LITHIUM ALKOXIDES

Unlike the alkoxides of the alkaline metals, Li alkoxides starting from ethoxide are sublimable and some of them were applied as MOCVD precursors (Table ).

Li ethoxide (Li etanolate) LiOEt

Lithium ethoxide LiOEt can be synthesized by the reaction of lithium with ethanol Li + EtOH (exothermic reaction).

Lithium ethoxide LiOEt has molar mass M = 52.00, is powder subliming at 100°C/vacuo, 150°C /10-2 torr, decomposes at 325°C. LiOEt is insoluble in hydrocarbons, soluble in EtOH (125g/l), α = 6, 4 (MS), ΔHform = -108.6

Lithium ethoxide was characterized by IR, 1H NMR, MS, DTA, thermochemically and by conductivity measurements.

LiOEt for LiTaO3 by MOCVD

Lithium ethoxide LiOEt was applied for the growth of LiTaO3 on sapphire [[i], [ii]]

[i] R. Xu, JOM, 1997, 49(10), JOM-e (http://www.tms.org/pubs/journals/JOM/9710/Xu/Xu-9710.html#Ref5)

[ii] A.A. Wernberg, G.H. Braunstein, H.J. Gysling, Appl. Phys. Lett., 63 (19), 1993, 2649-2651.

Lithium isopropoxide (lithium sec-propanolate) LiOiPr

Lithium isopropoxide (lithium sec-propanolate) LiOiPr has molar mass M = 66.03.

It is solid, sublimable at 170/0.1 Torr, very soluble in pentance (3,78 mol/l), insoluble in alcohols, α = 11.5 (cyclohexane), 4 (THF), 6, 4 (MS). LiOiPr was characterized by IR, 1H NMR, MS, cryoscopically, ebullioscopically.

Lithium isopropoxide LiOiPr due to its volatility is potentially applicable as precursor for MOCVD of lithium-containing layers.

Lithium isobutoxide (lithium sec-butanolate) LiOsecBu

Lithium isobutoxide (lithium sec-butanolate) LiOsecBu has molar mass M = 80.05. (obtained crystallised with some molecules of isobutanol Li(OiBu)·n iBuOH). It is solid, sublimable at 100°C /0.01 Torr, soluble in alcohols, in pentance (45.5%), α = 3-5. LiOiPr was characterized by IR and 1H NMR.

Lithium isobutoxide LiOiBu is potentially applicable as MOCVD precursor for the deposition of lithium-containing layers, due to its volatility.

Li tert-butoxide (Li tert-butanolate) LiOtBu

Fig. Vapor pressure of [Li(OtBu)]6 vs. precursor vessel temperature (vapor pressure equation log10P(Torr)=(9.6±1.1)-(4250±350)/T(K)

Lithium tertiary butoxide (Li tert-butanolate) LiOtBu has molar mass M = 80.05 (for monomer), it is solid having density d= 0.89 g/cm3, melting point 170-205°C, sublimes at 110°C/0.1 torr (or 110°C/0.01 Torr, or 90-110°C/10mbar, according to other reports). LiOtBu has vapor pressure 0.0016 Torr/ 70°C [4], is soluble in diethyl ether Et2O, hydrocarbons (f.e. heptane 80g/l), THF (196g/l, or 22%), tBuOH (12g/l), in boiling ROH 8%- Molecular association is α = 9 (Benzene), 6 (MS, Toluene, Et2O, CCl4), 5-4 (C6H12, Py, R2O, ROH); magnetic susceptibility μ = 0.74.

Lithium tert-butoxide can be synthesized by alcoholysis: LiOEt + tBuOH (excess) under reflux.

LiOtBu was characterized by IR, 1H, 13C, 7Li, 6Li NMR spectroscopy, mass-spectrometry, cryoscopycally, ebullioscopically, XRD (crystal parameters determined), it is hexamer (in vapor and benzene solution) [i]

Vapor pressure of [Li(OtBu)]6 was determined as a function of precursor vessel temperature (in the range 25-130°C) and fitted (see Fig. ) to the following vapor pressure equation:

log10P(Torr)=(9.6±1.1)-(4250±350)/T(K) [ii]

[i] D.C. Bradley, Phil Trans. R. Soc., Lond., 1990, A 330 167-171

[ii] D. Saulys, V. Joshkin, M.Khoudiakov, T.F.Kuech, A.B.Ellis, S.R.Oktyabrsky, L. McCaughan, J. Crystal Growth, 2000, Vol. 217, Iss. 3, p.287-301, « An examination of the surface decomposition chemistry of lithium niobate precursors under high vacuum conditions », https://doi.org/10.1016/S0022-0248(00)00412-7https://www.sciencedirect.com/science/article/pii/S0022024800004127

LiOtBu for LiNbO3 by low-pressure CVD

Solid lithium tert-butoxide, LiOtBu, was examined as Li precursor in the CVD of lithium niobate LiNbO3 thin films by low pressure MOCVD on SiO2/Si substrates at 450–630°C growth temperature (using Nb (OEt)5 as niobium source and argon gas as carrier). The deposited layers were found to be crystalline LiNbO3 by XRD measurements. [i].

