Composite Cathode Architectures Made By Freeze-Casting for All Solid State Lithium Batteries
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Composite Cathode Architectures Made By Freeze-Casting for All Solid State Lithium Batteries

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As part of a Battery 500 seedling project, MBRDNA partnered with LBNL and Montana State University (MSU) to develop solid-state batteries based on an Al-substituted LLZO (Li7La3Zr2O12) ceramic ion-conductor. All solid-state lithium batteries are attractive contenders for use as power sources for electric vehicles because of their potential for better safety and higher energy density than state-of-the-art lithium-ion batteries, although they are still at early stages of development. Al-substituted LLZO is one of the most promising solid electrolytes for battery applications based on high ionic conductivity, a wide operating voltage window, and apparent stability vs. reduction by lithium. However, difficulties with processing thin (< 20 um and dense LLZO membranes have hampered development of devices based on this material. It is non-compressible and must be sintered at high temperatures to densify components. It is also difficult to maintain contact between the active cathode material and LLZO in the composite cathodes, which is critical for successful operation. This research project aimed to use novel processing methods to overcome these issues. The end result was the first ever-reported truly all-solid-state battery based on LLZO operating at room temperature without application of exogenous pressure. During the course of the project, we also identified a number of issues that need to be addressed to improve the technology readiness level of this battery. The procedure is to fabricate a porous LLZO ceramic scaffold using freeze-tape casting methods and co-sinter it with a tape cast thin dense LLZO layer to make a bilayer. A cathode material (either LiNi0.6Mn0.2Co0.2O2 or LiNi0.8Mn0.1Co0.1O2, hereafter abbreviated NMC-622 or NMC-811), carbon, polyvinylidene fluoride (PVdF) and a secondary solid electrolyte based on succinonitrile complexed with lithium salts is then infiltrated into the porous layer, and lithium metal is attached to the opposite side to form the cell. A number of trouble-shooting configurations that were variants of this procedure were also assembled and tested at LBNL. MSU used their expertise in freeze tape casting to optimize the scaffold-making process, and LBNL developed cell making procedures and used advanced diagnostics such as synchrotron micro-tomography to understand the materials properties. MBRDNA assembled cells from components provided to them by either LBNL or MSU and tested them electrochemically. In some cases, already-assembled coin cells were provided to MBRDNA by LBNL. Tests carried out at MBRDNA included impedance spectroscopy, initial charge and discharge, rate capability tests, and extended cycling, all at room temperature. Some cells were de-crimped and subjected to post-mortem analysis after cycling. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) and optical imaging were carried out on the de-crimped cells. MBRDNA provided this data to LBNL. Quarterly reports were submitted to DOE summarizing these efforts and results.

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