Alkali-Ion Intercalation Chemistry and Phase Evolution of Sn₄P₃

Despite its high theoretical capacities, Sn4P3 anodes in alkali-ion batteries (AIBs) have been plagued by electrode damage and capacity decay during cycling, mainly rooted in the huge volume changes and irreversible phase segregation. However, few reports endeavor to ascertain whether these causes bear relevance to phase evolution upon cycling. Moreover, the phase evolution mechanism for alkali-ion intercalation remains imprecise. Herein, the structural transformations and detailed mechanisms upon various alkali-ion intercalation processes are systematically revealed, utilizing both experimental techniques and theoretical simulations. The results reveal that the energy storage of Sn4P3 occurs in a two-stage process, starting from an insertion process, followed by a transition process. As the cycle proceeds, the final delithiated/desodiated/depotassiated components gradually trap alkali ions (Li+, Na+, and K+), which is attributed to the incomplete electrochemical transition and difficulty in Sn4P3 regeneration due to the kinetic limitations in removing M (M = Li, Na, and K). Furthermore, Sn4P3 anode obeys the "shrinking core mechanism" in potassium-ion batteries (KIBs), wherein a minor fraction of Sn4P3 in the outer layer of the particles is initially involved in the potassiation/depotassiation processes, followed by a gradual participation of the inner parts until the entire particle is involved. It is worth mentioning that K-Sn alloys are not found to exist during the transition process of KIBs; instead, K-Sn-P phases are found, which makes it differ from that in lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs). These findings are expected to deepen the understanding of the reaction mechanism of Sn4P3 and enlighten the material designs for improved performance.