High-energy conversion electrodes undergo successive Li insertion and conversion during lithiation. A primary scientific obstacle to harnessing the potentially high lithium storage capabilities of conversion electrode materials has been the formation of insulating new phases throughout the conversion reactions. These new phases are chemically stable, and electrochemically irreversible if formed in large amounts with coarsening. Herein, we synthesized FeOF conversion material as a model system and mechanistically demonstrate that a thin solid electrolyte [lithium phosphorus oxynitride (LiPON)] atomic layer deposition-deposited on the composite electrode extends the Li insertion process to higher concentrations, delaying the onset of a parasitic chemical conversion reaction and rendering the redox reaction of the protected conversion electrode electrochemically reversible. Reversibility is demonstrated to at least 100 cycles, with the LiPON protective coating increasing capacity retention from 29 to 89% at 100 cycles. Pursuing the chemical mechanism behind the boosted electrochemical reversibility, we conducted electron energy-loss spectroscopy, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and electrochemical measurements that unrevealed the suppression of undesired phase formation and extended lithium insertion of the coated electrode. Support for the delayed consequences of the conversion reaction is also obtained by high-resolution transmission electron microscopy. Our findings strongly suggest that undesired new phase formation upon lithiation of electrode materials can be suppressed in the presence of a thin protection layer not only on the surface of the coated electrode but also in the bulk of the material through mechanical confinement that modulates the electrochemical reaction.