Silica-Based Ionogels Deposited By Spray Coating for All-Solid-State Lithium-Ion Microbatteries

Abstract

The emerging market of the Internet of Things, smart objects and others increase the demand for micro energy sources. Rechargeable Li-ion batteries are a well-known technology for energy storage. However, safety issues and high production costs constrain progress. Research on solid electrolytes, such as LiPON, was performed to evade leakage. But LiPON suffers from low ionic conductivity and a cost and time intensive production process. Another approach is the substitution of volatile and flammable organic electrolyte solvents with ionic liquids (IL), which display negligible vapor pressure and wide chemical, electrochemical, and thermal stability. Electrolyte solution based on ILs can be confined into inorganic porous networks forming so-called ionogels (IG), which are investigated as solid electrolyte materials. IGs combine low hazard and good ionic conductivity [1]. Silica-based IGs compatible with Li/LiCoO2 systems were prepared in a one-pot sol-gel process. The composition of the IG precursor solution and the influence of trifluoroacetic acid as catalyst were studied to obtain a fast condensation. Homogeneous and transparent IGs were obtained with a gelation time of less than 12 h. The physical properties of the host matrix were characterized by N2 sorption and SEM. The silica host matrix is a 3D network predominantly built from 3-fold condensed silicon centres. Its structure changes from densely packed silica particles to open porous architecture. The electrochemical performances of the IG deposited by spray coating under ambient contitions were evaluated by impedance spectroscopy measurements. First results show that these promising IGs may be successfully used as electrolyte in Li/LiCoO2 cells, with ionic conductivities up to 0.15 mScm-1. The coin cell batteries prepared with our IGs did not show dendritic growth after more than 100 cycles of galvanostatic cycling at a C/4 rate.