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Surface Promoted Enhanced Transport of Li-ions

Periodic Reporting for period 1 - SUPER-Lion (Surface Promoted Enhanced Transport of Li-ions)

Reporting period: 2017-03-15 to 2018-11-14

Solid-state batteries (SSB) can potentially solve the main challenges facing today’s lithium-ion batteries (LIB) with liquid electrolytes, namely, safety issues due to the flammability of the liquid electrolyte and limited cycle life time due to unwanted side-reactions at the solid/liquid interface. The emerging Internet of Things will lead to an exponential growth in wireless sensor networks and autonomous microsystems, however, improvements in energy storage is a critical aspect in order to enable future applications. Thin-film solid-state batteries (TFB) are the candidate of choice for powering this wide variety of microsystems, such as smart cards, radio-frequency identification tags and medical implants, due to their intrinsic safety and great flexibility in device design and integration. In order to meet the future demands in terms of power and energy density the need for all-solid-state 3D Li-ion microbatteries arises. One of the key challenges for the realization of 3D SSB is the conformality of the layers on high surface area substrates. This is especially true for the solid electrolyte layer, as a single pinhole will short-circuit the entire cell. Atomic layer deposition (ALD) and molecular layer deposition (MLD) are two of the few techniques capable of depositing conformal and pin-hole free layers on complex substrates, as they both are gas-phase deposition techniques based on sequential self-limiting surface reactions. The main purpose of SUPER-Lion was to combine these two powerful techniques for development of a novel thin-film nano-composite electrolyte (NCE) which would provide both high ionic conductivity and good electrochemical stability. The NCE consists of a mesoporous oxide matrix, obtained through MLD, which provides both mechanical stability and a high effective internal surface area. The internal surface of the mesoporous matrix is coated by ALD with nanometer thin layers of a Li-compound that supply the necessary Li+ ions. The increased ionic transport at the interface between the surface of the oxide and the lithium compound is exploited to make an NCE with enhanced ionic conductivity. SUPER-Lion has provided detailed insight into the potential of composite electrolytes for use in SSB for the first time. Several NCE systems were fabricated to study the effect of different Li-compound/oxide combinations, oxide matrix composition and structure on the resulting ionic conductivity, battery performance and stability. In addition, the feasibility of integrating composite electrolytes in battery systems was also demonstrated by utilizing NCE systems made by MLD/ALD in TFB cells. The battery devices were based on TiO2 as cathode material, NCEs as electrolyte, and lithium metal as anode. The TFBs were characterized in order to determine their performance and dependence on the NCE properties. The direct application of this work will enable the fabrication and development of novel solid-state electrolytes, paving the way for next generation TFBs.
Two different mesoporous oxides of Al2O3 were fabricated by MLD of hybrid alucone layers utilizing the precursor combinations of trimethyl aluminium (TMA) together with glycerol (GL) or ethylene glycol (EG). The EG and GL-based alucone layers were transformed into mesoporous oxides through calcination in air or water etching, respectively. Sol-gel processing based on tetraethyl orthosilicate (TEOS) were employed in order to fabricate and evaluate thin-films of mesoporous SiO2 as matrix material in the NCE. The composition and remaining carbon content of these oxide matrices were obtained by TOF-ERDA. Thickness of the oxide matrices were obtained by SEM and ellipsometry, and open porosity with ellipsometric porosimetry. The different mesoporous matrices were converted into NCE systems by deposition of nm-thin layers of Li2CO3 or Li3PO4 by ALD. The distribution of Li through the mesoporous oxide layer was confirmed by TOF-ERDA. Resulting NCE thicknesses were evaluated with SEM. Ionic conductivity values of the different NCE systems were extracted by temperature dependent solid-state impedance spectroscopy. An enhancement in Li-ion conductivity of two orders of magnitude, compared to the pure Li-compound, was achieved for a NCE consisting of a mesoporous Al2O3 matrix filled with Li2CO3. Clear evidence of the underlying enhancement mechanism suggested by space charge layer theory was demonstrated by the profound impact of the oxide material on the resulting NCE Li-ion conductivity. Finally, electrochemical performance of the NCE systems were evaluated by fabrication of TFB cells, utilizing thermally evaporated Li-metal as anode. The performance of these solid-state cells supported the SIS data, showing a clear improvement when the NCE system itself exhibited enhanced interface conductivity versus NCE systems without enhancement. The TFBs utilizing NCE systems with enhancement achieved 75% of the specific capacity of a reference cell with a liquid electrolyte (1M LiClO4 in PC) and showed stable cycling with an average coulombic efficiency of 100%. Data and results from the SUPER-Lion project have been presented at an international conference as well to IMEC stakeholders through the biannual industrial partner weeks, while journal publications are in progress.
In former studies composite solid electrolytes were usually synthesized by suspending insulator particles, typically of SiO2 or Al2O3 (both solid and mesoporous), in molten ionic conductors. With this approach, ensuring optimal utilization of the enhanced conductivity at the interface throughout the entire composite electrolyte layer can be difficult, as point-to-point contacts between particles could be limiting the performance. Furthermore, integration of these particle-based solid composite electrolytes into a solid-state battery stack can also be challenging from a fabrication point of view, as compact electrolyte pellets are typically obtained by compression under high pressure (~108 Pa). Within SUPER-Lion a new concept for fabrication of these composite systems has been demonstrated, offering two main advantages. The use of an oxide matrix, instead of particles, where the internal interconnected pore-structure allow for uninterrupted Li+ conduction pathways through the entire composite electrolyte layer. Compared to previously reported particle-based composite systems, the NCE systems developed in SUPER-Lion exhibit lower Li-ion activation energies and higher Li-ion conductivity enhancements. These observations highlights the potential benefit of exploiting the undisrupted conduction pathways provided by the mesoporous oxide matrix in composite electrolyte systems. Secondly, fabrication through conformal gas-phase deposition techniques enables precise thickness control in the nm-range and integration in TFB devices, even 3D-structured TFBs. Previously, composite electrolytes and LIB’s have only been investigated separately. By successfully integrating and demonstrating the concept of NCEs in TFB cells for the first time, SUPER-Lion has contributed to the unification of both domains and advanced state-of-the-art for composite electrolyte systems. The approach for development of novel solid-state electrolytes demonstrated in SUPER-Lion, offers additional flexibility in fabrication of SSB-powered devices that might be applied in consumer electronics in years to come.
Composite electrolyte system