Failure mechanisms involved in the high-voltage LiNi0.5Mn1.5O4 electrode are studied at an elevated temperature (60 °C). At ambient temperature (25 °C), this oxygen-deficient spinel-structured electrode shows a reasonable cycle performance, but a rapid capacity decay after 40 cycles at 60 °C. An increase of electrode polarization and formation of electrically isolated active material, both of which become prominent from the 40th cycle, suggest that the electrode failure is mainly caused by the breakdown of electrically conductive network that is made between the active material, carbon and current collector in the composite electrode. The post-mortem electron microscope study reveals a crack formation at the electrode layer/aluminum current collector interface as well as inside the composite electrode layer, which can account for the electrode polarization and electrical isolation. The failure can, however, be mitigated by increasing the load of conductive carbon or polymeric binder, and also by using an etched aluminum foil as the current collector. All these countermeasures seem to be beneficial to reinforce the electrically conductive network.