In Situ X-Ray Study of the Electrochemical Reaction of Li with ηʹ-Cu[sub 6]Sn[sub 5]

The electrochemical reaction of lithium with crystalline intermetallic has been studied by means of in situ X‐ray diffraction. The reaction of Li with proceeds in two distinct steps. A first discharge plateau at approximately 0.4 V vs. Li is attributed to the coexistence of and a ternary phase. Upon further reaction coexists with a Li‐rich tin phase and metallic Cu at a potential below 0.1 V. During the charge, the initial intermetallic is reformed in a multistep reaction. First, some Li can be removed from making with the same structure. As Li is removed from it reacts with the available Cu to make . Then, lithium can be removed from , making vacancies in the structure to form . When x in reaches about 1, the removal of further lithium results in the formation of a second phase, . The 0.8 V plateau during the charge of cells is associated with this coexistence of and . The in situ experiments show that both the and the phases can exhibit a range of Li stoichiometries, especially as Li is removed from their structures. When cells are charged and discharged above the lower plateau, to prevent the continual expulsion and reincorporation of Cu from the cathode grains, better cycling is obtained than when cycling includes the lower plateau where metallic copper is formed. © 2000 The Electrochemical Society. All rights reserved.