Enhancing the Utilization of Porous Li4Ti5O12 Layers for Thin-Film Lithium-Ion Batteries

Thin-film lithium-ion batteries (TFBs) are promising components for powering rigid and mechanically flexible electronic devices. The production of such all-solid-state power units is yet complex and cost-intensive, and the performance is to be improved. Because binders are often absent in TFBs, it is a key issue to retain the mechanical stability and reversible discharge capacities upon mechanical bending. One approach to reduce contact losses upon cell-bending may be the fabrication of electrodes with controlled residual porosities. The remaining voids could buffer occurring volume changes during cycling of typical cathode materials. To systematically approach the fabrication of long-living high-performance flexible TFBs, this work addresses the relationship between different residual porosities of pure TFB electrodes and their electrochemical performance in flat TFB systems. Crystalline and phase-pure Li4Ti5O12 (LTO) thin-film electrodes were deposited in situ with the flame spray pyrolysis technique and compressed at two different pressures. Zero-strain LTO has been chosen as a model material because it enabled us to understand the sole impact of porosity on electrochemical performances without interfering volume changes. Our results showed that denser LTO particle networks were accompanied by a larger number of LTO particle contacts. Also, the LTO contact densities to the neighboring interfaces in a TFB cell have increased. The improved electrode contacting led to reduced electrical sheet resistivities and significantly increased practicable capacities. Future projects will require the substitution of LTO with high-voltage cathode materials to maximize energy densities. Cycling under statically and dynamically bent condition will answer whether tuned residual porosities are useful for the production of long-living flexible TFBs.