A conversion reaction mechanism of Cu(3)B(2)O(6) during the electrochemical reduction in a Li-ion battery test cell was revealed. Different experimental techniques were used: X-ray diffraction (XRD), in situ synchrotron diffraction, quasi in situ X-ray photoelectron and Auger-electron spectroscopy (XPS and AES), and ex situ scanning electron microscopy (SEM). The reduction process changes the initial oxidation state of copper from Cu(II) to Cu(0). After the subsequent oxidation, only the Cu(I) state was recovered. However, a very high amount of carbon in the cathode mixture (65% w/w) enables the reversion of the Cu(II) state due to improved electronic conductivity. Synchrotron in situ diffraction did not indicate any significant change in the lattice parameters of Cu(3)B(2)O(6) during any stage of reduction, which excludes a Li-intercalation mechanism. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3597612] All rights reserved.