Atomic Layer Deposition for Thin‐Film Lithium‐Ion Batteries
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autonomous sensors, wearable devices, and medical implants will grow up to 100 billion USD. [1,2] The shrinking device sizes of portable electronics require microsized on-chip energy storage solutions with high-energy and high-power capability. These demands are beyond the abilities of liquid lithium (Li)-ion batteries due to limited miniaturization potential and inherent risks of the liquid electrolyte such as flammability and leakage.TFBs with solid-state electrolytes and binder-free electrodes are a promising alternative. In general, interfaces are more pronounced in thin-film devices, which remains challenging in all-solid-state batteries. [3] TFBs provide high-power density, long cycle life, low self-discharge, high-temperature and chemical stability, on-chip integration, and miniaturization. [4] These properties pave the way for future replacement of standard on-chip supercapacitors. [5] However, the small form factor and short Li diffusion length of TFBs come at the cost of low energy density. So-called 3D TFBs can partially compensate this by increasing energy density per footprint area. Thereby the battery layer stack is coated over a microstructured substrate with an enhanced surface area. [6] A first functional full cell 3D TFB was recently demonstrated by Pearse et al. [7] Moitzheim et al. reported an extensive overview of the current state, challenges, and outlooks of 3D TFBs. [8] ALD is the ideal technique enabling the required conformal, pinhole-free deposition and stoichiometric control of nanometer-thin films on highly structured surfaces. The vapor-phase technique based on sequential, self-limiting surface reactions is well understood and an industrial standard in integrated circuit manufacturing. [9] However, the deposition of Li metal is not possible and Li compounds remain challenging. [10,11] Functional ALD films of cathode materials such as LiMn 2 O 4 [12] or LiCoO 2 [13] and solid-state electrolytes such as LiPON [14] or Li x Al y Si z O [15] were demonstrated. However, Li-containing anode materials directly fabricated by ALD were not yet electrochemically evaluated. Only closely related ALD TiO 2 anodes were successfully investigated and optimized. [16,17] Spinel Li 4 Ti 5 O 12 (LTO) is a well-suited anode for 3D TFBs. The material undergoes a phase transition to the rocksalt-like structure during lithiation by rearranging the Li atoms with minimal volume change below 0.1%. [18] This so-called "zero-strain"The "zero-strain" Li 4 Ti 5 O 12 is an attractive anode material for 3D solid-state thin-film batteries (TFB) to power upcoming autonomous sensor systems. Herein, Li 4 Ti 5 O 12 thin films fabricated by atomic layer deposition (ALD) are electrochemically evaluated for the first time. The developed ALD process with a growth per cycle of 0.6 Å cycle −1 at 300 °C enables high-quality and dense spinel films with superior adhesion after annealing. The short lithiumion diffusion pathways of the nanostructured 30 nm films result in excellent electrochemical properties. Planar films reveal 9...
autonomous sensors, wearable devices, and medical implants will grow up to 100 billion USD. [1,2] The shrinking device sizes of portable electronics require microsized on-chip energy storage solutions with high-energy and high-power capability. These demands are beyond the abilities of liquid lithium (Li)-ion batteries due to limited miniaturization potential and inherent risks of the liquid electrolyte such as flammability and leakage.TFBs with solid-state electrolytes and binder-free electrodes are a promising alternative. In general, interfaces are more pronounced in thin-film devices, which remains challenging in all-solid-state batteries. [3] TFBs provide high-power density, long cycle life, low self-discharge, high-temperature and chemical stability, on-chip integration, and miniaturization. [4] These properties pave the way for future replacement of standard on-chip supercapacitors. [5] However, the small form factor and short Li diffusion length of TFBs come at the cost of low energy density. So-called 3D TFBs can partially compensate this by increasing energy density per footprint area. Thereby the battery layer stack is coated over a microstructured substrate with an enhanced surface area. [6] A first functional full cell 3D TFB was recently demonstrated by Pearse et al. [7] Moitzheim et al. reported an extensive overview of the current state, challenges, and outlooks of 3D TFBs. [8] ALD