Three-dimensional (3D) graphene can fully bring the excellent properties of graphene into wide utilization. However, its production requires the use of graphene oxide as the intermediate or harsh chemical vapour deposition conditions, which do not meet the requirements of high quality and low cost simultaneously. Herein, we report a room-temperature low-cost strategy to produce 3D graphene under ambient conditions, which is achieved via carbocation-induced interlayer oxygen release. Graphite layers can transfer electrons to hydrogen peroxide, forming carbocations, which in turn oxidize hydrogen peroxide to release O₂, substantially enlarging the interlayer space of graphite. We show that graphite can expand 1000 times or more into the liquid phase and no oxygen-containing groups are introduced on the basal plane. The obtained 3D graphene has an open porous structure and a specific surface area (SSA) of 1245 m² g⁻¹, which is comparable to that of CVD-grown 3D graphene networks and equivalent to that of 2-layer graphene. Moreover, the quality of 3D graphene is high, with I_D/I_G smaller than 0.2. The as-obtained 3D graphene can be easily exfoliated into graphene sheets with 100% yield. Gifted with well-maintained crystalline quality and highly accessible surface area, the 3D graphene-based composite exhibits ultra-long life when used as lithium anode, with slight capacity degradation after 1000 cycles.