Lithium Phosphate Thin Films Grown by Atomic Layer Deposition

Abstract

Lithium phosphate, Li 3 PO 4 , has been considered a potential electrolyte material for lithium ion batteries and CO 2 sensors in particular if the films can be made dense and of high quality already at low thickness. In this work, Li 3 PO 4 thin films were deposited by atomic layer deposition (ALD) between 225 and 350 • C using trimethyl phosphate and either of the two lithium sources, namely lithium hexamethyldisilazide or lithium tert-butoxide. The deposited films showed slightly crystalline Li 3 PO 4 structure in X-ray diffraction and the elastic recoil detection analysis confirmed this stoichiometry with some carbon and hydrogen impurities. The crystallinity and thermal stability of the films at elevated temperatures in N 2 were also examined. The long term stability of the deposited Li 3 PO 4 films under ambient air may be an issue for the applicability of these processes. Lithium phosphate, Li 3 PO 4 , can be applied as an electrolyte in solid state lithium ion batteries 1-3 and CO 2 gas sensors. 4-6 Also optical humidity sensors based on Li 3 PO 4 have been presented recently. 7 Thin Li 3 PO 4 film has also been used as an interfacial layer between LiCoO 2 electrode and solid polymer electrolyte in solid state lithium ion battery. 8 Crystallization of Li 3 PO 4 greatly decreases the ionic conductivity: amorphous Li 2.7 PO 3.9 has an ionic conductivity of 6.6 × 10 −8 S/cm while the polycrystalline γ-Li 3 PO 4 has been extrapolated to be highly resistive with as low conductivity as 4.2 × 10 −18 S/cm at 25 • C. 9 Even if the ionic conductivity is only moderate, amorphous Li 3 PO 4 is still a potential electrolyte material provided it can be made dense already at low thickness.In lithium ion battery applications the 3D structuring of the materials would be a way to effectively increase the energy storage capacity by increasing the specific surface area. 10, 11 Atomic layer deposition (ALD) 12-15 may prove to be useful for depositing thin films on demanding 3D structures as films grown by ALD inherently possess good conformality and uniformity because of the alternating, saturative precursor doses and self-limiting surface reactions. The resulting accurate thickness controllability and repeatability of the films is a major benefit of the layer by layer approach used in ALD. Some of the electrolyte material candidates for 3D batteries include for example Li 3 PO 4 and nitrogen mixed lithium phosphate known as LiPON. 10,11 ALD processes for phosphate films are very few in number, although aluminum phosphate thin films have been deposited already in the 90's. 16,17 Besides aluminum phosphate, only ALD of calcium phosphate has been published. 18 Also lithium containing ALD processes were absent until recent reports on ALD of lithium hydroxide, lithium carbonate, lithium aluminate, lithium silicate, lithium lanthanate and lithium lanthanum titanates. [19][20][21][22][23] In addition, ALD of LiFePO 4 and Ca:LaPO 4 materials have been recently presented. 24,25 Here we report ALD growth and characterization of lithium...

Atomic layer deposition for nanostructured Li-ion batteries

“…Nevertheless, an ALD process for Li 3 PO 4 has been recently presented. 49 LiLaTiO x (LLT) also has been suggested as an electrolyte material, but this material has been reported to have high electrical conductivity, causing short-circuit issues. 50 Because of the small thickness envisioned for ALD deposited films an extremely low electrical conductivity is required.…”

Atomic Layer Deposition of the Solid Electrolyte LiPON

“…ALD has emerged as the premier deposition process for fabrication of uniform, thin, conformal films on high aspect ratio scaffolds, 13−16 and ALD has previously been used to fabricate the solid electrolytes (Li,La) x Ti y O z , 17 Li 3 PO 4 , 18 Li x Al 2 O 3 , 19,20 and Li x Si y Al 2 O 3 . 21 Utilizing a unique integrated high-vacuum deposition, surface characterization, and battery assembly system, 22 we have developed a quaternary ALD process for the solid electrolyte LiPON.…”

Metal Fluorides as Lithium-Ion Battery Materials: An Atomic Layer Deposition Perspective

“…Li3PO4 can be deposited using either LiO t Bu or LiHMDS as the lithium source and TMPO (trimethyl phosphate, Figure 9) as the phosphate precursor [114,115]. The LiO t Bu + TMPO process showed a constant growth rate of approximately 0.7 Å/cycle between 225 and 275 °C [114].…”

“…Wang et al [115] and Létiche et al [116] have studied the Li3PO4 process using LiO t Bu and TMPO in an effort to measure the lithium-ion conductivity of these films. Wang et al reported an increasing growth rate as a function of the deposition temperature at 250-325 °C, which might be a result of the somewhat unsaturative behaviour of the process [114]. Electrochemical impedance spectroscopy showed that the films had rather good conductivities when deposited at 300 °C: 3.3 × 10 −8 S/cm was extrapolated for a film with a composition of Li2.8POz (as determined by XPS) [115].…”

“…In the PEALD process, LiO t Bu was used as the lithium source combined with a pulsing sequence of water, TMPO and nitrogen plasma [109]. Deposition of Li 2 O/LiOH before exposure to TMPO resulted in less carbon impurities as compared to the process used by Hämäläinen et al for Li 3 PO 4 [114]. By using nitrogen plasma after the TMPO pulse, nitrogen could be incorporated into the films, causing the amorphization of the crystalline Li 3 PO 4 .…”