Article pubs.acs.org/JPCC
Atomic Layer Deposition of Spinel Lithium Manganese Oxide by Film-Body-Controlled Lithium Incorporation for Thin-Film LithiumIon Batteries Ville Miikkulainen,*,† Amund Ruud,† Erik Østreng,† Ola Nilsen,† Mikko Laitinen,‡ Timo Sajavaara,‡ and Helmer Fjellvåg† †
Centre for Materials Science and Nanotechnology (SMN), Department of Chemistry, University of Oslo, P.O. Box 1126 Blindern, NO-0318 Oslo, Norway ‡ Department of Physics, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland S Supporting Information *
ABSTRACT: Lithium manganese oxide spinels are promising candidate materials for thin-film lithium-ion batteries owing to their high voltage, high specific capacity for storage of electrochemical energy, and minimal structural changes during battery operation. Atomic layer deposition (ALD) offers many benefits for preparing all-solidstate thin-film batteries, including excellent conformity and thickness control of the films. Yet, the number of available lithium-containing electrode materials obtained by ALD is limited. In this article, we demonstrate the ALD of lithium manganese oxide, LixMn2O4, from Mn(thd)3, Li(thd), and ozone. Films were polycrystalline in their asdeposited state and contained less than 0.5 at. % impurities. The chemical reactions between the lithium precursor and the film were found not to be purely surface-limited but to include a bulk component as well, contrary to what is usually found for ALD processes. In addition, we show a process for using Li(thd)/ozone and LiOtBu/water treatments to transform ALD-MnO2 and ALD-V2O5 into LixMn2O4 and LixV2O5, respectively. The formed LixMn2O4 films were characterized electrochemically and found to show high electrochemical capacities and high cycling stabilities.
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INTRODUCTION Lithium manganese oxides are promising cathode materials for lithium-ion batteries owing to their low cost and enhanced safety as compared to LiCoO2. The Li−Mn−O system offers a wide variety of compositions and crystalline phases, of which the most studied ones for battery application are the cubic spinels. The spinel framework provides a stable host structure for lithium intercalation with minimal volume change and isotropic lithium diffusion.1,2 In all-solid-state thin-film batteries, where the battery stack is rigid, structural stability and low volume change of the electrode materials upon charge−discharge cycling is of utmost importance. A phase or volume change would lead to delamination or pulverization of the electrode films. Lithium manganese oxide spinel electrodes have previously been deposited by radio-frequency magnetron sputtering,3 pulsed laser deposition,4 and reactive electron beam evaporation5 and by spin coating from solution.6 These methods limit the film stack to a planar geometry. The areal capacity of all-solid-state batteries can be significantly increased by applying a three-dimensional architecture rather than a plane. However, this also requires much thinner electrolytes than what are presently applied. At decreased film thicknesses, ionic and electronic diffusion becomes faster, and the battery performance is enhanced, especially at high currents. Several different approaches have © 2013 American Chemical Society
been suggested for such all-solid-state three-dimensional batteries.7−9 Atomic layer deposition (ALD) is a gas-phase thin-film deposition method based on saturative, self-limiting surface reactions. With ALD, highly conformal films can be deposited on complex three-dimensional substrates with accurate thickness control.10 Because of these appealing properties, ALD has been suggested as an enabling method for all-solid-state threedimensional batteries.11 There are currently few reports on applying atomic-layer-deposited electrode materials in batteries. Films of V2O5 and Co3O4 on planar substrates and TiO2 and SnO2 on three-dimensional substrates have been found to be electrochemically active.12−17 Recently, Donders et al. reported a plasma-ALD process for LiCoO2.18 To obtain fully functional battery film stacks, either of the electrode materials needs to be deposited in its lithium-containing reduced state or lithiated after the deposition process. However, the number of reports on ALD processes for lithium-containing transition-metal oxide materials is still limited.17−22 In this article, we report on the ALD growth of lithium manganese oxide through the combination of binary processes for Li2O/LiOH and MnO2 or MnO. The film growth Received: September 20, 2013 Revised: December 17, 2013 Published: December 17, 2013 1258
dx.doi.org/10.1021/jp409399y | J. Phys. Chem. C 2014, 118, 1258−1268
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analyses were performed with a Bio-Logic MPG-2 multichannel battery cycler.
characteristics for different precursor combinations are reported. A controllable ALD process for LixMn2O4 was found by combining Mn(thd)3, Li(thd), and ozone. The lithium content and crystalline phase of the films as a function of amount of lithium subcycles in the deposition sequence were studied. Furthermore, we studied film properties of ALD-MnO2 and V2O5 subjected to lithium ALD treatment. The findings indicate that chemical reactions between the lithium precursor and the film differ from what is usually found for ALD. Electrochemical analysis of the LixMn2O4 films was conducted to characterize their performance in a battery application.
