Hybrid Strategies for Sulfide Solid Electrolytes with Organic Materials: Toward Practical All-Solid-State Lithium-Ion Batteries

Department of Energy Engineering (Battery Science and Technology)Lithium-ion batteries (LIBs) should further deal with safety concerns as well as greater energy density so that they can become the core of future electrifications. In this regard, All-solid-state lithium-ion batteries (ASLBs) replacing flammable organic liquid electrolytes (LEs) with solid electrolytes (SEs) have been considered as the prospective system owing to following advantages: using solidified electrolytes (1) reinforces safety, (2) increases energy density of battery pack, (3) takes chance to adopt Li metal as an anode potentially, (4) realizes extremely high-loading density of electrodes. To guarantee impressive performance of ASLBs, tactical choice of SEs is important. Among various SEs, sulfide SEs generally exhibit high ionic conductivity in range from 0.1 to 21 mS cm???1 with unit transference number at room temperature (RT) and superb device integration due to their mechanical softness. However, the ASLBs based on sulfide SEs should leap out two hurdles to realize commercialization. First, ionic percolations within electrodes should be optimized because the performance of ASLBs is strongly affected by interfaces where heterogeneous solids are in 2-dimensional contact with each other. Second, scalable techniques for tailoring sheet-type electrodes must be further developed. Notably, most of prior reports in this field have been demonstrated based on small-scale-composite electrodes prepared in dry condition, which is improper for scalable production. To solve aforementioned issues and challenges, combining sulfide SEs and a small amount of soft organic materials (e.g., carbonate based liquid electrolytes, polymeric binders, N-Methyl-2-pyrrolidone (NMP) used in conventional LIBs) via wet-process, in terms of ???hybrid configurations???, would be appropriate to improve ionic percolations within the electrodes and processability for practical sheet-type ASLBs. However, highly reactive nature of sulfide SEs against polar organic materials used in typical LIBs has restricted extensive researches for hybridization. Herein, I report on novel strategies for hybridization of sulfide SEs with diverse organic materials for high performance ASLBs. The first part of my thesis is hybrid sulfide SEs employing solvate ionic liquids (SILs) to handle incomplete ionic percolations within all-solid-state electrodes. SIL is an equimolar mixture of Li salt and glyme, for instance, Li(G3)TFSI (referred as to "LiG3") comprised of G3 (triethylene glycol dimethyl ether or triglyme) and LiTFSI (lithium bis(trifluoromethanesulfonyl)imide). The excellent stability of sulfide SEs (Li3PS and Li10GeP2S12) with LiG3 and their application were successfully demonstrated. The poor solid-solid contacts within the electrodes healed by addition of a small amount of LiG3, providing alternative Li+-conductive pathways, resulting in drastically increased capacity of LiFePO4 (LFP) electrode. The second part of my thesis is sheet-type electrodes fabricated from wet-chemical route. In this work, scalable slurry-process for bendable sheet-type electrodes evolved from liquid-phase synthesis (LP) of sulfide SEs by adding active materials (LiNi0.6Co0.2Mn0.2O2 (NCM622) or graphite (Gr)) and polymeric binders (polyvinyl chloride (PVC) or nitrile-butadiene rubber (NBR)). These processes allowed to synthesize sulfide SEs and fabricate bendable sheet-type electrodes at the same time. A rocking-chair ASLBs employing sheet-type NCM622 and Gr electrodes exhibited a decent capacity of 110 mA h g-1 at extreme conditions (15C and 100 oC), emphasizing guaranteed performance of ASLBs at a wide range of temperatures. Although some literatures have reported wet-slurry process for sheet-type ASLBs while using non-polar solvents with polymeric binders, the binders result in below par electrochemical performance because electric pathways in the electrodes are interrupted by binders. In the last study, slurry-fabricable Li+-conductive polymeric binders enabled by SILs for sheet-type ASLBs were demonstrated to solve aforementioned obstacles. The less polar dibromomethane (DBM) allowed wet-slurry process to accommodate Li6PS5Cl (LPSCl) and SILs without undesirable dissolution problems. The NCM622 electrodes employing NBR???LiG3 showed higher capacity of 174 mA h g???1 at 30 ??C, which was far superior than using only NBR (144 mA h g???1). The facilitated Li+???ionic contacts at interfaces paved by NBR???LiG3 are evidenced by the complementary analysis from electrochemical characterizations and 7Li nuclear magnetic resonance measurements. These results provide not only breakthroughs for mass production but also rational guideline to design all-solid-state electrodes toward practical ASLBs. I believe that further investigation about compatible combinations of sulfide SEs with organic materials will contribute to understand complex chemical phenomena (e.g., reactivity, solubility, and stability) of sulfide SEs, providing creative opportunities for architecting electrodes as well as motivation and insights for a new knowledge in various battery systems.clos