In collaboration with group of Ni Zhao from the Chinese University of Hong Kong, in our recent publication on Advanced Materials, we reveal the mysterious role of small alkali ions in mixed perovskites in stabilizing the perovskite solar cells. Combining theoretical and experimental efforts from both groups, we demonstrate that incorporation of a very small amount of alkali cations (Rb+, K+, and Na+) in perovskite materials suppress ion migration and improve the device stability of the solar cells. The beneficial effects of these small cations is proven to have to do with their occupancy of interstitial sites in the perovskite lattice. Our findings provide new material design principles to manipulate the structural and ionic properties of complex perovskite materials.