The high-yield synthesis of Li2MnSiO4/C composites was realized under hot isostatic pressure (HIP) using a stainless steel capsule. We investigated the effect of temperature and pressure on the synthesis of Li2MnSiO4 and found that the synthesis temperature decreased with the increase of HIPing pressure and vice versa. By adjusting the reaction parameters (temperature, pressure, and time), we obtained Li2MnSiO4/C composites with different size and morphology. Among these various products, Li2MnSiO4/C films, which are composed of nanoparticles about 10–15 nm diameter, deliver a discharge capacity of 273 mAh g−1 at 0.05 C, and maintain a discharge capacity of 180 mAh g−1 after 50 cycles. When the current is increased to 5 C, the discharge capacity of 86 mAh g−1 can be retained after 20 cycles.