The electrochemical stability of lithium bis(oxatlato) borate (LiBOB) containing solid polymer electrolyte has been evaluated both by inert electrode and real cathodes. Enhanced intrinsic anodic stability and decreased interface impedance, are obtained by addition of nano-sized MgO to PEO20–LiBOB. It is also found that the LiBOB-containing SPEs exhibit prominent kinetic stability between 3.0 and 4.5V. For cells using SPEs as the separators, good cycling performance is obtained for real 4V class cathodes material LiNi1/3Co1/3Mn1/3O2 and LiCoO2. The Li|PEO20–LiBOB|LiNi1/3Co1/3Mn1/3O2 cell takes an initial capacity of 156.8mAhg−1, with retention of 142.5mAhg−1 after 20 cycles at 0.2C-rate. The cell also works well up to 1C-rate. The addition of nano-sized MgO into PEO20–LiBOB readily reduces the irreversible capacity per cycle, both for LiNi1/3Co1/3Mn1/3O2 and LiCoO2 cathodes. In addition, the critical role of LiBOB in obtaining kinetic stability and passivating ability towards cathodes are specially discussed.