Metal-organic　frameworks　(MOFs)　present　diverse　building　blocks　for　high-performance　materials　across　industries,　yet　their　crystallization　mechanisms　remain　incompletely　understood　due　to　gaps　in　nucleation　and　growth　knowledge.　In　this　study,　MOF　structural　evolution　is　probed　using　in　situ　liquid　phase　transmission　electron　microscopy　(TEM)　and　cryo-TEM,　unveiling　a　blend　of　classical　and　nonclassical　pathways　involving　liquid-liquid　phase　separation,　particle　attachment-coalescence,　and　surface　layer　deposition.　Additionally,　ultrafast　high-temperature　sintering　(UHS)　is　employed　to　dope　ultrasmall　Cobalt　nanoparticles　(Co　NPs)　uniformly　within　nitrogen-doped　hard　carbon　nanocages　confirmed　by　3D　electron　tomography.　Lithium-sulfur　battery　tests　demonstrate　the　nanocage-Co　NP　structure＇s　exceptional　capacity　and　cycling　stability,　attributed　to　Co　NP　catalytic　effects　due　to　its　small　size,　uniform　dispersion,　and　nanocage　confinement.　The　findings　propose　a　holistic　framework　for　MOF　crystallization　understanding　and　Co　NP　tunability　through　ultrafast　sintering,　promising　advancements　in　materials　science　and　informing　future　MOF　synthesis　strategies　and　applications.　Utilizing　in　situ　liquid　phase　transmission　electron　microscopy　and　cryo-TEM,　classical　and　nonclassical　pathways　in　MOF　structural　evolution　are　uncovered.　Ultrafast　high-temperature　sintering　uniformly　dopes　ultrasmall　cobalt　nanoparticles　within　nitrogen-doped　hard　carbon　nanocages,　confirmed　by　3D　tomography.　Lithium-sulfur　battery　tests　demonstrate　exceptional　capacity　and　cycling　stability,　signifying　significant　advancements　in　materials　science　and　MOF　synthesis　strategies.　image