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RESULTS AND DISCUSSION Depositions with Bis-Ethylcyclopentadienyl Manganese. First, saturation conditions were verified for MnO growth as reported in the literature.24 Films were uniform over the deposition chamber with pulsing parameters of a 0.5-s pulse and 1-s purge for Mn(EtCp)2 and a 0.25-s pulse and 1-s purge for water. Lithium was introduced into the MnO process by including either LiOtBu−H2O or LiN(SiMe3)2−H2O subcycles between the MnO cycles. In addition, a series of experiments with sodium was conducted for comparison and to simplify the alkali-metal analysis. The deposition temperature was 250 °C for experiments with LiOtBu and NaOtBu and 200 °C for those with LiN(SiMe3)2. Water-based lithium and sodium processes were selected rather than ozone-based processes to avoid oxidation of Mn during growth. The pulsing ratio of the alkalimetal and Mn subcycles was modulated, and the resulting film thickness was determined by ellipsometry. The variation in film growth per cycle as a function of alkali-metal pulsing ratio is presented in Figure 1. The growth per cycle was lower than
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EXPERIMENTAL SECTION Depositions employing bis(ethylcyclopentadienyl)manganese(II) [Mn(EtCp)2] were made with a Beneq TFS-500 ALD reactor. Mn(EtCp)2 from Strem (>98%) was used without further purification. The evaporation temperature for Mn(EtCp)2 was set at 90 °C. Deionized water, evaporated at room temperature, was used as the oxygen source for Mn(EtCp)2. Two lithium precursors, lithium tert-butoxide (LiOtBu) (97%, Aldrich) and bis(trimethylsilyl)amido lithium [LiN(SiMe3)2] (97%, Aldrich), both employing water as the oxygen source, were combined with the Mn(EtCp)2/H2O process. In addition, sodium tert-butoxide [Na(OtBu)] (Fluka, >97%) was used with water in an attempt at sodium incorporation into the MnO host. Depositions utilizing tris(2,2,6,6-tetramethylheptan-3,5dionato)manganese(III) [Mn(thd)3] were made with an ASM F-120 Sat reactor at a deposition temperature of 225 °C. Mn(thd)3 was made in-house by the method described in ref 23. Ozone, generated by an InUSA AC series ozone generator from oxygen (99.6% O2, AGA), was used as the oxygen source. The ozone concentration was ca. 200 g/Nm3. Three lithium precursors, namely, (2,2,6,6-tetramethylheptan-3,5-dionato)lithium [Li(thd); 98%, Aldrich], lithium tert-butoxide (LiOtBu), and bis(trimethylsilyl)amido lithium [LiN(SiMe3)2, LiHDMS] were used in combination with Mn(thd)3 and O3. Ozone was used as the oxidant for Li(thd), whereas water was used for LiOtBu and LiN(SiMe3)2. Film thicknesses of MnO-based films were analyzed by spectroscopic ellipsometry (J. A. Woollam Alpha SE) and fitted to a Cauchy function, whereas the thicknesses of MnO2-based films were characterized with X-ray reflectometry [Bruker AXS D8 instrument with a Cu tube, a Göbel mirror, a two-bounce Ge(220) monochromator, and a Lynx-Eye detector]. X-Ray diffraction analysis of the films was done with a Bruker AXS D8 powder diffractometer equipped with a Cu tube, a Ge(111) monochromator, and a LynxEye detector. X-ray photoelectron spectroscopy was performed with a Kratos Axis UltraDLD instrument. Compositional analysis was performed by timeof-flight elastic recoil detection analysis (TOF-ERDA) using an 8.5 MeV35 Cl-ion beam from 1.7 MV pelletron accelerator. For electrochemical analyses, LixMn2O4 depositions were conducted on 15.8-mm-diameter 316 stainless steel disks (MTI Corp., Richmond, CA). LixMn2O4-coated stainless steel disks were pressed with a Hohsen automatic crimping machine into CR2032 coin cells (MTI Corp., Richmond, CA) with Li metal (99.9% Li, Aldrich) as the anode and 1 mol/L LiClO4 in 1:1 ethyl carbonate/dimethyl carbonate solution as the liquid electrolyte. A Whatman glass microfiber sheet (grade GF/C) was used as the separator membrane. All handling and assembly was performed inside an MBraun Labmaster glovebox filled with argon atmosphere (99.999% Ar, AGA). Electrochemical
Figure 1. Film growth per cycle vs pulsing ratio (proportion of alkalimetal subcycles in a supercycle) using Mn(EtCp)2 as the Mn precursor. The line represents the theoretical growth per cycle with no contribution from alkali-metal subcycles to the film growth.
