Operando Investigations Toward Changeover Of Phase Assemblage And Associated Electrochemical Behavior During Lithiation/Delithiation Cycles Of Sn-Based Intermetallic Electrodes

Moving toward the next-generation alkali metal-ion battery systems, poor cyclic stability of "alloying-reaction"-based anode materials, such as Sn, is still an unresolved issue of immense significance. There is some interest/promise with Cu-Sn intermetallic-based anodes (viz., primarily Cu6Sn5-based), but with the stability still lagging behind the desired level and lithiation/delithiation mechanisms not being understood. Against this backdrop, starting with a less explored Cu-Sn intermetallic (viz., epsilon-Cu3Sn), operando synchrotron X-ray diffraction scans obtained during galvanostatic lithiation/delithiation cycles have indicated that changes in phase assemblage/evolution take place not only during an individual Li-alloying/dealloying cycle but also across different cycles. In this context, by the end of just one full lithiation/delithiation cycle, Cu3Sn gives way to a Sn-deficient Cu41Sn11 phase and some (ejected) beta-Sn. Cu41Sn11 behaves fairly similar to Cu3Sn during lithiation, forming primarily Li7Sn2 toward the end, but with itself getting reformed upon delithiation (unlike Cu3Sn). With continued cycling, Cu41Sn11 gives way to ternary LiCu2Sn as the primary phase, which becomes dominant by similar to 25 cycles. Upon lithiation, LiCu2Sn forms Li3CuSn (via Li2CuSn), which reverts back to LiCu2Sn upon delithiation. This process does not involve dissociation to first form Sn during lithiation; which, otherwise, is the norm with Cu-Sn binary intermetallics. Such changes in phase assemblage and associated lithiation/delithiation pathways resulted in reduction of potential hysteresis and progressive increase of Li-storage capacity upon cycling beyond 25 cycles. Accordingly, a reversible capacity of similar to 400 mA h/g could be obtained after 100 cycles, with the increasing trend promising further increment upon continued cycling. On a practical front, such excellent cyclic stability and "safe" operating potential of Cu3Sn-based electrodes are very encouraging and ultimate proofs toward their "activity", with the insights into various changes in phase assemblage/evolution and associated lithiation/delithiation mechanisms being expected to lead to the development of stable-cum-"safe" intermetallic anodes for Li-ion batteries and beyond.