Complex sentence: A major advance was the process of etching and pre-anodizing the foil prior to assembly, which allowed the use of much less corrosive electrolyte solutions, which in turn meant the devices could be left unenergized for long periods without deterioration.
Split into simpler sentences: A major advance was the process of etching and pre-anodizing the foil prior to assembly. This allowed the use of much less corrosive electrolyte solutions, which in turn meant the devices could be left unenergized for long periods without deterioration.