Formation of highly conformal spinel lithium titanate thin films based on a novel three-step atomic layer deposition process

Abstract

Lithium titanate (Li4Ti5O12) (LTO) has several promising properties with regard to energy storage. The most important is its low volume expansion during lithium (de-) intercalation enabling the material for complex three-dimensional battery anode designs. To employ this property at a small scale, e.g., for micro batteries (<100 nm active layer thickness), a highly conformal deposition process like atomic layer deposition (ALD) is needed. However, the ALD of lithium containing layers is quite ambitious. Particularly, thermally activated deposition of lithium containing layers with water as a coreactant is challenging due to the high reactivity and hygroscopic nature of many lithium compounds, e.g., lithium hydroxide. That is why a novel ALD process regime has been developed, which allows the deposition of highly conformal and single phase LTO layers with excellent step coverage and composition. The process uses two metalorganic precursors: one acting as lithium and another as a titanium source. In contrast to usual ALD processes, these two precursors are subsequently applied. The reactive pulse with water is applied after the two metal precursor pulses. In this work, this novel ALD process sequence has been introduced and successfully demonstrated on 200 mm wafers using standard industrial ALD equipment. The layers are transformed to single phase Li4Ti5O12 by rapid thermal processing as proven by crystal phase analysis. Elemental composition has been analyzed by time of flight secondary ion mass spectrometry and x-ray photoelectron spectroscopy (XPS). Results show that the amounts of contaminants like carbon and chlorine are below the detection limits of XPS. Also, a uniform element distribution and stoichiometry in good agreement with theoretical expectations for lithium titanate could be shown.

Atomic Layer Deposition of Textured Li4Ti5O12: A High‐Power and Long‐Cycle Life Anode for Lithium‐Ion Thin‐Film Batteries

“…A three-step ALD process was applied to suppress the so-called "water reservoir effect" occurring due to the hygroscopic nature of the material. [32] The novel concept is the introduction of a co-precursor step B (here TDMAT) between the LiHMDS (A) and the H 2 O (C) steps. Classic supercycle ALD processes consist of alternating subcycles with the basic order AC and BC.…”

“…The optimized ALD process with a 5 s LiHMDS pulse for the fabrication of polycrystalline spinel Li 4 Ti 5 O 12 is in good agreement with an earlier study following the same ALD three-step procedure. [32] There LTB was used as a Li precursor instead of LiHMDS. Films deposited with both precursors reveal a strong (111)-textured Li 4 Ti 5 O 12 film on SiO 2 after annealing.…”

“…Furthermore, ALD has the highest potential of all compared methods for upscaling to high volume production of 3D TFB via utilization of established 300 mm Si technology or roll-to-roll spatial ALD. [7,16,17,32,56]…”

“…[30,31] Recently, Bönhardt et al demonstrated a novel three-step ALD process with LTB and tetrakis (dimethylamino) titanium (TDMAT), achieving spinel Li 4 Ti 5 O 12 with ideal stoichiometry and highly conformal coating. [32] However, Li 4 Ti 5 O 12 films by ALD have not yet been electrochemically investigated.…”

Atomic Layer Deposition of Ni-Co-O Thin-Film Electrodes for Solid-State LIBs and the Influence of Chemical Composition on Overcapacity

“…Numerous studies have addressed the preparation of individual components of thin-film SSLIBs by ALD, such as cathodes [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ], anodes [ 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 ] and solid-state electrolytes [ 50 , 51 , 52 , 53 ]. Using several precursors in the ALD chamber allows the fabrication of all the SSLIB components using ALD equipment [ 12 ].…”