Study of the reactivity mechanism of M3B2O6 (with M = Co, Ni, and Cu) toward lithium

The electrochemical reactivity mechanism of M3B2O6 (with M = Co, Ni, and Cu) powders toward lithium was studied by a combination of transmission electron microscopy (TEM), infrared spectroscopy (IR), B-11 magic angle spinning nuclear magnetic resonance (NUS NMR), and electrochemical techniques. The electrochemical properties of these materials as anodes for lithium batteries were investigated and found to be similar to those of 3d-metal oxides. In fact, the reduction process of 3d-metal borates involves their decomposition in metal nanograins dispersed into a lithia matrix surrounded by an organic layer responsible for the observed extra capacity. The lithia matrix consists of a mixture of lithium oxide (Li2O) and lithium orthoborate (Li3BO3). During the subsequent charge, the organic layer vanishes and the metal grains are partially or fully oxidized with the concomitant decomposition of Li2O. The formation of Li2O and metal nanograins during the discharge, as well as that of oxides nanograins during the following charge, were identified by HRTEM studies while Li3BO3 was detected by IR spectroscopy and B-11 MAS NMR techniques.