Li2MnSiO4 is considered as a promising alternative cathode material for high-energy density lithium-ion batteries due to its high theoretical capacity. However, it suffers from poor electronic conductivity and lithium-ion diffusivity. Here we provide a series of Mn-deficiency Li2Mn(1-x)SiO4/C (x = 0.00, 0.02, 0.06 and 0.10) by a step-sintering method to address the problems. It is demonstrated that Mn deficiency improves the bulk electronic conductivity of Li2MnSiO4 by introducing Mn vacancy defects into its crystal lattices. The experimental and first principles calculation show that the Mn vacancy defects can decrease the band gaps and increase the carrier concentrations. The electronic conductivity of Li2Mn0.94SiO4/C is increased to 2.3 × 10−3 S cm−1, approximate three orders of magnitude higher than that of Li2MnSiO4/C. Besides, lithium-ion diffusivity is also raised by ∼2.1 times owing to the decreased particle sizes and prolonged LiO bond. Finally, Li2Mn0.94SiO4/C delivers an initial discharge capacity of 250.3 mA h g−1 at 25 mA g−1, which is 24.3% higher than that of Li2MnSiO4/C. And the discharge capacity of Li2Mn0.94SiO4/C is increased to 141.4 mA h g−1 at 250 mAh g−1 current density, compared with Li2MnSiO4/C (74.6 mA h g−1).