Thermo-electrochemical activation of Cu₃Sn negative electrode for lithium-ion batteries

A Cu₃Sn film electrode (thickness = ca. 3 μm) is prepared by DC magnetron sputtering deposition of Sn on Cu substrate and subsequent annealing at 300 °C for 30 h. At 25 °C, this Cu-Sn binary intermetallic compound is inactive for lithiation, but becomes active at elevated temperatures due to facilitation of Cu-Sn bond cleavage for the conversion-type lithiation. The lithiated product at 120 °C is the most Li-rich Li-Sn alloy (Li ₁₇Sn₄). Upon de-lithiation, the Cu-Sn intermetallics of different compositions are generated by the reaction between the metallic Sn that is restored from Li₁₇Sn₄ and the idling metallic Cu. The nature of the resulting intermetallics is dependent on the de-lithiation temperature: Cu₁₀Sn₃ at 120 °C and Cu ₆Sn₅ at 25 °C. Only the latter is active for lithiation in the subsequent room-temperature cycling. That is, Cu₃Sn is thermo-electrochemically activated to be Cu₆Sn₅ by lithiation at 120 °C and subsequent de-lithiation at 25 °C. The higher lithiation activity observed with the more Sn-rich phase (Cu₆Sn ₅) compared to the initial one (Cu₃Sn) has been accounted for by the higher equilibrium lithiation potential (thermodynamic consideration) and smaller number of Cu-Sn bonds to be broken (kinetic consideration).