A continuous hydrothermal process for producing LFP/LMFP cathode materials for lithium-ion batteries. The reactant solutions include: (1) a lithium precursor (LiOH) and a carbon source (15 wt % sucrose); (2) an iron precursor (FeSO4) and a phosphorus precursor (H3PO4); and in the case of LMFP, a manganese precursor (MnSO4) and a surfactant in solution 2. Reactant solutions are fed into a series of one or more continuous stirred tank reactors (CSTRs) at a constant flowrate. Active LFP/LMFP flows out of the CSTRs and into a collection tank, where it is cooled and depressurized. The product flows into a slurry tank, then a centrifugal separator to remove aqueous waste. The LFP/LMFP is transferred to a continuous rotary kiln for drying and sintering, and the carbon coating forms. The disclosed processes produce LFP/LMFP with small average particle size, high purity, high capacity, and high yield without any ball-milling or sieving steps.