Quasi-solid-state silver-zinc (Ag-Zn) batteries, featuring high energy density, stable voltage output, and outstanding safety, have been considered as promising power source for wearable electronics, while they suffer from poor areal capacity and insufficient rechargeability caused by low surface area and structural deterioration of cathode. In this work, we address these problems through redesigning the cathode with core-shell AgCl/carbon fiber structure decorated with MXene nanosheets. Benefiting from the unique structure with high surface area, the capacity is over two times higher than that with irregular morphology, and undesired Ag migration is suppressed owing to MXene protective layer, leading to enhanced structural integrity and ultralong cycle life. Theoretical calculations and experimental result reveal that a heterostructure is formed between MXene and Zn-coated AgCl, stabilizing the cathode structure. The battery demonstrates high capacity of 2.97 mAh cm−2 and impressive cyclability, maintaining 78% of initial capacity after 400 cycles at 4 mA cm−2, with nearly 100% coulombic efficiency. Moreover, robust mechanical flexibility is demonstrated in the separator-free batteries, and they can operate when twisted, cut, put on fire, and sealed in ice, suggesting the viability for practical application scenarios. This work offers pivotal guidance to construct stable electrodes and advanced batteries for powering electronics.