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 Cu 6 Sn 5 -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., ε-Cu 3 Sn), 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, Cu 3 Sn gives way to a Sn-deficient Cu 41 Sn 11 phase and some (ejected) β-Sn. Cu 41 Sn 11 behaves fairly similar to Cu 3 Sn during lithiation, forming primarily Li 7 Sn 2 toward the end, but with itself getting reformed upon delithiation (unlike Cu 3 Sn). With continued cycling, Cu 41 Sn 11 gives way to ternary LiCu 2 Sn as the primary phase, which becomes dominant by ∼25 cycles. Upon lithiation, LiCu 2 Sn forms Li 3 CuSn (via Li 2 CuSn), which reverts back to LiCu 2 Sn 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 ∼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 Cu 3 Sn-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.
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