Lithium transition metal orthosilicates such as Li2FeSiO4 (LFS) have been recognized as promising cathode materials for application in Li-ion batteries because of their high theoretical energy density, safety, and benign/abundant element composition. Their development, however, has been hampered by the challenge of obtaining phase-pure and defect-free Li2FeSiO4 nanoparticles as non-optimized crystal properties have an adverse effect on structural stability and electrochemical performance. Considering the sustainable potential of hydrothermal synthesis in producing electrode materials, here we employ this process to systematically study critical synthesis parameters for optimal control of particle size, morphology, phase purity, and defects of Li2FeSiO4 crystallized in the orthorhombic crystal system with space group Pmn2(1). It is shown that via a combination of elevated FeSO4 concentration regime and use of EDTA as a complexing agent, phase-pure Pmn2(1) particle formation can be controlled. Synchrotron-based XRD Rietveld refinement and Fe-57 Mossbauer spectroscopy reveal a significant reduction in Li-Fe antisite defects and presence of Fe3+. Additionally, the use of EDTA promotes the formation of unique peanut-shell looking hollow mesocrystals that are advantageous in maximizing electrode/electrolyte contact resulting in higher Li-ion storage capacity than dense LFS particles. On the basis of detailed XRD, SEM, and TEM characterizations, a four-step crystallization mechanism is proposed to explain the formation of the hollow mesocrystals. These findings bring new insight into our pursuit of optimization of Li2FeSiO4 as a cathode material for Li-ion batteries.