discharge in 56 s, the electrode still 202 retains 23 % of the full capacity. The high rate performance is not unexpected as according to 203 density functional theory calculations conducted by Zhang et al.11, the rate capability of LiTP 204 should inherently be excellent due to the low activation energy of lithium diffusion. Thus it is 205 apparent that the limited performance of bulk LiTP is indeed due to the poor electronic 206 conductivity, and this can be circumvented with the reduced dimensions of our thin-film electrodes 207 without relying on conductive additives. The reason for the sudden failure of the bare LiTP is 208 evident from the voltage curves recorded at increasing current rates shown in Figures 4b and 4c. 209 The bare LiTP electrode displays much higher overvoltages at the onset of the discharge plateau 210 as the current rate increases. At 12.8 C where the failure is observed, the initial overvoltage exceeds 211 the cut-off voltage ending the lithiation of the electrode prematurely. The LiPON protective layer 212 appears to stabilize the electrode resulting in a flat discharge plateau even at 12.8 C. Further 213 evidence for the resistive nature of the supposed SEI layer is revealed from electrochemical 214 impedance spectroscopy (EIS) measurements. In the Nyquist plots (Figure 4d) an additional 215 semicircle at mid-frequencies can be seen in the bare LiTP sample indicating increased cell 216 resistance. Since the measurements were conducted in a two electrode setup, further deciphering 217 of the EIS data was not attempted as it is not possible to separate the contributions of the LiTP 218 working electrode and the Li-metal counter electrode. 219 The cycle life of the LiPON-coated LiTP electrode was investigated at current rates of 3.2 and 220 6.4 C (Figure 4e). At 3.2 C the electrode performance is exceptionally stable: 97.4 % of the initial 221 capacity is retained even after 200 charge/discharge cycles. Accordingly, the coulombic efficiency 222 is very high, 99.7 % on the average. As the current rate is increased to 6.4 C, a steady capacity 223 fade can be observed. After 500 cycles the capacity remains at 81.8 % in respect to the 1st 6.4 C 224 charge/discharge cycle while the coulombic efficiency remains high at 99.8 % on the average. 225 From the voltage curves recorded at different stages of cycling, it could be confirmed that the 226 voltage profile remains stable throughout the cycling with no additional overpotentials associated 227 with the aging of the electrodes (Figure S5a and S5b). Combined with the high coulombic 228 efficiency, this seems to implicate that the capacity fade is mostly due to dissolution of the active 229 material, which has previously been noted to be a contributing cause to the capacity fade of LiTP,19 230 and not due to unwanted side reactions, e.g. formation of a resistive surface layer. Thus, while the 231 intended application for the LiTP thin films is in all-solid-state LIBs, the current results may prove 232 to be helpful also for bulk organic electrode materials highlighting the usefulness of a protective 233 surface coating on the stability of the materials. Moreover, although more sophisticated analysis 234 is needed, these results seem to implicate that also the LiTP-LiPON interface remains stable, which 235 is crucial for the performance of an all-solid-state LIB. 236 In conclusion, we developed a simple reproducible ALD/MLD process for Li-containing 237 inorganic-organic hybrid thin films. Our as-deposited Li terephthalate thin films were crystalline 238 exhibiting the same layered crystal structure previously reported for bulk LiTP. Moreover 239 demonstrated was that the films are electrochemically highly active showing excellent rate 240 capabilities without any conductive additives. Most remarkably, a thin LiPON electrolyte coating 241 applied using our recently reported ALD process was found to suppress the unwanted side 242 reactions with the LiPF6-based liquid electrolyte without increasing the cell resistance. Although 243 further studies using an all-solid-state setup are required, the present findings appear to indicate 244 that LiTP is a very promising candidate for a high-rate high-capacity anode material for thin-film 245 LIBs. We believe that our work is a step towards the all-solid-state organic-electrode-based LIB 246 technology, and has furthermore demonstrated the potential power of the ALD/MLD technique in 247 realizing this technology. 248 249 250 251 Experimental Section 252 The LiTP thin films were deposited using an F-120 flow-type hot-wall ALD reactor (ASM 253 Microchemistry Ltd.), from lithium 2,2,6,6-tetramethyl-3,5-heptanedionate (Li(thd)) and 254 terephthalic acid (TPA). Li(thd) was synthesized in-house by mixing 50-% EtOH solutions of 255 LiOH and Hthd. The resulting white precipitate was dried in vacuum and purified by sublimation. 