Mechanism Analysis of Enhanced Li-Ion Conductivity in Mesoporous Silica-Based Solid Nano-Composite Electrolytes
Authors: Vereecken Philippe M. , Chen Xubin , Gandrud Knut Bjarne , Put Brecht , Sagara Akihiko , Murata Mitsuhiro , Tomiyama Morio , Kaneko Yukihiro , Shimada Mikinari , Steele Julian , Roeffaers Maarten , Debucquoy Maarten , Mees Maarten J. 
Published: February 2020

Solid electrolytes with Li-ion conductivity higher than 1 mS/cm are required for the development of high capacity solid-state Li-ion batteries. In the past decade, several studies were done on the improvement of ion conductivity in composite materials by employing interface enhanced ion conduction at inorganic oxide surfaces such as silica, alumina or titania added to, for example, inorganic or polymer electrolytes. Also, composites of nanoparticles and mesoporous microparticles mixed with ionic-liquid electrolyte (ILE) have been proposed to promote the Li-ion conductivity along the particle or pore surface. However, so far the ionic conductivity was always lower than that of the original ILE due to the interrupted ionic paths by percolation from particle to particle. Monolithic nanoporous silica with ILE confined in the porous structures, also named “ionogels” have shown ion conductivities approaching the ILE bulk conductivity. These ionogels are fabricated by a sol-gel process e.g. by hydrolysis-condensation reaction with tetraethylorthosilicate (TEOS) precursor and typically formic acid where the ILE is added in the solution as the template for the silica to grow around. So far, the ionic conductivity of the confined ILE in the standard nanoporous oxide has never exceeded that of the ILE conductivity itself. By introduction of a surface functional group, an ion conductivity slightly higher than that of the bulk ILE was obtained, showing that surface interactions can be used to tailor the ion conductivity in these materials.

In this paper, we show nanocomposite electrolyte (nano-SCE) materials with enhancements in ion conductivity exceeding 200% and ion conductivities up to 3mS/cm. The nano-SCE was fabricated in a similar way as the ionogels: a single-step sol-gel process with a TEOS precursor and with the ionic liquid electrolyte in the homogeneous precursor solution. The processing conditions were such that molecular ordering of the IL molecules was favored and the adsorbed interface layers provided free Li+ ions for enhanced Li-ion conductivity along the surface. Figure 1 shows the ion conductivity of the obtained solid pellet for a model system with (N-butyl- N-methyl pyrrolidinium bis(trifluoromethanesulfonyl) imide ([BMP]TFSI) and bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI). Importantly, the graph shows the difference between an ionogel material with a confined ILE and our nano-SCE. For the ionogels, the conductivity versus temperature behavior is the same as that for the bulk ILE, albeit with lower conductivity due to the fraction of inactive silica. Both the ionogel composites and the bulk ILE show the melting point of the ILE with a lower conductivity for the solid phase. For the nano-SCE, however, the melting point is no longer observed and the slope indicates that the formed nanocomposite material has a lower activation energy for diffusion than that of the bulk ILE and of the ionogels with the confined ILE.

In this paper, we propose a mechanism for the observed behavior based on adsorption of the TFSI anion and subsequent molecular ordering and layering of the BMP cation and TFSI anions. The adsorbed layer has a solid-state like character and is therefore named as the mesophase layer. We will present experimental evidence for the interface interactions from FTIR spectra. Raman measurements confirm that the fraction of free Li+ ions increases in the composites. NMR measurements show that similar enhanced surface diffusion happens at nanoparticle systems but the interconnected pores in the SCE provide continuous pathways throughout the solid electrolyte nanostructure, ensuring the full effect of the surface enhancement is observed in contrast to analogous nanoparticle-ILE composites. To demonstrate the functionality of the nano-SCE as Li-ion electrolyte, cells with LFP cathodes were prepared. A technologically distinguishing feature of the nano-SCE, as for ionogels, is that it is applied as a liquid – via wet chemical coating – and only afterwards is converted into a solid. That way it is perfectly suited to be casted into dense powder electrodes where it fills all cavities and makes maximum contact, just as a liquid electrolyte does. A cell with 200Wh/L at 0.5C is demonstrated by casting of the nano-SCE precursor solution into the electrodes. The possibility of wet application of the nano-SCE precursor makes this technology also compatible with current Li-ion battery fabrication processes.

Fig. 1 Temperature dependence of the ion conductivity for the ionic liquid electrolyte (ILE) with Li-TFSI and BMP-TFSI, for two silica ionogels with changing ILE content (x refers to the ratio of ILE to silica) and for our nano-SCE showing different behavior between the nano-SCE and the ILE it contains as a result of the mesophase layer formed on the silica pore surface.

Figure 1