Phase change induced degradation of amorphous vanadium oxide cathode thin film during charge-discharge

Abstract

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/V2O5 full cell structure, charge-discharge performances were measured according to cycling number. Even though the full cell structure showed an average capacity of 15 muAh/cm(2) 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 V2O5 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.