Encapsulation　of　lithium　in　the　confined　spaces　within　individual　nanocapsules　is　intriguing　and　highly　desirable　for　developing　high-performance　Li　metal　anodes.　This　work　aims　for　a　mechanistic　understanding　of　Li　encapsulation　and　its　confined　growth　kinetics　inside　1D　enclosed　spaces.　To　achieve　this,　amorphous　carbon　nanotubes　are　employed　as　a　model　host　using　in　situ　transmission　electron　microscopy.　The　carbon　shells　have　dual　roles,　providing　geometric/mechanical　constraints　and　electron/ion　transport　channels,　which　profoundly　alter　the　Li　growth　patterns.　Li　growth/dissolution　takes　place　via　atom　addition/removal　at　the　free　surfaces　through　Li+　diffusion　along　the　shells　in　the　electric　field　direction,　resulting　in　the　formation　of　unusual　Li　structures,　such　as　poly-crystalline　nanowires　and　free-standing　2D　ultrathin　(1–2 nm)　Li　membranes.　Such　confined　front-growth　processes　are　dominated　by　Li　{110}　or　{200}　growing　faces,　distinct　from　the　root　growth　of　single-crystal　Li　dendrites　outside　the　nanotubes.　Controlled　experiments　show　that　high　lithiophilicity/permeability,　enabled　by　sufficient　nitrogen/oxygen　doping　or　pre-lithiation,　is　critical　for　the　stable　encapsulation　of　lithium　inside　carbonaceous　nanocapsules.　First-principles-based　calculations　reveal　that　N/O　doping　can　reduce　the　diffusion　barrier　for　Li+　penetration,　and　facilitate　Li　filling　driven　by　energy　minimization　associated　with　the　formation　of　low-energy　Li/C　interfaces.　©　2021　Wiley-VCH　GmbH