Solid polymer electrolytes (SPEs) are promising candidates for next-generation energy storage because of their excellent mechanical properties, weather resistance and easy processing. They are better than those of inorganic solid electrolytes (ISEs). Moreover, compared with liquid electrolytes, SPEs also perform a better thermal stability. However, the practical application of SPEs has been impeded by the interphase issues of electrodes/SPEs, especially, the high-capacity cathodes/SPEs interphase in a high energy density battery system. Here we review the key characteristics of SPEs and the interphase issues of high-voltage cathode/SPEs, including the poor interphase contact and interphase instability. In addition, we further analyze the main factors that lead to the serious interphase problems between highvoltage Ni-rich oxide cathodes and SPEs. The effective strategies to solve these problems are also summarized. Finally, we propose a prospect for the improvement of high-voltage Ni-rich cathode/SPE interphase, which provides a guideline for future research on solid Li batteries.