Thermally, Electrochemically, and Mechanically Tough Polymer Electrolytes Containing a Solvate Ionic Liquid for Lithium Secondary Battery
Authors: Hashimoto Kei , Tatara Ryoichi , Watanabe Masayoshi 
Published: December 2020

Lithium-ion rechargeable batteries (LIB) are energy storage devices with extremely high energy density, which play an essential role in portable electronic devices as well as electric vehicles. Conventional LIBs are composed of Li-intercalated cathode materials, graphite anode, and liquid electrolytes prepared by dissolving Li salts into organic solvents. Recently, a gel electrolyte, i.e., a three-dimensional polymer network swollen by a liquid electrolyte, is commercially used to bestow flexibility in the battery design, which contributes to the development of thin electronic devices. However, these gel materials are volatile, and their mechanical reliabilities are low.

In the last few decades, high-voltage cathode materials with over 4 V of operating voltage such as LiCoO2 (LCO, capacities of ca. 140 mA h g−1) and LiNi1/3Mn1/3Co1/3O2 (NMC111, capacities of ca. 150 mA h g−1) have been applied to lithium-ion batteries. Recently, next-generation cathode materials with a higher capacity and operating voltage, such as Ni-rich NMC, LiNi0.6Mn0.2Co0.2O2 (NMC622, capacities of ca. 180 mA h g−1 for 4.4 V of cut-off voltage) have been proposed for the higher capacity. The dramatic increase of the energy density of LIB contributes to the development of long-life portable electronic devices, however at the same time, causes a safety issue (i.e., short circuit, overheating, and explosion) due to the flammability of organic solvents. Most of the conventional gel electrolytes prepared by soaking a polymer network into an organic liquid electrolyte. Thus, the improvement of the thermal stability and mechanical property of the gel electrolytes is one of the critical issues.

In this study, to overcome these problems of gel electrolytes, we developed thermally, electrochemically stable, and mechanically tough electrolyte using a solvate ionic liquid (SIL) and a homogeneous polymer network. SIL, which is prepared by mixing equimolar glyme (oligoether, Gn, n = 3, 4) and a lithium salt, is known as a new class of ionic liquids (ILs). Triglyme (G3) or tetraglyme (G4) and Li cation form a stable cation complex ([Li(G3)]+ or [Li(G4)]+) by chelating solvation of O atoms toward Li cation, resulting in non-flammability, good thermal and electrochemical stability analogous to ILs as well as Li-ion conductivity. Herein, we used a equimolar mixture of G4 and lithium bis(trifluoromethanesulfonyl)amide ([Li(G4)][TFSA]). On the other hand, as a homogeneous network, we employed 4-armed poly(ethylene glycol) (TetraPEG) network with excellent mechanical property due to homogeneous dispersion of stress when they are deformed.

However, mixing polymers with high polarity including PEG with SIL results in deterioration of thermal and electrochemical stabilities of SIL. This is due to the interaction of electron-rich PEG chains with Li cation, which disrupts [Li(G4)]+ complex, resulting in generation of free G4, which is volatile and easily oxidative. To improve this, we prepared lithium-solvated tetraPEG (LS-TPEG) with ratio of O atoms to Li cation, [O]/[Li] = 5, which corresponds to that of [Li(G4)]+ complex. LS-TPEG/[Li(G4)][TFSA] showed high thermal and electrochemical stabilities comparable to [Li(G4)][TFSA] as well as good ionic conductivity (~ 1 mS cm−1 at 25 °C). The gel electrolyte also showed excellent mechanical properties inherent in homogeneous polymer network, including stretchability (>100%). We investigated the charge/discharge behavior of Li | gel electrolyte | NMC622, which showed a stable charge/discharge. The enhancement of thermal, electrochemical, and mechanical stabilities of the gel electrolyte was accomplished in the aspect of homogeneity of polymer network and solvation structures of polymer and solvent.