A method of synthesizing defect-free phospho-olivine materials is disclosed. The method is based on direct hydrothermal synthesis of phospho-olivine compound(s) and subsequent lattice reordering at or near the transition temperature to eliminate lattice defects or on one-pot in situ hydrothermal synthesis of phospho-olivine compound(s), where the cation ordering occurs during dwell time after rapid synthesis to eliminate lattice defects. The disclosed methods produce defect-free phospho-olivine compound(s) having a crystal lattice with a Pnma space group. In order to determine the exact transition temperature for complete removal of single- or mixed-transition metals from lithium sites or to monitor the crystal growth and removal of single- or mixed-transition metals from lithium sites during the hydrothermal synthesis, the method encompasses a procedure for determining and monitoring defects in the phospho-olivine phases using X-ray diffraction.
STATEMENT OF GOVERNMENT LICENSE RIGHTS
 The present invention was made with government support under contract number DE-AC02-98CH10886 awarded by the U.S. Department of Energy. The United States government may have certain rights in this invention.