<p>Side reactions between high-voltage cathodes and electrolytes remain a critical obstacle to the advancement of solid-state batteries—particularly for Na-ion systems—due to the higher Na⁺/Na redox potential. Despite recent extensive efforts, achieving a long cycle life is still challenging at the 4.2 V cut-off (versus Na⁺/Na). Here we design a room-temperature isotropic epitaxial growth to achieve a relatively uniform and dense metal–organic framework epilayer on Na₃V₂O₂(PO₄)₂F surfaces. Despite using polyethylene oxide, a typical ether-based solid polymer electrolyte, the cathode with isotropic epilayer exhibits enhanced cycling performance at the 4.2 V cut-off (retaining up to 77.9% of its initial capacity after 1,500 cycles). Combining experimental measurements and theoretical analyses, the key factor governing isotropic epitaxial growth behaviour is explicitly elucidated. Furthermore, we develop a self-designed high-sensitivity characterization method, in situ linear sweep voltammetry coupled with gas chromatography–mass spectrometry, to elucidate the failure mechanism of polyethylene oxide on Na₃V₂O₂(PO₄)₂F surfaces and and to reveal the excellent electrochemical stability of the isotropic epilayer. Interestingly, the universality of this approach has also been validated, highlighting its strong potential as an effective strategy for enabling high-energy-density batteries.</p>