The global energy crisis and environmental pollution have experienced an enormous push for development of alternative energy sources with an efficient energy storage system. Among the different energy storage systems, lithium ion battery is the most attractive one due to high energy density and long cycle life. One of the key components of these lithium or lithium ion batteries is the ion conducting polymer electrolyte. This book provides the scientific approaches to synthesize and characterize different gel and composite gel polymer electrolyte systems for possible application in lithium ion batteries. An attempt has been made in this book to investigate the basic ion conduction mechanism involved in the formation of polymer electrolytes and develop new polymer gel electrolyte systems which have high ionic conductivity and electrochemically stable. Another approach described in the book is the ion beam modification of materials. The effect of swift heavy ions irradiation on ionic conductivity of gel polymer electrolytes is also discussed in this context. This book will be beneficial for the students and researchers working on polymer electrolyte materials for lithium ion battery.