While lithium hexafluorophosphate (LiPF6) still prevails as the main conducting salt in commercial lithium-ion batteries, its prominent disadvantage is high sensitivity toward water, which produces highly corrosive HF that degrades battery performance. The hydrolysis mechanism and its correlation with high voltage in the battery environment remain poorly understood, despite the wide application of high voltage cathode. In this work, combining theoretical and experimental approaches, we identified the direct reaction between H2O and PF6 – as main source of HF based on the preferential solvation of PF6 – anion by water and the low energy barrier for the decomposition of PF6 ––H2O complex. Such a hydrolysis process would be accelerated by high voltage the electrolytes face at the cathode side. This important clarification of electrolyte failure mechanism points us to design more effective mitigation strategies with the purpose of stabilizing LiPF6-based electrolytes for high voltage LIBs.