Tetra-Peg Gels with Highly Concentrated Sulfolane-Based Electrolytes Exhibiting High Li-Ion Transference Numbers
Authors: Dokko Kaoru , Tasaki Natsumi , Kokubo Hisashi , Ueno Kazuhide , Watanabe Masayoshi 
Published: August 2024

To achieve high safety of lithium-ion batteries (LIBs), non-flammable electrolytes and prevention of liquid electrolytes are desirable. Sulfolane (SL) is a thermally stable and non-flammable solvent, and can dissolve high concentration Li salts. Recently, highly concentrated electrolytes (HCEs) containing Li salts over 3 mol dm−3 have attracted much attention for their attractive properties such as high thermal stability and wide electrochemical windows. We previously reported that SL-based HCEs exhibit Liion hopping conduction mechanism.1,2 In SL-based HCEs, a unique solvation structure is formed in which different Li+ ions are cross-linked by the SL and anions. In the cross-linked structure, Li+ ions dynamically exchange the ligands (SL and anion) and diffuse/migrate faster than SL solvent and anion, leading to high transference numbers of Li+ ion (>0.5). The high transference number of Li+ is effective in suppressing the concentration polarization during high-rate charging and discharging of a LIB.

Gelation of the HCEs can prevent the leakage of liquids and further improve the safety of LIBs. In the case of polymer gel electrolytes, liquid electrolytes are incorporated in the polymer network. Ionic conductivity of a gel electrolyte is generally lower than that of its parent liquid electrolyte. However, to prepare self-standing and mechanically robust gel electrolytes high polymer concentrations (typically over 20 wt%) are required. To achieve both high ionic conductivity and mechanical toughness of a gel electrolyte, the use of tetra-arm poly(ethylene glycol) (TPEG) has been proposed.3 TPEG gels obtained by the end-coupling reaction of two symmetrical TPEGs with different terminals exhibit excellent mechanical properties.4 TPEG forms a homogeneous polymer network, allowing the resulting gels to uniformly disperse external stresses. Consequently, TPEG gels possess excellent mechanical toughness even at low polymer concentrations (<10 wt%). In this work, we prepared self-standing TPEG gel membranes containing a high concentration LiN(SO2CF3)2/SL electrolyte.5 The mechanical, ion transport, and electrochemical properties of the gels were characterized. TPEG gels exhibited high transference numbers of Li+. The low polymer concentration and homogeneous polymer network in the gel electrolyte were useful in achieving both mechanical reliability and high Li+ transport ability. Li/LiCoO2 cell with a TPEG gel electrolyte membrane could discharge at a current density of 2.9 mA cm−2 despite the low ionic conductivity (0.26 mS cm−1) of the gel electrolyte.

Acknowledgements
: This study was partially supported by the JSPS KAKENHI (Grant No. JP19H05813, JP22H00340, and JP23K17370).

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