Phase Change Induced Degradation of Amorphous Vanadium Oxide Cathode Thin Film during Charge-Discharge

Amorphous vanadium oxide (V₂O₅) is a very good candidate as material for cathode thin film since it has a relatively high capacity. In addition, the room temperature deposition process is valuable in V₂O₅ thin film fabrication. Due to these advantages, much effort to grow amorphous V₂O₅ thin film has been made. In this research, we successfully grew amorphous V₂O₅ thin film using room temperature sputtering. Based on a Li/LiPON/V₂O ₅ full cell structure, charge-discharge performances were measured according to cycling number. Even though the full cell structure showed an average capacity of 15 μAh/cm² over more than 500 cycles, a capacity fade was shown after a few cycles. Many reports revealed that the phase change of V₂O₅ from amorphous to crystalline made this kind of capacity fade. In order to investigate this phenomenon, high-resolution transmission electron microscopy (HRTEM) was employed. The as-deposited V ₂O₅ thin film consisted of a homogeneous amorphous without any grain-boundary and/or polycrystalline island. However, the microcrystalline V₂O₅ phase was randomly distributed in the amorphous V₂O₅ thin film matrix after 450 cycles by cross sectional TEM (XTEM). That is, some amorphous phase in the V₂O₅ matrix changed to the crystalline phase. This crystalline phase strongly prevented the extraction of Li ions during the charge process, which induced the irreversible diffusion of Li ions from cathode to anode. From this result, a high efficiency thin film battery based on amorphous V₂O₅ can be fabricated by preventing amorphous-crystal phase transformation during cycling.