<p>Designing hybrid battery systems based on Li/Na coexisting silicate framework with synergized lithium’s high energy density with sodium’s economic advantages is still challenging. Herein, a series of Li_2-<i>x</i>Na_<i>x</i>FeSiO₄ (where <i>x</i> = 0, 0.25, 0.5, and 1.0) cathode materials were constructed through vibratory ball milling-assisted solid-state synthesis. The optimized sample at the composition <i>x</i> = 0.25 showed a single-phase monoclinic P2₁-Li₂FeSiO₄ phase, with exceptional electrochemical performances. By contrast, higher sodium contents (<i>x</i> ≥ 0.5) resulted in dual-phase mixtures of Na₂FeSiO₄ and Li₂FeSiO₄, along with some undesirable impurities of Li₅FeO₄ and Na₆Si₂O₇. The electrochemical characterization revealed that the introduction of sodium ions in the deintercalation reaction increased the interfacial charge transfer resistance (<i>R</i>_ct) due to the Na⁺ barrier, but also significantly improved the Li⁺ diffusion coefficient (<i>D</i>_Li⁺), suitable for enhancing ionic utilization efficiency for an optimized specific capacity. Overall, strategically incorporating sodium at lithium sites can effectively increase the storage capacity while reducing dependence on lithium resources for economical energy storage devices.</p>