In this work, a novel liquid nitrogen quenching treatment is introduced for the first time to design and prepare a grain boundary-rich intermetallic compound/metal/carbon composite, serving as an efficient electrode material for sodium ion removal from simulated seawater. The liquid nitrogen quenching method effectively enhances the grain boundary density within the composite due to the rapid cooling-induced forces, thereby creating abundant active sites. Consequently, Co7Fe3/Co/C-700 achieves optimal performance in sodium ion embedding and de-embedding, with a capacity of 207.5 mg/g. Furthermore, sodium ion adsorption/desorption mechanisms are substantiated through ex-situ techniques, revealing dynamic atomic and electronic structure evolutions under operational conditions. Density Functional Theory (DFT) calculations validate the pivotal role of grain boundary sites in enhancing activity, elucidating the correlation between sodium ion adsorption and grain boundaries. Overall, this work elucidates the significant correlation between adsorption behavior and grain boundaries, proposing an innovative strategy for developing electrode materials for capacitive deionization.