Solidification of High-Concentration Electrolytes with Faujasite As Nanosized Porous Zeolite Additive for Solid-Type Batteries
Authors: Hacatrjan Schanth , Nakamoto Kosuke , Yamada Izumi , Inui Naoki , Shibahara Kazuhiro , Inokuchi Daisuke , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Introduction: With a renewed push for commercialization of Li-metal anode batteries due to their potential to drastically increase battery performance, new electrolyte systems need to be investigated that can not only withstand the harsh reduction potential of Lithium, but also be used with cathodes operating above 4 V. Commercially used electrolytes such as ethylene carbonate (EC) and dimethyl carbonate (DMC), with lithium-salt concentrations in the range of 1 M exhibit high ionic conductivities, however, are susceptible to leakage, show high volatility which together with the high flammability of organic electrolyte solutions, lead to safety concerns and show incompatibilities with Li-metal anodes.[1][2][3] High-concentration liquid electrolytes (HCLE) due to their stability against Li-metal anodes and their wide electrochemical stability window are viewed as promising alternatives.[4][5] However, commercially used separators made from polypropylene (PP) suffer from low wettability with high-concentration electrolytes[6] and are susceptible to thermal shrinking leading to fatal malfunctioning of the cell.[7] Therefore, there is an interest in finding new separator materials with excellent wettability that can utilize the unique properties of high-concentration electrolytes while simultaneously increasing its performance by enhancing its safety properties.In this report we propose the use of nanosized lithiated faujasite-type zeolite (FAUX-Li) powder as solidification agent of HCLE made from DMC and LiFSI for the preparation of a quasi-solid-state electrolyte (QSSE) in lithium metal batteries. QSSE, electrolytes which consist of a solid-matrix for stability improvement and a liquid electrolyte for facilitating the ion-transport, with the potential to improve the safety by leakage prevention.

Experimental: HCLE was prepared by mixing lithium bis(fluorosulfonyl)imide (LiFSI, IONEL®, Nippon Shokubai Ltd.) with dimethyl carbonate (DMC, >99.5 %, Kishida Chemicals Ltd.) in a molar ratio of LiFSI:DMC= 1/1.5. For the preparation of the quasi-solid-state electrolyte (QSSE), lithiated faujasite (FAUX-Li, Nakamura-choukou Ltd., Si/Al = 1.15, average particle size of 100 nm) was mixed into the HCLE in a weight ratio of 1:3 and mixed hereafter via a shaker until the mixture showed a smooth texture. Low-concentration liquid electrolyte (LCLE) with 1 M LiFSI in DMC solution was also made. Electrochemical stability was investigated by CV and LSV with Cu||Li and Li||Al cells respectively. Stability of the electrolyte with Lithium metal was performed with Li||Li cells. Li-surface morphology was analyzed with SEM. Performance of electrolyte was investigated in LFP||Li 2023-type coin cells with HCLE, QSSE and LCLE. LFP dry electrodes with 70% LFP, 25 wt% AB and 5% PTFE with an active material load of 1.5 mAh cm-2 were used. TG-DTA was used for thermal analysis of the electrolyte and FT-IR for the electrolyte solvation structure analysis.

Results: Mixing of HCLE with the FAUX-Li resulted in the formation of a QSSE that displayed excellent wettability with a wide electrochemical stability window up to 5.25 V vs Li/Li+ and good stability with the Li-metal anode, analyzed by LSV and CV respectively. Li-metal compatibility was further investigated by employing the QSSE in a symmetrical Li||Li cell, cycled at 25 °C with a current density of 0.1 mA cm-2. While the QSSE was able to cycle stable for 500 cycles with no signs of dendrite formation, shown by SEM analysis, the HCLE showed instable cycling after 90 cycles and an irregular Li-metal surface. Stable cycling of the QSSE can be explained by the increase of LiTN to 0.68 from 0.49 for the HCLE. The increase of LiTN is due to the interaction of the FSI-anion with the Lewis-acidic functional group of the FAUX-Li hindering its movement when a potential is applied. LFP||Li cells with HCLE, QSSE and low-concentration electrolyte (1 M LiFSI in DMC, LCLE) were tested to compare the performance. Compared to the HCLE and LCLE, the QSSE showed a good initial capacity of 151 mAh g-1, negligible capacity loss after 100 cycles with a capacity retention of 98.6 % and excellent coulombic efficiency. This report shows that the QSSE has good stability and excellent properties when used in a lithium-metal battery.

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