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Layered transition metal oxides (LTMOs) are attractive cathode candidates for rechargeable secondary batteries because of their high theoretical capacity. Unfortunately, LTMOs suffer from severe capacity attenuation, voltage decay, and sluggish kinetics, resulting from irreversible lattice oxygen evolution and unstable cathode-electrolyte interface. Besides, LTMOs accumulate surface residual alkali species, like hydroxides and carbonates, during synthesis, limiting their practical application. Herein, a universal strategy is suggested to in situ convert surface residual alkali into a stable polymer coating layer for LTMOs, thus turning wastes into treasure. The formation process of polymer coating involves NH4F treatment to consume residual alkali, then utilizing generated fluorides to induce the ring-opening polymerization of tetrahydrofuran. Implementing this strategy to Li-rich Mn-based cathode materials (LRM) results in a notable reduction in voltage hysteresis, along with enhanced kinetics and cycling stability in lithium-ion batteries. With this layer of encapsulation, surface lattice oxygen release and layered-to-spinel phase transition of LRM are significantly alleviated with minimal mechanical degradation and surface parasitic reactions. Such strategy can also be applied to air-sensitive sodium-rich LTMOs in sodium-ion batteries, which showcases superior universality. This work might provide a promising solution to overcome residual alkali and interfacial instability issues for LTMOs in practical application.