expected throughout the series with few exceptions. It was even lower than assuming no growth from the alkali-metal subcycles (red line in Figure 1). The films were also nonuniform so that the film thickness was always decreasing along the flow direction. Also, the film thicknesses varied within parallel depositions. The numbers reported in Figure 1 are from samples at the middle of the deposition chamber. X-ray fluorescence spectroscopy on films deposited with NaOtBu showed that only trace amounts of sodium were found in the films throughout the deposition chamber, except for films with highest sodium pulsing ratio. Those films were rich in sodium. Analogously, analysis of the film with 75% LiN(SiMe3)2−H2O subcycles by X-ray photoelectron spectroscopy confirmed hardly any presence of lithium (Supporting Information). These findings suggest that the alkali-metal precursors react with the atomic-layer-deposited MnO material through etching or poisoning reactions. However, further 1259
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studies are required to characterize the nature of the surface reactions. Depositions with Tris(2,2,6,6-tetramethylheptan-3,5dionato) Manganese. According to the first set of experiments, depositions with Mn(thd)3 as the manganese precursor were performed using ozone as the oxidizer at a deposition temperature of 225 °C.25 This should result in tetravalent manganese oxide for depositions up to ca. 240 °C.25 First, we attempted to combine the LiN(SiMe3)2−water process with MnO2 to produce LixMnyOz films. The films produced using LiN(SiMe3)2 and water were nonuniform and amorphous by X-ray diffraction (XRD). The pulsing sequence was 200 × {19 × [Mn(thd)3 (1.5-s pulse/1-s purge) + O3 (5/ 5)] + [LiN(SiMe3)2 (1/3) + H2O (0.5/5)]}. A possible reason for the nonuniformity of the film might be that the Li precursor decomposes during deposition and forms silicates. Further studies on applying LiN(SiMe3)2 were postponed at this point. It should be noted that, for producing ALD-type Li3PO4, Li2SiO3, Li2CO3, and Li3N, LiN(SiMe3)2 has been reported to show self-limiting growth and give pure films.26−28 By combining the MnO2 ALD process with Li(thd) and ozone, films with high uniformity were produced. A series of experiments was carried out by varying the pulsing ratio of the lithium subcycle to the manganese subcycle. The pulsing sequence was A × {B × [Mn(thd)3 (1.5-s pulse/1-s purge) + O3 (5/5)] + [Li(thd) (30/3) + O3 (5/5)]}, where A and B are integers. A relatively long Li(thd) pulse time of 30 s was selected to ensure uniformity of the films. Even with such a long pulse time, some of the films showed signs of nonuniformity on the leading edge of the film. In addition, pure MnO2 films were deposited with the same pulse and purge parameters for reference and for further studies. Film thickness and density was characterized by X-ray reflectivity (XRR), crystallinity by XRD, and composition by TOF-ERDA. Results for film growth per cycle and relative lithium composition are presented in Figure 2. XRD diffractograms for films deposited using 4000 ALD cycles are presented in Figure 3. Full compositional data on the LixMnyOz films are reported in Table 1. Despite the low deposition temperature of 225 °C, all of the films had less than 0.5 at. % impurities in total. To verify the self-limiting nature of the surface reactions during the lithium precursor pulse, the pulse time of Li(thd) was varied, and the film growth per cycle and relative lithium concentration [Li/(Li + Mn)] was studied as a function of pulse length. The results illustrated in Figure 4 show that the relative lithium concentration increased as a function of pulse length, saturating at around 0.55, close to the maximum value found in the pulsing ratio study (Figure 2). Growth per cycle decreased as a function of pulse length, but was close to 0.22 Å, which was the growth per cycle observed for most of the lithium manganese oxide films throughout the present study, regardless of film thickness and Li/(Li + Mn) pulsing ratio. When the pulsing ratio of lithium subcycles was increased above 10%, the films became increasingly more nonuniform, with the thickest films at the precursor inlet. However, uniformity improved and the color of the film was restored if the purge time following the Li(thd) pulse was increased from 3 to 10 s and further to 30 s. Film uniformity, however, did not reach the uniformity level of the films with a Li pulsing ratio of
