Self-Limiting Electrospray Deposition (SLED) of Porous Polyimide Coatings as Effective Lithium-Ion Battery Separator Membranes

Electrospray deposition (ESD) is employed to produce separator membranes for coin-cell lithium-ion batteries (LIBs) using off-the-shelf polyimide (PI). The PI coatings are deposited directly onto planar NMC electrodes via self-limiting electrospray deposition (SLED). Scanning electron microscopy (SEM), optical microscopy, and spectroscopic microreflectometry are implemented in combination to evaluate the porosity, thickness, and morphology of sprayed PI films. Electrochemical performance of the PI separators are analyzed via charge/discharge cycle rate tests. Discharge capacities of cells with the unoptimized SLED PI separators are within 83-99.8% of those with commercial Celgard 2325 PP/PE/PP separators. Furthermore, ultraviolet-visual wavelength spectroscopy (UV-vis) is utilized to qualitatively assess variation in film porosity within a prescribed temperature range of 20-400oC. UV-vis results underscore the ability of the SLED PI separator to maintain its porous microstructure up to ~350oC. This study points to the unique possibility of SLED as a separator manufacturing technique for geometrically complex energy storage systems. Further research is needed to optimize the polymer-solvent system to enhance control of porosity, pore size, and coating thickness. This can lead to significant improvement in rate and cycle life performance in more advanced energy storage devices.