Phase Transformations and Phase Segregation during Potassiation of SnxPy Anodes

摘要

K-ion batteries(KIBs) have the potential to offer a cheaper alternative to Li-ion batteries(LIBs) using widely abundant materials.Conversion/alloying anodes have high theoretical capacities in KIBs,but it is believed that electrode damage from volume expansion and phase segregation by the accommodation of large K-ions leads to capacity loss during electrochemical cycling.To date,the exact phase transformations that occur during potassiation and depotassiation of conversion/alloying anodes are relatively unexplored.In this work,we synthesize two distinct compositions of tin phosphides,Sn4P3 and SnP3,and compare their conversion/alloying mechanisms with solid-state nuclear magnetic resonance(SSNMR) spectroscopy,powder X-ray diffraction(XRD),and density functional theory(DFT) calculations.Ex situ ~(31)P and ~(119)Sn SSNMR analyses reveal that while both Sn4P3 and SnP3 exhibit phase separation of elemental P and the formation of KSnP-type environments(which are predicted to be stable based on DFT calculations) during potassiation,only Sn4P3 produces metallic Sn as a byproduct.In both anode materials,K reacts with elemental P to form K-rich compounds containing isolated P sites that resemble K3P but K does not alloy with Sn during potassiation of Sn4P3.During charge,K is only fully removed from the K3P-type structures,suggesting that the formation of ternary regions in the anode and phase separation contribute to capacity loss upon reaction of K with tin phosphides.