Origins of Severe Structural\nChanges during Alloying–Dealloying\nReactions in Black Phosphorus

Li-alloying\nreactions facilitate the incorporation of\na large number\nof Li atoms into the crystalline structures of electrodes, such as\nblack phosphorus (BP). However, the reactions inevitably induce multistep\nphase transitions characterized by drastic atomic rearrangements and\nlattice collapse. Despite many theoretical and experimental studies\non alloying mechanisms, long-term debates persist regarding the structures\nof the intermediate phases, the accurate pathways of phase transitions,\nthe formation of specific configurations, and alloying/dealloying\nreversibility. Here, through a combination of operando electron diffraction\nmeasurements and <i>ab initio</i> simulations at the atomic\nand electronic scales, we identify key factors that govern the severe\nstructural changes during alloying–dealloying reactions in\nBP. P–P bonds of three-bond P atoms are continuously broken\nduring lithiation, generating two-bond P atoms with a high ability\nto accept inserted electrons and Li ions. Consequently, the pristine\nlayered structure in BP is transformed to P<sub>7</sub> cages in Li<sub>3</sub>P<sub>7</sub>, which then evolve to chain configurations in\nLiP and finally to isolated P atoms in Li<sub>3</sub>P. Specifically,\nthe preferential formation of the P<sub>7</sub> cage results from\nits lowest binding energy with three Li ions compared to other cage\nisomers. Furthermore, only LiP can be reversibly transformed to the\ncrystalline structure of Li<sub>3</sub>P<sub>7</sub> during charge,\nbut it is thermodynamically favorable for Li<sub>3</sub>P<sub>7</sub> and Li<sub>3</sub>P intermediates to be delithiated to amorphous\nstructures. Our findings offer unique insights into the alloying mechanisms\nand deepen the fundamental understanding of alloying anode systems.