Tailoring Slurries Using Cosolvents and Li Salt Targeting Practical All‐Solid‐State Batteries Employing Sulfide Solid Electrolytes

Abstract

classes of materials, including sulfides (e.g., Li 5.5 PS 4.5 Cl 1.5 : [14,15] ≈10 mS cm −1 ), oxides (e.g., Li 7 La 3 Zr 2 O 12 : [16] 0.5 mS cm −1 ), closo-borates (e.g., 0 . 7 L i ( C B 9 H 10 ) -0 . 3 L i ( C B 11 H 12 ) : [ 12,17] 6.7 mS cm −1 ), and halides (e.g., Li 3 YCl 6 : [18,19] 0.5 mS cm −1 , Li 2.25 Zr 0.75 Fe 0.25 Cl 6 : [20] 1 mS cm −1 ). Furthermore, consideration of multiple aspects, such as mechanical sinterability, cost, and lightness (e.g., Li 6 PS 5 Cl: [14] 1.86 g cm −3 , Li 7 La 3 Zr 2 O 12 : [21] 5.11 g cm −3 , Li 3 YCl 6 : [18] 2.43 g cm −3 ), indicates that sulfide materials are highly competitive. [14,22,23] To integrate SEs into large-scale ASLBs for mass production, sheet-type electrodes and SE films are required. [24][25][26][27][28][29][30][31][32][33] For this, it is necessary to use soft polymeric binders to avoid delamination and to supplement the brittleness of the inorganic components of the electrode active materials and SEs. [34][35][36] Moreover, stresses generated by volumetric strains in electrode active materials upon repeated cycling can be buffered by the polymeric binders. [34,36,37] However, the introduction of even a small amount of polymeric binder (e.g., 1-2 wt%) in composite electrodes severely degrades the electrochemical performance of ASLBs (e.g., as much as ≈30 mA h g −1 of capacity loss for LiNi 0.6 Co 0.2 Mn 0.2 O 2 electrodes), due to the disruption of interfacial Li + contact. [27,34,38] To address this issue, several approaches have been introduced. Yamamoto and coworkers reported that a binder-free sheet-type battery fabricated using thermally decomposable polymers of poly(propylene carbonate) provided good performance, [39] but at the expense of the mechanical properties of the battery. Moreover, it was shown that it is possible to use small amounts of binders via a dry process employing a fibrous polytetrafluoroethylene binder. [40,41] However, a wet-slurry process for ASLBs is still imperative, as it could take advantage of the already-developed manufacturing infrastructure for LIBs.Recently, our group reported the preparation of Li + conductive polymeric binders based on solvate ionic liquids (SILs), which are a solvent-salt complex of Li salt and glyme, such as Li(G3) TFSI (G3: triethylene glycol dimethyl ether, LiTFSI: lithium bis(trifluoromethanesulfonyl)imide). [34] The use of solvents with an intermediate polarity such as dibromomethane (DBM) for Polymeric binders that can undergo slurry fabrication and minimize the disruption of interfacial Li+ contact are imperative for sheet-type electrodes and solid electrolyte films in practical all-solid-state Li batteries (ASLBs). Although dry polymer electrolytes (DPEs) are a plausible alternative, their use is complicated by the severe reactivity of sulfide solid electrolytes and the need to dissolve Li salts. In this study, a new scalable fabrication protocol for a Li + -conductive DPE-type binder, nitrile-butadiene rubber (NBR)-LiTFSI, is reported. The less-polar dibromomethane and more-polar hexyl b...

Challenges, interface engineering, and processing strategies toward practical sulfide‐based all‐solid‐state lithium batteries

“…To eliminate the ionic transport block effect caused by binders, Jung et al conducted a series of works on favorable ionic contacts in the composite cathode. [389][390][391]396 They first found that the solvate ionic liquid (SIL) Li(G3)TFSI (G3: triethylene glycol dimethyl ether, LiTFSI: lithium bis [trifluoromethanesulfonyl]imide) is chemically stable against sulfides. The addition of trace amounts of Li(G3)TFSI provides an alternative ionic pathway across the commonly poor solid-solid contacts in the composite cathode.…”

“…A co-solvent consisting of less-polar dibromomethane and more-polar hexyl butyrate was employed to dissolve NBR and LiTFSI synergistically, which provides an alternative method to control the dispersion of binders (Figure 16D). 390 In addition, the ester solvents incorporating bulky alkyl groups, such as benzyl acetate, were discovered to dissolve both binders and Li salts while attenuating nucleophilic attacks on sulfides, which enable similar dry ionic conductive polymer binders to be used in the wet slurry process for sheet-type cathodes and SEs (Figure 16E). 391 Tuning the polarity of binders is another strategy to overcome the polarity mismatch among binders, solvents, and sulfides.…”

“…Reproduced with permission. 390 Copyright 2021, John Wiley and Sons. (E) Schematic illustrating reactivity with sulfides for different solvents.…”

“…(D) Schematic illustrating interactions of slurry‐processable solvents with LiTFSI and NBR. Reproduced with permission 390 . Copyright 2021, John Wiley and Sons.…”

The role of polymers in lithium solid-state batteries with inorganic solid electrolytes

“…137 Very recently, a blend of the less polar dibromomethane solvent and the more polar hexyl butyrate solvents was shown to work synergistically to dissolve NBR and LiTFSI, while preserving the argyrodite-type ISE, forming a SSB with the Li + -conducting NBR-LiTFSI binder. 141 In addition to solvents, the functionality of the polymer also inuences the properties of the ISE-binder composite. Riphaus et al compared the results of several binders poly(styrene-cobutadiene) (SBR), polyisobutene (PIB), polymethyl methacrylate (PMMA), HNBR, polyethylene vinyl acetate (PEVA) with Li 10 -SnP 2 S 12 (LSPS) ISE.…”