Role of electrochemically driven Cu nanograins in CuGa₂ electrode

Upon lithiation, the active (Ga) and inactive component (Cu) in a binary intermetallic CuGa₂ electrode are converted to nanograms (<50 nm) of Li_xGa and metallic Cu, respectively. It was found that the Cu nanograins are not idling as an inactive ingredient but have a strong influence on the thermodynamic and kinetic properties of Li_xGa phases through a partial bonding to Ga atoms of Li_xGa (Cu→Ga-Li). The Li _xGa phase diagram is altered by the presence of Cu nanograins, eloquently demonstrating that the surface energy becomes more important than internal energy in controlling thermodynamics of nanosized materials. The lithiation rate is slower than that for pure Ga electrode because of activation energy needed for bond cleavage of the partial bonding. The delithiation rate capability is, however, exceptionally good; the capacity at 26 C amounts to 91% of that at 0.13 C, which is indebted to a weakening in the Ga-Li bond by the Cu→Ga partial bonding.