Thin-film and bulk investigations of LiCoBO₃ as a li-ion battery cathode

The compound LiCoBO₃ is an appealing candidate for next-generation Li-ion batteries based on its high theoretical specific capacity of 215 mAh/g and high expected discharge voltage (more than 4 V vs Li ⁺/Li). However, this level of performance has not yet been realized in experimental cells, even with nanosized particles. Reactive magnetron sputtering was therefore used to prepare thin films of LiCoBO₃, allowing the influence of the particle thickness on the electrochemical performance to be explicitly tested. Even when ultrathin films (∼15 nm) were prepared, there was a negligible electrochemical response from LiCoBO ₃. Impedance spectroscopy measurements suggest that the conductivity of LiCoBO₃ is many orders of magnitude worse than that of LiFeBO ₃ and may severely limit the performance. The unusual blue color of LiCoBO₃ was investigated by spectroscopic techniques, which allowed the determination of a charge-transfer optical gap of 4.2 eV and the attribution of the visible light absorption peak at 2.2 eV to spin-allowed d → d transitions (assigned as overlapping ⁴A₂′ to ⁴A₂′ and ⁴E′ final states based on ligand-field modeling).