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 of Textured Li4 Ti5 O12 : A High-Power and Long-Cycle Life Anode for Lithium-Ion Thin-Film Batteries.

The "zero-strain" Li4 Ti5 O12 is an attractive anode material for 3D solid-state thin-film batteries (TFB) to power upcoming autonomous sensor systems. Herein, Li4 Ti5 O12 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 lithium-ion diffusion pathways of the nanostructured 30 nm films result in excellent electrochemical properties. Planar films reveal 98% of the theoretical capacity with 588 mAh cm-3 at 1 C. Substrate-dependent film texture is identified as a key tuning parameter for exceptional C-rate performance. The highly parallel grains of a strong out-of-plane (111)-texture allow capacities of 278 mAh cm-3 at extreme rates of 200 C. Outstanding cycle performance is demonstrated, resulting in 97.9% capacity retention of the initial 366 mAh cm-3 after 1000 cycles at 100 C. Compared to other deposition techniques, the superior performance of ALD Li4 Ti5 O12 is a breakthrough towards scalable high-power 3D TFBs.

Surface-Dependent Performance of Ultrathin TiN Films as an Electrically Conducting Li Diffusion Barrier for Li-Ion-Based Devices.

An in-depth understanding of lithium (Li) diffusion barriers is a crucial factor for enabling Li-ion-based devices such as three-dimensional (3D) thin-film batteries and synaptic redox transistors integrated on silicon substrates. Diffusion of Li ions into silicon can damage the surrounding components, detach the device itself, lead to battery capacity loss, and cause an uncontrolled change of the transistor channel conductance. In this study, we analyze for the first time ultrathin 10 nm titanium nitride (TiN) films as a bifunctional Li-ion diffusion barrier and current collector. Thermal atomic layer deposition (ALD) and pulsed chemical vapor deposition (pCVD) are employed for manufacturing ultrathin films. The 10 nm ALD films demonstrate excellent blocking capability with an insertion of only 0.03 Li per TiN formula unit exceeding 200 galvanostatic cycles at 3 µA/cm2 between 0.05 and 3 V versus Li/Li+. An ultralow electrical resistivity of 115 µΩ cm is obtained. In contrast, a partial barrier breakdown is observed for 10 nm pCVD films. High surface quality with low contamination is identified as a key factor for the excellent performance of ALD TiN. Conformal deposition of 10 nm ALD TiN in 3D structures with high aspect ratios of up to 20:1 is demonstrated. The measured capacities of the surface area-enhanced samples are in good agreement with the expected values. High-temperature blocking capability is proven for a typical electrode crystallization step. Ultrathin ALD TiN is an ideal candidate for an electrically conducting Li-ion diffusion barrier for Si-integrated devices.

Bending impact on the performance of a flexible Li4Ti5O12-based all-solid-state thin-film battery.

The growing demand of flexible electronic devices is increasing the requirements of their power sources. The effect of bending in thin-film batteries is still not well understood. Here, we successfully developed a high active area flexible all-solid-state battery as a model system that consists of thin-film layers of Li4Ti5O12, LiPON, and Lithium deposited on a novel flexible ceramic substrate. A systematic study on the bending state and performance of the battery is presented. The battery withstands bending radii of at least 14 mm achieving 70% of the theoretical capacity. Here, we reveal that convex bending has a positive effect on battery capacity showing an average increase of 5.5%, whereas concave bending decreases the capacity by 4% in contrast with recent studies. We show that the change in capacity upon bending may well be associated to the Li-ion diffusion kinetic change through the electrode when different external forces are applied. Finally, an encapsulation scheme is presented allowing sufficient bending of the device and operation for at least 500 cycles in air. The results are meant to improve the understanding of the phenomena present in thin-film batteries while undergoing bending rather than showing improvements in battery performance and lifetime.