Recently, ionogels, pseudo-solid-state electrolytes consisting of an ionic liquid electrolyte confined in a mesoporous inorganic matrix, have attracted interest for use as a solid electrolyte in Li+ batteries because of their high ionic conductivity, stability, and solution processability. Here-in, we report adapting the ionogel synthesis route through spin coat processing to obtain thin films. Films down to 600 nanometers were obtained that maintained the properties of the bulk ionogel. By using spin coating to achieve thin films, the ionogel can be processed directly on electrode surfaces as a sol. Thin film ionogels processed on lithium iron phosphate cathodes achieved capacities of 125 mAh g-1 at charge/discharge rates of 2 hours stably over 150 cycles, which corresponds to the highest reported power densities for silica ionogels. The sol-gel process was modified through the addition of a photoacid generator to add UV patternability. The UV crosslinked gel showed comparable properties and cycling to the spin-coated ionogel.
While having high achievable capacities, there have been few studies into deciphering the electrode-ionogel interface. The second section will review our work directed at understanding the interface for solution-processable ionogel electrolytes with various electrode materials. Using XPS, Raman spectroscopy, and electrochemical testing to probe the electrode-ionogel interface, surface reactions were identified as being the source of the interfacial barriers. Our results indicate that the acidity of the sol led to reduction of the transition metal and breakdown of the sol-vent and organic acid, forming an organic surface layer, which impedes Li+ transport. By adjusting the silica gelation route these interfacial reactions can be avoided, leading to stable cycling.
The high ionic conductivity of ionogels shown in Chapter 3, in comparison to other solid electrolytes, and its solution processability opens the possibility for incorporation into multidimensional cell designs. The last portion of my dissertation will explore using the ionogel electrolyte to form a 2.5D cell consisting of a 3D LiFePO4 cathode and 2D Li metal anode. The electrolyte is shown to penetrate uniformly throughout the 3D electrode array and into each individual post. High areal capacities (1.1 mAh cm-2) were achieved at 1 mA cm-2 with a stable capacity retention at 0.5 mA cm-2 over 50 cycles. The 2.5D cell is comparable or better than current 3D batteries in literature.