Big volume changes, the shuttle effect, and poor conductivity are well-known, critical issues of sulfur electrodes that prevent practical application of lithium-sulfur batteries. The design of active materials with a conductive shell provides an effective solution. Traditional strategies have long been limited for practical applications; however, by low productivity and time/energy consuming template-based methods. Here, a facile template-free self-caging nanotechnology for the scalable fabrication of graphene@sulfur nanocages with atomic-scale shells is reported. To do that, a new sulfur-graphene nanochemistry based on a reductive sulfur solution and oxidative sulfonated-graphene dispersion is developed for the first time. With only the help of mechanical mixing, sulfur particles are successfully synthesized in situ and encapsulated into reaction-induced self-assembled sulfonated-graphene nanocages. These unique nanocages not only provide accommodation of the big volume changes in the active materials, but also exhibit robust polysulfide trapping capability due to the synergistic effects from physical blocking and strong chemical absorption. As a result, the resultant sulfur cathodes deliver superior electrochemical performance and have shown an extremely slow capacity decay of 0.019% per cycle at 0.5 C for over 2000 cycles. This study introduces a new self-caging nanochemistry for scalable synthesis of functional nanocages with significant applications beyond lithium-sulfur batteries.