is the ideal technique enabling the required conformal, pinhole-free deposition and stoichiometric control of nanometer-thin films on highly structured surfaces. The vapor-phase technique based on sequential, self-limiting surface reactions is well understood and an industrial standard in integrated circuit manufacturing. [9] However, the deposition of Li metal is not possible and Li compounds remain challenging. [10,11] Functional ALD films of cathode materials such as LiMn 2 O 4 [12] or LiCoO 2 [13] and solid-state electrolytes such as LiPON [14] or Li x Al y Si z O [15] were demonstrated. However, Li-containing anode materials directly fabricated by ALD were not yet electrochemically evaluated. Only closely related ALD TiO 2 anodes were successfully investigated and optimized. [16,17] Spinel Li 4 Ti 5 O 12 (LTO) is a well-suited anode for 3D TFBs. The material undergoes a phase transition to the rocksalt-like structure during lithiation by rearranging the Li atoms with minimal volume change below 0.1%. [18] This so-called "zero-strain"The "zero-strain" Li 4 Ti 5 O 12 is an attractive anode material for 3D solid-state thin-film batteries (TFB) to power upcoming autonomous sensor systems. Herein, Li 4 Ti 5 O 12 thin films fabricated by atomic layer deposition (ALD) are electrochemically evaluated for the first time. The developed ALD process with a growth per cycle of 0.6 Å cycle −1 at 300 °C enables high-quality and dense spinel films with superior adhesion after annealing. The short lithiumion diffusion pathways of the nanostructured 30 nm films result in excellent electrochemical properties. Planar films reveal 9...
Interface‐Engineered Atomic Layer Deposition of 3D Li4Ti5O12 for High‐Capacity Lithium‐Ion 3D Thin‐Film Batteries
Upcoming energy‐autonomous mm‐scale Internet‐of‐things devices require high‐energy and high‐power microbatteries. On‐chip 3D thin‐film batteries (TFBs) are the most promising option but lack high‐rate anode materials. Here, Li4Ti5O12 thin films fabricated by atomic layer deposition (ALD) are electrochemically evaluated on 3D substrates for the first time. The 3D Li4Ti5O12 reveals an excellent footprint capacity of 20.23 µAh cm−2 at 1 C. The outstanding high‐rate capability is demonstrated with 7.75 µAh cm−2 at 5 mA cm−2 (250 C) while preserving a remarkable capacity retention of 97.4% after 500 cycles. Planar films with various thicknesses exhibit electrochemical nanoscale effects and are tuned to maximize performance. The developed ALD process enables conformal high‐quality spinel (111)‐textured Li4Ti5O12 films on Si substrates with an area enhancement of 9. Interface engineering by employing ultrathin AlOx on the current collector facilitates a required crystallization time reduction which ensures high film and interface quality and prospective on‐chip integration. This work demonstrates that 3D Li4Ti5O12 by ALD can be an attractive solution for the microelectronics‐compatible fabrication of scalable high‐energy and high‐power Li‐ion 3D TFBs.
Atomic layer deposition (ALD) of lithium-containing films is of interest for the development of next-generation energy storage devices. Lithium hexamethyl disilazide (LiHMDS) is an established precursor to grow this type of films. The LiHMDS molecule can either be used as a single-source precursor molecule for lithium, or as a dual-source precursor molecule for lithium and silicon. Single-source behaviour of LiHMDS is observed in the deposition process with trimethylphosphate (TMP) resulting in the deposition of crystalline lithium phosphate (Li 3 PO 4 ). In contrast, LiHMDS exhibits dual-source behavior when combined with O 2 plasma, resulting in a lithium silicate. Both processes were characterized with in situ ellipsometry, in situ time-resolved full-range mass spectrometry, x-ray photoelectron spectroscopy (XPS) and elastic recoil detection analysis (ERDA). When we combined both reactants into a three-step LiHMDS-TMP-O 2 * or LiHMDS-O 2 *-TMP process, the dual-source nature of LiH-MDS emerged again. By carefully combining our measurements, it is shown that film growth with LiHMDS (in combination with TMP and O 2 plasma) is driven by dipole-driven selfsaturated surface interactions combined with dissociative chemisorption. We show that when hydroxyl groups are present on the surface, silicon will be incorporated in the films. These insights benefit the general understanding of the behaviour of the LiHMDS and TMP precursors, and may facilitate their effective use in ternary or quaternary processes.
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