Sodium-rich metallic NaxCl has received significant attention as a low-cost alternative to the conventional electrode materials used in Li-ion batteries. However, the poor cyclability of NaxCl remains a major challenge to its practical application. Here, we developed a simple method for improving the electrochemical performance of NaxCl by controlling the upper limit of cut-off voltage. We demonstrated that additional Na-vacancy defects can be introduced in the NaCl structure during the high-voltage activation process at 4.5 V. The structure then accommodated more sodium ions during the next discharge, resulting in increased capacity. At the same time, Cl-ions released by NaCl decomposition were oxidized to form Cl-based organic species at the active material interfaces. This played a crucial role in protecting the NaCl electrode from undesired side reactions at high voltage. In short, this control of the charging protocol helped to induce more vacancies in the NaCl structure, as well as form stable interphases on the electrode surface, contributing to the increased capacity and enhanced cycle stability. This study will help in exploring a new approach for developing low-cost and high-capacity electrode material, which can potentially be applied in future energy-storage systems.