Abstract
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.
Highlights
Numerous emerging technologies such as wearables, smart clothes, smart cards, electronic papers, electronic skins, implantable medical devices, complementary metal-oxide semiconductor backups, various sensors, and micro-electromechanical systems benefit from flexible energy-storage systems.[1,2] All-solid-state flexible thin-film lithium-ion batteries (TFBs) enable ultrathin and bendable power source designs with very low weights and shape flexibility
The extracted discharge capacities per mass of active material, that is, “practicable capacities” were quite low. It was tested how the mechanical compression of Flame spray pyrolysis (FSP)-produced porous LTO thin films affects the practicable capacities within TFBs
By reducing the residual porosity of LTO(300) and concomitantly increasing the loading, the electrochemical performance may have approximated that of denser LTO layers synthesized by Schichtel et al.[59]
Summary
Numerous emerging technologies such as wearables, smart clothes, smart cards, electronic papers, electronic skins, implantable medical devices, complementary metal-oxide semiconductor backups, various sensors, and micro-electromechanical systems benefit from flexible energy-storage systems.[1,2] All-solid-state flexible thin-film lithium-ion batteries (TFBs) enable ultrathin and bendable power source designs with very low weights and shape flexibility. Typical thin-film preparation techniques for lithium-ion batteries (LIBs) include magnetron sputtering and pulsed laser deposition that produce amorphous dense films Crystallization of such particle layers, requires thermal treatment that endangers the integrity of flexible polymer substrates.[1]. It was tested how the mechanical compression of FSP-produced porous LTO thin films affects the practicable capacities within TFBs. Spinel-LTO is a wellknown negative electrode material for LIBs.[7,34−37] When connected as positive electrode material versus elemental lithium, it results in relatively low energy densities compared to typical high-voltage electrode materials. The additional electron[39] and lithium-ion diffusion pathways will, eventually, lead to enhanced practicable capacities of the LTO film To test this hypothesis, the as-prepared highly porous LTO electrodes were compacted at comparably high and low pressures. They provided valuable insights on the charge transport capabilities, particle coordination numbers, and particle contact densities of the LTO particle networks
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