Current technologies for electric vehicles and other applications require high energy, power density, and long cycle life. However, the energy density and safety of current batteries still need to be improved to satisfy these requirements. Typical Li-ion batteries consist of organic electrolytes that are toxic, flammable, and pose significant safety concerns in the events of thermal runaway and improper operation and handling. In recent years, we have explored advanced materials for Li-ion, Na-ion, Zn-ion and Mg-ion batteries that address these key challenges. A new revolutionary concept of an aqueous electrolyte for Li-ion batteries, termed water-in-salt electrolyte, has been developed, extending the electrochemical voltage stability window of aqueous electrolytes to >4.0V and significantly enhancing the energy density. Use of the water-in-salt gel electrolytes, a robust flexible Li-ion battery that can be cut open, exposed to air, and continue to operate despite the mechanical damage has been demonstrated.