Lithium metal silicates (Li2MSiO4, M = Fe or Mn) composed by earth abundant and nontoxic elements are important cathode materials for Li-ion batteries. In contrast to solid-state or sol-gel methods, hydrothermal synthesis conducted in pressure reactors is a favorable processing route as it provides the potential for scalable production at low cost and environmentally benign. However, the key is to control the precipitation reaction to obtain powders with desired properties meeting battery specifications. Through systematic study, we found that Li2MSiO4 with tunable size from nano- to micro-range can be produced by adjusting precursor concentrations, temperature and reaction time. Under optimum condition, high purity and low defect Li2MSiO4 was successfully produced and examined as LIB cathode. The use of complexing agents promoted the formation of unique hollow particles made via self-assembling of elongated crystals. A four-step formation mechanism is proposed based on extensive characterizations with synchrotron XRD, HR-TEM, SEM, and Mössbauer.