Herein, we develop a novel all-in-one cathode-separator-anode monolith architecture designed for high-capacity, ultra-thin flexible batteries. This architecture involves directly casting electrode slurry onto both sides of a polypropylene (PP) separator. Controlled volatility and wettability of the solvent system are critical for the formation of neat electrode coating layers on the PP separator. The monolith structure offers remarkable flexibility and intimate contact between the electrode and separator, which is especially advantageous when stacked for higher areal capacity. The monoliths are conjoined into an all-in-one multi-layered monolith structure, deploying electrode slurry as an 'electrochemically active adhesive' between them, enabling the creation of high-capacity, ultra-thin flexible batteries. The resulting pouch cell exhibits a high capacity of 44.5 mA h (areal capacity of 4.9 mA h cm-2) at 1 mA and a thickness below 1 mm. Notably, this cell boasts superior rate capability even at this high capacity, showing a discharge capacity of 36.3 mA h at 20 mA. Practical application of this high-capacity, ultra-thin flexible battery is demonstrated in a band-type light-therapy patch, which shows operational stability when bent around a human arm. This development marks a significant advancement in the design of ultra-thin, high-capacity flexible batteries, with potential applications in flexible and wearable battery technologies. All-in-one cathode-separator-anode monolith and multi-layered monolith systems for flexible batteries were elaborately designed, demonstrating superior rate capability and high areal capacity with ultra-thin thickness under 1 mm.