[i] A. Tanaka, K. Miyashita, T. Tashiro, M. Masakazu, T. Sukegawa, J. Cryst. Growth, 1995, 148, 324-326.

LiOtBu for LiNbOx by ALD

Lithium tert-butoxide LiOtBu (combined with Nb(OEt)5) was applied as Li precursor for the deposition of lithium niobium oxide LiNbOx thin films by ALD, at temperature of 235°C (with well-controlled film thickness and composition). Incorporation of higher Li content was achieved by increasing the Li-to-Nb subcycle ratio. The existence of Nb as Nb5+ in a distorted octahedral structure on the Nb L-edge was demonstrated by X-ray absorption near edge structure studies of the amorphous LiNbOx thin films; severe distortions in the octahedrons in Nb2O5 thin films existed, but were gradually alleviated upon the introduction of Li atoms into the thin films. The highest value of ionic conductivity of 6.39 × 10–8 S/cm at 303 K with an activation energy of 0.62 eV was achieved for the as-deposited LiNbOx thin films.[i]

[i] B. Wang, Y. Zh. Orcid, M. N. Banis, Q. Sun, K. R. Adair, R. Li, T.-K. Sham, X. S. Orcid, ACS Appl. Mater. Interfaces, 2018, 10, 2, 1654-1661, « Atomic Layer Deposition of Lithium Niobium Oxides as Potential Solid-State Electrolytes for Lithium-Ion Batteries », https://doi.org/10.1021/acsami.7b13467, https://pubs.acs.org/doi/abs/10.1021/acsami.7b13467

LiOtBu (+H2S) for ALD of Li2S

Lithium tert-butoxide (LTB) LiOtBu (98%, Strem Chemicals) combined with H2S (1% in Ar, Matheson Tri-gas), was applied as Li precursor for the deposition of Li2S thin films by ALD in a custom viscous-flow, hot-walled reactor. For the Li2S ALD, alternating 5 s exposures of LiOtBu and H2S (1% H2S in Ar), with 5 s Ar purging periods were used; 252 sccm ultrahigh purity Ar (99.999%) was used as carrier, reactor pressure was 1.2 Torr. The obtained Li2S layers proved to be extremely air-senisitive.[i]

[i] Y. Cao, X. Meng, J.W. Elam, Chem. Electrochem. Comm., 2016, Vol.3, Iss.6, p.858-863, « Atomic Layer Deposition of LixAlyS Solid‐State Electrolytes for Stabilizing Lithium‐Metal Anodes », doi.org/10.1002/celc.201600139, https://www.researchgate.net/profile/Xiangbo_henry_Meng/publication/299896180_Atomic_Layer_Deposited_LixAlyS_Solid_State_Electrolytes_for_Stabilizing_Lithium_Metal_Anodes/links/59e8f324aca272bc425a9024/Atomic-Layer-Deposited-LixAlyS-Solid-State-Electrolytes-for-Stabilizing-Lithium-Metal-Anodes.pdf

LiOtBu (+H2S) for ALD of LixAlyS

The alternating exposures between LiOtBu/ H2S (for Li2S) and Al(NMe2)3 (TDMA-Al)/ H2S (for Al2S3) were applied as LixAlyS ALD growth cycles; the growth process was investigated by in situ QCM measurements (the mass changes in ng/cm2 units were multiplied by Li2S and Al2S3 bulk densities (1.66 and 2.32 g/cm3). 50 nm LixAlyS ALD films were deposited on Li and Cu foils at 150°C. The obtained LixAlyS layers are extremely air-reactive.

Layer thicknesses of LixAlyS deposited on Si(100) substrates were determined by spectroscopic ellipsometry (J. A. Woollam Co. alpha-SE) in an Ar-purged glove bag. 50 nm LixAlyS ALD films were grown on SiO2/Si patterned substrates to prepare samples for ionic conductivity measurements with micrometer-scale interdigitated Pt electrodes. The charge/discharge testing was performed by Arbin 2043 electrochemical tester.[i]

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