Origins of Severe Structural Changes during Alloying-Dealloying Reactions in Black Phosphorus.

Li-alloying reactions facilitate the incorporation of a large number of Li atoms into the crystalline structures of electrodes, such as black phosphorus (BP). However, the reactions inevitably induce multistep phase transitions characterized by drastic atomic rearrangements and lattice collapse. Despite many theoretical and experimental studies on alloying mechanisms, long-term debates persist regarding the structures of the intermediate phases, the accurate pathways of phase transitions, the formation of specific configurations, and alloying/dealloying reversibility. Here, through a combination of operando electron diffraction measurements and simulations at the atomic and electronic scales, we identify key factors that govern the severe structural changes during alloying-dealloying reactions in BP. P-P bonds of three-bond P atoms are continuously broken during lithiation, generating two-bond P atoms with a high ability to accept inserted electrons and Li ions. Consequently, the pristine layered structure in BP is transformed to P cages in LiP, which then evolve to chain configurations in LiP and finally to isolated P atoms in LiP. Specifically, the preferential formation of the P cage results from its lowest binding energy with three Li ions compared to other cage isomers. Furthermore, only LiP can be reversibly transformed to the crystalline structure of LiP during charge, but it is thermodynamically favorable for LiP and LiP intermediates to be delithiated to amorphous structures. Our findings offer unique insights into the alloying mechanisms and deepen the fundamental understanding of alloying anode systems.