In Situ X-ray Diffraction of LiCoO₂ in Thin-Film Batteries under High-Voltage Charging

LiCoO2 has been used as the cathode material employed in lithium-ion batteries since their birth, and efforts to improve its performance are still in progress. For example, complete use of lithium provides the theoretical capacity as high as 274 mAh g−1; however, charge-discharge cycling with such a high capacity leads to rapid degradation. The degradation mechanism has been intensively studied in order to increase the practical capacity. Although phase transitions taking place in high-voltage charging have been considered to affect the cycling performance, side reactions induced by the high-voltage charging always overlap to blur the effects of phase transitions on the electrode properties. This study has unveiled the relation between the phase transition and electrode properties by employing a solid electrolyte that suppresses the side reactions efficiently. Electrochemical impedance spectroscopy combined with in-situ X-ray diffraction shows clear correlation between phase transition from O3 to H1-3 and drastic increase in the electrode resistance. The increasing resistance is attributable to formation of narrow interlayers with gallery height of 4.2 Å that impede lithium-ion diffusion.