Accelerated Removal of Fe-Antisite Defects while Nanosizing Hydrothermal LiFePO<sub>4</sub> with Ca<sup>2+</sup>

Based on neutron powder diffraction (NPD) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), we show that calcium ions help eliminate the Fe-antisite defects by controlling the nucleation and evolution of the LiFePO<sub>4</sub> particles during their hydrothermal synthesis. This Ca-regulated formation of LiFePO<sub>4</sub> particles has an overwhelming impact on the removal of their iron antisite defects during the subsequent carbon-coating step since (i) almost all the Fe-antisite defects aggregate at the surface of the LiFePO<sub>4</sub> crystal when the crystals are small enough and (ii) the concomitant increase of the surface area, which further exposes the Fe-antisite defects. Our results not only justify a low-cost, efficient and reliable hydrothermal synthesis method for LiFePO<sub>4</sub> but also provide a promising alternative viewpoint on the mechanism controlling the nanosizing of LiFePO<sub>4</sub>, which leads to improved electrochemical performances.