Long-life and safe lithium-ion batteries have been long pursued to enable electrification of the transportation system and for grid applications. However, the poor safety characteristics of lithium-ion batteries have been the major bottleneck for the widespread deployment of this promising technology. Here, we report a novel nonaqueous Li(2)B(12)F(12-x)H(x) electrolyte, using lithium difluoro(oxalato)borate as an electrolyte additive, that has superior performance to the conventional LiPF(6)-based electrolyte with regard to cycle life and safety, including tolerance to both overcharge and thermal abuse. Cells tested with the Li(2)B(12)F(9)H(3)-based electrolyte maintained about 70% initial capacity when cycled at 55 °C for 1,200 cycles, and the intrinsic overcharge protection mechanism was active up to 450 overcharge abuse cycles. Results from in situ high-energy X-ray diffraction showed that the thermal decomposition of the delithiated Li(1-x)[Ni(1/3)Mn(1/3)Co(1/3)](0.9)O(2) cathode was delayed by about 20 °C when using the Li(2)B(12)F(12)-based electrolyte.