The thermal and electrochemical properties of several succinonitrile (SN)-based organic ionic plastic crystals (OIPCs) containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(oxalate) borate (LiBOB) or mixtures of the two salts, were investigated with the goal of determining which is most promising for use as a catholyte in solid-state lithium batteries. The best combination of properties for this use was found for the mixture containing 3 mol% LiTFSI and 2 mol% LiBOB in SN. Based on these observations, several solid-state cells containing Al-substituted Li7La3Zr2O12 separators with composite cathodes consisting of LiNi0.33Mn0.33Co0.33O2 (NMC111), carbon black, and the succinonitrile-salt mixtures were assembled and cycled at room temperature, without exogenous pressure, using a simple design.

All-solid-state batteries (ASSB) have come into the spotlight as next-generation batteries because of their potential high energy density and low flammability. LLZO (Li7La3Zr2O12) is a prominent solid electrolyte material for ASSBs due to its high ionic conductivity (>1 x 10-4 S/cm at room temperature) and compatibility with Li metal. However, for its use in ASSBs, there are challenges including forming intimate contact with the active electrode material. Because both LLZO and the cathode active material are rigid, employing a soft solid catholyte between them can be a solution to ensure intimate contact. The solid catholyte must be ionically conductive, and stable against oxidation (> 4.5 V vs. Li/Li+). Organic Ionic Plastic Crystals (OIPC) are a class of materials that is soft and waxy at room temperature. When the OIPC is mixed with Li salts, the mixture has high ionic conductivity and a wide electrochemical window. Succinonitrile (SN) is an OIPC material, and SN with Li salts are reported to have high ionic conductivities (> 1x10-4 S/cm), good oxidation stability (>5 V vs. Li/Li+), and low flammability as well [1][2]. Despite the favorable properties of the SN-Li salt composite, poor chemical stability with Li metal makes it difficult to use it alone as a solid electrolyte [1]. For its use as a solid catholyte, chemical stability with Li metal is not an issue, making it a prominent candidate material. However, studies for its use as a solid catholyte with LLZO are limited. In this talk, we describe the properties of Succinonitrile-Lithium salt composites focusing on their suitability as solid catholytes in LLZO-based ASSBs. SN is mixed with different kinds of Li salts including LiTFSI and LiBOB to form composites. Their electrochemical properties are compared and the contact between the composite and LLZO is observed. The performance of all-solid-state cells with the mixtures will also be evaluated at room temperature. It is hoped that this work can motivate efforts toward developing LLZO cells using OIPC materials. [1] Alarco P.J, et al Nat. Mater., 3, 476 (2004) [2] Effat M.B, et al J. Power Sources, 226802, 436 (2019)

A challenge for lithium lanthanum zirconate (LLZO)-based solid-state batteries is to increase the critical current density (CCD) to enable high current cycling. A promising strategy is to modify the LLZO surface morphology to provide a larger contact area with the Li metal. Here, a surface-textured thin LLZO electrolyte was prepared through an easily scalable process. The texturing process is a simple pressing of green LLZO tapes between micro-textured substrates. A variety of textures can be produced, depending on the type of substrate, and texturing can be on either one side or both sides. For this work, after pressing and sintering, several micro-patterns are formed on thin LLZO (∼118 μm thick). The properties of the various samples were characterized to investigate the impact of surface texturing, and the most promising ones were selected for electrochemical testing in symmetrical lithium cells and full cells. Li symmetric cells using a coarse ridge-textured LLZO exhibit ∼2.5 times increased CCD compared to planar non-textured LLZO, and a solid-state full cell shows stable cycling and improved rate performance. We believe this process offers a favorable trade-off of processing complexity vs structural optimization to maximize CCD.

Ceramic solid electrolytes based on LLZO (Li7La3Zr2O12) are promising candidates for all-solid-state batteries due to their high ionic conductivity and good apparent stability vs. lithium metal, however they are prone to mechanical failure. Lithium metal intrusions, alongside cell stack pressure, transition polycrystalline solid electrolyte grains into a compressed state that promotes crack propagation and fracture. This work examines the mechanical response of Al-substituted LLZO to compressive forces by measuring ultimate strength under pillar compression with a flat punch tip. Failure modes characterized by in situ scanning electron microscopy show diverse splitting patterns arising from internal porosity, grain boundaries, and slip planes. Large correlated variations in compressive strength (0.93-2.63 GPa) and Young's modulus (72.1-150.97 GPa) are observed across microscale regions of the solid electrolyte. Molecular dynamics simulations of LLZO with different porosities describe the variation of compressive strength and Young's modulus, and enable a microscale porosity model to be fit accounting for Young's modulus reduction across the solid electrolyte. Overall, the results indicate the importance of microscale mechanical testing of ceramic solid electrolytes to identify preferential sites for mechanical degradation and Li intrusion, and ensure the robust design of all-solid-state lithium metal batteries.