Widespread commercial adoption of polymer electrolytes for lithium-ion batteries has been hindered by subpar transport properties, namely, ionic conductivities of <1 mS/cm at room temperature and slower Li+ compared to anion transport. The developed polymer and water-in-salt electrolyte demonstrated preferential Li+ transport compared to the anion via pulsed field gradient NMR, acceptable ionic conductivities of >1 mS/cm at 25 degrees C, and an extended electrochemical stability window compared to water-in-salt electrolytes. This polymer electrolyte has a flexible liquid/solid transition through polymer molecular weight tuning, and both liquid and solid iterations are investigated. MD simulations provided additional insight into the Li+ solvation environment and the mechanism of fast, preferential Li+ transport through percolation of water-rich Li+(H2O)(n) nanodomains inside the poly(ethylene oxide) matrix.