Cover and Barrier Layers for Thin-Film Rechargeable Lithium Battery

Introduction All-solid-state thin-film Li-ion battery with glassy inorganic electrolyte reported to show very high cycle stability. All non-annealing process and Li-metal free structure is developed aiming for high productivity. During its charging process the surface of anode collector deforms by Li precipitation between LiPON and the collector, and after discharging process the surface recovers totally flat by Li diffusion back to the cathode. Since the volume continuously changed by charging and discharging process, the cover barrier layer affected its physical behavior as well as its durability. Thus it is essential to control the property of the cover layer. In addition, since the battery consists largely of substrate and cover layer in volume, reducing the thickness of cover layer leads to higher energy density. Experimental Sample preparation is basically identical to the previous work. Battery size was 1 cm in the active area. Cover layer consists of UV curable resin formed by spin-coating, and for further protection from moisture SiN (25 nm) barrier layer was sputtered on top of the UV resin cover layer (Fig. 1). Charging and discharging was performed at 5 C rate for the thin film batteries with several different cover layer conditions. Water vapor transmission of those films was measured with TDS. The hardness of cover layer materials was measured with pico-indentor (Fischer HM500). Results and Discussion For the battery without the cover layer, it was shorted out during first charging due to deterioration of the anode current collector. Surface deformation was observed with AFM after charging. Height of plated Li was higher than the battery with cover layer. Then for the surface protection, inorganic layer (SiN or SiO2) was directly sputtered on the anode collector. It also resulted as short circuit after few cycles of charging, and its capacity was lower than theoretical value. With UV resin cover, the initial charging cycle drastically improved as shown in Fig.2. The anode surface was relatively flat compared to the battery without UV resin. Additionally, inorganic layer was sputtered on top of the UV resin to decrease the water vapor permeability. The capacity durability was enhanced as shown in the dashed line in Fig. 3. The improvement by UV resin cover is considered due to the difference of hardness of the cover material. Martens hardness of SiN and UV resin are 4268 and 199 N/mm respectively. Thus the suitable cover layer material is the material with moderate hardness. There is a clear correlation between capacity durability and physical property of the cover layer.