Ionogel Nanocomposite Electrolytes for Advanced Solid-State Lithium Batteries
Authors: Tan Guoqiang , Lu Jun , Amine Khalil 
Published: February 2020

Stringent requirements for safe, fast and long-life rechargeable lithium battery technologies are ever increasing with the rapid development of electronics and energy industries. Recently, solid-state lithium batteries have been regarded as one of the most promising technologies to achieve these requirements, because they can provide higher energy/power densities, longer lifetime and better safety than the state-of-the-art lithium ion batteries which contain organic electrolytes.1-3 However, the lack of advanced solid-state electrolytes with the high ionic conductivity, wide electrochemical window, and good thermal and mechanical stability, has delayed the battery’s application in many large autonomous devices, such as electric vehicles and intelligent grids.4-6 Herein, we propose a new approach to develop solid-state lithium battery technology, using an ionogel nanocomposite electrolyte composed of porous oxide matrices with in-situ immobilizing ionic liquid salt solutions. In such composites, the ionic liquid salt solutions maintain the liquid dynamics, so they are responsible for the ionic conducting and other electrochemical properties; the porous oxide matrices provide abundant channels to confine ionic liquid solutions while maintaining good mechanical properties, thus, the composites have a solid-state glassy structure. For example, a SiO2/[BMI][TFSI]/LiTFSI ionogel electrolyte system, which shows a high ionic conductivity (1.2~3.6 × 10‒3 S cm‒1 at room temperature), good electrochemical stability (3.9 ± 0.1 V vs Li+/Li), and excellent mechanical strength (Fracture strength: 0.8 ± 0.1 MPa). The solid-state cells tested with the various cathodes (LiCoO2, LiNi1/3Co1/3Mn1/3O2, and LiFePO4) exhibit good electrochemical properties, including high specific capacities, long cycling stability, and excellent high temperature performance. Therefore, the ionogel electrolytes exhibit the superior performance to conventional organic electrolytes with regard to safety and cycle-life. This solid-state battery technology will provide new avenues for the rational engineering of advanced lithium batteries and other electrochemical devices.
The nanocomposite electrolytes (NEs) look like glass monoliths (Fig. 1A), which are transparent, smooth, and homogeneous. They have high room-temperature ionic conductivities of 1.2‒3.6 × 10‒3 S cm‒1 and stable electrochemical windows of 3.9 ± 0.1 V vs Li+/Li (Fig. 1C). They can be easily processed into thin-film electrolytes, allowing their flexible incorporation into a solid-state battery configuration (Fig. 1B), where we use the NEs as the solid-state electrolyte, a lithium foil as the anode, and cathode material of LiCoO2, LiNi1/3Co1/3Mn1/3O2, or LiFePO4. Figs. 1D‒I show the electrochemical performance of NEs with four NEs compositions in these half cells. It is noteworthy that the specific capacities and cycling performance of all these cells are gradually increasing with higher ionic liquid content in NEs. This finding is mainly attributed to the improved charge transfer in the electrode−electrolyte interface by increasing their wetting properties. In a word, these solid-state cells exhibit good electrochemical performance.

Fig. 1
Electrochemical characterization of nanocomposite electrolytes (NEs). (A) Optical photograph of NEs. (B) Schematic showing the solid-state battery configuration. (C) Ionic conductivities, activation energies, and electrochemical windows of NEs. (D-F) Initial charge-discharge profiles, and (G-I) cycling performance of LiCoO2/NEs/Li cells, LiCo1/3Ni1/3Mn1/3O2/NEs/Li cells, and LiFePO4/NEs/Li cells cycled at C/10 rate and at 30 °C.

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Figure 1