256 TPA (>99.0 %) was acquired from Tokyo Chemical Industry Co., Ltd. Both precursors were kept 257 inside the reactor at temperatures of 175 °C for Li(thd) and 185 °C for TPA. Nitrogen (99.999%, 258 produced from air by Schmidlin UHPN 3000 nitrogen generator) was used both as purging and 259 carrier gas. The purging times were kept constant at 4 s for Li(thd) and 30 s for TPA. The films 260 were deposited on Si(100) substrates for structural characterization and on stainless steel disks 261 (15.5 mm diameter) for the electrochemical characterization. The reactor pressure was ~5 mbar. 262 For the deposition of the LiPON coatings, lithium hexamethyldisilazide and diethyl 263 phosphoramidate were used as the precursors.24 The pulse/purge lengths were 2 s/2 s for both 264 precursors with a total of 600 deposition cycles applied. The deposition temperature was 300 °C. 265 The thickness of the films was measured using a Semilab SE-2000 spectroscopic ellipsometer 266 equipped with a xenon lamp. The crystallinity of the films was studied by grazing incidence X-ray 267 diffraction using a PANanalytical X’Pert Pro diffractometer with a Cu K X-ray source while the 268 density of the films was deduced from X-ray reflectivity measurements using the same device. The 269 density of the films was calculated from the XRR patterns based on the dependency of the critical 270 angle, θc, on the mean electron density, ρe, of the material; namely ρe = (θcπ)/(λre), where λ is the 271 X-ray wavelength and re is the classical electron radius. By assuming the elemental composition 272 being that of pure LiTP, i.e. Li2C8H4O4, the mass density can be estimated from ρm = (ρe A)/(NAZ), 273 where A is the average molar mass, NA is the Avogadro constant and Z is the average atomic 274 number.26 We note that the assumption on chemical composition and ambiguity in determining the 275 exact critical angle result in uncertainties in the absolute value but still allows for extracting trends 276 within a sample series. For samples up to 100 deposition cycles, XRR was also used to cross check 277 the film thickness. The lattice constants were determined from the GIXRD pattern by Le Bail 278 profile fitting procedure using the FullProf Software Suite.27 The surface composition was 279 analyzed with X-ray photoelectron spectroscopy (Kratos Analytical AXIS Ultra) with 280 monochromatic Al-Kα radiation. The binding energy was calibrated based on C1s peak set to 285 281 eV. The FTIR measurements were conducted in a transmission mode on samples deposited on Si 282 with a Nicolet Magna 750 spectrometer in range of 400-4000 cm-1 using a resolution of 4 cm-1. 283 The SEM images were collected on a JEOL JSM-7500FA scanning electron microscope. AFM 284 measurements were conducted with a Veeco Dimension 5000 operated in tapping mode. For the 285 electrochemical measurements, the films deposited on stainless steel substrates were dried in 286 vacuum at 110 °C for 24 h and then used as the working electrode in a CR2016 coin cell. Lithium 287 metal was used as the counter electrode and the electrolyte was 1 M LiPF6 in 50:50 ethylene 288 carbonate/dimethyl carbonate solution. The cell assembly was conducted in an Ar filled glove box 289 with O2 level less than 1 ppm and H2O level less than 0.1 ppm. The cyclic voltammetry and 290 electrochemical impedance spectroscopy measurements were carried out using an Autolab 291 PGSTAT302N potentiostat/galvanostat. The EIS measurements were carried out in the frequency 292 range of 500 kHz 1 mHz using an amplitude of 10 mV. The galvanostatic measurements were 293 conducted using a Neware battery testing unit. 294 295 296 297 298 ASSOCIATED CONTENT 299 Supporting information. SEM image of LiTP thin film deposited on microstructured Si300 substrate, evolution of FWHM values of the 011 peak of LiTP from GIXRD as a function of film 301 thickness, FTIR and XPS analysis on the aging of LiTP thin films, voltage profiles recorded during 302 the cycling stability testing. This material is available free of charge via the Internet at 303 http://pubs.acs.org. 304 305 AUTHOR INFORMATION 306 Corresponding Author 307 *[email protected] 308 Notes 309 The authors declare no competing financial interest. 310 Author Contributions 311 The manuscript was written through contributions of all authors. All authors have given approval 312 to the final version of the manuscript. 313 314 315 ACKNOWLEDGEMENT 316 317 The present work has received funding from the European Research Council under the 318 European Union's Seventh Framework Programme (FP/2007-2013)/ERC Advanced Grant 319 Agreement (No. 339478). Dr. Erik Østreng is gratefully acknowledged for the ellipsometry 320 measurements and discussions related to the growth mode. Dr. Leena-Sisko Johansson is 321 thanked for carrying out the XPS measurements and Mr. Esko Ahvenniemi and Ms. Taina 322 Rauhala for their assistance in the AFM and SEM measurements, respectively. This work 323 made use of the Aalto Nanomicroscopy Center (Aalto NMC) facilities.324 REFERENCES3251) Long, J. W.; Dunn, B.; Rolison, D. R.; White, H. S. Chem. Rev. 2004, 104, 4463–4492.326(2) Oudenhoven, J. F. M.; Baggetto, L.; Notten, P. H. L. Adv. 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The deposition temperature was 200 °C and the pulse/purge369lengths of the other precursor were fixed to 4 s/4 s for Li(thd) and to 10 s/30 s for TPA. (b) GPC370(black squares) and film density (red circles) of LiTP as a function of deposition temperature. The371 pulse/purge lengths were 4 s/4 s and 10 s/30 s for Li(thd) and TPA, respectively. (c) Film thickness372(black squares) and density (red circles) of LiTP versus number of deposition cycles. The373deposition temperature was 200 °C and the pulse/purge lengths were 4 s/4 s and 10 s/30 s for374Li(thd) and TPA, respectively.375