High-Capacity Cathodes for All-Solid-State Thin-Film Batteries

Abstract

Lithium-ion batteries (LIBs) are critically important rechargeable energy storage devices, finding widespread application in consumer electronics and electric vehicles. However, safety issues associated with their flammability impede their development for many applications. As a fully solid-state device, the thin film battery (TFB) is a compelling candidate for advanced microscale applications, including wearable devices, smart ID cards, and microelectromechanical systems (MEMS). A typical TFB design consists of solid thin films of crystalline cathode, amorphous LiPON electrolyte, Li metal anode and current collectors supported on a rigid substrate. Polycrystalline cathodes are often employed, such as LiCoO2, Li2Mn2O4, LiNiO2, V2O5, etc., because they provide high specific capacity and good reversibility. However, these cathodes require thermal treatment to generate the crystalline microstructure for optimal electrochemical performance, demanding substrates that are stable to several hundreds of degrees Celsius. The industry is interested in lower-cost, flexible all-solid-state thin-film batteries that can be readily integrated into a wide range of applications, including wearable devices. Thus, there is strong demand for thin film cathodes that can be processed at low temperatures.Herein, we report a new amorphous inorganic thin-film cathode material, which is a solid state composite of Li2RuO3 and LiCoO2. (LRCO). Without any post-deposition annealing steps, as-deposited LRCO thin films possess a high discharge capacity of over 110 µAh cm-2 µm-1 at 0.3 C and capacity retention over 92% after 150 cycles owing to both cationic and anionic (oxygen oxidation) redox processes, which is competitive with conventional polycrystalline thin-film cathodes. The surface of LRCO cathode is smooth and isotropic, leading to uniform Li ion transport and thereby reducing the possibility of short-circuiting. More importantly, the entire preparation process is conducted at room temperature, enabling wider substrate options for TFBs, such as Kapton® films, polyethylene terephthalate (PET) films, or even paper sheets. Using PET plastic sheets to support LRCO TFBs; a specific capacity of 101 µAh cm-2 µm-1 at 0.3 C and capacity retention of 97.5% after 120 cycles was achieved, even when the substrate was mechanically bent (flexed).