Polymer solid electrolytes for solid-state batteries typically suffer from low ionic conductivity and low oxidative stability. We report a polymer electrolyte based on a polymerized ionic liquid and an ionic liquid plasticizer offering simultaneously a high room-temperature ionic conductivity of 0.8 mS cm^-1 and a high oxidative stability > 5.0 V vs Li⁺/Li. The electrolyte is compatible with lithium metal, exhibits high thermal stability up to 300 °C, is non-flammable upon direct flame exposure, shows decent lithium-ion transference number of 0.44, and possesses a low gravimetric density of only 1.6 g cm^-3, important when targeting batteries with high gravimetric energy density. In symmetric lithium metal cells, the electrolyte enables stable lithium plating and stripping at 0.1 mA cm⁻² with a capacity of 1 mAh cm⁻² per half cycle for over 1700 h at 25 °C. Cells with a lithium metal anode cycled against a state-of-the-art high-energy uncoated layered LiNi_0.8Mn_0.1Co_0.1O₂ cathode achieve an initial capacity of 162 mA h g^-1 and a capacity retention of 72% after 600 cycles to an upper cut-off voltage of 4.4 V at 0.1 mA cm^-2 and 25 °C. Cells with a high-voltage spinel LiMn_1.5Ni_0.5O₄ cathode reach an initial capacity of 132 mAh g^-1 and a capacity retention of 76% after 300 cycles to an upper cut-off voltage of 5 V at 0.1 mA cm^-2 and 25 °C, confirming the high oxidative stability and technological potential of this electrolyte for high-voltage solid-state lithium metal batteries.