Long-cycling dendrite-free solid-state lithium metal batteries (LMBs) require fast and uniform lithium-ion (Li+) transport of solid-state electrolytes (SSEs). However, the SSEs still face the problems of low ionic conductivity, low Li+ transference number, and unstable interface with lithium metal. In this work, a novel strategy of frustrated Lewis pairs (FLPs) modulating solid polymer electrolytes (SPEs) has been firstly proposed that enables durable Li reversible cycling. The tunable strength of Lewis acid and base dual-active sites of nickel borate FLPs can synergistically promote both the dissociation of lithium salts and the transfer of Li+. As a consequence, the FLPs modulated SPEs (SPE-NiBO-150) exhibit high ionic conductivity of 4.92×10−4 S cm−1, high Li+ transference number of 0.74, and superior interface compatibility with both lithium anode and LiFePO4 cathode at room-temperature. The Li//SPE-NiBO-150//Li symmetric cell demonstrates ultralong cycle stability (over 10,000 h (417 days) at both current density of 0.2 and 0.5 mA cm−2), and the assembled solid-state LiFePO4//SPE-NiBO-150//Li battery also shows excellent performance (86% capacity retention for 300 cycles at 0.5C). The present work supplies a new insight into designing high-performance SPEs for solid-state LMB applications.